Comparison of Tensile and Compressive Properties of Carbon/Glass Interlayer and Intralayer Hybrid Composites

materials

Article Comparison of Tensile and Compressive Properties of Carbon/Glass Interlayer and Intralayer Hybrid Composites

Weili Wu 1 ID , Qingtao Wang 1 ID and Wei Li 1,2,3,* ID

1 College of Textiles, Donghua University, No. 2999, Northern Renmin Rd., Songjiang District, Shanghai 201620, China; [email protected] (W.W.); [email protected] (Q.W.) 2 Key Lab of Textile Science & Technology, Ministry of Education, Shanghai 201620, China 3 Center for Civil Aviation Composites, Shanghai 201620, China * Correspondence: [email protected]; Tel.: +86-137-6402-2421

Received: 27 May 2018; Accepted: 25 June 2018; Published: 28 June 2018

Abstract: Tensile and compressive properties of interlayer and intralayer hybrid composites were investigated in this paper. The tensile modulus and compression modulus of interlayer and intralayer hybrid composites are the same under the same mixed ratio, the tensile strength is much superior to the compression strength, and while the tensile modulus and strength increase along with the carbon fiber content, the compression values change slightly. The influence of stacking structures on the tensile and compressive strengths is opposite to the ratio of T/C (tensile/compression) strength for interlayer hybrid composites, and while the tensile and compression strengths with glass fiber sandwiching carbon fiber can reach the maximum value, the ratio of T/C strength is minimum. For structures with carbon fiber sandwiching glass fiber, or with asymmetric structures, the tensile and compressive strengths are at a low value. For intralayer hybrid structures, while the carbon/glass (C/G) dispersion degree is high, the tensile and compression strengths are low. The experimental tensile and compressive strengths for interlayer and intralayer hybrid composites are greater than the theoretical values, which demonstrates that strength conforms well to the positive hybrid effect. The tensile fracture strain is greater than the compression fracture strain for hybrid composites, with both of them basically maintained at the same level.

Keywords: carbon/glass hybrid composites; interlayer hybrid; intralayer hybrid; tensile properties; compressive properties; ratio of tensile/compression strength

1. Introduction Due to high price of carbon fiber, hybrid composites composed of two or more fiber materials have been developed for cost reduction [1]. Hybrid composites not only embody the advantages of a single fiber, but can achieve complementary advantages of two materials; excellent physical and mechanical properties make hybrid composites widely applicable in many fields [2–7]. Currently, extensive work centers on the mechanical properties of hybrid composites [8–14]. Manders et al. [13] investigated tensile properties of carbon/glass hybrid composites with various fiber contents and layer structures to explore the impact factors of the hybrid effect. Phillips L N [15] studied hybrid composites and found that its mechanical properties, including tensile, impact, and fatigue properties, are better than in pure carbon fiber or glass fiber composites. Dong et al. [16,17] revealed that the hybrid effect is obvious since there is a distinct Young modulus difference between carbon and glass fiber, and an optimal mixed ratio has been obtained via theoretical analysis. Li et al. [18] studied the compression properties of UHMPEF/carbon hybrid composites and found the experimental compressive strength

Materials 2018, 11, 1105; doi:10.3390/ma11071105 www.mdpi.com/journal/materials Materials 2018, 11, 1105 2 of 13 is basically consistent with theoretical values via the rule of mixture (ROM), and compressive fracture strain exhibits the positive hybrid effect. In addition, a difference between the tensile and compressive properties of fiber and composites exists due to fiber buckling and production problems, which have been confirmed in many papers, with some authors having studied the tensile and compressive properties from the fiber viewpoint. Greenwood et al. [19] studied aramid fiber and found a highly oriented crystalline cellulose structure makes the tensile strength of fiber stiff, which makes it easy to form a kink band under the compressive loading contributing to compressive strength only accounting for 20% of the tensile strength. Oya et al. [20] found the compressive strength of PAN (polyacrylonitrile)-based carbon fiber only accounts for 30–50% of the tensile strength, and the compression modulus in the longitudinal direction was calculated by the Euler buckling method and accounts for 50% of the tensile modulus. Bos et al. [21] found that flax fiber has a high compression-to-tensile ratio of 80% in the loop test. Meng et al. [22] found the average ratio of tensile to compressive modulus is 0.9 due to misalignment of fiber and production problems (porosity content). In addition, the inclination angle of fiber was measured using an indirect method and the inclination angle distribution of fiber was given. In addition to the fiber investigation, some researchers studied the tensile and compressive properties of composites. Zobeiry et al. [23] developed a model to explain the failure mechanism of composite under tension and compression, where the tensile failure is mainly due to the kink band, which is the reason for the formation of fiber failure and resin fracture or yielding. The ratio of tensile and compressive strength of flax and bamboo composites was found to be 60%, and the compression properties of coir fiber composites were even better than the tensile properties [24]. Tensile and compressive testing combined with computed tomography (CT) were adopted to investigate the tensile failure in a laminate and the fiber wrinkling under compression [25]. Mujika et al. [26] determined the ratio of tension and compression moduli by two experiments of four-point bending. Liu et al. [27] studied the tensile/compressive properties of PVA (polyvinyl alcohol)-based, cement-matrix composites. The ratio of tensile/compressive strength was regarded as an important parameter to evaluate the brittleness of cement; a smaller ratio of value indicates a larger brittleness and a smaller toughness. Choi et al. [28] studied the tensile and compression properties of several concrete composites and found the ratio of tensile to compressive strength is 19.8% on average, which is almost twice that of normal concrete. Hartl et al. [29] confirmed that the compression/tensility of short glass fiber-reinforced polypropylenes has a stress-strain asymmetric behavior, and the ratio of compressive to tensile strength can reach 1.3 due to the fiber misalignment. The tensile and compressive properties of 3D woven carbon-fiber-reinforced composites were studied; while the tensile modulus and compression modulus were nearly the same, the tensile strength was far greater than the compressive strength, which is mainly due to the obvious crimp of 3D fabric. The resin breaks easily while subjected to the compressive loading, then delaminates, and then the sample fails [30]. Prabhakaran et al. [31] compared the tensile and compressive properties of hybrid composites and found the tensile and compressive moduli and strengths are consistent with previous conclusions [30]. This paper systematically designed carbon/glass interlayer and intralayer hybrid structures and studied the effect of mixed ratios and hybrid structures on the tensile and compressive properties of hybrid composites. In addition, ratios of tensile/compressive (T/C) strength were studied and analyzed.

2. Materials and Methods

2.1. Experimental Materials Carbon fiber, glass fiber, and epoxy resin were supplied by TORAY Inc. (Tokyo, Japan), CPIC Inc. (Chongqing, China), and SWANCOR Inc. (Shanghai, China), respectively. Five kinds of unidirectional Non-Crimp Fabrics (NCF), including pure carbon and glass fiber fabric and three kinds of hybrid fabrics, were designed and manufactured. Mechanical parameters of raw materials and fabric Materials 2018, 11, 1105 3 of 13 structures are given in Tables1 and2. Structure diagrams of intralayer hybrid fabric are shown in Figure1.

Table 1. Constituent materials and selected properties.

Material Tensile Strength (MPa) Tensile Modulus (GPa) CPIC ECT469L-2400 glass ﬁber 2366 78.7 TORAY T620SC-24K-50C carbon ﬁber 4175 234 SWANCOR 2511-1A/BS epoxy resin 73.5 3.1

Table 2. Speciﬁcations for hybrid fabric.

Areal Density (g/m2) Fabric Type Ratio of C/G Carbon Fiber Glass Fiber carbon 728.3 0 1:0 glass 0 944.9 0:1 C-G-C-G 364.2 472.4 1:1 C-G-G 242.8 629.9 1:2 Materials 2018, 11, x FORC-G-G-G-G PEER REVIEW 145.7 755.9 1:4 3 of 13

(a) C-G-C-G (b) C-G-G (c) C-G-G-G-G

FigureFigure 1.1. SchematicSchematic structuresstructures ofof threethree kindskinds ofof intralayerintralayer hybridhybrid NCFs.NCFs.

Table 1. Constituent materials and selected properties. 2.2. Layer Structures Schemes of Interlayer Hybrid Structures Material Tensile Strength (MPa) Tensile Modulus (GPa) With regard to interlayer hybrid composites, four C/G hybrid ratios with various hybrid structures CPIC ECT469L-2400 glass fiber 2366 78.7 were designed by altering stacking sequences of pure carbon and glass fiber fabric. Interlayer hybrid structuresTORAY are T620SC shown- in24K Table-50C3 .c Forarbon interlayer fiber hybrid structures4175 with various C/G hybrid234 ratios, the numberSWANCOR of layers in2511 a laminate-1A/BS epoxy was notresin the same. When73.5 C:G = 1:2, a laminate contained3.1 only three layers; for C:G = 1:4, a laminate contained five layers. Table 2. Specifications for hybrid fabric. 2.3. Schemes Design of Intralayer Hybrid Structures Areal Density (g/m2) Fabric Type Ratio of C/G Three kinds of intralayer hybridCarbon fabrics were Fiber used, Glass and Fiber dispersion degrees were achieved through variations in dislocationc arrangementsarbon of728.3 carbon and glass0 fiber bundles1:0 in various layers as shown in Table4. The number indicatesglass the dispersion0 degree;944.9 for example, for0:1 the structure [C-C-G-G]-0, the carbon and glass bundlesC-G-C-G were aligned364.2 with the472.4 upper and lower1:1 layers, and the structure [C-C-G-G]-0.5 indicates theC-G fabric-G translated242.8 horizontally 629.9 the half-width1:2 of one bundle. C-G-G-G-G 145.7 755.9 1:4

2.2. Layer Structures Schemes of Interlayer Hybrid Structures With regard to interlayer hybrid composites, four C/G hybrid ratios with various hybrid structures were designed by altering stacking sequences of pure carbon and glass fiber fabric. Interlayer hybrid structures are shown in Table 3. For interlayer hybrid structures with various C/G hybrid ratios, the number of layers in a laminate was not the same. When C:G = 1:2, a laminate contained only three layers; for C:G = 1:4, a laminate contained five layers.

Table 3. Stacking configurations of interlayer hybrid structures.

Hybrid Ratio Stacking Sequences

C:G = 1:1 [G/G/C/C] [G/C/C/G] [C/G/G/C] [G/C/G/C]

C:G = 1:2 [G/G/C] [G/C/G]

C:G = 1:3 [G/G/G/C] [G/G/C/G]

C:G = 1:4 [G/G/G/G/C] [G/G/G/C/G] [G/G/C/G/G]

MaterialsMaterials 2018Materials, Materials112018, x ,FOR 112018, 2018x PEER ,FOR 11,, 11x PEER REVIEWFOR, x FOR PEERREVIEW PEER REVIEW REVIEW 3 of 133 of 133 of3 13 of 13 MaterialsMaterials 2018, 112018, x ,FOR 11, x PEER FOR PEERREVIEW REVIEW 3 of 133 of 13 MaterialsMaterials 2018, 112018, x ,FOR 11, x PEER FOR PEERREVIEW REVIEW 3 of 133 of 13 MaterialsMaterials 2018Materials, 112018, x ,FOR 112018, x PEER ,FOR 11, x PEER REVIEWFOR PEERREVIEW REVIEW 3 of 133 of 133 of 13

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Modulus Modulus (GPa) Modulus (GPa) (GPa) (GPa) MaterialMaterial TensileTensile Strength Strength (MPa) (MPa) Tensile Tensile Modulus Modulus (GPa) (GPa) CPICCPIC ECT469LCPIC ECT469LMaterialCPIC -ECT469LMaterial2400 ECT469L- 2400glass- 2400g flass-iber2400 g flassiber glass fiberTensile fiber Tensile Strength2366 Strength 2366 (MPa) 23662366 (MPa) Tensile Tensile Modulus78.7 Modulus 78.7 (GPa) 78.778.7 (GPa) CPICCPIC ECT469L ECT469LMaterial-Material2400- Material 2400glass g flassiber f iber TensileTensile StrengthTensile2366 Strength 2366 Strength(MPa) (MPa) Tensile(MPa) Tensile ModulusTensile78.7 Modulus 78.7 Modulus (GPa) (GPa) (GPa) TORAYCPICTORAY CPICT620SC TORAYECT469LTORAY T620SC ECT469L-24K T620SC-2400 T620SC-50C24K- 2400g -lassc50C24Karbon- 24Kg f -lassciber50Carbon- f50C iber fciberarbon c fiberarbon fiber fiber 41752366 41752366 41754175 78.7234 78.7234 234234 TORAYCPICTORAY CPICT620SC ECT469L CPICT620SC ECT469L-24K -ECT469L2400-24K50C- 2400g -lassc50Carbon- 2400g f lassciberarbon fg iber flassiber f iberf iber 23664175 41752366 2366 78.7234 78.7234 78.7 TORAYSWANCORTORAYSWANCOR T620SCSWANCOR SWANCORT620SC2511-24K -25111A/BS-24K50C -25111A/BS 2511 -ec50Cpoxyarbon-1A/BS - e1A/BScpoxy arbonr esinfiber epoxy r e esin fpoxyiber r esin resin 417573.5 417573.5 73.573.5 2343.1 2343.1 3.13.1 TORAYSWANCORTORAYSWANCOR T620SCTORAY T620SC2511-24K T620SC-25111A/BS-24K50C-1A/BS -ec24K50Cpoxyarbon -ec50Cpoxy arbonr esinfiber c arbonr esin fiber fiber 417573.5 417573.5 4175 2343.1 2343.1 234 SWANCORSWANCOR 2511 -25111A/BS-1A/BS epoxy epoxy resin r esin 73.5 73.5 3.1 3.1 SWANCORSWANCORSWANCOR 2511 -25111A/BS -25111A/BS epoxy-1A/BS epoxy resin epoxy r esin r esin 73.5 73.5 73.5 3.1 3.1 3.1 TableTable 2. 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Stacking configurations configurations of interla of interlayer hybridyer hybrid structures structures. . HybridHybridHybrid Ratio TableRatioHybridHybrid TableRatio 3 . Stacking TableRatio 3 . RatioStacking 3 .configurations Stacking configurations configurationsStacking ofStacking interla Stacking of StackingSequences interlayerStacking of Sequences interlahybridyer Sequences hybrid yer Sequencesstructures hybrid structures .structures . . HybridHybrid Ratio Ratio StackingStacking Sequences Sequences HybridHybrid Ratio Ratio StackingStacking Sequences Sequences HybridHybrid RatioHybrid Ratio Ratio StackingStacking StackingSequences Sequences Sequences C:GC:G = 1:1 =C:G 1:1 =C:G 1:1C:G = 1:1 = 1:1 C:G =C:G 1:1 = 1:1 C:G =C:G 1:1 = 1:1 [G/G/C/C][G/G/C/C] [G/G/C/C] [G/G/C/C] [G/C/C/G] [G/C/C/G] [G/C/C/G] [G/C/C/G] [C/G/G/C] [C/G/G/C] [C/G/G/C] [C/G/G/C] [G/C/G/C] [G/C/G/C] [G/C/G/C] [G/C/G/C] C:G =C:G 1:1 =C:G 1:1 = [G/G/C/C]1:1 [G/G/C/C] [G/C/C/G] [G/C/C/G] [C/G/G/C] [C/G/G/C] [G/C/G/C] [G/C/G/C] [G/G/C/C][G/G/C/C] [G/C/C/G][G/C/C/G] [C/G/G/C][C/G/G/C] [G/C/G/C] [G/C/G/C] [G/G/C/C][G/G/C/C][G/G/C/C] [G/C/C/G] [G/C/C/G] [G/C/C/G] [C/G/G/C] [C/G/G/C] [C/G/G/C] [G/C/G/C] [G/C/G/C] [G/C/G/C] C:G =C:G 1:2 =C:G 1:2C:G = 1:2 = 1:2 C:GC:G = 1:2 =C:G 1:2 = 1:2 [G/G/C][G/G/C] [G/G/C][G/G/C] [G/C/G] [G/C/G] [G/C/G][G/C/G] C:G =C:G 1:2 = 1:2 [G/G/C][G/G/C] [G/C/G][G/C/G] C:G =C:G 1:2 =C:G 1:2 = 1:2[G/G/C] [G/G/C] [G/C/G] [G/C/G] Materials 2018, 11, x FOR PEER REVIEW[G/G/C] [G/G/C] [G/G/C] [G/C/G] [G/C/G] [G/C/G] 4 of 14 C:G =C:G 1:3 =C:G 1:3C:G = 1:3 = 1:3 C:G =C:G 1:3 = 1:3 2.3. SchemesC:G DesignC:G = 1:3 =C:G 1:3 of Intralayer= 1:3 [G/G/G/C] Hybrid[G/G/G/C][G/G/G/C] Structures[G/G/G/C] [G/G/C/G] [G/G/C/G] [G/G/C/G] [G/G/C/G] C:G =C:G 1:3 = 1:3 [G/G/G/C][G/G/G/C] [G/G/C/G] [G/G/C/G] C:G = [G/G/G/C]1:3 [G/G/G/C] [G/G/C/G][G/G/C/G] Three kinds of intralayer[G/G/G/C] [G/G/G/C]hybrid[G/G/G/C] fabrics [G/G/C/G] [G/G/C/G]were[G/G/C/G] used, and dispersion degrees were achieved C:G =C:G 1:4 =C:G 1:4C:G = 1:4 = 1:4 C:G =C:G 1:4 = 1:4 through variations in dislocation arrangements of carbon and glass fiber bundles in various layers as C:G =C:G 1:4 = 1:4 [G/G/G/G/C][G/G/G/G/C][G/G/G/G/C] [G/G/G/G/C] [G/G/G/C/G] [G/G/G/C/G] [G/G/G/C/G] [G/G/G/C/G] [G/G/C/G/G] [G/G/C/G/G] [G/G/C/G/G] [G/G/C/G/G] shown in C:GTableC:G = 1:44. =C:G The1:4 =C:G number1:4 [G/G/G/G/C]= 1:4 [G/G/G/G/C] indicates the [G/G/G/C/G] disper [G/G/G/C/G]sion degree; [G/G/C/G/G] [G/G/C/G/G] for example, for the structure [C-C- [G/G/G/G/C] [G/G/G/G/C] [G/G/G/C/G] [G/G/G/C/G] [G/G/C/G/G] [G/G/C/G/G] G-G]-0, the carbon and glass[G/G/G/G/C] bundle [G/G/G/G/C] [G/G/G/G/C]s were [G/G/G/C/G] aligned [G/G/G/C/G] with[G/G/G/C/G] the[G/G/C/G/G] upper [G/G/C/G/G] and[G/G/C/G/G] lower layers, and the structure

[C-C-G-G]-0.5 indicates the fabric translated horizontally the half-width of one bundle.

Table 4. Stacking conﬁgurations of intralayer hybrid structures. Table 4. Stacking configurations of intralayer hybrid structures.

Hybrid Fabric Stacking Sequences

[C-C-G-G]-0 [C-C-G-G]-1 [C-C-G-G]-2 C-C-G-G

[C-C-G-G]-0.5 [C-C-G-G]-1.5

C-G-G

[C-G-G]-0 [C-G-G]-1 [C-G-G-0].5 [C-G-G]-1.5

[C-G-G-G-G]-0 [C-G-G-G-G]-1 [C-G-G-G-G]-2 C-G-G-G-G

[C-G-G-G-G]-0.5 [C-G-G-G-G]-1.5 [C-G-G-G-G]-2.5

2.4.2.4. Experiments Experiments Vacuum-assistedVacuum-assisted resin resin transfer transfer molding molding proc processess (VARTM) (VARTM) was used to prepare composite laminates;laminates; the the detailed detailed process process is is referred referred to to in in reference reference [32]. [32]. Tensile Tensile testing testing and and compression compression testing testing were conducted accordingaccording toto thethe ASTMASTM D3039 D3039 Standard Standard [ 33[33]] and and ASTM ASTM D6641 D6641 [34 [34]] separately. separately. Due Due to tovariations variations in in failure failure speeds speeds and and modes modes of of carbon carbon and and glass glass ﬁber, fiber, the the force force attenuation attenuation rate rate was was set set to to50% 50% as as the the experimental experimental end end parameter. parameter. FiveFive specimens ofof each each laminate laminate were were tested, tested, and and sample sample width width of tensile of tensile and compression and compression testing testingfor interlayer for interlayer hybrid compositeshybrid composites based on based the standards on the standards was set to was 15 mm set andto 15 13 mm mm, and respectively. 13 mm, respectively.The specimen The width specimen for intralayer width for hybrid intralayer composites hybrid composites was set tothe was width set to ofthe a width minimum of a minimum repeating repeatingunit. In order unit. to In avoid order the to effect avoid of boundarythe effect conditionof boundary on the condition mechanical on properties,the mechanical carbon properties, and glass carbonbundles and in aglass specimen bundles were in a distributed specimen were symmetrically. distributed Specimen symmetrically. width andSpecimen cutting width positions and ofcutting three positionsintralayer of hybrid three compositesintralayer hybrid are shown composites in Figure are2 .shown in Figure 2. The fiber volume content was kept at 50% and the thickness of each layer in a laminate was 0.8 mm, therefore, dimensions of each laminate were not same and depended on the number of layers and the hybrid forms. The size parameters of composites are listed in Table 5.

(a) (b) (c)

Figure 2. Schematics of cutting location for intralayer hybrid composites: (a) [C-C-G-G]; (b) [C-G-G]; and (c) [C-G-G-G-G].

Materials 2018, 11, x FOR PEER REVIEW 4 of 13

2.3. Schemes Design of Intralayer Hybrid Structures Three kinds of intralayer hybrid fabrics were used, and dispersion degrees were achieved through variations in dislocation arrangements of carbon and glass fiber bundles in various layers as shown in Table 4. The number indicates the dispersion degree; for example, for the structure [C-C- G-G]-0, the carbon and glass bundles were aligned with the upper and lower layers, and the structure [C-C-G-G]-0.5 indicates the fabric translated horizontally the half-width of one bundle.

Table 4. Stacking configurations of intralayer hybrid structures.

Hybrid Fabric Stacking Sequences

[C-C-G-G]-0 [C-C-G-G]-1 [C-C-G-G]-2 C-C-G-G

[C-C-G-G]-0.5 [C-C-G-G]-1.5

C-G-G

[C-G-G]-0 [C-G-G]-1 [C-G-G-0].5 [C-G-G]-1.5

[C-G-G-G-G]-0 [C-G-G-G-G]-1 [C-G-G-G-G]-2 C-G-G-G-G

[C-G-G-G-G]-0.5 [C-G-G-G-G]-1.5 [C-G-G-G-G]-2.5

2.4. Experiments Vacuum-assisted resin transfer molding process (VARTM) was used to prepare composite laminates; the detailed process is referred to in reference [32]. Tensile testing and compression testing were conducted according to the ASTM D3039 Standard [33] and ASTM D6641 [34] separately. Due to variations in failure speeds and modes of carbon and glass fiber, the force attenuation rate was set to 50% as the experimental end parameter. Five specimens of each laminate were tested, and sample width of tensile and compression testing for interlayer hybrid composites based on the standards was set to 15 mm and 13 mm, respectively. The specimen width for intralayer hybrid composites was set to the width of a minimum repeating unit. In order to avoid the effect of boundary condition on the mechanical properties, carbon and glass bundles in a specimen were distributed symmetrically. Specimen width and cutting positions of three intralayer hybrid composites are shown in Figure 2. The fiber volume content was kept at 50% and the thickness of each layer in a laminate was 0.8 Materialsmm, therefore,2018, 11, 1105 dimensions of each laminate were not same and depended on the number of layers5 of 13 and the hybrid forms. The size parameters of composites are listed in Table 5.

(a) (b) (c)

Figure 2. Schematics of cutting location for intralayer hybrid composites: (a) [C-C-G-G]; (b) [C-G-G]; Figure 2. Schematics of cutting location for intralayer hybrid composites: (a) [C-C-G-G]; (b) [C-G-G]; and (c) [C-G-G-G-G]. and (c) [C-G-G-G-G].

The ﬁber volume content was kept at 50% and the thickness of each layer in a laminate was 0.8 mm, therefore, dimensions of each laminate were not same and depended on the number of layers and the hybrid forms. The size parameters of composites are listed in Table5.

Table 5. Size parameters of composites.

Laminate Structures C/G Hybrid Ratios Layers Laminate Thickness/mm Width/mm Span/mm pure carbon fabric 1:0 4 3.2 13 64 pure glass fabric 0:1 4 3.2 13 64 1:1 4 3.2 13 64 1:2 3 2.4 13 48 interlayer laminate 1:3 4 3.2 13 64 1:4 5 4 13 80 1:1 4 3.2 20 64 intralayer laminate 1:2 4 3.2 15 64 1:4 4 3.2 25 64

3. Results and Discussions

3.1. Tensile and Compressive Properties of Interlayer Hybrid Composites Tensile and compression moduli, strengths, and fracture strains of interlayer hybrid composites with various mixed ratios and stacking sequences were compared in Figures3–5, and the effect of hybrid ratios and layer structures on the tensile and compressive properties were analyzed. As seen from Figure3, the tensile and compression moduli of interlayer hybrid composites are very close and mainly determined by the factor C/G mixed ratio; the tensile and compression moduli increase gradually as the carbon fiber content increases. Under the same C/G mixed ratio, tensile and compression moduli of interlayer hybrid composites with various hybrid structures change slightly. Comparison of tensile and compression strengths of interlayer hybrid composites is shown in Figure4. It was found that the tensile strength is superior to the compression strength, moreover, the tensile strength increases obviously along with the carbon fiber content while the compression strength only changes slightly. Fiber-reinforced composites consist of fiber and resin matrix; while subjected to the tensile loading, fiber along the longitudinal direction mainly assumes the force, and the tensile strength is determined by the tensile properties of the reinforced fiber. However, when composites are subjected to a compression loading, the fiber and the resin assume the force, which will affect the compression strength of composites in common, therefore, the tensile and compression properties of composites exhibit a difference. Materials 2018, 11, x FOR PEER REVIEW 5 of 13 Materials 2018, 11, x FOR PEER REVIEW 5 of 13 Table 5. Size parameters of composites. Table 5. Size parameters of composites. Laminate Structures C/G Hybrid Ratios Layers Laminate Thickness/mm Width/mm Span/mm Laminate Structures C/G Hybrid Ratios Layers Laminate Thickness/mm Width/mm Span/mm pure carbon fabric 1:0 4 3.2 13 64 pure carbon fabric 1:0 4 3.2 13 64 pure glass fabric 0:1 4 3.2 13 64 pure glass fabric 0:1 4 3.2 13 64 1:1 4 3.2 13 64 1:21:1 34 2.43.2 1313 4864 interlayer laminate 1:2 3 2.4 13 48 interlayer laminate 1:3 4 3.2 13 64 1:41:3 54 3.24 1313 8064 1:4 5 4 13 80 1:1 4 3.2 20 64 intralayer laminate 1:21:1 44 3.23.2 1520 6464 intralayer laminate 1:41:2 44 3.23.2 2515 6464 1:4 4 3.2 25 64 3. Results and Discussions 3. Results and Discussions 3.1. Tensile and Compressive Properties of Interlayer Hybrid Composites 3.1. Tensile and Compressive Properties of Interlayer Hybrid Composites Tensile and compression moduli, strengths, and fracture strains of interlayer hybrid composites Tensile and compression moduli, strengths, and fracture strains of interlayer hybrid composites Materialswith2018 various, 11, 1105 mixed ratios and stacking sequences were compared in Figures 3–5, and the effect of6 of 13 with various mixed ratios and stacking sequences were compared in Figures 3–5, and the effect of hybrid ratios and layer structures on the tensile and compressive properties were analyzed. hybrid ratios and layer structures on the tensile and compressive properties were analyzed. 120 120 110 Tensile modulus 110 CompressionTensile modulus modulus 100 Compression modulus 100 90 90 80 80 70 70 60 60 50 50 40 Modulus(GPa) 40 Modulus(GPa) 30 30 20 20 10 10 C:G=0:1 C:G=1:4 C:G=1:3 C:G=1:2 C:G=1:1 C:G=1:0 C:G=0:1 C:G=1:4 C:G=1:3 C:G=1:2 C:G=1:1 C:G=1:0

FigureFigure 3. Tensile 3. Tensile and and compression compression modulimoduli ofof interlayer hybrid hybrid composites composites with with various various stacking stacking Figure 3. Tensile and compression moduli of interlayer hybrid composites with various stacking sequencessequences and and C/G C/G hybrid hybrid ratios. ratios. sequences and C/G hybrid ratios. 1800 1800 Tensile strength 1600 Tensile strength 1600 Compression strength 1400 Compression strength 1400 1200 1200 1000 1000 800 800 600 600 Strength (MPa)

Strength (MPa) 400 400 200 200 0 0 C:G=0:1 C:G=1:4 C:G=1:3 C:G=1:2 C:G=1:1 C:G=1:0 C:G=0:1 C:G=1:4 C:G=1:3 C:G=1:2 C:G=1:1 C:G=1:0

Figure 4. Tensile and compression strength of interlayer hybrid composites with various stacking Figure 4. Tensile and compression strength of interlayer hybrid composites with various stacking Figuresequences 4. Tensile and andC/G hybrid compression ratios. strength of interlayer hybrid composites with various stacking sequencessequences and and C/G C/G hybrid hybrid ratios. ratios. Materials 2018, 11, x FOR PEER REVIEW 6 of 13

2.6 Tensile fracture strain 2.4 Compression fracture strain 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8

Fracture strain(%) Fracture 0.6 0.4 0.2 0.0 C:G=0:1 C:G=1:4 C:G=1:3 C:G=1:2 C:G=1:1 C:G=1:0

FigureFigure 5. Tensile 5. Tensile and compressionand compression fracture fracture strain strain of interlayer of interlayer hybrid hybrid composites composites with with various various stacking sequencesstacking and sequences C/G hybrid and C/G ratios. hybrid ratios.

As seen from Figure 3, the tensile and compression moduli of interlayer hybrid composites are very close and mainly determined by the factor C/G mixed ratio; the tensile and compression moduli increase gradually as the carbon fiber content increases. Under the same C/G mixed ratio, tensile and compression moduli of interlayer hybrid composites with various hybrid structures change slightly. Comparison of tensile and compression strengths of interlayer hybrid composites is shown in Figure 4. It was found that the tensile strength is superior to the compression strength, moreover, the tensile strength increases obviously along with the carbon fiber content while the compression strength only changes slightly. Fiber-reinforced composites consist of fiber and resin matrix; while subjected to the tensile loading, fiber along the longitudinal direction mainly assumes the force, and the tensile strength is determined by the tensile properties of the reinforced fiber. However, when composites are subjected to a compression loading, the fiber and the resin assume the force, which will affect the compression strength of composites in common, therefore, the tensile and compression properties of composites exhibit a difference. Under the same C/G mixed ratio, the tensile and compression strengths of interlayer hybrid composites are highly correlated with the stacking sequences. Excellent strength can be achieved for glass fiber sandwiching carbon fiber, like [G/G/C/G/G], [G/C/G/G/G], [G/C/G], [G/C/C/G]. For symmetric structures with carbon fiber sandwiching glass fiber like [C/G/G/C], or asymmetric interlayer structures, like [C/G/G/G/G], [C/G/G/G], [C/G/G], [C/C/G/G], the tensile and compression strengths are maintained at a low level. Tensile and compression fracture strains of interlayer hybrid composites are shown in Figure 5. The tensile fracture strain is greater than the compression fracture strain, and both the strains drop slightly as the carbon fiber content increases.

3.2. Tensile and Compressive Properties of Intralayer Hybrid Composites In this part, tensile and compression moduli, strengths, and fracture strains of intralayer hybrid composites with various mixed ratios and hybrid structures were investigated and results are shown in Figures 6–8. In Figure 6, it was concluded that the tensile and compressive moduli increase as the carbon fiber content increases for intralayer hybrid composites. Under the same mixed ratio, the modulus maintains at the same level which is independent of the hybrid structures. The tensile and compression moduli of various layer structures are basically identical with little carbon fiber, like C:G = 1:4, 1:2, however, the compression modulus of C:G = 1:1 tends to be slightly lower than the tensile modulus.

Materials 2018, 11, 1105 7 of 13

Under the same C/G mixed ratio, the tensile and compression strengths of interlayer hybrid composites are highly correlated with the stacking sequences. Excellent strength can be achieved for glass fiber sandwiching carbon fiber, like [G/G/C/G/G], [G/C/G/G/G], [G/C/G], [G/C/C/G]. For symmetric structures with carbon fiber sandwiching glass fiber like [C/G/G/C], or asymmetric interlayer structures, like [C/G/G/G/G], [C/G/G/G], [C/G/G], [C/C/G/G], the tensile and compression strengths are maintained at a low level. Tensile and compression fracture strains of interlayer hybrid composites are shown in Figure5. The tensile fracture strain is greater than the compression fracture strain, and both the strains drop Materials 2018, 11, x FOR PEER REVIEW 7 of 13 slightly as the carbon fiber content increases. Figure 7 revealed that the tensile strength of intralayer hybrid composites is much greater than 3.2. Tensilethe and compressionMaterials Compressive 2018, 11strength, x FOR Properties PEER. The REVIEW difference of Intralayer is around Hybrid 100%, Composites and the tensile strength increases7 of 13 as the carbon fiberFigure content 7 revealed increases, that the while tensile the strength compression of intralayer strength hybrid appearscomposites to is be much indepe greaterndent than of the In thishybrid part,the structures compression tensile and and strength the compression hybrid. The ratios.difference moduli,Under is around the strengths, same 100%, mixed and the and ratio, tensile fracture the strength effect strains ofincreases hybrid of as structure intralayerthe s hybrid compositeson the with carbontensile various fiberand contentcompression mixed increases, ratios strength while and thes hybrid is compressionsmall, structures and the strength compressive were appears investigated to strength be indepe isndent at and a lower of results the level. are shown in FiguresWith6–8 .ahybrid high C/Gstructures mixed and dispersion the hybrid degree,ratios. Under such the as same[C-G -mixedG-G- G]ratio,-2.5, the [C effect-G-G] of- 1.5,hybrid and structure [C-C-Gs -G]-2, the tensileon the strength tensile and drops compression; it is assumed strength thes is carbon small, and fiber the breakage compressive results strength in the is at glass a lower fiber level. damage In Figure6, it was concluded that the tensile and compressive moduli increase as the carbon leadingWith to a a comparatively high C/G mixed low dispersion strength degree, of intralayer such as [C hybrid-G-G-G -composites.G]-2.5, [C-G- G]-1.5, and [C-C-G-G]-2, the tensile strength drops; it is assumed the carbon fiber breakage results in the glass fiber damage fiber contentIn increases Figure 8, it for can intralayer be concluded hybrid that the composites. tensile fracture Under strain theof intralayer same mixed hybrid ratio,composites the modulus leading to a comparatively low strength of intralayer hybrid composites. maintainsdecreases at the same slightlyIn Figure level as8, it whichthe can carbon be con is independentcluded fiber contentthat the tensile increases, of the fracture hybrid and strain the structures. of tensile intralayer strain Thehybrid is tensile highercomposites and than compression the moduli ofcompressive variousdecreases layerstrain. slightly structuresIn addition, as the carbon the are fiberfracture basically content strain increases,identical tends to and be with the lower tensile little if the strain carbon C/G is di higherspersion fiber, than like degree the C:G is = 1:4, 1:2, however,high. the compressioncompressive strain. modulus In addition, of the C:G fracture = 1:1 strain tends tends to to be lower slightly if the lowerC/G dispersion than thedegree tensile is modulus. high. 130 130 120 Tensile modulus 120 Tensile modulus 110 Compression modulus 110 Compression modulus 100 100 90 90 80 80 70 70 60 60 50 50 Modulus (GPa) 40 Modulus (GPa) 40 30 30 20 20 10 C:G=1:4 C:G=1:0 10 C:G=0:1 C:G=1:2 C:G=1:1 C:G=1:4 C:G=1:0 Figure 6. TensileC:G=0:1 and compression moduli of intralayerC:G=1:2 hybrid compositesC:G=1:1 with various stacking Figure 6. Tensilesequences and and compression C/G hybrid ratios. moduli of intralayer hybrid composites with various stacking sequencesFigure and 6.C/G Tensile hybrid and ratios.compression moduli of intralayer hybrid composites with various stacking sequences and C/G hybrid ratios. 1800 Tensile strength 1600 Compression strength 1800 Tensile strength 1400 1600 Compression strength 1200

14001000

1200 800 1000 600 Strength(MPa) 800 400 200 600

Strength(MPa) 0 400 C:G=1:4 C:G=1:0 C:G=0:1 C:G=1:2 C:G=1:1 Figure 7. Tensile200 and compression strengths of intralayer hybrid composites with various stacking sequences and C/G hybrid ratios. 0 C:G=1:4 C:G=1:0 C:G=0:1 C:G=1:2 C:G=1:1 Figure 7. Tensile and compression strengths of intralayer hybrid composites with various stacking Figure 7. Tensile and compression strengths of intralayer hybrid composites with various stacking sequences and C/G hybrid ratios. sequences and C/G hybrid ratios.

Materials 2018, 11, 1105 8 of 13

Materials 2018, 11, x FOR PEER REVIEW 8 of 13

2.6 2.4 Tensile fracture strain 2.2 Compression fracture strain 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 Fracture strain (%) 0.4 0.2 0.0 C:G=1:4 C:G=1:0 C:G=0:1 C:G=1:2 C:G=1:1 Figure 8. Tensile and compression fracture strains of intralayer hybrid composites with various Figure 8. Tensile and compression fracture strains of intralayer hybrid composites with various stacking stacking sequences and C/G hybrid ratios. sequences and C/G hybrid ratios. 3.3. Comparison of Tensile and Compressive Properties of Interlayer and Intralayer Hybrid Composites Figure7 revealedRatio of that tensile/compression the tensile strength (T/C) strength of intralayer is defined to hybrid illustrate composites the relationship is much between greater than tensile and compressive properties of hybrid composites as formula (1). A small T/C ratio indicates the compressionthe tensile strength. strength is The close difference to the compression is around strength. 100%, and the tensile strength increases as the carbon fiber content increases, while the compression strength appears to be independent of the hybrid : = (1) structures and the hybrid ratios. Under the same mixed ratio, the effect of hybrid structures on the tensile and compressionwhere RT:C denotes strengths the ratio of is T/C small, strength, and σ theT and compressive σC are the tensile strength strength is and at compression a lower level. With a high C/G mixedstrength dispersion of composites degree, (MPa), respectively. such as [C-G-G-G-G]-2.5, [C-G-G]-1.5, and [C-C-G-G]-2, the tensile Figure 9 reports ratios of T/C strength for interlayer hybrid composites with various mixed ratios strength drops;and hybrid it is assumed structures. It the can carbon be found fiberthat ratios breakage of T/C strength results increase in the along glass with fiber the carbon damage fiber leading to a comparativelycontent low overall, strength which of indicates intralayer the difference hybrid between composites. tensile and compression strengths becomes In Figurelarger.8, it Under can bethe concludedsame C/G mixed that ratio, the ratios tensile of T/C fracture strength with strain glass of fiber intralayer sandwiching hybrid carbon composites fiber, such as [G/G/C/G/G], [G/C/G/G], [G/C/G], [G/C/C/G], are the minimum. With regard to decreases slightlystructures as with the carbon carbon fiber sandwiching fiber content glass increases,fiber, like [C/G/G/C], and the or with tensile carbon strain fiber locating is higher in than the compressiveone strain. surface, In like addition, [C/G/G/G/G], the [C/G/G/G], fracture [G/G/C], strain ratios tends of T/C to strength be lower can reach if the up C/G to a high dispersion level. degree is high. Ratios of T/C strength of intralayer hybrid composites with various mixed ratios and hybrid structures are shown in Figure 10. There is not a clear law to describe the relationship between the ratio of T/C and the carbon fiber content, however, under the same C/G hybrid ratio, with a small 3.3. Comparison of Tensile and Compressive Properties of Interlayer and Intralayer Hybrid Composites dispersion degree like 0.5, structures have high ratios of T/C, while the composites with a high Ratio ofdispersion tensile/compression degree can reach low (T/C) ratios of strength T/C. is defined to illustrate the relationship between In order to evaluate the hybrid effect efficiently, ROM (Rule of Mixture) is introduced. ROM is tensile and compressivea method to calculate properties mechanical of hybrid properties composites of hybrid composites as formula according (1). Ato small the ratio T/C of two ratio indicates the tensile strengthmaterials: is close to the compression strength.

= + (2) + σT + RT:C = (1) σC where is the compression modulus of hybrid composite (GPa); , are the compression moduli of carbon fiber and glass fiber composite (GPa), respectively; , are the volume where RT:C denotes the ratio of T/C strength, σT and σC are the tensile strength and compression contents of carbon fiber and glass fiber composite, respectively. strength of composites (MPa), respectively. Figure9 reports ratios of T/C strength for interlayer hybrid composites with various mixed ratios and hybrid structures. It can be found that ratios of T/C strength increase along with the carbon fiber content overall, which indicates the difference between tensile and compression strengths becomes larger. Under the same C/G mixed ratio, ratios of T/C strength with glass fiber sandwiching carbon fiber, such as [G/G/C/G/G], [G/C/G/G], [G/C/G], [G/C/C/G], are the minimum. With regard to structures with carbon fiber sandwiching glass fiber, like [C/G/G/C], or with carbon fiber locating in one surface, like [C/G/G/G/G], [C/G/G/G], [G/G/C], ratios of T/C strength can reach up to a high level. Ratios of T/C strength of intralayer hybrid composites with various mixed ratios and hybrid structures are shown in Figure 10. There is not a clear law to describe the relationship between the ratio of T/C and the carbon ﬁber content, however, under the same C/G hybrid ratio, with a small dispersion degree like 0.5, structures have high ratios of T/C, while the composites with a high dispersion degree can reach low ratios of T/C. Materials 2018, 11, 1105 9 of 13

Materials 2018, 11, x FOR PEER REVIEW 9 of 13

3.0

2.8

2.6

2.4

2.2 T:C R 2.0

Materials 2018, 11, x FOR1.8 PEER REVIEW 9 of 13

1.63.0

1.42.8

1.22.6 C:G=0:1 C:G=1:4 C:G=1:3 C:G=1:2 C:G=1:1 C:G=1:0

2.4 FigureFigure 9. Ratio 9. Ratio of of T/C T/C strengthstrength of of interlayer interlayer hybrid hybrid composites. composites. 2.2

T:C 2.8 In order to evaluateR 2.0 the hybrid effect efﬁciently, ROM (Rule of Mixture) is introduced. ROM is a 2.6 method to calculate mechanical1.8 properties of hybrid composites according to the ratio of two materials:

2.41.6 ECVf C EGVf G E = + (2) ROM V + V V + V 1.4 f C f G f C f G 2.2 where EROM is the compression1.2 modulus of hybrid composite (GPa); EC, EG are the compression T:C moduli of carbon fiberR 2.0 andC:G=0:1 glass fiberC:G=1:4 compositeC:G=1:3 (GPa),C:G=1:2 respectively;C:G=1:1V , V C:G=1:0are the volume contents f C f G of carbon fiber and glass fiber composite, respectively. Figure 9. Ratio of T/C strength of interlayer hybrid composites. 1.8 2.8 1.6 2.6 1.4 2.4 C:G=0:1 C:G=1:4 C:G=1:2 C:G=1:1 C:G=1:0 Figure 10. Ratio of T/C strength of intralayer hybrid composites. 2.2 Experimental and theoretical values of tensile and compression moduli for interlayer and

intralayer hybridT:C composites are shown in Figure 11. It was found that tensile and compression R 2.0 moduli increase linearly along with the carbon fiber content, and the compression modulus is lower than the tensile modulus, but almost the same. 1.8 The tensile modulus is superior to the compression modulus resulting from the structure of NCF fabric. Figure 12a shows the ideal structure of the NCF fabric and Figure 12b is the real fabric. Due to 1.6 the weaving process, warp yarns in a fabric are constrained via stitching yarns in an unbalanced force, leading to a certain buckling of warp yarns. As the composite is subjected to a tensile loading, fiber and yarns along1.4 the longitudinal direction are gradually stretched, and high modulus of fiber will be fully utilized. However,C:G=0:1 as theC:G=1:4 specimen assumesC:G=1:2 a compressiveC:G=1:1 force alongC:G=1:0 the fiber axis, the buckling fiber and yarnsFigure have 10. noRatio chance of T/C to strength stretch, of therefore, intralayer thehybrid compression composites. modulus is low. Figure 10. Ratio of T/C strength of intralayer hybrid composites. Experimental and theoretical values of tensile and compression moduli for interlayer and Experimentalintralayer hybrid and composites theoretical are values shown in of Figure tensile 11.and It was compression found that tensile moduli and for compression interlayer and moduli increase linearly along with the carbon fiber content, and the compression modulus is lower intralayer hybrid composites are shown in Figure 11. It was found that tensile and compression than the tensile modulus, but almost the same. moduli increaseThe tensile linearly modulus along is superior with the to carbonthe compression ﬁber content, modulus and resulting the compression from the structure modulus of NCF is lower than thefabric. tensile Figure modulus, 12a shows but the almost ideal structure the same. of the NCF fabric and Figure 12b is the real fabric. Due to the weaving process, warp yarns in a fabric are constrained via stitching yarns in an unbalanced force, leading to a certain buckling of warp yarns. As the composite is subjected to a tensile loading, fiber and yarns along the longitudinal direction are gradually stretched, and high modulus of fiber will be fully utilized. However, as the specimen assumes a compressive force along the fiber axis, the buckling fiber and yarns have no chance to stretch, therefore, the compression modulus is low.

Materials 2018, 11, 1105 10 of 13 Materials 2018, 11, x FOR PEER REVIEW 10 of 13

120 Interlayer tensile 110 Intralayer tensile Interlayer compression Materials 2018, 11, x FOR PEER100 REVIEW Intralayer compression 10 of 13 90 ROM tensile modulus 120 ROM compression modulus 80 Interlayer tensile 110 Intralayer tensile 70 Interlayer compression 100 60 Intralayer compression

90 Modulus (GPa) ROM tensile modulus 50 ROM compression modulus 80 40

70 30 0 20 40 60 80 100 Carbon fiber content (%) 60

FigureModulus (GPa) 11. Experiment and theoretical values of tensile and compression moduli of hybrid composites. Figure 11.50Experiment and theoretical values of tensile and compression moduli of hybrid composites.

The tensile40 modulus is superior to the compression modulus resulting from the structure of NCF fabric. Figure 12a shows the ideal structure of the NCF fabric and Figure 12b is the real fabric. Due to the weaving30 process, warp yarns in a fabric are constrained via stitching yarns in an unbalanced force, 0 20 40 60 80 100 leading to a certain buckling of warpCarbon yarns. As fiber the composite content is(%) subjected to a tensile loading, fiber and yarns along the longitudinal direction are gradually stretched, and high modulus of fiber will be Figurefully utilized. 11. Experiment However, and as theoretical the specimen values assumes of tensile a compressive and compression force along moduli the of fiber hybrid axis, composites. the buckling fiber and yarns have no chance to stretch, therefore, the compression modulus is low.

(a) Ideal fabric structure (b) Real fabric pattern

Figure 12. NCF Fabric.

Theoretical stress and strain are not a simple superposition according to the ROM due to difference in fracture strain of carbon and glass fiber. As the compressive strain reaches the fracture strain of carbon fiber, carbon fiber fails first, and glass fiber assumes the residual load until the specimen fails. Calculations of compression stress are shown in (3) and (4).

Before carbon fiber fracture: = ( + ) (3)

After carbon fiber fracture: = (4) (a) Ideal fabric structure (b) Real fabric pattern where is the compression stress of hybrid composite (MPa); , are the volume contents of carbon fiber and glass fiber compositeFigure,Figure respectively 12. 12. NCFNCF; Fabric. Fabric., are the compression moduli of carbon fiber and glass fiber composite (GPa), respectively; and is the strain of hybrid composite. The relationship between the carbon fiber fraction and strength is illustrated in Figure 13. TheoreticalTheoretical stress stress and and strain strain are are not not a simple a simple superposition superposition according according to the ROM to due the to ROM difference due to Comparisons of experimental and theoretical results showed that tensile strength of hybrid differencein fracture in fracture strain strain of carbon of carbon and glass and fiber. glass As fiber. the compressive As the compressive strain reaches strain the reaches fracture the strain fract ofure composites appears to increase much more obviously than the compressive strength along with the strain carbon of carbon fiber, fiber, carbon carbon fiber fails fiber first, fails and first,glass fiber and assumes glass fiber the residual assumes load the until residual the specimen load until fails. the carbon fiber content, and the compressive strength tends to drop first and then rise later. specimenCalculationsE xperimentalfails. Calcul of compressionvaluesations are of higher compression stress than are showntheoretical stress in (3) areones and shown, which (4). inindicates (3) and the (4). tensile and compression strengths of hybrid composites conform well to the positive hybrid effect. BeforeBefore carbon carbon fiber fiber fracturefracture: : σ = (VE + VE))ε (3)(3) When the carbon fiber content ranges from 0% toHY 20%, theC CtensileG andG compression fracture strains of hybrid composites,After shown carbon in Figurefiber fracture: 14, decrease slightly.= Nevertheless, the decrease of (4) After carbon fiber fracture : σ = V E ε (4) fracture strain drops slowly with the carbon fiber content rangingHY fromG G 20% to 100%. As the fracture where is the compression stress of hybrid composite (MPa); , are the volume contents of whereenergyσHY releaseis the of compression little carbon fiber stress is ofnot hybrid sufficient composite to cause a (MPa); completeVC failure, VG are of thethe specimen volume contents rapidly, of carbon fiberthe damageand glass exhibits fiber a composite progressive, failure.respectively When ; the , carbon are fiber the content compression increases moduli to a certain of carbon carbon fiber and glass fiber composite, respectively; EC,EG are the compression moduli of carbon fiber fiber andand glass fiberfiber composite composite (GPa), (GPa) respectively;, respectively and; andε is the strainis the ofstrain hybrid of composite.hybrid composite. The relationship between the carbon fiber fraction and strength is illustrated in Figure 13. Comparisons of experimental and theoretical results showed that tensile strength of hybrid composites appears to increase much more obviously than the compressive strength along with the carbon fiber content, and the compressive strength tends to drop first and then rise later. Experimental values are higher than theoretical ones, which indicates the tensile and compression strengths of hybrid composites conform well to the positive hybrid effect. When the carbon fiber content ranges from 0% to 20%, the tensile and compression fracture strains of hybrid composites, shown in Figure 14, decrease slightly. Nevertheless, the decrease of fracture strain drops slowly with the carbon fiber content ranging from 20% to 100%. As the fracture energy release of little carbon fiber is not sufficient to cause a complete failure of the specimen rapidly, the damage exhibits a progressive failure. When the carbon fiber content increases to a certain

Materials 2018, 11, 1105 11 of 13

The relationship between the carbon fiber fraction and strength is illustrated in Figure 13. Comparisons of experimental and theoretical results showed that tensile strength of hybrid composites appears to increase much more obviously than the compressive strength along with the carbon fiber Materials 2018, 11, x FOR PEER REVIEW 11 of 13 content, and the compressive strength tends to drop first and then rise later. Experimental values are higherfraction, than the fracture theoretical energy ones, release which of carbon indicates fiber leads the tensileto a rapid and failure compression of hybrid composites, strengths ofand hybrid compositesthe fracture conform strain wellis close to to the the positive pure carbon hybrid fiber effect. composites.

1800 Theory tensile strength Theory compression strength 1600 Inter-layer tensile Intra-layer tensile Inter-layer compression Materials 2018, 11, x 1400FOR PEER REVIEW Intra-layer compression 11 of 13

fraction, the fracture1200 energy release of carbon fiber leads to a rapid failure of hybrid composites, and the fracture strain is close to the pure carbon fiber composites. 1000

8001800 Theory tensile strength

Strength(MPa) Theory compression strength 6001600 Inter-layer tensile Intra-layer tensile Inter-layer compression 4001400 Intra-layer compression

2001200 0 20 40 60 80 100 1000 Carbon fiber content (%)

FigureFigure 13. Experimental 13. Experimental800 and andtheoretical theoretical values values of tensile of tensile and compression and compression strengths strength of hybrids of hybrid composites .

composites. Strength(MPa) 600 When the carbon fiber content ranges from 0% to 20%, the tensile and compression fracture strains 2.4 of hybrid composites,400 shown in Figure 14, decrease slightly. Interlayer Nevertheless, tensile the decrease of fracture strain drops slowly2.2 with the carbon fiber content ranging from Intralayer 20% to tensile 100%. As the fracture energy 200 Interlayer compression release of little carbon2.0 fiber is not0 sufficient20 to cause40 a complete60 Intralayer failure80 compression of the100 specimen rapidly, the damage exhibits a progressive failure. WhenCarbon the carbon fiber content fiber (%) content increases to a certain fraction, the 1.8 fracture energyFigure release 13. Experimental of carbon and fiber theoretical leads valuesto a rapid of tensile failure and of compr hybridession composites, strengths of and hybrid the fracture 1.6 strain is closecomposites. to the pure carbon fiber composites. 1.4

1.22.4 Interlayer tensile 1.02.2 Intralayer tensile Fracturestrain (%) Interlayer compression 0.82.0 Intralayer compression 0.61.8

1.6 0 20 40 60 80 100 Carbon fiber content (%) 1.4 Figure 14. Tensile1.2 and compression strengths of interlayer and intralayer hybrid composites obtained by experiment and ROM. 1.0 Fracturestrain (%) 4. Conclusions 0.8 Tensile and compressive0.6 properties of interlayer and intralayer hybrid composites were studied in this work. 0 20 40 60 80 100 (1) The tensile modulus and compressionCarbon fiber modulus content of (%) interlayer and intralayer hybrid composites are basically the same under the same mixed ratio, and they increase linearly along with the carbonFigure fiber 14. Tensilecontent and. Variations compression in hybridstrength structuress of interlayer have and no intralayer effect on hybrid the tensile composites and compression obtained Figure 14. Tensile and compression strengths of interlayer and intralayer hybrid composites obtained moduliby. experiment and ROM. by experiment and ROM. 4. Conclusions

Tensile and compressive properties of interlayer and intralayer hybrid composites were studied in this work. (1) The tensile modulus and compression modulus of interlayer and intralayer hybrid composites are basically the same under the same mixed ratio, and they increase linearly along with the carbon fiber content. Variations in hybrid structures have no effect on the tensile and compression moduli.

Materials 2018, 11, 1105 12 of 13

4. Conclusions Tensile and compressive properties of interlayer and intralayer hybrid composites were studied in this work. (1) The tensile modulus and compression modulus of interlayer and intralayer hybrid composites are basically the same under the same mixed ratio, and they increase linearly along with the carbon fiber content. Variations in hybrid structures have no effect on the tensile and compression moduli. (2) Analysis of tensile strength and compression strength of interlayer and intralayer hybrid composites revealed that the tensile strength is significantly greater than the compression strength resulting from the warp bulking. The tensile strength increases along with the carbon fiber content, and the compression strength changes slightly. (3) The tensile fracture strain is greater than the compression fracture strain for interlayer and intralayer hybrid composites; both of them drop slightly as the carbon fiber content increases, and for intralayer hybrid composites, the strain tends to be lower as the C/G dispersion degree is high. (4) Ratios of T/C strength for interlayer and intralayer hybrid composites increase as the carbon fiber content increases. For interlayer hybrid composites with the same C/G mixed ratio, the ratio of T/C strength with glass fiber sandwiching carbon fiber is the minimum. By contrast, the ratio of T/C strength with carbon fiber sandwiching glass fiber or with carbon fiber distributed in one side can reach the maximum level. For intralayer hybrid composites, the effect of stacking structures is small, and for some dispersion degree, the structures can achieve a low ratio of T/C. Tensile and compression strengths of hybrid composites are larger than the theoretical values, which conform to the positive hybrid effect.

Author Contributions: The manuscript was completed through contributions of all authors. W.L. provided the idea for this program and participated in the discussion for experimental plans. Q.W. designed the hybrid fabrics and performed the testing. W.W. analyzed the data and wrote the paper. Funding: This research was funded by the Innovation Funding for Graduates of Donghua University, grant number CHSF-DH-D-2016019. Acknowledgments: I sincerely appreciate the funding from Donghua University, so I could complete the research work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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