The Residual Stress Characteristics and Mechanical Behavior of Shot Peened Fiber Metal Laminates Based on the Aluminium-Lithium Alloy

Delft University of Technology The residual stress characteristics and mechanical behavior of shot peened fiber metal laminates based on the aluminium-lithium alloy Li, Huaguan; Wang, Hao; Alderliesten, René; Xiang, Junxian; Lin, Yanyan; Xu, Yingmei; Zhao, Haidan; Tao, Jie DOI 10.1016/j.compstruct.2020.112858 Publication date 2020 Document Version Final published version Published in Composite Structures Citation (APA) Li, H., Wang, H., Alderliesten, R., Xiang, J., Lin, Y., Xu, Y., Zhao, H., & Tao, J. (2020). The residual stress characteristics and mechanical behavior of shot peened fiber metal laminates based on the aluminium- lithium alloy. Composite Structures, 254, [112858]. https://doi.org/10.1016/j.compstruct.2020.112858 Important note To cite this publication, please use the final published version (if applicable). Please check the document version above. 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Composite Structures 254 (2020) 112858 Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct The residual stress characteristics and mechanical behavior of shot peened fiber metal laminates based on the aluminium-lithium alloy ⇑ Huaguan Li a,c, , Hao Wang b, René Alderliesten c, Junxian Xiang b, Yanyan Lin b, Yingmei Xu b, Haidan Zhao a, ⇑ Jie Tao b, a Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing Institute of Technology, Nanjing 211167, China b College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China c Structural Integrity & Composites Group, Faculty of Aerospace Engineering, Delft University of Technology, Delft 2600 GB, The Netherlands ARTICLE INFO ABSTRACT Keywords: The effect of shot peen forming on the mechanical behavior of fiber metal laminates (FMLs) based on Fiber metal laminates aluminium‐lithium alloy was investigated to reveal the strengthening mechanism and to dispel the suspicion Aluminum‐lithium alloy that shot peen forming may result in the performance deterioration of FMLs. The interlaminar, static strength Shot peen forming and fatigue properties of shot peened FMLs were investigated. The residual stress characteristics of the shot Mechanical behavior peened FMLs was also involved with finite element analysis to help understanding the unique mechanical Residual stress behavior. The results indicated that shot peening caused non‐negligible work hardening in external metal layers, which increased the tensile strength of the laminates. But the work hardening did not deteriorate the elon- gations of FMLs since the failure still dominated by the limitation of fiber failure strain. Moreover, two yield stages were observed in the tensile tests of shot peened FMLs owing to the great difference in stress states between external and internal metal layers. The compressive stress introduced by shot peening effectively improved the FCG properties of FMLs. All metal layers possessed similar crack propagation rates despite that the stress difference was up to 300 MPa, which indicated that the fiber bridging effect still dominated the FCG of FMLs. 1. Introduction FML components are usually manufactured referring conventional forming method of metal panels. However, limited failure strain of Fiber Metal Laminates (FMLs) possess excellent fatigue and impact the glass fiber increase forming difficulties, which is one of the biggest resistance [1,2], which are still attractive in aviation and high‐speed factors limiting the application of FMLs [12,13]. Recent years, the shot train. The well‐known GLARE laminates usually choose 2024‐T3 alu- peen forming [14,15] has been developed for FMLs. It provides effec- minum alloy as the metal layer and S‐glass fiber composites as the tive as well as economical solution for the manufacturing of complex fiber layer [3,4]. Except outstanding impact and fatigue, it also exhi- FML components. Moreover, the residual stress field introduced by bits good residual and blunt notch strength. Also, GLARE always pos- shot peening also beneficial to the fatigue properties. Lots of investiga- sesses stable crack propagation behavior after suffering serious tions have focused on the conventional shot peening or laser forming irreversible damage [5,6]. Though carbon fiber composites are getting of FMLs. The unique failure behavior and deformation rules, compar- more attention, GLARE is still an important material in the field of avi- ing with metal panels, are revealed [16,17]. Meanwhile, the suitable ation, rail transit, and other lightweight components that both require balls and shot peen parameters are obtained for both conventional shot impact resistance and fatigue. Besides, GLARE has the potential to fur- peening or laser forming [18–20]. In view of the essential problem of ther improve the overall performance. Considering the lower costs and “stress dominant deformation” in shot peen forming, our previous better damage tolerance, third generation aluminium‐lithium alloy research also focused on the residual stress evolution of FMLs during [7,8], replace 2024‐T3 aluminum alloy, is one of the desired choose the shot peen forming. The relationship between processes, residual to develop novel FMLs [9–11]. stress distribution and forming curvature is finally established [21]. ⇑ Corresponding authors at: Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing Institute of Technology, Nanjing 211167, China (H. Li). E-mail addresses: [email protected] (H. Li), [email protected] (J. Tao). https://doi.org/10.1016/j.compstruct.2020.112858 Received 19 June 2020; Revised 30 July 2020; Accepted 17 August 2020 Available online 20 August 2020 0263-8223/© 2020 Elsevier Ltd. All rights reserved. H. Li et al. Composite Structures 254 (2020) 112858 Nomenclature p positive pressure d damage variable q Mises equivalent stress Lc characteristic length ωD state variables related to plastic deformation ɛf maximum strain Àpl ɛ equivalent plastic strain t thickness of the cohesive element ɛf ;T failure strain of tension η mixed failure index ɛf ;C failure strain of compression G fracture toughness However, studies have never addressed on the mechanical behavior variation of FMLs caused by shot peening. FMLs has more complex mechanical behavior than conventional aluminum alloy owing to the laminating structure and large difference in mechanical response of each component. Firstly, the single metal layer of FMLs or conventional GLARE is 0.3–0.5 mm, which means that the local plastic deformation caused by shot peening occupies a large proportion through that thickness. The serious work‐hardening and embrittlement of shot peened metal layers may promote the formation and propagation of cracks. Meanwhile, the depth of the pit is non‐ ignorable as the single metal layer is extremely thin. Every pit may act as the apparent defect, inducing fracture during the loading. More- Fig. 1. The FMLs studied constitute an FML3-3/2-0.3 lay-up. over, the previous work indicated that large size balls or high shot peen intensity could results in the delamination of metal/fiber inter- faces. Even the optimized forming parameters did not cause obvious defects in FMLs, the interlaminar properties of shot peened laminates should be verified. Besides, the complicated residual stress characteristics greatly affect the mechanical behavior of FMLs. For conventional aluminium‐lithium panels, the shot peened surface with a certain depth possesses obvious compressive stress while most other areas pre- sent tensile stress. The change in stress is continuous and gradual along the thickness. However, non‐negligible residual stress field has already formed in FMLs during the curing process owing to different thermal expansion coefficient of each component [22]. The metal layers usually possesse a tensile stress of 40–45 MPa while the fiber layers exhi- bit a compressive stress of 65–70 MPa. Then, shot peening introduces significant compressive stress in the external aluminium‐lithium layers, which results in a stress rebalance within the whole laminate. Residual stress change discontinuously on the fiber/metal and 0° fiber/90°fiber interface due to the different modulus of each component. Hence, our research primarily tried to investigate the properties Fig. 2. The curing process of the FMLs in autoclave. variation of FMLs after shot peening through evaluating the interlaminar properties, tensile properties and fatigue crack growth. This work revealed the strengthening mechanism of shot peening on FMLs, and 2.2. Shot peen forming of FMLs by doing so further demonstrated the feasibility of shot peen forming. FMLs were shot peened in the production line of AVIC Xi’an Air- 2. Experimental craft Industry (group) Co., Ltd (see Fig. 3(a)), since the motivation of this work was to verify whether shot peen forming could be used 2.1. Manufacturing of FMLs in the real manufacturing of actual FMLs components. Differ from conventional metal panels, FMLs were shot peened using AZB425 ceram- The 2060 Al‐Cu‐Li alloy with the initial thickness of 0.3 mm mm ics balls instead of cast steel balls to achieve a good surface quality and was used as the metal layer [9], while S4‐glass/epoxy prepregs used avoid possible failure behavior [18].

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