Compression Creep and Thermal Ratcheting Behavior of High Density Polyethylene (HDPE)

Article Compression Creep and Thermal Ratcheting Behavior of High Density Polyethylene (HDPE)

Rahul Palaniappan Kanthabhabha Jeya * ID and Abdel-Hakim Bouzid

Department of Mechanical engineering, École de Technologie Supérieure, 1100 Notre Dame O, Montréal, QC H3C1K3, Canada; [email protected] * Correspondence: [email protected]

Received: 25 December 2017; Accepted: 2 February 2018; Published: 7 February 2018

Abstract: The characterization of thermal ratcheting behavior of high density polyethylene (HDPE) material coupled with compressive creep is presented. The research explores the adverse influence of thermal cycling on HDPE material properties under the effect of compressive load, number of thermal cycles, creep time period, and thermal ratcheting temperature range. The compressive creep analysis of HDPE shows that the magnitude of creep strain increases with increase in magnitude of applied load and temperature, respectively. The creep strain value increased by 7 and 28 times between least and maximum applied temperature and load conditions, respectively. The creep modulus decreases with increase in compressive load and temperature conditions. The cumulative deformation is evident in the HDPE material, causing a reduction in the thickness of the sample under thermal ratcheting. The loss of thickness increases with increase in the number of thermal cycles, while showing no sign of saturation. The thermal ratcheting strain (TRS) is influenced dominantly by the applied load condition. In addition, the TRS decreases with increase in creep time period, which is cited to the extended damage induced due creep. The results highlight the need for improved design standard with inclusion of thermal ratcheting phenomenon for HDPE structures particularly HDPE bolted flange joint.

Keywords: compressive creep; thermal ratcheting; HDPE characterization; creep modulus; thermal ratcheting strain; bolted ﬂange joints

1. Introduction In recent times, the polymer or plastic materials have seen a rapid growth in replacing the conventional metallic piping structures, mainly due to their economical production cost and minimal dependence and impact on the environment. The typical advantages of polymers over metallic materials are extensive protection against chemical and corrosion attacks, extended service life, and that they are lightweight with high strength and modulus. Amongst the different types of polymeric materials commercially available, high density polyethylene (HDPE) polymer has the second largest share of spoils behind polyvinyl chloride (PVC). HDPE is a good candidate for application in chemical ﬂuid and slurry transfer pipes, because of its excellent chemical resistance and near frictionless ﬂow characteristics. The applicability of HDPE material has seen a recent boom in the piping system against PVC, due to its excellent resistance to fatigue and UV radiation. The dominance of HDPE pipe in urban service piping network for water and gas, nuclear service water, and desalination piping, is evident. Similar to most polymer materials, the research on characterizing HDPE material properties is abundant. Since the early eighties, a large number of research studies have focused on the creep property of polymers, as this phenomenon is perceived as a hindrance and a drawback of polymer materials. Quantitative data on creep, and other perennial properties of two varieties of PVC and polyethylene materials under liquid pressure at different temperatures, was published [1].

Polymers 2018, 10, 156; doi:10.3390/polym10020156 www.mdpi.com/journal/polymers Polymers 2018, 10, 156 2 of 13

The creep behavior of thermoplastics at temperatures close to the glass transition region of polymers was studied [2]. The viscoelastic creep response of high density polyethylene is explored in two creep models, a viscoplastic model and a nonlinear viscoelastic model, which on being fitted with experimental data, gave near perfect and moderate accuracy, respectively [3,4]. The developed nonlinear creep model of high density polyethylene has a good agreement with the experimental data, including the effect of ageing [5]. The recent enthusiasm towards viscoelastic property has lead into probing of the viscoelastic and viscoplastic behavior of HPDE under cyclic loading conditions [6]. Crack initiation and propagation of ductile and brittle polyethylene resin under creep damage show that only the brittle resin exhibits a lifespan controlled by slow crack growth (SCG) [7]. Researchers [8,9] studied the impact of strain rate and temperature on tensile properties of the post-consumer recycled HPDE. A large quantity of research focused on the mechanical cycling or fatigue behavior of HDPE material [10], HDPE geogrid [11], solid extruded HDPE [12], HDPE composite [13], HDPE pipe joints [14], but almost none on the thermal ratcheting effect. The mechanical property of filled HDPE and the effect of loading and manufacturing method on the properties of HPDE are well documented in the studies [15,16], respectively. Additionally, statistical analysis of HDPE fatigue life is performed [17]. The influence of cyclic loading rates under different temperature conditions on the cyclic creep behavior of polymers and polymer composites was examined [18]. The brittle and ductile failure under creep rupture testing of high density polyethylene pipes are thoroughly researched [19]. There are no typical references available on the creep data of polymeric bolted flange joint subjected to compression. In metallic bolted flange gasketed connections, the gasket component is usually blamed for relaxation, and hardly ever the flange material itself [20]. However, such is not the case with polymeric flanges, hence, quintessential analysis of compressive creep behavior is a necessity. Furthermore, most polymer materials have restrictive features of low operational temperature conditions, thereby making them vulnerable to any temperature fluctuations. Consequences of thermal ratcheting or cycling of temperature on polymers can be severe, yet remain a relatively rare phenomenon in reported scientific literature. The work on the hot blowout testing procedure for polytetrafluoroethylene (PTFE) based gaskets [21], and the creep–thermal ratcheting analysis of PTFE based gaskets [22], are a few examples of the rarest research on the thermal ratcheting behavior of polymers. In the present work, a detailed characterization of high density polyethylene under compressive creep, thermal ratcheting phenomenon, and the coupled effect of compressive creep and thermal ratcheting at different temperature and stress conditions, is presented. Even though the compressive creep problem of the polymeric bolted flange joint is the driving factor for this research, the output can be adapted to a wider range of polymer applications.

2. Materials and Methods

2.1. Experimental Setup The universal gasket rig (UGR) is an innovative in-house built experimental test bench for performing mechanical and leak characterization of polymeric materials, shown in Figure1a. The signiﬁcance of UGR is highlighted through the capacity to perform intricate compressive creep and thermal ratcheting analysis of HDPE material. Conceptually, the UGR generates a simple distributed compressive load on the specimen with hydraulic pump and two platens. A central stud, screwed to the hydraulic tensioner head, transmits the required compressive stress to the sample. The conservation of load on the material is achieved through an accumulator connected with hydraulic system. The UGR can facilitate ring shaped samples with a maximum outer and minimum inner diameter of 100 and 50 mm in between the two platens. The polymer test pieces are limited to an allowable thickness of 10 mm. This simple and sophisticated machine supports the complex analysis of material properties through the ability to apply an integrated load of internal pressure, compression, and heat. Typically, Polymers 2018, 10, 156 3 of 13

Polymers 2018, 10, x FOR PEER REVIEW 3 of 13 the maximum operating condition of UGR unit is restricted to 5 MPa of internal pressure on a controlled compression, and heat. Typically, the maximum operating condition of UGR unit is restricted to 5 high temperature environment of 450 ◦C. MPa of internal pressure on a controlled high temperature environment of 450 °C.

(a)

(b)

FigureFigure 1. 1.Universal Universalgasket gasket rig: ( aa)) entire entire unit; unit; and and (b ()b heating) heating system. system.

The real-time reduction in the thickness of the sample under compressive creep is measured usingThe real-timea high sensitive reduction linearly in variable the thickness differenti ofal the transformer sample under (LVDT). compressive The samples creep are heated is measured by usingmeans a high of an sensitive external linearly ceramic variableband, which differential encloses transformeraround the platens (LVDT). to apply The samplesheat on the are sample heated by meansin form of an of externalconduction ceramic (illustrated band, in which Figure encloses1b). A proportional around the integral platens deri tovative apply (PID) heat controller on the sample is in formused ofto conductioncontrol the temperature (illustrated of in the Figure heater1b). by A monitoring proportional theintegral temperature derivative of gasket (PID) through controller a is usedthermocouple, to control which the temperature is connected to of a the computer heater by by RS232 monitoring serial port. the The temperature rate of heating of gasket is set at through 1.5 a thermocouple,°C/min, while whichthe cooling is connected is accomplished to a computer through na bytural RS232 convection serial port. by shut-off The rate of the of heating heater once is set at 1.5 ◦theC/min, desired while temperature the cooling is attained. is accomplished The rigidity through is controlled natural convectionusing an appropriate by shut-off number of the heaterof onceBelleville the desired washers. temperature is attained. The rigidity is controlled using an appropriate number of Belleville washers.

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PolymersA special 2018, 10 insulation, x FOR PEERcover, REVIEW comprising fiber materials and a stainless-steel cap, successfully4 of 13 accomplishes the prevention of heat loss to the surroundings. A Wheatstone bridge circuit strain gauge isA affixed special at insulation the bottom cover, of the comprising central stud, fiber to ma measureterials and the straina stainless- inducedsteel oncap, the successfully test piece due to theaccomplishes application the of prevention load. The machineof heat loss can to apply the surroundings. a maximum A stress Wheatstone of 70 MPa bridge on acircuit sample strain area of 645.16gauge mm is2 .affixed Specially at the designed bottom of inlet the andcentral outlet stud, ports to measure in the the upper strain platens induced are on handy the test in piece pressurizing due the internalto the application surface and of load. measuring The machine the leak can rate apply under a maximum different teststress conditions. of 70 MPa The on a exclusive sample area feature of of 645.16 mm2. Specially designed inlet and outlet ports in the upper platens are handy in pressurizing UGR is the thermal ratcheting analysis, which is accomplished by the combined use of PID controller and the internal surface and measuring the leak rate under different test conditions. The exclusive feature LabVIEW program. The system requires a complete definition of ratcheting temperature range, number of UGR is the thermal ratcheting analysis, which is accomplished by the combined use of PID of temperaturecontroller and cycles, LabVIEW and time program. periodof The hold-off system between requires thermal a complete cycles todefinition achieve thermalof ratcheting ratcheting. temperature range, number of temperature cycles, and time period of hold-off between thermal 2.2. Test Procedure and Material Specifications cycles to achieve thermal ratcheting. The mechanical characterization of ring shaped HDPE material is achieved through the sophistication2.2. Test Procedure of the and UGR Material test bench. Specifications Typically, the physical measurement of polymer dimensions is the startThe point mechanical for the test characterization procedure (Figure of 2ring), followed shaped byHDPE initialization material ofis LabVIEWachieved through program the to set up allsophistication the measuring of the sensors. UGR test The bench. measured Typically, polymer the physical ring measurement dimensions areof polymer fed as inputs, dimensions which is are usedthe in start the point evaluation for the oftest applied procedure compressive (Figure 2), foll stressowed from by initialization the measured of LabVIEW compressive program strain to byset full bridgeup all strain the measuring gauge. Initially, sensors. manual The measured tightening polymer of the ring nut dimensions is required are to fed hold as theinputs, ring which specimen are in positionused betweenin the evaluation two the of contacting applied compressive platen surfaces stress before from the any measured application compressive of load through strain by hydraulic full pump.bridge The strain zero positiongauge. Initially, for LVDT manual sensor tightening and sample of the gasket nut is stressrequired is defined to hold atthe the ring instant specimen of locking in position between two the contacting platen surfaces before any application of load through hydraulic the platens manually. Subsequently, depending on the requisite of test to be performed, the chosen pump. The zero position for LVDT sensor and sample gasket stress is defined at the instant of locking load level is exerted on to the polymer specimen, before or after the application of heat. In accordance the platens manually. Subsequently, depending on the requisite of test to be performed, the chosen withload the level industrial is exerted fluid on process to the polymer heating specimen, rate, the ceramicbefore or bandafter the electrical application heater of heat. is set In at accordance a heating rate ◦ of 1.5withC/min. the industrial Specially fluid developed process heating LabVIEW rate, the program ceramic provides band electrical for facile heater real-time is set at monitoringa heating rate of all the sensorsof 1.5 °C/min. of the Specially test rig, developed while also LabVIEW enabling program for options provides to modify for facile the real-time temperature monitoring and of pressure all conditions.the sensors The of system the test has rig, the while capacity also toenabling monitor for changes options into themodify test conditionsthe temperature at a minimum and pressure interval of 10conditions. s, with option The system to record has values the capacity at a time to intervalmonitor betweenchanges in 10 the to 600test s,conditions as necessitated at a minimum by behavior of theinterval material of 10 over s, with time. option The to creeprecord andvalues thermal at a time ratcheting interval between characterization 10 to 600 s, ofas HDPEnecessitated polymer by is conductedbehavior in of two the phases. material The over first time. phase The involvescreep and the thermal short-term ratcheting compressive characterization creep analysis of HDPE of the HDPEpolymer samples is conducted for 4 to 5 daysin two under phases. a variety The first of ph temperaturease involves and thestress short-term settings. compressive The second creep phase is dedicatedanalysis of to the the HDPE analysis samples of thermal for 4 to 5 ratcheting days under phenomenon, a variety of temperature which is evaluatedand stress settings. by preforming The second phase is dedicated to the analysis of thermal ratcheting phenomenon, which is evaluated by 10–20 thermal cycles between two target temperatures, with or without a day of creep. The information preforming 10–20 thermal cycles between two target temperatures, with or without a day of creep. provided in Tables1 and2 elaborates, in detail, the types of tests performed. The ring sample of HDPE The information provided in Tables 1 and 2 elaborates, in detail, the types of tests performed. The material respects 3 inch iron pipe size (IPS), standardized to 72 mm and 90 mm as inner and outer ring sample of HDPE material respects 3 inch iron pipe size (IPS), standardized to 72 mm and 90 mm diameter,as inner along and outer with diameter, a material along thickness with a of material 6.35 mm. thickness of 6.35 mm.

Figure 2. Specimen sample. Figure 2. Specimen sample.

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Table 1. Creep test parameters.

High Density Polyethylene Test No. Temperature (◦C) Stress (MPa) Test Time Period 1 23 7, 14 & 21 5 days 2 50 7 & 14 5 days 3 60 7 & 14 5 days 4 70 7, 14 & 21 5 days

Table 2. Thermal ratcheting test conditions.

Days of Creep + No. of Test No. Applied Stress (MPa) Creep Temp (◦C) Ratcheting Temp (◦C) Thermal Cycles High Density Polyethylene T1 7 23 28–55 1 + 20 T2 14 – 28–55 0 + 20 T3 14 23 28–55 1 + 20 T4 14 23 28–55 4 + 20 T5 14 – 28–60 0 + 20 T6 14 – 28–40 0 + 10

3. Results and Discussion

3.1. Creep Strain The vulnerability of polymer materials to creep and fatigue phenomenon are widely known, and high density polyethylene is no exception. Unlike metallic materials, the variation in creep strain of polymers under tensile and compressive load is distinct. Furthermore, the HDPE material is suspected to be prone to thermal ratcheting damage, due to inherent low melting temperature. Therefore, a quantitative assessment of thermal ratcheting behavior, coupled with compressive creep of HDPE material, is essential. The experimental creep test results highlight the importance of both applied compressive load and temperature on the creep strain of HDPE. Influence of applied load on creep strain is shown in Figure3, where the magnitude of induced creep strain increases with increase in the value of applied compressive stress. Importance of applied load on the transition time from primary to secondary creep stage is evident, as the specimen demonstrates different time periods to reach the secondary phase. Yet, all HDPE samples demonstrate secondary creep, mostly within the first few hours of test. At ambient temperature, the creep strain of HDPE at 14 MPa of stress grew by six times the value of creep strain at 7 MPa of compressive stress. Whereas, on comparing the creep strain of HDPE at 14 and 21 MPa of compressive stress, the sample demonstrates a growth of 4.7 times the creep strain value at the lower load. The jump in magnitude of creep strain between 7 and 21 MPa of load is quite significant, which is nearly 28 times the former. The magnitude of the primary creep strain tends to increase with increase in applied load, causing subtle variations in the transition from primary to secondary creep for all the tested HDPE polymer samples. Alike to applied load, on varying the sample temperature under the same load (Figure4), the creep strain tends to rise with intensification of temperature. Roughly, there is 20 percent increase in the creep strain with 10 ◦C escalation of temperature for HDPE material under the tested load. Even though the material’s response to creep under the two types of loading is similar, their magnitude of deformation is straight out different. The magnitude of primary creep phase amplifies with increase in the sample temperature under the same compressive creep load. Analogous to compressive stress, the magnitude of primary creep strain is proportional to applied sample temperature. A compressive creep test at a sample temperature of 70 ◦C was performed to understand if there is any drastic change in the material response, as the particular temperature is above the standard maximum operating temperature. The consequent reaction of HDPE sample is consistent with rest of the test results. Polymers 2018, 10, 156 6 of 13 Polymers 2018, 10, x FOR PEER REVIEW 6 of 13 Polymers 2018, 10, x FOR PEER REVIEW 6 of 13

0.2

0.18

0.16

0.14 c c at 21 MPa 0.12 at 21 MPa at 14 MPa at 14 MPa 0.1 at 7 MPa 0.1 at 7 MPa 0.08 Creep Strain ε Strain Creep Creep Strain ε Strain Creep 0.06

0.04

0.02

0 050100150 Elapsed Time (hrs) Elapsed Time (hrs) Figure 3. CreepCreep strain strain under under different different loadsloads atat ambientambient temperature.temperature.

0.16

0.14

0.12 c c 0.10

atat 70°C70°C 0.08 atat 60°C60°C at 50°C 0.06 at 50°C Creep Strain ε Strain Creep Creep Strain ε Strain Creep atat 23°C23°C 0.04

0.02

0.00 0 20406080100 Elapsed Time (hrs)

Figure 4. Creep strain under different temperatures at 14 MPa. Figure 4. Creep strain under different temperatures at 14 MPa.

3.2. Creep Modulus 3.2. Creep Modulus Creep modulus is another important characteristic property of polymer material. The creep Creep modulus is anotheranother importantimportant characteristiccharacteristic propertyproperty ofof polymerpolymer material.material. The creep modulus is defined as the instantaneous elastic modulus of the material that varies with time. The modulus isis deﬁneddefined as as the the instantaneous instantaneous elastic elastic modulus modulus of theof the material material that that varies varies with with time. time. The ratioThe ratio of creep stress over creep strain computes the creep modulus of the material. Since the creep ofratio creep of creep stress stress over creepover creep strain strain computes computes the creep the creep modulus modulus of the of material. the material. Since theSince creep the stresscreep stress is maintained constant, the creep modulus is inversely proportional to the creep strain. The isstress maintained is maintained constant, constant, the creep the moduluscreep modulus is inversely is inversely proportional proportional to the to creep the strain.creep strain. The creep The creep modulus is dependent of the applied stress, where an increase of compressive load amplifies moduluscreep modulus is dependent is dependent of the of applied the applied stress, stress, where where an increase an increase of compressive of compressive load load ampliﬁes amplifies the the loss in creep modulus of the material. The drop-in value of HDPE creep modulus over time under lossthe loss in creep in creep modulus modulus of theof the material. material. The The drop-in drop-in value value of of HDPE HDPE creep creep modulus modulus overover timetime under different compressive stress levels at room temperature is illustrated in Figure 5. The trend of creep different compressive stress levels at room temperaturetemperature is illustrated in Figure 55.. TheThe trendtrend ofof creepcreep modulus curve is similar to the creep strain curve, butbut inin thethe inverseinverse direction,direction, wherewhere thethe materialmaterial losesloses aa substantialsubstantial amountamount ofof creepcreep modulusmodulus initiallyinitially followedfollowed byby aa gradualgradual saturationsaturation overover time.time. TheThe descent of creep modulus is maximum with the highest applied stress, while the magnitude of loss

Polymers 2018, 10, 156 7 of 13 modulus curve is similar to the creep strain curve, but in the inverse direction, where the material loses a substantial amount of creep modulus initially followed by a gradual saturation over time. The descent of creep modulus is maximum with the highest applied stress, while the magnitude of loss in creep modulus decreases as the value of applied compressive stress lowers. The saturation of creep modulusPolymers happens 2018, 10, x swiftly FOR PEER for REVIEW the sample compressed under 21 MPa of stress, whereas the7 materialof 13 requires a minimum of 10 and 30 h for transition to near saturation state under the compressive stressin of creep 14 and modulus 7 MPa, decreases respectively. as the value The of change applied in compressive creep modulus stress lowers. between The 7 saturation and 14 MPa of creep of stress is highermodulus than happens the difference swiftly for in creepthe sample modulus compressed between under 14 21 and MPa 21 of MPa stress, of stress,whereas even the material though the increaserequires in magnitude a minimum of of compressive10 and 30 h for stress transition remains to near constant saturation at state 7 MPa. under The the probablecompressive reason stress cited of 14 and 7 MPa, respectively. The change in creep modulus between 7 and 14 MPa of stress is higher for this behavior is the greater extent of irreversible damage induced in the specimen tested at higher than the difference in creep modulus between 14 and 21 MPa of stress, even though the increase in compressive load, leading to earlier saturation. The inﬂuence of applied temperature on the creep magnitude of compressive stress remains constant at 7 MPa. The probable reason cited for this modulusbehavior of HDPE is the isgreater presented extent in of Figure irreversible6, which damage exhibits induced the loss in ofthe creep specimen modulus tested with at higher different chosencompressive temperature load, under leading the to appliedearlier saturation. compressive The influence stress of of 14 applied MPa. Consistenttemperature with on the the creep applied compressivemodulus stress of HDPE condition, is presented the trend in Figure in loss 6, ofwhich creep exhibits modulus the underloss of creep different modulus temperatures with different is similar wherechosen the highest temperature reduction under in the creep applied modulus compressive happens stress at the of highest14 MPa. appliedConsistent temperature with the applied under the samecompressive compressive stress stress. condition, The magnitude the trend ofin reductionloss of creep in creepmodulus modulus under betweendifferent temperatures the room and is high temperaturesimilar where samples the ishighest enormous. reduction Besides, in creep the modu saturationlus happens of creep at the modulus highest dropapplied is evidenttemperature with the highestunder temperature the same compressive test, while stress. the ambient The magnitude temperature of reduction did notin creep yield modulus over the between tested the time room period. and high temperature samples is enormous. Besides, the saturation of creep modulus drop is evident The compressive creep test at 70 ◦C shows that the creep modulus drops by 8 times the initial value with the highest temperature test, while the ambient temperature did not yield over the tested time after saturation. By comparing the value of creep modulus at different temperatures, the consequence period. The compressive creep test at 70 °C shows that the creep modulus drops by 8 times the initial ◦ of temperaturevalue after issaturation. apparent. By At comparing 70 C, the the material value of loses creep nearly modulus 75% atof different the creep temperatures, modulus valuethe at ◦ 23 Cconsequence of temperature. of temperature is apparent. At 70 °C, the material loses nearly 75% of the creep modulus value at 23 °C of temperature. Constant applied stress Creep Modulus = σ /ε = , (1) Creep Modulusc =c σc/εc = , Creep strain at the instant (1)

2000

1600 at 21 MPa at 14 MPa at 7 MPa 1200

800 Creep Modulus (MPa) Modulus Creep 400

0 050100150 Elapsed Time (hrs)

FigureFigure 5. Creep 5. Creep modulus modulus under underdifferent different loads at at ambient ambient temperature. temperature.

Polymers 2018, 10, 156 8 of 13

Polymers 2018, 10, x FOR PEER REVIEW 8 of 13

800

700

600

500

400 at 70°C 300 at 60°C at 50°C

Creep Modulus (MPa) Modulus Creep 200 at 23°C

100

0 0 20406080100 Elapsed Time (hrs)

FigureFigure 6. Creep6. Creep modulus modulusunder under different temperatures temperatures at at14 14MPa. MPa.

3.3. Thermal Ratcheting 3.3. Thermal Ratcheting As mentioned earlier, the thermal ratcheting behavior is of utmost importance for polymeric Asmaterials, mentioned primarily earlier, due to the their thermal inherent ratcheting property of behavior low melting is of temperature. utmost importance It is fair to forsay polymericthat a materials,large selection primarily of polymer due to their material inherents can operate property only of in low a moderate melting temperature temperature. conditions, It is fair hence, to say a that a largesmall selection oscillation of polymer in temperature materials can cancause operate a notewo onlyrthy in change a moderate in the temperature physical dimensions conditions, of the hence, a smallstructure. oscillation Thermal in temperature cycling or thermal can causeratcheting a noteworthy induces a permanent change in cumula the physicaltive deformation dimensions in the of the structure.structure Thermal as a result cycling of cycling or thermal of temperature ratcheting under induces load. Similarly a permanent to mechanical cumulative ratcheting deformation or fatigue, in the structurethermal as aratcheting result of cyclinginduces ofcumulative temperature deformation under load.on the Similarly material, towhere mechanical the scientific ratcheting publications or fatigue, on this phenomenon is near to zilch. A proper understanding of thermal ratcheting phenomenon is thermal ratcheting induces cumulative deformation on the material, where the scientific publications important for HDPE bolted flange joint application, as a small loss in flange thickness would lead to a on this phenomenon is near to zilch. A proper understanding of thermal ratcheting phenomenon is significant reduction in the bolt load, thereby causing a leakage failure. The test bench, universal gasket importantrig, facilitates for HDPE the thermal bolted ratcheting flange joint analys application,is of high density as a small polyethylene loss in flange material. thickness would lead to a significantQuantitatively, reduction infive the different bolt load, combinations thereby causing of thermal a leakage ratcheting failure. tests The were test performed bench, universal to gasketdistinguish rig, facilitates the consequence the thermal of ratchetingcreep time, analysisapplied load, of high and densitytemperature polyethylene on the thermal material. ratcheting Quantitatively,strain of high density five differentpolyethylene. combinations Under all thermal of thermal ratcheting ratcheting tests, the tests cyclic were escalation performed and to distinguishreduction the of consequence thickness is apparent, of creep with time, a appliednet decrea load,se in andthickness. temperature The wavy on nature the thermal of the graphs ratcheting strain(Figures of high 7–9) density illustrates polyethylene. the change Underin thickness all thermal under each ratcheting thermal tests, cycle. the The cyclic rise and escalation fall of and reductionthickness of thickness corresponds is apparent,to the increase with and a netdecrease decrease of temperature in thickness. during The the wavy thermal nature cycling, of thewhich graphs is clearly visualized with thermal cycles and thickness change plots in Figure 7. The material (Figures7–9) illustrates the change in thickness under each thermal cycle. The rise and fall of thickness accumulates damage with each thermal cycle, causing an overall reduction in thickness. The corresponds to the increase and decrease of temperature during the thermal cycling, which is clearly significance of time period of creep and applied load on the thermal ratcheting strain is presented in visualizedFigures with 8 and thermal 9, respectively. cycles andFrom thickness the assessment change of plotsFigure in 8, Figurea substantial7. The swift material in thickness accumulates of damageHDPE with samples each thermaltested at the cycle, same causing thermal an ratcheting overall reductiontemperature, in but thickness. with different Thesignificance initial period ofof time periodcreep, of creep is noticeable. and applied The amplification load on the of thermal the amou ratchetingnt of deformation strain is or presented reduction inin Figuresthickness8 andis 9, respectively.partially Fromcited to the the assessment initial creep ofof Figurethe material,8, a substantial while the rest swift is the in thicknesscoupled interaction of HDPE of samples creep and tested at thethermal same thermalratcheting, ratcheting as the material temperature, simultaneously but with creeps different and cumulates initial deformation, period of creep, due to is thermal noticeable. The amplificationratcheting. On of physical the amount evaluation of deformation of the two-test or reductionspecimen mentioned in thickness in Figure is partially 8, the cited difference to the in initial creepthickness of the material, reduction while is evident the rest after is removal the coupled of the interaction applied load. of Both creep samples and thermal demonstrated ratcheting, radial as the flow with increase and decrease of external and internal diameter, respectively. With the addition of material simultaneously creeps and cumulates deformation, due to thermal ratcheting. On physical one day of creep, an increase of 2% in the overall reduction of thickness is observed under similar evaluation of the two-test specimen mentioned in Figure8, the difference in thickness reduction is ratcheting temperature. The impact of magnitude of applied compressive load on the thermal evident after removal of the applied load. Both samples demonstrated radial flow with increase and decrease of external and internal diameter, respectively. With the addition of one day of creep, an increase of 2% in the overall reduction of thickness is observed under similar ratcheting temperature. The impact of magnitude of applied compressive load on the thermal ratcheting deformation is Polymers 2018, 10, 156 9 of 13 demonstratedPolymers 2018 in, 10 Figure, x FOR PEER9. HDPE REVIEW samples tested under the same ratcheting temperatures,9 but of 13 with Polymers 2018, 10, x FOR PEER REVIEW 9 of 13 differentratcheting applied deformation stressesof is 7demonstr and 14ated MPa, in wereFigure evaluated 9. HDPE samples to understand tested under the the importance same ratcheting of applied load condition.ratchetingtemperatures, deformation The but specimen with isdifferent demonstr tested applied atated 14 in MPastresses Figure of of stress9. 7HDPE and lost 14 samples MPa, in excess were tested ofevaluated under 7-fold the of to same theunderstand thickness ratcheting the of the sampletemperatures,importance tested at of 7 butapplied MPa with after load different 20condition. thermal applied The cycles.stresses specimen Sinceof 7 test and theed 14 at two MPa, 14 MPa tests were of were evaluatedstress performed lost toin understandexcess without of 7-fold the creep, the reductionimportanceof the thickness in of thickness applied of the load issample the condition. coupled tested The at interaction 7specimen MPa after test of 20 instantaneoused thermal at 14 MPa cycles. of compressivestress Since lost the in excesstwo creep tests of due 7-fold were to load and thermalofperformed the thickness ratcheting without of only.thecreep, sample The the cumulative reductiontested at 7in deformationMPa thickness after 20is sustainedthethermal coupled cycles. during interaction Since the initialthe of two instantaneous cycles tests forwere HDPE materialperformedcompressive under 14without creep MPa due iscreep, higher to load the in andreduction comparison thermal in ratchethickness withting the only.is sample the The coupled under cumulative 7interaction MPa deformation of stress. of instantaneous This sustained distinctive behaviorcompressiveduring is consistentthe initial creep cycles withdue to thermalfor load HDPE and cycles material thermal after underratche 1 day ting14 ofMPa only. creep is Thehigher under cumulative in 14 comparison MPa, deformation where with the the sustained magnitude sample of during the initial cycles for HDPE material under 14 MPa is higher in comparison with the sample initialunder damage, 7 MPa especially of stress. This in the distinctive first four behavior cycles, is consistent great. A probablewith thermal reason cycles cited after for 1 day this of behavior creep is under 714 MPa MPa, of wherestress. theThis magnitude distinctive of behavior initial damage, is consistent especially with thermal in the first cycles four after cycles, 1 day is ofgreat. creep A a change in the co-efficient of thermal expansion. underprobable 14 reasonMPa, where cited forthe thismagnitude behavior of is initial a change damage, in the especially co-efficient in ofthe thermal first four expansion. cycles, is great. A probable reason cited for this behavior is a change in the co-efficient of thermal expansion. 5.6 75 5.6 75

65 5.5 65 5.5 55 5.4 55 5.4 45 45 5.3 Temperature(°C)

Thickness (mm) Thickness 5.3 35 Temperature(°C)

Thickness (mm) Thickness 35 5.2 25 5.2 at 14 MPa and 60°C atThermal 14 MPa Cycles and 60°C 25 Thermal Cycles 5.1 15 5.1 020406015 0204060Elapsed Time (hrs)

Elapsed Time (hrs)

FigureFigure 7. Thickness 7. Thickness variation variation of of highhigh density polyethylene polyethylene (HDPE) (HDPE) under under 14 MPa 14 MPaof stress of stressand a and a thermalFigurethermal ratcheting 7. ratcheting Thickness temperature temperature variation of rangerange high of density 28 28 to to 60 polyethylene 60 °C.◦C. (HDPE) under 14 MPa of stress and a thermal ratcheting temperature range of 28 to 60 °C. 5.80 65 5.80 65 60 5.70 60 5.70 55 55 5.60 50 5.60 50 45 5.50 45 5.50 40 40 5.40 35 Thickness (mm) Thickness 5.40 35 (°C) Temperature Thickness (mm) Thickness 30 (°C) Temperature 5.30 at 14MPa, TR 55°C & 1C 30 5.30 at 14MPa, TR 55°C & 1C0C 25 atThermal 14MPa, cycles TR 55°C & 0C 25 5.20 Thermal cycles 20 5.20 0 102030405020 0 1020304050Elapsed Time (hrs) Elapsed Time (hrs) Figure 8. Ratcheting of HDPE with and without 1 day creep, at 14 MPa of stress. FigureFigure 8. Ratcheting 8. Ratcheting of of HDPE HDPE with with andand with withoutout 1 1day day creep, creep, at at14 14MPa MPa of stress. of stress.

Polymers 2018, 10, 156 10 of 13 Polymers 2018, 10, x FOR PEER REVIEW 10 of 13

6.00 62

5.90 57

5.80 52 5.70 47 5.60 42 5.50 37 Thickness (mm) Thickness 5.40 (°C) Temperature 32 5.30 at 14MPa & TR 55°C 27 5.20 at 7MPa & TR 55°C Thermal cycles 5.10 22 0 1020304050 Elapsed Time (hrs)

FigureFigure 9. Ratcheting9. Ratcheting of of HDPE HDPE underunder 7 7 and and 14 14 MPa MPa of ofstress. stress.

3.4. Thermal Ratcheting Strain 3.4. Thermal Ratcheting Strain The thermal ratcheting strain (TRS) is the cumulative strain induced in the material as a Theconsequence thermal ratchetingof thermal strain ratcheting (TRS) isphenomenon. the cumulative The strain thermal induced ratcheting in the materialstrain increases as a consequence with of thermalincrease ratcheting in the number phenomenon. of cycles, which The isthermal observed ratchetingin Figures 10–12. strain The increases prominence with of increase creep time in the numberperiod of cycles, on thermal which ratcheting is observed strain in is Figuresseen in the 10– comparison12. The prominence between HDPE of creep samples time under period the on same thermal ratchetingcycling strain conditions, is seen but in at the different comparison initial betweencreep time HDPE period. samples From the under thermal the ratcheting same cycling strainconditions, graph but at(Figure different 10), initialit is noticeable creep time that the period. TRS is From higher the for thermalthe sample ratcheting tested without strain creep graph than (Figure the sample 10), it is noticeabletested thatwith the one TRS and isfour higher days of for creep. the sample This phenom testedenon without indicates creep the than effect the of creep sample on testedthe thermal with one ratcheting strain. Therefore, the creep time period is inversely proportional to the thermal ratcheting and four days of creep. This phenomenon indicates the effect of creep on the thermal ratcheting strain. strain. Furthermore, the difference in thermal ratcheting strain between 1 day of creep, and without Therefore, the creep time period is inversely proportional to the thermal ratcheting strain. Furthermore, creep, is quite small, while maintaining a similar trend of thermal ratcheting strain. The thermal the differenceratcheting in strain thermal increased ratcheting by 10 percent strain for between the test 1performed day of creep, without and 1 day without of creep, creep, in comparison is quite small, whileto maintaining the test conducted a similar with trend one day of thermal of creep. ratcheting Moreover, strain.the difference The thermal in TRS between ratcheting 4 days strain of creep increased by 10and percent zero fordays the of testcreep performed is significant, without where 1 daythe TRS of creep, reduced in comparisonby 36%, indicating to the that test the conducted effect of with one daythermal of creep. ratcheting Moreover, is severe the without difference initial in creep. TRS The between hardening 4 days of the of creepmaterial and under zero creep days is of cited creep is signiﬁcant,as the wherereason thefor TRSincreased reduced resistance by 36%, to indicating thermal ratcheting, that the effect and ofconsequently, thermal ratcheting reducing isthe severe withoutmagnitude initial creep.of TRS Thewith hardening increase of creep of the time material period. under The vulnerability creep is cited of HPDE as the to reason thermal for cycling increased resistanceis undoubtedly to thermal exposed, ratcheting, prompting and consequently, for further inve reducingstigation. theThe magnitude change in TRS of TRSas an with outcome increase of of difference in applied compressive load is revealed in Figure 11. The variation in TRS between 7 and creep time period. The vulnerability of HPDE to thermal cycling is undoubtedly exposed, prompting 14 MPa of stress is substantial, where the latter is higher than the former by approximately 1.9 times. for further investigation. The change in TRS as an outcome of difference in applied compressive load It is noteworthy to mention that the TRS curve is much smoother at lower stress than at higher stress, is revealedwhere after in Figure the 7th 11 thermal. The variationcycle, a change in TRS in slope between of the 7curve and is 14 evident. MPa of Both stress the thermal is substantial, ratcheting where the lattertests isat higherthe same than ratcheting the former temperature by approximately range and different 1.9 times. compressive It is noteworthy stress, are to executed mention after that the TRS curveone day is of much creep. smoother at lower stress than at higher stress, where after the 7th thermal cycle, a change in slope of the curve is evident. Both the thermal ratcheting tests at the same ratcheting Thermal Ratcheting Strain εTRSx = (LTR1 − LTRX)/Lo, (2) temperature range and different compressive stress, are executed after one day of creep. where, LTR1 refers to the thickness of the sample after the first thermal cycle, while LTRX represents the thickness of the sample with each thermal cycle, and Lo is the initial thickness of the specimen. Thermal Ratcheting Strain εTRSx = (LTR1 − LTRX)/Lo, (2) where, LTR1 refers to the thickness of the sample after the ﬁrst thermal cycle, while LTRX represents the thickness of the sample with each thermal cycle, and Lo is the initial thickness of the specimen.

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Polymers 2018, 10, x FOR PEER REVIEW 11 of 13 Polymers 2018, 10, x FOR PEER REVIEW 11 of 13 0.045 0.045 0.040 0.040 0.035 TR 0.035 TR 0.030 0.030 0.025 0.025 0.020 0.020 0.015 0.015 at 14MPa, TR 55°C & 4C Thermal ratcheting strain ε strain ratcheting Thermal 0.010 atat 14MPa,14MPa, TRTR 55°C55°C && 4C1C Thermal ratcheting strain ε strain ratcheting Thermal 0.010 atat 14MPa,14MPa, TRTR 55°C55°C && 1C0C 0.005 at 14MPa, TR 55°C & 0C 0.005 0.000 0.000 0 5 10 15 20 25 0 5 10No. of cylces 15 20 25

No. of cylces

FigureFigure 10. Thermal 10. Thermal ratcheting ratcheting strain strain under under differentdifferent ti timeme periods periods of initial of initial creep creep at 14 MPa. at 14 MPa. Figure 10. Thermal ratcheting strain under different time periods of initial creep at 14 MPa. 0.045 0.045 0.040 0.040 TR

TR 0.035 0.035 0.030 0.030 0.025 0.025 0.020 0.020 0.015 0.015 0.010 at 14MPa , TR 55°C & 1C 0.010 at 14MPa , TR 55°C & 1C Thermalratchetingstrain ε at 7MPa, TR 55°C & 1C

Thermalratchetingstrain ε 0.005 0.005 at 7MPa, TR 55°C & 1C 0.000 0.000 0 5 10 15 20 25 0 5 10 15 20 25 No. of cycles No. of cycles Figure 11. Thermal ratcheting strain at different applied loads. Figure 11. Thermal ratcheting strain at different applied loads. Figure 11. Thermal ratcheting strain at different applied loads. Finally, the importance of variation in ratcheting temperature range while maintaining a constantFinally, load the is demonstrated importance inof Figurevariation 12. Thein ratctrenhetingd of the temperature three thermal range ratcheting while strains maintaining is similar, a Finally,constantwith thea slender load importance is demonstratedvariation of in variation the in magnitude.Figure in 12. ratcheting The The tren testsd of temperature were the three performed thermal range at ratcheting three while different maintaining strains ratchetingis similar, a constant load iswithtemperature demonstrated a slender ranges variation in (28–40 Figure in °C, the 12 28–55 magnitude.. The °C, and trend The 28–60 oftests °C the ).were Notably, three performed thermal the thermal at three ratcheting ratcheting different strainsstrain ratcheting of the is similar, temperature ranges (28–40 °C, 28–55 °C, and 28–60 °C). Notably, the thermal ratcheting strain of the with a slenderHDPE tends variation to decrease in the with magnitude. an increase Thein ratc testsheting were temperature performed range. at During three differentthe initial ratchetingfew HDPEcycles, tendsthe difference to decrease in TRS with for an all increase the three in ratcheratchetingting temperaturetemperature rangesrange. isDuring minimal, the butinitial a clear few temperature ranges (28–40 ◦C, 28–55 ◦C, and 28–60 ◦C). Notably, the thermal ratcheting strain of the cycles,deviation the occurs difference after inthe TRS 7th forcycle all of the thermal three ratcheratcheting.ting temperatureNevertheless, ranges the thermal is minimal, ratcheting but a strain, clear HDPE tendsdeviationafter 10 to thermal decrease occurs aftercycles, with the was an7th increase reducedcycle of thermalonly in ratcheting as ratclittleheting. as temperature 5% Nevertheless, between 28–40 range. the °C thermal During and 28–55 ratcheting the °C initial TR strain, tests, few cycles, the differenceafterwhile 10 the inthermal difference TRS forcycles, allis 4.7% thewas between three reduced ratcheting 28–55 only °Cas andlittle temperature 28–60 as 5% °C between tests. ranges Unlike 28–40 is gasket minimal,°C and materials 28–55 but °C [22], a clearTR HDPE tests, deviation occurs afterwhilematerial the did difference 7th not cycle saturate is of4.7% thermalunder between 20 ther ratcheting. 28–55mal °C cycles and Nevertheless,in28–60 the tested°C tests. conditions. Unlike the thermalgasket materials ratcheting [22], HDPE strain, after 10 thermalmaterial cycles, did wasnot saturate reduced under only 20 as ther littlemal ascycles 5% in between the tested 28–40 conditions.◦C and 28–55 ◦C TR tests, while the ◦ ◦ difference is 4.7% between 28–55 C and 28–60 C tests. Unlike gasket materials [22], HDPE material did not saturate under 20 thermal cycles in the tested conditions. Polymers 2018, 10, 156 12 of 13

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0.045

0.040

TR 0.035

0.030

0.025

0.020

0.015 at 14MPa & TR 40°C 0.010 at 14MPa & TR 55°C Thermalratcheting strainε at 14MPa & TR 60°C 0.005

0.000 0 5 10 15 20 25 No. of cycles

FigureFigure 12. Thermal 12. Thermal ratcheting ratcheting strain strain under under diffe differentrent ratcheting ratcheting temperature temperature ranges. ranges. 4. Conclusions 4. Conclusions The compressive creep and thermal ratcheting characterization of high density polyethylene The(HDPE) compressive material is creep successfully and thermal studied ratchetingthrough universal characterization gasket rig. The of highlights high density of mechanical polyethylene (HDPE)characterization material is successfully tests are summarized: studied through universal gasket rig. The highlights of mechanical characterization• The HPDE tests material are summarized: exhibits substantial creep damage under different compressive and thermal load conditions. The specimen shows a growth of 28 times the creep strain at 21 MPa from 7 • The HPDEMPa materialof compressive exhibits stress, substantial while the increase creep damagein creep strain under from different the lowest compressive tested temperature and thermal load conditions.to highest tested The specimen temperature shows is 7-fold. a growth of 28 times the creep strain at 21 MPa from 7 MPa of compressive• The creep stress,strain is whiledirectly the proportional increase to in th creepe applied strain load from and applied the lowest temperature, tested temperatureexposing to highestthe tested vulnerability temperature of the material. is 7-fold. • Creep modulus is dependent on the applied stress. The magnitude of creep modulus decreases • The creepwith strain increase is in directly applied proportionalcompressive load to and the ma appliedterial temperature. load and applied The maximum temperature, loss of creep exposing the vulnerabilitymodulus occurred of the at material. the highest applied stress and temperature, respectively. • Creep• modulusThe impact is of dependent thermal ratcheting on the appliedis evident, stress. where The the magnitudeextent of cumulative of creep deformation modulus decreases is with increasedominated in applied by the compressive compressive load, load followed and materialby material temperature. temperature. TheThe thermal maximum ratcheting loss of creep modulusis very occurred similar at tothe the highestmechanical applied ratcheting stress or fatigue, and temperature, causing an accumulation respectively. of deformation with each thermal cycle. • The• impactIn addition, of thermal all HPDE ratcheting specimens is demonstrate evident,where thinning the of extentstructural of thickness cumulative under deformation thermal is dominatedratcheting, by the and compressive none of the specimens load, followed show any by materialsign of saturation temperature. of deformation The thermal under the ratcheting 20 is very similartested tothermal the mechanical cycles. ratcheting or fatigue, causing an accumulation of deformation with each• thermalThe thermal cycle. ratcheting strain of HDPE material is influenced by applied load, temperature, time period of creep, and number of thermal cycles. TRS is higher for the tests without creep, • In addition, all HPDE specimens demonstrate thinning of structural thickness under thermal suggesting the deformation due to thermal ratcheting is critical during the initial period of the ratcheting,operation. and none of the specimens show any sign of saturation of deformation under the 20 tested thermal cycles. Finally, the coupled behavior of compressive creep and thermal ratcheting phenomenon of • TheHDPE thermal material ratcheting raises the strain question of HDPEon considering material a common is inﬂuenced design criterion by applied for polymeric load, temperature, materials, time periodespecially of creep, in bolted and numberflange joint of thermalapplication. cycles. The results TRS is clearly higher indicate for the the tests necessity without for creep, upgraded suggesting thedesign deformation standards due with to inclusion thermal of ratcheting thermal ratcheting is critical effect. during the initial period of the operation.

Finally, the coupled behavior of compressive creep and thermal ratcheting phenomenon of HDPE material raises the question on considering a common design criterion for polymeric materials, especially in bolted ﬂange joint application. The results clearly indicate the necessity for upgraded design standards with inclusion of thermal ratcheting effect.

Author Contributions: Rahul Palaniappan Kanthabhabha Jeya has performed the experimental tests, analyzed the data, wrote the paper under the supervision of Abdel-Hakim Bouzid. Conﬂicts of Interest: The authors declare no conﬂict of interest. Polymers 2018, 10, 156 13 of 13

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