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Development of a holistic approach for force and strain measurements in high-speed tensile testsLars Gerdes1, Steffen Dittmann1, Mustafa Awd1, Olesia Khafizova2, Frank Walther1

1 Department of Materials Test Engineering (WPT), TU Dortmund University, Dortmund 2 European Innovation Center, Shimadzu Europa GmbH, Duisburg

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No. SCA-300-070

Comparison and dependency of HITS-T10 system integrated and additional techniques for measuring test forces and stresses, displacements and strains.

Innovative measurement with HPV-X2 camera and DIC-analysis, with which true test stresses and strains can be calculated.

IntroductionCrash loads with highstrain rates can often befound in automotive andaerospace industries, sothat a material charac-terization on dynamicbasis is required to guar-antee safety propertiesof highly loaded compo-nents. Most exteriorparts nowadays aremade of fiber-reinforcedpolymers (FRP), whichshow strain rate depend-ent properties [1-3]. Forthis purpose, the deter-mination of characteris-tic parameters in high-speed tensile tests is acommon method. SinceFRP show an in-creasing ultimate tensilestrength (σUTS) with………higher strain rates, a high potential regardingmaterial savings and lightweight design is enabled.Consequently, a comprehensive understanding ofthe data, which is measured in high-speed tensiletests, is necessary. Typically, the test force isprovided by a piezo-electric load cell, whereas thepiston movement is recorded by a capacitivesensor. This is also how it is realized with the high-speed impact testing machine HITS-TX, Figure 1.Furthermore, HITS-TX provides an additional gripdisplacement sensor (GDS) for measuring thespecimen movement just inside the grips andtherefore excluding influences of additional parts inthe load train. This is in accordance with existingknowledge about oscillations and stress waves inload train, influencing recorded force and displace-ment signals. [4,5]A general challenge of high-speed material testing…

Figure 1: HITS-TX system

is the impact occurring between the approach jigand actuator piston at test beginning. An approachdevice is usually used in high-speed systems inorder to accelerate the piston to the desired testspeed, before the specimen is loaded. As a resultof the impact inside this device, oscillations, whichare travelling as elastic stress waves, aretransmitted to the load train. These waves arereflected in the load cell at the bottom of the testsystem and, as a consequence, influencing forceand displacement measurements. In order to avoidor at least to reduce these oscillations, it isessential to install a mechanism for damping theforce application. For this purpose, differentmaterials or geometries at the contact surfaces canbe used, as well as installing damping elements.However, the effect of damping should no longerhave an influence on the measurement at a load of25% of the yield strength [5]. Materials and methodsFor investigations, a composite material made ofglass fiber-reinforced epoxy (GFR-EP) is used, asa good representation of the material groupmentioned above. The material is reinforced withbidirectional glass fibers in 0/90°orientations.Based on several studies, a positive strain ratedependency is expected [1-3]. The specimengeometry, Figure 2, is a further development of thespecimen geometry and was specially adapted tothe measurement with a high-speed video camera.In detail, the specimen provides an additionaldynamometer section to enable force calculationwith the help of digital image correlation (DIC).

Figure 2: Specimen geometry

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Figure 3: Test setup with HPV-X2 camera

The test setup (Figure 3) consists of a HITS-TXsystem with a piston stroke sensor, connected to a4870 type controller, along with an additional gripdisplacement sensor (GDS), as well as a hypervision high-speed video camera (HPV-X2, Figure4). In addition, the HITS-TX system providesvarious approach jigs in order to optimally adaptthe geometry and damping effects to the selectedtest speed. On the one hand, there is a taperedversion with two lengths available. On the otherhand, a flat version of the approach jig can beused. While the flat version is equipped with ashock-absorbing rubber, the tapered versions areusing the contact surface to absorb test forces, aswell as a combination of two rubber rings toprevent multiple collisions. In summary, the flatversion should be used up to 8 m/s, while thetapered version is then used up to 20 m/s(72 km/h). In this work, high-speed tensile tests at5, 6.5, 8 and 12 m/s in combination with the flatand tapered version of the approach jig are carriedout. According to [6], the speeds are corres-ponding to nominal strain rates of 920, 1,200,1,480, and 2,220 s-1. In order to achieve highestrecording rates with the HPV-X2 camera, anexposure time as short as possible is needed.Consequently, three light sources are installed,one high-power LED with 15,000 lumen just belowthe lens and two HMI-lights with each 400 Wplaced on either side. Another challenge thatcomes with high-speed imaging, is the triggering ofthe camera [7]. For this purpose, the 4870 typecontroller of the HITS-TX system provides a TTL5 V signal, while timing is controlled via pistonstroke. With the help of a synchronization signalfrom HPV-X2 camera to HITS-TX system, repro-ducible results could be achieved.

Since the HPV-X2 camera has a limited ringbuffer, the number of images recorded at fullresolution is limited to 128. However, it is possibleto set the number of images to 256 when using aspecial interpolation, which leads to a smalleroverall resolution. Therefore, it is mandatory toknow the exact test time and to set the recordingrate accordingly. In this case, the test timesobserved are between 100 and 200 μs, dependingon the test speed. Test times of 200 μs arecorresponding to a maximum theoretical recordingrate of 640,000 frames per second (fps), providedthat the trigger point in time is set perfectly. Tocompensate for deviations in timing, the actualrecording rate is set to 250,000 fps.…

Figure 4: High-speed video camera HPV-X2

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Results and discussionIn order to determine material properties in high-speed tests in general, different strainmeasurement techniques, such as strain gages,optical extensometers, laser systems or high-speed cameras, can be used [6]. By additionalDIC-analysis of high-speed images the visuali-zation of strain distribution on the entire specimenbecomes available. The same applies to forcemeasurements, which can either be done bysystem-integrated load cells or inertia-freetechniques.

a. Force measurementIn order to assess the quality of the forcemeasurement with a load cell, a comparison toforces calculated by DIC-analysis and HPV-X2camera is done. A precise measurement isbecoming available, when designing a dynamo-meter section for the specimen. Therefore, aslightly asymmetrical specimen is used for theseinvestigations (Figure 5). This geometry allows astrain measurement in the gage section and aforce measurement in the dynamometer sectionwith DIC-analysis at the same time.Normally, the dynamometer section is an area,where no plastic deformation occurs. Transferredfor FRP’s a section is meant, where the deter-mination of Young’s modulus is allowed. Since thedetermination of Young’s modulus is allowedwithin the limits of 0.05 to 0.25% [8], a varyingmodulus versus the total strain is implemented,which is suitable for the visco-elastic materialbehavior of GFR-EP. Thus, the force, which isactually appearing on the specimen, can bedetermined by σ = Eꞏε (Hooke’s Law) F = EꞏεꞏA.However, the Young’s modulus has to bedetermined on a dynamic basis, because quasi-static material properties cannot be transferred tohigh-speed testing. Thus, the only possibility todetermine a correct Young’s modulus is then acalculation within the short period of time inbetween of the impact at the approach jig and theelastic stress waves travelling until reaching theload cell at the bottom of the test system.Furthermore, it is important to average themeasured strain in the dynamometer section overthe same specimen width, which the cross sectionA is related to.Figure 5 shows the comparison of test forcesmeasured by load cell versus DIC, when using atapered approach jig and a test speed of 8 m/s.The varying modulus mentioned above is relatedto the course of the mean strain (blue box) of 0 to0.8% throughout the entire test.

It can be observed that the two curves do not differgreatly, which leads to assuming that using theload cell without any additional measurementtechnique is efficient as well. With higher testforces, there are deviations, caused by oscillatingforces and reflected stress waves in the load train.At the beginning of the test, stress waves areinduced into the load train, which leads tooscillations in the force signal, as seen between 25and 75 μs. In the course of the test, these stresswaves travel through the load train and load cell,where they are reflected. Depending on theirphase shift and amplitude the waves can canceleach other out or amplify, which in turn leads tofluctuating deviations in the force signal, as seenbetween 75 and 125 μs. This shows that forcemeasurement by DIC-analysis is often essential toobtain reliable results. So the propagation andinterferences cannot be predicted and hence theuse of a correction factor is not effective. However,the force signal is less influenced by the stresswaves than expected, so it is possible to deter-mine correct material properties with the help ofthe integrated load cell. The first explanation forthis behavior could be the use of the taperedapproaching jig, as it does not contain a rubberring. Therefore, no influences on the materialproperties are expected, especially at testbeginning. However, as the test progresses, theinfluences of the stress waves become greater asthey are being reflected at the load cell.

b. Displacement and strain measurementWith the presented test setup, there are severaltechniques for strain measurement available:Piston stroke, GDS, and DIC. While the pistonstroke is measuring the actual piston movement atthe top of the load train, the GDS is just measuringthe displacement between the grips. With DIC-analysis, on the other hand, displacements andstrains can be recorded directly at the specimen.

Figure 5: Test forces measured by load cell and DIC

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Thus, only with DIC-analysis it is possible todetermine true strains. Differences between thesemethods are expected, which could be due to thefollowing influences:• Components with different rigidities involved in

the load train• Elastic deformation of components in the load

train• Dampening effects by compression and force-

absorbing properties of the rubber• Measuring distance to the specimen.As can be seen in Figure 6, the effects mentionedabove could lead to significant deviations in theresulting strain curves. As expected, the pistonstroke sensor is quite linear and differs greatlyfrom the other curves, since most components andtheir deformation are included in this measure-ment. Furthermore, the impact and multiplecollisions with the approach jig could be recordedby piston stroke. The deviations between GDS andDIC is small at low strains, but the increases asthe test time progresses. But, as can be seen inthe enlarged section, the slopes of the appliedtangents within the given limits are equaling, thusthe GDS is working well within small strains,where effects like specimen bending, clampingissues or clearance come into account.In order to visualize the strain distribution at thespecimen, Figure 7 shows the total strain atdifferent test times. It is obvious, that the specimengeometry is working well, since the straindistribution in the gage section is homogenous.However, an inhomogeneous strain distributioncan be seen in the dynamometer section, which isdue to the bidirectional fiber orientation. Since thespecimen is made of continuous fibers, the load in0°-layers in the middle of the specimen is beingtransferred from gauge to dynamometer section,…

whereas the outer fibers are subjected to a smallerforce. From these findings it can be concluded thatevery local strain measurement, like strain gages,will not work properly for measuring force in thedynamometer section, since an inhomogeneousstrain distribution can be found there.

Figure 7: Specimen strain distribution at diff. test times

Conclusions and outlookWithin this work at high-speed impact testingmachine HITS-TX, a comparison of systemintegrated and additional techniques for measuringtest forces and displacements was conducted.Especially these two parameters were defined ascritical, since they can be influenced byoscillations, which are consequences of the impactinside the approach device. An innovativemeasurement with the help of HPV-X2 cameraand DIC-analysis is presented, with which true testforces and total strains can be calculated.It could be shown that the force signal of the loadcell shows only slight deviations from DIC-analysis, so that generating material properties ismade possible especially with low strains, as in…..

Figure 6: Displacements measured by piston stroke, GDS and DIC

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SCA-300-070

modulus determination. This procedure can beused for applications like creating material data forsimulation purposes, especially if componenttesting in high-speed or crash load regime is cost-intensive.Regarding displacement and strain measure-ments, a clear dependency on the use of theappropriate measuring technique was pointed out.Where GDS can be used at low strains, it is

essential to use DIC-analysis when determiningtotal strains at the specimen failure. With the helpof DIC, a local strain measurement and a straindistribution on the entire specimen could also berealized. Due to the triggering and synchronizationby connecting HPV-X2 and HITS-TX, a sufficientnumber of images for strain evaluation is alwaysensured as well as a comfortable data analysis.

References

[1] K. Naresh, K Shankar, B.S. Rao, R. Velmurugan: Effect of high strain rate on glass/carbon/hybridfiber reinforced epoxy laminated composites. Composites Part B 100 (2016) 125-135.doi: org/10.1016/j.compositesb.2016.06.007

[2] Y. Ou, D. Zhu, L. Huang, Y. Yao: Mechanical characterization of the tensile properties of glassfiber and its reinforced polymer (GFRP) composite under varying strain rates and temperatures.Polymers 8 (5) (2016) 1-16.doi: 10.3390/polym8050196

[3] J. M. L. Reis, F. L. Chaves, H. S. da Costa Mattos: Tensile behaviour of glass fibre reinforcedpolyurethane at different strain rates. Materials and Design 49 (2013) 192-196.doi: 10.1016/j.matdes.2013.01.065

[4] SAE J2749: High strain rate tensile testing of polymers.[5] M. Keuerleber: Determination of Young’s modulus of plastics at high strain rates by the example

of PP (in German), PhD work, Stuttgart (2006).[6] DIN EN ISO 26203-2: Metallic materials - Tensile testing at high strain rates - Part 2: Servo-

hydraulic and other test systems (in German).[7] Myslicki, S.; Ortlieb, M.; Frieling, G.; Walther, F.: High-precision deformation and damage

development assessment of composite materials by high-speed camera, high-frequency impulseand digital image correlation techniques. Materials Testing 57 (11-12) (2015) 933-941.doi: org/10.3139/120.110813

[8] DIN EN ISO 527-1: Plastics - Determination of tensile properties - Part 1: General principles (inGerman).