Appel 43.1 - Micro

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YIELD STRESS IN MPA VERSUS THE TESTING TEMPERATURE, TESTED WITH THE PURE ENDLOADING METHOD.This linear relation can be applied for single deformation behavior, typicallyintra-lamellar. It expresses the relation between the yield stressof a polymer and the logarithm of the applied strain rate. However, compositesoften exhibit multiple deformation behaviors; an α -phase inthe plies, intra-lamellar, and a β-phase on the interfaces between theplies, the inter-lamellar. At high temperatures or low strain rates, intralamellardeformation is the dominant type to determine the yield stress,whereas at lower temperatures or higher strain rates, inter-lamellar deformationis dominantly present. Since these deformations act in parallel,they can be described according to the principle of the stress additive,resulting in the modified Ree-Eyring equation. For higher stresses,this can be written as follows:FAILURE OF SAMPLES, LEFT -20 C AND RIGHT 110 C, BOTH TESTED WITH PURE ENDLOADING.38 DE APPEL
MICROSCOPY IMAGE OF A TESTED SAMPLE, TESTED AT ROOM TEMPERATURE.This equation describes the contribution of both the strain rate (I) aswell as the temperature (II) dependence of the yield stress. When thistrend is modelled, a line with a characteristic sharp bend can be recognized,as seen in the figure. This bend separates the α-phase is dominantand where both the α-phase and β-phase contribute. At a constanttemperature and increasing strain rate, the true yield stress will increase,whereas with a constant strain rate and increasing temperature,the true yield stress will decrease. This is typical for a viscoelastic material,such as polypropylene, and research shows the same behavior forglass-polypropylene (GPP) composites in tension.To validate whether this same behavior can be regarded in a compressiveload state, multiple tests have been performed. As testing method,a method based on pure loading and comparable with ASTM D695 isused. To this extent, small, cubic samples with a 3.77 mm thickness willbe used. Crucial in the production phase of these samples is the planparallelism,to obtain the required accuracy during testing. A deviationof on average 21 micron was established.With this method, multiple tests were performed, with sample setsof five. The temperature was varied between -20 deg. C and 110 deg. C,and the strain rate between 10^-5 and 10^-2 /s. This resulted in figurescomparable to the ones which were expected based on the modifiedRee-Eyring equation. An interesting observation during the test wasthe fact that with increasing strain rates or decreasing temperatures,the deviation between the multiple samples in a single set increased.The temperature effect can be related to the fact that with increasingtemperature, the ductility of a material increases. Consequently, at lowtemperatures the material tends to behave more brittle, increasing itssensitivity for for example stress concentrations. Stress concentrationscan be an initiator for sample failure, which could explain the increasedstandard deviation. Furthermore, the samples tested at elevated temperaturesexhibited little exterior damage after failure, whereas samplestested at lower temperatures often split or had excessive damage.For the tests with different constant strain speeds, a similar trend canbe recognized, as mentioned above. At higher testing rates, the deviationamong the samples tends to increase. Similarly, the damage to thesample after failure is much more excessive compared to the samplestested at lower strain rates. This can be related to the fact that at elevatedstrain rates, the sensitivity for local defects and stress concentrationsin the material is increased.When a close look is taken at a sample after failure, interesting observationscan be done. In the figure, a sample tested at room temperatureat a strain rate of 10^-3 can be seen. Approximately ¾ of the sample hasbeen removed, after it was embedded in an epoxy. Clearly, the fibers canbe recognized in the picture. The elliptic shapes which can be regardedare fibers which have bend in the z-direction. The load during testingwas exerted in the vertical direction on the sample. From the figure, itcan be regarded that most of the fibers are still intact but have changedlocation. This means they could move freely through the matrix duringdeformation, which would be another indication that the matrix is dominantduring compression of the sample, resulting in a viscoelasticnature of the composite, based on the polymer properties.Based on the observations done it can be stated that under a compressiveload, the GPP polymer tends to exhibit a viscoelastic behavior. Inpractice this means that there is a positive relation between the appliedstrain rate and the yield stress, and a negative relation between thetemperature and the yield stress. The addition of fibers, in this caseglass fibers, does increase the compressive properties of the composite,however, the polymer properties remain dominant in the failure mode. nDE APPEL 39
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