Stress-strain curves tensile testing

Fig. 11-20. Tensile stress-strain curve and test specimen appearance for a polymer which yields and cold draws.

For accuracy, modulus values should not be determined from the results of one stress-strain curve. Several tests should be made on the material and average tensile modulus should be calculated. 

J7 In a tensile test on a plastic, the material is subjected to a constant strain rate of 10 s. If this material may have its behaviour modelled by a Maxwell element with the elastic component f = 20 GN/m and the viscous element t) = 1000 GNs/m, then derive an expression for the stress in the material at any instant. Plot the stress-strain curve which would be predicted by this equation for strains up to 0.1% and calculate the initial tangent modulus and 0.1% secant modulus from this graph. 

The mechanical properties were obtained using a tensile machine at room temperature and for a strain rate of 1000%/h. Each reported value of the modulus was an average of five tests. The tensile modulus Et was taken as the slope of the initial straight line portion of the stress-strain curve. 

Real differences between the tensile and the compressive yield stresses of a material may cause the stress distribution within the test specimen to become very asymmetric at high strain levels. This cause the neutral axis to move from the center of the specimen toward the surface which is in compression. This effect, along with specimen anisotropy due to processing, may cause the shape of the stress-strain curve obtained in flexure to dif-... 

Even plastics with fairly linear stress-strain curves to failure, for example short-fiber reinforced TSs (RPs), usually display moduli of rupture values that are higher than the tensile strength obtained in uniaxial tests wood behaves much the same. Qualitatively, this can be explained from statistically considering flaws and fractures and the fracture energy available in flexural samples under a constant rate of deflection as compared to tensile samples under the same load conditions. These differences become less as the... 

It should be recognized that tensile properties would most likely vary with a change of speed of the pulling jaws and with variation in the atmospheric conditions. Figure 2-14 shows the variation in a stress-strain curve when the speed of testing is altered also shown are the effects of temperature changes on the stress-strain curves. When the speed of pulling force is increased, the material reacts like brittle material when the temperature is increased, the material reacts like ductile material. 

The important tensile modulus (modulus of elasticity) is another property derived from the stress-strain curve. The speed of testing, unless otherwise indicated is 0.2 in./min, with the exception of molded or laminated TS materials in which the speed is 0.05 in./min. The tensile modulus is the ratio of stress to corresponding strain below the proportional limit of a material and is expressed in psi (pounds per square inch) or MPa (mega-Pascal) (Fig. 2-7). 

The test can provide compressive stress, compressive yield, and modulus. Many plastics do not show a true compressive modulus of elasticity. When loaded in compression, they display a deformation, but show almost no elastic portion on a stress-strain curve those types of materials should be compressed with light loads. The data are derived in the same manner as in the tensile test. Compression test specimen usually requires careful edge loading of the test specimens otherwise the edges tend to flour/spread out resulting in inacturate test result readings (2-19). 

Fig. 6.2. Yield strengths from tensile tests at 23 °C are plotted against the glass transition temperatures (T,max) of the five polymers [] result of extrapolated stress-strain-curve...

Fig. 11.16 Representative stress-strain curves of HEMA-MPS nano-hybrid, TCP-HEMA-MPS and CPP-HEMA-MPS before and after soaking in SBF for 1 day. Tensile test, the others compressive test.

The maximum in the curve denotes the stress at yield av and the elongation at yield v. The end of the curve denotes the failure of the material, which is characterized by the tensile strength a and the ultimate strain or elon gation to break. These values are determined from a stress-strain curve while the actual experimental values are generally reported as load-deformation curves. Thus (he experimental curves require a transformation of scales to obtain the desired stress-strain curves. This is accomplished by the following definitions. For tensile tests ... 

Typical tension stress-strain curves of baseline and irradiated unidirectional T300/934 composites tested in [0] and [90] orientations at three different temperatures (121 are shown in Figures 11 and 12. Irradiation had essentially no effect on the fiber-dominated tensile modulus of the [0] specimen and caused only a small (10-15%) reduction in strength at the low and elevated temperatures. For the matrix-dominated [90] laminates, irradiation caused a very substantial decrease in strength at three test temperatures (-38% at -157°C, -26% R.T., -13% 121°C). Irradiation increased the modulus at -157°C and R.T. (10 - 15%), but lowered it at 121°C (-15%). These results are consistent with results obtained on the neat resin specimens discussed above. 

In measurement of tensile stress-strain properties, a test piece is stretched to breaking point and the force and elongation are measured at different stages. Tensile strength, elongation at break or work to failure (the area under the stress-strain curve) provide... 

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