Tensile testing, stress-strain curve from

O3, h). .. etc. from this family of curves, which form the relaxation curve. Furthermore, by determining the stress which belongs to a specified strain in the relaxation test after a long test time, one can experimentally verify the only calculated tensile test stress-strain-curves for very small deformation rates. The isochronous stress-strain diagram also directly indicates the non-linearity of the visco-elastic behaviour otherwise the curves would be straight lines. 

Figure 7.3 Stress-strain curves from tensile testing experiments performed on electrospun nanofiber scaffolds made by blends of collagen/elastin and various synthetic polymers. Reproduced with permission from Ref. 80, J. Biomed. Mater. Res. A, 2007, 83, 999-1008. Doi 10.1002/jbm.a.31287. Copyright 2007, Wiley Periodicals, Inc.

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. 

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... 

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). 

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...

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 ... 

Storage modulus measurements. All measurements were taken at temperatures near 45 °C above the network Tg s. Representative network true stress versus strain curves from the tensile experiments are shown in Fig. 2. The ordinate axis, true stress, is normalized by 3eRT to account for the different test temperatures employed. The resultant curves are thereby directly comparable for structural differences, since the instantaneous slopes are proportional to l/M, after Eq. (2). The curves of all five networks are linear and reversible up to strains of around 10 percent. The reversibility suggests that the measurements were performed under near-equilibrium conditions and that the networks were stable at the high test temperatures employed. 

As mentioned earlier, the Gc value required to define the CZ model is obtained from TDCB tests. The remaining parameter Gm is chosen as the UTS, and was extracted from the stress-strain curves at the corresponding rates. This was an arbitrary choice, since the level of the constraint near the crack tip is higher than that in uniaxial tensile tests used to obtain the stress-strain curves. Therefore, a sensitivity study on this parameter was performed. For illustration purposes, a numerical analysis carried out on TDCB test specimens bonded with the two adhesives under investigation is shown in this section. The value of a was varied from 20 to 80 MPa and numerical predictions of load versus time were compared against the experimental results. Fig. 5 shows a comparison of the FV and experimental results for different values for TDCB tests performed at 0.1 mm/min. The best fit G value should be able to predict correctly both the experimental force and crack history. (Note that the latter was found to be less sensitive to changes of the cohesive strength.)... 

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