Uniaxial tension response

Uniaxial tension testing with superposed hydrostatic pressure has been described by Vernon (111) and Surland et al. (103). Such tests provide response and failure measurements in the triaxial compression or tension-compression-compression octants. 

Generally, when testing materials with a nonlinear stress-strain behavior, the tests should be conducted under uniform stress fields, such that the associated damage evolution is also uniform over the gauge section where the material s response is measured. Because the stress field varies with distance from the neutral axis in bending tests, uniaxial tension or compression tests are preferred when characterizing the strength and failure behavior of fiber-reinforced composites. 

Since spallation is controlled by the response to tensile stress pulses, the measurements of yield behavior were performed in uniaxial tension rather than in shear, and a tensile yield stress criterion was required. Bouwens-Crowet et ah (6) rearranged Equation 1 to give an expression for the uniaxial-tension yield stress [Pg.201]

The Poisson ratio, like the bulk, tensile, and shear creep compliance, is an increasing function of time because the lateral contraction cannot develop instantaneously in uniaxial tension but takes an infinite time to reach its ultimate value. In response to a sinusoidal uniaxial stretch, the complete Poison ratio is obtained ... 

Fig. 1. Response of the bulk material for uniaxial tension at room temperature under adiabatic conditions.

The volumetric properties (Chapter 3) and the mechanical properties are interrelated since the mechanical properties are defined in terms of the response of the volume and the shape of a specimen to an applied mechanical deformation. Each type of modulus is defined in terms of the stress a required to deform a specimen by a strain of e, in the limit of an infinitesimally small deformation of the type quantified by that modulus. For example, Young s modulus is defined by Equation 11.1, in the limit of —>0 under uniaxial tension. This equation shows that the stress cr required to achieve a small strain of under uniaxial tension is proportional to E. 

The Poisson s ratio can be determined by measuring the transverse strain during uniaxial tension or compression experiments. Due to the small magnitude of the transverse strain, it is difficult to accurately determine the Poisson s ratio. Instead, it is often sufficient to assume a value for the Poisson s ratio of about 0.4. Unless the fluoropolymer component is highly confined, the Poisson s ratio has only very weak influence on the predicted material response. 

Figure 9.28 shows the stress train eurves of a whole eomplement of deformation modes with all flow stresses normalized with tq, giving the dependenees of e/ro, the normalized global equivalent deviatorie shear resistances, on Se, the global equivalent plastic strain. The predieted stress strain eurve for plane-strain eompression agrees well with the data points of the Gal ski et al. experiments. We note that the predicted response for uniaxial tension is also elose to the predietion for plane-strain compression and that these two, as examples of irrotational flow, differ markedly from the simple shear results and also from the experimental results and the predictions for uniaxial compression, in comparison with the experimental results of Bartczak et al. (1992b). 

The generic fracture response of polymers in uniaxial tension... 

Many semi-crystalline polymers have remarkable toughness in uniaxial tension at room temperature but show brittleness at low temperatures, under high strain rates and in notched impact loading. Since toughening of HDPE and of Nylon-6 and -66 is of primary interest, their baseline response is considered first. 

The predicted results for the uniaxial tension, and the tension-compression data are shown in Figures 14.17 and 14.18, respectively. It is clear that the HM accurately captures both the large strain time-dependent response and the intermediate strain cyclic response. 

If purely elastic behaviour is considered, elastic response of the adhesive may be obtained by attaching extensometers to the adherend either side of the bondline, making a suitable correction for adherend strain. Beyond the elastic limit, yield of the adhesive may be suppressed by the triaxial stress state, occurring at a stress higher than that in uniaxial tension. On the other hand, brittle adhesives which fracture before they yield will probably indicate a low failing stress because of stress concentrations. 

Fig. 5 Stress-strain response of the MWCNT/NR composites under uniaxial tension (a) MWNT1/ NR composite. The inset figure shows a magnified view of NR stress-strain curve [75], and (b) comparison of stress-strain curves of NR, NR/CNT and NR/CB composites [94]...

The behavior of filled elastomers can be primarily described as hyperelastic under static or quasi-static loading dissipative effects are negligible. There have been numerous experimental studies addressing the response of rubber under quasi-static loading conditions, including uniaxial tension/compression, shear, equibiaxial tension [53-56]. 

Other less well-known types of nonlinearities include interaction and intermode . In the former, stress-strain response for a fundamental load component (e.g. shear) in a multi-axial stress state is not equivalent to the stress-strain response in simple one component load test (e.g. simple shear). For example. Fig. 10.3 shows that the stress-strain curve under pure shear loading of a composite specimen varies considerably from the shear stress-strain curve obtained from an off-axis specimen. In this type of test, a unidirectional laminate is tested in uniaxial tension where the fiber axis runs 15° to the tensile loading axis. A 90° strain gage rosette is applied to the specimen oriented to the fiber direction and normal to the fiber direction and thus obtain the strain components in the fiber coordinate system. Using simple coordinate transformations, the shear response of the unidirectional composite can be found (Daniel, 1993, Hyer, 1998). At small strains in the linear range, the shear response from the two tests coincide. 

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