Uniaxial tensile loading, yield stresses

For most practical purposes, the onset of plastic deformation constitutes failure. In an axially loaded part, the yield point is known from testing (see Tables 2-15 through 2-18), and failure prediction is no problem. However, it is often necessary to use uniaxial tensile data to predict yielding due to a multidimensional state of stress. Many failure theories have been developed for this purpose. For elastoplastic materials (steel, aluminum, brass, etc.), the maximum distortion energy theory or von Mises theory is in general application. With this theory the components of stress are combined into a single effective stress, denoted as uniaxial yielding. Tlie ratio of the measure yield stress to the effective stress is known as the factor of safety. 

The reduction in the tensile load capacity of the drill pipe is 311,400 -260,500 = 50,900 lb. That is about 17% of the tensile drill pipe resistance calculated at the minimum yield strength in uniaxial state of stress. For practical purposes, depending upon drilling conditions, a reasonable value of safety factor should be applied. 

Uniaxial tensile tests were carried out to determine the stress-strain curves and document the damage growth on a computer-controlled Instron model 8516 servo hydraulic testing machine operating at a strain rate of 5% min . The macroscopic tensile yield stress was considered equal to the maximum stress on the loading curve. The Young s modulus was determined as the plateau value of a plot of the secant modulus as a function of the strain. 

Where a, is the uniaxial tensile yield stress and for polymer-based materials this is usually taken as the maximum load if a distinet yield point is not exhibited. However shear yielding in tensile tests with most polymers can be achieved by carefully polishing the specimen edges in order to remove surface blemishes and thus avoid premature failure. If yielding does not oecur and brittle failure is obtained, the stress at failure should be used in the criteria which gives a conservative size value. Alternatively 0.7 times the compressive yield stress may be used. The loading time to yield (or equivalent) should be within 20% of the loading duration in the fracture test. 

While the true stress-true strain response is qualitatively similar in both compression and tension, the resulting deformation states are very different. Tensile loading leads to uniaxial molecular orientation along the loading axis. Compression on the other hand results in a biaxial orientation state in a plane perpendicular to the loading direction and so it is expected that quantitatively different stress-strain curves are seen. In addition, as discussed below, the hydrostatic pressure difference between tension and compression leads to differences in yield strength because yield in polymers is pressure dependent. 

If the linearity criteria is violated, a possible option is to increase W for the same value of B. Values of WIB up to 4 are permitted. The uniaxial tensile yield stress maximum load in the tension test, as described in Chap. 19. If the linearity and size criteria are met, then Kq is recorded as Kjc, that is, the plane strain value. The corresponding fracture energy, Qc can be obtained using ... 

Thus far, the discussion has centered on uniaxial tension. Let us see what happens when the bar in tension is unloaded first and then loaded in compression. The typical stress-strain curve is represented in Figure A.6. The loading is initially in tension up to a stress of Si at which the unloading process starts and follows a straight line parallel to the elastic curve. Upon unloading as the strain reaches zero, a tensile strain of Sp remains. Subsequently as the material is loaded in compression, it may yield in... 

Yield criteria are mathematical tools to decide whether the stress state in a material will cause plastic deformation or not. In a polycrystalline metallic tensile test specimen, which can be assumed to be isotropic and uniaxially loaded, the material yields at a stress ... 

If we deform a kinematically hardening material in uniaxial tension and compression, its behaviour differs drastically from the isotropically hardening material discussed above (figure 3.32). Upon load reversal, the material yields at a stress yield surface remains unchanged. In the extreme case, this may lead to plastic deformation while the stress is still tensile (figure 3.32(b)). 

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