Tensile loading, yield stresses

Under tensile loading, the stress concentrating effect of an unbonded spherical particle is dmilar to that of a void. Nicolais and co-workers have studied the tensile stress-strain behaviour of composites based on SAN, ABS, PPO, and epojgr resins " 4-36) jjj jj g g polymers, unbonded glass beads cause yielding... 

Figure 11.3. Variation of tensile yield stress with melt flow index (10 kg load at 190 C) and isotactic index. (After Crespi and Ranalli )...

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. 

Figure 7.6 Idealized stress-strain curve. Material deforms elastically until yield stress, oyield or Oy is reached. Load increases until the ultimate tensile strength (UTS), which marks the onset of necking. Fracture occurs at a reduced load in the necked region. Toughness is measured by the area under the stress-strain curve.

FIGURE 4.19 A Zr705 C-ring specimen under tensile loading to 98% of yield stress that has been tested in HI acid. 

Oys = yield strength = 9,8 MPa (at 80°C) for this copoly(ester ester) and Ki is the stress intensity factor (18) under tensile loading, which for a double notched tensile specimen is ... 

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. 

Because of the difference in form between Eqs. (2) and (3), the mechanisms of deformation and fracture change with the state of stress. For example, polystyrene yields by shear band formation under ccm ression, but crazes and frachues in a brittle matmer under tensile loading. Chants in failure nwchanian with state of stress are e cially important in particulate conqx tes, since the second phase can alter the local state of stress in the surrounding matrix. 

Figure 8.9. Tensile yield stress of PVC vs. fiUer loading. [Adapted, by permission, from Wiebking H E, Antec 95. Volume 111. Conference proceedings. Boston, Ma., 7th-l 1th May 1995, 4112-6.]...

The same phenomenon occurs in the deflection of a column imder a compression loading. In this type of failure, a critical load is reached beyond which collapse occurs as a result of a rapid increase in the stresses beyond the yield point of the material. The critical pressure that causes collapse is not a simple function of the induced stress, as with tensile loads. In fact, it is directly proportional to the modulus of elasticity of the material and the moment of inertia of the shell and is inversely related to the cube of the radius of the curvature. 

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