Tensile load, plastic deformation under

The explanation of this aging effect can be based on the common concepts of aging. The initial material is ductile and tough enough that the plastic deformation under tensile load proceeds relatively slowly, yielding weak acoustical signals which do not surpass the noise background. 

Boniface L, Ogin SL, Smith PA. Damage development in woven glass/epoxy laminates under tensile load. In Proceedings second international conference on deformation and fracture of composites, Manchester, UK. London Plastics and Rubber Institute 1993. 

Of course, metals may experience plastic deformation under the influence of applied compressive, shear, and torsional loads. The resulting stress-strain behavior into the plastic region is similar to the tensile counterpart (Figure 6.10a yielding and the associated curvature). However, for compression, there is no maximum because necking does not occur furthermore, the mode of fracture is different from that for tension. 

Prior to the advent of fracture mechanics as a scientific discipline, impact testing techniques were estabhshed to ascertain the fracture characteristics of materials at high loading rates. It was realized that the results of laboratory tensile tests (at low loading rates) could not be extrapolated to predict fracture behavior. For example, under some circumstances, normally ductile metals fracture abruptly and with very little plastic deformation imder high loading rates. Impact test conditions were chosen to represent those most severe relative to the potential for fracture —namely, (1) deformation at a relatively low temperature, (2) a high strain rate (i.e., rate of deformation), and (3) a triaxial stress state (which may be introduced by the presence of a notch). 

Plastic deformation is commonly measured by measuring the strain as a function of time at a constant load and temperature. The data is usually plotted as strain versus time. Deformation strain can be measured under many possible loading configurations. Because of problems associated with the preparation and gripping of tensile specimens, plastic deformation data are often collected using bend and compression tests. 

Some plastic materials have different tensile and compressive characteristics. For example, polystyrene is tough under compressive load but very brittle in tension. However, for most elastoplastic materials, the stress-strain curves in compression are the same as in tension. Hence, the deformation properties of these materials in tension may also be applied to those in compression, which is of great interest to gas-solid flows. 

The intersection of the crack arrest curve with the yield curve (Curve B) is called the fracture transition elastic (FTE) point. The temperature corresponding to this point is normally about 60°F above the NDT temperature. This temperature is also known as the Reference Temperature - Nil-ductility Transition (RTj dt) is determined in accordance with ASME Section III (1974 edition), NB 2300. The FTE is the temperature above which plastic deformation accompanies all fractures or the highest temperature at which fracture propagation can occur under purely elastic loads. The intersection of the crack arrest curve (Curve D) and the tensile strength or ultimate strength, curve (Curve A) is called the fracture transition plastic (FTP) point. The temperature corresponding with this point is normally about 120°F above the NDT temperature. Above this temperature, only ductile fractures occur. 

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