Pseudo-macrostress

When a specimen is stretched plastically a few percent and then unloaded, x-ray measurements show a line shift indicating residual compressive macrostress in the direction of prestrain. The effect is symmetrical after plastic compression, x-rays indicate residual tensile stress. It is not a surface effect, because x-ray measurements made after successive removal of surface layers show that the stress persists throughout the specimen. On the other hand, dissection measurements show that a true macrostress does not exist, and, in fact, none would be expected after uniform deformation. The stress indicated by x-rays is called pseudo-macrostress, pseudo because it is not a true macrostress causing strain on dissection and macro because it causes an x-ray line shift. Pseudo-macrostress is actually an unusual kind of microstress, in which the portions of the material that are in tension and in compression are unequal in volume. It has been discussed in various reviews [16.26-16.28]. 

The effect was first observed after uniaxial deformation, but such deformation is not restricted to pure tension and compression. Plastic bending, for example, causes true macrostress (Fig. 16-2), but the deformation mode is predominantly a tension or compression of layers parallel to the neutral axis of the beam. The longitudinal residual stress indicated by x-rays is therefore the sum of true macrostress and pseudo-macrostress, and the x-ray result will be numerically larger at either surface than the result obtained by dissection. 

Deformation by rolling or die drawing has a certain uniaxial character, but the forces on the material at the roll or die surface are inclined to the specimen axis. Macrostress is produced, superimposed on a complex system of microstresses the latter probably include a pseudo-macrostress. As a result, the x-ray method does not measure the true macrostress, but rather the sum of macrostress and some component of the microstress that causes an additional line shift. 

On the other hand, plastic deformation by grinding or shot peening produces maCrostresses that are accurately measurable by x-rays, as shown by excellent agreement between x-ray and dissection measurements. Figure 16-11 shows an example. Deformation by these processes appears to be multiaxial, rather than uniaxial, and pseudo-macrostress is accordingly absent. 

In summary, the x-ray method does not reveal the true macrostress in specimens that have been plastically deformed by tension, compression bending, rolling, or die drawing. It would be wrong to conclude, however, that the x-ray method is in error. Pseudo-macrostress is just as real as macrostress. X-rays indicate the sum of the two, dissection only the latter. 

When the sin / method (Sec. 16-4) is used, some specimens that have been plastically deformed in the region examined yield values of df that vary with sin in an oscillatory manner, rather than linearly [16.33, 16.34, 16.26]. These oscillations in di are not fully understood. They must be caused by a system of micro-stresses more complex than pseudo-macrostress, because pseudo-macrostress, like true macrostress, yields a linear variation of di with sin ij/. When oscillations occur, the standard two-exposure method of stress measurement (Sec. 16.4) can be seriously in error.)... 

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See also in sourсe #XX -- [ ]

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