Macro-residual stress

It is known that for a materials with two or more phases the stress field is the superposition of stresses at two levels Macroscopic stresses which exist between the different layers and result from the internal force balance through the whole material. Microscopic stresses which appear between grains or phases in the material. Thus, the micro residual stresses stemming from the two-phase system have to be added to the results from finite element analysis (where only macro residual stresses are determined) allowing direct comparison with the total stresses experimentally measured. Figure 9 shows the macro residual surface stresses from the numerical analysis for the two and three layer specimens. One can see that the results from X-ray measurements agree fairly well with the predicted values. 

Fig. 20 Critical scratching load L, macro-indentation failure load F and residual stresses SiC in the triple layer system, e. corresponds to the thickness of the Ta interlayer.

Thermal residual stresses are inherent to fibre reinforced composites due to the heterogeneity of the thermo-mechanical properties of their two constituents. Such stresses build up when composite structures are cooled down from the processing temperature to the test temperature. Residual stresses will be present on both a fibre-matrix scale (micro-scale), and on a ply-to-ply scale (macro-scale) in laminates built up from layers with different orientations. It is recognised that these stresses should be taken into account in any stress analysis. 

Alternatively, Clement et al. proposed that the micro PS found under illumination could result from shattering of the macro PS into fine filaments due to residual stress. Dissolution-precipitation was also considered to be a possible mechanism for the formation of micro PS althongh it is inconsistent with the single-crystalline nature of the miCTO PS in many two-layer PS. 

Residual stresses can be distinguished in macro-stresses (Type I) and micro-stresses (Type II -I- IB) which are characterized by the scale at which they exist within a material. Macro-stresses occur over Iraig distances within a material whereby micro-stresses exist only locally between workpiece grains or inside a grain (Macherauch et al. 1973). 

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]. 

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