Micro-residual stress

The micro-residual stresses arise from the differential CTE of the fiber and matrix, and the temperature difference. Table 7.4 gives the linear CTE values for various types of reinforcing fibers and matrix materials used widely for composite fabrication. The CTEs of most fibers and ceramic matrices are relatively lower... 

Three-dimensional distributions of the micro-residual stresses are very complicated, and are affected by the elastic properties, local geometry and distribution of the composite constituents within a ply. Many analytical (Daniel and Durelli, 1962 Schapery, 1968 Harris, 1978 Chapman et ah, 1990 Bowles and Griffin, 1991a, b Sideridis, 1994) and experimental (Marloff and Daniel, 1969 Koufopoulos and Theocaris, 1969 Barnes et ah, 1991 Barnes and Byerly, 1994) studies have been performed on residual thermal stresses, A two-dimensional photoelastic study identified that the sign and level of the residual stresses are not uniform within the composite, but are largely dependent on the location (Koufopoulos and Theocaris,... 

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. 

The first and foremost step in failure analysis of ceramics consists of identifying the fracture origin and the type of cracking, which throws light on the type of failure such as failure due to impact, residual stress combined with load, thermal shock, improper machining, oxidation and corrosion. This is aided by micro- and macrofracto-graphy, examination of microstructure by SEM, chemical analysis and metallographic examination. 

Mohrbacker, H., Van Acker, K., Blanpain, B., Van Houtte, R, and Cehs, J.R, Comparative measurement of residual stress in diamond coatings by low-incident-beam-angle-diffraction and micro-Raman spectroscopy, J. Mater. Res., 11, 1776, 1996. 

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. 

The results obtained at the end of 1989, and the laboratory tests and examinations identified the most probable scenario involving the nature of the drum steel (ferritic 15 D3) and the simultaneous presence of three factors the existence of start sites (micro-crackmg) m zones of high hardness, residual stresses close to the elastic limit of the matenal, and lastly, the contnbutions of hydrogen which allowed the bottling phenomenon to occur... 

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). 

A mechanical interaction of the abrasive grains with the workpiece usually leads to compressive residual stresses by localized elastic deformation and plastic flow. The predominance of mechanical process effects can be achieved by chip formation with increased ratio of micro-plowing. This usually occurs in grinding with small chip thickness and low cutting speeds (Fig. 2). 

Hardness values, indentation moduli, strain hardening exponents and viscoelastic properties can be measured with the instrumented indentation test, also the fracture toughness of very brittle polymers as well as the influence of residual stresses. If needed and a suitable device provided measurements can be done with high spatial resolution and with very small indentation depths. A special application of the testing devices is the characterization of the elastic behaviour of miniaturized components or the realization of micro compression tests, i.e. using the machines like a small universal testing machine. 

D. Baneijee, H. Rho, H. E. Jackson and R. N. Singh, Characterisation of Residual Stresses in a Sapphire-Fiber-Reinforced Glass-Matrix Composite by Micro-Fluorescence Spectroscopy, Comp. Sci. Technol. 61, 1639-1647(2001). 

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