A method for the distribution analysis of residual stress in epoxy resins by Raman microspectroscopy was developed. By using the uniaxial tensile method, a negative correlation between the peak shift of the aromatic C─H stretching vibration and external stress was obtained. By optimizing the precision of the Raman shift measurement, a reliable stress image was obtained for a bulk epoxy resin. This method was applied to the evaluation of the residual stress distribution in epoxy resin/aluminum bonded materials. Raman spectra imaging revealed that stress concentration strongly occurred in the interface of the cured epoxy resin with aluminum, and it was found that Raman spectra images of the small residual stress of the resin interface were affected by the curing process and physical properties of the epoxy resin. Therefore, Raman spectra imaging would appear to have significant potential as a technique for the evaluation of local thermal residual stress in the interface of multimaterial structures.
K E Y W O R D Sadhesion interface, epoxy resin, stress distribution, thermal residual stress
The mechanism of shell mold cracking and its prediction during casting of aluminum alloy were elucidated. A cylindrical shell mold made of silica sand fractures easily when filled with aluminum alloy melt. The cracking mechanism can be considered as follows. The immediate inner surface of a shell mold undergoes a sudden temperature rise from heating by the melt and attempts to expand. This thermal expansion is restrained by the other part of the mold that is still low in temperature. Consequently, compressive stress in the area near the inner surface and tensile stress in the area near the outer surface develop respectively, causing the shell mold to fracture when the tensile stress exceeds the tensile strength of the shell mold. With some part of a cylindrical shell mold cut to a thinner thickness, a higher tensile stress acts on the outer surface of the thinner part and a crack is formed in a shorter time after the mold has been filled with aluminum alloy melt. The criterion for shell mold cracking can be described by the relation of fracture stress and effective volume based on the Weibull's statistical method, which is utilized for evaluating the strength of brittle materials. The relation of fracture stress and effective volume enabling us to predict the shell mold cracking was obtained from the statistical properties of the tensile strength of the shell mold material.
Static fatigue crack growth tests of several silicon nitride ceramics with various microstructures were carried out by a constant moment method. Correlations between the static fatigue crack growth behavior and microstructures, such as the size and distribution of grains and the fraction of rod-like grains, were investigated.The experimental results showed the following tendency. The larger average grain size, the higher fatigue crack growth resistance. The more uniform distribution of grain size, the higher gradient of fatigue crack growth resistance curve for crack length ranging to about 5mm. And the fraction of rod-like grains was not so correlative to crack growth resistance. Furthermore, a grain bridging model which may explain the influence of microstructure on fatigue crack growth behavior was proposed. In this model, the bridging force reducing effective stress intensity is described as a function of crack opening displacement and size and interval of bridging grains. The simulation showed that the more uniform distribution of grain size, the higher fatigue crack growth resistance. Also, the larger average grain size and the more fraction of rod-like grains, the higher fatigue crack growth resistance. These results were conformed qualitatively by the experimental results.
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