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A fiber reinforced nickel was developed base on the concept of active composites due to the thermal deformation, and its reproducibility at elevated temperature has been investigated. It was reported that curvature of the composite changed non-linearly during heating/cooling cycle. This hysterisis behavior may be involved with microfracture in the composites. Acoustic emission (AE) technique is very useful to monitor a dynamic microfracture. However, conventional AE technique has a limit in application at elevated temperature. We have investigated the noncontact laser AE technique using laser interferometer as a sensor. In this study, we tried to evaluate thermal deformation process of the active composite by this method. Specimens of pure nickel plate as matrix, SiC continuous fiber as reinforcement fiber and pure aluminum plate as insert layer were prepared by hot pressing. AE signals during heating and cooling processes were detected at the reverse of the specimen using a heterodyne type laser interferometer. Observation results showed that three failure modes such as cracking in matrix layer, debonding of matrix/ fiber interface, and breakage of SiC fiber occurred during thermal deformation process. These failure modes were discussed based on AE source models. AE behavior of the composite showed thermal Keiser effect. This indicates thermal stresses in the composite cause microfracture during thermal deformation process. As a result of the test with the maximum temperature of 1073 K, AE event rate increased rapidly at 960 K and AE signals were detected even in cooling process. This temperature was identified to be the transition temperature of microfracture process in this composite. AE generation temperature was also in good agreement with the critical temperature of hysterisis of curvature.

Evaluation of Thermal Deformation Process of Nickel Based Active Composites by Laser AE Technique