Monitoring the fracture behavior of SiCp/Al alloy composites using infrared lock-in thermography

his work deals with the study of fracture behavior of silicon carbide particle-reinforced (SiCp) A359 aluminum alloy matrix composites using an innovative nondestructive method based on lock-in thermography. The heat wave, generated by the thermo-mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the sample, was detected by an infrared camera. The coefficient of thermo-elasticity allows for the transformation of the temperature profiles into stresses. A new procedure was developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue: (a) The distribution of temperature and stresses at the surface of the specimen was monitored during the test. To this end, thermal images were obtained as a function of time and saved in the form of a movie. (b) The stresses were evaluated in a post-processing mode, along a series of equally spaced reference lines of the same length, set in front of the crack-starting notch. The idea was that the stress monitored at the location of a line versus time (or fatigue cycles) would exhibit an increase while the crack approaches the line, then attain a maximum when the crack tip was on the line. Due to the fact that the crack growth path could not be predicted and was not expected to follow a straight line in front of the notch, the stresses were monitored along a series of lines of a certain length, instead of a series of equally spaced points in front of the notch. The exact path of the crack could be easily determined by looking at the stress maxima along each of these reference lines. The thermographic results on the crack growth rate of the metal matrix composite (MMC) samples with three different heat treatments were correlated with measurements obtained by the conventional compliance method, and found to be in agreement.