An Experimental Procedure to Estimate Surface Crack Density Using Thermography and Acoustic Emissions

The current study discusses two aspects of an infrared thermography-based local SHM system for monitoring damage evolution during fatigue of composites materials, namely, pre-macrocrack damage (Stage I) followed by subsequent crack growth (Stage II). The crack density is a very well-known damage parameter representing the actual mechanical state of the material in terms of stiffness degradation. In effect, for laminates presenting off-axis laminae, crack density is useful for determining the “characteristic damage state” (CDS) that is related the load carrying capability of the laminate. In present research, a novel procedure is proposed for performing contactless measurements of crack density during static tensile tests by using temperature signal. The results have been critically compared with the strain waves signal acquired by acoustic emission sensors during the same tests. The proposed technique and procedure lead to estimate the crack density in those applications where it is difficult to detect transverse crack using a direct measurement from common experimental techniques.