Damage assessment using fracture mechanics and NDT approaches

Assessment of damage in materials and structures can be carried out by applying elements of both fracture mechanics and nondestructive testing. This is particularly important for determining the remaining life of structures subjected to cyclic or fatigue loading. In fatigue, the relationship defining the rate of crack growth dA / dN is given by the Paris law as dA / dN = a(ΔK)^b where A is the crack length, N is the number of cycles, a is a constant, K is the stress intensity factor and b is an exponent which has a value of about 3 for pearlitic steels and 4 for aluminum alloys. In the present research, we establish a similar relationship between elements of signals obtained from monitoring using nondestructive testing techniques, such as ultrasonics and acoustic emission. Signal characteristics include features of pattern recognition which are then related to AK by equations similar to the Paris law given earlier. The application of these relationships using custom software allows obtaining in real time the extent of crack growth and hence the accumulated damage incurred during the normal operation of a critical structure or component to be attained in real time. Examples of applications of these algorithms for specific structures are presented.