Small crack initiation and formation - an investigation into equivalent initial flaw size and parallel offset surface crack interactions
Abstract
Crack initiation and formation is an area of much importance in both the manufacturing and use of materials; and the frequency and size of inhomogeneities in a material has a significant effect on its fatigue life. Equivalent Initial Flaw Size (EIFS) is a method that can be used to characterize the microscopic quality of a material; however, traditionally the cracks are grown backwards from larger crack lengths. It is known that small crack growth behaves differently to large crack growth; hence, the EIFS for 7050-T7451 Aluminum Plate was calculated using the small crack growth rate for this material. It was found that the distribution of the EIFS was very similar to the distribution of the actual inhomogeneities as measured using a Scanning Electron Microscope. The mean of the EIFS was comparable to the inhomogeneity mean, whilst early crack length measurements were utilized to reduce the effects of crack growth scatter on the EIFS distribution. The early formation and coalescence of cracks can determine which cracks will become dominant. Predicting how these cracks interact by developing a set of stress intensity solutions provides an additional tool when studying coalescence. A commercial Finite Element Method (FEM) package was used to model various geometries of parallel overlapped surface cracks with a depth to half surface length ratio of 0.9. The magnitude of interaction was found to increase as the height and horizontal separation decreases, with the crack tip interaction being more sensitive to overlap than height. The model showed that a mixed mode crack develops on the leading third of the crack profile as the two cracks approach each other, which leads to a circular growth pattern as the tips overlap. A full solution space of mode I, mode II and crack propagation angle was created for the inner tips between an overlap to crack half length ratio (S/a) of -0.5 to 0.5, and a height separation to crack half length ratio (H/a) of 0.3 - 0.9.
Degree
M.S.A.A.
Advisors
Grandt, Purdue University.
Subject Area
Aerospace engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.