| An investigation of fatigue crack initiation and propagation was carried out on structural adhesive joints. The joints consisted of 7075T6 aluminum adherends bonded with a mineral filled structural epoxy (Cybond 4523GB, American Cyanamid). Three types of joints were tested to achieve mode I (double-cantilever beam specimen, DCB), mixed mode I-II (cracked lap shear specimen, CLS), and mode II (end notch flexure specimen, ENF). All tests were conducted under ambient conditions with load ratio of 0.1 at a frequency of 30 Hz. Fatigue loading significantly reduces the strain energy release rate (G) required to initiate a crack compared to static and quasi-static loading. Negligible differences in crack initiation times were observed for most specimens with cracks starting from fast mode I precracks, intact fillets and fatigue precracks. However, for the load ranges tested, fatigue precracks doubled the crack initiation time in DCB joints. Mixed mode I-II fatigue tests had shorter crack initiation times and faster crack propagation rates than mode I and mode II tests at a given percentage of the quasi-static critical energy release rate, G{dollar}rmsb{lcub}c{rcub}.{dollar} The threshold G for no crack growth ranges from 3% to 22% of G{dollar}rmsb{lcub}c{rcub}.{dollar} It is recommended that adhesive joint design should be based on threshold values for zero crack growth. Crack propagation rates show too much scatter to be relied upon for the prediction of in-service subcritical crack growth. |
