COMPOSITE-TO-METAL TUBULAR LAP JOINTS - STRENGTH AND FATIGUE RESISTANCE

The axial strength and fatigue resistance of thick-walled, adhesively bonded E-glass composite-to-aluminum tubular lap joints have been meas ured for tensile and compressive loadings. The joint specimen bonds a 63 mm OD aluminium tube within each end of a 300 mm long, 6 mm thick E -glass/epoxy tube. Untapered, 12.5 mm thick aluminium adherends were u sed in all but four of the joint specimens. The aluminum adherends in the remaining four specimens were tapered to a thickness of 1 mm at th e inner bond end (the bond end where the aluminum adherend terminates) . For all loadings, joint failure initiates at the inner bond end as a crack grows in the adhesive adjacent to the interface. Test results f or a tension-tension fatigue loading indicate that fatigue can severel y degrade joint performance. Interestingly, measured tensile strength and fatigue resistance for joints with untapered adherends is substant ially greater than compressive strength and fatigue resistance. The jo int specimen has been analyzed in two different ways: one approach mod els the adhesive as an uncracked, elastic-perfectly plastic material, while the other approach uses a linear elastic fracture mechanics meth odology. Results for the uncracked, elastic-plastic adhesive model ind icate that observed bond failure occurs in the region of highest calcu lated stresses, extensive bond yielding occurs at load levels well bel ow that required to fail the joint, and a tensile peel stress is gener ated by a compressive joint loading when the aluminum adherends are un tapered. This latter result is consistent with the observed joint tens ile-compressive strength differential. Results of the linear elastic f racture mechanics analysis of a joint with untapered aluminum adherend s are also consistent with the observed differential strength effect s ince a mode I crack loading is predicted for a compressive joint loadi ng. Calculations and a limited number of tests suggest that it may be possible to selectively control the differential strength effect by ta pering the aluminum adherends. The effect of adherend material and thi ckness on fracture mechanics parameters is also investigated. The pape r concludes by examining the applicability of linear elastic fracture mechanics to the joints tested.