Royal Aircraft Establishment

Fatigue crack growth experiments have been carried out on cracks in the parent plate and the weld zone of alloy 7017.The initial tests were carried out to examine the parameters C and m in the Paris power law to understand the deformation behavior during fatigue crack growth in weldments.Using linear elastic fracture mechanics approaches, characterization of fatigue and deformation behavior can be used to describe the kinetics of crack growth in both the microstructure of the parent plate and the weld zone.This was achieved by growing fatigue cracks at constant ΔK and by monitoring the changes in crack closure as a function of increasing crack length.It is clearly shown that the effective driving force ΔK_eff is reduced as the crack length increased and this effect is most pronounced in the white-zone microstructure of the weldment.These results were verified in a characterization study using the shadow optical method during both static deformation and fatigue testing.The aim was to observe crack-closure development as a function of increasing crack length.The concept of an effective ΔK range, based on the back-face strain method of crack-opening loads, is critically examined in contrast with the shadow optical method.Both methods show good agreement in finding the effective crack-opening loads, but in addition the effectiveness of the entire fatigue cycle can be reassessed by interpretation of results from the shadow optical method.

Plastic yielding behavior of 3 different crosslink d. carboxy-terminated nitrile rubber-modified epoxy resins, at different rubber levels and temperature, was studied.All the systems showed decreases in Young's modulus and yield stress with increasing temperature and rubber content.The deformation process was analyzed using both Bowden and Argon theories.Mol. parameters from each theory were then compared with chem. structures of the epoxy systems.

The composition of lubricating oil obtained from the top ring zone of a spark ignition, natural gas, internal combustion (i.c.) engine at top dead center is very much different from that of the oil in the sump.The implications of this observation for wear in the bore of i.c. engines and piston fouling are briefly discussed.