No abstract

SynopsisResults are presented from a recent study of the influence of tensile overloads on fatigue crack growth in polycarbonate. Fatigue cracks were grown under conditions of constant range in stress intensity factor in four-point bend specimens. The data presented here indicate that tensile overloads may significantly retard subsequent fatigue crack growth in polycarbonate. The period of delay in crack growth was shown to increase with the magnitude of the overload. Recovery of stable crack extension following the overload appeared to involve reinitiation of separate crack growth sites a t the tip of the blunted crack tip, similar to the original crack initiation a t sharp V-notches.

The foreign-object damage problem associated with jet engines was investigated by impacting both titanium (6Al-4V) and boron-aluminum specimens with either steel or room-temperature vulcanizing rubber spheres to simulate the two principal types of ingested foreign objects. To simulate engine operating conditions, some specimens were impacted while under a tensile load. Results indicate that a prestressed composite suffers much greater damage than simple cantilevered specimens. Ti-6Al-4V exhibits excellent impact resistance, loosing only 10 percent of its ultimate tensile strength up to the highest prestresses and impact velocities. Composite specimens react much differently depending on the type of impactor. Steel spheres cause severe damage at very low impact velocities. Room-temperature vulcanizing rubber, on the other hand, causes little damage up to a velocity threshold. Above this point, specimen failure was observed at some prestresses upon impact. The residual tensile and low-cycle fatigue strength of both boron-aluminum and titanium was documented for various impacting velocities and prestresses. A fracture-mechanics analysis is presented which successfully predicts the effects of room-temperature vulcanizing rubber impact on boron-aluminum composites.