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An exploratory reliability analysis of composites under periodic proof tests in service is presented. The ultimate strength of composites is a random variable. A residual strength degradation model for composites under service loads is employed. Failure occurs as soon as the residual strength of composites is exceeded by statistical service loads, for example, gust turbulence and maneuver loads. Meanwhile, the composites are subjected to periodic proof tests in order to eliminate weak components and ensure an acceptable level of reliability. When a component fails under the proof test, a new component is manufactured and proof-tested for replacement, so that the strength of composites is renewed. Taking into account the statistical strengths, strength degradation characteristics, statistical service loads, design stresses, periodic proof tests, and renewal processes due to replacement, the probability of composite failure in service is derived. Two numerical examples, a boron/titanium bonded joint of a fighter aircraft and a glass/epoxy component of a transport-type aircraft, are worked. It is demonstrated that significant fatigue reliability improvement for composites can be achieved by the application of periodic proof tests.
composite materials, reliability, ultimate strength, fatigue strength, residual strength degradation, bonding, periodic proof tests
Associate professor, The George Washington University, Washington, D.C.