Crack Growth Data Collection and Reduction Methodology Survey.

摘要

Engineers responsible for predicting solid rocket motor performance and ensuring reliability know that during manufacture, transport, and storage of motors, cracks may appear in the propellant that threaten this reliability. when they discover cracks, engineers use fracture mechanics principles to assess the crack stability. Structural analysis shows the critical loads for the cracked motor, and testing of specimens determines the tendency of the material toward crack growth initiation, as well as subsequent growth rates. The measurement of crack growth rates in propellant is complicated by inhomogeneity of the microstructure and by time-dependent behavior. The local microstructure affects the crack growth, so that growth does not increase uniformly with load. Instead, the crack growth is sporadic, reacting to local stresses and strains in the microstructure; crack growth may even stop at some points during the test. Also, the high ductility of the viscoelastic matrix causes large dimensional changes, resulting in crack tip blunting and damage zones near the crack tip that deviate from the mathematical ideal of an infinitely sharp, well-defined crack. Another source of difficulties is that material properties often vary among specimens because of trouble maintaining uniformity during processing of large rocket grains, resulting in high statistical scatter in measurements compared with other materials.