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An investigation was made to determine the relevance of impacter shape to nonvisible damage and tensile residual strength of graphite/epoxy cases for the solid rocket motors of the Space Shuttle. Impacters were dropped onto 30.5-cm (12-in.)-long rings (short cylinders) that were 76.2 cm (30 in.) in diameter and 36 mm (1.4 in.) thick. The kinetic energies ranged from 17.0 to 136 J (12.5 to 100 ft∙lb). Some rings were filled with inert propellant and some were empty. A 5 kg (11 lb) impacter was used with a 12.7-mm (0.5-in.)-diameter hemisphere and a sharp corner attached. The rings were impacted numerous times around the circumference and cut into 51-mm (2-in.)-wide specimens. Because of a shortage of rings, impacts with a 6.3-mm (0.25-in.)-diameter bolt-like rod were simulated by quasi-statically pressing the rod against the face of individual specimens. All of the specimens were uniaxially loaded to failure in tension. Results from a previous impact study using a 25.4-mm (1.0-in.)-diameter hemisphere were also considered. For the range of impact energies investigated, the corner and rod always made visible damage on the surface, but the hemispheres did not. The damage on the surface consisted of a crater shaped like the indenter, and the damage below the surface consisted of broken fibers that appeared to result from shear failure of the matrix. The damage initiated when the contact pressure exceeded a critical level but did not become visible on the surface until an even higher pressure was exceeded. The contact pressures for the rod and corner exceeded the critical level to cause visible damage. The kinetic energy to initiate damage and to cause visible damage on the surface increased approximately with hemisphere diameter to the third power. For a given kinetic energy, the residual strengths did not vary significantly with indenter shape. However, the reduction in strength for nonvisible damage increased dramatically with increasing hemisphere diameter, 9 and 30% for the 12.7-mm (0.5-in.) and 25.4-mm (1.0-in.)-diameter hemispheres, respectively. Factors of safety for nonvisible damage increased with increasing kinetic energy. With the factor of safety for the filament-wound case (1.4), the maximum allowable kinetic energy was 123 J (91 ft ∙lb). The effects of hemisphere diameter on impact force, damage size, damage visibility, and residual tensile strength were predicted quite well assuming Hertzian contact and using maximum stress criteria and a surface crack analysis.
low-velocity impact, nonvisible impact damage, impacter shape, residual tensile strength, graphite/epoxy composite, motor case, composite materials, fracture, fatigue (materials)
Senior research engineer, NASA Langley Research Center, Hampton, VA