Analysis of impact cracking in the composites stern compartment

In the impact test of the simulated emission, when the shell material of a spacecraft stern compartment was improved from Aluminum alloy 2A14 to composites, several stern compartments axially cracked at the right connection hole of the engine vice joint. Crack characteristics were obtained by observations of macrography, metallography, scanning electron microscopy (SEM) and thermal deplying test in the damage regions. Based on the analyses of structure, stress variation in the slip impact process, material properties and processing, the formation and reasons of cracks were deduced. It concludes that because of the inconsistent stiffness between stern compartment and engine, the shell bulges, and generates a relative axial displacement in the impact process, which leads to the fact that the connection hole region in the stern compartment shell axially cracks on both ends under a large impact load. The rear-end crack forms in the impact instant with a positive fracture, while the front-end crack forms in the rebound instant with an oblique fracture. Missing of partial plies aggravates the inconsistent deformation, but it has little effect on shell cracking. Furthermore, as no old cracks and initial defects are found, the cracks have little relation with processing, and the root cause to the cracking of the stern compartment is that the composite possesses poor impact toughness. Based on these conclusions, a proposal is given for cracking prevention and design improvement.

► The rear-end crack occurred under the impact overload in the impact instant, which is characterized by positive fracture.
► The front-end crack occurred under the extruding overload in the rebound instant, which is characterized by oblique fracture.
► The formation of the cracks has nothing to do with the stern compartment processing.
► Missing and incompletion of partial plies in the crack nearby regions have limited influences on the crack formation.
► The root cause of the cracks is the composites poor impact toughness.