Spiral Tunneling Cracks Induced by Environmental Stress Cracking in LaRC™-TPI Adhesives |
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| Authors: | David A. Dillard Jeffrey A. Hinkley W. Steven Johnson Terry L. St. Clair |
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| Affiliation: | a Engineering Science and Mechanics Department, Virginia Polytechnic Institute and State University, Blacksburg, VA, USAb Materials Division, NASA-Langley, Hampton, VA, USA |
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| Abstract: | Some currently-available formulations of LaRC™-TPI, a thermoplastic polyimide originally developed at NASA-Langley, were found to be highly susceptible to environmental stress cracking when exposed to solvents such as acetone, toluene, diglyme and methyl ethyl ketone. The combination of stress and solvent led to rapid cracking in films and adhesive layers of this material system. Residual cool-down stresses induced when the LaRC-TPI is used as an adhesive or coating led, in the presence of a solvent, to dense “mud crack” patterns which relieve a portion of the stored energy. Because these through-the-thickness cracks are not able to relieve the stored energy in the vicinity of the adherends, additional fractures in the form of curious spiral tunnel cracks initiated and grew inward within each adhesive fragment. Micrographs of the spiral fractures are given, along with a qualitative explanation for the failure process as observed in adhesives and coatings. |
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| Keywords: | spiral cracks adhesives coatings LaRC-TPI polyimide environmental stress cracking residual stresses tunnel cracks |
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