Application of fracture mechanics to the interpretation of bond strength data from ASTM standard C633-79

The debonding specimen used in ASTM Standard C633- 79 has a nonuniform stress distribution at the interface between the coating and the substrate.[1] This means that bond strengths determined according to the standard could be significantly lower than actual strengths. A new specimen, 50% longer than the standard specimen, was developed to alleviate this problem. The elongated specimen has a uniform stress distribution that is equal to the uniform stress assumed by ASTM Standard C633- 79. Thus, bond strengths obtained using the elongated specimen are higher and more representative of the actual bond strength than estimates obtained from the standard specimen. In this work, a procedure is developed to transform the existing bond strength values obtained using the C633- 79 Standard specimens to the more representative bond strength values that would be obtained if the tests were repeated using the elongated specimens. A combination of finite- element analyses and laboratory test data is used to identify the relation between the bond strength values of standard specimens and those of elongated specimens. Examples are presented and the procedure is verified by comparisons with bond strength data for Colmonoy No. 6 and aluminum oxide coatings.