New Practice for Mode I Dominant Fracture Toughness Fatigue Testing of Sandwich Constructions
1. Scope
This practice provides instructions for modifying the single cantilever beam test method, ASTM WK56166, to determine the Mode I dominant fracture toughness of sandwich constructions subjected to constant-amplitude fatigue loading. Fatigue loading is performed using the same composite sandwich specimen and Mode I dominant loading configuration specified in WK56166. Composite sandwich specimens initially are loaded quasi-statically for precracking and to measure the initial specimen compliance and Gc value. A series of constant-amplitude fatigue loading increments are performed, the duration of each increment based upon obtaining a specified increase in specimen compliance. The crack length is measured at the end of each fatigue loading increment and used to construct an a/N versus Gcmax plot from all of the fatigue loading increments. Knowledge of the fatigue fracture toughness of the facesheet-to-core separations of sandwich composites is of use in material selection, product development, research and development activities, and design allowables.
Considerable interest exists in the sandwich composites community, particularly in the CMH-17 (Composite Materials Handbook) Sandwich Disbond Working Group, for a Mode I dominant fracture toughness test method performed under constant-amplitude fatigue loading. The proposed standard practice, based on the proposed Mode I Single Cantilever Beam test method (ASTM WK56166), will be used by the sandwich composites community for measuring the resistance to disbond growth under mode I dominant fatigue loading conditions.
