A Double Compliances Method for Measuring the Mode I Interlaminar Fracture Toughness of Composite: Theory and Applications

Abstract

Delamination is one of the most serious failure modes in laminated composite materials. From the material testing and characterization point of view, it is highly desirable to have a testing procedure which is easy to prepare, conduct and yields consistent Interlaminar fracture toughness. When the ASTM test standard D5528 is applied to measure the fracture toughness, the applied load and its associated displacement and crack length are required. However, it is tedious to measure the growing crack length and to record the corresponding load and displacement simultaneously, especially for unstable crack growth and DCB test in certain environment. Recently, we devised a double compliances method to measure the mode I fracture toughness for composite materials. Only the load and displacement recorded from the test machine are required for the determination of the fracture toughness. The theoretical foundation of this method will be briefly given in this paper. It is then used for two special cases, where the crack growth length is difficult to get access. One is a DCB test in a cold temperature environment. The other is a DCB test for a 2D plain weave woven textile composite. There are several finite crack growths and associated load drops in the load displacement curve in the DCB test for woven textile composites, which increase the difficulty in measuring the crack growth length, displacement and load simultaneously. Using the double compliances method, the finite crack length and static fracture toughness can be obtained from the loaddisplacement relation. It is demonstrated that the double compliances method is very useful for determining the mode I interlaminar fracture toughness of composites, especially for the case where the crack growth length is hard to obtain.