Large Displacement J-Integral Double Cantilever Beam (DCB) Test Method for Mode I Fracture Toughness

Abstract

The J-integral is used to develop an alternative double cantilever beam (DCB) test method for the Mode I fracture toughness suitable for both small and large displacements. The current focus is the experimental determination of the Mode I interlaminar fracture toughness of composite materials, but the method is generally applicable to other similar tests and material systems, such as to the Mode I fracture toughness of adhesives. A series of five identical specimens are tested to compare the linear-elastic fracture mechanics method recommended by ASTM, which makes use of linear beam theory with root rotation, large displacement, and end block corrections, with the new nonlinear-elastic and elastic-plastic fracture mechanics method, which does not require these corrections. Experimental results show excellent agreement between the two methods over a series of five tests of primarily linear-elastic DCB specimens subjected to moderate to large displacements as defined in the ASTM standard. Furthermore, an agreement is found between the results of the derivations for the two methods being compared, whereby the large displacement equation for JIc presented in this work is identical to the equation given by J. G. Williams (1987) and which he found to be the true value of GIc. It is the true value of GIc that the large displacement and root rotation correction factors were intended to approximate, and the test method presented here allows for direct measurement of its parameters and evaluation. This method has the added benefit that specimens can be primarily linear-elastic or nonlinear-elastic at the crack tip and may extend to those that are elastic-plastic at the crack tip.