Fracture toughness data have provided the basis for estimating the crack length-residual strength behavior of aerospace structures since the late fifties.  Initial correlation tests for airplane skin-stringer type structures were typically conducted using wide, center crack panel tests of the skin material.  It was soon realized that such tests were inappropriate for estimating the fracture behavior of thicker material/structure for a number of reasons.  By the late sixties, ASTM had evolved a fracture toughness test for materials that fail by abrupt fracture.  This test method eventually became the plane-strain fracture toughness (K_Ic) test standard, ASTM E399, in 1972.

Additional work by ASTM throughout the seventies resulted in several additional fracture toughness methods.  One such method appropriate for tougher (or thinner) materials which fail by tearing fracture is ASTM Standard E561, which covers the development of the K_R resistance curve.  The K_R resistance curve test has found wide acceptance in the aircraft industry since calculation procedures were already in place to utilize the data for residual strength estimates.  Another recently-approved standard, ASTM E1820, covers the determination of fracture toughness using several methods.  One such method applicable to materials which lack sufficient thickness for plane-strain fracture toughness (K_Ic) per ASTM E399 is the J-integral approach to determine the plane-strain toughness J_Ic.