Section 126.96.36.199. Fracture Toughness Testing Methods
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 skininger 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 planeain fracture toughness (KIc) 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 KR resistance curve.
The KR 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 planeain fracture toughness (KIc)
per ASTM E399 is the J-integral approach to determine the planeain