Fracture Toughness Testing at Alcoa Research Laboratories

Abstract

At Alcoa Research Laboratories, fracture toughness determinations are made on commercial and promising experimental alloys and tempers with center-cracked specimens, single- and double-edge-notched specimens, and notched round specimens. Compliance techniques are usually used to detect the initiation of unstable crack growth. Kahn-type tear tests and sharp-notch tension tests are used as screening tests to supplement fracture toughness testing in the evaluation of the effects of composition, fabrication procedure, temper, and environment on the fracture characteristics of aluminum alloys. Tear tests have the advantage that direct measures of the relative amounts of energy required to propagate a crack can be obtained even for the toughest of aluminum alloys; furthermore, the unit propagation energy from the tear test is directly correlated with the valid values of Kc and KIc. Sharp-notch tension tests are economical and, particularly when general yielding is not obtained, the ratio of notch strength to yield strength provides a meaningful measure of relative toughness. Specific examples are given of instances in which these techniques have been used to: (1) determine which series of aluminum alloys shows the most promise for the development of high-toughness alloys; (2) establish optimum compositions and fabricating procedures for specific alloys; (3) develop optimum tempers; and (4) indicate the outstanding alloys for cryogenic applications.

A systematic study of the effect of interrupted ageing conditions on the strength and toughness development of AA6061

“…Fracture toughness was studied by means of Kahn tear test based on ASTM B-871-01. Details of the test and its application for estimating plain stress toughness can be found elsewhere [19]. Tensile and fracture toughness properties were determined for all heat treatment conditions.…”

A semi-quantitative model of pop-in behavior

“…These three considerations along with equations (13) and (16) In order to compare the model to the experimental evidence, three factors had to be described -K ICt Kc and Z: The yield strength was already knmm to be 78 Ksi for this heat of 7075-T6 alumin~(2). K Ie was taken as 28 Ksi -iril/2 as this \oms the lowest stress intensity at which pop-in was obtained, This is also not unreasonable compared to other experimental· data (16). K was ta~\.en as 55 Ksi-iri l / 2 since this was the average valucof K at instabilitYCin those tests where plane stress failure was observed.…”