Stable crack growth in adhesively bonded aluminum alloy laminates

Abstract

Fabricating relatively thick sheets or plates by bonding together a number of thin layers has considerable potential for improving fracture toughness. This is because of the well-known size effect which leads to high values of the so-called plane stress fracture toughness K at thicknesses which are relatively small in high strength materials [~]. Laminated composites in which the crack front is perpendicular to the plane of the laminae, Fig. i, can take advantage of this effect; these have been termed crack-divider laminates [2]. To achieve high fracture toughness in such a laminate, the layers must be able to fracture independently of one another.A simple way of ensuring sufficiently low interlayer strengths is to use adhesive bonding [3,4].Because the achievement of maximum toughness in a laminate of high-strength material requires the use of thin layers which will typically exhibit considerable slow crack growth under increasing load, it is desirable to examine crack growth resistance or K R curves [5] for such laminates in preference to simple values of K at instability. This allows comparisons to be made among va~rious . c specimen sizes and types; the use of K_ curves is the most rational way of trying to find that thickness whic~ will offer the highest fracture toughness for a given material.In this work K_ curves have been determined for laminates of clad R 7075-T6 aluminum and compared to those for monolithic specimens having a similar total metal thickness.The specimen preparation and testing procedure have been described elsewhere [4]. Briefly, the laminates consisted of 8 layers, each 0.033 in thick, bonded with an epoxy adhesive, the bond layers being about 0.0035 in thick. Monolithic specimens for comparison were machined from a 0.256 in thick clad plate. The ordinary tensile properties of the laminated and solid materials were comparable, as shown in Table I. All fracture testing was done using compact tension specimens, Fig. I, which were pre-cracked by low-level fatigue cycling.The crack mouth opening displacement was measured during the final loading to failure using a conventional clipon displacement gage. The load-displacement trace obtained was then analyzed using a procedure suggested by Sullivan and Stoop [6]. This method relies upon an independently derived crack length-displacement calibration.The resulting K R curves are shown in Fig. 2. Good consistency was found among the various specimens tested, which showed a considerable range in slow crack growth, Aa, prior to instability.This behavior is caused primarily by differences in the initial crack lengths. The values of K at instability show the expected increase with crack extension, Aa. Inspection.of the fracture surfaces of the laminates, Fig. 3, indicates that crack growth does proceed independently in the Int Journ of Fracture II (1975)