DAMAGE EVOLUTION IN COMPOSITE BONDED JOINTS UNDER FATIGUE LOADING
In the present paper the evolution of fatigue damage in composite bonded joints is extensively analysed and discussed, with particular reference to the influence of layer orientation at the adhesive-adherend interface, corner geometry at the end of the overlap area and stacking sequence. Single lap bonded joints manufactured from autoclave-moulded laminates were investigated under tension-tension fatigue loading. Two laminate lay-ups [45/02]s and [452/0]s , two overlap lengths (20 and 40 mm) and two corner geometry (square edge and fillet) were considered. The evolution of fatigue damage, a crack initiation followed by propagation up to a critical length, was investigated at macroscopic and microscopic level, by monitoring stiffness trends and by optical and scanning electron microscopy and it is extensively discussed in the paper. The presence of a 45° oriented layer at the adhesive-interface layer makes damage patterns and crack paths much more complicated with respect to 0° interface joints, previously investigated by the authors. As a result the resistance to crack propagation is increased.
CRACK PROPAGATION ASSESSMENT IN COMPOSITE BONDED JOINTS
In the present paper the problem of crack propagation assessment of single lap bonded joints in composite material has been analysed. Experimental tests on carbon fabric-epoxy 1.6-mm-thick single lap specimens were conduced in tension-to-tension fatigue. Crack length was monitored by means of an optical microscope at scheduled number of cycles during each test. Different over lap lengths (20 mm and 40 mm), configurations of corner geometry (square edge or spew fillet) and stacking sequence ([45/02]s and [452/0]s) were taken into account. Being a scientific definition of the crack initiation and propagation phases beyond our scope, it has been assumed that initiation phase ceases when a technical crack having size equal to 0.3 mm is observed at the adherend-adhesive interface. Since the tested joint geometry induces combined opening and sliding modes at the crack tip, experimental crack growth rates have been summarized in terms of an equivalent value of the Strain Energy Release Rates (SERR). However, crack monitoring by means of manual optical microscope inspection resulted to be a very time –consuming procedure: then it is proposed to adopt the specimen’s stiffness loss, which is easier to be automatically monitored during the fatigue test, as an indicator of the evolution of cracks’ length. Calibration by means of a so-called master curve enabled us to convert the stiffness values into corresponding crack lengths and finally crack growth rates, which resulted consistent with the experimental ones. The existence of a unique Paris-like curve correlating crack growth rates in terms of SERR, to be used for engineering crack propagation assessments, has been finally checked.
The durability of some epoxide adhesive-bonded joints on exposure to moist warm air
Abstract Single lap joints bonded with an epoxide adhesive have been exposed to warm moist air for periods up to one year, and then subjected to strength measurement. Surface treatment of the aluminum alloy adherends had an effect upon the ability of joints to resist exposure, the order of surface treatment efficiency in this respect being chromic acid anodize > chromic acid etch > sandblast > degrease. A linear relationship has been observed between joint strength and the water content of joints, leading to the conclusion that water enters a joint by diffusion through the adhesive, rather than by passage along the interface.