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.