Issue 36

F.A. Stuparu et alii, Frattura ed Integrità Strutturale, 36 (2016) 69-77; DOI: 10.3221/IGF-ESIS.36.08 70 elastic analysis and the experimental results generated further studies of da Silva et al. [4]. The adherends were made from steel as to diminish the level of deformations. Their conclusion suggests that the stresses at the interface adherend- adhesive are responsible for the strength decrease when the thickness of the adhesive increases. A different approach is proposed by Matihas and Lemaire [5] which emphasize that in engineering applications the thickness of the adhesive is seldom constant and a probabilistic analysis is needed to study the reliability of such adhesive joints. They used aluminium and carbon fibre adherends of constant thickness and using Volkensen's model calculated a coefficient of safety for which the probability of failure should be below 0.01%. Their results pointed out that a thicker adhesive will help in reaching the reliability goal, that is contrary to the experimental findings obtained in [1, 2, 4]. The increase of the adherend thickness diminished the peeling effect at the extremities of the adhesive length and led to the increase of the shearing strength of an epoxy adhesive [6], and the increase of the adhesive overlap length increased the failure force of the assembly [7], but in fact the strength of the adhesive layer diminishes. In [8] different adhesives were used for single-lap joints of carbon fibre adherends with an overlap between 10 and 80 mm. For a ductile adhesive the failure force increases with the increase of the overlap, but for a fragile adhesive the force increases only up to 30 mm overlap and afterwards decreases due to the interlaminar failure of the adherends. In [9], when increasing the thickness of the steel adherends from 1 to 5 mm the maximum force and failure strength increases as already established in other studies. A complex analysis was performed by da Silva et. al [10] which analysis the influence of several parameters (stiffness of adherend, thickness of adherend, thickness of adhesive, overlap length) on the strength of the single-lap joint by using the Taguchi method. Starting from an initial configuration the influence of each parameter is quantified as giving a maximum percentage increase of the strength of the assembly. The increase of the values of the analyzed parameters is beneficial to obtain strength increase with specified values, with one exception, the thickness of the adhesive, by which increase the strength of the single-lap is diminished. It was also noticed that the different procedures used to prepare the surfaces to be glued haven't influenced the obtained results. The digital image correlation (DIC) method has inspired several researchers to analyze the strength of lap-joints. Moreira and Nunes [11] investigated the behaviour of a flexible adhesive and the critical shearing deformations which decrease towards the ends of the overlap, suggesting that the peeling strains are responsible for the initiation of the failure. They pointed out that it is essential to consider the peeling effects for the correct interpretation of the strength of the joint. Moutrille et al. [12], Nunes and Moreira [13], and Silva and Nunes [14] used also DIC for studying several geometrical configurations and successfully analyzed the influence of the aforementioned different parameters on the shearing strength of the joints. In this article the type and the thickness of the adhesive as well as the overlap length are kept constant. The single-lap joints are configured by using aluminium and carbon fibre adherends of 3 mm and 5 mm thickness combined differently. DIC is used to monitor the local failure in the adhesive and give insides on the particular phenomena. Peeling (opening or mode I) and shearing (mode II) deformations are analyzed in detail and some conclusions concerning the particularities of using dissimilar adherends are drawn. It is emphasized that only local measurements, in the overlap region, can provide correct information about the deformation and failure of the adhesive. T ESTED CONFIGURATIONS AND MATERIALS he single-lap joints used in the investigations have the dimensions presented in Fig. 1. The thickness of the adhesive is kept constant to 0.5 mm and the overlap length is 20 mm. The thickness t of the adherends was changed from one configuration to another. The adherends were made from aluminium or carbon fibre. At the ends of the overlap a 5 mm gap is kept on each side of the overlap as used to control the thickness of the adhesive layer with a wax layer of 0.5 mm. The adhesive used in the experiments is Araldite 2015 with the elastic constants established with DIC on bulk specimens as: longitudinal modulus of elasticity E = 1790 MPa and Poisson's ratio  = 0.32. This adhesive has a ductile behaviour. The adherends were made from aluminium 6060 T6 and unidirectional carbon fibre of 250 g/m 2 with epoxy resin matrix. The considered thicknesses of the adherends for both materials were either 3 mm or 5 mm, having all of them a width of 30 mm. The adherends were further denoted as aluminium and carbon having the thickness indicated afterwards. The elastic constants of these adherends were established through traction tests on bulk ISO standardized specimens, as indicated in Tab.1. Tests were done on a Zwick Z010 (10 kN) machine. Speed of testing was of 1 mm/min. The increase of stiffness of the 3 mm carbon adherend can be explained due to the higher volume fraction of carbon fibres which resulted for this thickness. T