Abstract

Abstract

In riveted joints, typically two groups of failure mechanisms must be verified, those that affect the joining elements and those that affect the parts that must be drilled to be joined.

In this last group, at least, three failure mechanisms must be taken into account: tension, shear and bearing. In each of these failure mechanisms, it must be verified that the component of the stress state responsible for the failure does not exceed a certain critical value that causes the failure.

In the case of riveted joints with composite materials, the task is doubly complex. On the one hand, the stress state in the neighbourhood of the hole is complex, not only because of the non-linearity of the problem (the contact of the rivet with the walls of the hole) but also because of the non-isotropic nature of the material. This makes the stresses to have a strong dependence on the type of fiber, stacking sequence, and in general, on the mechanical properties (non-isotropic) of the laminate. On the other hand, the determination of the critical values, associated with each component of the stress state, responsible for each failure mechanism, is difficult to obtain, due to the fact that these strength values also depend on the particular orientation along the hole contour, where they are evaluated.

In the present work, the stress state in the neighbourhood of the hole is evaluated in riveted joints with composite materials, analyzing different types of laminates, to serve as a basis for the prediction of the different failure mechanisms potentially acting.

Abstract

Abstract

Results and conclusions of the analysis of a campaign of pull-off tests of single T-stiffened specimens are shown in this paper. The test intend to reproduce in a specimen the symmetrical local deformation shape that occurs in a stiffened panel during its postbuckling evolution. Four different groups have been considered combining two geometries and two stacking sequences for the skin and stringer. A total of 10 specimens have been tested. Tests were monitored with strain gauges. Visual inspections were performed during the process to detect the presence of debonding and/or delamination and to study the damage evolution. After the colapse, visual and microscopic analysis (optical and electronic) of the fracture surfaces were carried out. Inspections have shown the appearance of migrations of cracks from the skin-stiffening interface to delaminations between the first layers of the skin or vice versa. The effect that the different configurations of each specimen had on the onset and development of the damage has also been studied.

Abstract

Adhesive joints are characterized by non-uniform distribution of stresses and strains along the bond-lines, with maximum values near the ends of the overlaps. This phenomenon can cause the premature failure of the joints, being specially critical when brittle or semi-brittle structural adhesives are considered. This has motivated the research in the field of ‘functionally graded adhesive’ (FGA) or ‘multiple-adhesive’ joints, which reduce the phenomena described through a combination of rigid and brittle adhesives with more flexible and ductile ones. In this way, these strategies allow to notably increase the joints maximum load capacity. In this field, this article presents the work carried out by ITAINNOVA within the project SAFEJOINT. Specifically, the potential of the techniques based on ‘banded’ gradings as approximations of the continuum gradings is analysed. The study is focused in aluminium-composite single lap shear (SLS) joints with epoxy adhesives. The work done covers both the numerical optimisation of the joints and the fabrication and testing of samples in order to verify experimentally the potential for improvement of the technique comparing with mono-adhesive joints. Finally, an example of application of the technology is presented.