Surface modification for improving the formability of SPA laminated composite
Introduction
Adhesive bonding as a method of fabrication has attracted special interest recently, as it offers potential advantage of saving the material costs and simplified manufacturing processes. In enlarging the application of adhesive joints to the mechanical elements, however, there is not enough of the necessary basic data on mechanical properties of adhesive joints [1].
In recent years, automobile manufacturers have seriously considered using Steel–Polymer–Steel (SPS) laminates for automobile parts such as fenders, doors, interior panels and others, because of their superior properties in vibration damping and weight reduction [2], [3]. Although these laminates are produced for both the application fields and are commercially available in the market, they are not used widely because of their uncontrollable geometric distortions during the processes of formation. Therefore, their applications are restricted to structural parts [4].
In order to increase the formability of the laminated Steel–Polymer–Aluminium (SPA) composite, failure mode must be studied well. In the deep drawing of a homogenous sheet, failure of the metal normally follows localised thinning caused by tension in the plane of the sheet surface. In contrast, failure of laminated sheet usually occurs in the relatively weak adhesive or in the metal/adhesive interface, leading to separation or delamination of the laminate long before thinning of the metal sheets [5], [6].
It has already been published [7] that durable and strong adhesion between incompatible can only be achieved by ion beam techniques. Unfortunately, ion beam techniques are unsuitable for metal–polymer–metal applications as its application is impossible to produce laminated composites. Therefore, new approaches are necessary to increase the bonding strength between polymer and metal components [8].
The approach used in this study for increasing mechanical bonding between these incompatible materials is the modification of surfaces of metal layers [9] by means of laser surface processing technique.
The effect of surface modifications on the mechanical properties of SPA was determined by tensile, OSU formability and mixed mode bending (MMB) tests. After finding the mechanical properties, an optimum surface model was decided, and the effect of thickness was then investigated by finite element method (FEM). The modified surface was modelled elasto plastically in l-Deas Software and OSU formability testing simulated. The model was transferred to Ansys program and then run. The results were transferred to l-Deas 2.01 Master Series program for the purpose of postprocessing.
Section snippets
Experimental analysis
In the study, using commercially available SSA sheets produced SPA laminates. These sheets were cut into pieces, after the sheet pieces were cut to specimen dimensions.
Specimens were modified by means of laser cutting technique to obtain small size cavities or holes on the surface of the sheets. The shapes of hole and cavity on the surface of SPA laminated composite is given in Fig. 1. Shape of cavities was assumed as hemispherical. In reality, they look like hemispherical.
There were four
Tensile test results
Tensile test results were obtained for both the specimens with modified surface and the specimens without any modification for all the adhesive types. The effective stress–effective strain relations of the adhesives used for laminated SPA composites are given in the following figure. As is seen from the figure, adhesives used show different stress–strain relationships.
As shown in Fig. 5, SPA specimens without any surface modifications show ladder like stress–strain behaviour. B3, B2 and B1
Conclusions
The conclusions obtained from testing and investigations for SPA laminated composites are summarised as follows:
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The surface roughness or modification of laminated composites plays a dominant role in the separation behaviour.
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The size and shape of the modification are restricted by the fact that the strength of the metal layers must be protected.
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OSU formability test helped in conducting many trials for different surface roughness conditions and many layer thickness in a short time with respect to
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