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EXPERIMENTAL STUDY OF THE STRESS RELIEF IN PATCHED ALUMINIUM SPECIMENS

This paper presents an experimental procedure enabling to assess the stress relief in aluminium specimens reinforced by composite patches. Infrared thermography is used to measure temperature fields during cyclic tensile tests. Temperature variations are directly linked to the stress variations within the framework of thermoelasticity. Measurements are performed through the thickness of the aluminium substrate. A data processing is performed to fit the temperature amplitude: a function based on a one-dimensional model is used for this purpose. The stress transfer relief in the aluminium is observed along the transfer length, enabling to assess the shear stress in the adhesive. Introduction Composite patches are often used to reinforce or to repair damaged structures, especially aeronautical components [1]. The mechanical properties of the bonded joint between metallic substrate and composite patch clearly influence the quality of the reinforcement. Objective of this work is to study in detail the mechanical response of such a joint. The response in terms of strains, displacements and stresses of an adhesive bonded joint and especially the stress transfer between substrate and adherent has been intensively studied in the literature. Many theoretical and numerical mechanical models are available [2] [3] for instance. The load transfer from substrate to adherent induces some shear stress concentrations in the adhesive near the free edges, along the so-called “transfer length”. To the knowledge of the authors, this shear stress concentration has seldom been experimentally studied whereas many theoretical or numerical studies are available in the literature. This lack of experimental data has recently led the present authors to use Digital Image Correlation (DIC) to measure shear strain fields in the adhesive along the transfer length [4]. The aim of the present work is to complete these first results with the experimental evidence of the normal stress decrease in the aluminium substrate since this decrease reveals the actual relief of the substrate. It is also directly linked to the shear stress peak in adhesive. Temperature variations measured with an infrared camera are used for this purpose. Heat diffusion equation From the principles of thermodynamics, the following heat diffusion equation is obtained for an isotropic material producing thermoelastic heat sources: t T T t T C ii ∂ ∂ − = ∆ − ∂ ∂ σ α λ ρ (1)