Extension of the Deep-Hole Drilling Method for the Measurement of in-Plane Residual Stresses in Composite Laminates

Deep-Hole Drilling (DHD) is a residual stress measurement method generally used in isotropic materials. This study provides an evaluation into applying DHD to determine residual and fit-up stress fields in anisotropic composite materials. In this method, a reference hole with a small diameter is firstly drilled through the thickness of the component. The diameter of the hole is measured accurately and then a cylindrical core of material around the hole is trepanned from the component, relaxing the residual stresses in the core. Finally, the diameter of the reference hole is re-measured and the change in diameter used to calculate the residual stress. For anisotropic materials, the calculation of residual stresses requires the evaluation of distortion coefficients which rely on the mechanical properties of the components. In this work, Lekhnitskii’solution for the distortion of a circular hole in an anisotropic plate and finite element analysis are used to determine these coefficients. Using this technique, the in-plane residual stresses in AS4/8552 composite laminates were experimentally measured and compared to finite element predictions as well as to classical lamination theory. The obtained results indicate that, when using DHD in laminated materials, an optimal ratio between the layers’ thickness, the reference hole and trepan diameters is of significant importance since unrelaxed thermally induced residual stresses can lead to inaccurate measurements. Introduction Deep-hole drilling (DHD) is a technique that allows the measurement of residual stress fields through very thick components. This technique requires that a small diameter hole is drilled through a component that contains residual stresses. The hole diameter is accurately measured (generally using an air probe) as a function of both depth and angular position inside the hole. The residual stresses in the component are afterwards released by machining away a core of larger diameter from around the hole. The relaxation of residual stress distorts the shape of the reference hole. The reference hole diameter is subsequently re-measured at the same angular positions and depths. The reference hole distortion is lastly related to the residual stresses that existed in the component before drilling the hole. The main assumption of this method is that the drilling of the reference hole has no effect on the residual stress state and that removing the core causes the residual stresses around the hole to be fully released in an elastic manner [1]. The core is additionally assumed to consist of independent lengths. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 377-382 doi: http://dx.doi.org/10.21741/9781945291173-64 378 This indicates that a thick specimen is equivalent to a set of layers unconnected by through-thickness shear stresses [2]. The deep hole drilling technique has usually been used on metal parts such as in welds [3]–[5]and railway tracks [6]. In metallic components, EDM can be used to trepan a central core around the reference hole since the cutting stresses introduced by this method are not significant. An attempt to apply this technique to a laminated carbon-fibre composite was made by Bateman et al. [2]. In their investigation, EDM could not be used to machine the core around the reference hole because CFRP is not conductive and for this reason a diamond encrusted hole saw was selected instead, as it was for this study as well. Analytical model The DHD technique is based on the measured radial distortion to determine the components of the residual stress from the reference hole. The formulation to the case of far field biaxial stresses plus shear stress can be stated as 1 i i i x y xy u f g h E θ θ θ σ σ τ   = + +   (1) where for the case of isotropic materials (measurement angle θ ) 1 2cos 2 1 2cos 2 4sin 2 i