Measurement of in-plane residual stresses in an AS4/8552 composite laminate using the deep-hole drilling method

The deep-hole drilling (DHD) method is a residual stress measuring technique commonly used in isotropic materials. This paper provides an investigation into using DHD to determine residual stress fields in orthotropic laminated composite materials. In this method, a reference hole with a small diameter is drilled through a component that has residual stresses. The diameter of the hole is carefully measured using an air probe as a function of depth and angular position inside the hole. The residual stresses are then released by trepanning a core of larger diameter from around the hole. The diameter of the hole is afterwards re-measured at the same angular positions and depths as in the original measurements. Changes in the shape of the hole are related to the residual stresses that were present before the hole was drilled. For orthotropic materials, the calculation of residual stresses requires the evaluation of distortion coefficients which rely on the mechanical properties of the components. In this work, the finite element method is used to determine these coefficients. Using this technique, the in-plane residual stresses in an AS4/8552 composite laminate are experimentally measured and compared to finite element predictions as well as to classical lamination theory. It was determined that when using DHD in laminated materials the ratio between the thickness of the layers and the reference hole and trepan diameter needs to be sufficiently high, otherwise remaining interlaminar shear stresses in the trepanned core leads to inaccurate measurements.