Mechanics of delamination-free drilling in polymer matrix composite laminates: A review

There is a critical limit of thrust force during drilling operation in a composite laminate below which no delamination takes place, which is known as the “critical thrust force”. In this paper, various critical thrust force models have been discussed in detail for circular- and elliptical-shaped delaminations around the drilled hole. Critical thrust force models for circular-shaped delamination for isotropic composite laminates and anisotropic composite laminates for different loadings like point load, uniformly distributed load, triangular load, and their combinations using different drill geometries have been discussed. With elliptical-shaped delamination, critical thrust force models for anisotropic composite laminates are discussed for: point load, uniformly distributed load, combined point plus distributed load, and point load for antisymmetric composite laminates. At several locations, graphs have been created to illustrate the dependence of various parameters on the critical thrust force. These graphs will be of use for practising engineers while selecting tool or/and machining parameters during drilling of polymer matrix composites. A tool designer can get good insight from these graphs while designing an optimal drilling tool. The paper highlights the importance of load distribution across the drill causing deflection leading to damage due to delamination in composite laminates. Special drill geometries help in better distribution of thrust forces towards periphery and improve the critical thrust force for the prevention of damage due to delamination during drilling in composite laminates.

Analytical and experimental investigation of the delamination during drilling of composite structures with core drill made of diamond grits: X-ray tomography analysis

Among the various forms of material damage, exit-ply delamination has been identified as one of the most deleterious damage processes associated with drilling fibre-reinforced plastics. The thrust force has been cited as the primary cause for drilling-induced exit-ply delamination. Only one analytical model for the prediction of the critical thrust force responsible for delamination using core drills can be found in the literature. In this study, a realistic model to predict critical thrust force responsible for drilling-induced exit-ply delamination in a multi-directional carbon fibre-reinforced plastic laminate with core drill has been proposed. A comparison between the proposed model, literature model as well as the experimental tests conducted during punching tests is presented. The proposed model is found to correlate well with experimental punching tests. In fact, the maximum relative errors recorded between the experimental values of the critical thrust force and the measured values are around 15%. Micro-tomography experiments have also been conducted that capture the drilling-induced damage in multi-directional carbon fibre-reinforced plastics in great detail. The X-ray images highlight the difficulty in controlling the thickness of the uncut plies located under the core drill during punching tests that can be attributed to some deviations in predictions of critical thrust force. Postmortem examination of the blind holes after punching tests also confirms the presence of a net delamination near the vicinity of the nominal diameter of the core drill, which correlates well to the hypothesis of the analytical model.

Critical Thrust Force for On-Set Delamination of Hybrid FRP Composite during Drilling Process

Hole quality is one of the important criteria for hybrid composite components when assessing drilling behaviour because it influences the strength of composite parts post assembly. Nonetheless, some unique characteristics of hybrid Fibre-Reinforced Polymer (FRP) composites make them difficult to obtain the required quality and strict final dimensional accuracy. Based on previous studies, delamination has been recognized as one of the critical failure mechanisms in the drilling operation of FRP composites. It can often be the limiting factor for the final composite materials applications. Thus, in order to achieve a delamination-free in the drilling of hybrid FRP composites, an analytical model and a series of thrust force experiments are endeavoured in this study. The main purpose of the model is to compute the critical thrust force at the on-set of delamination during the drilling process. Results of this analytical study indicated that the delamination damage can be alleviated if the applied thrust force is lower than the critical thrust force value. Importantly, a good agreement was evident between the estimated critical thrust force and the measured thrust force in this particular study.