Frédéric Lachaud

Experimental analysis of drilling damage in thin carbon/epoxy plate using special drills

Abstract The aim of this study is drilling with a twist drill and a specific cutting tool of structural thin backing plates in carbon/epoxy. Drilling with a twist drill of the bolts’ holes to fix a stiff plate reinforcement in front of the damage leads to defects and damages at the entrance, on the hole wall and at the plate exit. The possibility to manufacture carbon/epoxy with a conventional cutting tool was analysed and the limits of the twist drill were shown. Consequently we defined a specific cutting tool. Series of comparative experiments were carried out using a conventional twist drill and this specific cutting tool. The results showed the capabilities of the specific cutting tool because several defects and damages usually encountered in twist drilled holes were minimised or avoided (entrance damage, roundness and diameter defects and plate exit damage).

Drilling of composite structures

Structural parts made of composites have frequently to be drilled in the aircraft industry. However, little is know about the interacting conditions between the drilling tool and the material, which may be multi-type and multi-size. This study proposes a model which links the axial penetration of the drill bit to the conditions of delamination (crack opening mode I) of the last few plies. Several types of tool/material contact conditions were analyzed and were compared with experimental measurements, and with a model taken from the literature. Our study shows a close correlation between experiment and calculation when the thrust force of the drill is modeled by taking into account the geometrical nature of the contact between the tool and a laminate composite material.

Study into causes of damage to carbon epoxy composite material during the drilling process

There is increasing interest from industry in the machining of composite materials, especially the drilling of thick composite panels and of multi-material assemblies. Damage to laminates due to drilling has an influence on the resistance of plates to the various stresses applied to the structure. The present paper aims firstly to highlight the different defects generated by drilling thick carbon epoxy plates at the entry, on the wall and on the whole exit. Following this, the main aim of this work is the observation in the real time of the initiation and the propagation of such defects. This enables to understand and to show the causes of these defects and then allow operating procedures to be proposed that are likely to reduce such damage.

Damage localization map using electromechanical impedance spectrums and inverse distance weighting interpolation: Experimental validation on thin composite structures

Piezoelectric sensors are widely used for structure health monitoring technique. In particular, electromechanical impedance techniques give simple and low-cost solutions for detecting damage in composite structures. The purpose of the method proposed in this article is to generate a damage localization map based on both indicators computed from electromechanical impedance spectrums and inverse distance weighting interpolation. The weights for the interpolation have a physical sense and are computed according to an exponential law of the measured attenuation of acoustic waves. One of the main advantages of the method, so-called data-driven method, is that only experimental data are used as inputs for our algorithm. It does not rely on any model. The proposed method has been validated on both one-dimensional and two-dimensional composite structures.

Edge impact damage scenario on stiffened composite structure

Low-velocity/low-energy edge impact and quasi-static experiments have been carried out on carbon fiber-reinforced plastic structures. A drop-weight testing machine has been used to impact four different uni-directional laminates at 10, 20 and 35-J impact energy levels. In parallel, a quasi-static study has been carried out to compare its results with the impact ones. The residual behavior will be provided by the compression after impact tests. The impact results show that the static and dynamic behaviors are different. The difference between static and dynamic edge impacts, to understand the impact damage scenario, is explained with the help of an analytical approach irrespective of the stacking or impact energy. This approach provides good results regarding the dynamic and static initial stiffness along with the crushing plateau. It has been observed that the fiber properties control the initial impact stiffness, while in the quasi-static indentation case, the properties of the matrix control the initial indentation stiffness; whereas the crushing plateau is also controlled by the matrix properties.

Modelling Strategies for Simulating Delamination and Matrix Cracking in Composite Laminates

The composite materials are nowadays widely used in aeronautical domain. These materials are subjected to different types of loading that can damage a part of the structure. This diminishes the resistance of the structure to failure. In this paper, matrix cracking and delamination propagation in composite laminates are simulated as a part of damage. Two different computational strategies are developed: (i) a cohesive model (CM) based on the classical continuum mechanics and (ii) a continuous damage material model (CDM) coupling failure modes and damage. Another mixed methodology (MM) is proposed using the continuous damage model for delamination initiation and the cohesive model for 3D crack propagation and mesh openings. A good agreement was obtained when compared simple characterization tests and corresponding simulations.

Analysis of hole wall defects of drilled carbon fiber reinforced polymer laminates

The hole wall defects created during drilling of carbon fiber reinforced polymer laminates are analyzed. First, an analysis of the location of the defects on the wall is performed. It is shown that, using results of orthogonal cutting, it is possible to predict the location of the main defects. Then refined scanning electron microscopic observation shows the different patterns of the defects. These observations raise the question of the quantification and measurement of the quality of holes drilled in composite laminates. Two roughness parameters, Ra and the bearing surface are compared and significant differences are found. This study is a contribution to a better definition of quality indicators for machined surfaces in composite structures, which should help to limit overquality and production costs.

Experimental and Numerical Investigation of Delamination in Curved-Beam Multidirectional Laminated Composite Specimen

Composite corners are generally tested under four point-bending to identify Inter-Laminar Tensile Strength design values. Hereafter several lay-ups have been defined in order to characterize very different damage evolution processes. Damage monitoring has been performed with the help of Digital Image Correlation, Acoustic Emission and Fast Video Recorder. The different processes leading to final failures have been identified. A deterministic continuous composite material model is used to investigate these phenomena. Initiation and evolution up to saturation and fracture are implemented for various damage mechanisms and for delamination. A first comparison between experiments and numerical simulations is presented.

Elasto-plastic analysis of bonded joints with macro-elements

The Finite Element (FE) method could be able to address the stress analysis of bonded joints. Nevertheless, analyses based on FE models are mainly computationally cost expensive and it would be profitable to develop simplified approaches, enabling extensive parametric studies. Firstly, a one-dimensional 1D-bar and 1D-beam simplified models for the bonded joint stress analysis, assuming a linear elastic adhesive material, are presented. These models derive from an approach, inspired by the FE method using a formulation based on a four-node macro-element, which is able to simulate an entire bonded overlap. Moreover, a linear shear stress variation in the adherend thickness is included in the formulation. Secondly, a numerical procedure is then presented to introduce into both models an elasto-plastic adhesive material behavior, while keeping the previous linear elastic formulation. Finally, assuming an elastic perfectly plastic adhesive material behavior, the results produced by simplified models are compared with the results predicted by FE using 1D-bar, plane stress, and three-dimensional (3D) models. Good agreements are shown.

Drilling thick composite materials using large diameter drills

Composite materials are widely used in aeronautical structures. Assembling the various parts involved requires machining operations, especially drilling. When drilling, a number of defects that diminish the structure breaking strength are propagated. Delamination on hole exit is considered to be the main such defect, this being directly related to the drilling axial force. Determining the drilling critical thrust force at delamination is thus crucial. A number of studies have been conducted into this question but are only applicable to small diameter drilling operations cases. This paper proposes an orthotropic analytical model with the aim of calculating the drilling critical thrust force with a large diameter drill. New assumptions are then proposed. The plate element located under the cutter is broken down into a number of zones in relation to the cutter different active parts. A digital model is developed to validate analytical modelling. Punching tests were also conducted to validate the choices of boundary conditions.