Robert Piquet

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.

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.

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.

Modelling Strategies for Predicting the Residual Strength of Impacted Composite Aircraft Fuselages

Aeronautic Certification rules established for the metallic materials are not convenient for the composite structures concerning the resistance against impact. The computer-based design is a new methodology that is thought about to replace the experimental tests. It becomes necessary for numerical methods to be robust and predictive for impact. Three questions are addressed in this study: (i) can a numerical model be “mechanically intrinsic” to predict damage after impact, (ii) can this model be the same for a lab sample and a large structure, and (iii) can the numerical model be predictive enough to predict the Compression After Impact (CAI)? Three different computational strategies are used and compared: a Cohesive Model (CM), a Continuous Damage Model (CDM) coupling failure modes and damage, and a Mixed Methodology (MM) using the CDM for delamination initiation and the CM for cracks propagation. The first attempts to use the Smooth Particle Hydrodynamics method are presented. Finally, impact on a fuselage is modelled and a numerical two-stage strategy is developed to predict the CAI.

Finite Element Model Matching Based on Optical Measurement Fields on Single Shear Lap Joint

This paper presents a study of Finite Element Model Matching, based on the use of displacement field measurements. The wealth of information provided by full-field measurement techniques is extremely useful in experimental mechanics, because it is without contact. This study is a three-dimensional mechanical problem. An investigation of the secondary bending on aeronautical structures is presented, especially on the single shear lap joint. First we introduce the optical method and the results of field measurements applied to a tensile test on single-shear lap joint specimen. The experimental technique to determine the displacement field is presented. Afterwards we outline the use of Finite Element Modelling to achieve a correlation between numerical fields and their experimental counterparts. This study is applied to one type of joint; numerical and experimental example is presented: metal/metal lap joints, with one fastener (LGPL type). We describe the matching process. Finally, a parametric study is presented to show the effect of fastener preload clamping and fit clearance between plate hole and shank fastener on the secondary bending.