Hervé Wargnier

An experimental criterion to detect onset of delamination in real time

This study about the detection of delamination initiation is integrated into a wider project on the definition of a criterion of delamination on composite materials. It must therefore provide experimental data on the behavior of material to allow the formulation of a criterion of edge delamination as accurate as possible. Reliable experimental methods were used to identify the intrinsic parameters of the studied material. One of the most important procedures of this study consists in determining the onset of delamination in real time in order to determine stress level of delamination. Acoustic emission was used for detection of real-time damage initiation, using a location algorithm and specific pattern recognition. After correlation between experimental results and data processing, an acoustic emission signature of a delamination initiation was defined. Almost all experiments can be stopped before failure, in order to characterize damage. Extensive experimental work enabled us to verify the validity, the accuracy and the reliability of real-time delamination acoustic emission criterion.

A progressive intra- and inter-laminar damage model to predict the effect of out-of-plane confinement on pin-bearing behaviour of laminated composites:

In this paper, a progressive intra- and inter-laminar damage model is proposed to study the effect of out-of-plane confinement on the pin-bearing behaviour of laminated composite. The constitutive law for fibre direction is formulated at the ply scale and is related to classical models dealing with matrix and inter-laminar damage. The influence of shear and transverse stresses on fibre damage is represented through a modified damage evolution law. Specific issues resulting from the use of a softening law in a structural analysis are discussed from a physical point of view in order to select a relevant modelling strategy whose limitations are well understood. The experimental procedure is based on an extensive set of tests in order to identify separately most of the material constants. Pin-bearing tests on configurations with and without confinement are used to evaluate the model performances. Results show the model’s ability to predict the damage scenario and the loss of stiffness implied by first damage stages.

Tolerance synthesis of fastened metal-composite joints based on probabilistic and worst-case approaches

Abstract Variabilities in geometrical and material properties occur systematically after manufacturing and joining operations on different parts of an assembly. The behaviour and structural performance of composite/composite or composite/metal fastened joints are particularly sensitive to some of these variabilities. Controlling the effect of variabilities by tolerancing uncertain parameters is therefore crucial to avoid the failure of the structure. However, performing a variability study is generally costly because a large number of configurations need to be evaluated, especially when the behaviour model of the system is a numerical one. This paper presents an approach for tolerance synthesis of uncertain parameters in fastened metal-composite joints. The low time-cost of the approach is ensured by using a reduced finite element model of the joints and a strategy to reduce the number of calculations. Both probabilistic and worst-case approaches for the propagation of uncertainties can be applied through the proposed tolerancing synthesis. An allowable tolerance value for an uncertain parameter can then be easily calculated by identifying an analytical law which links tolerance to a structural performance criterion. The robustness of the proposed approach is illustrated by its application to a 4-bolt metal-composite single-lap joint where several sources of variability are introduced (i.e. hole-location error, pin/hole clearance, fastener preload).