Elsevier

Composite Structures

Volume 134, 15 December 2015, Pages 302-310
Composite Structures

On the modelling of low to medium velocity impact onto woven composite materials with a 2D semi-continuous approach

https://doi.org/10.1016/j.compstruct.2015.08.067Get rights and content

Abstract

An explicit finite element modelling of the 5-harness satin woven composite material is proposed in this paper. It is based on the semi-continuous approach which consists in separating fibre and matrix mechanical behaviours. The bundles are modelled with rod elements and a specific damageable shell element is used to stabilize this truss structure. As the woven pattern geometry plays a key role in damage initiation and propagation, a modification has been made in the failure strain of the rods located at the crimp regions where warp and weft yarns cross each other.

The method has been implemented into the explicit finite element code RADIOSS and is computationally efficient to model low to medium velocity impact (1–200 m/s) at the structure scale. The modelling strategy is validated by representing a drop weight test and an oblique impact test and provides good prediction of impact force history and damage size and location.

Introduction

This article deals with the modelling of woven composite laminates under impact loading. Indeed, a modelling strategy that accounts for the internal structure of 5-harness satin woven composite plies is investigated.

In the field of aeronautical designing, 5-harness satin carbon/epoxy woven composites are increasingly used for the manufacturing of the structure. A minor weakness in this part can have catastrophic consequences. Thus, understanding and predicting impact damage mechanisms in this material is required.

The behaviour of composite structures under impact loading has been widely studied. Comprehensive reviews by Abrate [1], [2] describe impact damage mechanisms for a large range of composite materials. Typically, cracks in the resin, delamination and fibre breakages can be observed.

Damage mechanisms in composite materials highly depend on its structure. In the specific field of woven laminate composites, the characteristic structure of the plies leads to complex behaviour [3], [4], [5]. Indeed, even for static loadings, the damage first accumulates in the crimp region where the yarns cross each other. Local behaviour has an important influence on the global response, damage initiation and propagation. For impact loading, Nilakantan et al. [6] showed that the local variation of the strength of the yarns can have a major effect on the impact response of woven fabrics.

Three levels of representation can be distinguished for the modelling strategies of impacts on woven composite laminates. The first modellings are build at a macroscopic scale. Customized damageable energy based material laws are used with FEM [7], [8], [9], [10], [11], [12], [13]. In these models, the woven fabric is represented using homogenised shell elements. The second level of representation is the bundle scale. Navarro et al. [14], [15], [16] have developed a semi-continuous modelling where the bundles of fibres are represented with rod elements stabilized with specific damageable shell elements. The idea is to model the fact that when the resin is fully damaged, the woven fabric behaves as a discrete truss structure. This strategy provides a good representation but cannot represent the local damages due to the weaving pattern. The third strategy is at the pattern scale. The models are based on properties calculated from a deforming unit cell. This unit cell is generally very detailed [17], [18], [19], [20]. This strategy is able to represent the local strain and stress fields due to the weaving pattern, but it can hardly be used for large structures.

In this paper, an extension of the semi-continuous approach presented in [15] is described. The modelling is modified in order to take into account the weaving pattern geometry. The properties of the rods are adapted to represent local strain concentrations.

In the first part, the modelling strategy is presented. The damage law used for the specific shell elements and the failure behaviour of the rods are described. Then a parameter sensitivity study is proposed. Finally the strategy is validated by comparing with drop weight normal impact tests and gas gun oblique impact tests.

The presented approach is accurate enough to predict the size and the shape of the damage. The mechanisms observed experimentally leading to the final rupture are well represented.

Section snippets

Modelling strategy

The proposed modelling strategy for the woven composite laminate extends previously published models that use a “semi-continuous approach” [14], [15], [16]. In this model, the ply bundles are represented by 1D rod elements connected by nodes to the 4 edges of a quadrilateral shell element (Fig. 1). The element length respects the woven fabric pattern i.e. the distance between two bundles. For delamination modelling, each ply are connected with a shell-to-shell interface element with a cohesive

Implementation

The developed woven element is implemented into the explicit finite element code RADIOSS (Altair Hyperworks) through a 4-nodes user element (SUSER Fortran subroutine). The four nodes are used to model both the shell element and the four rod elements. Therefore, the rod elements do not need an additional meshing work since they are embedded into the user element formulation. Besides, the section of each rod is half the real bundle one since each bundle is represented with two rods (one from each

Validation and discussion

In this section, the ability of the model to predict the failure of composite structures is investigated upon two kind of tests: drop weight normal impact and gas gun oblique impact.

Conclusion

A 2D modelling of the 5-harness satin woven composite material has been presented in this paper. A modelling strategy based on the semi-continuous approach and at the woven pattern mesh scale is used. Bundles of fibres are represented with rod elements and matrix is modelled with a damageable shell element.

In order to be more in agreement with experimental observations, two main improvements have been carried out from the original modelling presented in [15]. First, a work on the shell damage

Acknowledgement

The authors acknowledge the supercomputing centre CALMIP for granting access to the HPC resources under the allocation 2015-P09105.

References (22)

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