Damage evolution of novel 3D textile-reinforced composites under fatigue loading conditions

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Abstract

For the simulation of the material degradation process during multiaxial fatigue loading of 3D textile-reinforced composites a new physically based damage model is developed based on the fracture mode concept (FMC) of CUNTZE and the continuum damage mechanics. For the damage analysis and the model parameter identification cyclic tests under superposed tension/compression–torque loading are performed.

Introduction

The novel class of 3D textile-reinforced polymers with multi-layer knitted fabric reinforcement is characterised by its straight fibre arrangement. These textile composites are virtually predestined for applications in lightweight structures under cyclic loadings in automotive engineering as well as general machine building due to their high specific strength and stiffness, their low tendency toward delamination as well as their selectively adjustable damping characteristics [1]. However, for the development of structural concepts there is a lack of well-founded knowledge of the fatigue behaviour of 3D textile-reinforced composites under superposed load conditions.

In contrast to general engineering materials fibre-reinforced composites show a significant degradation of the stiffness and strength during cyclic loading. For the prediction of the material degradation and life many models have been developed [2], [3], [4]. The existing fatigue damage theories for fibre- and textile-reinforced composites can be categorised into:

  • General damage accumulation theories.

  • Stiffness and strength degradation theories.

  • Progressive damage models with damage initiation and evolution.

General damage accumulation theories such as the Miner rule have been investigated extensively for the application in fibre-reinforced materials. Many investigators found the miner rule insufficient for the description of the damage behaviour of fibre composites under cyclic loading [2], [4], [5], [6].

The group of stiffness and strength degradation theories is based on the phenomenological description of the macroscopic material degradation with the help of stiffness and strength degradation functions. Shokrieh developed a damage model for multi-layered carbon fibre-reinforced epoxy based on the stiffness and strength degradation of the single layers [2], [5]. With the modified failure criterion of Hashin the lifetime of the composite can be calculated for various stress ratios and stacking sequences. The model however does not take into account the difference in the damage behaviour of separately tested single layers and embedded layers in multi-layered or textile-reinforced composites.

In contrast to the stiffness and strength degradation theories, the new group of progressive damage models is based upon a multiscale damage evolution approach formulated with the help of the continuum damage mechanics. For the evaluation of the macroscopic stiffness and strength degradation the development and growth of damage is modelled on the meso- and microscale [3], [7], [8]. The progressive damage model recently presented by Paepegem is based on anisotropic damage evolution functions with separated terms for the damage initiation, growth and the final progressive damage evolution. The damage evolution is formulated by means of the material effort, given by the global failure criterion according to Tsai/Wu. The experimental verification have been done by displacement controlled cantilever bending fatigue experiments. Most of the investigators used standardised strip specimens for the characterisation of the degradation behaviour of fibre-reinforced materials [2], [4], [5], [7], [9], [10], [11], [12]. The evaluation of the damage behaviour under multiaxial states of stresses is usually done with the help of off-axis tests on flat specimens. Recently fibre-reinforced tube specimens are used in tension/compression–torsion fatigue tests for the characterisation under multiaxial states of stresses [13], [14].

The experimental and theoretical investigations of this study are focused on constant amplitude tests with pulsating load – selectively allocated to the respective type of fracture. The fatigue strength tests under uniaxial and defined superposed stress conditions are carried out on textile-reinforced tube specimens on a servo-hydraulic tension/compression–torsion (T/C–T) test machine. The determined fatigue strength, residual strength as well as dynamic stiffness values are used to develop corresponding design criteria and strength diagrams and provide an initial tool for the design of 3D textile-reinforced polymer components under cyclic loads.

Section snippets

Textile reinforcement and specimen definition

The focus of experimental studies is directed towards novel 3D textile-reinforcements, which are developed at the TU Dresden. These glass fibre multi-layer knits (GF-MLG) exhibit high levels of stiffness and strength, because the load-bearing warp and weft threads are in straight orientation without any ondulations. In addition, the glass fibre knitting loop threads that secure the fibre interlock prevent the delamination between the individual layers [15]. Composites with multi-layer knit

Determination of the material degradation under T/C–T load and experimental damage analysis

Extensive constant amplitude tests with uniaxial and in-phase multiaxial load conditions were carried out for the fracture-type related damage analysis of GF-MLG reinforced polymers. On the basis of the characteristic stiffness drop during cyclic loads, as well as the visually verifiable growth of damage, the evolution of damages and its effect on the degradation of material parameters is analysed. Corresponding to usual quasistatic test methods with continuous loading, cyclic loads without

Damage model for 3D reinforced-textile composites under cyclic loads

Reliable calculation methods for cyclically stressed textile composite structures assume that the development of damages and the damage evolution are modelled realistically. In particular the effect of the damage on mechanical properties, such as residual stiffness and strength, must be modelled correctly. General global fatigue live analysis models, as currently implemented in finite element method (FEM) software, are able to illustrate material degradation and stress relocations only

Summary and outlook

The fatigue strength behaviour and the damage phenomenology of glass fibre 3D textile-reinforced composites under cyclic loads was elaborated in uni- and multiaxial stress tests on tubular specimens. The visual inspection of the material condition and the evaluation of photographic and light microscopic images with the aid of damage type related crack measurement make it possible to quantify the material damages developing under cyclic loads and to gain approaches to the formulation of the

Acknowledgement

The authors gratefully acknowledge the financial support of this research by the Deutsche Forschungsgemeinschaft (DFG) at TU Dresden.

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