Study of post-impact behaviour of thin carbon/epoxy laminates subject to fatigue tensile loading

Highlights

• Post-impact damage propagation in tension in woven carbon/epoxy laminates is studied.

• The monitoring of damages is based on Digital Image Correlation and RX Tomography.

• The propagation of cracks is governed by the resin damages and by delamination.

• A FEM based on Semi-Continuous strategy is developed to represent these propagations.

• This new modelling strategy well represent the damage initiation and propagation.

Abstract

This article concerns the experimental and numerical study of post-impact damage propagation in thin carbon/epoxy woven composite laminates loaded in fatigue tension. Low velocity normal drop weight impact tests are first performed. Post-impact fatigue tensile tests are then carried out. They are controlled in displacement. The monitoring is based on Digital Image Correlation and RX tomography. The influence of the impact energy and the tensile fatigue loading on the post-impact damage propagation is studied. The damage propagation is governed by matrix damage, with the emergence of tows/resin splittings and intra-tows crackings as well as delamination when the plies have different orientations. When the impact energy or the displacement level increases, the post-impact damage initiates sooner and propagate faster. In some cases, that can lead to a quasi-instantaneous failure identical to that observed for quasi-static tensile loading. The FEM based semi-continuous approach, initialy developped for the modelling of impact damage, is extended to fatigue loading for carbon/epoxy woven laminates. Fatigue damage laws, based on experimental observations are implemented. The modelling well correlates the experimental results in terms of damage propagation scenario and speed depending on the number of cycles for laminates made up of plies with the same orientation.

Introduction

This article deals with the study of the post-impact behaviour of thin carbon/epoxy woven composite laminates under fatigue tensile loading. This paper is dedicated to the experimental and numerical study of samples submitted to a fatigue tensile loading after having been impacted with a low velocity in a drop tower device.

Carbon woven composite structures are widely used, especially in the field of aeronautics. However, these structures are known to be very sensitive to impact loadings. In fact, an impact can generate matrix cracking, delamination and fibre breakage. The impact can perforate the structure or the damages can be barely detectable. In any case, that induces a decrease of the mechanical properties and a loss of the strength. More, the damages can propagate if the structure is loaded after having been impacted, what can lead to the final failure.

For all these reasons, understanding and identifying the post-impact damage propagation phenomenons that occur in a carbon woven composite structures loaded with a fatigue tensile loading is important.

In general, fatigue studies on composite structures are conducted at a macroscopic scale in order to obtain the residual mechanical properties of the laminate such as the residual strength [1], [2], [3], [4], [5], the residual strain or the residual stiffness [6]. Other authors try to characterize the fatigue behaviour of the material through the establishment of a Wöhler curve [5]. The visualization of the damage propagation is sometimes performed at a microscopic scale.

Existing studies concerns mainly compression-compression loading [7] or tension–compression loading [8], [9], [10] rather than tension–tension loading. Indeed, the delamination that can be generated by impact can provoke local buckling when loaded in compression that could lead to an unstable propagation of the damage [11]. For instance, Melin et al. [8] concluded that, during tension–compression fatigue loading on impacted carbon/epoxy samples, the compressive part of the cycle had a more important role in failure than the tensile part. More, Beheshty et al. [7] noticed that the effects of impact damage on carbon/epoxy laminates were more severe for compression-compression fatigue loading than for tension–compression loading.

However, few studies concern the influence of an impact on the fatigue performances of a carbon woven composite under tensile loading. Cantwell et al. [1] have shown that substituting UD carbon plies by woven plies in a laminate could improve the fatigue behavior of the composite structure and Ding et al. [2] have shown that the increase of the impact energy lead to a decrease of the residual strength of the laminate. More, the residual stiffness decreases with the impact energy [6]. It decreases in three steps: an initial fast decrease, a stable state which indicates the beginning of the transverse cracking and a final fall just before the failure.

Concerning the monitoring of the fatigue tests, various techniques are used in the literature to follow the damage during the fatigue test. For example, digital image correlation (DIC) [12], [13], [14], [15], acoustic emission [16] or self-heating methodology [17]. Ambu et al. [12] monitored their tension–tension fatigue tests on CFRP samples with DIC. They found that DIC was a useful to identify the roles and the influence of each failure modes on the residual properties of notched samples, the mechanisms of damage development and the strain distribution around a hole. More, De Rosa et al. [16] made a correlation between the amplitude of acoustic emission and the type of damage occurring in the laminate. Gornet et al. [17] made also a link between heating effects and damage mechanisms during fatigue loading.

Concerning the modelling of post-impact damage propagation in fatigue, the literature on the subject is barely existing. Most of the studies have been conducted on laminates with an initial defect such as notches, holes or delamination. Continuum damage modelling (CDM) is often used to model resin damaging [18], [19], [20], [21]. The principle is to degrade the stiffness matrix by one or several damage variables, calculated through thermodynamical forces.

Some authors use empirical criterions, introduced by Whitney and Nuismer [22], such as the point stress criterion (PSC) and the average stress criterion (ASC). Miot [18] used the ASC method for holed and notched carbon laminates. The mean stress is calculated into a characteristic volume in order to evaluate the damage propagation. But, even if the implementation of these criterions is simple, they are very dependant on the testings parameters.

Concerning the delamination modelling, cohesive elements, introduced by Hilleborg et al. [23], are widely used for impact loading. Robinson et al. [24] used it for the modelling of delamination propagation in fatigue.

These elements are also used in some studies to model the matrix crackings propagation. For example, Hallett et al. [25] used them for a tensile simulation on a holed sample. For this purpose, they have first noted the position of the crackings after an experimental test. Then, they meshed the sample placing the cohesive elements on potential paths of the crack propagation.

This paper focuses on the post-impact damage propagation in three thin carbon/epoxy woven composite laminates. In a first time, the objective is to investigate the propagation phenomenons specific to carbon/epoxy thin woven laminates. Low velocity impacts are first performed with a drop weight device. Then fatigue tensile tests are carried out on the impacted samples. Impact energies and levels of loading are varied. An analysis of the damage propagation scenario is based on measures from Digital Image Correlation and RX tomography. In a second time, the semi-continuous approach, already validated for post-impact quasi-static tensile loading on impacted carbon/epoxy thin woven laminates [26], is extended to fatigue loading. The innovative idea is to use the same model to calculate the damage due to an impact and its propagation. To do so, developments, based on the experimental observations are implemented. A strategy is developed in order to perform fatigue simulation on impacted samples in the explicit finite element code Radioss. Finally, the results of the simulation are compared to the experimental ones.