Joris Degrieck

Fatigue damage modeling of fibre-reinforced composite materials: Review

This paper presents a review of the major fatigue models and life time prediction methodologies for fibrereinforced polymer composites, subjected to fatigue loadings. In this review, the fatigue models have been classified in three major categories: fatigue life models, which do not take into account the actual degradation mechanisms but use S-N curves or Goodman-type diagrams and introduce some sort of fatigue failure criterion; phenomenological models for residual stiffness/strength; and finally progressive damage models which use one or more damage variables related to measurable manifestations of damage (transverse matrix cracks, delamination size). Although this review does not pretend to be exhaustive, the most important models proposed during the last decades have been included, as well as the relevant equations upon which the respective models are based.

A new coupled approach of residual stiffness and strength for fatigue of fibre-reinforced composites

Abstract During the last decades, fibre-reinforced composites have been established as competitive materials for naval, automotive and aerospace industry. However, the fatigue behaviour of fibre-reinforced composites is so diverse and complex that present knowledge is far from complete. Two commonly used approaches to model fatigue damage are the residual stiffness and the residual strength approach. In this paper, the change in the modulus of elasticity due to fatigue damage is studied for uni-axial bending. The proposed modelling approach is new in two ways: (i) the damage growth rate—a measure for stiffness loss—is expressed by two separate terms representing the initiation and propagation phase of damage respectively, (ii) a static failure criterion is modified to represent the decreasing reserve to ultimate static strength. In that way this coupled approach of residual stiffness and strength is capable of simulating the three stages of stiffness degradation: initial decline, gradual reduction and final failure, as well as the stress redistribution due to the loss of stiffness in the damaged zones. The model has been applied to displacement-controlled bending fatigue experiments of plain woven glass/epoxy specimens.

Strain Measurements of Composite Laminates with Embedded Fibre Bragg Gratings: Criticism and Opportunities for Research

Embedded optical fibre sensors are considered for structural health monitoring purposes in numerous applications. In fibre reinforced plastics, embedded fibre Bragg gratings are found to be one of the most popular and reliable solutions for strain monitoring. Despite of their growing popularity, users should keep in mind their shortcomings, many of which are associated with the embedding process. This review paper starts with an overview of some of the technical issues to be considered when embedding fibre optics in fibrous composite materials. Next, a monitoring scheme is introduced which shows the different steps necessary to relate the output of an embedded FBG to the strain of the structure in which it is embedded. Each step of the process has already been addressed separately in literature without considering the complete cycle, from embedding of the sensor to the internal strain measurement of the structure. This review paper summarizes the work reported in literature and tries to fit it into the big picture of internal strain measurements with embedded fibre Bragg gratings. The last part of the paper focuses on temperature compensation methods which should not be ignored in terms of in-situ measurement of strains with fibre Bragg gratings. Throughout the paper criticism is given where appropriate, which should be regarded as opportunities for future research.

Effects of Load Sequence and Block Loading on the Fatigue Response of Fiber-Reinforced Composites

The vast majority of fatigue loading experiments are constant-amplitude tests, although this type of fatigue loading is hardly present in real in-service fatigue loading conditions. However, due to the expensive and time-consuming nature of variable-amplitude experiments, their effect is often assessed by performing block loading experiments with various low-high and high-low sequences. In this article, the effects of load sequence and block loading on the fatigue damage development in fiber-reinforced polymer composites is investigated. First it is shown that the opinions in the open literature on the damaging effect of low-high and high-low load sequences are divided. Next the effect of block loading on the bending fatigue behavior of composites is experimentally tested and numerically simulated with a newly developed fatigue damage model. Finally, numerical simulations show that the transitions from low to high stress levels are the most damaging, and that the number of transitions and their relative importance in particular determine which block loading sequence (low-high or high-low) is the most devastating.

Transversal Load Sensing With Fiber Bragg Gratings in Microstructured Optical Fibers

We present fiber Bragg grating based transversal load sensing with a highly birefringent microstructured optical fiber. For the bare fiber, the change of the Bragg peak separation under a transverse line load was simulated with a finite-element model and experimentally verified. We also show that microstructured optical fibers with fiber Bragg gratings can be successfully embedded in a carbon fiber reinforced composite material. The linear dependence of the Bragg peak separation to a transversal stress in the composite sample was measured to be 15.3 pm/MPa.

Experimental set-up for and numerical modelling of bending fatigue experiments on plain woven glass/epoxy composites

In general fatigue of fibre-reinforced composite materials is a quite complex phenomenon, and a large research effort is being spent on it today. Due to deficiencies in the current life prediction methodologies for these materials, composite structures are often overdesigned: large factors of safety are adopted and extensive prototype-testing is required to allow for an acceptable life time prediction. This paper presents an investigation of the fatigue performance of plain woven glass/epoxy composite materials and of the numerical modelling of these composites’ behaviour under fatigue. First the experimental setup which has been developed for bending fatigue experiments, is discussed. The materials used are plain woven glass/epoxy specimens in two configurations: [#0o]8 and [#45o]8. Experiments show that these two specimen types, although being made of the same material, have a quite different damage behaviour and that the stiffness degradation follows a different path. Next a numerical model is presented which allows one to describe the degradation behaviour of the composite specimen during its fatigue life. This model has been implemented in a mathematical software package (Mathcad TM ) and proves to be a useful tool to study the fatigue degradation behaviour of composite materials.

The tribological behaviour of paper friction plates for wet clutch application investigated on SAE#II and pin-on-disk test rigs

Abstract The friction behaviour of wet clutches for automatic transmission applications strongly influences the dynamic behaviour of the entire machine or vehicle including the transmission. The wear, but also the friction curve, determines the lifetime of the clutch. The role of wear is obvious. The friction coefficient of the material couple friction plate/separator plate decreases with number of engagement cycles. As a result, the possible torque of the transmission decreases with time. Under a certain threshold, the clutch has to be revised. But because manufacturers tend to oversize their clutches the decrease in friction coefficient does not yield a limitation to the lifetime of the clutch. In this paper, both SAE#II and pin-on-disk tests are used to investigate the wear and friction characteristics of the friction material. Although the pin-on-disk tests fail in correctly reproducing the wear rate of the SAE#II tests, they can be used for a qualitative analysis of the influence of material parameters and operating conditions on both friction coefficients and wear rate.

Monitoring of fibre reinforced composites with embedded optical fibre Bragg sensors, with application to filament wound pressure vessels

Tests carried out on bare optical fibres with a Bragg sensor show the feasibility of using these sensors for strain sensing. They have been embedded into simple composite laminates and have been subjected to static loading in bending tests. The measured strain from the Bragg sensor is perfectly linear with the applied force. Optical fibres with a Bragg sensor have also been embedded into filament wound pressure vessels. Tests carried out on such a pressure vessel include both static and slowly varying load schemes. The Bragg signal is nearly perfectly linear with the applied pressure. The results demonstrate the applicability of Bragg sensors for continuous monitoring of composite materials.

Coupled Residual Stiffness and Strength Model for Fatigue of Fibre-reinforced Composite Materials

The fatigue behaviour of fibre-reinforced composite materials is complex and present knowledge is far from complete. Several classes of models attempt to predict the fatigue life and/or fatigue degradation of fibre-reinforced composites. Two major classes are the residual stiffness models and the residual strength models. This paper presents a phenomenological residual stiffness model which predicts the stiffness degradation as well as final failure of the composite component. The reserve to failure has been evaluated by means of a modified use of the Tsai-Wu static failure criterion. The fatigue damage model has been applied to displacement-controlled bending fatigue experiments of plain woven glass/epoxy specimens. The damage and stress (re)distribution, as well as the force-cycle history have been simulated and compared to experimental results. Due to the consistent integration of continuum damage mechanics and the residual stiffness approach, the implementation of the fatigue model in a commercial finite element code has been possible, which allows for an accurate simulation of the successive damage states during fatigue life.

Application of digital phase-shift shadow Moiré to micro deformation measurements of curved surfaces

The shortcomings of conventional shadow Moire topography have in the past been improved by means of the phase-shift method which enhances the sensitivity and allows to process the fringe patterns automatically. This paper presents a digital implementation of the phase-shifting process, which requires only one image to be taken. The grating lines, projected onto the deformed object surface, are captured directly with a digital camera. Next the reference grating is superimposed numerically onto the projected grating lines. Then a number of phase-shifts are performed taking into account the non-linearities in the expression for the height-dependent intensity field. Experimental results prove that these non-linearities can considerably affect the micro deformation measurements of curved surfaces. The proposed method is very efficient and eliminates all causes of erroneous measurements due to the miscalibration of phase-stepping devices.