Fracture Response and Cumulative Damage of E-Glass Fiber Reinforced Composite Under Tensile and Flexural Stresses
CHAPTER ONE
Objectives of Research
The research objectives are as follows:
- To study the homogenous behaviour of fatigue damage in a selected E-glass fibre composite subject to cyclic tensile loading.
- Study the basic nature of damage development, fundamental to the understanding of cyclic degradation and failure behaviour.
- To determine the effects of these micro-cracks experimentally through
- The change in material stiffness
- An empirical fatigue damage growth law in existence
- An experimental program is conducted first to provide information on micro-crack initiation, growth and associated characteristics for subsequent analysis.
CHAPTER TWO
LITERATURE REVIEW
Existing Fatigue information
In general fatigue of fibre-reinforced composite materials is a quite complex phenomenon, and a large research effort is being spent on it today. Fibre-reinforced composites have a rather good rating as regards to life time in fatigue. The same does not apply to the number of cycles to initial damage nor to the evolution of damage. Composite materials are inhomogeneous and anisotropic, and their behaviour is more complicated than that of homogeneous and isotropic materials such as metals. The main reasons for this are the different types of damage that can occur (e.g. fibre fracture, matrix cracking, matrix crazing, fibre buckling, fibre-matrix interface failure, delaminations, etc.), their interactions and their different growth rates. Among the parameters that influence the fatigue performance of composites are: – fibre type, – matrix type, – type of reinforcement structure (unidirectional, mat, fabric, and braiding), – laminate stacking sequence, – environmental conditions (mainly temperature and moisture absorption), – loading conditions (stress ratio R, cycling frequency, etc.) and boundary conditions. As a consequence the microstructural mechanisms of damage accumulation, of which there are several, occur sometimes independently and sometimes interactively, and the predominance of one or other of them may be strongly affected by both material variables and testing conditions. There are a number of differences between the fatigue behaviour of metals and fibre-reinforced composites. In metals the stage of gradual and invisible deterioration spans nearly the complete life time. No significant Author to whom correspondence should be addressed. Reduction of stiffness is observed during the fatigue process. The final stage of the process starts with the formation of small cracks, which are the only form of macroscopically observable damage. Gradual growth and coalescence of these cracks quickly produce a large crack and final failure of the structural component. As the stiffness of a metal remains quasi unaffected, the linear relation between stress and strain remains valid, and the fatigue process can be simulated in most common cases by a linear elastic analysis and linear fracture mechanics. In a fibre-reinforced composite damage starts very early and the extent of the damage zones grows steadily, while the damage type in these zones can change (e.g. small matrix cracks leading to large size delaminations). The gradual deterioration of a fibre-reinforced composite – with a loss of stiffness in the damaged zones – leads to a continuous redistribution of stress and a reduction of stress concentrations inside a structural component. As a consequence an appraisal of the actual state or a prediction of the final state (when and where final failure is to be expected) requires the simulation of the complete path of successive damage states. According to Fong (1982), there are two technical reasons why fatigue damage modelling in general is so difficult and expensive. The first reasons are the several scales where damage mechanisms are present: from atomic level, through the subgrain, grain and specimen levels, to the component and structural levels. The second reason is the impossibility of producing ‘identical’ specimens with well-characterized microstructural features. Fong also draws the attention to some pitfalls of fatigue damage modelling: – confusion over scale: information from measurements on different scale levels, is combined improperly and leads to erroneous results, – false generalization: for example stiffness reduction can often be divided in three regimes: sharp initial reduction – more gradual decrease – final failure (Schulte et al (1985), Daniel and Charewicz (1986)), but the related models are not always valid in the three stages, – oversimplification: curve fitting of experimental data is done by using oversimplified expressions. This last statement was confirmed by Barnard et al (1985). He presented evidence that much of the scatter of the S-N curve drawn from his experimental data was caused by a change in failure mode, generating a discontinuity in the S-N curve. Indeed a Students t -distribution indicated that his test data were falling apart in two distinct and statistically significant populations. The remaining scatter was a consequence of static strength variations. Next, many models have been established for laminates with a particular stacking sequence and particular boundary conditions, under uniaxial cyclic loading with constant amplitude, at one particular frequency,… The extrapolation to real structures with a stacking sequence varying from point to point, and more complex variations of the loads, is very complicated, if not impossible. Indeed some serious difficulties have to be overcome when fatigue life prediction of composite materials under general loading conditions is pursued: – the governing damage mechanism is not the same for all stress level states (Barnard et al (1985), Daniel and Charewicz (1986)). Failure patterns vary with cyclic stress level and even with number of cycles to failure, – the load history is important. When block loading sequences are applied in low-high order or in high-low order, there can be a considerable difference in damage growth (Hwang and Han (1986a)), – most experiments are performed in uniaxial stress conditions (e.g. uniaxial tension/compression), although these stress states are rather exceptional in real structures, – the residual strength and fatigue life of composite laminates have been observed to decrease more rapidly when the loading sequence is repeatedly changed after only a few loading cycles (Farrow (1989)). This so- called ‘cycle-mix effect’ shows that laminates that experience small cycle blocks, have reduced average fatigue lives as compared to laminates that are subjected to large cycle blocks, although the total number of cycles they have been subjected to, is the same for both laminates at the end of the experiment, – the frequency can have a major impact on the fatigue life. Ellyin and Kujawski (1992) investigated the frequency effect on the tensile fatigue performance of glass fibre-reinforced [± 45°] 5S laminates and concluded that there was a considerable influence of test loading frequency. Especially for matrix dominated laminates and loading conditions, frequency becomes important because of the general sensitivity of the matrix to the loading rate and because of the internal heat generation and associated temperature rise. Clearly a lot of research has still to be done in this domain. However several attempts have been made to extend models for uniaxial constant amplitude loading to more general loading conditions, such as block-type and spectrum loading and to take into account the effect of cycling frequency and multiaxial loads.
CHAPTER THREE
EXPERIMENTAL PROCEDURE
Experimental Materials.
The polymer matrix used was a low-viscosity unsaturated polyester resin of the orthophthalic type (Boytek Ltd, Turkey), cured using a cobalt/MEKP curing system (Merck, Germany). The glass reinforcements used were a continuous random fibre mat (CNBM Ltd, China) treated with a high solubility binder. It had a nominal areal weight of 600 g m-² and thickness of 1.60 mm.
E-glass fibres having density of 256 g/cm³ and modulus of 72×35 GPa were used as the reinforcing material in polyester resin composites and N–N dimethyl aniline were used as catalyst, accelerator and promoter, respectively.
Sample preparation
Woven roving and chopped strand mat (see fig. 3.1a and fig. 3.1b).
Samples with fibre rovings were prepared by taking 12 rovings of E-glass and soaked in Polyester resin with the catalyst, accelerator and promoter in 1: 1: 1 ratio to the weight of the resin designated as GFRP 12. The soaked E-glass rovings were then pultruded by hand through a cylindrical glass tube of ~ 300 mm length and 6 mm diameter. The samples were cured for 24 h at room temperature. The glass tube jacket was then broken and separated from the composite samples. Each rod was cut into pieces of 290 mm length for the flexural and bending stresses study.
CHAPTER FOUR
RESULTS
The end result of building a laminate is to have one material property table that can be assigned to elements in the finite element anaylsis (FEA) model. After solving the composite problem, then processing is required to interrogate the results of the analysis in terms of both laminate and ply orientations. The term analysis in engineering typically means the application of an acceptable analytical procedure to a design problem based on established engineering principles. One performs analysis to verify the structural or thermal integrity of a design.Sometimes this can be done using handbook formulas or analytical procedures for simple designs. More often, however, this analysis is being performed using numerical analysis and computers to predict structural or product performance. The predominate type of engineering software used in these analyses is based on the finite element method, and this type of analysis is term finite element analysis (FEA), using matlab software a fundamental understanding of materials of the strength, mechanical behaviour and fatigue resistance of composite materials of E-glass fiber reinforced is gained and developed.
CHAPTER FIVE
DISCUSSION
Microscopic fatigue damage.
The microscopic cracks in the composite possess strong directionality relative to fatigue loading. In general, in a matrix-dominant area with sparsely dispersed fibres, almost all micro-cracks were formed nearly normal to the loading direction. In a fibre-dominant region with fibres oriented parallel to the loading, micro-cracks were developed mainly in the matrix, also normal to the loading, but had rather small crack lengths limited by the inter fibre spacing. In a fibre strand oriented with an angle to the loading direction, micro-cracks generally grew along the interface between the fibre and matrix.
Fibre-bundle (or ligament) fracture, which is commonly observed in fatigue of continuous filamentary composites (Mishnaevsky, Jr. 2007), was rarely observed in fibre reinforced composite subjected to longitudinal tensile loading.
CHAPTER SIX
CONCLUSION
A probabilistic treatment of micro-crack developments is conducted. Quantitative information on damage statistics in the form of cumulative distribution of micro-crack length is obtained. Based on the results obtained from this study, the following conclusions may be reached.
- Mechanisms of the fatigue damage involved continuous initiation and growth of various kinds of micro-cracks. The homogenous fatigue damage will be characterized by the cumulative distribution functions of micro-crack length.
- The initiation of a fatigue crack is influenced only by conditions in a small volume near the point of origin, while the propagation is affected by conditions throughout the cross section of the test piece.
- In the first stage, the material undergoes bulk deformation and work hardening; Slip lines which gradually thicken are then formed. The process continues for a while leading to final rupture of the lattice and sub microscopic cracks.
- Many small cracks form in the matrix and in the reinforcement thereby, describing fatigue damage of composites as a “collective failure mode” not as “single failure mode”.
- All the E-glass fiber reinforced composites exhibited significant damage as a result of flexural test. They also exhibit fiber fracture as a result of matrix cracking.
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