CHAPTER ONE
INTRODUCTION
Insufficient knowledge of composite durability and the lack of life prediction methodologies for predicting glass fiber-reinforced composite material durability and damage tolerance are the mitigating factors against the readily acceptance of fiber-reinforced plastics (FRP) in the marine and civil infrastructure. In order to increase the use of composite materials in the infrastructure arena, the nature and effect of the service environment on the durability of glass fiber-reinforced composite materials must be investigated and appropriate methods established for assessing service life. The knowledge of the mechanics and kinetics of glass-polymer system degradation is essential for the formulation of analytical tools for the characterization of fiber- reinforced composites. The absence of a unified theory for the complete characterization of glass fiber-reinforced composites systems is the major challenge facing the composite industry. In addition to lack of predictive tools, the composite industry is also confronted with little data. The data currently available is industry-specific. Most of the data belong to the aerospace and petrochemical industries where years of exposure to composites have resulted in a databank while little data is currently available for the marine and infrastructure sectors. The absence of glass fiber-reinforced data for marine and infrastructure application where longevity is the objective function has been responsible for the slow acceptance of the use composites in the marine and infrastructure arena. This growing interest in the application of composite materials in the infrastructure sector has begun a more rigorous approach in the evaluation of these materials to ensure that they perform within expected hygro-thermal-mechanical environment. The absence of significant data characterizing the long-term durability of glass fiber-reinforced polymeric composites and the absence of adequate established standards for the repair, design, and maintenance of glass fiber-reinforced composite have been the mitigating factors against the introduction of composites to industry. In order to circumvent the restrictions imposed by the absence of long-term data, simulation and other stochastic methods have been envisioned. These simulations provide insights into the long-term response of glass fiber-reinforced composite materials or their constituents to combined environments. The lack of long-term data is not only restricted to the physical response of composite materials in the application environment but also the environmental and chemical synergism responsible for the premature failure of glass fiber-reinforced composite materials. Another salient advantage of a reliable simulation technique is that it allows for the establishment of performance bounds for the material. Performance bounds are essential because even though we may know the mechanics of environ-mechanical degradation and can describe it, we still have no predictive way of assessing the mechanical and environmental loading (severity and sequence) that the application environment will impose. Thus, the path-dependent damage process that the composite experiences will never fully allow one precise assessment of the remaining strength and life.
The failure of glass fibre reinforced composites under single and repeat impact (fatigue) has been of concern to the designer and users of aerospace structures. because their specific properties make them attractive for mobile applications which often experience cyclic loading of the component materials of fibre reinforced plastics, the dominant reinforcement, E-glass, is known to suffer from a loss of strength with time under load due to a stress- corrosion mechanism common in inorganic glasses. Thus, clear understanding of the fatigue behaviour is essential in the proper use of the composite materials.
The concept of “fatigue” was introduced by wohler to classify material degradation or failure which was proportional to the number of cycles of applied load. Recently, the term has been associated with self-similar single crack growth in homogeneous materials, and crack growth rates have become the single most common design approach for dealing with such behavior. The fatigue behavior of the composite materials cannot be described in that way since the complexity of the internal microstructure of those materials introduces a wide range of fatigue damage modes that normally act in concert with one another to produce a collective result.
Specifically, quantitative information on damage evolution and accumulation, and accompanying property degradation is necessary in the analysis, design and life prediction to insure structural integrity and reliability.
1.1 Relevance of Research
The research has shown us that beyond a certain crack length of (2.00mm), many small cracks form in the matrix and in the reinforcement the material is expected to fail catastrophically without warning.
More so, the fatigue damage of composites has described the failure mode of composites as a “collective failure mode” not as “single failure mode”.
1.2 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
(i) The change in material stiffness
(ii) An empirical fatigue damage growth law in existence
4) An experimental program is conducted first to provide information on micro-crack initiation, growth and associated characteristics for subsequent analysis.
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