CHAPTER ONE

INTRODUCTION

1.1 Background of the study

Composite is a material that is made by combining a binding resin with small filaments (strands) of solid materials whose individual constituent’s strength is less than the composite’s strength. Composite materials can best be described as having the strength of metal, the light weight of plastic and the rigidity of ceramics.

Since the discovery and introduction of glass reinforced polyester composite material as a structural engineering material in the early 1950s, it has found wide application in automobile, energy, marine, military, building, electronics, and aerospace engineering and household appliances and has continue to gain popularity as structural engineering material. The use of glass reinforced composite materials by manufacturers according to Wood (1980) was largely because of the mouldability, good strength to weight ratio of the combined constituents and the promise of design freedom by the elimination of expensive panel beating operations in the building of custom and short run work as well as the comparative simplicity of the equipment and tooling required for the production activities. Over the past six decades, glass reinforced polyester composites have gained popularity in engineering applications due to their flexibility in obtaining the desired mechanical and physical properties in combination with its light weight, and several developments has led to its application in structural and load bearing designs. Due to manufacturing flaws on a micro scale, these materials are susceptible to damages and subsequent failure on exposure to impact force. Griffiths’ flaw hypothesis and catastrophic fractures of tankers and cargo ships almost immediately after the Second World War led to the development of fracture mechanic discipline, borne to address the inadequacies that was beyond the scope of conventional design criteria in providing answer to reason why materials fail at nominal stress level. Fracture mechanics is based on the principle that all engineering materials contains initial defects in the form of cracks, debonding in composites, voids, inclusions, resin starved regions which can affect the load carrying capacity of engineering structures.

During the service life of reinforced polyester composite, they are exposed to varying intensity of loading and environmental conditions, as in the presence of oxidation and harsh acidic or alkaline environments which have seriously limited the use of engineering materials in various capacities as stated in (Edelugo, 2009) and these could also constitute a threat to their load carrying capacity which sometimes results in catastrophic failure if defects are not noticed and monitored. According to Gdoutos (2005), these materials are heterogeneous and have several types of inherent flaws. Failure of fibre composites is generally preceded by an accumulation of different types of internal damage. Failure mechanisms on the micromechanical scale include fibre breaking, matrix cracking, and interface debonding. They vary with type of loading and are intimately related to the properties of the constituents, i.e., fibre, matrix and interface/ interphace. While the above failure mechanisms are common in most composites, their sequence and interaction depend on the type of the loading and the properties of the constituents. The damage is generally well distributed throughout the composite and progresses with an increasingly applied load. It coalesces to form a macroscopic fracture shortly before catastrophic failure. Study of the progressive degradation of the material as a consequence of growth and coalescence of internal damage is of utmost importance for the understanding of failure.

Fracture mechanics development according to Gdoutos was strengthened due to two key factors: the size effect and the inadequacy of traditional failure criteria. It was discovered that the traditional failure criteria were inadequate because they could not explain failures which occurs at a nominal stress levels considerably lower than the ultimate strength of the material. A major objective of fracture mechanics, as applied to engineering design, is the determination of the critical load by accounting for the size and location of initial defects. Thus, the problem of initiation, growth and arrest of cracks play a major role in the understanding of the mechanism of failure of structural components. Three ways in which defects can appear in a structure are: first, they could exist in a material due to its composition, as second phase particles, debonds in composites, e.t.c.; second, they can be introduced into the structure during fabrication, and third, they can be created during the service life of a component, like fatigue cracks, environment assisted or creep cracks. To components made from such material, fracture is the primary threat to their integrity, safety and performance as it applies to nearly all mechanical structures highly stressed (Foreman and Beek, 2006).According to Reed (2004) the need for such assessment in the design of polymeric materials and of items fabricated from plastic and polymer composite materials arises from the increased use of these materials in critical load bearing situations.

1.2Why Reinforced Composites are Prone to Impact Damage?

Impact strength can be said to be the ability of a material to resist high-rate loading. During an impact, the energy absorbed by a material is the objective parameter to be determined, resulting to fracture of the material at high velocities of varied magnitudes. Impact response study of materials is important because, impact resistance of a material is the most important properties for a designer to consider and without question, the most difficult to quantify. The impact resistance of a part is, in many applications, a critical measure of service life. More importantly these days, it involves the perplexing problem of product safety, liability and reliability. The energy absorbed during impact is illustrated graphically below;

Figure 1.1: Load – Displacement diagram

Figure 1.2: Principal types of load-displacement plots obtained during fracture test.

Source: Adapted from Gdoutos(2005)

Mathematically,

The energy absorbed is;

• A measure of material strength and ductility.
• Graphically, the area beneath the Stress – Strain curve.

In real world situations, impacts in materials are biaxial rather than unidirectional. Brittle materials take little energy to start a crack, little more to propagate it to a shattering climax. In highly ductile materials, failure starts by puncture in a drop weight testing and this requires a high energy load to initiate and propagate the crack.

According to Reid and Zhou (2004), the lack of plastic deformation in composites means that once a certain stress level is exceeded permanent damage, resulting in local or structural weakening, occurs. Unlike a metal, which may undergo plastic deformation but can retain its integrity (e.g. water tightness), composites stressed above a certain level, though possibly retaining some structural properties, are permanently damaged. A blow with energy of ~1 J or less at ~2ms-1 can cause irreversible damage in a realistic composite laminate. To summarize, the reasons for low impact strength are:

1. Low transverse and interlaminar shear strength.
2. Laminar construction, which is required if the reinforcing fibres are to be used efficiently and anisotropy reduced.
3. No plastic deformation.
4. High stress concentration in the neighbourhood of pre-existent defects in the form of micro cracks, manufacturing flaws, scratches are responsible for low impact and fracture strength.

1.3 Failure Mechanisms in Composite Materials Exposed to Sudden Impact Force

Damage and subsequent fracture of composites exposed to sudden impact force begins with a deflection, matrix cracking, bending tensile and compressive stresses build-up, experienced by the reinforcement and matrix just below and above the neutral axis respectively. The sudden impact loading time is of the order  to where T is the period (Agwibilo, 2010).  The impact force adversely affect the internal structure in a wave-like manner starting from the contact point of the impactor, due to a small plastic zone developed around the point of impact and hence, propagation of failure of the reinforcement and finally the failure of the lamina in the presence of micro cracks, developed within the impacted zone and beyond. Under the influence of sudden impact force, the kinetic energy of the impactor goes into the damage process as described above and hence, making the response of the material inelastic as a result of the static deflection and other nonlinear effects near the crack tip which precedes the fracture.

The principal mechanisms of failure in fibre-reinforced composites are:

• Matrix cracking
• Fibre matrix debonding – the separation of fibres from the matrix
• Delamination – the failure mode in which cracks propagate between the layers (lamina) of the composite
• Fibre fracture – failure of the reinforcement and normally occurs across the diameter of the fibre
• Fibre pull-out, ply gaps
• Resin rich/resin starved areas
• Fibre waviness, wrinkles, miscollimation
• Inclusions, contamination, foreign particles
• Improper stacking sequence
• Dents, scratches.
• Micro bulking

1.4 Failure Criteria of Composite Materials

Assuming an extended through thickness plane crack on the edge of a plate, let the crack plane occupy the plane xz and the crack front be parallel to the z-axis. Place the origin of the system Oxyz at the midpoint of the crack front. There are three independent kinematic movements of the upper and lower crack surfaces with respect to each other. These three basic modes of deformation are illustrated in Figure 1.3, which presents the displacements of the crack surfaces of a local element containing the crack front. Any deformation of the crack surfaces can be viewed as a superposition of these basic deformation modes as generalised in fracture mechanics analysis, which are defined as follows:

1. Crack Opening modeI: The crack surfaces separate symmetrically with respect to the planes xy and xz.
2. In-plane Sliding mode II: The crack surfaces slide relative to each other symmetrically with respect to the plane xy and skew-symmetrically with respect to the plane xz.
3. Tearing mode III: The crack surfaces slide relative to each other skew symmetrically with respect to both planes xy and xz.

Figure 1.3: The three basic modes of crack extension

Source: Adapter from Gdoutos (2005)

In the analysis of impact damage and fracture of composites, the review of the sources, nature and curvature of impactor and how these fracture initiation takes place, propagate from a microstructural scale at the sites of flaws, and how the coalescence of these flaws leads to a visible separation process that manifests on a macrostructural level before resulting in a catastrophic failure are vital.

Below are some applications where reinforced polyester structures are exposed to sudden impact force resulting from projectiles of varying magnitude, and the fashion in which damage on the surface nucleates to failure if inappropriate reinforcements are not selected for optimum performance. From Làszló and George (2003), the mechanical and thermal behaviours of a structure depend on the properties of the fibres and the matrix and on the amount and orientations of the fibres. In the structural analysis of composites, the design steps from the micromechanics (which takes into account the fibre and matrix properties) through macromechanics (which treats the properties of the composite) are taken into consideration.

Figure 1.4: Reinforced composite automobile parts and region that are exposed to impact forces resulting from collision or fallen objects

Figure 1.5: Reinforced composite crash helmet and regions that are exposed to impact force as result of collision with hard objects or crash

Figure 1.6: Reinforced composite shield used by riot police or fire fighters

 

Figure 1.7: Fracture and damage mechanics of reinforced composite laminate

Figure 1.8: Boat haul stress state under calm and disturbed sea condition

From the above indications, it is practically essential to address improving the impact performance of reinforced composite by using multidirectional reinforcement. For instance, the boat haul under constant bending stress resulting from the disturbance of sea waves, needs reinforcement that can arrest crack growth in the eventuality of the tendency for crack to propagate, since stress is usually at the highest level at the crack tip in the region of material defects.

1.5 Statement of the Problem

It has been observed that typical events such as cracking of coating, matrix, fibre exposure and breaks, and instantaneous fibre/matrix debonding in reinforced polyester composite materials, are influencing conditions which dominate crack growth under loading conditions. These conditions thereby pose significant threat to the fracture and impact performance of these structures in service. And also, fracture events in composites are associated with finite increase in fracture area which leads to eventual catastrophic failure.These failures are imminently progressive due to the absence of sufficient and effective reinforcement and monitoring, which could prevent crack propagation through fibre bridging and crack arrest mechanism.

1.6 Aim and Objectives of the Research

In order to analyse the fracture mechanism of reinforced polyester composite when subject to suddenly applied force, focus will be on parameters that are sufficient to predict the strength on impact conditions, obtainable in structural applications of the material. These include;

1. To maximise the advantage of good strength-to-weight ratio of reinforced polyester composite by combination of reinforcements.
2. The determination of the mode-I stress intensity factor, K_I due to crack mouth opening.
3. The determination of the mode-II stress intensity factor, K_II due to in-plane shearing.
4. The determination of the fracture toughness, K_ICand K_IIC.
5. The determination of the critical stress, σ_cand shear stress, τ_ntrequired to extend a crack of specified size and determine whether the crack extension is stable or unstable.
6. The determination of the strain energy, E and impact strength, U of the laminates.
7. The determination of reinforcement combination with the highest fracture toughness and impact resistance.
8. The determination of the volume fraction, V_f of each laminate.

1.7 Significance of the Research

The experimental fracture mechanics reality and analysis for reinforced composites material is necessary, since crack growth is often not observable at the structural level of application; all that could be observed is the occurrence of sudden fracture events. Thus, through this research, designers can respond to the question of the possibility of preventing fracture and low impact performance of reinforced polyester composite, by addressing the condition for safe design of polyester composite structures from two perspectives, namely; the determination of the safe operating load for specific combinations of reinforcement present within the structure; or determine the reinforcement combinations that could be used in a structure, given the operating load. Thereby, providing a window for the inspection and monitor of the stress state and crack growth that may like result in a catastrophic damage and/or failure. The stress intensity factor, K-curves obtained can be practically useful in the following ways:

1. Data will be useful in directly choosing between when a reinforced composite material should be or not be used, or the loading limits they could be subjected to, for a particular application in high strength automobile material selection.
2. Fracture mechanics data will be available for the combination of glass reinforcement in polyester composite subjected to fracture and impact loading.
3. The contribution of fibre volume fraction to the response of reinforced composite, when exposed to sudden impact force will be known with respect fracture toughness and impact damage.

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