Chapter 1

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Chapter 1 Introduction

CHAPTER 1

Introduction

1.1 Composite Materials

Many of our modern technologies require materials with an unusual combination of properties that cannot be met by the conventional metal alloys, ceramics or polymeric materials. This is especially true for materials that are needed for aerospace, underwater or transportation applications. For example, aircraft engineers are increasingly searching for structural materials that have low densities, are strong, stiff, abrasion and impact resistant and are not easily corroded. This is a rather formidable combination of characteristics. Frequently, strong materials are relatively dense whilst increasing the strength or stiffness generally results in a decrease in impact strength.

Material property combinations and ranges have been, and continue to be, extended by the development of composite materials. Generally speaking, a composite is considered to be any multiphase material that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties is realized. According to this principle of combined action, better property combinations are fashioned by the judicious combination of two or more distinct materials. Property trade-offs are also made for many composites.

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A composite, in the present context, is a multiphase material that is artificially made, as opposed to one that occurs or forms naturally. In addition, the constituent phases must be chemically dissimilar and separated by a distinct interface. Thus, most metallic alloys and many ceramics do not fit this definition because their multiple phases are formed as a consequence of natural phenomena.

In designing composite materials, scientists and engineers have ingeniously combined various metals, ceramics and polymers to produce a new generation of extraordinary materials. Most composites have been created to improve combinations of mechanical characteristics such as stiffness, toughness and ambient and high-temperature strength.

Many composite materials are composed of just two phases; one is termed the

matrix, which is continuous and surrounds the other phase, often called the dispersed phase or reinforcement. The properties of composites are a function of

the properties of the constituent phases, their relative amounts and the geometry of the dispersed phase.

A simple scheme for the classification of composite materials can be used. It consists of three main divisions: particle-reinforced, fiber-reinforced and structural composites. The dispersed phase for particle-reinforced composites is equiaxed, for fiber-reinforced composites the dispersed phase has the geometry of a fiber (i.e., a large length-to-diameter ratio). Structural composites are combinations of composites and homogenous materials [1].

1.2 Laminar Composites

A structural composite is normally composed of both homogeneous and composite materials, the properties of which depend not only on the properties of

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the constituent materials but also on the geometrical design of the various structural elements. Laminar composites and sandwich panels are two of the most common structural composites.

A laminar composite is composed of two-dimensional sheets or panels that have a preferred high-strength direction such as that found in wood and continuous, aligned fiber-reinforced plastics. These layers can be stacked such that the orientation of the high-strength direction varies in each successive layer. Thus a laminar composite can have a relatively high strength in a number of directions in the two-dimensional plane. However, the strength in any given direction is, of course, lower than it would be if all the fibers were oriented in that direction [1].

1.3 Carbon

Fibre

Reinforced

Polymer

(CFRP)

Composites

The use of fibre reinforced polymer composites is encountered in numerous applications. Carbon fibre reinforced polymer composites are popular with the most demanding of them, due to their high in-plane stiffness to weight ratio, high strength, high energy absorption capacity, chemical resistance, thermal and low wear properties, excellent fatigue characteristics and their complex forming potential. Despite the wide range of manufacturing techniques now available for composites, most carbon fibre laminate applications make use of high-grade pre-preg in order to achieve the highest possible fibre volume content. Most composite material applications are sized in terms of stiffness and strength, both directly related to the fibre volume content [2].

Composite materials and in particular CFRP are highly orthotropic. Their in-plane shear strength and especially their out-of-in-plane shear strength are only a

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fraction or their in-plane principal strengths. The low out-of-plane strength is due to the lack of fibre reinforcement in the thickness direction. The properties of the matrix dominate the response of the composite in the case of out-of-plane loading, as well as in some in-plane loading cases. The importance and role of the matrix in composite materials is well documented [3]. In particular, delamination toughness (or the critical strain energy release rate ‘G’) is a matrix dominated property, which in turn governs the response to out-of-plane loading.

1.4 Impact Damage: Delamination of Composites

The primary weakness of laminated composite materials is their low strength in the through-the-thickness direction, making interlaminar delamination a major concern in the application of these materials in damage critical structures [4].

Impact, depending on the speed of the projectile and the peak load generated, can result in either penetration of the laminate or in delamination. Penetration occurs for high speed impacts and is related to matrix density and modulus. Lower speed impact produces delaminations which are hidden inside the laminate. This kind of damage is usually referred to as barely visible impact damage (BVID). The delaminated area is a function of the impact energy and the laminate stiffness and is thus dependant on the laminate thickness and fibre properties. The location for delamination initiation is typically where the shear stress is maximum, namely at the centre of the laminate in the thickness direction [5].

Delamination in composite parts on aircraft are commonly caused by low energy impacts, such as a dropped tool on the wing skins, debris projected from the runway during take off and landing or vehicle impact on loading bay doors [6].

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1.5 Through-the-Thickness

Reinforcement

(TTR):

Z-Pinning

Different attempts at overcoming the problem of interlaminar delamination have included toughening of the resin matrix, control of the fibre-resin interface, resin interleaving and, more recently, through-the-thickness reinforcement (TTR).

TTR such as textile stitches, fibre ‘tufts’ or rods bridges delamination cracks and shields the highly stress concentrated tips from the applied load thus reducing the crack diving force. Depending on the structure geometry and on the applied loading mode, delamination crack growth can be rendered stable. This consideration of stabilisation of the failure process is critical in the design of damage tolerant composite structures. The ultimate strength can be increased and the notch and impact sensitivities reduced [6].

Z-fibre pinning represents an entirely new form of TTR. Thin, rigid, carbon fibre rods called Z-Fibres® are inserted orthogonally to the plane of the composite plies during the manufacturing process, before the resin matrix is cured, effectively pinning the individual layers together [4].

1.6 Thesis Topic

Assembly design of a structural component of a commercial aircraft such as a stabiliser or an empennage requires the presence of bonded and, more significantly, bolted joints. Frequent servicing and loading of aircraft also results in frequent, low energy impact of the structure resulting in delamination damage. In fact, airframe structures are subjected to all kinds of out-of-plane loads during service and are very likely to be damaged.

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Improving the delamination resistance of CFRP is of high importance, in order to exploit fully the remaining advantageous material properties.

Through-the-thickness reinforcement of polymer matrix composites by the incorporation of rigid Z-pins has been proven to be effective in limiting delamination crack growth under quasi-static, fatigue and impact loading conditions in laboratory coupons [4]. The magnitude of the effect is highly dependent on the geometry of the coupon (particularly the thickness) and on the nature of the fibre architecture (i.e. UD or fabric). For any given sample geometry, the significant increase in the delamination resistance is achieved at the cost of a certain reduction in the in-plane stiffness and strength. However, the occurrence of alternative failure modes/locations in such highly reinforced structures is inevitable and needs to be considered in the design stages.

Thin walled, patch joined I-sections were selected as a representative aerospace structural element, to investigate the effects of localised through-the-thickness reinforcement on its ultimate load carrying capability. Tensile loading of the I-section was selected so as to mimic their loading on the outside of a curved stiffened panel, such as an aircraft nacelle. This has the effect of placing the Z-pinned patch under shear loading. Some of the I-sections were impacted on the patch with a round tip impactor prior to being tensile tested so as to mimic an impact induced delamination damage.

The aim of the study was to monitor the effect Z-pin reinforcement has on the strength and on the failure mode of these joints.