Composite Joints and Connections

Principles, Modelling and Testing

P Camanho, Stephen R. Hallett

• Publisher: Woodhead Publishing
• Year: 2011
• Print ISBN: 9781845699901
• eBook ISBN: 9780857094926
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The growing use of composites over metals for structural applications has made a thorough understanding of the behaviour of composite joints in various applications essential for engineers, but has also presented them with a new set of problems. Composite joints and connections addresses these differences and explores the design, modelling and testing of bonded and bolted joints and connections.Part one discusses bolted joints whilst part two examines bonded joints. Chapters review reinforcement techniques and applications for composite bolted and bonded joints and investigate the causes and effects of fatigue and stress on both types of joint in various applications and environments. Topics in part one include metal hybridization, glass-reinforced aluminium (GLARE), hybrid fibre metal laminates (FML), glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) composites. Topics in part two include calculation of strain energy release rates, simulating fracture and fatigue failure using cohesive zone models, marine and aerospace applications, advanced modelling, stress analysis of bonded patches and scarf repairs.Composite joints and connections is a valuable reference for composite manufacturers and composite component fabricators, the aerospace, automotive, shipbuilding and civil engineering industries and for anyone involved in the joining and repair of composite structures. - Explores the design, modelling and testing of bonded and bolted joints and connections - Reviews reinforcement techniques and applications for composite bolted and bonded joints - Investigates the causes and effects of fatigue and stress on bolted and bonded joints in various applications and environments

Part I

Bolted joints

1

Reinforcement of composite bolted joints by means of local metal hybridization

A. Fink, German Aerospace Centre (DLR), Germany

P.P. Camanho, University of Porto, Portugal

Abstract:

The use of a local hybridization with metal represents a suitable and technologically feasible means of increasing the mechanical efficiency of highly loaded composite bolted joints. This technology allows significant improvement of the overall structural efficiency of real composite structures. Contrary to conventional reinforcement concepts with metal, the local laminate hybridization entails the implementation of thin high-strength metal foils within the laminate by means of either ply substitution or ply addition techniques. This chapter addresses some fundamental aspects of the mechanical behaviour of hybrid joints in terms of its reinforcement effect, the damage behaviour and the impact of geometric and material-related parameters.

Key words

hybrid materials

composite bolted joints

bearing strength

local laminate reinforcement

fibre metal laminates

1.1 Introduction

Composite materials and their specific manufacturing technologies allow for the development of highly integral design concepts that minimize the amount of individual structural parts and thereby reduce the need of structural joining. Fundamental advantages associated with reduced structural joining are a reduction in structural weight, assembly and inspection efforts, production times and production costs, and an increase in robustness and structural efficiencies. Despite these potentials, structural joining remains an unavoidable need, which arises from technological limitations for the production of geometrically complex structures and their size. These parts require elaborate tooling and complex manufacturing processes. In addition, there are specific inspection and repair requirements as well as associated restrictions in terms of logistics, process technology, materials and even politics. Joints represent structural discontinuities, which excite local stress intensities and considerably affect the overall structural design by setting important penalties on the efficient use of composite materials. For many practical applications, structural joining ‒ related to either initial design or repair requirements ‒ even represents a design driver for the basic laminate configuration of the structure.

Mechanical fastening techniques represent the main joining methods currently used in composite design due to their well-established acceptance and reliability in metallic structural design. Mechanical fastening mainly stands out due to its suitability for thick and highly loaded laminates, its detachability and its easy inspectability and repairability. However, the use, design and sizing of mechanical fastened joints for composite materials are less favourable and less straightforward than for metals, especially in relation to static strength requirements. The inherent anisotropic and brittle nature of composite materials and their laminated configuration result in a complex fracture behaviour with a wide variety of failure modes, which all have to be accounted for to create an optimal joint design. Attaining a satisfactory joint efficiency represents a challenging task considering the strong impact of geometrical parameters, stacking and installation characteristics and environmental effects as well as material and process-related variabilities.

The laminate failure modes of a single fastener joint include net tension failure, shear-out failure, cleavage (a simultaneous combination of the first two modes), pure bearing and pull-through failure. Bearing failure modes are the less catastrophic ones and hence are the most desirable, although they are not necessarily the most efficient. To avoid shear-out failures and premature net-tension failures, a large edge-to-diameter ratio and a large width-to-diameter ratio are required.

Composite bolted joints at best reach a strength efficiency of 40‒50% in terms of ultimate static strength requirements, whereas the usual design range for ductile metal alloys features efficiencies between 70% and 80% (Hart-Smith, 2003). In terms of weight efficiency, mechanical joints in composite material show even lower performances in comparison to metals. Typically, thicker skins, larger edge distances and greater bolt distances or a larger number of bolt rows result in the need for larger and heavier fasteners and larger overlap lengths.

One of the most outstanding properties of composite materials is the ability to tailor the fibre orientations and laminate configuration in accordance with the loading conditions. Hence, structural elements, which are predominant under uniaxial loading, can be efficiently designed taking advantage of a pronounced orthotropic laminate configuration. However, highly orthotropic properties, in terms of tensile or compressive strength and stiffness, are coupled with reduced shear, bearing and even damage tolerance properties. Contrary to metals, bearing and shear strength decrease with rising directional material performance, as shown in Fig. 1.1. Purely unidirectional laminates fail in shear-out independently of the edge distance. Considering the complex three-dimensional stress state of a loaded hole and its variety of failure modes in composite laminates, precise attainment of a strong mechanical fastening (especially for single-row bolted joints or lug joints) requires the simultaneous presence of high notched strengths, bearing and shear strengths.

1.1 Bearing strength of metals and CFRP laminates vs. the material’s tensile strength.

Considerable effort has been devoted to address the material’s mechanics and its multifaceted fracture response in order to predict the damage behaviour of bolt-loaded holes in fibre reinforced laminates. Multiple influencing parameters have been taken into consideration to finally lead to their optimization. However, the joint efficiencies are inherently insufficient irrespective of the amount of fasteners used, the number of fastener rows or the optimization refinement achieved (in general terms, robustness and high optimization do not correlate); as noted above, this is especially severe for highly orthotropic laminate configurations. To overcome this well-known deficiency of composite materials, reinforcement measures have to be int...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Introduction
• Part I: Bolted joints
• Part II: Bonded joints
• Index