Residual Stresses in Composite Materials

Mahmood M. Shokrieh

• Publisher: Woodhead Publishing
• Year: 2014
• eBook ISBN: 9780857098597
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Residual stresses are a common phenomenon in composite materials. They can either add to or significantly reduce material strength. Because of the increasing demand for high-strength, light-weight materials such as composites and their wide range of applications in the aerospace and automotive industries, in civil infrastructure and in sporting applications, it is critical that the residual stresses of composite materials are understood and measured correctly.The first part of this important book reviews destructive and non-destructive testing (NDT) techniques for measuring residual stresses. Various mathematical (analytical and numerical) methods for calculation of residual stresses in composite materials are also presented. Chapters in the first section of the book discuss the simulated hole drilling method, the slitting/crack compliance method, measuring residual stresses in homogeneous and composite glass materials using photoelastic techniques, and modeling residual stresses in composite materials. The second part of the book discusses residual stresses in polymer matrix, metal-matrix and a range of other types of composites. Moreover, the addition of nanoparticles to the matrix of polymeric composites as a new technique for reduction of residual stresses is discussed.Residual stresses in composite materials provides a comprehensive overview of this important topic, and is an invaluable reference text for both academics and professionals working in the mechanical engineering, civil engineering, aerospace, automotive, marine and sporting industries. - Reviews destructive and non-destructive testing (NDT) techniques for measuring residual stresses - Discusses residual stresses in polymer matrix, metal-matrix and other types of composite - Considers the addition of nanoparticles to the matrix of polymeric composites as a new technique for reduction of residual stresses

Part I

Measurement and modelling

1

The importance of measuring residual stresses in composite materials

M.M. Shokrieh

A.R. Ghanei Mohammadi Iran University of Science Technology, Iran

Abstract

This chapter discusses categories of residual stress in composite materials, their effects and the importance of their measurement. It also summarizes issues in measuring residual stresses and introduces the range of techniques available.

Key words

composite materials

residual stress

measurement

experimental techniques

1.1 Introduction

In the modern world there is an increasing need for high strength, lightweight materials such as composites for a wide range of applications, including the aerospace and automotive industries, civil infrastructure, sporting goods, etc. In order to get the best out of such materials, a good understanding of the different aspects of their behavior is required. An important aspect that needs proper investigation is the effect of the manufacturing process on the mechanical behavior of the material. A good example is residual stresses in materials created by processes such as heating. Such stresses have played an important role in manufacture since the beginning of civilization. In the manufacture of sword blades, for example, repeated hammering at a controlled elevated temperature creates a thin layer of compressive residual stress which strengthens the blade.

Residual stresses can be defined as stress fields that exist in the absence of any external loads and are the result of any mechanical process which can cause deformation. As an example, non-uniform heating or cooling causes thermal strain. Incompatible deformation is induced by plastic deformation, and mismatched thermal expansion coefficients produce discontinuity in deformation due to temperature change. The two main factors that affect residual stress are the processes that the component has undergone, and the material properties that relate the mechanical process to deformation behaviour (Cheng and Finnie, 2007).

Operations, such as mechanical forming procedures, heat treatment or welding, can cause residual stresses during manufacture and/or use. Processes resulting in stress concentrations close to surfaces can boost failure resistance.

Manufacturing processes, such as shot peening, chemical surface treatment and laser surface hardening, are used to induce useful surface compression to improve resistance to fatigue failure. However, those leading to surface tension normally facilitate the formation of cracks, which can cause untimely fracture (Colpo, 2006; Fitzpatrick and Lodini, 2003). The distortion of laminate composites is typically the result of residual stresses. This makes it particularly important to understand, measure, model and control residual stresses in composite and other materials.

Residual stresses arise for several reasons: on the macroscopic scale, they may emanate from heat treating, machining and secondary processing, and assembly. On the microscopic scale, they usually result from the discontinuities between the thermal expansion coefficients, yield stresses, rigidities or phase changes (e.g. cure shrinking) of different constituents. In any component or material, both kind of stress may co-exist (Colpo, 2006).

A fiber reinforced polymer (FRP) composite is usually subject to a process wherein the resin is heated, the fibers are wetted and cure is performed at high temperatures. The need for high temperatures in the curing process results in formation of residual stresses in the final laminate structure. These residual stresses have two major causes: the mismatch in thermal expansion of the constituents, and chemical shrinkage of the polymers in the composite. Measurement and characterization of these stresses is complex (Liu, 1999). Residual stresses typically arise due to the discrepancies between the mechanical properties of the matrix and the reinforcing fibers. Other mechanisms that cause residual stresses include cure shrinkage, moisture, ageing, elevated post-cure temperature, differences in material properties at the microscopic scale, differences in fiber volume across the matrix and non-uniform degree of cure (Tsouvalis et al., 2009).

1.2 Categories of residual stresses

Barnes and Byerly (1994) explained the various types of residual stress at work in continuous carbon-fiber-reinforced thermoplastic composites. They identified three levels of stress in laminated structures: the ‘micro-stresses’ present between distinct fibers within each ply, the ‘macro-stresses’ forming in multi-axial laminates at the ply-to-ply scale, and a third, more prevalent level of stress resulting from different thermal histories of distinct parts of a laminate during the cooling stage.

The discontinuity between the thermal expansion coefficients of the fiber and matrix, along with the development of chemical shrinkage, create residual strains and stress at the ply scale. As a lamina is cured, its matrix constituent is subject to polymerization. Epoxy resins unde...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• Woodhead Publishing Series in Composites Science and Engineering
• Introduction
• Part I: Measurement and modelling
• Part II: Residual stresses in different types of composites
• Index