1 The importance of measuring residual stresses in composite materials M.M.SHOKRIEH and A.R.GHANEI MOHAMMADI, Iran University of Science and Technology,Iran D01:10.1533/9780857098597.1.3 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. 3 Woodhead Publishing Limited,2014© Woodhead Publishing Limited, 2014 3 1 The importance of measuring residual stresses in composite materials M. M. SHOKRIEH and A. R. GHANEI MOHAMMADI, Iran University of Science and Technology, Iran DOI: 10.1533/9780857098597.1.3 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. 1.1Introduction 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 defi ned as stress fi elds 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 coeffi cients 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