Woodhead Publishing Series in Composites Science and Engineering: Number 48 Residual stresses in composite materials Edited by Mahmood M.Shokrieh WP WOODHEAD PUBLISHING Oxford Cambridge Philadelphia New Delhi Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 Woodhead Publishing Series in Composites Science and Engineering: Number 48 Residual stresses in composite materials Edited by Mahmood M. Shokrieh
Contributor contact details (*main contact) Editor Chapter 4 M.M.Shokrieh A.R.Ghasemi Composites Research Laboratory Department of Mechanical Center of Excellence in Engineering Experimental Solid Mechanics University of Kashan and Dynamics Kashan 87317-51167,Iran School of Mechanical Engineering Iran University of Science and F.Taheri-Behrooz and M.M. Technology Shokrieh* Narmak Composites Research Laboratory Tehran 16846-13114,Iran Center of Excellence in Experimental Solid Mechanics E-mail:shokrieh @iust.ac.ir and Dynamics School of Mechanical Engineering Iran University of Science and Chapters 1,2 and 3 Technology M.M.Shokrieh*and A.R.Ghanei Narmak Mohammadi Tehran 16846-13114,Iran Composites Research Laboratory E-mail:shokrieh@iust.ac.ir Center of Excellence in Experimental Solid Mechanics and Dynamics Chapter 5 School of Mechanical Engineering M.M.Shokrieh*and S.Akbari Iran University of Science and Composites Research Laboratory Technology Center of Excellence in Narmak Experimental Solid Mechanics Tehran 16846-13114,Iran and Dynamics School of Mechanical Engineering E-mail:shokrieh@iust.ac.ir Iran University of Science and Technology Narmak Tehran 16846-13114,Iran E-mail:shokrieh@iust.ac.ir 灯 Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 xi Contributor contact details Editor M. M. Shokrieh Composites Research Laboratory Center of Excellence in Experimental Solid Mechanics and Dynamics School of Mechanical Engineering Iran University of Science and Technology Narmak Tehran 16846-13114, Iran E-mail: shokrieh@iust.ac.ir Chapters 1, 2 and 3 M. M. Shokrieh* and A. R. Ghanei Mohammadi Composites Research Laboratory Center of Excellence in Experimental Solid Mechanics and Dynamics School of Mechanical Engineering Iran University of Science and Technology Narmak Tehran 16846-13114, Iran E-mail: shokrieh@iust.ac.ir Chapter 4 A. R. Ghasemi Department of Mechanical Engineering University of Kashan Kashan 87317-51167, Iran F. Taheri-Behrooz and M. M. Shokrieh* Composites Research Laboratory Center of Excellence in Experimental Solid Mechanics and Dynamics School of Mechanical Engineering Iran University of Science and Technology Narmak Tehran 16846-13114, Iran E-mail: shokrieh@iust.ac.ir Chapter 5 M. M. Shokrieh* and S. Akbari Composites Research Laboratory Center of Excellence in Experimental Solid Mechanics and Dynamics School of Mechanical Engineering Iran University of Science and Technology Narmak Tehran 16846-13114, Iran E-mail: shokrieh@iust.ac.ir (* = main contact)
xii Contributor contact details Chapter 6 M.Safarabadi School of Mechanical Engineering H.Aben*and A.Errapart College of Engineering Institute of Cybernetics University of Tehran Tallinn University of Technology North Amirabad 21 Akadeemia tee Tehran,1135716914,Iran 12618 Tallinn,Estonia E-mail:msafarabadi@ut.ac.ir E-mail:aben@cs.ioc.ee Chapter 9 J.Anton GlasStress Ltd M.M.Aghdam*and S.R.Morsali 21 Akadeemia tee Thermoelasticity Center of 12618 Tallinn,Estonia Excellence Department of Mechanical Chapter 7 Engineering Amirkabir University of M.M.Shokrieh*and S.M.Kamali Technology Shahri Hafez Ave. Composites Research Laboratory Tehran,Iran Center of Excellence in Experimental Solid Mechanics E-mail:aghdam@aut.ac.ir and Dynamics School of Mechanical Engineering Iran University of Science and Chapter 10 Technology H.Wu Narmak Department of Materials Tehran 16846-13114,Iran Loughborough University Leicestershire LEl1 3TU.UK E-mail:shokrieh@iust.ac.ir E-mail:h.wu2@lboro.ac.uk Chapter 8 M.M.Shokrieh* Chapter 11 Composites Research Laboratory R.Jaeger*and C.Koplin Center of Excellence in Biomedical Materials and Implants Experimental Solid Mechanics Group and Dynamics Fraunhofer Institute for Mechanics School of Mechanical Engineering of Materials IWM Iran University of Science and Freiburg,Germany Technology Narmak E-mail:raimund.jaeger@iwm. Tehran 16846-13114,Iran fraunhofer.de;christof.koplin@iwm. fraunhofer.de E-mail:shokrieh@iust.ac.ir Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 xii Contributor contact details Chapter 6 H. Aben* and A. Errapart Institute of Cybernetics Tallinn University of Technology 21 Akadeemia tee 12618 Tallinn, Estonia E-mail: aben@cs.ioc.ee J. Anton GlasStress Ltd 21 Akadeemia tee 12618 Tallinn, Estonia Chapter 7 M. M. Shokrieh* and S. M. Kamali Shahri Composites Research Laboratory Center of Excellence in Experimental Solid Mechanics and Dynamics School of Mechanical Engineering Iran University of Science and Technology Narmak Tehran 16846-13114, Iran E-mail: shokrieh@iust.ac.ir Chapter 8 M. M. Shokrieh* Composites Research Laboratory Center of Excellence in Experimental Solid Mechanics and Dynamics School of Mechanical Engineering Iran University of Science and Technology Narmak Tehran 16846-13114, Iran E-mail: shokrieh@iust.ac.ir M. Safarabadi School of Mechanical Engineering College of Engineering University of Tehran North Amirabad Tehran, 1135716914, Iran E-mail: msafarabadi@ut.ac.ir Chapter 9 M. M. Aghdam* and S. R. Morsali Thermoelasticity Center of Excellence Department of Mechanical Engineering Amirkabir University of Technology Hafez Ave. Tehran, Iran E-mail: aghdam@aut.ac.ir Chapter 10 H. Wu Department of Materials Loughborough University Leicestershire LE11 3TU, UK E-mail: h.wu2@lboro.ac.uk Chapter 11 R. Jaeger* and C. Koplin Biomedical Materials and Implants Group Fraunhofer Institute for Mechanics of Materials IWM Freiburg, Germany E-mail: raimund.jaeger@iwm. fraunhofer.de; christof.koplin@iwm. fraunhofer.de
Contributor contact details xiii Chapter 12 Chapter 13 F.Dai M.M.Shokrieh*S.Akbari and A Center for Composite Materials and Daneshvar Structures Composites Research Laboratory Harbin Institute of Technology Center of Excellence in 92 West Dazhi Street Experimental Solid Mechanics Nan Gang District and Dynamics Harbin,People's Republic of China School of Mechanical Engineering 150001 Iran University of Science and Technology E-mail:daifh@hit.edu.cn Narmak Tehran 16846-13114,Iran Email:shokrieh@iust.ac.ir Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 Contributor contact details xiii Chapter 12 F. Dai Center for Composite Materials and Structures Harbin Institute of Technology 92 West Dazhi Street Nan Gang District Harbin, People’s Republic of China 150001 E-mail: daifh@hit.edu.cn Chapter 13 M. M. Shokrieh*, S. Akbari and A. Daneshvar Composites Research Laboratory Center of Excellence in Experimental Solid Mechanics and Dynamics School of Mechanical Engineering Iran University of Science and Technology Narmak Tehran 16846-13114, Iran Email: shokrieh@iust.ac.ir
Woodhead Publishing Series in Composites Science and Engineering 1 Thermoplastic aromatic polymer composites F.N.Cogswell 2 Design and manufacture of composite structures G.C.Eckold 3 Handbook of polymer composites for engineers Edited by L.C.Hollaway 4 Optimisation of composite structures design A.Miravete 5 Short-fibre polymer composites Edited by S.K.De and J.R.White 6 Flow-induced alignment in composite materials Edited by T.D.Papthanasiou and D.C.Guell 7 Thermoset resins for composites Compiled by Technolex 8 Microstructural characterisation of fibre-reinforced composites Edited by J.Summerscales 9 Composite materials F.L.Matthews and R.D.Rawlings 10 3-D textile reinforcements in composite materials Edited by A.Miravete 11 Pultrusion for engineers Edited by T.Starr 12 Impact behaviour of fibre-reinforced composite materials and structures Edited by S.R.Reid and G.Zhou 13 Finite element modelling of composite materials and structures F.L.Matthews,G.A.O.Davies,D.Hitchings and C.Soutis 14 Mechanical testing of advanced fibre composites Edited by G.M.Hodgkinson 15 Integrated design and manufacture using fibre-reinforced polymeric composites Edited by M.J.Owen and I.A.Jones 16 Fatigue in composites Edited by B.Harris 17 Green composites Edited by C.Baillie XV Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 xv 1 Thermoplastic aromatic polymer composites F. N. Cogswell 2 Design and manufacture of composite structures G. C. Eckold 3 Handbook of polymer composites for engineers Edited by L. C. Hollaway 4 Optimisation of composite structures design A. Miravete 5 Short- fi bre polymer composites Edited by S. K. De and J. R. White 6 Flow- induced alignment in composite materials Edited by T. D. Papthanasiou and D. C. Guell 7 Thermoset resins for composites Compiled by Technolex 8 Microstructural characterisation of fi bre- reinforced composites Edited by J. Summerscales 9 Composite materials F. L. Matthews and R. D. Rawlings 10 3-D textile reinforcements in composite materials Edited by A. Miravete 11 Pultrusion for engineers Edited by T. Starr 12 Impact behaviour of fi bre- reinforced composite materials and structures Edited by S. R. Reid and G. Zhou 13 Finite element modelling of composite materials and structures F. L. Matthews, G. A. O. Davies, D. Hitchings and C. Soutis 14 Mechanical testing of advanced fi bre composites Edited by G. M. Hodgkinson 15 Integrated design and manufacture using fi bre- reinforced polymeric composites Edited by M. J. Owen and I. A. Jones 16 Fatigue in composites Edited by B. Harris 17 Green composites Edited by C. Baillie Woodhead Publishing Series in Composites Science and Engineering
XVI Woodhead Publishing Series in Composites 18 Multi-scale modelling of composite material systems Edited by C.Soutis and P.W.R.Beaumont 19 Lightweight ballistic composites Edited by A.Bhatnagar 20 Polymer nanocomposites Y-W.Mai and Z-Z.Yu 21 Properties and performance of natural-fibre composite Edited by K.Pickering 22 Ageing of composites Edited by R.Martin 23 Tribology of natural fiber polymer composites N.Chand and M.Fahim 24 Wood-polymer composites Edited by K.O.Niska and M.Sain 25 Delamination behaviour of composites Edited by S.Sridharan 26 Science and engineering of short fibre reinforced polymer composites S-Y.Fu.B.Lauke and Y-M.Mai 27 Failure analysis and fractography of polymer composites E.S.Greenhalgh 28 Management,recycling and reuse of waste composites Edited by V.Goodship 29 Materials,design and manufacturing for lightweight vehicles Edited by P.K.Mallick 30 Fatigue life prediction of composites and composite structures Edited by A.P.Vassilopoulos 31 Physical properties and applications of polymer nanocomposites Edited by S.C.Tjong and Y-W.Mai 32 Creep and fatigue in polymer matrix composites Edited by R.M.Guedes 33 Interface engineering of natural fibre composites for maximum performance Edited by N.E.Zafeiropoulos 34 Polymer-carbon nanotube composites Edited by T.McNally and P.Potschke 35 Non-crimp fabric composites:Manufacturing,properties and applications Edited by S.V.Lomov 36 Composite reinforcements for optimum performance Edited by P.Boisse 37 Polymer matrix composites and technology R.Wang.S.Zeng and Y.Zeng 38 Composite joints and connections Edited by P.Camanho and L.Tong 39 Machining technology for composite materials Edited by H.Hocheng 40 Failure mechanisms in polymer matrix composites Edited by P.Robinson,E.S.Greenhalgh and S.Pinho 41 Advances in polymer nanocomposites:Types and applications Edited by F Gao Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 xvi Woodhead Publishing Series in Composites 18 Multi- scale modelling of composite material systems Edited by C. Soutis and P. W. R. Beaumont 19 Lightweight ballistic composites Edited by A. Bhatnagar 20 Polymer nanocomposites Y-W. Mai and Z-Z. Yu 21 Properties and performance of natural- fi bre composite Edited by K. Pickering 22 Ageing of composites Edited by R. Martin 23 Tribology of natural fi ber polymer composites N. Chand and M. Fahim 24 Wood- polymer composites Edited by K. O. Niska and M. Sain 25 Delamination behaviour of composites Edited by S. Sridharan 26 Science and engineering of short fi bre reinforced polymer composites S-Y. Fu, B. Lauke and Y-M. Mai 27 Failure analysis and fractography of polymer composites E. S. Greenhalgh 28 Management, recycling and reuse of waste composites Edited by V. Goodship 29 Materials, design and manufacturing for lightweight vehicles Edited by P. K. Mallick 30 Fatigue life prediction of composites and composite structures Edited by A. P. Vassilopoulos 31 Physical properties and applications of polymer nanocomposites Edited by S. C. Tjong and Y-W. Mai 32 Creep and fatigue in polymer matrix composites Edited by R. M. Guedes 33 Interface engineering of natural fi bre composites for maximum performance Edited by N. E. Zafeiropoulos 34 Polymer- carbon nanotube composites Edited by T. McNally and P. Pötschke 35 Non- crimp fabric composites: Manufacturing, properties and applications Edited by S. V. Lomov 36 Composite reinforcements for optimum performance Edited by P. Boisse 37 Polymer matrix composites and technology R. Wang, S. Zeng and Y. Zeng 38 Composite joints and connections Edited by P. Camanho and L. Tong 39 Machining technology for composite materials Edited by H. Hocheng 40 Failure mechanisms in polymer matrix composites Edited by P. Robinson, E. S. Greenhalgh and S. Pinho 41 Advances in polymer nanocomposites: Types and applications Edited by F. Gao
Woodhead Publishing Series in Composites xvii 42 Manufacturing techniques for polymer matrix composites(PMCs) Edited by S.Advani and K-T.Hsiao 43 Non-destructive evaluation (NDE)of polymer matrix composites:Techniques and applications Edited by V.M.Karbhari 44 Environmentally friendly polymer nanocomposites:Types,processing and properties S.S.Ray 45 Advances in ceramic matrix composites Edited by I.M.Low 46 Ceramic nanocomposites Edited by R.Banerjee and I.Manna 47 Natural fibre composites:Materials,processes and properties Edited by A.Hodzic and R.Shanks 48 Residual stresses in composite materials Edited by M.M.Shokrieh 49 Health and environmental safety of nanomaterials:polymer nanocomposites and other materials containing nanoparticles Edited by J.Njuguna,K.Pielichowski and H.Zhu Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 Woodhead Publishing Series in Composites xvii 42 Manufacturing techniques for polymer matrix composites (PMCs) Edited by S. Advani and K-T. Hsiao 43 Non- destructive evaluation (NDE) of polymer matrix composites: Techniques and applications Edited by V. M. Karbhari 44 Environmentally friendly polymer nanocomposites: Types, processing and properties S. S. Ray 45 Advances in ceramic matrix composites Edited by I. M. Low 46 Ceramic nanocomposites Edited by R. Banerjee and I. Manna 47 Natural fi bre composites: Materials, processes and properties Edited by A. Hodzic and R. Shanks 48 Residual stresses in composite materials Edited by M. M. Shokrieh 49 Health and environmental safety of nanomaterials: polymer nanocomposites and other materials containing nanoparticles Edited by J. Njuguna, K. Pielichowski and H. Zhu
Introduction Residual stresses are self-balanced stresses that exist in engineering components, even when they are not under external loads.Although residual stresses exist in many engineering components,due to the complexity of their nature some designers ignore them in the design process.Often the magnitude of these stresses is significant and ignoring them at the design stage may result in a risky design. However,residual stresses can be useful and improve the performance of the component under load-bearing conditions.Therefore,methods of determination, measurement,simulation and reduction of residual stresses are important research topics. In composite materials,micro-residual stresses are created during the manufacturing process,due to the mismatch of the physical and mechanical properties of the matrix and reinforcement.The shrinkage of the matrix after curing is also another source of such stresses.In laminated composites,the physical and mechanical properties of each ply are functions of the direction of the reinforcement.This is the source of macro-residual stresses in laminated composites.Also,heat treatment processes after manufacturing,machining and environmental conditions,such as absorption or release of the moisture,are some of the other sources of residual stresses. Although residual stresses can occasionally be beneficial,they are usually detrimental.In some circumstances these stresses can cause warping,undesired distortion and dimensional instability in composite specimens.Experimental observations show that residual stresses can cause matrix cracking.Although this mode of failure is not catastrophic,cracks can be a dangerous source of failure initiation;especially when the specimen is under cyclic loading conditions.Also, matrix cracking can be a source of delamination in laminated composites,which is a catastrophic mode of failure. There are different experimental methods for measuring residual stresses in various materials.Measurement of residual stresses can be performed by destructive,semi-destructive and non-destructive techniques.As a general classification,these methods can be categorized as mechanical,optical,diffraction and stress-relevant properties methods.By considering the physical and XIX Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 xix Residual stresses are self- balanced stresses that exist in engineering components, even when they are not under external loads. Although residual stresses exist in many engineering components, due to the complexity of their nature some designers ignore them in the design process. Often the magnitude of these stresses is signifi cant and ignoring them at the design stage may result in a risky design. However, residual stresses can be useful and improve the performance of the component under load- bearing conditions. Therefore, methods of determination, measurement, simulation and reduction of residual stresses are important research topics. In composite materials, micro- residual stresses are created during the manufacturing process, due to the mismatch of the physical and mechanical properties of the matrix and reinforcement. The shrinkage of the matrix after curing is also another source of such stresses. In laminated composites, the physical and mechanical properties of each ply are functions of the direction of the reinforcement. This is the source of macro- residual stresses in laminated composites. Also, heat treatment processes after manufacturing, machining and environmental conditions, such as absorption or release of the moisture, are some of the other sources of residual stresses. Although residual stresses can occasionally be benefi cial, they are usually detrimental. In some circumstances these stresses can cause warping, undesired distortion and dimensional instability in composite specimens. Experimental observations show that residual stresses can cause matrix cracking. Although this mode of failure is not catastrophic, cracks can be a dangerous source of failure initiation; especially when the specimen is under cyclic loading conditions. Also, matrix cracking can be a source of delamination in laminated composites, which is a catastrophic mode of failure. There are different experimental methods for measuring residual stresses in various materials. Measurement of residual stresses can be performed by destructive, semi- destructive and non- destructive techniques. As a general classifi cation, these methods can be categorized as mechanical, optical, diffraction and stress- relevant properties methods. By considering the physical and Introduction
XX Introduction mechanical properties of composite materials,special methods for measurement of residual stresses in these types of materials are available.Among different measurement techniques for measuring the residual stresses,mechanical techniques are those most often used by various researchers for measurement of residual stresses in composites. Application of the other techniques for measurement of the residual stresses in composite materials can sometimes be very difficult or even impossible.A deeper understanding of the physics and mechanics of these stresses in composites and finding suitable techniques for their measurement are still needed.As already mentioned,there are many complexities and difficulties in the use of some of the experimental techniques for the measurement of residual stresses in composites and further research is also needed to eliminate these obstacles. There are also various mathematical (analytical and numerical)methods for calculation of residual stresses in composite materials.These methods have been developed based on assumptions made on the curing process of composites.The generality of the model and the proper characterization of the constituent materials (reinforcement and matrix)play important roles in successful simulation of the process.For this purpose,a precise characterization of the matrix and the reinforcement properties with time and temperature is necessary.Studies on the mechanisms of generation of additional residual stresses due to tool-part interaction are an ongoing research area and their influence on the final residual stresses should be understood.Thus,there are many unanswered questions in the mathematical modeling of residual stresses that need to be clarified by further research. There are a few techniques available in the literature for reduction of residual stresses.One of these techniques is heat treatment after the curing process of composites.Addition of nanoparticles to the matrix,to reduce the mismatch of the physical and mechanical properties of the matrix and the reinforcement(also reducing the shrinkage behavior of the matrix),is a new method that has been recently studied by some authors.This field also seems attractive for further research. In this book,the latest available results of research in the field of residual stresses in polymer,metal and ceramic matrix composites are comprehensively reviewed and provided.The present state of knowledge and the future trends of research on this subject are analyzed and interpreted.A comprehensive review of the available knowledge of residual stresses in composite materials shows that further research is required,in modeling and experimental characterization,for a proper and deeper understanding of this complicated subject Mahmood M.Shokrieh Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 xx Introduction mechanical properties of composite materials, special methods for measurement of residual stresses in these types of materials are available. Among different measurement techniques for measuring the residual stresses, mechanical techniques are those most often used by various researchers for measurement of residual stresses in composites. Application of the other techniques for measurement of the residual stresses in composite materials can sometimes be very diffi cult or even impossible. A deeper understanding of the physics and mechanics of these stresses in composites and fi nding suitable techniques for their measurement are still needed. As already mentioned, there are many complexities and diffi culties in the use of some of the experimental techniques for the measurement of residual stresses in composites and further research is also needed to eliminate these obstacles. There are also various mathematical (analytical and numerical) methods for calculation of residual stresses in composite materials. These methods have been developed based on assumptions made on the curing process of composites. The generality of the model and the proper characterization of the constituent materials (reinforcement and matrix) play important roles in successful simulation of the process. For this purpose, a precise characterization of the matrix and the reinforcement properties with time and temperature is necessary. Studies on the mechanisms of generation of additional residual stresses due to tool- part interaction are an ongoing research area and their infl uence on the fi nal residual stresses should be understood. Thus, there are many unanswered questions in the mathematical modeling of residual stresses that need to be clarifi ed by further research. There are a few techniques available in the literature for reduction of residual stresses. One of these techniques is heat treatment after the curing process of composites. Addition of nanoparticles to the matrix, to reduce the mismatch of the physical and mechanical properties of the matrix and the reinforcement (also reducing the shrinkage behavior of the matrix), is a new method that has been recently studied by some authors. This fi eld also seems attractive for further research. In this book, the latest available results of research in the fi eld of residual stresses in polymer, metal and ceramic matrix composites are comprehensively reviewed and provided. The present state of knowledge and the future trends of research on this subject are analyzed and interpreted. A comprehensive review of the available knowledge of residual stresses in composite materials shows that further research is required, in modeling and experimental characterization, for a proper and deeper understanding of this complicated subject. Mahmood M. Shokrieh
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