14 Residual stresses in composite materials Raju,I.S.and Newman,J.C.(1979)Stress-intensity factors for a wide range of semi-elliptical surface cracks in finite-thickness plates,Engineering Fracture Mechanics,.11(4),817-29. Reid,R.G.(2009)The Measurement of Longitudinal Residual Stresses in Unidirectional Glass Fibre Reinforced Plastic,PhD Thesis,University of the Witwatersrand, Johannesburg. Reid,R.G.and Paskaramoorthy,R.(2009)A novel method to measure residual stresses in unidirectional GFRP,Composite Structures,88,388-93. Schajer,G.S.and Prime,M.B.(2006)Use of inverse solutions for residual stress measurement,Los Alamos National Laboratory,Engineering Sciences and Applications Divisions,LA-UR-04-5890,Journal of Engineering Materials,40(3),375-82. Seif,M.A.and Short,S.R.(2002)Determination of residual tresses in thin-walled composite cylinders,Experimental Techniques,26(2),43-6. Seif,M.A.,Khashaba,U.A.and Rojas-oviedo,R.(2006)Residual stress measurements in CFRE and GFRE composite missile shells,Composite Structures,79(2),261-9. Stamatopoulos,K.(2011)Measurement of Residual Stresses on Composite Materials with the Incremental Hole Drilling Method,Diploma Thesis,National Technical University of Athens,School of Naval Architecture and Marine Engineering,Athens. Stone,M.A.Schwartz,I.F.and Chandler,H.D.(1997)Residual stresses associated with post-cure shrinkage in GRP tubes,Composites Science and Technology,57(1), 47-54. Tsouvalis,N.,Margelis,G.and Dellis,D.(2009)Residual Stresses in Composite Materials: A Review,NTUA Report No.MAR-R4-3-ntua-24(2)for MARSTRUCT. Twigg,G.,Poursartip,A.and Fernlund,G.(2004)Tool-part interaction in composites processing.Part I:Experimental investigation and analytical model,Composites:Part A,35,121-33. Warrier,S.G.,Rangaswamy,P.,Bourke,M.A.M.and Krishnamurthy,S.(1999) Assessment of the fiber/matrix interface bond strength in Sic/Ti-6Al-4V composites, Materials Science and Engineering A,259(2),220-7. Zheng,M.H.(2000)Strength formulae of unidirectional composites including thermal residual stresses,Materials Letters,43(1-2),36-42. Woodhead Publishing Limited,2014
14 Residual stresses in composite materials © Woodhead Publishing Limited, 2014 Raju , I. S. and Newman , J. C. ( 1979 ) Stress- intensity factors for a wide range of semi- elliptical surface cracks in fi nite- thickness plates , Engineering Fracture Mechanics , 11 (4) , 817 – 29 . Reid , R. G. ( 2009 ) The Measurement of Longitudinal Residual Stresses in Unidirectional Glass Fibre Reinforced Plastic , PhD Thesis, University of the Witwatersrand , Johannesburg . Reid , R. G. and Paskaramoorthy , R. ( 2009 ) A novel method to measure residual stresses in unidirectional GFRP , Composite Structures , 88 , 388 – 93 . Schajer , G. S. and Prime , M. B. ( 2006 ) Use of inverse solutions for residual stress measurement, Los Alamos National Laboratory, Engineering Sciences and Applications Divisions, LA-UR–04–5890 , Journal of Engineering Materials , 40 (3) , 375 – 82 . Seif , M. A. and Short , S. R. ( 2002 ) Determination of residual tresses in thin- walled composite cylinders , Experimental Techniques , 26 (2) , 43 – 6 . Seif , M. A. , Khashaba , U. A. and Rojas- oviedo , R. ( 2006 ) Residual stress measurements in CFRE and GFRE composite missile shells , Composite Structures , 79 (2) , 261 – 9 . Stamatopoulos , K. ( 2011 ) Measurement of Residual Stresses on Composite Materials with the Incremental Hole Drilling Method , Diploma Thesis, National Technical University of Athens, School of Naval Architecture and Marine Engineering , Athens . Stone , M. A. Schwartz , I. F. and Chandler , H. D. ( 1997 ) Residual stresses associated with post- cure shrinkage in GRP tubes , Composites Science and Technology , 57 (1) , 47 – 54 . Tsouvalis , N. , Margelis , G. and Dellis , D. ( 2009 ) Residual Stresses in Composite Materials: A Review , NTUA Report No. MAR-R4–3-ntua–24(2) for MARSTRUCT. Twigg , G. , Poursartip , A. and Fernlund , G. ( 2004 ) Tool- part interaction in composites processing. Part I: Experimental investigation and analytical model , Composites: Part A , 35 , 121 – 33 . Warrier , S. G. , Rangaswamy , P. , Bourke , M. A. M. and Krishnamurthy , S. ( 1999 ) Assessment of the fi ber/matrix interface bond strength in Sic/Ti–6Al–4V composites , Materials Science and Engineering A , 259 (2) , 220 – 7 . Zheng , M. H. ( 2000 ) Strength formulae of unidirectional composites including thermal residual stresses , Materials Letters , 43 (1–2) , 36 – 42
2 Destructive techniques in the measurement of residual stresses in composite materials:an overview M.M.SHOKRIEH and A.R.GHANEI MOHAMMADI. Iran University of Science and Technology,Iran D0:10.1533/9780857098597.1.15 Abstract:Existing methods of measuring residual stresses in composite materials are often categorized into two broad groups:destructive and non- destructive.Destructive testing implies damaging or removing a section of material so that the specimen may no longer be usefully employed.This chapter aims to introduce the main methods that fall into the category of destructive methods.For each method,the notable contributions are discussed in detail. Key words:composite materials,residual stress,measurement,destructive techniques. 2.1 Introduction This chapter reviews the following destructive methods for testing residual stresses in composite materials: the layer removal method; the Sachs (boring)method; hole-drilling methods; the ring-core method; 。the cutting method ·the contour method the ply sectioning method; the radial cutting method; matrix removal methods; micro-indentation methods; 。the slitting method the first-ply failure method; the measurement of curvature method; 。heating methods. 15 Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 15 2 Destructive techniques in the measurement of residual stresses in composite materials: an overview M. M. SHOKRIEH and A. R. GHANEI MOHAMMADI, Iran University of Science and Technology, Iran DOI: 10.1533/9780857098597.1.15 Abstract: Existing methods of measuring residual stresses in composite materials are often categorized into two broad groups: destructive and nondestructive. Destructive testing implies damaging or removing a section of material so that the specimen may no longer be usefully employed. This chapter aims to introduce the main methods that fall into the category of destructive methods. For each method, the notable contributions are discussed in detail. Key words: composite materials, residual stress, measurement, destructive techniques. 2.1Introduction This chapter reviews the following destructive methods for testing residual stresses in composite materials: • the layer removal method; • the Sachs (boring) method; • hole- drilling methods; • the ring- core method; • the cutting method; • the contour method; • the ply sectioning method; • the radial cutting method; • matrix removal methods; • micro- indentation methods; • the slitting method; • the fi rst- ply failure method; • the measurement of curvature method; • heating methods
16 Residual stresses in composite materials 2.2 The layer removal method The layer removal method was first considered as early as the 1940s (Letner, 1953:Richards,1945;Timoshenko,1941)and was originally developed by Treuting and Read (1951)for isotropic materials such as metals,but has been modified for use with composites.It depends on the removal of thin layers of material from one surface of a plate.The internal stresses originally present in this layer are thus eliminated and the plate consequently curves to restore force equilibrium.By measuring the strain and curvature of the laminate as successive layers are removed,it is possible to derive the stress profile of the original laminate (Reid,2009;Stamatopoulos,2011). Abrasion or milling can be used to remove one or more plies of the composite material.Removal of composite layers can also be done by machining (Cowley and Beaumont,1997;Eijpe and Powell 1997;Jeronimidis and Parkyn,1988), splitting with a knife (Ersoy and Vardar,2000)or by placing separation films within a laminate during cure(Chapman et al.,1990;Manson and Seferis,1992). The use of methods such as abrasion can reduce the accuracy of the technique. This is because during the layer removal,the surface temperature increases and microcracks are created,and this results in the relaxation of residual stresses. Moreover,the irregularities caused at the plies from abrasion,for example alteration of the thickness of the plies,influence the final shape of the resulting deformation.In order to prevent any damage to the laminate,the Process Simulated Laminates(PSL)technique is used.PSL composite materials consist of prepreg plies and other thin separating films placed between the plies.The plies between two thin separating films create a constitutive laminate (CL)(Fig.2.1).These CLs may be removed from the composite material after manufacturing (Parlevliet et al.,2006).The use of separation films provides more reproducible data than machining (Chapman et al.,1990;Reid,2009). There are two methods for calculating residual stresses.The first uses the CL deformation technique,that is,the measurement of the dimension changes (curvature)of the PSLs and CLs before and after their removal.The second is by the PSL-strain gage technique,where strain gages are attached to the surface of the composite.These monitor the strain changes as CLs are removed from the other side(Parlevliet et al.,2006).The PSL-strain gage technique is more precise. The layer removal method cannot be used to measure the micro-scale residual stresses in a uni-directional glass fiber-reinforced plastic (GFRP)laminate.Each layer that is removed has a thickness far larger than the fiber diameter and consequently it is not possible to remove either matrix material or fibers individually (Reid,2009). Jeronimidis and Parkyn (1988)investigated residual stresses in APC-2 cross- ply laminates using a number of experimental techniques,including the layer removal method.They showed that accurate predictions can be made,provided that the changes in thermo-elastic properties of the materials with temperature are Woodhead Publishing Limited,2014
16 Residual stresses in composite materials © Woodhead Publishing Limited, 2014 2.2 The layer removal method The layer removal method was fi rst considered as early as the 1940s (Letner, 1953; Richards, 1945; Timoshenko, 1941) and was originally developed by Treuting and Read (1951) for isotropic materials such as metals, but has been modifi ed for use with composites. It depends on the removal of thin layers of material from one surface of a plate. The internal stresses originally present in this layer are thus eliminated and the plate consequently curves to restore force equilibrium. By measuring the strain and curvature of the laminate as successive layers are removed, it is possible to derive the stress profi le of the original laminate (Reid, 2009; Stamatopoulos, 2011). Abrasion or milling can be used to remove one or more plies of the composite material. Removal of composite layers can also be done by machining (Cowley and Beaumont, 1997; Eijpe and Powell 1997; Jeronimidis and Parkyn, 1988), splitting with a knife (Ersoy and Vardar, 2000) or by placing separation fi lms within a laminate during cure (Chapman et al ., 1990; Manson and Seferis, 1992). The use of methods such as abrasion can reduce the accuracy of the technique. This is because during the layer removal, the surface temperature increases and microcracks are created, and this results in the relaxation of residual stresses. Moreover, the irregularities caused at the plies from abrasion, for example alteration of the thickness of the plies, infl uence the fi nal shape of the resulting deformation. In order to prevent any damage to the laminate, the Process Simulated Laminates (PSL) technique is used. PSL composite materials consist of prepreg plies and other thin separating fi lms placed between the plies. The plies between two thin separating fi lms create a constitutive laminate (CL) ( Fig. 2.1 ). These CLs may be removed from the composite material after manufacturing (Parlevliet et al ., 2006). The use of separation fi lms provides more reproducible data than machining (Chapman et al ., 1990; Reid, 2009). There are two methods for calculating residual stresses. The fi rst uses the CL deformation technique, that is, the measurement of the dimension changes (curvature) of the PSLs and CLs before and after their removal. The second is by the PSL-strain gage technique, where strain gages are attached to the surface of the composite. These monitor the strain changes as CLs are removed from the other side (Parlevliet et al ., 2006). The PSL-strain gage technique is more precise. The layer removal method cannot be used to measure the micro- scale residual stresses in a uni- directional glass fi ber- reinforced plastic (GFRP) laminate. Each layer that is removed has a thickness far larger than the fi ber diameter and consequently it is not possible to remove either matrix material or fi bers individually (Reid, 2009). Jeronimidis and Parkyn (1988) investigated residual stresses in APC-2 crossply laminates using a number of experimental techniques, including the layer removal method. They showed that accurate predictions can be made, provided that the changes in thermo- elastic properties of the materials with temperature are
Destructive techniques in the measurement of residual stresses 17 (Compressive)(Tensile) -Constitutive laminate(CL) 2.1 Schematic view of the process simulated laminate(PSL) configuration with constitutive laminates(CLS)for determination of laminate skin-core residual stress distribution(gray area)through this type of layer removal method (Parlevliet et al.,2007). taken into account.Chaoui et al.(1988)evaluated residual stress distribution in a medium density polyethylene (MDPE)pipe using a modified layer removal procedure.Paterson and White (1989)analysed residual stresses in parallel-sided polymer moldings.Chapman et al.(1990)analysed the macroscopic in-plane residual stress state of uni-directional graphite (AS4)reinforced polyetheretherketone(PEEK)laminates. Crasto and Kim (1993)studied the curing-induced strains in graphite/epoxy (AS4/3501-6)composites at different temperatures.These results were used to predict the stress at first-ply and ultimate failure of [090 laminates.Gungor and Ruiz (1996)used layer removal methods to measure residual stresses in continuous fiber composites.Cowley and Beaumont (1997)carried out experiments to measure the residual stresses in a thermoplastic matrix and a toughened thermosetting matrix,both reinforced with carbon fiber.They showed that,under certain conditions,the residual tensile stress can approach closely to the transverse ply tensile strength.Eijpe and Powell (1997,1998)validated the modified layer removal method using a compression-molded continuous-fiber laminate(PEI/glass)and an injection-molded short-fiber-reinforced laminate(PC/ glass).They showed that the modified layer removal method produced good results.Ersoy and Vardar(2000)also used a layer removal technique to measure macroscopic residual stresses in layered composites, 2.3 The Sachs (boring)method The Sachs method (also known as the boring method)is similar to the layer removal method,except that it is applied to rods and tubes rather than plates.In this method,tube-shaped parts are progressively removed from the center of a circular section of the material.As each radial increment is removed,the residual stresses within this material are released and the remaining material responds Woodhead Publishing Limited,2014
Destructive techniques in the measurement of residual stresses 17 © Woodhead Publishing Limited, 2014 taken into account. Chaoui et al . (1988) evaluated residual stress distribution in a medium density polyethylene (MDPE) pipe using a modifi ed layer removal procedure. Paterson and White (1989) analysed residual stresses in parallel- sided polymer moldings. Chapman et al . (1990) analysed the macroscopic in- plane residual stress state of uni- directional graphite (AS4) reinforced polyetheretherketone (PEEK) laminates. Crasto and Kim (1993) studied the curing- induced strains in graphite/epoxy (AS4/3501-6) composites at different temperatures. These results were used to predict the stress at fi rst- ply and ultimate failure of [0 2 /90 2 ] s laminates. Güngör and Ruiz (1996) used layer removal methods to measure residual stresses in continuous fi ber composites. Cowley and Beaumont (1997) carried out experiments to measure the residual stresses in a thermoplastic matrix and a toughened thermosetting matrix, both reinforced with carbon fi ber. They showed that, under certain conditions, the residual tensile stress can approach closely to the transverse ply tensile strength. Eijpe and Powell (1997, 1998) validated the modifi ed layer removal method using a compression- molded continuous- fi ber laminate (PEI/glass) and an injection- molded short- fi ber- reinforced laminate (PC/ glass). They showed that the modifi ed layer removal method produced good results. Ersoy and Vardar (2000) also used a layer removal technique to measure macroscopic residual stresses in layered composites, 2.3 The Sachs (boring) method The Sachs method (also known as the boring method) is similar to the layer removal method, except that it is applied to rods and tubes rather than plates. In this method, tube- shaped parts are progressively removed from the center of a circular section of the material. As each radial increment is removed, the residual stresses within this material are released and the remaining material responds 2.1 Schematic view of the process simulated laminate (PSL) confi guration with constitutive laminates (CLS) for determination of laminate skin- core residual stress distribution (gray area) through this type of layer removal method (Parlevliet et al. , 2007)
18 Residual stresses in composite materials elastically.Axially and circumferentially aligned strain gages are used to record the response on the outer surface of the section(Reid,2009).An alternative way of using this method is to remove material from the outer surface of tubes.In this case,the strain gages are placed on the inner surface of the tube (Kovac et al., 1989;Stacey and Webster,1988).Despite the fact that these techniques can be used for measuring the residual stresses in laminated composite tubes,the Sachs method cannot be used to find the micro-scale residual stresses in a uni-directional GFRP laminate,since the technique cannot differentiate between fiber and matrix stresses (Reid,2009). The Sachs method technique allows axial,circumferential and radial residual stresses to be determined(Chen et al.,2000;Garcia-Granada et al.,2001;Sachs, 1927;Sharman et al.,1997).It has been used to measure residual stress distribution in metal and ceramic components (Hung,1989;Jones and Martin,1977;Kovac et al.,1989;Kuboki et al.,2000;Lacarac et al.,2004;Olson and Bert,1966: Ozdemir and Edwards,1996;Rasty et al.,2007;Smith et al.,1998;Stacey and Webster,1988;Voyiadjis and Hartley,1987;Voyiadjis et al.,1985;Waki et al., 2003;Yamada et al.,1990).However,the technique does seem to have been used for composite materials. 2.4 Hole-drilling methods 2.4.1 The hole-drilling method The hole-drilling technique for determining residual stresses was first proposed by Mathar (1934).When a hole is introduced into a stressed body the stresses relax,which leads to a change in the surrounding strain field that can be measured and correlated to the relaxed stresses.There are three variations of this method: 1.Center-hole drilling involves drilling a hole normal to a surface to investigate surface strain: 2.Through-hole drilling is used to measure uniform through-thickness residual stresses;and 3.Incremental-hole drilling is used to measure residual stresses varying through the thickness,by drilling in successive depth increments(Reid,2009) Originally,this semi-destructive method was restricted to macroscopically homogeneous isotropic materials,but attempts have been made to extend it to anisotropic,fiber reinforced composites(Prasad et al.,1987)and,since the mid- 1960s,to orthotropic materials (Lake et al.,1970;Rendler and Vigness,1966),but it is numerically intense and several assumptions must be made to simplify the resulting solutions.The highly orthotropic nature of composites further complicates the measurements themselves,because it is extremely difficult to obtain measurement precision around the hole,particularly in the fiber direction, even with high precision techniques,such as Moire interferometry (Nicoletto, 1991). Woodhead Publishing Limited,2014
18 Residual stresses in composite materials © Woodhead Publishing Limited, 2014 elastically. Axially and circumferentially aligned strain gages are used to record the response on the outer surface of the section (Reid, 2009). An alternative way of using this method is to remove material from the outer surface of tubes. In this case, the strain gages are placed on the inner surface of the tube (Kovač et al ., 1989; Stacey and Webster, 1988). Despite the fact that these techniques can be used for measuring the residual stresses in laminated composite tubes, the Sachs method cannot be used to fi nd the micro- scale residual stresses in a uni- directional GFRP laminate, since the technique cannot differentiate between fi ber and matrix stresses (Reid, 2009). The Sachs method technique allows axial, circumferential and radial residual stresses to be determined (Chen et al ., 2000; Garcia-Granada et al ., 2001; Sachs, 1927; Sharman et al ., 1997). It has been used to measure residual stress distribution in metal and ceramic components (Hung, 1989; Jones and Martin, 1977; Kovač et al ., 1989; Kuboki et al ., 2000; Lacarac et al ., 2004; Olson and Bert, 1966; Özdemir and Edwards, 1996; Rasty et al ., 2007; Smith et al ., 1998; Stacey and Webster, 1988; Voyiadjis and Hartley, 1987; Voyiadjis et al ., 1985; Waki et al ., 2003; Yamada et al ., 1990). However, the technique does seem to have been used for composite materials. 2.4Hole- drilling methods 2.4.1The hole- drilling method The hole- drilling technique for determining residual stresses was fi rst proposed by Mathar (1934). When a hole is introduced into a stressed body the stresses relax, which leads to a change in the surrounding strain fi eld that can be measured and correlated to the relaxed stresses. There are three variations of this method: 1. Center- hole drilling involves drilling a hole normal to a surface to investigate surface strain; 2. Through- hole drilling is used to measure uniform through- thickness residual stresses; and 3. Incremental- hole drilling is used to measure residual stresses varying through the thickness, by drilling in successive depth increments (Reid, 2009). Originally, this semi- destructive method was restricted to macroscopically homogeneous isotropic materials, but attempts have been made to extend it to anisotropic, fi ber reinforced composites (Prasad et al ., 1987) and, since the mid- 1960s, to orthotropic materials (Lake et al ., 1970; Rendler and Vigness, 1966), but it is numerically intense and several assumptions must be made to simplify the resulting solutions. The highly orthotropic nature of composites further complicates the measurements themselves, because it is extremely diffi cult to obtain measurement precision around the hole, particularly in the fi ber direction, even with high precision techniques, such as Moiré interferometry (Nicoletto, 1991)