also to verify biaxial strength criteria because,as will be discussed,uniaxial loading will lead to a combined state of stress in the principal material system. 9.1 Deformation and Stress in an Unconstrained Specimen Because of the off-axis configuration of the specimen,the in-plane response is characterized by a fully populated compliance matrix,as shown in Equation (2.16) Ex 52 516 Ox Ey S12 y (9.1) 56 526 where the x-y system is defined in Figure 9.1,and expressions for the trans- formed compliance elements Su are given in Appendix A. For an ideal,uniformly stressed off-axis tensile coupon,the only stress acting is ox,(oy=txy=0),and Equations (9.1)give the state of strain in the specimen, Ex Ey S (9.2) S.c] Consequently,the off-axis coupon subjected to a uniform uniaxial state of stress thus exhibits shear strain()in addition to the axial and transverse strains(Ex and )(Figure 9.2). A set of material properties may be evaluated based on measurement of axial stress (and axial,transverse,and shear strains (x yYy).It is customary to determine the axial Young's modulus(E)and Poisson's ratio (Vxy)of the off-axis specimen (9.3) Ex Vxy= y (9.4) Ex ©2003 by CRC Press LLCalso to verify biaxial strength criteria because, as will be discussed, uniaxial loading will lead to a combined state of stress in the principal material system. 9.1 Deformation and Stress in an Unconstrained Specimen Because of the off-axis configuration of the specimen, the in-plane response is characterized by a fully populated compliance matrix, as shown in Equation (2.16) (9.1) where the x-y system is defined in Figure 9.1, and expressions for the trans￾formed compliance elements Sij are given in Appendix A. For an ideal, uniformly stressed off-axis tensile coupon, the only stress acting is σx, (σy = τxy = 0), and Equations (9.1) give the state of strain in the specimen, (9.2) Consequently, the off-axis coupon subjected to a uniform uniaxial state of stress thus exhibits shear strain (γxy) in addition to the axial and transverse strains (εx and εy) (Figure 9.2). A set of material properties may be evaluated based on measurement of axial stress (σx) and axial, transverse, and shear strains (εx, εy, γxy). It is customary to determine the axial Young’s modulus (Ex) and Poisson’s ratio (νxy) of the off-axis specimen (9.3) (9.4) x y xy x y xy SSS SSS SSS ε ε γ σ σ τ               =                           11 12 16 12 22 26 16 26 66 x y xy x S S S ε ε γ σ               =               11 12 16 Ex x x = σ ε ν ε ε xy y x = − TX001_ch09_Frame Page 132 Saturday, September 21, 2002 5:01 AM © 2003 by CRC Press LLC