The solution of Equation(9.22)yields two roots,the positive associated with the tensile strength and the negative associated with the compressive strength of the off-axis specimen. As mentioned early in this chapter,the 10 off-axis tension test has been proposed for measuring the in-plane shear strength(S)of unidirectional composites [1].However,because failure occurs under the influence of nor- mal stresses o,and o,(Figure 9.4),which separate the specimen in two pieces, this test is not recommended for the generation of shear strength [6]. 9.4 Test Procedure 1.Prepare off-axis tension coupons from a unidirectional,six-to eight-ply-thick panel.The specimens should be about 230 mm long and between 12.5 and 25 mm wide.Select at least three different off-axis angles,e.g.,15,30,and 60.Use the same tolerances as for the tension specimen discussed in Chapter 5,and bond end tabs as described in Chapter 4. 2.The off-axis test specimen is instrumented with a three-element strain gage rosette with one of the elements aligned with the coupon axis(x-direction in Figure 9.1),one element at 45,and one element at45°. 3.Measure the specimen cross-sectional dimensions (average six measurements). 4.Mount the specimen in a properly aligned and calibrated test frame.Set the crosshead rate at about 0.5 to 1 mm/min. 5.Monitor the load-strain response of the specimen (all three elements). Take strain readings at small load intervals to collect at least 25 data points in the linear region.Load the specimen to failure. 9.5 Data Reduction 9.5.1 Elastic Properties Axial modulus,Poisson's ratio and shear coupling ratio may be determined from measured stress-strain data according to Equations(9.3)-(9.5).The axial strain,ex,is obtained directly from the axially oriented strain gage.Trans- verse strain and shear strain,y and Yy,are obtained from the +45 gages using Equations(2.13). ©2003 by CRC Press LLCThe solution of Equation (9.22) yields two roots, the positive associated with the tensile strength and the negative associated with the compressive strength of the off-axis specimen. As mentioned early in this chapter, the 10° off-axis tension test has been proposed for measuring the in-plane shear strength (S6) of unidirectional composites [1]. However, because failure occurs under the influence of nor￾mal stresses σ1 and σ2 (Figure 9.4), which separate the specimen in two pieces, this test is not recommended for the generation of shear strength [6]. 9.4 Test Procedure 1. Prepare off-axis tension coupons from a unidirectional, six- to eight-ply-thick panel. The specimens should be about 230 mm long and between 12.5 and 25 mm wide. Select at least three different off-axis angles, e.g., 15, 30, and 60°. Use the same tolerances as for the tension specimen discussed in Chapter 5, and bond end tabs as described in Chapter 4. 2. The off-axis test specimen is instrumented with a three-element strain gage rosette with one of the elements aligned with the coupon axis (x-direction in Figure 9.1), one element at 45°, and one element at –45°. 3. Measure the specimen cross-sectional dimensions (average six measurements). 4. Mount the specimen in a properly aligned and calibrated test frame. Set the crosshead rate at about 0.5 to 1 mm/min. 5. Monitor the load-strain response of the specimen (all three elements). Take strain readings at small load intervals to collect at least 25 data points in the linear region. Load the specimen to failure. 9.5 Data Reduction 9.5.1 Elastic Properties Axial modulus, Poisson’s ratio and shear coupling ratio may be determined from measured stress-strain data according to Equations (9.3)-(9.5). The axial strain, εx , is obtained directly from the axially oriented strain gage. Trans￾verse strain and shear strain, εy and γxy, are obtained from the ±45° gages using Equations (2.13). TX001_ch09_Frame Page 138 Saturday, September 21, 2002 5:01 AM © 2003 by CRC Press LLC