D. Koch et al. Composites Science and Technology 68(2008)1165-1172 Matrix shear failure o Experimental data Fiber buckle failure Fiber tensile failure -100 Fig. 6. Summary of measured strength values and failure modes depending on fiber orientation and loading direction. induced cracks and pores within and between the fiber bun- shear stresses additionally reduce the overall compression dles. Further details are described elsewhere [ll]. trength In+45/45 orientation the specimen fails sim- The mechanical properties were investigated at room ilar to the tensile test with large strain to failure and temperatures under ambient atmosphere. The specimens extended nonlinear stress-strain behavior with various geometries were tested in tension, compres- The results from tension, shear and compression tests sion, and shear modes in a spindle testing machine(Zwick, are summarized in Fig. 6 showing 0I-T12 plane with I Germany)using different angles (0/90, +10/-800, +15/ and 2 representing the fiber orientation in the 2D rein- 75°,+30°/-60°,+45°-45°) between fiber orientation forced material. Depending on load and fiber orientation and loading direction. Strain was measured with strain gauges and with a laser based contactless strain measure- ment system. Complex loaded specimens as DEN (double end notch) coupons are tested in tensile mode in order to Stress- Strain Curve 2500 investigate the influence of stress concentrations on the mechanical behavior of the composites. Amor 2000 hensive description of the tests is found in [4, 9, 11, 13] 1500 5. Experimental results 0cQEu Depending on the angle between fiber orientation and 1000 loading direction significant changes of the stress strain o) 100 curves are observed in tensile as well as in compressive mode(Fig. 5). In 0/90 orientation the material behaves almost linear-elastic up to failure, the fibers that are ori ented in loading direction carry the load. Transversal strain 0.000.050.100.150200.250.300.350 is almost negligible due to the 90 fibers. Failure occurs Axial Strain [% when the fiber strength is reached resulting in large volume damage throughout the total gauge length. With increasing b35 Stress-Strain Curve 6000 angle between fiber orientation and loading direction(off axis loading) strength and Youngs modulus sharply decrease Due to the weak matrix damage occurs already at low stresses resulting in a reduced stifness of the com- 4000 posite. Under off-axis loading shear failure is always observed. The failure processes are not distributed over the whole gauge length but locally restricted. The fracture 2000 surface develops along the fiber axis under similar shear stresses and independent of the ofi-axis angle Under compression mode the material behaves in a sim ilar manner. In 0/90 orientation a linear-elastic behavior is observed, however, the specimens fail at much lower 00000.0020.0040.0060.0080.0100.012 stresses just above 200 MPa which is only half of tensile strength. The weak matrix is not able to prevent fiber buck ling which is also observed at specimens with +10/-80 tensile test and(b) pure shear test with associated acoustic emission and +15/-75 orientation. In these cases superimposed signalsinduced cracks and pores within and between the fiber bun￾dles. Further details are described elsewhere [11]. The mechanical properties were investigated at room temperatures under ambient atmosphere. The specimens with various geometries were tested in tension, compres￾sion, and shear modes in a spindle testing machine (Zwick, Germany) using different angles (0/90, +10/80, +15/ 75, +30/60, +45/ 45) between fiber orientation and loading direction. Strain was measured with strain gauges and with a laser based contactless strain measure￾ment system. Complex loaded specimens as DEN (double end notch) coupons are tested in tensile mode in order to investigate the influence of stress concentrations on the mechanical behavior of the composites. Amore compre￾hensive description of the tests is found in [4,9,11,13]. 5. Experimental results Depending on the angle between fiber orientation and loading direction significant changes of the stress strain curves are observed in tensile as well as in compressive mode (Fig. 5). In 0/90 orientation the material behaves almost linear-elastic up to failure, the fibers that are ori￾ented in loading direction carry the load. Transversal strain is almost negligible due to the 90 fibers. Failure occurs when the fiber strength is reached resulting in large volume damage throughout the total gauge length. With increasing angle between fiber orientation and loading direction (off- axis loading) strength and Young’s modulus sharply decrease. Due to the weak matrix damage occurs already at low stresses resulting in a reduced stiffness of the com￾posite. Under off-axis loading shear failure is always observed. The failure processes are not distributed over the whole gauge length but locally restricted. The fracture surface develops along the fiber axis under similar shear stresses and independent of the off-axis angle. Under compression mode the material behaves in a sim￾ilar manner. In 0/90 orientation a linear-elastic behavior is observed, however, the specimens fail at much lower stresses just above 200 MPa which is only half of tensile strength. The weak matrix is not able to prevent fiber buck￾ling which is also observed at specimens with +10/80 and +15/75 orientation. In these cases superimposed shear stresses additionally reduce the overall compression strength. In +45/45 orientation the specimen fails sim￾ilar to the tensile test with large strain to failure and extended nonlinear stress–strain behavior. The results from tension, shear and compression tests are summarized in Fig. 6 showing r1–s12 plane with 1 and 2 representing the fiber orientation in the 2D rein￾forced material. Depending on load and fiber orientation Fig. 6. Summary of measured strength values and failure modes depending on fiber orientation and loading direction. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0 50 100 150 200 250 300 350 400 Stress-Strain Curve Stress [MPa] Axial Strain [%] 0 500 1000 1500 2000 2500 Sum of Acoustic Emission Signals Acoustic Emission Signals 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0 5 10 15 20 25 30 35 Stress-Strain Curve Shear Stress [MPa] Shear Strain [%] 0 1000 2000 3000 4000 5000 6000 Acoustic Emission Signals Sum of Acoustic Emission Signals Fig. 7. Stress–strain curves with unloading reloading cycles of (a) on-axis tensile test and (b) pure shear test with associated acoustic emission signals. D. Koch et al. / Composites Science and Technology 68 (2008) 1165–1172 1169