1 RESIDUAL STRENGHT OF LAMINATED GRAPHITE-EPOXY COMPOSITE CIRCULAR PLATES DAMAGED BY TRANSVERSAL LOAD Giangiacomo Minak Andrea Zucchelli DIEM - Facoltà di Ingegneria Università degli Studi di Bologna Italia ABSTRACT Fibre-reinforced composite materials are increasingly used in airframes and the tendency is to built the main fuselage in Carbon Fibre Reinforced Polymer (CFRP).In general CFRP has a wide application in light-weight structural members, because it is characterized by a high strength-to-weight and stiffness-to-weight ratio, but it is vulnerable to damage caused by transverse loads, such as those arising from indentation and impact loading. One of the main reasons that inhibits more widespread applications of composite materials is their lack of resistance to low velocity and low energy impact damage[1-3], particularly in the case of thermoset matrix like epoxy, while the probability of such loadings occurring during the manufacture, service or maintenance of composite structures is very high [4] The research dealt with the correlation between damage and tensile residual strength in quasi-isotropic carbon fibre reinforced epoxy resin laminate loaded at the centre. Load was applied by means of a servo hydraulic machine and it was supposed to simulate a low velocity impact. The Acoustic Emission (AE) technique was used to detect damage progression. Tensile resistance after indentation was investigated and correlated with acoustic emissions parameters. This was been done for different lamination sequences on specimens cut from the damaged plates and for two different damage levels, one corresponding to the first load drop in the load- displacement curve[5] and to the complete perforation of the plate. Composite plates square 250x250 mm graphite/epoxy laminates were studied; their thickness was 1.6 mm. They have been made in autoclave from pre-pregs by stacking eight unidirectional plies with quasi-isotropic orientations, [0°,90°,45°,-45°] s . The specimens were placed in a circular clamping fixture with an internal diameter of 200 mm (Figure 1) and they were loaded at the centre by a hemispherical hardened steel ball with a radius 7 mm. Figure 1- Fixture System A servo hydraulic Instron 8033 testing machine controlled by a MTS Teststar II system and equipped by a 25kN load cell was used. The specimens were loaded monotonically in control of displacement and the head speed was of 0.05mm/sec. The test were stopped after the penetration of the steel ball into the laminate or at the first load drop During the test, the AE has been monitored by a Physical Acoustic Corporation (PAC) PCI-DSP4 device with four transducers PAC R15 setting up the amplitude threshold at 40 dB. After each quasi static test the damaged plate was sliced by a diamond saw in one of the different directions shown in figure 2. The indented zone was in the centre of these tensile specimens to be tested to find the residual tension strength of the laminate composite as a function of the stacking sequence. The tensile specimens had the same geometry suggested by ASTM D 5766 for open hole testing of CFRP, a width of 40 mm and a length of 250 mm.