Characterization For The Cracking And Shearing Behaviour Of Carbon/Epoxy Laminated Composites

Steel plates have shown an effective and convenient method of improving the performance of structures when used for rehabilitation or upgrading of deteriorated members of constructions. However, recent research has shown that using fibre-reinforced polymers have overcome the disadvantages encountered with steel plates. The fibre-reinforced polymers provide the advantages of a non-corrosive and a more versatile strengthening system.Composite structural materials may contain aligned continuous fibres and/or angle-ply laminates to form rods, plates, and other different profiles. Fibres that are mostly used for reinforcing polymers are carbon, aramid, and glass.The rehabilitation process may be done either by mechanical bonding or adhesive bonding. Each method has got its advantages and disadvantages together with the appropriate field of application. Thus, fibre composites are getting more popular as structural materials due to their stiffness, strength, and toughness.Recently, fibre reinforced plastics have found applications in civil engineering. Owing to their mechanical properties, these materials have replaced steel and steel plates for external crack repairs in deteriorated structures. Fibre reinforced plastic sheets can be fixed to the deteriorated structural parts using suitable adhesives.In this work, a new clamping system capable of holding specimens, to be tested, is developed to enable tensile cracking and shear cracking tests. This system simulates, as accurately as possible, the cracking criterion. It is also designed to allow making tests with different gauge lengths. These gauge lengths represent possible expected spaces which are made when cracking effect takes place in any body. This is a very important feature of the system since repairing of cracks needs external fixing of materials onto the surface of the body at the area of the crack. For deep cracks, a more or less narrow material may be used, and thus it needs to be fixed at the appropriate position. The gauge length criterion of the system solves the problem of where to fix the material inorder to prevent more cracking effect. It can also be considered as a matter of changeable overlap length of the joint made between the substrate and the laminated composite material.The system is also designed to represent the single-lap joint, which is believed to be more representative for the testing of adhesive joint strengths upon shearing. Another important feature of the clamping system is its capability of being mounted on the SHIMADZU AG-10 tensile meter.Carbon fabrics and carbon fibre reinforced epoxy laminated composite materials were used to join together two parts of sand-cement mixture by an adhesive at three different gauge lengths. To study the effect of cracking, the performance of the adhesive joint was tested for tensile and shearing effects. The angle-ply laminated composites were prepared with the designations of [0°, +45°, -45°] and [90°, +45°, -45°].During the tensile cracking test, the materials of both fabrics and laminated composites were adhered to the substrates only at an angle of 90° to the loading direction.On carrying out the shear-cracking test, carbon fabrics were adhered to the substrates at angles of 90°, +45°, and -45° to the loading direction, whereas the angle-ply laminates were adhered at angles of 0° and 90° to the loading direction. At every testing stage, photographs of the fracture modes were taken and presented.When dealing with tensile cracking, better results were attained when medium gauge length was used. The joint strength showed good results when carbon fabric was adhered to the parts with a gauge length of 10 mm.The 3-iayered angle piy laminated carbon fibre composite material is round to be more suitable for cracking force resistance than the unidirectionally woven carbon fabric. The joint showed the highest strength value with this material at a gauge length of 10 mm too.Thus, better results have been attained in repairing cementitious cracks by positioning the joining material and the adhesive in the middle of the crack and not at the sideswhere narrow or wide crack space is found. It is preferable to use an angle-ply laminated composite rather than a unidirectionally woven fabric by joining it in the central part of the crack meant for repair. The critical adhering length where peeling and/or slippage does not exist has been found to be 5.1 cm when using a fabric and 3.7 cm when a laminate is used.The shearing test results have shown that the fabrics laid at -45° were damaged firstly and then followed by an adhesive fracture. Those laid at +45° have shown fabric fracture followed by joint fracture at 10 mm gauge length, and fabric damage was occurred at gauge lengths of lmm and 20 mm. Joint failure was observed when the fabric is laid at 90° at lmm and 10 mm gauge lengths while at 20 mm gauge length the fabric fractured firstly and followed by joint failure. Substrate failure mode was noticed with the angle-ply laminated composite materials with both of their layouts and at the entire gauge lengths used. However, the [0°, +45°, -45°]. designated laminate showed the maximum shear stress at the largest gauge length while the [90°, +45°, -45°] designation showed the maximum shear stress to exist at the smallest gauge length. The critical adhering length was found to be 5.5 cm and 5.4 cm for each designation respectively.Axial tensile tests were also carried for the aim of knowing the effect of preform structure on the tensile performance of composite materials. Three different materials: ends, preforms, and composites were studied to determine the effect of preform structure on the fracture behaviour of the composite materials. The preform structures are unidirectionally laid ends (UDF), a non-woven fabric (NWF), and a warp-knitted (WKF) fabric. The tensile performance and characteristics were, measured at different stages;single ends, preforms, and composite materials.Sudden fibre fracture and epoxy cracking occurred in UDF composites, whereas a gradual fracture was noticed for the NWF and WKF composites. For the UDF composite, the fracture occurred parallel and perpendicular to the loading direction at different positions with fibre/matrix interfacial fracture. The NWF fractured in the same mode but in the middle of the specimen. The WKF material showed a fracture that isperpendicular to the loading direction and the process involved a brittle fracture. Thus the structure of the preform affected the tensile performance of the composite materials.From the data of the tensile tests, a comparison was made between the UDF and the WKF composites to lcnow the effect of the transversely aligned filaments on the axial tensile characteristics. The results have shown that the transversely aligned fibres had increased the load values and the elongation percentage of the WKF composite material. However, they reduced the modulus of the composite materials.