The effect of cure cycle on microstructure and mechanical properties of interlayer toughened composites

Carbon fibre reinforced thermoset composites make for lighter and stronger structures and are widely used for secondary and primary structures of commercial airplanes. The cure cycle is important to achieve a high quality of composite components. Cure path dependency for thermoset composites has received increasing interest from automotive and aerospace researchers as fast cure cycles are required to achieve high-volume and low-cost composites manufacturing. Furthermore, toughened composites are using increasingly complex resin formulations and laminate microstructures to increase fracture and damage resistance. These advances raise the question whether modifying the cure cycle affects microstructures and chemical, physical and mechanical properties of toughened composites. This dissertation studies the processing-microstructure-property relationship for the interlayer toughened thermoset prepreg composite T800SC/3900-2B. The work has three main parts. The first part investigates curing effects on interlaminar microstructure and properties of composite constituents. Laminates were processed to the same degree of cure using different cure cycles. Interlaminar microstructure and particle morphology were examined using optical microscopy and scanning electron microscopy. In-situ elastic modulus of the base resin and toughening particles was characterized using nanoindentation. The second part studies curing effects on elastic properties during cure. Lamina shear modulus development was measured using dynamic mechanical analysis. The third part investigates curing effects on mechanical properties after complete cure. Mode I and Mode II interlaminar fracture toughness were measured using double cantilever beam and end-notched flexure tests. Damage resistance was examined using quasi-static indentation and low-velocity impact test. The work revealed that cure path dependency for T800SC/3900-2B prepreg composites is mainly driven by the glass transition of interlaminar toughening particles. Microstructural characteristics such as the interlayer thickness, particle shape and particle volume fraction, and manufacturing anomalies such as fibre migration into the interlayer, are affected by particle deformation which is dependent on the material state of particles in the pre-gelation stage. Cure-dependent interlaminar microstructures affect both elastic properties during cure and mechanical properties after complete cure. This dissertation discovered a new mechanism for cure path dependency of interlayer toughened composites, glass transition of the toughening particles, which is governed by the cure path in a conversion-temperature-transformation process map.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat