The effect of curing schedules on fiber-matrix adhesion in carbon fiber–epoxy resin composites

Fiber-reinforced polymeric composites are used in a large and growing number of applications, all requiring different property sets including the nature of the fiber-matrix adhesion to which the present work is addressed. Specifically, the number of curing cycles, curing temperature and schedule, degree of cure, use of accelerants, annealing, and the use of fiber handling agents are investigated for systems of Hexcel IM7 carbon fibers embedded in Epon862 (resin) and Epikure Curing Agent W (hardener) using the single-fiber fragmentation method. The fractional extent of cure is monitored using differential scanning calorimetry (DSC), so that comparisons are made at the same degree of cure (99%). Single-stage curing at the highest temperature produces the highest apparent adhesion, and the use of accelerants significantly increases the curing rate while maintaining the same level of adhesion. Accelerants in some cases, however, decrease the plastic yield strength of the specimens. Annealing reduces induced residual stress and apparent adhesion, but not below the baseline achieved at lower curing temperatures. Plastic yield strength and apparent adhesion decrease for any degree of cure lower than 95%, while the use of handling agents shows no effect on adhesion.

Polyvinyl Alcohol Fiber Length Optimization for High Ductility Cementitious Composites with Different Compressive Strength Grades

The fiber length has a significant impact on the fiber bridging capacity and the mechanical properties of high ductility cementitious composites (HDCCs), which is related to fiber/matrix interfacial bonding. However, this fundamental knowledge of HDCCs design has rarely been investigated systematically. To this end, this study deeply investigates the effect of the fiber length on the bridging stress and the complementary energy with various fiber/matrix interfacial bonds in theory. Then, the mechanical performances of HDCCs with various fiber lengths and compressive strengths were evaluated experimentally. In micromechanical design, longer fibers can achieve stronger bridging stress and more sufficient complementary energy regardless of the fiber/matrix interfacial bonding properties. However, it should be noted that the increase in bridging capacity was quite slow for the overlong fibers and excessive interfacial bonding. The experiments indicated that overlong fibers (18 mm and 24 mm) easily twined on the mixer blade and were hard to disperse evenly. The HDCCs with shorter fibers displayed better workability. The compressive strength was less affected by the fiber length, and most striking differences were less than 5.0%, while the flexural properties and the tensile properties first increased and then decreased when the fiber length ranged from 6 mm to 24 mm. Consequently, the fibers with lengths of 9 mm and the fibers with lengths of 12 mm were better candidates for the HDCCs with compressive strengths of 30 MPa to 80 MPa, and fibers with lengths of 9 mm caused the HDCCs to exhibit higher ductility properties in general.