Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: IV. Mode I and Mode II Fracture Toughness of Graphite/Epoxy Composites

Abstract

To optimize the level of fiber-matrix adhesion an understanding of the relationship between fiber-matrix interfacial bond strength and the mechanical and frac ture behavior of composites is essential. This study establishes the relationship between fiber-matrix interfacial shear strength (ISS) and interlaminar fracture toughness (both Mode I and Mode II) and failure modes for graphite/epoxy composites. A well defined and characterized graphite fiber/epoxy system was chosen in which the level of adhesion be tween fiber and matrix was changed by using the same graphite fibers with different sur face treatments. These surface treatments changed the level of adhesion between the fiber and matrix thus resulting in an increase of the fiber-matrix ISS by over a factor of two while the fiber and matrix properties remained unchanged. The Mode I and Mode II tests were conducted by the double cantilever beam (DCB) and end-notch flexure (ENF) tests methods, respectively. The Mode I fracture toughness ( GIC) of composites having low fiber-matrix ISS could not be determined from the DCB test because of extensive fiber bridging and crack meandering. For the composites having higher values of the ISS, the GIC increased with the ISS. The experimental results demonstrated that there is a strong dependency of Mode II fracture toughness ( GIIC) on fiber-matrix adhesion. Increased fiber-matrix adhesion in one group of composites significantly improved the GIIC , but the presence of brittle interphase around graphite fibers in another group of composites tended to cancel part of the improvement resulting from increased adhesion. Based on the major failure modes occurring during the Mode I and Mode II loading conditions, a causal link age between fiber-matrix adhesion and interlaminar fracture behavior of graphite/epoxy composites is established.

Experimental optimization of the impact energy absorption of epoxy–carbon laminates through controlled delamination

“…Most of the above mentioned methods have been reviewed by Kim and Mai [26], who also pointed out the key role played by the fibre-matrix interfacial shear strength in determining the interlaminar resistance of composite laminates. In fact, a marked improvement of the interlaminar resistance when the fibrematrix adhesion increases is reported in several studies [27][28][29][30]. On the other hand, the improvement of damage resistance and tolerance in interlaminar fracture is at the expenses of other important mechanical properties, such as the the energy absorption under impact conditions [31] and the fatigue resistance.…”

Validating the Classical Failure Criteria for Applicability to the Notched Woven-Roving Composite Materials

“…The debond between the fiber filaments and matrix was clear. The justification for this is that the load is mainly transferred by the interfacial region between the fiber and matrix and the existence of shear stress component 6 [1,[15][16][17][18]. Another attestation for our interpretation is the X-radiograph taking a graphite-epoxy [0 2 /90 2 ] s laminate having a center notch by Binienda et al [34].…”

“…They also proposed an optimization of fiber coating procedure to reduce the stress concentration at the fiber matrix interaction locations. At the macrolevel scale, Madhukar and Drazal [15][16][17][18] published a series of papers describing the considerable effect of the interphase/interface on macrolevel strength and stiffness properties.…”

Fracture toughness of adherends bonded with two-part acrylic-based adhesive: double cantilever beam tests under static loading

“…The term C is the compliance of the specimen, expressed as The compliance is measured by an appropriate polynomial function, where A 1 , A 2 , and A 3 are experimentally determined constants of the specimen.The compliance can also be measured using classical beam theory. In a double cantilever beam, the compliance [24] can be expressed as where E is the modulus of elasticity and I is the moment of inertia of the cross section, given by where h is the thickness. The moduli of elasticity for the aluminum alloy, GF/PP, and CF/ EP specimens were taken as 70, 20, and 50 GPa, respectively.…”