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This study reviews several of the recent and commonly used fracture models for predicting the notched strength of composite laminates. Emphasis has been placed on semi-empirical fracture models which are operationally simple to utilize. The review is sufficiently detailed so that it is self-contained.

East Lansing. MI 48824 (Raeivod December 12. B89) (Revised Augur1 21. 1990)AB!TlXACl': An optimum level of interfacial bond strength between reinforcing fiber and a polymeric matrix in which it is placed is essential for acceptable composite mechanical properties and performance. The interfacial bond strength can be optimized only when the relationship between the level of fiber-matrix adhesion and thc mechanical and fracture behavior of composites is clearly understood. This study establishes the relationship between the fiber-matrix interfacial shear strength and 0" and 9 0 ' tensile and flexure p r o p t i e s of graphitdepoxy composites. A well defined and characterized graphite fiberlepoxy system was chosen in which the level of adhesion between fiber and matrix Mas changed by using the same gmphite fibers through the use of surface treatment and finish. The level of adhesion between the fiber and matrix associated with these changes resulted in an increase of fiber-matrix interfacial shear strength (ISS) by Over a factor of tvm while the fiber and matrix properties remained unchanged. The experimental results demonstrated that thc fiber surface modification did not have much effect on the tensile and flexural moduli and on the fiber dominated properties. However, the strengths and maximum strains that are governed by the matrix and interface properties were highly sensitive to the fiber surface modification. In addition, the major failure modes were also found to be affected by the fiber-matrix interfacial shear strength. 1. BACKGROUND HE SENSITIVITY OF the mechanical behavior of composite materials to the T fiber-matrix interfacial bond strength has long been realized [l-31. Longitudinal tensile failure of carbon fiber composites having low interfacial bond strength displays a broom-like fracture surface, while those having a high interfacial bond display a brittle fracture surface [4,5]. It has been qualitatively known 95 8 Joiiriial Of COhfPOSlTE MATERIALS, k' d. 25-Altgltst 1991

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.

An experimental investigation was performed to establish the relationships between fiber-matrix adhesion as determined by single fiber interfacial shear strength tests with the inplane and interlaminar shmr properties of gnphite/epoxy composites. *45"tension, Iosipescu, and short beam shear tests were conducted on three identical S~I S of composites differing only in their fiber-matrix interfacial shear strength. The fiber-matrix interphase and consequently the interfacial shear strength HBS varied by using the same graphite fiber was different surface modifications. namely untreated, surface treated, and surface treated and coated with a thin layer of epoxy. The surface modification changed the interfacial shear strength by more than a factor of IWD, while the properties of fibers remained unchanged. The experimental results showed that both inplane and interlaminar shear strengths increased approximately in the same ratio as the interfacial shear strength, however, the inplane shear modulus Has relatively insensitive to the fiber surface modifications. The fracture surface analpis revealed that when the fiber matrix interfacial shear strength was increased from low to intermediate to high values. the major failure modes changed from primarily interfacial failure to a combination of interfacial and matrix failure to primarily matrix failure, respectively. in a manner identical to that observed with the single fiber fragmentation tests.