Fracture toughness epoxies

A proposed mechanism for toughening of mbber-modifted epoxies based on the microstmcture and fracture characteristics (310—312) involves mbber cavitation and matrix shear-yielding. A quantitative expression describes the fracture toughness values over a wide range of temperatures and rates. 

Fig. 5. Interlaminar fracture toughness, for a number of thermosetting and thermoplastic composites (36,37). Open white bars represent glass-fiber composites shaded bars are for carbon fibers. The materials are A, polyester (unidirectional) B, vinyl ester (CSM = chopped strand mat) C, epoxy (R/BR1424) D, epoxy (T300/914) E, PPS F, PES and G, PEEK. To convert J/m to fdbf/in. multiply by 2100.

A number of amorphous thermoplastics are presently employed as matrices in long fiber composites, including polyethersulfone (PES), polysulfone (PSU), and polyetherimide (PEI). AH offer superior resistance to impact loading and higher interlaminar fracture toughnesses than do most epoxies. However, the amorphous nature of such polymers results in a lower solvent resistance, clearly a limitation if composites based on such polymers are to be used in aggressive environments. 

Two wooden beams are butt-jointed using an epoxy adhesive (Fig. A1.3). The adhesive was stirred before application, entraining air bubbles which, under pressure in forming the joint, deform to flat, penny-shaped discs of diameter 2fl = 2 mm. If the beam has the dimensions shown, and epoxy has a fracture toughness of 0.5 MN mT , calculate the maximum load F that the beam can support. Assume K = cT Tra for the disc-shaped bubbles. 

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

Calculation of Stress Distribution in Macroporous Epoxies Fracture Toughness of Solvent-Modified and Macroporous Epoxies Prepared via CIPS. 

The mechanical properties of the blend of silane/size and bulk epoxy matrix (at concentrations representing likely compositions found at the fiber-matrix interface region) also suggest that the interaction of size with epoxy produces an interphase which is completely different to the bulk matrix material (Al-Moussawi et al., 1993). The interphase material tends to have a lower glass transition temperature, Tg, higher modulus and tensile strength and lower fracture toughness than the bulk matrix. Fig. 5.4 (Drown et al., 1991) presents a plot of Tg versus the amount of... 

Summary of Mode I and Mode II interlaminar fracture toughness values for unidirectional carbon fiber 828 mPDA epoxy matrix composites with different fiber surface treatments ... 

Madhukar, M.S. and Drzal, L.T. (1992b). Fiber-matrix adhesion and its effect on composite mechanical properties IV. Mode 1 and Mode II fracture toughness of graphite/epoxy composites. J. Composite Mater. 26, 936-968. 

Fig. 6.3. A theoretical plot of fracture toughness, R, with variation of frictional shear stress, Tf, compared with experimental fracture toughness values (A) Charpy impact and (A) slow bend tests for carbon-epoxy composites ( ) Charpy impact and (O) slow bend tests for carbon polyester composites. After...

Fig. 6.12. Toughness maps depicting contours of predicted fracture toughness (solid lines in kJ/m ) for (a) glass-epoxy composites as a function of fiber strength, Uf, and frictional shear stress, tf and (b) Kevlar-cpoxy composites as a function of at and clastic modulus of fiber, Ef. The dashed line and arrows in (a) indicate a change in dominant failure mechanisms from post-debonding friction, Rif, to interfacial debonding, Sj, and the effect of moisture on the changes of Of and Tf, respectively. Bundle debond length...

Fig. 6.25. Maximum fracture toughness, ATr, as a function of relative crack length, 2a/W, for carbon fiber-epoxy matrix [0°/ 45°/0°]j and (0°/90°]2 laminates. After Ochiai and Peters (1982).

Bathias, C., Esnault, R. and Bellas, J. (1983). On the increasing fracture toughness at increasing notch length of 0/90 and 0/ 45/0 graphite/epoxy laminates. Composites 14, 365-369. 

Caprino, G. Halpin, J.C. and Nicolais, L. (1980). Fracture toughness of graphite/epoxy laminates. Composites 11, 105-107. 

Gershon B. and Marom G. (1975). Fracture toughness and mechanical properties of glass fiber-epoxy composites. J. Mater. Sci. 10, 1549-1556. 

Tough matrices, such as thermoplastics and rubber-modified epoxies, are particularly useful for high fracture toughness and damage tolerance against... 

Fig. 7,2. Fracture toughness, R, of Kevlar 49-epoxy matrix composites (a) under varying strain rates in three-point bending and (b) at different temperatures under impact loading (O) uncoated fibers (0)41%, (Q) 63% and ( ) 100% Estapol coated fibers (A) silicone vacuum fluid (SVF) coated fibers. After Mai...

Fig. 7.4. Fracture toughness (O) and flexural strength ( ) of silicone rubber coated carbon fiber-epoxy matrix composites as a function of coating thickness. After Hancox and Wells (1977).

Fig. 7.5. (a) Transverse impact fracture toughness and (b) fiber pull-out length versus testing temperature for carbon fiber-epoxy matrix composites with and without PVAL coatings on fibers. After Kim and... 

Fig. 7.8. (a) Normalized impact fracture toughness and (b) interlaminar shear strength (ILSS) of carbon fiber-epoxy matrix composites as a function of glycidyl acrylate/methyl acrylate (GA/MA) interlayer... 

Fig, 7.16, Impact fracture toughness (O) and interlaminar shear strength (ILSS, ) of carbon fiber-epoxy matrix composites with varying number of nylon sheets as delamination promoters. After Havre (1977). 

Atkins A.G, and Mai Y.W. (1976). Effect of water and ice on strength and fracture toughness of intermittently bonded boron-epoxy composites J. Mater. Sci. II, 2297-2306. 

Jea L.C. and Felbeck D.K. (1980). Increased fracture toughness of graphite epoxy composites through intermittent interlaminar bonding. J. Composite Mater. 15, 245-159,... 

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