Flexural strength, composition

Those stmctural variables most important to the tensile properties are polymer composition, density, and cell shape. Variation with use temperature has also been characterized (157). Flexural strength and modulus of rigid foams both increase with increasing density in the same manner as the compressive and tensile properties. More specific data on particular foams are available from manufacturers Hterature and in References 22,59,60,131 and 156. Shear strength and modulus of rigid foams depend on the polymer composition and state, density, and cell shape. The shear properties increase with increasing density and with decreasing temperature (157). 

Carbon-Fiber Composites. Cured laminates of phenoHc resins and carbon-fiber reinforcement provide superior flammabiHty resistance and thermal resistance compared to unsaturated polyester and epoxy. Table 15 shows the dependence of flexural strength and modulus on phenoHc—carbon-fiber composites at 30—40% phenoHc resin (91). These composites also exhibit long-term elevated temperature stabiHty up to 230°C. 

Figure 22 Influence of fiber content on flexural strength and fracture toughness of (O) softwood-cement composites and ( ) hardwood-cement composites (air-cured) [78].

However, these values are less than those recorded for composite resins used in dentistry. Goldman (1985) reports values of 29 to 49 MPa for anterior composite resins and Lloyd Adamson (1987) values of 76 to 125 MPa for posterior composite resins. A typical amalgam has a flexural strength of 6 MPa (Lloyd Adamson, 1987) (Table 5.16). However, the flexural strengths of some glass-ionomer cements increase with time and values as high as 59 MPa (after 3 months) and 70 MPa (after 7 days) have been reported (Pearson Atkinson, 1991). 

These low values for flexural strength and fracture toughness compared with the values for composite resins and dental amalgams make the glass-ionomer cement less suitable than these materials in high-stress situations. 

The mechanical strength of the composite disk specimens was tested, before and after immersion, under biaxial flexure conditionsn [8-10] with a universal testing machine (United Calibration Corp., Huntington Beach, CA). The biaxial flexure strength (BES) of the specimens was calculated according to mathematical expression (1) [8-10] ... 

Berthelot, J.M., Cupcic, A. and Maufras, J.M. (1978). Experimental flexural strength-deflection curves of oriented discontinuous fiber composites. Fiber Sci. Technol. 11, 367-398. 

Improvement in flexural strength due to silane treatments in glass liber thermoplastic matrix composites"... 

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).

Effect of through-the-thickness stitches on flexural strength and Mode I interlaminar fracture toughness of carbon fiber-epoxy matrix composites manufactured using unidirectional prepregs". 

A number of mechanical properties have been studied that may affect the clinical success of dental composite restorative materials. Among these are diametral tensile strength (DTS), flexural strength, fracture toughness, elastic modulus, hardness, and fatigue resistance. The mechanical properties should approximate those of tooth structure [183], but correlation of clinical success to any of these properties is limited. 

Flexural strength and flexural modulus are being used as the screening criteria for a new ADA specification 27 which adopts the new ISO standards. The minimum flexural strength required will be 50 MPa. In addition, the criterion makes it mandatory that the flexural strength must exceed a value, N, based on the flexural modulus, where N = (flexural modulus 0.0025) + 40. This stipulation will require higher flexural moduli from the stiffer composites like the conventional, small particle and hybrid systems. 

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