Stress flexural

Flexural stress SiC mpture curves are shown in Figure 3 (27). AU. the forms tend to be fairly resistant to time-dependent failure by elevated temperature creep. In addition, SiC shows outstanding resistance to oxidation even at 1200°C as a result of formation of a protective high purity siUca surface layer (28). 

Table 10-56 gives values for the modulus of elasticity for nonmetals however, no specific stress-limiting criteria or methods of stress analysis are presented. Stress-strain behavior of most nonmetals differs considerably from that of metals and is less well-defined for mathematic analysis. The piping system should be designed and laid out so that flexural stresses resulting from displacement due to expansion, contraction, and other movement are minimized. This concept requires special attention to supports, terminals, and other restraints. 

In Table 28.9 we have evaluated the cross-breaking, shearing and bending (flexural) stresses, that may act at such locations, to establish the suitability of the supports used. 

There are a number of different modes of stress-strain that can be taken into account by the designer. They include tensile stress-strain, flexural stress-strain, compression stress-strain, and shear stress-strain. 

The stress-strain behavior of plastics in flexure generally follows from the behavior observed in tension and compression for either unreinforced or reinforced plastics. The flexural modulus of elasticity is nominally the average between the tension and compression moduli. The flexural yield point is generally that which is observed in tension, but this is not easily discerned, because the strain gradient in the flexural RP sample essentially eliminates any abrupt change in the flexural stress-strain relationship when the extreme fibers start to yield. 

The stress-relaxation behavior of a material is normally determined in either the tensile or the flexural mode. In these experiments, a material specimen is rapidly elongated or compressed to produce a specified strain level and the load exerted by the specimen on the test apparatus is measured as a function of time. Specimens of certain plastics may fail during tensile or flexural stress-relaxation experiments. 

A maximum flexural stress of 9,500 psi is assumed for polycarbonate. This conservative stress value should account for degradation in ultraviolet stabilized polycarbonate exposed to long term solar exposure. While more research is required in this area, it is reasonable to expect at least a ten year useful life for ultraviolet stabilized polycarbonate. A Young s modulus of 345,000 psi and a Poisson s ratio of 0.38 are also assumed for polycarbonate. 

To incorporate the effect of material strength increase with strain rate, a dynamic increase factor (DIF) is applied to static strength values. DIFs are simply ratios of dynamic material strength to static strength and are a function of material type as well as strain rate as described above. DIFs are also dependent on the type of stress (i.e. flexural, direct shear) because peak values for these stresses occur at different times. Flexural stresses occur very quickly while peak shears may occur relatively late in time resulting in a lower strain rate for shear. 

Note A related quantity is the flexural stress which is somewhat arbitrarily defined as the amplitude of the stress in the convex, outer surface of a material specimen in forced flexural oscillation. 

A specific example of this can be found in the evaluation of composites for heat exchanger applications.13 In this case, the heat exchanger design calls for a tubular construction which will be pressurized. Under these conditions, a flexural stress will be present in service, and consequently, a C-ring test specimen configuration provides a reasonable way to examine the properties of the composite. 

Testing conditions (stresses, temperatures, environments) will ultimately have to match up with service conditions. If flexural stresses are expected in service, then a flexural test is one that should be performed as part of the test suite. If thermal or stress cycles are expected, then simulation of these conditions should also be included in the evaluation of the material. 

ASTM D790-71 was used to obtain flexural strengths and tangent moduli. The tests were performed in water at 37 °C on specimens preconditioned in water 50 hr at 37 °C to approximate bioconditions. Comparative fracture energies were calculated from the flexural stress-deflection curves by taking the area under the curves. 

Tangent modulus of elastcitv, ASTM D790-71. c Area under flexural stress-deflection curve. 

FIGURE 12.11 Improvements of the mechanical properties of three-dimensional reinforced CMCs by hybrid infiltration routes (a) R.T. flexural stress-strain plots for a three-dimensional carbon fiber reinforced composite before and after cycles of infiltration (comparison between eight cycles with zirconium propoxide and fonr cycles pins a last infiltration with aluminum-silicon ester (b) plot of the mechanical strength as a fnnction of the final open porosity for composites and matrix of equivalent porosity, before and after infiltration (Reprinted from Colomban, R and Wey, M., Sol-gel control of the matrix net-shape sintering in 3D reinforced ceramic matrix composites, J. Eur. Ceram. Soc., 17, 1475, 1997. With permission from Elsevier) (c) R.T. tensile behavior (d) comparison of the R.T. mechanical strength after thermal treatments at various temperatures. (Reprinted from Colomban, R, Tailoring of the nano/microstructure of heterogeneous ceramics by sol-gel routes, Ceram. Trans., 95, 243, 1998. With permission from The American Ceramic Society.)... 

It was shown above that the flexural stress at center-point load is described by the formula (7.20)... 

As it was indicated above, to apply a uniformly distributed load testing is a technically difficult task therefore, testing is usually done using three- or four-point load. As it was shown above, Eq. (7.42)-(7.44) describe equivalency between these loads in terms of their equal flexural stress at the outer surface ... 

Figure 3.52 Flexural stress-strain behavior of two grades (MFRs) of ETFE at room temperatureJ i...

Flexural Fatigue - Progressive localized permanent structural change occurring in a material subjected to cyclic flexural stress that may culminate in cracks or complete fracture after a sufficient number of cycles. 

Flexural Properties - Properties describing the reaction of physical systems to flexural stress and strain. 

Flexural Stress - The maximum fiber stress in a specimen at a given strain in a bending test. The maximum fiber stress is a function of load, support span, and specimen width and depth. It depends on the method of load application relative to the... 

The response of a polymer blend to tensile, compressive and flexural stresses is examined in the initial section. Its rigidity, fatigue and failure characteristics are also studied. The toughened... 

Flexural stress (at conventional deflection) Flexural stress at maximum load Flexural Stress at rupture Fracture... 

The flexural stress at a deflection equal to 1.5 times the thickness of the test piece. 

The flexural stress developed when the load reaches the first maximum. The flexural stress developed at the moment of rupture. 

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