Flexural tensile stress

Class 2 flexural tensile stresses but no visible cracking. 

Class 3 flexural tensile stresses but surface width of cracks not exceeding 0.1 mm for members in very severe environments and not exceeding 0.2 mm for all other members. 

The effect of temperature on PSF tensile stress—strain behavior is depicted in Figure 4. The resin continues to exhibit useful mechanical properties at temperatures up to 160°C under prolonged or repeated thermal exposure. PES and PPSF extend this temperature limit to about 180°C. The dependence of flexural moduli on temperature for polysulfones is shown in Figure 5 with comparison to other engineering thermoplastics. 

Textile fibers must be flexible to be useful. The flexural rigidity or stiffness of a fiber is defined as the couple required to bend the fiber to unit curvature (3). The stiffness of an ideal cylindrical rod is proportional to the square of the linear density. Because the linear density is proportional to the square of the diameter, stiffness increases in proportion to the fourth power of the filament diameter. In addition, the shape of the filament cross-section must be considered also. For textile purposes and when flexibiUty is requisite, shear and torsional stresses are relatively minor factors compared to tensile stresses. Techniques for measuring flexural rigidity of fibers have been given in the Hterature (67—73). 

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. 

Fig. 2-16 Flexural specimen subjected to compressive and tensile stresses.

Although the creep behavior of a material could be measured in any mode, such experiments are most often run in tension or flexure. In the first, a test specimen is subjected to a constant tensile load and its elongation is measured as a function of time. After a sufficiently long period of time, the specimen will fracture that is a phenomenon called tensile creep failure. In general, the higher the applied tensile stress, the shorter the time and the greater the total strain to specimen failure. Furthermore, as the stress level decreases, the fracture mode changes from ductile to brittle. With flexural, a test specimen... 

Young s modulus may be calculated from the flexure of other kinds of beams. Examples are given in Table 1 (11,12). The table also gives equations for calculating the maximum tensile stress am ix and the maximum elongation en,.,x, which are found on the surface at the center of the span for beams with two supports and at the point of support for cantilever... 

Lay up the reinforcements according to the stresses so that they really withstand the loads the planes of the reinforcements will be parallel to the tensile stresses, and perpendicular to the shear, flexural or compressive stresses. 

Flexural strength The strength of a material in bending, expressed as the tensile stress of the outermost fibres of a bent test sample at the instant of failure. With plastics, this value is usually higher than the straight tensile strength. 

Because of its higher rigidity at warm temperatures, sand Thermopave formulations are not as flexible as asphalt concrete mixes. A typical sand Thermopave mix (6 wt % asphalt 12 wt % sulfur) exhibits a flexural strain at break of 0.004 cm/cm under the same test conditions as indicated in Table IV. Although this is below the strain values for asphalt concrete, lower flexibility in Thermopave can be tolerated as the tensile stresses and strains developed at the underside of the pavement are lower than for an asphalt pavement of equivalent thickness and subjected to the same loading. Performance of test pavements to date, some over six years old, have not indicated flexibility to be a problem as yet. 

The intrinsic strength of boron can be estimated in a flexure test. Assuming that in a flexure test the core and interface are near the neutral axis, critical tensile stresses would not develop at the core or interface Flexure tests on boron fibers lightly etched to remove any surface defects yield a strength of 14 GPa (DiCarlo, 1985). Without etching, the strength is about half this value... 

In thermomechanical analysis (TMA) the deformation of the sample under stress is monitored against time or temperature while the temperature increases or decreases proportionally to time. Changes are detected by mechanical, optical, or electrical transducers. The stress may be a compression, penetration, tension, flexure, or torsion. Generally the instruments are also able to measure the sample dimensions, a technique called thermodilatometry. The stress F/A) expressed in N/m or Pa may be a normal tensile stress cr, a tangential shearing stress x, or a pressure change Ap the force applied is F and A is the area. 

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

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