Compressive transverse stresses

To conclude the analysis of the influence of biaxiality ratios, it is worth mentioning the work by Perevozchikov et al. [19]. They investigated the fatigue behavior of unidirectional glass/epoxy mbular samples under the combined effect of shear stresses and transverse tension or compression stresses and reported the negative effect of cyclic shear stress components in the case of both tensile and compressive transversal stress. It was also shown that the increase of Xu ratio induced a reduction in the fatigue strength. 

In the present multiscale approach, SAXS data analysis reveals that already at low deformations (lower than e(start)) the lamellar crystals oriented with their normal parallel to the tensile force, in order to alleviate compressive transversal stress, undergo buckling instability, with consequent formation of undulated chevron-like super-structures (Fig. 11.12b ). However, at deformations close to e(end) up to the breaking the effect of lamellar stacking and parallel orientation of the lamellar normal to the stretching direction on the SAXS intensity becomes buried by the large diffuse scattering localized on the equator due to cavitation (Fig. 11.12c"). 

The curves for 3M XP251S fiberglass-epoxy are shown in Figures C-1 through C-5 [C-1]. Curves are given for both tensile and compressive behavior of the direct stresses. Note that the behavior in the fiber direction is essentially linear in both tension and compression. Transverse to the fiber direction, the behavior is nearly linear in tension, but very nonlinear in compression. The shear stress-strain curve is highly nonlinear. The Poisson s ratios (not shown) are essentially constant with values v.,2 =. 25 and V21 =. 09. 

Figures 12 and 13 show interfacial out-of-plane transverse and shear stress distributions of the co-cured double lap joints with [0]i6t and [ 45]4s stacking sequences along the interfaces between steel and composite adherends, respectively. It is important to consider interfacial out-of-plane shear stress rather than interfacial out-of-plane transverse stress because of the compressive stress distribution due to the symmetric configuration of the co-cured double lap...

One may attempt to derive the ideal shear strength So of the van der Waals solid normal to the chain axis from the value of the lateral surface free energy, a. This value is well known for common polymers such as PE or polystyrene (PS) (Hoffman et al, 1976) or else can be calculated from the Thomas-Stavely (1952) relationship a = /a Ahf)y, where a is the chain cross-section in the crystalline phase, Ahf is the heat of fusion, and y is a constant equal to 0.12. If one now assumes that a displacement between adjacent molecules by Si within the crystal is sufficient for lattice destruction then the ultimate transverse stress per chain will be given by So = cr/31. The values so obtained are shown in Table 2.1 for various polymers. In some cases (nylon, polyoxymethylene, polyoxyethylene (POE)) the agreement with experiment is fair. In the others, deviations are more evident. In order to understand better the discrepancy between the experimentally observed and the theoretically derived compressive strength one has to consider more thoroughly the micromorphology of polymer solids and the phenomena caused by the applied stress before lattice destruction occurs. 

The results of tension tests upon refractories in the hot state are not available in the literature nor are data relating to the transverse strength of bricks and tiles. This is to be regretted since in many furnace constructions transverse loads must be considered. Again, it seems very probable that the compression test of firebricks will ultimately be replaced by one involving transverse stress. 

Fig. AIA.2 Theoretical and experimental transverse stresses for a ja = 0.50 expressed as 1 ratio of the uniform compressive stress...

The results for a 25-4 mmx 25-4 mm (1 inxl in) lap joint show that the form of the adherend tensile stress and the adhesive shear stress (in the longitudinal direction) were much as would be expected from a Volkersen (1938) type solution (Fig. 15). However, the transverse stresses show direct (tension or compression) and shear stress maxima at the ends of the joint (see Figs 16(a) and (b)). 

The strength of laminates is usually predicted from a combination of laminated plate theory and a failure criterion for the individual larnina. A general treatment of composite failure criteria is beyond the scope of the present discussion. Broadly, however, composite failure criteria are of two types noninteractive, such as maximum stress or maximum strain, in which the lamina is taken to fail when a critical value of stress or strain is reached parallel or transverse to the fibers in tension, compression, or shear or interactive, such as the Tsai-Hill or Tsai-Wu (1,7) type, in which failure is taken to be when some combination of stresses occurs. Generally, the ply materials do not have the same strengths in tension and compression, so that five-ply strengths must be deterrnined ... 

The moduli of elasticity, G for shear and E for tension, are ratios of stress to strain as measured within the proportional limits of the material. Thus the modulus is really a measure of the rigidity for shear of a material or its stiffness in tension and compression. For shear or torsion, the modulus analogous to that for tension is called the shear modulus or the modulus of rigidity, or sometimes the transverse modulus. 

Strength can be measured in compression, in tension, in shear and transversely (flexural strength). However, if we exclude plastic flow as a means of failure, then materials can only fracture in one of two ways (1) by the pulling apart of planes of atoms, i.e. tensile failure, or (2) by the slippage of planes of atoms, i.e. shear failure. Strength is essentially a measure of fracture stress, which is the point of catastrophic and imcontrolled failure because the initiation of a crack takes place at excessive stress values. 

A powder compact s TS is the stress required to separate its constituent particles in tensile mode. This is measured for the tableting indices by transverse compression of the square compacts, using narrow platens. Stresses build within the sample until it fails in a tensile mode that is perpendicular to the direction of platen movement. Tablets that are manufactured on a traditional tablet press and that have high TS are considered hard and generally robust, and so this is a highly desired attribute for immediate release and other tablet types. 

The TS of the compacted samples was determined by transverse compression with a custom-built tensile tester. Tensile failure was observed for all the rectangular compacts when compressed between flat-faced platens at a speed ranging between 0.006 and 0.016 mm/sec. Platen speed was adjusted between materials to maintain a time constant of 15 2 seconds to account for viscoelastic differences the constant is the time between the sample break point and when the measured force equals Fbreak/e in the force versus time profile, where the denominator is the mathematical e. Specially modified punch and die sets permitted the formation of square compacts with a centrally located hole (0.11 cm diameter) that acted as a stress concentrator during tensile testing. This capability permitted the determination of a compromised compact TS and thus facilitated an assessment of the defect sensitivity of each compacted material. At least two replicate determinations were performed for each mechanical testing procedure and mean values are reported. 

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