Tensile stress, with transverse

Edges of the test specimens should preferably be sawed or machined. If specimens are obtained by shearing, the plastically deformed edges should be dressed by machining, sanding, or filing to a depth equal to the specimen thickness to remove residual short transverse tensile stresses. With very susceptible matericils stress-corrosion cracks parallel to the surface may initiate in the edges. 

In this expression ex is the modulus perpendicular to the chain axis and wj is the work of fracture for the transverse tensile stress. Equation 39 demonstrates that the Griffith length Lq increases rapidly with decreasing orientation of the chain angle 0. 

Type 2 is ductile crack propagation with the crack opening to form a V-notch. Finally, the crack becomes catastrophic. It is found in nylon and other melt-spun synthetics. Classical illustrations of this type are readily available (3, 4). However, variations can occur. Figures la and lb show a polyester fiber subjected to tensile stress in which, after the V-notch formed, failure continued along a plane parallel to the fiber axis before eventually crossing the fiber. Thus, a split-level transverse break had occurred. 

When a solid is subjected to a tensile stress, it extends in the direction of the stress but contracts in the perpendicular direction. This is quantified by the Poissson s ratio which is the ratio of transverse strain to the longitudinal strain. This can be understood with reference to the application of a uniaxial stress as shown in Figure 10.01 (a) where the elongation in x-direction is associated with a shrinkage in y and z directions. Thus v is defined by the transverse, -e, (e, has a negative sign) and the longitudinal strain, e/, as. 

Poisson s ratio is a measure of the reduction in the cross section accompanying stretching and is the ratio of the transverse strain (a contraction for tensile stress) to longitudinal strain (elongation). Poisson s ratio for many of the more brittle plastics such as polystyrene, the acrylics, and the thermoset materials is about 0.3 for the more flexible plasticized materials, such as cellulose acetate, the value is somewhat higher, about 0.45. Poisson s ratio for rubber is 0.5 (characteristic of a liquid) it decrease to 0.4 for vulcanized rubber and to about 0.3 for ebonite. Poisson s ratio varies not only with the nature of the material but also with the magnitude of the strain for a given material. All values cited here are for zero strain. 

In orthotropic laminates with transverse plies (as shown before) the small Poisson s ratio of these plies introduces large stresses perpendicular to the loading direction, especially if other plies with a large Poisson s ratio are present. An example from Rohwer [11], shown in Fig. 5.4, shows the displacements in thickness and width direction of a quasi-isotropic laminate. The deformation of the edge element of the grid in the 90° ply indicates severe tensile stresses in the thickness direction. Thus, for this laminate the tensile stresses in the 90° ply at the specimen edge are very critical, which is also shown in Fig. 5.5. 

The transverse tensile strength (TTS) was measured with the 3-point bending fixture at l/h=5, but now with the fibres parallel to the rollers. Fracture initiates at the point of maximum tensile stress in the bottom face of the beam below the central roller. Samples with a length of 10 mm (this is the width in the test) were cut between perspex plates with a diamond saw in order to obtain smooth cuts. The transverse tensile strength was calculated as TTS = 3/2 F/(w h) (1/h). 

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