Shear stress intensity

Earlier work in this laboratory and others demonstrated in vitro dysfunction of both platelets and leukocytes following application of low-level shear stress (1 4). The effects of mechanical trauma can be attributed to bulk shear stress effects rather than surface-mediated ones, since changing the surface-to-volume ratio by a factor of three did not alter the leukocyte or platelet response to a given shear stress intensity and exposure duration (2). Platelet microaggregate formation and adhesion alterations occurred following shear stress exposure in the range of 150 dyn/cm2 for 2 min, while... 

The smallest value is 3,500 in-lb. If, for example, the beam were a simple beam carrying a load Won a 10-inch span, the bending moment at the center of the span would be WL/A. Setting this equal to 3,500 in-lb gives the load Was 1,400 lb. Shear Fis W/2 or 700 lb. Using this value, the shear stress intensity at various horizontal planes in the beam may be computed by means of the shear stress equation. For planes b-b, c-c, and d-d, for example ... 

In all probability tiie shear stress intensity is actually higher adjacent to the rib, and lower near the outer ends of the flange, but in any event it is not likely to be excessive. 

High-Intensity Mixer. Mixers such as that shown in Fig. 18-44 combine a high shear zone with a fluidized vortex mixing action. Blades at the bottom of the vessel scoop the batch upward at peripheral speeds of about 40 m/s (130 ft/s). The high shear stress (to 20,000 s" ) and blade impact easily reduce agglomerates and aid intimate dispersion. Since the energy input is high [200 kW/m (about 8 hp/fE)h even powdery material is heated rapidly. 

Let US now look at how this contact geometry influences friction. If you attempt to slide one of the surfaces over the other, a shear stress fj/a appears at the asperities. The shear stress is greatest where the cross-sectional area of asperities is least, that is, at or very near the contact plane. Now, the intense plastic deformation in the regions of contact presses the asperity tips together so well that there is atom-to-atom contact across the junction. The junction, therefore, can withstand a shear stress as large as k approximately, where k is the shear-yield strength of the material (Chapter 11). 

But, there is no need to rely on hugonium. The theory and practice of the deformation of solids under other, less intense, loadings are well developed and show that the fluidlike flow of shock deformation is the expected consequence of the motion of defects in response to applied shear stresses that exceed the shear strength of solids. In most shock loadings, the shear stresses are well in excess of that shear strength and there is certainly ample theory and experiment to qualitatively identify overall features of the defect genera-... 

The defect question delineates solid behavior from liquid behavior. In liquid deformation, there is no fundamental need for an unusual deformation mechanism to explain the observed shock deformation. There may be superficial, macroscopic similarities between the shock deformation of solids and fluids, but the fundamental deformation questions differ in the two cases. Fluids may, in fact, be subjected to intense transient viscous shear stresses that can cause mechanically induced defects, but first-order behaviors do not require defects to provide a fundamental basis for interpretation of mechanical response data. 

The symmetric stress-intensity factor k, is associated ith the opening mode of crack extension in Figure 6-10. The skew/-symmetric stress-intensity factor l<2 is associated ith the fonward-shear mode. These plane-stress-intensity factors must be supplemented by another stress-intensity factor to describe the parallel-shear mode. The stress-intensity factors depend on the applied loads, body geometry, and crack geometry. For plane loads, the stress distribution around the crack tip can always be separated into symmetric and skew-symmetric distributions. 

The stress-intensity factors are quite different from stress concentration factors. For the same circular hole, the stress concentration factor is 3 under uniaxial tension, 2 under biaxiai tension, and 4 under pure shear. Thus, the stress concentration factor, which is a single scalar parameter, cannot characterize the stress state, a second-order tensor. However, the stress-intensity factor exists in all stress components, so is a useful concept in stress-type fracture processes. For example. 

An analysis of loading mode effects has also provided evidence of the critical role of hydrogen. A stress-intensity factor (K) can be achieved in either a tensile loading mode (mode I) or a shearing mode (mode III) (Section 8.9). Under mode I conditions the volume of metal immediately in... 

The maximum intensity of stress allowed will depend on the particular theory of failure adopted in the design method (see Section 13.3.2). The maximum shear-stress theory is normally used for pressure vessel design. 

---