Local stress

Elastohydrodynamic Lubrication (EHL). Lubrication needs in many machines ate minimized by carrying the load on concentrated contacts in ball and toUet beatings, gear teeth, cams, and some friction drives. With the load concentrated on a small elastically deformed area, these EHL contacts ate commonly characterized by a very thin separating hydrodynamic oil film which supports local stresses that tax the fatigue strength of the strongest steels. 

Piping supports, guides, and anchors increase local stresses on the pipe wall at the point of attachment. These stresses derive from continuously acting loads owing to the weight of the piping system carried at these points (pipe, contents, insulation), the pressure in the pipe, and any other loads such... 

However, conventional systems ia natural mbber do provide better flex life than EV cures, and this is one of the limitations of EV curiag. The short monosulftde bonds are less able to rearrange to reheve localized stresses which build duriag flexing, whereas the longer S bonds can. This abiUty for stress rehef is thought to be the mechanism for the superior flex life of conventional cures. 

Stress concentration Kis defined (1) as local stress/mean stress in a particle and calculated according to if = 1 + 2 LR) length and R is the radius of the crack tip. 

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

The closer one approaches to the tip of the crack, the higher the local stress becomes, until at some distance r from the tip of the crack the stress reaches the yield stress, o,, of the material, and plastic flow occurs (Fig. 14.2). The distance r is easily calculated by setting crio ai = o y in eqn. (14.1). Assuming r to be small compared to the crack length, a, the result is... 

Large differences in microstructure between parent metal, heat-affected zone and weld bead. Sharp changes in mechanical properties give local stress concentrations. 

Fig. 28.13. A sharp change of section induces stress concentrations. The local stress can be many times greater than the nominal stress.

Fig. 2. Schematic of energy dissipation in a commonly used peel test. The energy dissipation can occur in the adhesive and/or the adherends. The extent of energy dissipation depends on the elasto-plastic properties of the adhesive and the adherends under the test conditions as well as the local stresses and strains near the crack tip.

Aesthetically more pleasing than screws and bolts, while also introducing less localized stress on the substrates. 

Stress relieving at 600-700°C for steels decreases operation at temperatures some 20°C lower. Unfortunately, suitable furnaces generally are not available, and local stress relieving of welds, etc., is often not successful because further stresses develop on cooling. 

In each case the stresses in the global directions x, y may also be expressed in terms of the local stresses or strains. For example,... 

The local stresses and strains are then obtained from the stress and strain transformations... 

The significance of interlaminar stresses relative to laminate stiffness, strength, and life is determined by Classical Lamination Theory, i.e., CLT stresses are accurate over most of the laminate except in a very narrow boundary layer near the free edges. Thus, laminate stiffnesses are affected by global, not local, stresses, so laminate stiffnesses are essentially unaffected by interlaminar stresses. On the other hand, the details of locally high stresses dominate the failure process whereas lower global stresses are unimportant. Thus, laminate strength and life are dominated by interlaminar stresses. 

To decrease the thickness of a laminate, some of the laminae must be selectively stopped, dropped, or terminated. Moreover, the stopped laminae are covered with at least one continuous lamina as in Figure 7-31. There, resin-rich zones occur naturally in the vicinity of the ply drop. Such regions are also local stress-concentration sites. However, the tapered laminate creates a globally lessened stress concentration. 

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