Stress build

When a fiber breaks, the normal stress at each of its broken ends become zero. Over a distance of 1 /2 from each end, stress builds back up to the average value by shear stress transfer at the fiber-matrix interface. Also, the stress state in a region close to the broken ends contain the following ... 

Under thermal cycling conditions, the principal source of stress within the oxide scale is the temperature change . Christl et have noted that, when cooling 2.25%Cr-l%Mo steel from 600°C in air, compressive stresses build up in the haematite, whilst tensile stresses build up in the magnetite and spinel layers. This arises because the thermal expansion coefficients of the individual oxide layers increase in the order a metal < a spinel < a magnetite < a haematite . ... 

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

Weaver et al., formed superlattices with CdSe and CdS, using EC-ALE, without an automated system [163]. They studied their relatively thin deposits by surface enhanced Raman (SERS), examining stress build-up in the deposits. 

Where q0 is the length efficiency factor related to the aspect ratio of the filler and thus the stress build-up along its length it can take values from 0 to 1 and can be calculated using theoretical models, q, is an orientation factor that takes values of 1 for perfect alignment, 3/8 for alignment in the plane and 1/5 for random orientation. Equation (8.1) can be arranged as ... 

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. 

Stress builds up at a coherent interface between two phases, a and / , which have a slight lattice mismatch. For a sufficiently large misfit (or a large enough interfacial area), misfit dislocations (= localized stresses) become energetically more favorable than the coherency stress whereby a semicoherent interface will form. The lattice plane matching will be almost perfect except in the immediate neighborhood of the misfit dislocation. Usually, misfits exist in more than one dimension. Sets (/) of nonparallel misfit dislocations occur at distances... 

Eq. (15.98) strongly resembles Eq. (15.61), that we obtained for shear stress build up after starting a steady shear flow with shear rate q at time t = 0. Accordingly, the elongational or extensional viscosity, which is defined to be... 

The existence of a stress-free meniscus introduces a boundary discontinuity on both upper and lower rims. This discontinuity is bound to result in sharply enhanced stress build-up at the upper and lower contact lines. It may produce an effective slip layer on the sample/plate interfaces at the meniscus, yielding an overall torque, which is less than anticipated on the basis of Fig. 7a, since a large portion of the measured torque on the plate arises from the stress contribution at the rim r=R. This correction may increase with lowering the gap distance. Without a proper analytical treatment, it remains unknown how the magnitude of such an apparent slip depends on the rheological properties of the sample and whether the reported slip like behavior [ 19,33] is a manifestation of such a plausible edge effect. 

Thermal residual stresses are inherent to fibre reinforced composites due to the heterogeneity of the thermo-mechanical properties of their two constituents. Such stresses build up when composite structures are cooled down from the processing temperature to the test temperature. Residual stresses will be present on both a fibre-matrix scale (micro-scale), and on a ply-to-ply scale (macro-scale) in laminates built up from layers with different orientations. It is recognised that these stresses should be taken into account in any stress analysis. 

This cure shrinkage stress build-up is another point favoring the application of this type of membrane over a hot asphalt membrane on concrete. The asphalt has sufficient cold flow to relieve much of the stresses in the resin squares during the 7-day cures. 

Transient shear flows involve examining the shear stress and viscosity response to a time-dependent shear. The stress build up at the start of steady flow (<7+) and at the cessation of steady flow (a ) and the stress decay (ff(0) after a dynamic instantaneous impulse of deformation strain (y) can be used to characterize transient rheological behaviour. 

Another example of a transient measurement is given in Figure 3.73. Figure 3.73(a) shows the start up of an imposed strain rate and the subsequent measurement of the stress build-up ((t" " (0) over time. Figure 3.73(b) shows the relaxation of an applied strain rate and the subsequent measurement of the stress decay (polymer melt is shown in Figure 3.74. 

Figure 3.73. A transient rheological test showing (a) the start up of an imposed strain rate and the subsequent measurement of the stress build-up, and (b) the cessation of an imposed shear rate and the subsequent stress decay.

Figure 3.74. An example of a typical stress build-up during start up of shearing for a polymer melt.

Besides the highest thermomechanical stress build up due to the different thermal expansion behaviour, this system requires the largest temperature gradient over the compositional gradient for pressureless densification. 

---