Compressive surface stress, caused

Thermal shock resistance can also be improved by the surface treatments used to increase mechanical strength (see 17.3.3.2). The stresses induced by thermal shock must exceed the compressive surface stresses to cause a crack to propagate, resulting in failure during quenching. 

For the description of surface rheological properties as we have said we need to consider the surface stress caused by dilatation/compression as well as by shear. As in the case of area changes two coefficients of surface shear flow exist. Using the same symbols for the shear and stress tensors, as given in Fig. 3.5., we obtain... 

Shot-peening is used to alter the surface condition of stainless steel subjected to fatigue conditions. Essentially the initial imposition of extra surface stresses causes local deformation that re-adjusts the stress state within the region. This process relaxes high local stresses. The subsequent creation of a thin layer of compacted steel introduces a residual compressive surface stress. Under usage conditions, externally applied tensile stresses are offset by this inherent compressive stress, as a... 

Adhesive joints often contain internal stress, that is, stress that is caused by internal movements as the joint is made, not by externally applied forces. These stresses are also called residual stresses because they remain in the joint after all external stresses have been removed. Such internal stresses are important because they cause premature failure, or even spontaneous fracture of the joint. However, in particular circumstances, an adhesive joint can be strengthened by inserting appropriate internal stress, just as glass is strengthened by blowing cold air on its surface during solidification, to promote residnal compressive surface stress. 

The failure of ceramic materials almost always results from a crack that is initiated at the surface by an applied tensile stress. To cause fracture of a tempered glass piece, the magnitude of an externally applied tensile stress must be great enough to first overcome the residual compressive surface stress and, in addition, to stress the surface in tension sufficient to initiate a crack, which may then propagate. For an untempered glass, a crack is introduced at a lower external stress level, and, consequently, the fracture strength is smaller. 

Figure 8.2.17b. Early calculations proposed that, in fact, compressive surface stress can soften this Ai phonon mode down to zero [32]. Later stress measurements using the cantilever-bending technique found carbon-induced compressive surface stress in the order of magnitude required for the phonon freezing [33]. While reconstruction and phonon softening are clearly related to each other, recent first principles calculations question that surface stress is the sole cause of the surface reconstruction [34-36].

Macrostrain is often observed in modified surfaces such as deposited thin films or corrosion layers. This results from compressive or tensile stress in the plane of the sample surface and causes shifts in diffraction peak positions. Such stresses can easily be analyzed by standard techniques if the surface layer is thick enough to detect a few diffraction peaks at high angles of incidence. If the film is too thin these techniques cannot be used and analysis can only be performed by assuming an un-... 

Real differences between the tensile and the compressive yield stresses of a material may cause the stress distribution within the test specimen to become very asymmetric at high strain levels. This cause the neutral axis to move from the center of the specimen toward the surface which is in compression. This effect, along with specimen anisotropy due to processing, may cause the shape of the stress-strain curve obtained in flexure to dif-... 

The results presumably were distorted by the unavoidable heating of the microcrystals occurring in the electron beam and other side effects. However, if the A a detected in the (111) reflexions is real, then the surface stress or rather surface tension 7S causing the compression of the lattice can be calculated. If R i and R2 in Eq. (37) are supposed to be identical and equal to 0.5 D, see above, then Pc= 4 ys/D. Let k be the compressibility of the micro-crystal, i.e., -dV/V.dp. Then the relative compaction under the pressure, that is -dV/V, is 4 ysk/D. The ratio dV/V may be approximated as 3 A aja. Hence... 

The force acting on any differential segment of a surface can be represented as a vector. The orientation of the surface itself can be defined by an outward-normal unit vector, called n. This force vector, indeed any vector, has direction and magnitude, which can be resolved into components in various ways. Normally the components are taken to align with coordinate directions. The force vector itself, of course, is independent of the particular representation. In fluid flow the force on a surface is caused by the compressive (or expansive) and shearing actions of the fluid as it flows. Thermodynamic pressure also acts to exert force on a surface. By definition, stress is a force per unit area. On any surface where a force acts, a stress vector can also be defined. Like the force the stress vector can be represented by components in various ways. 

From the decrease of the lattice constant in small crystals caused by the compression due to the surface stress The lattice spacings can be measured with the help of X-ray diffraction or by LEED experiments (see Section 8.7.2). 

If a piezoelectric plate (Fig. 6.1), polarized in the direction indicated by P, carries electrodes over its two flat faces, then a compressive stress causes a transient current to flow in the external circuit a tensile stress produces current in the opposite sense (Fig. 6.1(a)). Conversely, the application of an electric field produces strain in the crystal, say a negative strain reversal of the field causes a positive strain (Fig. 6.1(b)). The changes in polarization which accompany the direct piezoelectric effect manifest themselves in the appearance of charges on the crystal surface (see Eq. (2.71)) and, in the case of a closed circuit, in a current. 

Generally, in a TM problem, an increase in temperature causes an expansion of the material which results in compressive stresses. The presence of free surfaces often make it possible to reduce compressive stresses in the normal direction on the surface and cause tensile stresses in the orthoradial. 

This effect, which applies to microscale texture, induces a macroscopic evolution of the network. At the onset of drying, the surface of the liquid is flat. The curvature increases as the liquid of the gel evaporates. The liquid is then in tension, and as a consequence the solid part of the gel is under compression. This effect causes the gel network to shrink. That shrinkage continues as long as the solid network (depending on the nature of the gel) is not stiff enough to resist the compressive stress. 

---