Residual stresses near surface

X-ray diffraction Slow 40 MPa Only polls the surface Position-sensitive detector Residual stress near the surface... 

There are three primary sources of tensile stresses for RPVDs These are (1) fabrication induced stresses, (2) primary stresses, (3) and secondary stresses. Fabrication induced stresses consist of stresses introduced during manufacture and installation (i.e., fit-up and assembly in the shop or field plus those introduced by machining or forming operations and welding). As is the case for weld residual stresses, hard machining, abusive grinding can produce surface residual stresses near or above the yield point of the material. 

Attack at welds due to bacteria is possible, but it is not nearly so common as is often supposed. Because of residual stresses, microstruc-tural irregularities, compositional variation, and surface irregularities, welds show a predisposition to corrode preferentially by most corrosion mechanisms. Attack is common along incompletely closed weld seams such as at butt welds in light-gauge stainless steel tubing (Fig. 6.9A and B). Attack at carbon steel welds may occur. Figure 6.10 shows a severely corroded carbon steel pipe from a service water sys-... 

Several experimental procedures can be used to measure the residual stresses. The three preferred methods involve diffraction (X-ray or neutron), beam deflection, and permanent strain determination. X-ray diffraction measurements have the limitation that the penetration depth is small, such that only near-surface information is obtained. Moreover, in composites, residual stresses are redistributed near surfaces.47 Consequently, a full stress analysis is needed to relate the measured strains to either q or a. ... 

Plastic flow can also set up residual stresses. The beam shown in Fig. I6-2(a) is supported at two points and loaded by two equal forces Fapplied near each end. At any point between the two supports the stress in the outside fibers is constant, tensile on the top of the beam and compressive on the bottom. These stresses are a maximum on the outside surfaces and decrease to zero at the neutral axis, as indicated by the stress diagram at the right of (a). This diagram shows how the longitudinal stress varies across the section AA, when all parts of the beam are... 

The small diameter of the incident beam from a stress camera or a Fastress unit is an advantage when one wishes to measure stress variations from point to point on a surface, as in the region near a weld. The stress distribution shown in Fig. 16-16, determined by a photographic method, simulates the residual stresses due to spot welding. The specimen was a steel strip 10 x 3 x inch (25 x 8 x 0.6 cm). A circular area of about I inch (1 cm) diameter, whose size is indicated on the graph, was heated locally to about 700°C for a few seconds by clamping the strip at its center between the two electrodes... 

Crack propagation by the Vickers indentation was observed to prove the presence of residual stresses. When the surface of SiC layer was indented, the crack propagation was serious near disk center. This means that tensile stress expanded the cracks on SiC surface. Figure 4 shows the crack propagation on the section of the ceramic part. It was found that the cracks parallel to interface were longer than those perpendicular to interface. This means that tensile stress was large in the perpendicular direction to expand the cracks. These results are well consistent with the expectation from the calculation of stress distributions. 

Near-surface structure occurs up to approximately 50 pm and is affected by surface hardening or reaction film formation in this regime. Near-surface structure is particularly susceptible to severe deformation and is important because the residual stresses in these regions will affect crack propagation during lubrication failure. 

The residual stresses in the CD are—equally biaxial compression near the surface and equally biaxial tensions in the core. According to the stress optic law (Eq. 8.9), the birefringence is proportional to the difference between the principal stresses. Therefore, the birefringence due to residual stresses should be nearly zero. 

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