Stress risers

Corners on the edges of the sole plate should have at least a two-ineh radius to prevent the ereation of stress risers and subsequent eraeking of the eorners. 

Avoid surface discontinuities such as pits, slip marks (notches) and other damage that act as stress risers. Stresses concentrate at the tip of the notch. Therefore, stress-corrosion cracks usually originate from the base of a pit. 

The simultaneous action of cyclic stress alternating tensile and compressive and corrosive attack is known as corrosion fatigue. Corrosive attack can be in the form of pitting. These pits function as notches, acting as stress risers and initiate cracks. Once a crack is formed, the probability of pipe failure is enhanced by further corrosion as corrosion is accelerated by action of stress. The tip of the crack deep within the fracture, the area under the greatest stress, is anodic to the wider part of the crack. As corrosion progresses, the metal at the tip of the crack goes into the solution, the crack deepens and eventually penetrates the wall of the tube. 

Requirements for vessels fabricated by forging in Part UF include unique design requirements with particular concern for stress risers, fabrication, heat treatment, repair of defects, and inspection. Vessels fabricated by brazing are covered in Part UB. Brazed vessels cannot be used in leth service, for unfired steam boilers, or for direct firing. Permitted brazing processes as well as testing of brazed joints for strength are covered. Fabrication and inspection rules are also included. 

Flat end pieces (plates) are frequently chosen for production reasons. For monobloc forgings of high pressure vessels or welded on end pieces it is important to avoid stress risers at the transition locations. For conical transition the stresses yield only slightly larger than those in the cylindrical wall (Fig. 4.3-8). 

In hot mixing or elevated-temperature curing of an epoxy system, vapor pressure could also be of concern relative to the quality of the adhesive bond. If the components in an epoxy system become too hot, boiling can occur, resulting in gas bubbles. If gas bubbles become trapped in the cured adhesive film, they can lead to reduction of cohesive strength and stress risers. For many adhesive applications, particularly those in the electrical and electronic industries (due to possible ionization of air voids), complete removal of any gas bubbles from the epoxy is essential. 

Brittle Failure (8). Brittleness is a principal consideration in selecting construction materials for liquid hydrogen service. Brittle fracture can result in the essentially instantaneous release of a vessel s contents, the hazard being a combined one of PV energy release and the possibility of fire and/or explosion. Three conditions must exist for a brittle fracture to occur 1) a stress riser, a crack, notch, or other discontinuity, 2) a section where the actual stress exceeds the yield stress of the material, and 3) a temperature below which failure occurs without appreciable plastic deformation. Metals that are satisfactory for liquid hydrogen service include aluminum, stainless steels, brass, and copper. Carbon steel is not suitable. 

The mechanical response of composites, as shown in these exploratory studies, indicates dependence on the ease with which fracture can occur between fibers, yarns, and plies. Poorly crystallized matrices result in composites that are strong and stiff but with little yield so that failure occurs catastrophically. In contrast, more crystalline matrices seem to be not quite as strong and to have a lower effective modulus, but their increased strain capability ensures that failure is not catastrophic the composited strength decays gradually as further strain is applied. Thus, the energy required for total failure is increased, and the composite with more crystalline matrix is more tolerant of defects or stress risers. 

As a second example, we show in Fig. 27 the nucleation of intergranular stress corrosion cracks in sensitized Type 304SS as observed by Diercks and Dragel at the Argonne National Laboratory (see [56]). The cracks were found to nucleate from pits or at intergranular penetrations (e.g., intergranular attack, IGA). In other cases, cracks are found to nucleate from MnS inclusion but, in this case too, the primary event is probably the formation of a pit which acts as a stress riser. Examination of many micrographs indicates that the critical nucleus is of the order of 30-1 (X) pm deep. 

In addition to appearance, EP products offer other advantages such as an extremely smooth surface which minimizes the adherence of debris, an increased chromium to iron ratio which improves corrosion resistance, the creation of a passive layer that is free from iron contamination, improved ability to visually detect surface defects, and improved mechanical property performance through the minimization of stress risers. 

Nevertheless, failures have been known to occur during hydrostatic testing because the metal temperature was below the NDT temperature of some plate and a stress riser, such as the presence of two unreinforced nozzles near one another or a large defect, was present. 

Fillets and Radii. All shaft shoulder fillets and radii shall be made as large as practical and finished to reduce additional stress risers. 

Solid oxide fuel cells (SOFCs) involve structures composed of multiple ceramic layers, or interleaved layers of ceramic membranes and metal interconnects. To eliminate premature mixing of fuel and air gases or leaking of these gases from interior regions of the structure, the interfaces of adjacent layers are sealed with a glass or ceramic seal. These seals must withstand the high-temperature environment of the SOFC over its lifetime. Therefore these materials must be thermally matched with the adjacent layers to minimize transient stress risers and eliminate the potential for consequent seal failure. 

In Fig. 4.6A, the product shows a notch effect, a severe stress riser causing a serious weakness in the corner. A small radius (Fig. 4.6B) is better, and a large radius (Fig. 4.6C).or a slope (Fig. 4.6D) is best for strength in the comer area, provided the slope blends into the straight portion with a large radius and without a notch. 

As we discussed earlier, any change in plastic thickness has several disad-to the flow and to the creation of stress risers, which, unless very can severely affect the product strength in the area of the thicken-... 

An inside radius in a steel part is easy to produce using any machining method (milling, turning, or grinding). This radius is also important for the life expectancy of the steel part, as it avoids stress concentration points (stress risers) and consequent breakage. The radius on the core pin will then produce a radius hi the molded piece where, in normal design practice, a chamfer would be expected (Fig. 5.8). 

On the other hand, a comer at the end of the core pin will be easier to chamfer than to make a radius such chamfer will reproduce a chamfer on the inside of the plastic where normally a radius would be expected and desired. The (inside)-chamfer in the plastic (Fig. 5.9B) now has two comers (stress risers) Jherefore, it is recommended that the designer specify the core pin chamfer to have well-rounded comers. 

However, the opposite is true when designing a plastic stud. The entrance of e hole (in the mold), which creates the plastic stud, is easy.to make with a Cham er as shown in Fig. S.lOl but this point in the plastic should have well-rounded comers. It is far better to have the comer well roMded, as shown in Fig. 5.9. There should also be aradius at the deep end of the nnie. This radius is a good protection against stress risers in the mold cavity. 

ASTM G 58, Practice for Preparation of Stress Corrosion Test Specimens for Weldments— This covers the manufacture and application of welded test specimens in stress corrosion testing. This practice is used to evaluate a total weldment, weld metal, or presence of notches and stress risers in weldments with respect to SCC in an environment. In addition, ASTM G 58 contains a method for evaluating the critical stress levels that will produce SCC in a weldment. The specimens are evaluated after exposure to an environment by microscopic methods. This standard includes typical specimens (stressed and tension), welding considerations, test specimen preparation, and inspection after exposure. 

In general, the addition of nonadherent filler is an undesirable method of filling plastics materials. The fillers add bulk and improve some properties such as the compressive strength and modulus, and they generally improve the heat resistance and reduce the coefficient of thermal expansion. Because of the nonadherence, however, they make the materials brittle and weak in tension and bending. This results from the fact that each particle acts as a stress riser, increasing... 

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