![]() ![]() If the operating stresses, including the residual stresses, exceed the strength of the part, it will result in part failure and fractures ( “F”)ĭepending on the scale of their effects, residual stresses are grouped into three categories (Refs. "Processes to build up desired resiudual stresses") and/or fracture the part. In this case, cracking will reduce the residual stresses ( Fig. ![]() thermal fatigue), very high residual stresses can be induced. In a deformation-restricting situation (e.g. At least in the case of cracking, this development should unallowably worsen the operating behavior.Ĭhanges in the residual stresses also occur when the operating stresses exceed the flow limit and cause plastic deformation. This effect also occurs when the material yields through cracking ( “D”), such as that caused by stress corrosion cracking or liquid metal embrittlement (“ E”). If the residual stresses lead to plastic deformation or, at higher temperatures, creep ( relaxation, “C”) they will be reduced. When the bow is drawn by an external force ( “B”), the stresses from this “operating force” overlay with the residual stresses. In its “designed” resting state ( “A”), the bowstring is under tensile stress, and the bow is under bending stress (tension and compression). As long as they don`t change, they won`t make themselves outwardly apparent. 16.2.2.4-15).įigure "Effects of residual stresses": Residual stresses are stresses in a body that are in a state of equilibrium ( Fig. However, it is still more accurate to speak of stress relief heat treatment (annealing) rather than stress-elimination heat treatment (Ill. "Residual stresses in spite of stress-relief annealing"). Through the use of creep effects of longer annealing times ( relaxation), residual stresses can be reduced very significantly ( Fig. If undesired residual stresses are present in a part, heat treatments can reduce them considerably, but not eliminate them completely. At the surface, the plastic deformations caused by machining will always create residual stresses near the flow limit. This makes this method problematic for evaluating their effects on the operating behavior of a part. Non-destructive methods can only be used to determine the residual stresses in a very thin surface zone (10 mm range). It is possible to measure residual stresses using both destructive and non-destructive methods (Ills. The expansion of the lattice under tensile stress promotes the absorption of hydrogen, and therefore hydrogen embrittlement. "Chlorine in process baths causing stress corrosion") and liquid metal embrittlement (Ills. They are the cause of stress corrosion cracking ( Fig. Tensile stresses promote and/or are a prerequisite for cracking under the influence of certain media. 16.2.2.4-11 and 16.2.2.4-12) because they worsen the behavior (dynamic fatigue, creep) of the parts, especially when they overlay with tensile operating stresses. In contrast, tensile residual stresses are usually undesirable (Ills. Case hardening makes use of high compressive stresses in the hardening layer, which are based on an increase in volume of the hardening structure. Structural differences can also create compressive residual stresses. "Increasing fatigue strength by shot peening"). The most common of these is shot peening ( Fig. In order to improve dynamic fatigue strength, finishing processes that specifically induce compressive residual stresses in parts are frequently used. However, they can also have damaging effects if they promote the delamination of coatings (e.g. 16.2.2.4-7 and 16.2.2.4-10) because they improve the operating behavior (dynamic fatigue strength) of the parts. "Equilibrium of residual stresses").Ĭompressive residual stresses are usually desirable (Ills. The process parameters have a decisive effect on whether tensile or compressive residual stresses are created, and how powerful these are ( Fig. There is probably no stage in the finishing process in which residual stresses are not created or changed (Ills. Changes in micro residual stresses during the finishing process can cause dimensional changes. "Increasing fatigue strength by compressive residual stresses"). relaxation, hardening) and/or during operation (creep, cracking ( Fig. These effects can be observed in finishing (e.g. Changes in macro residual stresses (type I residual stresses) cause movement, i.e. Residual stresses in a part are in a state of equilibrium ( Fig. Residual stresses are grouped into three categories as described in Reference 16.2.2.4-1: type I, type II, and type III ( Fig. Therefore, thermal stresses resulting from temperature gradients are also residual stresses. In this chapter, residual stresses (also referred to as internal stresses or self-equilibrating stresses) are defined as stresses existing in a part without external loads. ![]()
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