Residual Stress Zone Research Articles

A procedure for treating secondary stress in failure assessment diagram

The CEGB R6 Rev.3 failure assessment method, for structures containing defects subjected to combined primary and secondary loads, is widely adopted all over the world. A set of ϱ factors, which was derived by Ainsworth according to a deformation bounding theorem and the concept of reference stresses, has been given in the revision. However, the results obtained with those ϱ factors are unsafe when a small defect is located within a high tensile residual stress zone, and are over-conservative in other situations. In this paper, recommended ϱ values based on R6 option 1 are given by redefining the reference stress under pure secondary loads. The recommended ϱ factors overcome the above-mentioned shortcomings of R6 Rev.3. This procedure has been accepted in the Chinese “A Guidance of Safety Assessment for Pressure Vessel Containing Defects” (SAPV-95).

ｘ線材料強度 正面研削加工したセラミックスの曲げ強度

The surface of the bending specimens of silicon nitride was ground by the face grinding method. Face grinding was conducted with a cup-shaped diamond wheel whose mesh size was #200/230. The feed direction of face grinding was parallel and perpendicular to the longitudinal direction of the specimen. The surface roughness of the face-ground specimen was one-fifth of that of the surface-ground specimen, when a diamond wheel with the same mesh size was used for grinding. The residual stress state was compressive at the ground surface. The depth of the compressive residual stress zone was about 25μm. The compressive residual stress took the maximum of about 1GPa which was slightly smaller than that of the surface-ground specimen. When compared with the lapped specimen, the face-ground specimens had higher bending strength in the directions both parallel and perpendicular to the feed direction, in contrast to the surface-ground specimens whose strength reduced by about 200MPa in the direction perpenducular to the grinding direction. The reasons for the increment of bending strength for the face-ground specimens were as follows: (1) The main grinding flaw induced by face grinding was in the plane which was formed between the feed direction and the rotating direction of wheels, so that the grinding flaws were hardly remained in the face-ground surface. (2) The compressive residual stress contributed directly to the increment of bending strength of the face-ground specimens, because the size of grinding flaws was smaller than the depth of the compressive residual stress zone.It was concluded that the face grinding method was recommended for machining of ceramics, because of high efficient machining and a high quality of finished surfaces.

窒化ケイ素の研削き裂寸法の破壊力学的評価

The surfaces of bending specimens of silicon nitride were ground in two different grinding directions with a diamond wheel. One was parallel and the other was perpendicular to the longitudinal direction of specimens. The Weibull distribution of the bending strength of ground specimens was compared to that of lapped specimens. The average fracture strength normal to the grinding direction fell by about 25% in comparison with the average strength of lapped specimens. The residual stress distribution near the ground surface was measured by the cosφ method of X-ray stress measurement. The stress state was biaxial. One of the pricipal directions of stresses was along the grinding direction. Both of the principal residual stresses, σ1 and σ2, were compressive and decreased exponentially with increasing the distance from the surface. The residual stress was diminished at about 30μm in depth. The increment of strength due to compressive residual stresses was determined by calculating the stress intensity factor of a semi-elliptical surface crack in the residual stress field. The size of grinding flaws degrading bending strength was estimated from the relation between the fracture strength and the surface crack size. The depth of grinding flaws parallel to the grinding direction ranged approximately from 40 to 70μm; the deepest point of flaws was out of the zone of compressive residual stresses.

Fatigue crack growth in polymers subjected to fully compressive cyclic loads

It is experimentally demonstrated in this work that the application of cyclic compression loads to polymeric materials, specifically high-density polyethylene and polystyrene, results in the nucleation and propagation of stable fatigue cracks. The cracks grow at a progressively slower rate along the plane of the notch in a direction perpendicular to the far-field cyclic compression axis. The overall characteristics of this compression fatigue fracture are macroscopically similar to those seen in metals, ceramics, as well as discontinuously reinforced inorganic composites. It is reasoned that the origin of this Mode I compression fatigue effect is the generation of a zone of residual tensile stress locally in the vicinity of the notch-tip upon unloading from the maximum far-field compressive stress. The residual tensile field is generated by permanent damage arising from crazing and/or shear deformation ahead of the notch-tip. Evidence for the inducement of residual tensile stresses on the crack plane is provided with the aid of micrographs of near-tip region where crazes are observed along the plane of the crack, i.e. normal to the compression loading axis. Compression fatigue crack growth in polystyrene is also highly discontinuous in the sense that the crack remains dormant during thousands of fatigue cycles following which there is a burst of crack extension, possibly in association with fracture within the craze. This intermittent growth process in cyclic compression is analogous to the formation of discontinuous growth bands during the tension fatigue of many crazeable polymers. The exhaustion of the near-tip residual tensile field and the increase in the level of crack closure with increasing crack length cause the fatigue crack to arrest. The universal features of this phenomenon are discussed in the context of ductile and brittle, non-crystalline and crystalline, as well as monolithic and composite materials.

Fatigue crack growth in unidirectional graphite-epoxy composites under cyclic compression

It is shown that in addition to the well-known failure process involving kink band formation, the application of cyclic compressive loads to notched plates of graphitr-epoxy composites can lead to the nucleation and growth of a single fatigue crack normal to the compression axis and to the fibre direction. This stable mode I compression fatigue fracture is a consequence of the generation of a zone of residual tensile stresses within the notch-tip region where deformation occurs in the form of matrix cracking, fibre breakage, fibre buckling nad/or interfacial debonding

Fatigue crack growth in cementitious materials under cyclic compressive loads

This paper presents experimental results and a discussion of the mechanisms of fatique crack growth under uniaxial cyclic compressive loads in cementitious materials containing stress concentrations. It is demonstrated that the application of cyclic compressive loads to single edge-notched specimens of mortar induces Mode I fatigue cracks from the notch-tip in a direction normal to the far-field compression axis. The experiments of this study, in conjunction with the recent work of Suresh and coworkers on the compression fatigue behavior of a wide range of ceramics and ceramic composites, suggest that the driving force for Mode I crack advance is the generation of a zone of residual tensile stresses in the near-tip region of the stress concentration upon unloading from the far-field compressive stress. Numerical simulations of notch-tip residual stresses in brittle solids, including mortar and plain concrete, indicate that the residual tensile stresses are of sufficient magnitude to induce fracture along the plane of the notch. This Mode I fatigue crack growth mechanism in cyclic compression, observed at nominal compressive stress values far below the uniaxial compressive strength, is vastly different from the splitting mode of failure generally found in cementitious materials under monotonic compression. Practical implications of the cyclic compression fracture mechanism are highlighted.

Relationship between the amount of ? phase and residual stresses in weldments of steel 12Kh18N10T

1. Residual deformation and stresses that reach 700–800 MPa occur in the process of welding steel 12Kh18N10T. These stresses are several times higher than the yield strength of the material. Decomposition of austenite and formation of α phase occur in the zone of peak tensile stresses, the amount of α phase reaching about 10% in the zone of highest stresses and then declining along the width of the sample by a nonlinear rule. 2. Alpha phase is formed only in the region of elastic-plastic deformation in the zone of residual tensile stresses. The point of transition from residual tensile stresses to compressive stresses is the boundary of the zone in which α phase occurs. With increaseing first-order residual stresses and degree of plastic deformation, the amount of α phase increases sharply by a nonlinear rule.

転がり接触問題解析の一手法 (第2報)

A theoretical approach is presented to the plane theory of rolling contact with friction. An analysis is given of plastic deformation which affects elastic-plastic and residual stress fields in rolling contact. Some effects are discussed of sress fields due to tangential load. In case of static loads, because of friction the tensile zone of residual stress σχis constructed in the neighbourhood of the rear contact area, and the negative shearing residual stress zone is extended widely than that of positive one. The axial residual stress field σz is affected by the secondary plastic flow due to unloading. When the combined loads ran away the values of residual stresses do not vary because of friction, while the tangential load can be understood that specifies the depth of plastic zone.

Analysis of Thermal Stress during Welding

This report is concerned with the development of an analytical system for calculating thermal stresses during welding and resulting residual stresses.The present analysis concerns only longitudinal stresses, that is, stresses in the direction parallel to the weld line. Computer programs developed in this study calculate both temperature and stress distribution during welding. Using these programs, an analysis was made of effects of welding parameters on thermal stresses during bead-on-plate welding of ASTM 2219-0 aluminum alloy.Important results are : (a) The maximum stress at the weld center is little affected parameters. However, the width of tensile residual stress zone is significantly affected by welding parameters, especially heat input. With an increased heat input, a wider residual stress zone is formed.(b) However, when a weld is made with very low heat input, considerably high tensile stresses are produced in area just behind the arc.(c) Fairly high compressive stresses occur in area ahead of the welding arc.Experiments were conducted. Temperature and strain changes were recorded during bead-on-plate welding of 2219-0 aluminum alloy plate, 30 inches long, 18 inches wide, and 1/4 inch thick.Measured temperature changes coincide well with calculated ones by assuming the arc efficiency, η, to be 0.7. As far as the general trend is concerned, measured longitudinal strains coincide fairly well with calculated ones. However, transverse strains and shearing strains reach considerable amounts in area near the welding arc. The current analysis is not adequate for studying complex stress changes in area near the welding arc.