Crack tip constraints in a weldment and their effect on. For engineering assessments it is common to utilize a calculation procedure sometimes called the crackface pressure method. K vs k max plot for al 7475 t7351 alloy tested in the lt orientation for two crack growth rates sadanada et al 1999. Defect assessment of welded specimen considering weld. Residual stressinduced constraint ctodqr formulation in welded components, cracked specimens may suffer from residual stresses and the residual stresses will in. Measurement of swellinginduced residual stress in ion. Test temperatures ranged from 196c to room temperature, in order to provoke a range of behaviour from fully brittle to fully ductile. Residual stress is often a cause of premature failure of critical components, and was probably a factor in the collapse of the silver bridge in west virginia, united states in december 1967. By combining the ctod crack tip opening displacement and qstress odowd and shih. The thermal insulation of a crack was found to have a great impact on the fracture parameters hutchinson and lu, 1995. Lets keep it really simple with a basic examplethink about those oneuse disposable chopsticks that one gets at an asian restaurant. Effect of residual stresses on the cracktip constraint in. Recently, the effect of residual stress on the crack tip stress field has been studied by liu et al. Determination of k and cod in residual stress fields the stress intensity factor and crack opening displacement for a crack in a weld residual stress field can be determined in more than one way.
Crack tip constraints in a weldment and their effect on ductile crack growth zhiliang zhang and erling ostby norwegian university of science and technology ntnu and sintef materials and chemistry esis tc1 autumn meeting, 56 december 2012, freiburg. Residual stress induced crack tip constraint, engineering. How does compressive residual stress increase fatigue life. In general, compressive residual stress in the surface of a component is beneficial. For three different cases, crack length and residual stress intensity factor sif are calculated and its. The effects of residual stress magnitude on crack driving forces and cracktip constraint are also studied. The stresses measured in this work were several gpa in magnitude.
Ideally, knowledge of the impact on nucleation and crack growth within the region of residual stress is required. Results also show that the r value decreases with the increase in the external load. A description of the compliance method for measuring residual stress. Residual stress and its role in failure iopscience. Residual stress induced by machining of steel the specimen and the method of measurement of residual stress are shown in table 1 and machining conditions are shown in table 2 as a 1, b 1, c 1.
Residual stress induced constraint ctodqr formulation. The maximum region of residual stress in the crack tip was obtained by numerical simulations which calculated the effect of notch radius and penetrated stress. Effect of welding residual stresses on crack opening. In terms of the stress intensity factor, the stress very near to the crack tip can be approximated as. Residual stresses at a blunted crack tip springerlink. Residual stress induced crack tip constraint request pdf. However, crack tip constraint r elevated with the increase of residual stress. Results show that the residual stresses slightly reduced the ductile crack growth resistance. Process induced residual stress in fibrereinforced thermoset polymermatrix composites was analysed using a unit cell model and the finite element method, with the consideration of chemical shrinkage of the epoxy resin and thermal cooling contraction of the whole fibre and resin system. The crack tip opening displacement ctod and constraint parameter r were calculated for different specimens and residual stresses. Nevertheless, there must be some sort of mechanism or property of the material that prevents such a crack from propagating. The external load and crack tip geometry constraint in the model are controlled by a remote boundary condition, which is a function of k and t stress. Residual stresses and their effects on fatigue resistance to improve fatigue resistance we should try to avoid tensile mean stress and have compressive mean stress.
To break them apart, we simply pull on the two of them at the ends and they split at the junction. Residual stress induced by machining of steel the specimen and the method of measurement of residual stress are shown in table 1 and machining conditions are shown in table 2. The constraint parameter r introduced by the authors can be used to rank the crack tip constraint induced by the biaxial residual stresses. To compare analytical computations with results of experiments, the initial residual stress field, indicated in fig. Analytical modeling and applications of residual stresses. The fourbubble failure criterion does not indicate whether a fatigue crack initiated on the fillet surface can propagate through or arrest in the compressive residual stress zone induced by the rolling process. By virtue of the outcomes from the residual stress free specimen, fatigue crack closure characteristics are decided in residual stress fields on. Includes qualitative and theoretical descriptions, actual experimental examples, and comparisons with other methods.
Modified pathindependent jintegral and crack tip constraints were calculated for each model, using computer software developed for the postprocessing of the finite element results obtained through abaqus analyses, with and without residual stresses. For engineering assessments it is common to utilize a calculation procedure sometimes called the crack face pressure method. By virtue of the outcomes from the residual stressfree specimen, fatigue crack closure characteristics are decided in residual stress fields on. Only in case b do the residual stresses affect the stress intensity range. The investigations are performed on compact specimens made of 10mnmoni55 highductility bainitic steel with a computerized xray diffractometer. K eff, due to residual stress induced crack closure, this acts to limit the exposure of the cracktip to the full applied.
Recently, the effect of residual stress on the crack tip stress. The crack length of specimens with compressive residual stress is shorter. Rail cracking due to residual stress from manufacturing and. The constraint parameter r is defined to characterize the residual stress induced crack tip constraint. The tip of a blunted crack is regarded as a stress concentrator of a special kind. Processinduced residual stress in fibrereinforced thermoset polymermatrix composites was analysed using a unit cell model and the finite element method, with the consideration of chemical shrinkage of the epoxy resin and thermal cooling contraction of the whole fibre and resin system. In toughened glass, compressive stresses are induced on the surface of the glass, balanced by tensile stresses in the body of the glass. A very nice use of measurements made in situ during fatigue testing in order to examine residual stress effects on fatigue crack growth. The effects of residual stress magnitude on crack driving forces and crack tip constraint are also studied.
Weld induced longitudinal residual stress profile can be obtained through sintap procedure, fea or experimental analysis. Not about measuring residual stress but about calculating ki in a dented pipe. Numerous surveys have been conducted by various organizations in recent years. Assessing the effects of riveting induced residual stresses. The residual stress induced constraint is also coupled with the t stress. In welded components, cracked specimens may suffer from residual stresses and the residual stresses will influence the crack tip stress field. Modified pathindependent jintegral and cracktip constraints were calculated for each model, using computer software developed for the postprocessing of the finite element results obtained through abaqus analyses, with and without residual stresses. Most recently, the residual stressinduced cracktip constraint has been investigated by liu et al. It has been shown that the crack tip stress eld is strongly in uenced by the residual stresses and a new parameter,r, is proposed to characterize the residual stress induced crack tip constraint. The external load and crack tip geometry constraint in the model are controlled by. Stress triaxiality constraint and crack tip parameters.
In this method, rather than calculate shrinkage strain we directly calculate a residual stress distribution for each element. The entire residual stress field, throughout the cross section of a component is evaluated. Reynolds, ap, evaluation of residual stresses during fatigue test in an fsw joint, strain, 442, p. Residual stress induced crack tip constraint residual stress induced crack tip constraint liu, j zhang, z. The remodelled crank shaft shows that the residual stress is less when compared with conventional crankshaft. Residual stresses were induced in the other half of the specimens via a thermal loading technique, with the residual stress distribution induced before the introduction of the crack. In this study, the objectives of the investigation are twofold. Abstract a relationship between crack tip constraints of specimens and those defined by the hutchinsonricerosengren hrr solution in the plane strain case has been derived from the corrected stressstrain field of anderson 1989, by taking account of losses in triaxiality of specimens.
To evaluate the effect of residual stresses on rail integrity, an analysis technique was sought that could quantify the effect of these stresses on web crack propagation. The eyebar links were castings which showed high levels of residual stress, which in one eyebar, encouraged crack growth. This can often be achieved by using residual stresses. It has been shown that the crack tip stress eld is strongly in uenced by the residual stresses and a new parameter,r, is proposed to characterize the. R is chosen to characterize crack tip constraint induced by the residual stress. The approach proposed in bs 7910 combines global relaxation of residual stress q m under high mechanical load with local enhancement of crack tip driving force through the adoption of a simplified primarysecondary stress interaction factor. Residual stress induced crack tip constraint sciencedirect. This effect results in a temperature discontinuity across crack faces and a complex temperature distribution very near the crack tip. The r value decreases with the increase in the applied jintegral. The stress near the crack tip, is dependent on the nominal applied stress, and a correction factor, which depends on the geometry of the specimen, and is inversely dependent on the radial distance from the crack tip. Even under purely compressive farfield loading, fatigue cracks can initiate and propagate. Effects of residual stresses on cracktip constraints. A combined effect of mechanical stresses by the applied load and residual stress on the crack tip constraint is.
However, limited literatures have studied the combined effects of prestrain history and strength mismatched effect on welded joint specimens. The origin of the compressive biaxial stress is radiationinduced swelling constrained by the undamaged substrate. It has been shown that the crack tip stress field is strongly influenced by the residual stresses and a new parameter, r, is proposed to characterize the residual stress induced crack tip constraint. Three initial residual stress distributions were considered. The role of residual stresses and straininduced martensite in delayed cracking of metastable austenitic stainless steels was studied by means of swift cup tests, measurement of residual stresses by xray diffraction and ring slitting, and martensite content determination. Numerical simulation on residual stress generation in the. Most recently, the residual stress induced crack tip constraint has been investigated by liu et al. For a throughthickness crack in an infinite plate, the tensile stress distribution ahead of the crack tip is accurately described by the equation. Effects of processing residual stresses on fatigue crack. In this study, the effects of residual stresses on crack behavior are investigated in a large disk shaped model. Due to the residual compressive stress on the surface, toughened glass is more resistant to cracks, but shatter into small shards when the outer surface is broken. Residual stress shrinkage prediction method concept. It accounts for the stress developed while the material cools under pressure in the mold. These assume that cracktip plasticity occurs in thin strips, with cracktip plastic displacements and crack advance calculated stepwise by considering the region broken down into.
A crack opening residual stress field, similar to a longitudinal residual stress field in butt welded plates, is introduced into the. Although residual stresses affect only the mean stress, or stress ratio, in fatigue, these have been shown to influence crack initiation, propagation, and closure e. The distribution of compressive residual stresses near the crack tip is established after its blunting under the conditions of plastic deformation. Residual stress effects on fatigue life via the stress. Effect of residual stresses on ductile crack growth in. A new constraint parameter r is proposed to characterize the residual stress induced crack tip constraint and a threeparameter approach ctodqr has been suggested to describe the crack tip stress field. Effect of plastic prestrain on the crack tip constraint of. Residual stresses are in equilibrium within a part, without any external load. Optical photograph of an indentation crack induced at a load of 19. Effects of thermal gradient and residual stresses on thermal. The evolution of cracktip stresses during a fatigue overload. I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the requirements for the degree of doctor of philosophy, with a major in engineering science. The effect of specimen and load conditions, load level as well as residual stress patterns on the validity of the threeparameter.