About the heat sources generated during fatigue crack growth: What consequences on the stress intensity factor? (original) (raw)
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International Journal of Fatigue, 2014
Because of the reverse cyclic plastic zone at the crack tip, there is plastic dissipation in heat at the crack tip under cyclic loading. That creates a heterogeneous temperature field around the crack tip. A thermomechanical model is proposed in this paper for evaluating the consequence of this temperature field on the Mode I stress intensity factor. Two cases are studied: (i) the theoretical problem of an infinite plate with a semi-infinite through crack under Mode I cyclic loading, and (ii) a finite specimen with a central through crack. In the first case, the main hypothesis and results are presented from the literature but no heat loss is taken into account. In second case, heat loss by convection is taken into account with a finite element analysis, while an analytical solution exists in the literature for the first case. In both cases, it is assumed that the heat source is located in the reverse cyclic plastic zone. The heat source within the reverse cyclic plastic zone is quantified by experiments on a mild steel under R = 0.1. It is shown that the crack tip is under compression due to thermal stresses coming from the heterogeneous temperature field around the crack tip. The effect of this stress field on the stress intensity factor (its maximum, minimum and its range) is calculated. This paper shows that experiments have to be carried out to determine the heat source within the reverse cyclic plastic zone. This is the key parameter to quantify the effect of dissipation at the crack tip on the stress intensity factor.
Engineering Fracture Mechanics, 2011
Plastic dissipation at the crack tip under cyclic loading is responsible for the creation of an heterogeneous temperature field around the crack tip. A thermomechanical model is proposed in this paper for the theoretical problem of an infinite plate with a semi-infinite through crack under mode I cyclic loading both in plane stress or in plane strain condition. It is assumed that the heat source is located in the reverse cyclic plastic zone. The proposed analytical solution of the thermo-mechanical problem shows that the crack tip is under compression due to thermal stresses coming from the heterogeneous stress field around the crack tip. The effect of this stress field on the stress intensity factor (its maximum and its range) is calculated analytically for the infinite plate and by finite element analysis. The heat flux within the reverse cyclic plastic zone is the key parameter to quantify the effect of dissipation at the crack tip on the stress intensity factor.
2013
Plastic dissipation at the crack tip under cyclic loading is responsible for the creation of an heterogeneous temperature field around the crack tip. A thermomechanical model is proposed in this paper for two problems: (i) an infinite plate with a semi-infinite through crack and (ii) a centre cracked plate specimen, both under mode I cyclic loading. The heat source is assumed to be located in the reverse cyclic plastic zone (RCPZ). The solution of the thermomechanical problem shows that the crack tip is under compression due to thermal stresses coming from the heterogeneous temperature field around the crack tip. The effect of this stress field on the stress intensity factor (its maximum and its range) is calculated. The heat flux within the RCPZ is the key parameter to quantify the effect of dissipation at the crack tip on the stress intensity factor. Introduction During a cyclic loading of a crack, the cyclic plasticity is located in the reverse cyclic plastic zone (RCPZ) near the...
On the effect of fatigue crack plastic dissipation on the stress intensity factor
2013
In metals, during plastic strain, a significant part of the plastic energy is converted into heat. This generates a heterogeneous temperature field around the crack tip which depends on the intensity of the heat source associated with the plasticity and the thermal boundary conditions of the cracked structure under cyclic loading. Due to the thermal expansion of the material, the temperature gradient near the crack tip creates thermal stresses which contribute to stress field around the crack tip. This paper shows how this thermal effect modifies the mode one stress intensity factor for two cases: (i) the theoretical problem of an infinite plate with a semi-infinite through crack and (ii) a finite plate specimen with a central through crack. The comparison of the two cases allows the authors to discuss the effect of convection. The comparison of the simulated and experimental temperature field variation at the specimen surface (infra-red measurement on a mild steel) leads to identif...
2013
The research project presents a fundamental understanding of the fatigue crack growth mechanisms of AISI 420 martensitic stainless steel, based on the comparison analysis between the theoretical and numerical modelling, incorporating research findings under isothermal fatigue loading for solid cylindrical specimen and the theoretical modelling with the numerical simulation for tubular specimen when subjected to cyclic mechanical loading superimposed by cyclic thermal shock.The experimental part of this research programme studied the fatigue stress-life data for three types of surface conditions specimen and the isothermal stress-controlled fatigue testing at 300 °C - 600 °C temperature range. It is observed that the highest strength is obtained for the polished specimen, while the machined specimen shows lower strength, and the lowest strength is the notched specimen due to the high effect of the stress concentration. The material behaviour at room and high temperatures shows an ini...
Crack Growth Behaviour in a Thermal Fatigue Test. Experiments and Calculations
Fatigue & Fracture of Engineering Materials and Structures, 1989
This study is concerned with the results of experiments in which thermal cycles have been repeatedly applied through the wall thickness of axisymmetrically cracked tubular specimens. The investigated material is a Cr-Mo steel used for the moulds when fabricating centrifugally cast iron pipes. Crack growth rates have been measured by using the interrupted tests technique. A methodology is proposed to model the crack growth rates under such thermal fatigue loadings. The elastic and plastic stress-strain fields are calculated on the uncracked specimen by means of a finite element code. Special attention was paid to reach a mechanical steady state regime. Fatigue crack growth rates data were obtained, both under isothermal and anisothermal conditions, on CT and SEN specimens. The latter specimens were tested under large-scale yielding in order to obtain the data appropriate to the cyclic stress-strain field calculated in the thermal fatigue specimens. An effective stress intensity factor, which takes into account both plastic strains and crack closure effect, was used to correlate the results of isothermal tests on CT and SEN specimens and to calculate the thermal fatigue crack growth rates in tubular specimens. It is shown that the use of the effective stress intensity factor gives a satisfactory agreement between the observed and the calculated crack growth rates.
Experimental study of heat dissipation at the crack tip during fatigue crack p.PDF
This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE and titanium alloy OT4-0 samples. The investigation of the fatigue crack propagation was carried out on flat samples with different geometries and types of stress concentrators. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at the crack tip.
Effect of thermomechanical processing on fatigue crack propagation
Metallurgical Transactions, 1973
The effect of thermomechanical processing on fatigue crack propagation (FCP) is examined for 70/30 brass and 305 stainless steel. It is found that grain size and cold work induced changes in yield strength, ductility, and preferred orientation have a minor effect on FCP. Rather, cyclically stabilized properties of material in the crack tip plastic zone are believed to control the FCP process. Although mechanical processing fails to significantly alter the rate of FCP, it is apparently responsible for the unique fracture path observed in specimens oriented at an angle (A) to the rolling direction. Deviation of the crack path out of the plane of maximum net section stress is believed to be associated with mechanical fibering and/or crystallographic texturing effects. The complex fracture mode transition observed in cold worked 70/30 brass also is associated with the deformation texture of the starting material. For the cold-worked 305 stainless steel, striation spacings are correlated with the stress intensity range for specimens tested in the longitudinal, transverse, and "angle" orientations. Comparison of these data with corresponding macroscopic data indicate that an approximately one-to-one correspondence exists between macroscopic and microscopic fatigue crack growth rates over the range investigated.
An Analytical Thermal Fatigue crack growth approach
2009
This paper presents recent works on the thermal fatigue crack growth approach in structure integrity analysis proposed by nuclear standard codes such as A16 Appendix of RCC-MR. The proposed approach for crack growth is used to study the mechanisms leading to cracking of piping as a result of thermal loading in mixed flow zones. To accurately determine the mechanical response of a complex component such as a pipe or a mixing zone due to thermal loading, it is necessary to be able to take into account in modelling the effects due to the structure, i.e. the effects due to the boundary conditions, the geometry and the additional loadings (pressure). The first part of our proposed approach consists of the description of our analytical model of the thermo-mechanical problem. By using this analytical model, a study to determine the critical range of frequency of the thermal loadings for a particular thermal fatigue problem is done. And the second part treated the used of analytical model t...
Effect of thermo-mechanical residual stresses on fatigue crack propagation
2022
In this paper we present the influence of residual stresses on the behavior of fatigue cracks under thermo-mechanical loading. Propagation modeling is performed using the global stress intensity factor approach, which describes the simultaneous influence of residual stress field and applied stress on crack propagation. The residual stresses are generated by a thermal loading. These stresses are the result of incompatibilities of deformation related to the heterogeneity of the plastic deformation. They will be superimposed on the loading of fatigue and thus modify the average stress. These residual stresses relax under the effect of fatigue loading, by keeping the crack closed or open, according to the stress cycle.