Measuring stress intensity factors during fatigue crack growth using thermoelasticity (original) (raw)
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Development of fatigue crack growth testing under thermo-mechanical fatigue conditions
Materials at High Temperatures, 2013
As the need for the prediction of component life and maintenance interval schedules becomes more demanding, there is an increasing requirement for thermo-mechanical fatigue (TMF) test data including fatigue crack growth rates under such conditions. The test equipment requirements to meet this challenge are discussed and finally a conventional servo-electric load frame is utilised in combination with a radiant lamp furnace to generate the desired thermal cycles. The radiant lamp furnace enables reasonably consistent temperature gradients to be achieved with notched test pieces.
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.
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...
Comparing crack growth testing and simulation results under thermo-mechanical fatigue conditions
2013
As the need for improved prediction of component life and development of new materials for use at high temperatures becomes more demanding, there is increasing requirement for a detailed understanding of thermo-mechanical fatigue (TMF) behaviour including the combined effects of fatigue and dwell on the overall crack growth rates under such conditions. To generate experimental TMF crack growth test data, a conventional servo-electric load frame, used in combination with a radiant lamp furnace, has been employed. The method for the measurement of crack growth under the TMF load cycle is also described. The performance of the experimental method is demonstrated with trials on an advanced nickel-based superalloy, RR1000. To reduce future testing requirements, simulation via the finite element method provides a means for crack growth prediction. This clearly requires validation with real test data at the outset. A method is described whereby separate fatigue and time dependent growth da...
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.
Theoretical and Applied Fracture Mechanics, 2020
During cyclic loading of a cracked metallic alloy at room temperature, heat sources are generated and produce a heterogeneous temperature field around the crack tip. Those heat sources are: (i) the thermo-elastic coupling source, (ii) the intrinsic dissipation due to microplasticity in the material, and (iii) the cyclic plasticity dissipated into heat in the reverse cyclic plastic zone (RCPZ) ahead of the crack tip. The thermoelastic source is computed by finite element analysis in agreement with classic linear thermoelasticity theory. The intrinsic dissipation due to microplasticity is experimentally estimated by carrying out self-heating fatigue tests on uncracked specimens, and then approximating its values in the cracked specimens by using self-heating curves. The cyclic plastic strain energy dissipated into heat in the RCPZ is also experimentally quantified by carrying out fatigue crack growth tests and using infrared measurements. The temperature fields, generated by the three types of heat sources, are separately computed by using the linearity of the heat diffusion equation. Afterward, the stress fields, associated with each thermal effect and induced by the material thermal expansion, are computed by considering the hypothesis of the linear elastic fracture mechanics (LEFM). Thus, the mode I stress intensity factor is calculated by taking into account the thermal effect associated with each heat source. The consequences on K K , Δ and = R K K / K m i n m a x are discussed. It is shown that the heat sources do not modify significantly K Δ , but the modification of R K can be significant since the effects are proportional to the loading frequency.
Royal Society open science, 2017
This article presents an experimental study on the fatigue behaviour of cracks emanating from cold-expanded holes utilizing thermoelastic stress analysis (TSA) and synchrotron X-ray diffraction (SXRD) techniques with the aim of resolving the long-standing ambiguity in the literature regarding potential relaxation, or modification, of beneficial compressive residual stresses as a result of fatigue crack propagation. The crack growth rates are found to be substantially lower as the crack tip moved through the residual stress zone induced by cold expansion. The TSA results demonstrated that the crack tip plastic zones were reduced in size by the presence of the residual compressive stresses induced by cold expansion. The crack tip plastic zones were found to be insignificant in size in comparison to the residual stress zone resulting from cold expansion, which implied that they were unlikely to have had a notable impact on the surrounding residual stresses induced by cold expansion. Th...
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.
This work is part of a global study performed, among a research partnership between three industries and a research laboratory, on the development of a method which allows to better estimate the fatigue life of welded structures (armoured vehicles, ships, Floating Production Storage Off-loading units, wrecking cranes, cars …) submitted to variable loading conditions. The complexity of these structures has lead to adopt a multiscale approach, based on the use of finite element codes associated to various levels of modelling, going from the global cartography of damaged zones to the local calculation with cracks inserted in the models. The aim of this project is to develop an industrial process, avoiding successive re-meshing, being an efficient and easy tool to apply. It is also open enough to provide tools allowing the engineer to assess crack initiation, propagation until failure. The crack initiation is calculated by the use of a multi-axial fatigue damage criterion based on the local approach. Coupled with an extension of the Line Spring Method, multi-initiation of fatigue cracks in welds and through crack growth are then considered, in order to calculate the stress intensity factors for various loads and geometries. Furthermore, sets of tools were developed to predict crack bifurcation and take into account the influence of the loading history on fatigue crack growth, such as crack growth retardation effect, as a result of overloads. This approach is then applied to an overall aluminium welded structure experiment, which was designed to allow several cracks to initiate and propagate. Local micro-geometries and residual stresses were measured at weld toe, as needed for local stress calculations. Furthermore, a complete instrumentation and test of this welded structure allowed to determine precisely crack initiations and to follow crack propagation. The results are in good agreement with calculations and point out the industrial necessity to measure the local characteristics of welds and to control the quality for fatigue design.