Papers by Keyword: Creep-Fatigue Interaction

Abstract: The study deals with the interaction of creep and high cycle fatigue of cast polycrystalline nickel-based superalloy IN 713LC at high temperatures. Previous works indicated that creep lifetime of superalloy structures was un-affected or even slightly increased in the cases with superimposed vibrations. The reason for this behaviour was not well described up to now. Therefore, set of fatigue tests was conducted at high mean stresses level to observe this phenomenon. The mean stress was kept constant while the stress amplitudes were selected in order to measure wide range of conditions from pure creep to pure fatigue. Fractographic analysis by scanning electron microscopy (SEM) was done with the aim to identify governing damage mechanisms for particular test conditions as a preliminary evaluation of conducted tests.

Abstract: Strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of K447A are conducted at 760oC in order to investigate the effects of different dwell times and strain ratios on the fatigue behavior and life. For the cases of stain ratio R_ε=-1 with balanced hold time, the tensile and compressive mean stresses will generate. For the case of stain ratio R_ε=-1 with compressive holding 60s, the tensile mean stress will produce. For the case of stain ratio R_ε=-1 with tensile holding 60s, the compressive mean stress will produce. For the cases of stain ratio R_ε=0.1 and R_ε=-1with no hold time, the tensile mean stress will produce. The cyclic damage accumulation (CDA) method and modified CDA method were employed to predict the fatigue life for K447A, respectively. The fatigue life predicted by CDA method is within the scatter band of 18.2X. The fatigue life predicted by the modified CDA method agrees very well with the experimental life and the predicted life is well within the scatter band of 3.1X, which means that the modified CDA method is able to consider the influences of dwell time and strain ratio on the fatigue life of K447A.

Abstract: This paper presents an integrated creep-fatigue (ICF) theory to describe the non-linear creep-fatigue interaction during thermomechanical loading. The ICF theory recognizes the damage evolution as a holistic process consisting of nucleation and propagation of surface or subsurface cracks in coalescence with internally distributed damage, leading to final fracture. In a polycrystalline material under combined cyclic and dwell loading, crack nucleation and propagation occurs by fatigue or oxidation mechanisms, whereas internally distributed damage often occurs in the form of grain boundary cavities or microcracks due to creep or dwell effects, particularly at high temperatures. Based on the above mechanism, a damage evolution equation is mathematically derived, and the generality of the above physical mechanisms warrants the applicability of the ICF theory over a wide range of stresses and temperatures. This paper uses Mar-M 509, a cobalt base superalloy, as an example to illustrate how the ICF theory describes creep and low cycle fatigue (LCF).

Abstract: Strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of an equiaxed crystal nicke-base superalloy were conducted at 850oC in order to investigate the effects of different dwell times and strain ratios on the fatigue behavior and life distribution. The cyclic damage accumulation (CDA) method and modified CDA method were employed to predict the fatigue life for the superalloy under complex loading condition, respectively. CDA method is employed to predict the fatigue life for the superalloy and the predicted fatigue life is within the scatter band of±6X. The fatigue life predicted by the modified CDA method agrees very well with the experimental fatigue life and the predicted fatigue life is well within the scatter band of±3X, which means that the modified CDA method is able to consider the influences of dwell time and strain ratio on the fatigue life for the superalloy.

Abstract: Reduced activation ferritic/martensitic (RAFM) steels are candidate materials for the test blanket modules of ITER. Several degradation mechanisms such as thermal fatigue, low cycle fatigue, creep fatigue interaction, creep, irradiation hardening, swelling and phase instability associated irradiation embrittlement must be understood to estimate the component lifetime. The current work focuses on the effect of tungsten and tantalum on low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of four RAFM steels with varying W and Ta contents. Total strain controlled LCF experiments were performed under various strain amplitudes in the range +0.25% to +1% and temperatures (300 K to 873 K) in air at a constant strain rate of 3×10^-3s^-1 using a servo hydraulic fatigue testing system. CFI experiments were carried out at total strain amplitude of +0.6% and by applying strain hold of different durations (10 min and 30 min) in peak tension and peak compression. Both LCF and CFI life of the RAFM steels improved with the increase in tungsten and tantalum contents. Based on the amount of softening during continuous cycling, tungsten content was optimized at 1.4 wt. % and the tantalum content at 0.06 wt%. Stress relaxation obtained during creep-fatigue interaction studies showed close relation with the chemical composition of the RAFM steels. Other damaging parameters influencing fatigue life were dynamic strain ageing (DSA) occurring in the intermediate temperature regime and oxidation at elevated temperatures. Keywords: RAFM steel, low cycle fatigue, dynamic strain ageing, creep-fatigue interaction, oxidation

Abstract: Crack propagation tests of lead-free solder were conducted at room temperature in air using center-notched plates under load-controlled conditions with three waveforms: triangular pp waveform having fast loading and unloading rates, cp-h waveform having a hold time under tension, and cc-h waveform having a hold time under tension and compression. The J integral was evaluated from load-displacement curves. For fatigue loading of pp waveform, the crack propagation rate was expressed as a power function of the fatigue J-integral range. The creep component due to the hold time greatly accelerated the crack propagation rate. The creep crack propagation rate was found to be a power function of the creep J integral range for each case of cp-h and cc-h waveforms. The creep crack propagation rate for cp-h waveform was higher than that for cc-h waveform. Displacement-controlled tests were also performed under four triangular strain waveforms: pp, cp, cc and pc. For the case of pp waveform, the crack propagation rate was also expressed as the same power function of the fatigue J integral range as in the case of load-controlled tests. The creep crack propagation rate was expressed as a power function of the creep J integral range for each case of cp, pc and cc waveforms. Microscopic observations were conducted to clarify micromechanisms of creep-fatigue crack propagation.

Abstract: Low cycle fatigue (LCF) and Creep-fatigue interaction (CFI) behavior of 316LN austenitic stainless steel alloyed with 0.07, 0.11, 0.14, .22 wt.% nitrogen is briefly discussed in this paper. The strain-life fatigue behavior of these steels is found to be dictated by not only cyclic plasticity but also by dynamic strain aging (DSA) and secondary cyclic hardening (SCH). The influence of the above phenomenon on cyclic stress response and fatigue life is evaluated in the present study. The above mentioned steels exhibited both single-and dual-slope strain-life fatigue behavior depending on the test temperatures. Concomitant dislocation substructural evolution has revealed transition in substructures from planar to cell structures justifying the change in slope. The beneficial effect of nitrogen on LCF life is observed to be maximum for 316LN with nitrogen in the range 0.11 - 0.14 wt.%, for the tests conducted over a range of temperatures (773-873 K) and at ±0.4 and 0.6 % strain amplitudes at a strain rate of 3*10^-3 s^-1. A decrease in the applied strain rate from 3*10^-3 s^-1 to 3*10^-5 s^-1 or increase in the test temperature from 773 to 873 K led to a peak in the LCF life at a nitrogen content of 0.07 wt.%. Similar results are obtained in CFI tests conducted with tensile hold periods of 13 and 30 minutes. Fractography studies of low strain rate and hold time tested specimens revealed extensive intergranular cracking.

Abstract: The effect of process parameters on the creep-fatigue behavior of a hot-work tool steel for aluminum extrusion die was investigated through a technological test in which the specimen geometry resembled the mandrel of a hollow extrusion die. Tests were performed on a Gleeble thermomechanical simulator by heating the specimen using joule’s effect and by applying cyclic loading up to 6.30 h or till specimen failure. Displacements during the tests at 380, 490, 540 and 580°C and under the average stresses of 400, 600 and 800 MPa were determined. A dwell time of 3 min was introduced during each of the tests to understand the creep behavior. The results showed that the test could indeed physically simulate the cyclic loading on the hollow die during extrusion and reveal all the mechanisms of creep-fatigue interaction.

Abstract: A methodology for evaluating and predicting component lives in creep-fatigue interaction region was investigated for Waspaloy. A unified viscoplasticity constitutive equation including multi-back stresses was used to describe cyclic material behaviors. Also, a continuum damage model coupling with the creep-fatigue damage rules was established based on the analysis of creep and low cycle fatigue behavior. Multi-axial fatigue and creep equivalent stress concepts were employed to predict three dimensional component lives. Notched cyclic tests under various stress conditions in the creep-fatigue interaction region were carried out to validate the life prediction methodology with FEM simulation based on the continuum damage model. The comparison of experimental data and prediction results indicates that the continuum damage model is a powerful approach for the prediction of component lives.

Abstract: A series of laboratory investigations concerned about fatigue assessment with acoustic emission method was presented. Fatigue aspects including cumulative fatigue damage, fatigue crack growth and creep-fatigue interaction were considered. As a basic approach, residual strength and acoustic emission characteristics of fatigue damaged materials were considered from the nominal stress-life (S-N) viewpoint. Acquired signal indicated that counts emission quantity can be a good measure of cumulated fatigue damage. In the fatigue crack growth approach, interrelationship between acoustic emission parameter and stress intensity factor was examined with different stress level and crack length. Experimental results were somewhat scattered since sensitive characteristics of acoustic emission method. However, their empirical relation indicated that counts rate correlated with fracture mechanics parameter. Finally, acoustic emission application was extended to the creep-fatigue interaction at elevated temperature. Emission response under each damage mode was compared and characterized. Based on these characteristics, creep-fatigue interaction was evaluated by use of acoustic emission parameter. Overall investigations concluded acoustic emission is very effective tool of fatigue assessment.