Papers by Keyword: Life Prediction

Abstract: Low Cycle Fatigue (LCF) is a prominent failure mechanism in many design components; therefore, an evaluation of cycles to failure in this regime is of high importance. Most international standards recommend a closed loop strain-controlled mode specimen testing in this regime. However, the ꜪN data obtained from this test is not suitable for life evaluation of parts enduring force-controlled history during actual service without correction for control mode. Many existing procedures, which accounts for cyclic strain stabilization during force-controlled loading may significantly complicate the finite elements analysis (FEA) at solving or post processing stages and are often an inherent source of uncertainty. A heuristic, cost effective and sufficiently accurate approach for LCF life estimation is advocated. The method involves only two force loading FEA simulations, one of the actual parts and the other of the test specimen, using initial unstabilized stress strain curve, followed by a limited number of force-controlled specimens testing. Actual part and specimen life correlation performed using first loading unstabilized equivalent plastic strain value Ꜫ_p1 under locality and similitude assumptions. Unstabilized strain vs. number of cycles to failure curve Ꜫ_p1N is constructed and discussion regarding specimen geometry considerations for providing sufficient accuracy is included. Method validation and crack propagation study are provided.

Abstract: Based on the constant amplitude CF test of oil well tube material TP140small specimen, the procedures of predicting the variable amplitude corrosion fatigue (VACF) life of given reliability of TP140 were developed. Through the multi-sample sinusoidal loading constant amplitude CF life texts, the reliability distribution of CF life was analyzed. The CF life prediction of the specified reliability was obtained under 4 different stress levels, and the corresponding P-S-N curve expressions were obtained. Using VACF loading block spectrum and P-S-N curve expressions, the VACF life was calculated without considering the loading sequence effect. Furthermore, 5 VACF life tests were carried out using the same loading block spectrum. The VACF life with reliability was predicted by the principle of reliability and statistics. VACF life was calculated and compared with the predicted life. Test results and analysis show that the predicted results agree well with the experimental results, and CF life of TP140 casing steel follows a lognormal distribution at the given equivalent stress level, which has been substantiated.

Abstract: The main exploitation process of heavy oil is cyclic steam huff and puff. Cyclic operation causes cumulative damage to thermal recovery casing and causes a large number of failures. Failure analysis shows that the main failure factor of thermal recovery casing is the low cycle fatigue process caused by thermal cycle, which results in the decrease of strength of casing material, thus causing casing fracture and failure. Based on the failure analysis results, the low cycle fatigue tests of 80SH thermal recovery casing steel under different temperature, strain, pre-strain and other conditions were carried out. The influencing factors, life prediction model and criterion of service safety of casing materials under thermal recovery service conditions were first proposed, which provides a strong theoretical basis for the service safety design theory of thermal recovery casing materials. The research shows that the strain limit and low cycle fatigue life are the two core issues of service life. Life predictions need to satisfy two criteria. The first is the strain criterion. The strain limit should be lower than the total strain during the long service life. And the second is the low cycle fatigue criterion, which satisfies the expectation of low cycle fatigue life under double conditions of strain amplitude and mean strain.

Abstract: This paper is concerned with the creep life prediction of cast 20Cr32NiNb alloy, an alternative candidate material to wrought Inconel alloys for use in the gas collector pipes of CO reformers which suffer from long-term creep damage due to high temperatures and stresses. Uniaxial creep tests of 20Cr32NiNb alloy were performed at 890 °C and 950 °C for different stresses. The Omega method for creep life prediction is applied to the 20Cr32NiNb tests and shown to give reasonably accurate prediction, particularly at low stress levels. A new method, based on the use of a hyperbolic sine function for stress correlation at specific temperatures for identification of the characteristic Omega parameters is presented and validated.

Abstract: P92 heat-resistant steel was used to demonstrate that creep rupture life evaluation period could be shorted by the assistant of the creep data from short-period stress relaxation test without reducing the prediction precision. Research showed that the minimum creep rate and the relaxation creep rate were exchangeable, and the stress exponent and the apparent activation energy analysis of the constant strain creep and the constant stress creep showed a similar deformation mechanism at the condition of T and . The creep rupture life predicted through the combination of these two kinds of creep data was closer to the real creep data than that evaluated by the traditional method based on the time to rupture only, and the precision of the evaluated creep strength increased at last 14.5 %.

Abstract: In this paper, four types of creep damage models (Kachanov-Robotnov, Liu-Murakami, Cocks-Ashby and Wen-Tu model) were used to study the creep crack growth (CCG) behavior in compact tension (CT) specimen of Hastelloy C-276/BNi-2 brazed joint. The results show that the creep damage model has a great influence on the CCG behavior of brazed joint. The crack-tip stress states, da/dt-C* curves, crack initiation time and rupture life are different for the different damage models. The Kachanov-Rabotnov model can lead to higher CCG rate and shorter rupture life, while the Cocks and Ashby model can reduce CCG rate and prolong the rupture life. The model order in terms of the CCG rate from high to low is K-R, L-M, W-T, C-A model, which is opposite order of crack initiation time. In the simulation of CCG of brazed joint, a precious damage model should be employed for life prediction.

Abstract: With the purpose of long-cycle safe operation of cold stretched austenitic stainless steel pressure vessels so as to achieve unification of economy and safety, prediction of fatigue life of S31603 austenitic stainless steel at high temperature is systematic studied. Based on the Hull-Rimmer cavity theory, a fatigue life prediction model applicable to stress controlled is developed. Fatigue test is carried out on the solution annealed and cold stretched S31603 steel at high temperature and corresponding test data is obtained. The fatigue life of the solution annealed and cold stretched materials is predicted by the model and the prediction results are in good agreement with the experimental results. On this basis, the life prediction model coupled with the strain level of cold stretching is further established. Compared with the test data, the prediction results is found to be very satisfactory with an error band less than ±1.5 times. The fatigue life prediction model suitable for stress control at high temperature is simple in form and has a clear and obvious physical significance which points out a new way to predict fatigue life of metal materials.

Abstract: To evaluate the fatigue damage accumulation and predict the residual life of components at different stress levels, this paper proposed a modified cumulative damage model based on the strain energy density parameter. Noting that mean stress and load interaction under uniaxial fatigue loading exhibit significant effects on fatigue damage accumulation and life prediction. According to this, a new model based on damaged stress model which considers the effects of mean stress and load interaction was presented in this paper. The proposed model was verified by using four experimental data sets of aluminium alloys and steels. The experimental results are compared with those of the Miner’s rule, damaged stress model (DSM) and damaged energy model (DEM). Results show that the proposed model agrees better with the experimental observations than others.

Abstract: Considering the effects of mean stress, the progressive accumulation inelastic strain occurs in engineering components under the direction of mean stress, it is simply known as ratcheting. Based on the ductility exhaustion theory, a new model is proposed to account for the effects of mean stress and ratcheting on the component fatigue life. The capability and accuracy of the proposed model are compared with those of Walker, Xia-Ellyin, Goswami, GDP and Peng models. A comparison between the model prediction and tested life is found to be quite satisfactory in the cases of 9 sets of experimental data available in the literature under different loading conditions.

Abstract: In this study, creep properties of lead-free solders based on Sn1.0Ag0.7Cu were investigated. In the solders, germanium, nickel and bismuth were added to improve their mechanical properties. The effect of element addition on their microstructure and creep properties were studied. Creep properties were improved with the element addition. Creep rupture lifetime was the longest for SnAgCuBiNiGe, and that of SnAgCuBi was the second longest. It was made clear that a minimum creep strain rate is useful for lifetime prediction of the solders. An observation of microstructure was performed and coarsening of intermetallics after creep test was identified, resulting in creep rupture. It was found that the addition of bismuth hinders the coarsening of intermetallics and is effective to improve the creep properties of the solders.