CREEP FATIGUE, PART I: Compilation of Data and Trends in the Creep-Fatigue Behavior of Low Alloy Steels

This paper describes an attempt to compile the creep-fatigue data of low alloy steels. In part I, international data have been collected to compare the variability that exists in a particular low alloy steel when characterized in different laboratories. From this work of compilation, trends in the creep-fatigue behavior of low alloy steels have been identified in Part II of this paper and a review of life prediction methods and assessments will be discussed in subsequent papers. The creep-fatigue behavior, in general, improved with the increase in chromium content; however, when additional alloying elements were added to a standard alloy system, the creep-fatigue behavior of that alloy deteriorated. There was a threshold temperature limit as well as a threshold hold time beyond which only interactions of creep-fatigue and oxidation occurred and reduced the life considerably. However, limiting values of threshold temperature and hold times for different low alloy steels have not yet been determined

Dwell Fatigue I : Damage Mechanisms

The mechanisms controlling deformation and failure under high temperature creep-fatigue conditions of materials are examined in this paper. The materials studied are solder alloys, copper alloys, low alloy steels, stainless steels, titanium alloys, and Ni-based alloys. The deformation and failure mechanisms were different (fatigue, creep, oxidation and their interactions) depending upon test and material parameters employed. Deformation mechanisms, such as cavity formation, grain boundary damage, intergranular (IG) and transgranular (TG) damage, oxidation, internal damage, dislocation cell concentration, and oxide mechanisms are very important in order to gain more knowledge of fatigue behavior of materials. The observed mechanisms can be categorized as follows: • Depending upon the test temperature, higher NCR resulted with higher strain range, dwell time and lower strain rates. The damage was due to creep-fatigue interaction by mixed (TG) and/or (IG) creep damage by cavity formation, and oxidation striated surface damage. Oxidation damage was found to depend upon a critical temperature and compression and tension dwell periods in a cycle. • Dwell sensitivity was effective only below a certain strain range; once this threshold was exceeded NCR value was not affected -by further increase in dwell time. • Microstructure changed depending upon test temperature, dwell period, and strain range. Triple point cracking favored mechanisms such as cavitation. New metal precipitates formed depending upon the temperature, strain range and dwell time. Some precipitates were beneficial in blocking the grain boundary damage from creep, whereas other precipitates changed the dislocation sub- structure, promoting more damage. New cells formed during tests that coarsened with longer dwell times. In some cases dynamic strain aging occurred enhancing fatigue behavior of materials. • Depleted regions developed due to high temperature exposure, which was a function of dwell time applied in a cycle, and material composition that aided in the formation and/or propagation of (IG) cracks. • Dwell cycles evolved mean stresses in tension and compression directions. Mean stresses in tension were more deleterious and caused dwell sensitivity. • Dwell sensitivity was also dependent on material conditions, and discontinuities present in a material. These parameters together with test parameters produce damage interactions in a particular fashion that evolve different micro-mechanisms. The dwell sensitivity micro-mechanisms are summarized in this paper

Creep Fatigue, Part III: Diercks Equation: Modification and Applicability

Creep-fatigue data of low allow steels were compiled from international sources, and trends in creep-fatigue behavior were identified in Paper I. Methods of life prediction and their trends were examined in Paper II with respect to compiled data. Diercks equation, a multivariate creep-fatigue life extrapolation equation, for SS, 304, in terms of strain range, temperature, hold time and strain rate parameters is modified and extended to the life prediction of low alloy steels in this paper

A New Creep-Fatigue Life Prediction Model

A published creep-fatigue life prediction model is modified in this paper and assessed with data on Inconel 718 under different test conditions. It has been shown that the method developed within the viscosity concepts correlates the low-cycle fatigue data well. This paper reports the modification over the previous model

Creep Fatigue, Part II: Creep Fatigue Life Prediction: Methods and Trends

Creep-fatigue data of low allow steels were compiled from international sources, and trends in creep-fatigue behavior were identified in Paper I. This paper reviews the methods of creep-fatigue life prediction that were assessed from the compiled data. The methods reviewed in this paper are phenomenological in nature. However, an empirical method has also been discussed which offers very high reliability of the prediction capability. Various test requirements, from which material parameters are determined for each method, have been tabulated. No single method has been generalized as a best method of life prediction for all types of creep-fatigue test conditions

Applicability of Modified Diercks Equation with NRIM Data

The applicability of the modified Diercks equation (MDE) was assessed with elevated temperature low cycle fatigue (ETLCF) data generated by the National Research Institute for Metals (NRIM) for lCr-Mo-V, 1.25Cr-Mo, 2.25Cr-Mo and 9Cr-lMo steels respectively. The modified Diercks equation was assessed with data generated with symmetrical, slow-fast and hold-time waveforms for low alloy steels. The following characteristics were observed: Symmetrical waveforms: Five strain rates were used with these waveforms where predicted life was by a factor of ± x2 for 77%, 87%, 82% and 92% of data points for lCr-Mo-V, 1.25Cr-Mo, 2.25Cr-Mo and 9CrlMo steels respectively. Slow-fast waveforms: Diercks equation was not applicable when the test parameters in tension and compression changed. However, when assessed with such data, predicted life was by a factor of ± x2 for 93%, 66% and 66% of data for lCr-Mo-V, 2.25Cr-Mo and 9Cr-lMo steels respectively. Holdtime waveforms: Tensile only holds of 0.1 and 1 hour were applied where the predicted life was by a factor of ± x2 for 88%, 50% and 100% of test data for lCr-Mo-V, 2.25Cr-Mo and 9Cr-lMo steels respectively

A New Model of High Temperature Low Cycle Fatigue Life Prediction - Applicability with Low Alloy Steels

Creep-fatigue data on low alloy steels were collected from National Research Institute for Metals (NRIM) Tokyo, Japan. These data were generated for lCr-Mo-V, 2.25Cr-Mo, and 9Cr-lMo steels under a wide range of test conditions. A new creep-fatigue life prediction method was developed and data compiled were assessed to examine the applicability of the new method. A brief review of the empirically based, phenomenological life prediction methods was presented where no method was found to be applicable universally to all the creep-fatigue data. The new model was developed within the viscosity concepts, in which the damage parameter was accounted for in terms of dynamic viscosity. The deformations, represented in terms of flow characteristics, generated every cycle, were proposed to culminate in the failure of the specimen when the specimen can no longer accommodate viscous flow or deformations. This method was found to be conservative under all the test conditions employed by the NRIM. In lack of assessments of the same data with other life prediction models, comments on the comparison of the analyses by various methods cannot be established

Creep-fatigue behaviour and life prediction

This thesis describes an investigation into the creep-fatigue behaviour and life prediction for high temperature materials. The methodology adapted in this research was not experimental, but, analytical using data compiled from several sources. High temperature low cycle fatigue (HTLCF) data generated internationally on 0.5Cr-Mo-V, ICr-Mo-V, 1.25Cr-Mo, 2.25Cr- IMo, 2.25Cr-lMo-V and 9Cr-lMo low alloy steels were compiled and analysed to identify trends in creep-fatigue behaviour and life prediction for those steels. Effects of alloying elements such as chromium and vanadium were investigated and it was shown that with increase in chromium content the life improved, but with vanadium addition to a 2.25Cr-Mo steel the life was lowered. For the annealed condition, in which the material tensile properties were nearly half the value for the normalized and tempered condition, the 2.25Cr-lMo steel had higher life

Flange Bolt Failure Analysis

Failure cause investigation of flange bolts from the bi-axial shaft fatigue test rig was carried out and crack growth rates in combined bending and torsion conditions were determined. Flange bolts are used in gas turbine engines that are pre-torqued exerting bending and torsion fatigue situation arising from high centrifugal stresses. The fracture surface features were characteristic of bending fatigue (BF), where striations were observed, a distinct area within which conjoint bending and torsion fatigue (CBTF) features were documented, and a region between the BF and CBTF, a region of overloading (OL), where ductile dimples were observed. Striations dominated within BF and CBTF areas. The CBTF fracture comprised predominantly 80 percent or more of the total fracture surface area. However, as these two modes progress from opposite sides, the region between them experiences overloading and fails as a result. These features were documented for each case. Fatigue striations were counted for CBTF mode and crack growth rate as a function of crack length and crack length as a function of cumulative striations were presented. Since the failure occurred with the application of 60,000 minor cycles, cumulative striation counts were 1/5 of this life. The fatigue crack nucleation stage was affected since the part contained burr marks

Wear Characteristics of Wright State University Total Ankle Replacement Under Shear and Torsion Loads

Introduction: Total ankle replacement (TAR) involves replacement of the damaged bone with prosthetic components and is usually performed in patients suffering with arthritis to relieve pain and maintain motion. There are several different factors that contribute to failure of TARs but aseptic loosening is the primary method of failure in total ankle replacements. Because of its superior mechanical properties UHMWPE is used as a liner material in TARs. Wear generated from the liner due to high contact stresses during gait causes osteolysis resulting in early loosening of the prostheses. Most of the earlier studies have focused on wear due to axial loads. However, the aim of this study was to conduct finite element analysis for determining wear characteristics of Wright State University (WSU) TARs under shear and torsion loads which play a significant role in ankle joint motion. Methods: WSU-patented TAR models were used for applying loads. Finite element analysis was conducted to determine the wear rate by deriving contact stresses generated in the liner during the gait cycle. Forces acting on the ankle joint during the gait cycle1, 2 were used to simulate the mechanical environment in the ankle joint for measuring the Von Mises and contact stresses developed in the liner by applying shear and torsion loads. Based on Hertzian contact theory and Archard’s wear law, a wear equation (3) was used to determine the yearly wear rate in the liners based on contact pressure. Results and Discussion: Shear loads cause signifi cantly lower contact stresses and wear when compared with the axial load. Under torsion load, the stress values increased with an increase in the degree of rotation. The TAR models under torsion have shown signifi cantly greater stress values when compared with stress values obtained under shear load. Conclusions: Stress analysis on TAR showed that torsion load causes higher stress values than shear load. With increase in the degrees of rotation, the stress values were increased in the liner under torsion load. From this study it can be concluded that shear and torsion loads acting on the ankle joint during gait plays a major role in affecting the contact stresses, but the axial load plays a more signifi cant role in generating wear