Estimation Of Fatigue Strength Research Articles

Analysis of Thermal Fatigue Crack Propagation Based on Local Approach to Fracture

Local approach to fracture based on continuum damage mechanics and finite element method is a powerful method to estimate fatigue strength, which can reduce computing time and cost for the analysis of fatigue crack initiation and propagation. In the present study, this approach has been applied to the analysis of the behaviors of ring-shaped cracks in stainless steel pipes under thermal cyclic loading. The obtained solutions have been compared with the experimental results to discuss the accuracy of uncoupled, locally-coupled and fully-coupled approaches. A new technique to remove the mesh-dependence of the solutions has been proposed and its validity has been discussed through numerical studies on the low-cycle fatigue crack propagation.

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process. So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows: W = K × P × S , where W is the wear volume (depth), K the wear coefficient, P the contact pressure, S the slippage. And then the stress intensity factor for cracking due to fretting fatigue was calculated by using contact pressure and frictional stress distributions, which were analyzed by the finite element method. The S– N curves of fretting fatigue were predicted by using the relationship between the calculated stress intensity factor range (Δ K) with the threshold stress intensity factor range (Δ K th) and the crack propagation rate (d a/d N) obtained using CT specimens of the material. And then fretting fatigue tests were conducted on Ni–Cr–Mo–V steel specimens. The S– N curves of our experimental results were in good agreement with the analytical results obtained by considering fretting wear process. Using these estimation methods we can explain many fretting troubles in industrial fields.

Fatigue life prediction of in-plane gusset welded joints using strain energy density factor approach

Fatigue crack growth behavior of in-plane gusset welded joints is studied using the strain energy density factor approach. Fatigue tests were performed in order to estimate fatigue strength under tension. Fatigue crack growth analysis was carried out to show the effects of the initial crack shape, the initial crack length, and the stress ratio on the crack types of in-plane gusset welded joints. The assumed crack types were edge crack, semi-elliptical crack, and corner crack. Fatigue crack growth parameters were obtained from crack growth curves assuming constant crack shapes for the given crack types. The results of analysis for the assumed crack types agreed well with the experimental data. The fatigue life did not change as initial crack shape varied for a given initial crack length.

Estimation of Crack Propagation by The Analysis of Thermal Fatigue Based on Local Approach Fracture

Local approach to fracture based on continuum damage mechanics and finite element method is a powerful method to estimate fatigue strength, which can reduce computing time and cost for the analysis of fatigue crack initiation and propagation. In the present study, this approach has been applied to the analysis of the behaviors of circular cracks in stainless steel pipes under thermal cyclic loading. The obtained solutions have been compared with the experimental results to discuss the accuracy of uncoupled, locally-coupled and fully-coupled approaches. A new technique to remove the mesh-dependence of the solutions has been proposed and its validity has been discussed through numerical studies on the low-cycle fatigue crack propagation.

Fatigue life prediction of out-of-plane gusset welded joints using strain energy density factor approach

Fatigue growth behavior of out-of-plane gusset welded joints is studied using the strain energy density factor approach. Fatigue tests on two types of specimens with curvatures of ρ = 0 and ρ = 30 were performed in order to estimate fatigue strength under tension. Fatigue crack growth analysis is carried out to show the effects of initial crack shape, initial crack length and stress ratio. Fatigue crack growth parameters were obtained from crack growth curves assuming constant crack shapes. The results of analysis for the assumed crack shapes agreed well with the experimental data. Fatigue propagation life of the ρ = 30 specimen was larger than that of the ρ = 0 specimen.

Fatigue Life Estimation of Adhesively Bonded Scarf Joints Based on a Continuum Damage Mechanics Model

An estimation method of fatigue strength of adhesively bonded joints with various stress triaxialities in the adhesive layer has been proposed based on a damage evolution model for high cycle fatigue. To realize various triaxial stress states, fatigue test was conducted for adhesively bonded butt and scarf joints with various scarf angles bonded by a rubber-modified epoxy adhesive. An equation for estimating the damage evolution in the adhesive layer of the butt and scarf joints was derived from the damage model, where undefined parameters in the equation were determined by comparing the experimentally obtained damage evolution curves of the butt joints with the estimated damage evolution curves. Furthermore, an equation for the estimation of fatigue strength was derived under the assumption that fatigue failure occurs when the damage variable reaches to a critical value. When compared the experimental S-N data of scarf joints with the estimated ones, the estimated fatigue strengths agree well with the experimental data with various scarf angles. This finding suggests that the CDM model is applicable for estimating fatigue strength of adhesively bonded joints with different stress triaxialities.

Fretting Fatigue Strength and Life Estimation in Ultra High Cycle Region Considering the Fretting Wear Process

Fretting Fatigue Strength and Life Estimation in Ultra High Cycle Region Considering the Fretting Wear Process

Development of Fatigue Detecting Sensor

A fatigue detecting sensor that enables easy and precise estimation of fatigue damage of machines and structures was developed, based on the crack growth characteristics of metal. The sensor is composed of a metal foil with a single edge notch and a base metal foil. The two metal foils are bonded at the both far ends. The sensor is attached with an adhesive to the surface of structural member which is subjected to cyclic loads. The fatigue damage of the member is detected as the fatigue crack initiation from the notch tip of the sensor. The fatigue crack growth rate is independent of the crack length. The metal foil of the sensor is given pre-tension, so the sensor has stable crack growth characteristics independent of the average strain. The crack growth length is measured in a certain period after the installation. The length is converted to the fatigue damage of the member during the sampling period of sensor installation. Since no measuring instrument and signal wiring is necessary, the diagnosis procedure becomes easy and the cost for structural health monitoring could be reduced. With such small size of the sensor as conventional strain gages, the pin-pointed application of the sensor to the hot spot of structural stress concentration, where is important for the fatigue strength estimation of welded structures, is possible. Therefore, precise fatigue remaining life assessment could be carried out. Sensors with several strain ranges were developed, so application to vast strain range is possible. Even such a small strain range as 1/10000 is able to be detected. The sensors have been applied to various products like steel bridges, rolling stocks, ships, etc.

Fatigue strength estimation of adhesively bonded tubular joint using genetic algorithm approach

The bonding strength of adhesives is influenced by many factors such as, the surface roughness, bonding clearances, interference fit, temperature, and material of the joining parts, etc. Since all these factors affect the strength of the adhesively joined parts, the effects of these parameters need to be investigated. The present paper describes the use of stochastic search process that is the basis of Genetic Algorithm (GA), in developing fatigue strength estimation of adhesively bonded cylindrical components. Nonlinear estimation models are developed using GA. Developed models are validated with experimental data. Genetic Algorithm Fatigue Strength Estimation Model (GAFSEM) is developed to estimate the fatigue strength of the adhesively bonded tubular joint using several adherent materials, such as steel, bronze and aluminum materials.

Tension–tension fatigue behavior of hydroxyapatite reinforced polyetheretherketone composites

Hydroxyapatite (HA) particle reinforced polyetheretherketone (PEEK) composites with 5, 10, 20, 30, and 40% (by volume) HA content were subjected to tension–tension fatigue under load-controlled mode. Results of quasi-static tensile tests indicated that the tensile stiffness of the HA/PEEK composites increases while their tensile strength decreases, with an increase in HA content, up to 40% (by volume). Stress-life (SN) and stiffness degradation data were obtained. The fact that all of the specimens survived cyclic load at 50% ultimate tensile strength (and below) and the high estimated fatigue strength at 1 million cycles suggest that HA/PEEK composite is a promising fatigue-resistant material for biomedical applications. Microscopic examination of cyclically loaded specimens revealed evidence of interfacial failure between the filler and the PEEK matrix, and subsequent initiation and propagation of micro cracks in the matrix that cause fatigue failure.

Application of multiaxial fatigue criteria to materials containing defects

ABSTRACT Estimation of fatigue strength under biaxial and multiaxial loading is usually based on the assumption that fatigue limit is the limit stress for crack nucleation. However, in presence of defects and inhomogeneities the fatigue limit is determined by the threshold condition of small cracks. Nevertheless, multiaxial criteria, especially the Dang Van's criterion, could be successfully applied to defective materials. The scope of this paper is to discuss the differences in the two approaches (multiaxial criteria versus mechanics of the small cracks) for fatigue design of components containing defects subjected to biaxial and multiaxial in‐phase loading. In particular it will be discussed, on the basis of the fatigue properties of large springs, the need of a correct description of the fatigue mechanism in order to determine the lower bound of fatigue strength in presence of defects.

鋳造アルミニウム合金ＡＣ４ＣＨにおける疲労き裂発生および微小き裂成長の応力比依存性

This paper describes the stress ratio dependence of fatigue behaviour in a cast aluminium alloy, AC4CH. Axial fatigue tests have been conducted using smooth specimens at three different stress ratios, R, of -1, 0.05 and 0.4. Particular attention has been paid to crack initiation and small crack growth behaviour. The Goodman relationship gave fairly good or slightly conservative estimation of fatigue strength at 107 cycles. Cracks initiated from a casting porosity or Sb-segregated defect and the latter defect acted more predominantly as crack initiation site. Regardless of stress ratio, small cracks initially grew faster than the corresponding large cracks characterized in terms of the effective stress intensity factor range, ΔKeff, then gradually approached the da/dN-ΔKeff relationship for large cracks and coincided with it at a ΔK value of 2-3MPa√m. Based on detailed fractographic analyses, shear-type growth region following the crack initiation was clearly discerned, the size of which showed remarkable stress ratio dependence, with increasing crack size with decreasing stress ratio, particularly very large at R=-1. This shear-type crack growth was responsible for the acceleration of small cracks because of a different growth mechanism from large cracks.

201 単一微小欠陥材の疲労強度推定

201 単一微小欠陥材の疲労強度推定

Expert System for Fatigue Design of Spot-welded Joint.

In establishing a long life fatigue design criterion and estimating the reliability of spot-welded thin sheet structure such as the automobile and train, it is necessary to estimate fatigue strength of spot-welded joint. So far, many investigators have numerically and experimentally studied on the method of systematic fatigue strength estimation for various spot-welded joint. Also considerable amount of data on the fatigue strength of spot-welded joint has been accumulated. But, it is difficult to determine the fatigue design criterion based on the existing data when fatigue design is modified. Therefore, many tests were required to determine a new criterion for modified design. Then it takes a lot of money and time. Thus, an expert system was developed to predict the fatigue design criterion including fatigue strength and fatigue life of spot-welded joint. The expert system consists of the material destination database, the spot-welding condition database and the fatigue design module. And this expert system was developed based on the knowledge base, the numerical calculation module, interface engine and user interface. It is expected that the expert system can economically provide the designer and the manufacturer with useful information on the material properties and the fatigue design criterion for a modified design.

Effect of residual stress on fatigue strength of resistance spot weldment

The estimation of fatigue strength of spot welded joints is crucial to the strength design of spot welded steel sheet structures. In the present paper, the effect of residual stresses on the fatigue life of resistance spot weldments was studied. Residual stress fields in the weldments were calculated using thermoelastic–plastic finite element analysis and the equivalent fatigue stress considering the residual stress effect was obtained. This was followed by fatigue life predictions, which included the effect of the residual stresses and the actual loading stresses. The calculation and experimental results showed good agreement. Therefore, the proposed calculation model can be considered to be sufficiently powerful for the prediction of fatigue life

Effect of Residual Stress on Fatigue Strength in Resistance Spot Weldment

Estimation of fatigue strength on the spot welded joint is very Important for strength design of spot welded steed sheet structures. In this paper, the effect of residual stress on the fatigue life of resistance spot weldment was studied. Residual stress fields of weldment were calculated by using thermo elastic plastic finite element analysis and equivalent fatigue stress considering residual stress effect was obtained. And then we predicted fatigue life, which included the effect of the residual stresses and the actual loading stresses. The calculation and experimental results were in good agreement. Therefore, the proposed calculated model can be considered to be sufficiently powerful for the prediction of fatigue life.

Estimation of Fatigue Strength of Spheroidal Graphite Cast Irons under Combined Stress.

Spheroidal graphite cast iron has many graphite nodules and other casting defects such as microshrinkage cavities in the structure. These small defects cause reduction and scatter in the fatigue strength. In this study, rotating bending, reversed torsion and in-phase combined axial/torsional fatigue tests were carried out on smooth round-bars of FCD 400, FCD 600 and FCD 700. The fatigue limit was determined by the threshold condition for propagation of a small mode I crack emanating from small defects. Using both the multiaxial fatigue fracture criterion derived based on this observational result and the √(area) parameter model, a unified method for predicting the lower bound of fatigue limit of spheroidal graphite cast irons subjected to combined loading was proposed. The predicted and experimental results agreed well.

The use of hot spot stress for estimating the fatigue strength of welded components

The relatively complex geometry of welded joints makes an assessment of fatigue strength rather difficult in many cases, especially if the common nominal stress approach is applied. An alternative to this method is the hot spot stress approach which considers structural stress concentration factors at a welded discontinuity, in which a unified S-N curve is used to estimate fatigue strength, rather than using numerous curves corresponding to welded joint categories of welded structures. In this paper, using numerical and experimental results for a variety of welded joint models, a unified S-N curve was derived, which is independent of structural stress concentration factors. Based on a fatigue test of a real welded joint in a train bogie frame, it could be confirmed that the unified S-N curve can be directly applied to the estimation of fatigue strength and fatigue design for general welded steel constructions.

Estimation of the Fatigue Strength of Granite Subjected to Long-period Cyclic Loading.

The effects of excitation frequency on fatigue life and fatigue damage were investigated by carrying out a series of cyclic loading tests at high stress levels on Inada granite. An attempt was made to estimate fatigue strength under long-period loading based on the experimental results. The amount of fatigue damage per cycle and fatigue life decreased with increases in the excitation frequency at the same maximum stress. On the other hand, they did not depend on the excitation frequency between 2.5 × 10 - 4 Hz and 0.5 Hz when the same maximum stress ratio, which was normalized by the static strength under a strain rate corresponding to the excitation frequency, was applied. Cyclic fatigue tests at the frequency of 0.5 Hz were performed under several stress ratios. It was clarified that : (1) there is no fatigue limit within 4 × 104 cycles of loading, (2) the relationship between stress ratio S and fatigue life Nf can be represented by S = A - B log Nf (S - Nf curve), (3) fatigue strength increases with increases in confining pressure, and (4) fatigue strength decreases in the presence of water. Estimation of fatigue strength under long-period loading might be possible using both the S - Nf curve and the relationship between static strength and strain rate.

Small defects and Inhomogeneities in fatigue strength: experiments, models and statistical implications

The method explained in this paper for quantitative evaluation of fatigue limit for materials containing defects is based on the experimental evidences that inhomogeneities and micro-notches can be treated like cracks. First, the basic concept of the √area parameter model is explained introducing the various data obtained by the first author's group for over last 15 years. Evidences are shown that small cracks, defects and nonmetallic inclusions having the same value of the square root of projection area, √area, have the identical influence on the fatigue limit regardless of different stress concentration factors. Various applications of these concepts to various defect types and microstructural inhomogeneities are shown. Since the estimation of fatigue strength is related to the estimation of the size of maximum defects occurring in a piece, the methods for searching the defects and the quality control of materials with respect to inclusion or defect rating as well as their statistical implications are discussed.