Gigacycle fatigue data sheets for advanced engineering materials

Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 10¹⁰ cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 10⁸ cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete.     

Gigacycle fatigue behavior of a high chromium alloyed cold work tool steel

The influence of carbides and the effect of surface residual stresses (RS), resulting from heat treatment or from the grinding/polishing process, on the fatigue behavior in the gigacycle regime of ingot metallurgy produced D2 type tool steel was examined. RS were found to be responsible for the occurrence of two failure modes: Internal cracks initiating at large primary carbides (clusters) were observed in the cycle number range of 10⁵–10⁶, while in the gigacycle regime near-surface cracks originating at primary carbides caused failure, which was related to degradation of the RS by cyclic loading. Simple models were employed estimating the RS degradation process and the local fatigue strength along the specimen cross section as a function of active RS. In absence of considerable RS predominantly near-surface crack initiation was obtained.     

Durability of welded aluminium extrusion profiles and aluminium sheets in vehicle structures

Different approaches - the nominal stress, structural hot spot stress and notch stress approach - to analyse the fatigue strength of welded structures made from wrought aluminium alloys were studied. Experimental and numerical investigation was carried out for this purpose on detail specimens and components. The results shown here were generated during the research project “Extrusion profile and sheet metal structures of wrought aluminium alloys in vehicle construction” [1].The studies show that due to the existing guidelines, welds on structures made from aluminium alloys are sometimes designed very conservatively. It is possible to optimise and reliably design welded joints of thin sheet structures by applying the notch stress approach using the reference radius r_ref = 0.05 mm and the reference SN curves derived here.     

Evaluation of nominal and local stress based approaches for the fatigue assessment of seam welds

In the context of the German joint research project “Applicability of fatigue analysis methods for seam welded components”, fatigue tests were performed by five universities and institutes on welded components, welded parts of larger structures as well as component-like samples of weld details. The sheet thickness t was in the range 1 mm ? t ? 20 mm. The welding parameters for all test coupons and structures tested were chosen according to the industrial production process. Based on the data acquired, nominal, structural and notch stress approaches were analysed with regard to applicability and quality of assessment. The actual weld geometry except the real notch radii was taken into account within the notch stress approach. For the notch radii various values, the reference radii 0.05, 0.3 and 1 mm, were applied.Experimental and numerical results for welded steel components are presented.Approximately equivalent scatter ranges were obtained when applying the various approaches based on the current state of the art. It should be noted that both the nominal and the structural stress approaches are limited in their application compared to the notch stress approach. A comparison of the scatter bands obtained for the various approaches is subject to limitations because it was necessary, in each case, to use different test series as the basis for determining the scatter bands.     

Fretting fatigue in engineering alloys

The experimental procedures which have been used to carry out fretting fatigue tests are reviewed and the preferred specimen and contact pad geometries and method of testing are identified. The S–N curves generated with and without fretting and subsequent analysis have been used to satisfy a number of objectives: (1) to establish the important variables which can significantly affect fretting fatigue behaviour; (2) to increase our fundamental understanding of the fretting fatigue process; and (3) to give a ranking of a diverse range of materials in terms of their resistance to fretting fatigue. The analytical methods which have been used to predict fretting fatigue crack initiation are briefly discussed. With some specimen/fretting pad material combinations, small fretting fatigue cracks are introduced at a very early stage in life and fracture mechanics methods are developed in order to model their growth. Analytical procedures for fretting fatigue based on either S–N endurance or fracture mechanics methodologies are discussed.     

Gigacycle fatigue properties of 1800 MPa class spring steels

Fatigue tests up to 10⁸ cycles were carried out for two spring steels (Heats A and D1) and one valve spring steel (Heat F) with tensile strength, σ _B, of 1720, 1725 and 1764 MPa, respectively. The size and composition of inclusions in Heats Dl and F were controlled. The surface‐type fracture occurred at shorter lives below 10⁶ cycles, while the fish‐eye‐type fracture occurred at longer lives. The fatigue limit, σ _W, at 10⁸ cycles was 640 MPa for Heats A and D1 and 700 MPa for Heat F. Al₂O₃ inclusions for Heat A and both TiN inclusions and matrix cracks, i.e. internal facets, for Heat F were observed at the fish‐eye‐type fracture sites, while only matrix cracks were observed for Heat Dl. ODA, i.e. optically dark area, which is considered to be related to hydrogen effects, were formed around Al₂O₃ and TiN inclusions. Fatigue tests were also conducted after specimens were heated up to 573 K in high vacuum of 2 × 10^–6 Pa. The heat treatment eliminated matrix cracks for Heat D1 and the fatigue limit at 10⁸ cycles recovered to the estimated value of 920 MPa from the equation σ _w= 0.53 σ _B for the surface fracture. These results suggest that inclusions control and hydrogen influence the gigacycle fatigue properties for these high strength steels. In addition, it is expected that the creation of a martensite structure with a high resistance to hydrogen effects in the inclusion‐controlled steel could achieve the higher fatigue limit estimated for the surface‐type fracture.     

A comparison of maximum likelihood models for fatigue strength characterization in materials exhibiting a fatigue limit

In this study, various probabilistic models were considered to support fatigue strength design guidance in the ultra high-cycle regime (beyond 10⁸ cycles), with particular application to Ti-6Al-4V, a titanium alloy common to aerospace applications. The random fatigue limit model of Pascual and Meeker and two proposed simplified models (bilinear and hyperbolic) used maximum likelihood estimation techniques to fit probabilistic stress-life curves to experimental data. The bilinear and hyperbolic models provided a good fit to large-sample experimental data for dual-phase Ti-6Al-4V and were then applied to a small-sample data set for a beta annealed variant of this alloy, providing an initial probabilistic estimate of beta annealed Ti-6Al-4V fatigue strength in the gigacycle regime. The bilinear and hyperbolic models are recommended for use in estimating probabilistic fatigue strength parameters in support of very high-cycle design criteria for metals with clearly defined fatigue limits and fairly constant scatter in fatigue strength.     

Application of different concepts for fatigue design of welded joints in rotating components in mechanical engineering

Several concepts are used for the fatigue design of welded joints. In this paper investigations are presented, which were carried out in a joint project between five research institutes [1]. The aim is to investigate currently applied fatigue concepts with respect to their limitations, compatibility and reliability, in order to improve the accuracy of lifetime estimation and to simplify the choice of the optimum fatigue concept. Here, the results of the investigation of welded joints in rotating universal joint shafts are shown [2]. In the critical weld, a structural steel and a quenched and tempered steel are joined. In practice, stresses result from rotating bending, torsion and also residual stresses are sometimes present. Several welding techniques, MAG, TIG and laser welding, and two seam geometries were investigated with regard to their influence on fatigue strength. Experiments were conducted with welded tube specimens representative of the actual component application and with derived flat specimens as detail specimens. The welded sheet thickness was 5.5 mm. Fatigue strength was investigated from 10⁴ to 10⁷ numbers of cycles. In numerical analyses, nominal stress, structural hot spot stress and elastic notch stress with reference radii of 0.3 mm and 0.05 mm were calculated. In the comparison of the concepts, their respective advantages and disadvantages have been demonstrated. A comparison of the results with the IIW recommendation for fatigue design of welded joints and components [3] has been carried out and improvements have been suggested.     

Comparison of different statistical models for description of fatigue including very high cycle fatigue

In this paper the fatigue behaviour of welded joints and helical compression springs are analysed using two different statistical models. The data consist of results from low cycle, high cycle and very high cycle fatigue and different number of investigated specimens. In particular, the software program ProFatigue is used for derivation of the probabilistic S–N field from experimental fatigue data. The program, based on a former regression Weibull model, allows the estimation of the parameters involved in the S–N field model, providing an advantageous application of the stress based approaches in the fatigue design of mechanical components. The results obtained are compared with the customary Wöhler-curve, represented as a straight line in a double-logarithmic scale. Application to probabilistic assessment of cumulative damage and further program enhancement can be now envisaged.     

Mechanisms for corrosion fatigue crack propagation

ABSTRACT The corrosion fatigue crack growth (FCG) behaviour, the effect of applied potential on corrosion FCG rates, and the fracture surfaces were studied for high‐strength low‐alloy steels, titanium alloys, and magnesium alloys. During investigation of the effect of applied potential on corrosion FCG rates, polarization was switched on for a time period in which it was possible to register the change in the crack growth rate corresponding to the open‐circuit potential and to measure the crack growth rate under polarization. Due to the higher resolution of the crack extension measurement technique, the time rarely exceeded 300 s. This approach made possible the observation of a non‐single mode effect of cathodic polarization on corrosion FCG rates. Cathodic polarization accelerated crack growth when the maximum stress intensity (K_max) exceeded a certain well‐defined critical value characteristic for a given material‐solution combination. When K_max was lower than the critical value, the same cathodic polarization, with all other conditions (specimen, solution, pH, loading frequency, stress ratio, temperature, etc.) being equal, retarded or had no influence on crack growth. The results and fractographic observations suggested that the acceleration in crack growth under cathodic polarization was due to hydrogen‐induced cracking (HIC). Therefore, critical values of K_max, as well as the stress intensity range (ΔK) were regarded as corresponding to the onset of corrosion FCG according to the HIC mechanism and designated as K_HIC and ΔK_HIC. HIC was the main mechanism of corrosion FCG at K_max > K_HIC (ΔK > ΔK_HIC). For most of the material‐solution combinations investigated, stress‐assisted dissolution played a dominant role in the corrosion fatigue crack propagation at K_max < K_HIC (ΔK < ΔK_HIC).     

On the development of fatigue failure maps for titanium matrix composites

The purpose of the present article is to demonstrate how fatigue failure maps can be constructed for fiber-reinforced titanium alloys. The maps are constructed using a combination of micromechanical models and experimental measurements of fatigue cracking and fracture. The maps are used to identify the regimes in which various failure mechanisms dominate. Moreover, they provide information about the fatigue life and the critical amount of crack extension at failure. Examples of such maps for a well-characterized titanium matrix composite are presented and used to illustrate the sensitivity of fatigue life and critical crack extension to both the applied stress and the length of pre-existing cracks or notches.     

Fatigue strength of steel rollers with failure occurring at the weld root based on the local strain energy values: modelling and fatigue assessment

Weldments geometry with failures occurring at the weld toe or at the weld root cannot, by its nature, be precisely defined. Parameters such as bead shape and toe or root radius vary from joint to joint even in well-controlled manufacturing operations. The worst case configuration can be achieved by modelling as a sharp, zero radius, notch both the toe and the weld root. The intensity of asymptotic stress distributions obeying Williams’ solution is quantified by means of the Notch Stress Intensity Factors (NSIFs). For steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch, units for NSIFs are the same as conventional SIFs used in LEFM. The different dimensionality of NSIFs for different notch opening angles does not allow a direct comparison of failures occurring at the weld toe or at the weld root. In order to overcome the problem related to the variability of the V-notch opening angle, a simple scalar quantity, i.e. the value of the strain energy density (SED) averaged in the structural volume surrounding the notch tip, has been introduced. This energy is given in closed form on the basis of the relevant NSIFs for modes I, II and III. The radius R_c of the averaging zone is carefully identified with reference to conventional arc welding processes being equal to 0.28 mm for welded joints made of steel.The local-energy based criterion is applied here to steel welded rollers produced by Rulmeca and subjected to prevailing mode I (with failures at the weld root). The aim of the paper is firstly to describe the employed methodology for the fatigue assessment and secondly to show the first synthesis of fatigue data by means of local SED for a specific geometry.     

A fatigue model for sensitive materials to non-proportional loadings

The present study proposes a novel fatigue model based on virtual strain energy. This model separates loading paths based on their non-proportionality where directly takes into account the loading in fatigue life prediction. The proposed fatigue model is expressed in two tension-based and shear-based equations for two tensile and shear cracking failure modes. The model was validated against several experimental datasets available in the literature. In addition, obtained results were compared to predicted lives through some well-known fatigue models comprising maximum shear strain, Smith–Watson–Topper, and Fatemi–Socie. The results were strongly correlated with the experimental data indicating accuracy of the model.     

Total fatigue life prediction for Ti-alloys airframe structure based on durability and damage-tolerant design concept

Based on the concept of the damage-tolerance and durability design, the total fatigue life of titanium alloys is divided into three phases: crack initiation (0–0.3 mm), short crack growth (0.3–2 mm) and long crack growth (2 mm–a_C). Among these three phases, different prediction models are accepted due to different failure mechanisms. A computer program was developed to predict the total fatigue life of the titanium alloy structure. Fatigue testing is also conducted for two types of ELI grade titanium alloy to verify the prediction models. The predicted fatigue life agrees well with experimental results.     

Development and validation of a probabilistic model for notch fatigue strength prediction of tool steels based on surface defects

A new model for the high cycle notch fatigue strength prediction of tool steels subjected to axial loading is proposed, based on previous literatures studies and experimental tests carried out on six different tool steels, including rotating bending fatigue tests on notched specimens, fractographic analyses, hardness, residual stress, and roughness measurements. The novelty is the assumption that surface defects are the main cause of notch fatigue failures of such steels. A probabilistic approach was implemented by modeling size distributions of defects, resulting in the prediction of normal distributions of fatigue strength. Like to other previous models, the effect of steel hardness, surface residual stress, notch severity, and specimen size was also taken into account. Model calibration and validation were performed using the data collected by the experimental activity. Model behavior was investigated by performing a sensitivity analysis, aiming to verify the response to variations of the considered input variables. Prediction errors of only 1.3% (on average) and 3.1% (maximum) resulted from the comparison between model-predicted and experimental notch fatigue strength.     

Relations between fatigue strength and other mechanical properties of metallic materials

The relations between fatigue strength and other mechanical properties especially the tensile strength of metallic materials are reviewed. After analyzing the numerous fatigue data available, the qualitative or quantitative relations between fatigue strength and hardness, strength (tensile strength and yield strength) and toughness (static toughness and impact toughness) are established. Among these relations, the general relation between fatigue strength σ_w and tensile strength σ_b, σ_w = σ_b(C ? P ? σ_b), where C and P are parameters, (hereafter, the general fatigue formula) can well predict the fatigue strength with increasing the tensile strength in a wide range for many materials such as conventional metallic materials, newly developed materials and engineering components. On the basis of the experimental results of many materials, the fatigue damage mechanism, especially for high‐strength steels, is proposed. It is suggested that the general fatigue formula can provide a new clue to predict the fatigue strength and design the materials by adjusting material parameters P and C adequately.     

New Developed Welding Electrode for Improving the Fatigue Strength of Welded Joints

A new welding electrode, low transformation temperature electrode (LTTE), was introduced in this paper. It was described in design principle, mechanics, chemical compositions of their deposited metal and manufacturing methods. It was proved that the best transformation starting temperature from austenite to martensite of the deposited metal of LTTE was at about 191℃ and it was obtained by adding alloying elements such as Cr, Ni, Mn and Mo. The microstructure of the weld metal of the LTTE was low carbon martensite and residual austenite. The compressive residual stress was induced around the weld of the LTTE and the -145 MPa in compression could be obtained in middle of weld metal. The fatigue tests showed that the fatigue strength of the longitudinal welded joints welded with the LTTE at 2×106 cycles was improved by 59% compared with that of the same type of welded joints welded with conventional E5015 and the fatigue life was increased by 47 times at 162 MPa. It is a very valuable method to improve th     

Solving some key failure analysis problems using advanced methods for materials testing

A dilemma encountered in engineering practice is a proliferation of newly designed (mostly high-strength and/or corrosion-resistant) steels and alloys that are unusable in industry as they are highly susceptible to failure under operating conditions including environmentally assisted cracking. The problem of materials failure has several sources, the most significant of which is how engineers select which material to use in which industry. As a rule, selection is based solely on assessing the mechanical properties of materials with little or no consideration of how these mechanical properties will interface with specific operating parameters found within different industries. The functional design, selection and use of materials aimed at preventing in-service failures depends, therefore, on finding testing methods, standards and approaches appropriate to real operating conditions guided by analysis of material performance under those conditions.