Simple Tensile Data Research Articles

Low-cycle fatigue life estimation of steam turbine rotor steels at high temperature

Nine methods currently available for estimation of fatigue properties from simple tensile data are discussed in detail. The estimation accuracies of these methods were compared according to the test data of nine types of steam turbine rotor steels. The results showed that the values calculated using the nine methods resulted in unacceptable values. In double logarithmic coordinates, the nine method functions were linearly modified, where the predicted lives were all less than quadruple the observed lives, except for the mixed method. A new method that takes the temperature influence into account is presented, which is suitable for fatigue life estimation of steam turbine rotor steels at high temperature (400 °C–600 °C). The estimation results show that the new method can provide acceptable accuracy.

A Modified Method to Estimate Fatigue Parameters of Wrought Aluminum Alloys

Several methods for estimating fatigue properties of wrought aluminum alloys from simple tensile data or hardness were discussed. Among them, Park-Song’s modified Mitchell’s method provided the best estimation results in low fatigue life regime. Roessle-Fatemi’s hardness method tends to be very erroneous in the present estimations. None of the investigated methods provide the satisfactory estimation results in the case of Nf > 3 × 104 cycles. Besides, correlation between ultimate tensile strength and Brinell hardness was developed. Then, the modified Mitchell’s method utilizing the ultimate tensile strength predicted from Brinell hardness was proposed in this study and successfully applied to estimate fatigue properties for wrought aluminum alloys. This simple method requires only Brinell hardness and modulus of elasticity as inputs, both of which are either commonly available or easily measurable. Prediction capability of this method was evaluated for wrought aluminum alloys with hardness between 120 and 157 HB. Results show that the proposed method provides the best life predictions for wrought aluminum alloys.

Implementation of expert system on estimation of fatigue properties from monotonic mechanical properties including hardness

This paper describes implementation of the expert system on estimating fatigue properties from simple tensile data or hardness data. The fatigue properties are usually obtained by performing fatigue tests. However, as fatigue test requires time and high cost, many methods have been proposed to estimate the strain-life curves from monotonic mechanical properties including hardness. Though many useful estimation methods have been proposed and they afford every facility, a mechanical designer has some difficulties in making a decision to select the best method among the several methods for the fatigue strength design or assessment under given conditions. And then several researchers including authors evaluated the methods and proposed the best method according to material. In spite of those guidelines, it does not seem to be always easy for ordinary mechanical designer to apply. In this study, an expert system was developed to facilitate it. A commercial expert system shell, Acquire, was used and the user interfaces of the expert system were developed using Visual Basic. The expert system helps mechanical designer to select the best estimation method among methods for estimating fatigue properties from monotonic mechanical properties including hardness, depending on his or her knowledge level of fatigue and has some special functions to describe recommendation and remarks in detail and display the accuracy and predictability of estimation method.

An Expert System for Estimation of Fatigue Properties from Simple Tensile Data or Hardness

Abstract Fatigue properties such as strain-life (ε-N) curves are usually obtained by performing fatigue tests. However, as fatigue testing requires time and high cost, many methods have been proposed to estimate the strain-life curves from simple tensile data or hardness. Based on results of recent studies, an expert system is developed to estimate with high accuracy fatigue properties (strain-life curves) from simple tensile data or hardness. The expert system is developed by using a commercial expert system shell, Acquire. The expert system consists of two modules, Fatigue Properties Estimation module and Knowledge Base Update module. An important feature of the expert system is to provide results of fatigue properties estimation with information of their anticipated accuracy. The fatigue data used for the performance testing of the expert system are 96 ε-N curves consisting of 663 ε-N data points obtained for 52 materials. The expert system has very good performance for steels, aluminum alloys, and titanium alloys, even when only hardness data are available as well as when simple tensile data are available.

Estimation methods for strain-life fatigue properties from hardness

Several methods for estimating fatigue properties from hardness are discussed, along with all existing estimation methods. The (direct) hardness method proposed by Roessle and Fatemi provides excellent estimation results for steels. So-called indirect hardness methods utilizing the ultimate tensile strength predicted from hardness were proposed in this study and successfully applied to estimate fatigue properties for aluminum alloys and titanium alloys. The medians method proposed by Maggiolaro and Castro is found to provide the best estimation results for aluminum alloys. Based on the results obtained, some guidelines are provided for estimating fatigue properties from simple tensile data or hardness. In addition, a new relationship of ultimate tensile strength versus hardness is proposed for titanium alloys.

New Estimation Method of Fatigue Properties of Aluminum Alloys

A new method on estimating fatigue properties from simple tensile data was proposed for aluminum alloys. The method is based on experimental strain-life curves. An optimization technique was used to get the best fatigue properties. The four-point correlation method, the universal slopes method, Mitchell’s method, the modified universal slopes method, the uniform material law by Ba¨umel and Seeger, the modified four-point correlation method by Ong and a new method proposed in this work were evaluated in a quantitative manner. Mitchell’s method, the uniform materials law and the new method give good life predictions for aluminum alloys. In particular, the new method provided the best results.

An expert system for estimation of fatigue properties of metallic materials

An expert system for estimation of fatigue properties from simple tensile data of material is developed, considering nearly all important estimation methods proposed so far, i.e. seven estimation methods. The expert system is developed using an expert system shell, UNIK, and the knowledge base is constructed with production rules and frames. Forward chaining is employed as a reasoning method. The expert system has three major functions including the function to update the knowledge base. The performance of the expert system is tested using the 54 ϵ– N curves consisting of 381 ϵ– N data points obtained for 22 materials. It is found that the expert system developed has excellent performance especially for steel materials, and reasonably good for titanium alloys.

Detailed evaluation of methods for estimation of fatigue properties

Using extensive experimental strain-life curve data on 116 steels, 16 aluminium alloys and six titanium alloys, nearly all methods currently available for estimation of fatigue properties from simple tensile data are discussed in detail. The four-point correlation method, the universal slopes method, Mitchell's method, the modified universal slopes method, the uniform material law by Baumel and Seeger. and the modified four-point correlation method by Ong are evaluated in a quantitative manner by using new criteria proposed in this work, along with conventional error criterion. The modified universal slopes, Seeger's and Ong's methods give good life predictions. Among them, the modified universal slopes method provides best results.

A Modified Universal Slopes Equation for Estimation of Fatigue Characteristics of Metals

Using extensive data on forty-seven materials including steels, aluminum, and titanium alloys, a new equation has been derived for estimating fatigue properties from simple tensile data at the temperature of interest in the sub-creep range. Compared to the Universal Slopes Equation previously proposed by Manson and Hirschberg, the new equation provides predictions that agree more closely with the measured fatigue properties over the entire life range. The earlier Universal Slopes Equation now in common use involves only ductility in the estimation of the plastic line whereas the improved equation involves both tensile strength and ductility in the estimation of the plastic line. A new equation was also derived for a large class of materials suitable for use in the cryogenic temperature range (ten different structural alloys and metals tested at liquid helium and liquid nitrogen temperatures). Here, as in the original Universal Slopes Equation, the plastic line did not require a tensile strength term. Although new constants were derived for the cryogenic range, the final predictions provided about the same degree of accuracy as the original Universal Slopes Equation.

Stored Energy Function of Rubberlike Materials Derived from Simple Tensile Data

An explicit formulation to obtain the stored energy function W from simple tension experiments alone, based on the Valanis-Landel separable symmetric stored energy function, W(λ1, λ2, λ3) = w(λ1) + w(λ2) + w(λ3), is developed. For a simple extension stress-strain law of the form λσ=Eε, the analytical formula of W in the limited range 1≤λ≤2.5 is found to be W=E ∑ i=l3 [λi−1−lnλi−16 (lnλi)2+118 (lnλi)3−1216 (lnλi)4],where the limitation originates with the stress-strain law. The expression is used to verify the validity of the Valanis-Landel postulation through prediction of the stress-strain behavior in multiaxial deformations.

The effect of stress-strain properties on notched tensile failure in plane stress

An approximate analysis for crack opening displacements for finite sheets of work hardening materials using a modified Dugdale model is given. This is combined with a fracture criterion based on percentage reduction in area and an overall plastic instability failure mode. Predictions using simple tensile data in this analysis are compared with experimental results for hard and soft aluminium, copper, brass and PVC and good agreement is obtained.