Marta Kurek

Estimation of Fatigue Life of Materials with Out-of-Parallel Fatigue Characteristics under Block Loading

The paper presents the algorithm of fatigue life estimation for materials with out-of-parallel fatigue characteristics under block loading. Brass CuZn40Pb2, medium-alloy steel 30CrNiMo8 and high-alloy steel 35NCD16 belong to such materials. Brass CuZn40Pb2 was used for analysis. The experimental results were compared with those calculated according to the assumed model, and satisfactory results were obtained.

Fatigue Life Estimation under Cyclic Loading Including Out-of-Parallelism of the Characteristics

The paper presents an algorithm of fatigue life determination for materials with no parallel fatigue characteristics under pure bending and pure torsion. The presented model uses the iteration method, and the applied fatigue criterion is function of the ratio of normal and shear stresses coming from bending and torsion, respectively. Three materials were applied for analysis: CuZn40Pb2 brass, 30CrNiMo8 medium-alloy steel and 35NCD16 high-alloy steel.

Life estimation by varying the critical plane orientation in the modified Carpinteri-Spagnoli criterion

The modified Carpinteri-Spagnoli (C-S) criterion is a multiaxial high-cycle fatigue criterion based on the critical plane approach. According to such a criterion, the orientation of the critical plane is linked to both the averaged directions of the principal stress axes and the fatigue properties of the material. The latter dependence is taken into account through a rotational angle, ?. Then, the multiaxial fatigue strength estimation is performed by computing an equivalent stress amplitude on the critical plane. In the present paper, some modifications of the original ? expression are implemented in the modified C-S criterion. More precisely, such modified expressions of ? depend on the ratio between the fatigue limit under fully reversed shear stress and that under fully reversed normal stress (in accordance with the original expression), and can be employed for metals ranging from mild to very hard fatigue behaviour. Some experimental data available in the literature are compared with the theoretical results in order to verify if the modified ?expressions are able to improve the fatigue strength estimation capability of the modified C-S criterion.

Estimation of fatigue life of selected construction materials under cyclic loading

In the literature, there are many criteria of multiaxial fatigue. They are based on various assumptions and parameters describing the process of fatigue. Among them, there is a special group of criteria based on the concept of critical plane. Some of them in their equations take into account the ratio of normal and shear stresses. Macha has formulated the criterion of maximum normal and shear stress in fracture plane which can be generalised for the scope of random loading of numerous criteria. In the present study authors estimated the fatigue life of several construction materials. For the purposes of the analysis, the authors proposed modified Carpinteri et al. method to find orientation of critical plane, which is used in multiaxial fatigue criterion defined in critical plane. This plane is turned through the angle of ? in relation to the plane defined by maximum normal stresses. In this study authors analyzed the variability calculation of fatigue life, depending on the angle ?. Simulation studies were conducted in which it was assumed that ? ? . For each of the 46 angles, we calculated parameters B and K appearing in the formula defining equivalent stress. Then we calculated fatigue life according to the proposed model for each of the obtained angles ?. Fatigue life analysis was carried out in order to verify which angle ? gives the most similar results.

The application of the criteria of multiaxial fatigue in the critical plane for the topology optimization of a structure

The article presents the proposal of a simplified fatigue criterion based on a critical plain concept and convenient for the topological optimization of structures. To demonstrate the functioning of a fatigue optimization with critical plain constraints, an example of the optimization problem is checked for two types of material S355JOWP (formerly 10HNAP) and 6082–T6. In the investigation, an evolutionary topology optimization algorithm is used and a new numerical procedure for calculating equivalent stresses is proposed. On the basis of the obtained results, one may notice that for alloy steel the critical plain criterion gives values of the minimum volume of optimum structures similar to comparative Dang Van criterion. In contrary, for the aluminum alloy, the critical plain criterion is much more conservative.

Life time assessment of an aluminum alloy under complex low cycle fatigue loading

Abstract This publication presents an expression for the equivalent strain based on fatigue tests under complex strain state. The respective aluminum alloy was tested under a very low number of cycles. The criterion uses the critical plane determined according to the material kind function. The determined equivalent amplitude for complex proportional and non-proportional loading states is included into the scatter band of the same coefficient for simple loading states. The model based on the proposal by Carpinteri for determination of the critical plane position under a complex strain state can be a useful tool for fatigue life assessment in the case of a very low number of cycles. The expression for the equivalent strain includes both the normal and shear strain histories on the critical plane with suitable weight functions. The determined equivalent amplitude for complex proportional and non-proportional loading states is also included into the scatter band of the same coefficient for simple loading states.

Determination of the critical plane orientation depending on the fatigue curves for bending and torsion

This paper contains a proposition setting out a new way of determining an orientation angle of the critical plane. The applied method involves an analysis of fatigue life results’ scatter of several selected materials on the basis of a new criterion of multiaxial fatigue. The expression for the equivalent stress was derived on the basis of pure bending and pure torsion. The paper also contains a verification of the effectiveness of the proposed model by analyzing scatter bands and the value of the ratio of normal to shear stresses.

The Use of a Power Law Function for Fatigue Life Estimation for Block Loads

A new damage accumulation model based on a power law for fatigue life estimation is presented. The model has been created on the basis of functions of memory taken from psychology. The model derivation is presented as well as its basic properties. It takes into account the loading sequence. The experimental verification of the model is performed for experimental results taken from the literature for four material groups tested with block loading programs. The obtained calculation results are compared with results obtained with the use of Palmgren-Miner model. The model can be used for the determination of fatigue life, especially for block load fatigue tests.

Estimation of fatigue strength under multiaxial cyclic loading by varying the critical plane orientation

The main purpose of this paper is to examine the influence of the critical plane orientation on the estimated fatigue strength of metals under multiaxial loading. The algorithm employed to evaluate fatigue strength implements the criterion of maximum normal and shear stress on a suitable damage plane (critical plane). The angle beta defining the critical plane orientation is measured with respect to the direction that maximises the applied normal stress. Eleven (11) structural materials under combined bending and torsion cyclic loading are examined. For each analysed material, the value of beta angle is selected so that the value of the scatter, defined by a root-mean-square value, is minimum. On the basis of such a calculation, an empirical expression for beta is proposed, that takes into account the values of bending and torsion fatigue strengths at a reference number of loading cycles. According to such an expression, beta is constant for a given material.

Accumulation of Fatigue Damages for Block-Type Loads with Use of Material Memory Function

The paper proposes a new model of fatigue damage accumulation based on the memory of the material and developed on the basis of memorizing of the meaningless material. The comparison of damage accumulation between the classical and proposed models for chosen materials are in favor of the authors model.