Dariusz Skibicki

Multiaxial fatigue life and strength criteria for non-proportional loading*

Abstract In the article a proposal of loading non-proportionality measure H for sinusoidal biaxial loading has been presented. On the basis of non-proportionality measure, there has been proposed a bending and torsional fatigue strength, with a phase shift of components and the mean value has been proposed. Moreover, a high-cycle fatigue life criterion based on the same non-proportionality measure has been proposed. The predictive capability of both criteria were demonstrated by analysing experimental results from the literature. The predicted results are generally in good agreement with the experimental ones.

Steel X2CrNiMo17-12-2 Testing for Uniaxial, Proportional and Non-Proportional Loads as Delivered and in the Annealed Condition

The article presents the results of fatigue life and fractographic testing of steel X2CrNiMo17-12-2 exposed to proportional and non-proportional fatigue loads. The following load types were applied: tension-compressive strength, torsion, proportional combined/complex loads produced by tension-compressive strength and torsion as well as non-proportional combined load – by tension-compressive strength and torsion by the phase shift angle φ=90°. The paper analyses the effect of the load method on the fatigue life and fractography of fatigue fractures recorded, and especially the effect of non-proportional load.

Analysis of CDM Model in the Scope of Low-Cycle Fatigue Life Estimation

The article presents calculations of fatigue life acc. to CDM model performed analytically and in program LS-DYNA with the model implemented there. On the basis of comparison of the obtained results with experimental fatigue lives, the possibility of estimation of fatigue life in LS-DYNA program acc. to CDM model was evaluated.

Verification of FEM Modelling Capabilities Allowing for the Effects of Strain Rate

The effects of strain rate on the changes in material properties are commonly known [2,5]. This phenomena is also the subject of studies related to composite materials due to fact that composites are widely used as an energy absorbers in road safety structures [6]. Those effects are more often considered as required to be included in the design of structures subject to dynamic loads [1,3]. The example structure is the road infrastructure subject to high strain due to the vehicle impact characterized by a high strain rate [1].Various numerical methods are being employed at the design stage to reduce the costs of experiments on actual structures. One of the most important factors in preparation of the analysis is to select a suitable material model to allow for the effects of strain rate on the changes in material properties. One of the material models allowing for the effects of strain rate is the model developed by G.R. Johnson and W.H. Cook.The study aims to verify the FEM modelling capabilities of the effects characterized by the effects of high strain rate on the material properties. Thus, the strength tests required to determine the material constants for Johnson-Cook model were performed and modelled using FEM method and the results including the effects of strain rate on the mechanical properties were compared.

Verification of the Charpy Impact Test Capability to Determine the Constants of the Johnson-Cook Model

Feasibility analysis of replacing split Hopkinson bars test by Charpy impact test for determination of Johnson-Cook’s material model parameters. The results show that the Charpy impact test may, due to the strain rates achieved, successfully replace the mentioned experimental test. Moreover the results shows that some further studies should be conducted to improve efficiency of the proposed method.

The Influence of Fatigue Loading on the Microstructure of an Austenitic Stainless Steel

Abstract The rotation of principal stress/strain axes, which takes place under conditions of non-proportional fatigue loading, may significantly worsen the fatigue life. An explanation of this phenomenon can be found through microstructure examination. In this paper, we present the results of a microstructure examination of a X5CrNi18-10 steel subjected to fatigue loading, both, proportional and non-proportional. The variability of the positions of the principal stress axes was controlled by a fatigue loading program that consisted of alternatively realised blocks of torsion and tension-compression. The microscopic investigations were carried out by X-ray structural analysis and the micro-hardness of the material was examined. The results revealed that the application of cyclic loading caused a plasticity-induced martensitic transformation. Comparative analysis showed that the transformation depended on intensity and the kind of applied loading.

Determination of the Johnson-Cook damage parameter D4 by Charpy impact testing

Abstract This study proposes to design a road support structure subjected to high-strain-rate loading occurring during vehicle collisions. The strain-rate affects both the hardening process and the material failure process. For modeling the strain-rate influence on material, various mathematical material models are used including the Johnoson-Cook model. The main goal of the study is to presenta method for determining the parameters for the Johnson-Cook damage model, a hybrid method which requires Charpy impact tests on a standard specimen with an annular notch and a series of calculations using finite element method simulating the test. The advantages of the presented method compared to existing methods are the high availability of the equipment, simple and quick processing of results and significantly lower costs.

Method for determining the strain rate sensitivity factor for the Johnson-Cook model in Charpy tests

Abstract The basis for the study was the necessity to design a supporting structure of the road infrastructure subjected to high strain rates occurring during vehicle accidents. Mathematical models of hardening due to the high strain rates are reviewed. Commonly used methods for determining constants for one of the models, i. e., Johnson-Cook model are characterized. The main section of the study presents a proposed method for determining the strain rate sensitivity of the material. It is a hybrid method which requires Charpy tests using unnotched specimens and a series of calculations using the finite element method to simulate the tests. Several variants of strain measurement of specimen subjected to failure in Charpy tests, as required for the proposed method are analyzed. Using the suggested method, a strain rate sensitivity factor was obtained with a value similar to the value obtained using the split Hopkinson pressure bar technique. The advantages of the suggested method compared to the existing methods are the high availability of the instruments, simple and quick processing of the results and significantly lower costs.

Determination of Johnson-Cook model constants by measurement of strain rate by optical method

The goal of the research program implemented by the authors was to develop methodology to be used for determination of constants for Johnson-Cook material model using Charpy impact test. This model is used in programs for modeling by the FEM method in order to account for the influence of strain rate on the strength properties. The method we propose involves selection of the material constants by comparing behavior of a specimen during Charpy test with results of simulation using the FEM method, in LS-DYNA program. Measurement of strain on the specimen surface involved recording Charpy test by means of a fast video camera and time –lapse measurement of changes in the distance of markers.

The Development of an Experimental Research Plan Based on Local Strain-Life Method for an Air Handling Unit Subjected to Seismic Loads

This paper presents procedures to be used for development of an experiment plan which is supposed to provide the tested structure with operating conditions most similar to those in real life. The research object is an air handling unit subjected to random loading such as earthquakes.