R. Pawliczek

The influence of the mean stress on fatigue life of 10HNAP steel under random loading

Abstract This paper contains the results of uniaxial random load fatigue tests with different mean values performed on 10HNAP steel specimens. The experimental fatigue life was compared with the life determined according to different methods of amplitude transformation of the cycles. Cycles were counted with the rain flow algorithm and damage was cumulated with the Palmgren–Miner hypothesis. It has been shown that in the case of “symmetric” stress histories with zero expected values, the mean values of selected cycles do not strongly influence the calculated fatigue life and transformation of the cycle amplitudes; hence, no mean stress correction for cycle mean values different from zero is required. As for the “asymmetric” histories, the algorithm of transformation of the stress history (because of its global expected value different from zero) gives similar results to the algorithm of amplitude transformation of cycles. A suitable choice of the transformation relationship is very important. In the case of 10HNAP steel, stress amplitude transformation according to the Goodman relationship gives good results.

Energy Based Characterization of Fatigue Behaviour of Cyclically Unstable Materials

The paper presents a definition of a new energy based parameter which allows to better describe fatigue proprieties of materials, especially cyclically unstable materials in comparison with the Lagoda-Macha parameter. The proposed parameter distinguishes positive and negative work of external force and depends on the sign of the stresses in paths of strain in materials. It is used in laboratory tests for control of a fatigue process on hydraulic stand. A close-loop control system has been equipped with computer program based on MATLAB/Simulink module.

Influence of the Mean Load Value in Fatigue Block Loading on Strains

This paper presents the results of the fatigue tests of the specimens made from S3555 steel and aluminum alloy AlCu4MgSi subjected to bending block loading with mean load value. The influence of the changes of the mean load conditions was investigated. During the tests strains were registered and relation between the load and the strains was analyzed. In the case of S355 steel it was observed, that additional hardening occurs when higher value of the mean load is applied at the beginning of the block loading. Other case, when the mean load was increased from zero to maximum, a creep effect of the material was registered. For the specimens made from aluminum alloy similar processes were not observed. The strain levels are almost the same apart from changes of the mean load in load sequence and the fatigue life of the specimens is the same.

Measurement and Control System for Analysis of the Operation of the Stepper Motor

The paper describes a control and measurement system for controlling and analysing operation of a stepper motor. The system design is based on the concept of virtual instrument, where data acquisition and transmission use standard solutions, whereas software serves both as a generator and a measurement system – an oscilloscope. The generator allows efficient control of motor operation in the following control modes: wave, full-step and half-step control. There were no winding powering sequence errors observed. The measurement system allows presenting and analysing voltage waveforms in individual windings with possibility to detect couplings and overvoltages. It was also shown that it is possible to detect locking of rotor rotation.The proposed virtual instruments have open structure, which enables extended analysis based on available measurements or expand range of measurements as long as measurement and control system instrumentation allows that.

Modeling of the stress-strain relationship for specimens made of S355J0 steel subjected to bending block loading with mean load

The paper presents results of calculation for modelling of the stressstrain relationship in the case of block loads with mean load value. A model, based on the stable hysteresis loops, was assumed and modified to use for block loading analysis. For stress history calculation, the proposed model and two other constitutive models were used. Results of fatigue test of specimens made of S355J0 steel subjected to bending block loading with mean load value are presented and used to verify the proposed model. In the tests, the mean load was increased and decreased in subsequent blocks. The changes of strain recorded during the tests shown in the paper indicate a different behavior of the material. Damage accumulation degree for block sequence was used to compare the results of calculations. It was shown, that stress history parameters (stress amplitude and mean stress value in this case) are similar for all investigated models.

Mean Stress in Algorithms Estimating Fatigue Life of Selected Structural Materials

The paper presents comparison of the mathematical models for fatigue life calculations including influence of the mean load value. Several model based on stress analysis on the critical plane and energy density parameter were investigated. In this paper three types of materials were tested and subjected to bending, torsion and combination of bending with torsion loading with the participation of mean value of the load. It was found, that the best fatigue life estimations obtained by models taking into account changes of the material behavior under fatigue loading related to the specified numbers of cycles of the load.

Evaluation of fatigue life of specimens made of S355J0 steel under block loading with mean value

This paper presents the results of fatigue tests carried out for specimens made of S355 steel under bending block loading with the mean value of loading. The tests consisted in increasing and decreasing the mean value of loading in subsequent blocks. Experimental tests show a significant difference in the fatigue life of the material depending on whether the mean value of loading is increased or decreased in the next sequences of the loading block. Models taking into account the influence of mean loading on the fatigue life calculated by means algorithms were verified. It was shown that computational models based on the SWT parameter appropriately describe the results of the experiment; however, when introducing the coefficient of the material sensitivity to the stress ratio, its change together with the number of cycles to failure should be taken into account.

Mechatronic Concept for Airflow Tests Laboratory Equipment

The paper presents application of the mechatronic concept for laboratory test to support a research activity. Laboratory test is taken into account as set of processes for which some number of environmental parameters must be controlled. This paper describes the laboratory stand for investigations of airflow phenomena, where modern data acquisition system is applied to control pressure, temperature and velocity of the air stream. To acquire and process a large number of data, the idea of virtual instruments is introduced to control system. Modern, high-level graphical language was used and the author’s program was developed. The presented solution of the laboratory stand allows increasing the accuracy of measurement, it is possible to observe all necessary parameters as rescaled to standard units and use this information in process steering. All technical solutions applied in the system assume flexibility for changes because of additional sensors, actuators and new signal processing and steering procedures.