Papers by Keyword: Strain Control

Abstract: For strain-controlled fatigue tests, especially at elevated temperature, implementation of knife-edges and punching dimples within gauge length of specimen may cause premature failure on the test section. In this paper, an alternative approach is introduced to control and measure the strains in fatigue tests. Two dimples outside the gauge length section are punched and used to mount the extensometer. The correlation between the controlled (measured) strain and the strain within the gauge length section is established. By comparing the hysteresis loops obtained through the correlation and the one directly measured within the gauge length, it is found that the established correlations are in good agreement with the latter result.

Abstract: The paper presents a comparison of the fatigue life curves based on test of 15Mo3 steel under cyclic, pendulum bending and tension-compression. These studies were analyzed in terms of a large and small number of cycles where strain amplitude is dependent on the fatigue life. It has been shown that commonly used Manson-Coffin-Basquin model cannot be used for tests under cyclic bending due to the impossibility of separating elastic and plastic strains. For this purpose, some well-known models of Langer and Kandil and one new model of authors, where strain amplitude is dependent on the number of cycles, were proposed. Comparing the results of bending with tension-compression it was shown that for smaller strain amplitudes the fatigue life for both test methods were similar, for higher strain amplitudes fatigue life for bending tests was greater than for tension-compression.

Abstract: In this article, a kind of structural strain controlled reliability method was presented. The interval extension of multi-objective control algorithm was applied to control the structural strain reliability. The method has realized the goal of controlling the multiple static interval reliability indexes by the control of the structural interval parameters. In order to accelerate the speed of the structural reanalysis, the Epsilon algorithm was used in the process of the structural reanalysis when a wide range of modification happened in the interval parameters. This method can both get a satisfactory accuracy, and improve the speed of the reanalysis. Numerical examples show that the method is effective and feasible.

Abstract: The low cycle fatigue behaviors of TI-6AL-4V alloy controlled by strain were investigated by experiment. The fatigue tests were performed at room temperature, and cyclic strain and stress ratio are 0.1 with triangle load wave. The results show that TI-6AL-4V alloy is soften rapidly under the cyclic tensile stresses and it is harden rapidly under the cyclic compressive stresses during the initial-stage of strain controlled fatigue, and the rates of cyclic soften and cyclic harden are decreased with the fatigue progress. The soften rate is related to the cyclic strain but little to the cyclic stress during the overall fatigue progress. The change of cyclic stress is related to the macro friction stresses. The results of experiment show that obvious cyclic creep occurs under the stress controlled low cycle fatigue conditions, and the magnitude of cyclic creep strain is related to the maximum cyclic stress. The softening of tensile friction stresses is the main factor of cyclic creep.

Abstract: Unreliable and inflexible analog control systems of fatigue test stands applied in laboratories are being replaced by systems enabling easy development and implementation of new test ideas. This paper presents a digital control system applied in the fatigue test stand SHM 250b. It is based on a new electronic system and the I/O board RT-DAQ4/PCI with the software Matlab/Simulink used for communication with the board. The control system for the fatigue test stand SHM 250b gives the possibility of performing fatigue tests under controlled strain ε(t), stress σ(t) and energy parameter W(t) which is calculated from the stress and strain signals. The applied control system is now used, depending on the performed fatigue tests, and can be developed for specific needs of material strength analysis.