Identification of Fatigue Constants by Means of 3D Method

Abstract

Abstract:The conventional method for evaluation of the fatigue constants uses one set of experimental data from strain-controlled uniaxial fatigue tests. However, these constants do not ensure the compatibility conditions. The new 3D method retains the mathematical and physical relationships between curves considered. This paper presents a way of implementation of the identification procedure and shows results obtained for three types of materials. KEYWORDS:Fatigue constant, low-cycle fatigue, 3D method. IntroductionNowadays, fatigue behaviour of materials is described mostly by the stress-strain curve in the form of Ramberg-Osgood equation and by the Manson-Coffin-Basquin equation in the form of the strain-life curve. It is well known that the assumption of equality of the plastic and elastic components in both equations leads to the so called compatibility condition. The conventional evaluation method of fatigue constants uses one set of experimental data from strain-controlled uniaxial fatigue tests. However, these constants do not ensure the compatibility conditions. A new method proposed by A. Nieslony [1] for determining the stress-strain and strain-life curves retains the mathematical and physical relationships between the curves considered. This paper presents a way of implementation of the identification procedure and shows some interesting results obtained for three types of materials: aluminum alloy 2124T851, stainless steel 316L and structural steel ST52. Identification of fatigue constantFor evaluation of low cycle fatigue tension-compression tests, the Manson-CoffinBasquin equation [2] is applied usually. It illustrates the relationship between the amplitude of total strain and number of cycles to crack initiationwhere -amplitude of elastic strain, -amplitude of plastic strain, , -fatigue ductility coefficient, c -fatigue ductility exponent, , -fatigue strength coefficient, b -fatigue strength exponent.

Identification of Fatigue Cracks on the Basis of Measurable Changes in System Dynamics

“…Among other parameters, the following reduce strength properties of materials due to the occurrence of damage in material structure during service: component shape, material type, load type and the state of stress. Due to multi-aspect nature of material fatigue phenomenon and considering continuously expanding level of knowledge in this regard, individual aspects of this phenomenon are currently under thorough investigation, including: the impact of load type [4], [5], cumulation of damage during random loads [6], [7], reduction of multiaxial state of stress to an equivalent uniaxial state of stress [8], [9], or else the impact of an average value of load [10], [11]. Are also known in the literature approaches which takes into account the grain structure and the content of respective alloying elements for the purpose of estimating the fatigue life [12].…”