A fatigue damage indicator parameter for P91 chromium‐molybdenum alloy steel and fatigue pre‐damaged P54T carbon steel

Abstract

A B S T R A C T Two grades of structural steel were subjected to fully reversible, constant stress amplitude cyclic loading. The local strain response of the material was measured and recorded during the test, with the applied testing technique enabling the monitoring of hysteresis loop variation for the narrowest cross-section of the hourglass specimen. Changes in hysteresis loop width, representing the local inelastic response of the material, were recorded in order to monitor the density of structural imperfections. Material ratcheting behaviour was observed as changes in the mean strain for selected load cycles. Ratcheting was attributed to local deformation of the material in the vicinity of imperfections such as voids or inclusions, as well as deformation induced by the propagation of microcracks. Definitions of a damage indicator parameter and damage parameter were proposed. The fatigue behaviour of the two investigated grades of steel was finally illustrated in the form of damage curves for different stress amplitudes and for undamaged and fatigue pre-damaged material.Keywords fatigue damage; inelastic strain; ratchetting; damage accumulation; damage parameter. N O M E N C L A T U R Ea = crack length D = damage parameter N, n = number of load cycle N f = number of cycles to failure R = cycle asymmetry ratio ε i = inelastic strain range for load cycle ε m = mean strain for load cycle φ = damage indicator parameter σ a = stress amplitude I N T R O D U C T I O NIn the two hundred years since fatigue in metal alloys was first investigated, 1 numerous studies have been carried out with the aim of identifying the physical phenomena underlying the accumulation of fatigue damage and leading to final failure of construction components. Although significant progress has been achieved, with the results of investigations published in many papers, the basic procedure for the estimation of fatigue life in construction components is still based on Wöhler's proposition. 1 This is because of the fact that observations of changes in internal structure have revealed the huge complexity of physical phenomena underlying fatigue damage accumulation. Many attempts have been made to relate identified phenomena to changes in the macroscopically observed mechanical properties of the investigated alloys. A summary regarding the elaborated damage models can be found in a number of review books 2 and papers. [3][4][5] Because finding relationships between changes in material mechanical characteristics and the physical phenomena underlying fatigue damage accumulation is very difficult, none of the mathematical models developed in this way have as yet been accepted by the engineering community for the estimation of component fatigue life.One obvious result of fatigue damage accumulation is the formation of dominant crack. The rapid development of fracture mechanics theory and testing methods have Correspondence: G. Socha.

Deformation Based Fatigue Damage Accumulation Model

“…In the paper [7] authors used such simplified definition of damage indicator parameter for investigation of fatigue damage accumulation of pre-damaged (after service) P54T carbon steel. Definition of damage indicator parameter φ was simplified in the following way:…”

“…,where φ f stands for value of damage indicator parameter at failure. Value of damage indicator parameter as the function of load cycle number for P54T carbon steel reported in [7] is shown in fig. 4 for applied stress amplitudes ranging from 320 to 410 MPa.…”

Study on deformation-induced damage evolution for Inconel718 superalloy with the use of innovative single-specimen method

“…This approach was already followed by one of the authors of this paper in the investigation of the fatigue damage accumulation. The results were published in (Socha and Dietrich, 2014). Using the accumulated equivalent plastic strain as the damage indicator parameter has some disadvantages.…”

Fatigue Damage of Al/SiC Composites - Macroscopic and Microscopic Analysis

“…Many attempts have been made to evaluate an appropriate fatigue damage parameter [1][2][3][4][5][6][7][8]. Kaczanow proposed as a damage indicator the density of microcracks identified at the surface in a given volume.…”

“…The parameter was successfully applied for steels where the main fatigue damage mechanism is based on cyclic plasticity. Dietrich proposed the mean inelastic strain as a fatigue damage indicator for materials which exhibit ratcheting instead of cyclic plasticity [6][7][8]. There are materials, e.g.…”