A Study of Overload Effects on Fatigue Crack Propagation under Cyclic Loading Using ESPI System

For many fatigue-critical parts of machines and structures, load history under service conditions generally involves variable amplitude loading rather than constant amplitude loading. An accurate prediction of fatigue crack propagation life under variable amplitude loading requires a thorough evaluation of load interaction effects. In this study, fatigue tests under both constant and variable amplitude loading are carried out to investigate the overload effects on fatigue crack propagation of the notched specimens. Strain distribution around the crack tip before and after a tensile overloading is measured using ESPI (Electronic Speckle Pattern Interferometry) system. The size of plastic zone is determined from the measured strain distribution. The study proposes a crack propagation prediction model that incorporates the overload effect. The overload effect factor for fatigue crack propagation life is used to characterize the load interaction effect in load spectrum. A comparative work demonstrates that the prediction by the proposed model is in good agreement with the experimental results. Introduction In ship and offshore structures composed of fatigue-critical parts, crack propagation under service conditions is more attributable to random or variable amplitude loading than constant amplitude loading. Under variable amplitude loading, crack growth rate can be accelerated or decelerated. Accordingly, to accurately predict fatigue life, an adequate evaluation of load interaction effects is required. Single overload cycle is the primitive building block of the variable amplitude load. It also represents the simplest situation involving crack retardation [3]. A thorough understanding of the behavior of materials to single overload would be beneficial for fatigue crack propagation analysis under variable amplitude loading. There are a number of empirical and semi empirical models proposed for retardation, which contain one or more fitting parameters that must be obtained experimentally. Some models assume that crack closure effects are responsible for retardation [2], while others consider the plastic zone in front of the crack tip [1]. In the present paper, an improved crack retardation model, based on crack-tip plasticity measured via an ESPI system, is proposed and examined. Results of experiments on crack growth under constant-amplitude loading with a single tensile overload are presented. Subsequently, the observed crack growth behaviors and the predictions from the Wheeler’s model are compared with the predictions based on the proposed model. Experiment Engineering Materials and Test Specimens. The specimens are the engineering steel (SM490B) and their chemical composition and the mechanical properties are shown in Table 1 and 2. They are DENT (Double Edge Notch Tension) and ESET (Essentially-Loaded Single Edge Crack Tension) Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 709-714 doi:10.4028/www.scientific. net/KEM.270-273.709 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-13/03/20,17:37:54) Title of Publication (to be inserted by the publisher) specimen designed on the basis of ASTM E338-99 and ASTM E647-99. The geometry is shown in Fig. 1 and Fig. 2. The test area of the specimen is of a rectangular cross section. The V notches are machined at the middle of the specimen using an electrical discharge machining [1]. Table 1. Chemical composition of SM490B Table 2. Mechanical properties of SM490B Composition (weight %) Material C Si Mn P S SM490B 0.18 0.55 1.60 0.035 0.035 Fig. 1. Geometry of the DENT specimen Fig. 2. Geometry of the ESET specimen (unit: mm) Experiment Procedure. All experiments were conducted in a servo-hydraulic, closed-loop mechanical test machine with 500kN load capacity, interfaced to a computer for machine control, a 3D-ESPI system to measure strain distributions, and a microscope (200×) with a resolution of 1 m μ to identify micro crack length. DENT specimen tests were performed under cyclic constant amplitude loading with a mean load of 39 kN, a load amplitude of 26 kN and a frequency of 3 Hz, while ESET specimen tests with a mean load of 8.25 kN and a load amplitude of 6.75 kN and a frequency of 3 Hz. As a measure of overloads, the overload ratio (OLR) is defined as follows: 100 (%) max max ×       − = P P P OLR OL (1) where OL P and max P are the amplitude of overload and the maximum constant-amplitude load, respectively. The tests were conducted for OLR of 50% and 75%. (a) DENT -174,250 cycles(Overloading) (b) ESET -163,721 cycles(Overloading) Fig. 3. Strain distribution measured by ESPI system according to cyclic loading When a crack growth rate is below cycle mm / 10 1 5 − × , the crack growth rate exhibits sensitive changes. Accordingly, the amplitude of cyclic constant amplitude loading was determined on the basis of crack Yield stress (MPa) 325 Ultimate tensile stress (MPa) 490 Young’s modulus (MPa) 202,000 Poisson’s ratio 0.3 710 Advances in Nondestructive Evaluation