IFMASS10

FATIGUE ANALYSIS USING FRACTURE MECHANICS

Zijah Burzić

Military Technical Institute, Belgrade, Serbia,

Meri Burzić

Institute GOŠA, Belgrade, Serbia

1. INTRODUCTION

Fatigue properties offer information for structural life assessment in many engineering

applications. They are a critical element in the path relating the materials of construction

to the components and must take into account many influences to reflect the actual

situation. In application, fatigue requests a detail analysis, trying to assess what will occur

at a particular location of a component or assembly under cyclic loading.

The topic of fatigue properties is very broad and is typically based on testing coupons.

To be applicable, determined properties must support proper fatigue design approach.

Three general approaches to fatigue design are considered here, and their individual pro-

perty requirements are described. The purpose is to provide the basic insights necessary

to examine those properties that can be found, review some of the common presentation

formats, and recognize their inherent characteristics, which may be critical in actual

application. The susceptibility of mechanical properties to variation through micro-

structural manipulation and structural consideration can be substantial.

The testing for property characterisation is reviewed briefly, with the discussion of

relations between material, property, and structure. Three sections cover specific proper-

ties to the major design approaches: stress-life, strain-life, and fracture mechanics, with

selected examples of properties that reflect some detail of each approach.

Fatigue life assessment is obviously one of the main areas requiring properties deve-

lopment. Basically, data for test coupons are only good for life assessment of test cou-

pons; other structures may not be as amenable to assessment. The substantial amount of

scatter in results is one of the contributing features to these difficulties. Verification of

life estimations should be considered as an important activity to confirm the calculations.

For the sake of brevity, only the constant-amplitude loading will be discussed. But, it is

essential to understand that variable-amplitude loading can produce different rankings

than constant-amplitude results. Another concession to brevity is that within the fracture

mechanics area, only plane-strain state is included /1/.

2. FATIGUE DESIGN PHILOSOPHY

Fatigue properties have to be consistent with one of three general fatigue design

philosophies (Table 1). Each of these has a concomitant design methodology and at least

one way of representing testing data that provide the properties of interest.

These "lifing" or assessment techniques correspond to the historical development and

evolution of fatigue technology over the past minimum 150 years.