Fatigue analysis from market data

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Discussion Overview

The discussion revolves around analyzing the fatigue life of a low carbon steel component that fails after an average of 540 days. Participants explore theoretical methods to simulate this fatigue life, considering various approaches and definitions of fatigue life.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to simulate the fatigue life of a component that fails after 540 days, questioning how to approach this theoretically.
  • Another participant asks for clarification on what is meant by "simulate by theory," indicating confusion over the terminology used.
  • It is suggested that two primary methods could be used: the Paris equation with experimental coefficients and Finite Element Method (FEM) for rapid calculations.
  • Discussion includes the definition of "fatigue life," with some participants noting that it may involve both crack initiation and propagation, which could influence the choice of analytical methods.
  • Classical fatigue analysis methods are mentioned, including the IIW rules and general lower bounds for Paris law coefficients, depending on material type and fatigue conditions.
  • One participant requests assistance with classical fatigue analysis, indicating a need for further guidance on the topic.
  • Links to introductory resources on fatigue analysis are shared, suggesting that a classical 'stress-life' or S-N approach might be applicable in this case.

Areas of Agreement / Disagreement

Participants express varying interpretations of how to simulate fatigue life and the definitions involved, indicating that multiple competing views remain without a clear consensus on the best approach.

Contextual Notes

There are unresolved aspects regarding the assumptions made in defining fatigue life, the applicability of different methods, and the complexity of the component being analyzed.

chandran
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I am working on a project for a company to analyse the fatigue life of a component. There is an existing component that the company
manufactures and the component fails by fatigue on the average 540 days. The component is a tube made of low carbon steel of yield 260N/sqmm and ultimate of 340N/sqmm. But the company gives a guarantee of 700 days to the customers. How can i simulate
by theory the fatigue life of 540 days for that component.
 
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What exactly do you mean by "simulate by theory?" I'm a little lost at that phrase.
 
Yes. Simulate by basic theory.
 
minger said:
What exactly do you mean by "simulate by theory?" I'm a little lost at that phrase.

I recall there are two ways:

-Using Paris equation, which is equated with experimental coefficients.

-Using FEM.

For a rapid calculation see first case.
 
Would also consider how you define "fatigue life" of the component, i.e. whether it consists of crack initiation and/or propagation, if only the latter then Paris law is the best way to go, if former is included methods of "classical" fatigue analysis come into play. Depending on the complexity of your component I think you can have decent enough estimates using "desktop" solutions, such as the IIW rules and there are some general lower bounds for Paris law coefficients (or Nasgro if a more general form of FCP law is required) depending on type of material and under what conditions the fatigue occurs.
 
can anyone help with classical fatigue analysis
 
Found these general intros to different aspects of fatigue analysis - one by D. Socie and another lecture paper:

http://www.mie.uiuc.edu/content/files/FCP%202001%20Basic%20Short%20Course/4%20Analysis.pdf
http://www.engr.ku.edu/~rhale/ae510/fatigue.pdf

... good starts in familiarizing the different approaches and concepts, I think a classical 'stress - life' / S-N approach might do it in this case (?).
 
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