Probabilistic Crack Growth Analysis

If needed, please go to the Constant Amplitude section for a review of the general terms and terminology related to Crack Growth fatigue analysis.

Enter as much data as you know. If it is not enough, you will be asked for more. Fields with a light blue/gray background represent the minimum required data to begin calculations. Other data may become necessary as calculation proceeds. Pressing the button provides help in the form of an equation or default information for a parameter.

Experienced user mode is off. Turn experienced user mode on for a more concise form.

Click on the button below to learn by example:

Description of Distribution Types

Loading

Loading on structures frequently follows a Normal or LogNormal distribution. In a well controlled situation, such as may be encountered in a test track or in an electric motor, the coefficient of variation COV is typically 0.1. The COV increases to as much as 0.5 for uncontrolled customer usage. In the absence of any other information, a reasonable value of 0.2 or 0.3 may be assumed. If you enter a value of zero for either the normal or the uniform distribution types, you should enter the standard deviation for the scale parameter. This allows you to generate a distribution around zero.

Loading Units		MPa ksi kN lbs mm/mm in/in % strain micro-strain
				Distribution Type	Scale Parameter
Maximum	Smax or emax =			None Normal Log-Normal Uniform Gumbel Weibull
Minimum	Smin or emin =			None Normal Log-Normal Uniform Gumbel Weibull
OR
Range	ΔS or Δe =			None Normal Log-Normal Uniform Gumbel Weibull
R Ratio	R =			None Normal Log-Normal Uniform Gumbel Weibull

Material

Material properties are the most common source of uncertainty in a fatigue life calculation. Variability in crack growth properties and resulting fatigue lives is smaller than traditional fatigue properties. Typical values for the COV range between 0.05 and 0.1.

You may load a material from the database by selecting it and clicking on "Load Material", or browse the database by clicking the "Material Property Finder" button, or specify individual properties directly. Clicking "Material Property Estimator" will show the default properties that are computed from the input values.

For registered users, the Material Property Estimator will display a curve plot. Registered users may also save this material in their personal database by clicking the "Save Material" button.

public/A356-T6 public/Aluminum 2024-T3 public/Aluminum 2024-T3 public/Aluminum 2024-T351 public/Aluminum 2219-T851 public/Aluminum 7075-T6 public/Man-Ten public/RQC-100 public/Stainless Steel 316 public/Steel 1045, Normalized, BHN=153 public/Steel 3.5NCMV public/Steel 4340 public/Steel 4340 public/Steel 4340M public/Steel A27, Cast, BHN=135 public/Steel A514-B public/Steel A517 public/Steel A533-B public/Steel A533B public/Steel A542 public/Steel A588 public/Steel EN24 public/Steel EN30B

Name
Type		steel aluminum nickel stainless steel other
				Distribution Type	Scale Parameter	Correlation Coefficient
Crack Growth Intercept	C =		m/cycle mm/cycle in/cycle	None Normal Log-Normal Uniform Gumbel Weibull
Crack Growth Exponent	m =			None Normal Log-Normal Uniform Gumbel Weibull
Crack Growth R Ratio	Rmat =
Threshold Stress Intensity	ΔKTH =		MPa sqrt(m) ksi sqrt(in)	None Normal Log-Normal Uniform Gumbel Weibull
Elastic Modulus	E =		MPa ksi	None Normal Log-Normal Uniform Gumbel Weibull

Stress Intensity Factor

The initial crack size can vary by an order of magnitude and the variability in crack sizes is much greater than the materials variability. Coefficients of variation between 0.1 to 0.5 are reasonable.

When you enter a distribution for F(a), a distribution with mean = 1 will be generated. The entire F(a) curve will be multiplied by this number. This will add variability the F(a) curve to account for modeling uncertainties.

Calculate