Fatigue Damage including Mean Stress

Options for including mean stress have been added to the Vibrationdata GUI package’s Fatigue Toolbox, for both stress time histories and PSDs.

Four methods are available:

Gerber
Goodman
Morrow
Soderberg

Matlab script: Vibrationdata Signal Analysis Package

Here are some charts from Iowa State University: Fatigue Mean Stress

– Tom Irvine

NASGRO Coefficients

The following scripts calculate the A, B, C, P coefficients to model a set of SN curves with varying R values: sin_curve_fit_R.zip

sin_curve_fit_R.m is the main script.

The remaining scripts are supporting functions.

The equations are given in: sn_curvefit_equation.pdf

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References:

1. MIL-HDBK-5J

2. NASGRO NASFORM manual

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– Tom Irvine

Webinar 37 – Acoustic Fatigue

PowerPoint Slides:  webinar_37_acoustic_fatigue.pptx

Audio/Visual File:

NESC Academy Acoustic Fatigue – Recommend viewing in Firefox with Sliverlight Plugin

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References:

Rainflow Fatigue Posts

Acoustic Fatigue of a Plate

Acoustic Power Spectra

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Matlab script: Vibrationdata Signal Analysis Package

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See also: Vibrationdata Webinars

Thank you,

Tom Irvine

Low Risk Parts for Spaceflight

NASA-STD-5019, Fracture Control Requirement for Spaceflight Hardware (excerpt)

4.1.1.12 Low-Risk Part

This section addresses parts that can be classified non-fracture critical because of large structural margins and other considerations that make failure from a pre-existing flaw extremely unlikely.

a. For a part to be classified low risk, it shall be constructed from a commercially available material procured to an aerospace standard or equivalent.

b. Aluminum parts shall not be loaded in the short transverse direction if this dimension is greater than 7.62 cm (3 in).

c. A part whose failure directly results in a catastrophic hazard shall be excluded from being classified low risk, except when the total (unconcentrated) stresses in the part at limit load are less than 30 percent of the ultimate strength for the material used and requirements (1) through (3) and either (4) or (5) are met.

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Thus fatigue and fracture analyses are not required for parts with peak stress less than 0.3*ultimate.

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The peak stress can also be compared to the endurance limit, but some materials do not have identified endurance limits.  Here is a rule-of-thumb for these cases from NASA-HDBK-5010.

Perform endurance limit analysis to show the maximum stress does not exceed the endurance limit or

Smax < Ftu/( 4{1-0.5 R} )

where

Smax is the local concentrated stress
Ftu is the tensile ultimate stress
R is the ratio of minimum stress to maximum stress in a fatigue cycle

Note that R=-1 for fully reversed stress with zero mean stress.  For this case:  Smax < Ftu/6

This formula has some limitations and needs further research.

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– Tom Irvine

SDOF Response to Sine or Sine Sweep Base Input, Rainflow

sdof_base_image

Rainflow fatigue cycles can be easily calculated for a single-degree-of-freedom subjected to a sine or sine sweep base input.  The reason is that each pair of consecutive positive and negative response peaks forms a half-cycle.

The relative fatigue damage can then be calculated from the rainflow cycles.

Here are Matlab scripts for performing the rainflow and damage calculations.  rainflow_sine.zip

rainflow_sine.m is for the case where the natural frequency is known.

rainflow_sine_fds.m gives the fatigue damage spectrum for a family of natural frequencies.

The remaining scripts are supporting functions.

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See also:

Rainflow Cycle Counting

ramp_invariant_base.pdf

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– Tom Irvine

Acoustic Fatigue of a Plate

Here is a paper showing how fatigue damage can be calculated from a stress response PSD for a plate excited by an acoustic pressure field:  acoustic_fatigue_plate.pdf

The calculation method is given at:
Fatigue Damage for a Stress Response PSD

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The Matlab scripts for calculating the plate responses are included in the vibroacoustics section of the Vibrationdata GUI package, available at:   Vibrationdata Signal Analysis Package

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See also:

Steady-State Response of a Rectangular Plate Simply-Supported on All Sides to a Uniform Pressure:  ss_plate_uniform_pressure.pdf

Steady-State Vibration Response of a Plate Fixed on All Sides Subjected to a Uniform Pressure: fixed_plate_uniform_pressure.pdf

Aircraft Fuselage Fluctuating Pressure

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– Tom Irvine

Fatigue Damage for a Stress Response PSD

Here is a paper.

Estimating Fatigue Damage from Stress Power Spectral Density Functions: estimate_fatigue_psd.pdf

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This following Matlab program calculates the cumulative rainflow fatigue damage for an input stress PSD using the following wideband methods:

1. Wirsching & Light
2. Ortiz & Chen
3. Lutes & Larsen, Single-Moment
4. Benasciutti & Tovo, alpha 0.75
5. Dirlik
6. Zhao & Baker

Reference:

Random Vibrations: Theory and Practice (Dover Books on Physics)

The stress PSD and the fatigue strength coefficient must have consistent stress units.

The input PSD must have two columns: freq(Hz) & stress(unit^2/Hz)

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Main scripts:

stress_psd_fatigue.zip

Vibrationdata Signal Analysis Package

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The following values are “For Reference Only.”

m = fatigue exponent
A = fatigue strength coefficient

Aluminum 6061-T6 with zero mean stress

m=9.25
A=9.7724e+17 (ksi^9.25)
A=5.5757e+25 (MPa^9.25)

Butt-welded Steel Joints

m=3.5
A=1.255e+11 (ksi^3.5)
A=1.080e+14 (MPa^3.5)

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See also:

Rainflow Fatigue

Mrsnik, Janko Slavic, Boltezar, Frequency-domain methods for a vibration-fatigue-life estimation – Application to real data:  mrsnik_article_vib_fatigue.pdf

Spectral Moments Notes

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– Tom Irvine

Using Random Vibration Testing to Cover Shock Requirements

Aerospace and military components must be designed and tested to withstand shock and vibration environments.

Some of this testing occurs as qualification, whereby a sample component is tested to levels much higher than those which it would otherwise encounter in the field. This is done to verify the design.

Now consider a launch vehicle component which must withstand random vibration and pyrotechnic shock. The random vibration specification is in the form of a power spectral density (PSD). The shock requirement is a shock response spectrum (SRS).

Pyrotechnic-type SRS tests are often more difficult to control and thus more expensive than shaker table PSD tests. Furthermore, some lower and even mid-level SRS specifications may not have the true damage potential to justify shock testing.

The fatigue damage spectrum (FDS) can be used to further determine whether the PSD specification covers the SRS requirement.  If so, then shock testing can be omitted in some cases.

Here is a paper: random_cover_shock.pdf

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See also:

Shock Severity Limits for Electronic Components

Rainflow Cycle Counting

Fatigue Damage Spectrum

Matlab Mex – fds_main script

SDOF Response to an acceleration PSD Base Input – VRS script with FDS option

Matlab script: Vibrationdata Signal Analysis Package  – SRS damped sine time history synthesis function

Direct Fatigue Damage Spectrum Calculation for a Shock Response Spectrum

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– Tom Irvine