Tech Briefs

The Automated Fatigue Calculation Program (FATPLUS)

Since it was developed in the late 1980s, the Automated Fatigue Calculation Program (FATPLUS) has consistently demonstrated its ability to predict fatigue life, independently validate proprietary fatigue methodologies, identify fatigue-sensitive parts, and train engineers in predicting fatigue lives. FATPLUS enables its users to create, save, and modify specific stress spectra and properties of materials.

A conservative methodology yields valid estimates of fatigue lives as long as reasonable engineering practices are maintained. However, FATPLUS can do more than this. The program is also a powerful, menu-driven, interactive design software tool for studying and analyzing effects of a spectrum of stresses, effects of changes in design, effects of changes in materials, and changes in concentrations of stresses.

To make FATPLUS into a program that predicts fatigue lives and a very powerful tool for analyzing systems, the developers of FATPLUS accounted for (1) the spectrum of stresses, (2) the number of loadings per stress event, (3) fatigue properties of materials, and (4) safety factors. Of these considerations, safety factors were found to be the largest variables in estimating fatigue lives because safety is affected by (1) constant life reduction, which multiplies total damage or reduces the number of cycles by a constant factor, and (2) statistical reduction, according to which allowable stress is reduced by a percentage based on the known degrees of variability of properties of materials.

When performing a system analysis or study, FATPLUS uses an online or preprogrammed spectrum, which is applied to a materials curve (a complex curve based on the more conservative of constant life reduction or statistical life reduction). Total damage and allowable life are determined from a combination of input from the user and cycles in the input spectrum. Because all users will not have access to corporate or private fatigue data, the developers of FATPLUS provided MIL-HNDBK-5 data on materials.

FATPLUS is therefore a superior program. Since release, this multifaceted program has proved itself strong and adaptable. FATPLUS can be expected to become used widely across private industry and government as users become more aware of all the program has to offer.

FATPLUS is written in FORTRAN for execution on PC-compatible computers.

This work was done by Thomas Minyard and James A. Babb, formerly of Lockheed Electronics and Space Corporation for Johnson Space Center. The software with complete documentation is available from James Babb, Johnson Engineering, telephone (281) 228-7710.   MSC-22537