The material property calculator provided by Vibration Research will plot the gRMS vs. time to failure data, determine the slope of the S-N curve, and calculate the material property (m) based on the input data.
Material property can be calculated based on repeated test runs. The steps include:
- Enter statistical information: gRMS and time or cycles to failure
- Plot gRMS vs. time (or cycles) on a log-log plot
- Create a trendline (power-law function)
- Calculate the slope of the power-law function trendline (b)
- Calculate m, where:
- m = (-(1/b))*80% for random waveforms
- m =(-(1.b))*60% for sinusoidal waveforms
Download Calculator (Excel)
What is the Material Property (m)?
Fatigue Damage Spectrum (FDS) software creates an FDS using weighted time-history files representative of a product’s end-use environment. With the FDS, an engineer can generate a random power spectral density (PSD), resulting in a random profile that is the damage equivalent to the product’s operation.
To accurately calculate the FDS, the engineer must enter the product’s m and Q values. The “m” value is the material property. There are general values for analysis or investigative purposes, but for more in-depth analysis and precise calculation, we can determine an m and Q value for a product.
The m value is more significant in the FDS calculation, and its accuracy can improve the accuracy of the generated FDS. The process to determine the m value is also more involved than that of determining Q.
To accurately determine m, we must create an S-N graph (stress to number of cycles) (Wöhler, 1870). One method to determine the S-N graph is to repeatedly test a product to failure at varying GRMS levels and record the amount of time required to achieve failure. When enough failure runs have been recorded, it is possible to back-calculate a strong approximation of the S-N curve by plotting the data points on a log-log graph and then plotting the power-law model of the data. The slope of the power-law model is equal to b. From b, it is possible to calculate the value of m.