When metal equipment operates at temperatures higher than half the melting temperature, it undergoes creep: deformation under smaller forces than those that would cause deformation at normal temperatures. Creep depends on the operating temperature and the amount of time the equipment has been in operation. Engineers typically test metal to determine its usable lifespan, but tests under operating conditions are not feasible when lifespan is measured in years. Instead, engineers test the metal at higher temperatures, and calculate the Larson-Miller parameter based on the temperature and time to failure. Once you know the Larson-Miller parameter, you can calculate the lifespan of the metal at the operating temperature for the equipment.
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Divide the Larson-Miller parameter by the operating temperature of the equipment in Kelvin. If the parameter is 25,000 and the temperature is 1,000K, for example, 30,000/1,000 = 25.
Subtract 20 from the result. For example, 30 - 25 = 5.
Take the inverse base 10 logarithm of the result to find the time to failure in hours. For example, invlog(5) = 100,000 hours.
Divide the time to failure in hours by 8,760 hours per year to find the life of the equipment in years. For example, 100,000 hr/(8,760 hr/yr) = 11.4 years.
Tips and warnings
- Taking the inverse base 10 logarithm of a number is the same as raising 10 to the power of that number.
- Add 273.15 to the temperature in degrees Celsius to convert to Kelvin.
- Subtract 32 from the temperature in degrees Fahrenheit and divide by 1.8 to convert to degrees Celsius. Convert from degrees Celsius to Kelvin.
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