S-N Fatigue is two separate programs (one for the fatigue design, the
other for material properties) that work together to design against fatigue
failure for any structure or component. The programs are the only tools you will
need for parts that are known to be (almost) free of cracks. The fatigue models
are based on the Coffin-Manson strain model which has been improved and enhanced
Material Selection. The materials module serves as the preprocessor to the fatigue design module and generates the material-property file for your application. To access the design module, you must first enter information to define the material for your design. The minimum information required is the basic material properties such as modulus of elasticity, yield, and ultimate strength. S-N Fatigue will estimate the fatigue properties such as fatigue limit and both stress- and strain-cycle curves. The material file has a complete definition of the material, as well as the generated stress and strain-cycle curves.
Fatigue Design. Now that the material definition is complete, you can proceed with the load definition. Loading conditions can be either single load or cumulative with a variable load spectrum.
For the single-load option in the elastic range, you can choose between single alternating stress or combined static and alternating stress. The single alternating stress can originate from bending, axial, or torsional loading. For plastic range design, single strain in bending or an alternating strain range can be used. Alternatively, the single load option lets you enter cycles in bending, torsion, or axial fatigue for S-N Fatigue to estimate the equivalent alternating stress and strain. The loading state, which can be either plane stress or strain, combined with a selected fatigue failure probability will determine the life in cycles, the equivalent alternating stress, and total strain for a smooth specimen.
A new option lets you enter the cycles to failure in order to generate a portion of what is known as constant life diagram. S-N Fatigue will create a table of static and alternating stresses for a given cycle-to-failure level. Running the program at different cycles to failure and plotting the data will give you a better idea of what to expect at any fatigue life requirement for that material, for the same probability to failure, and for fully reversed loading.
For variable load history, S-N Fatigue accepts information in terms of stress pairs from rain flow analysis or any other load-spectrum data acquisition system. To save time,you can have stress pairs that are clustered into number of cycles per pair. Clustering the data reduces the calculation time three to ten fold. If your data are strain instead of stress pairs, the stress- and strain-cycle curves generated by the materials module will help you convert them.
If you specify the number of cycles per block (the number of cycles a loading sequence is repeated), S-N Fatigue will calculate the cumulative damage per block and the number of blocks to failure. The calculation is done for any combination of discrete loads, or when repeating the same loading condition at various levels. For either loading option (single or cumulative) the program asks for safety factors for alternating stress or strain. The combined effects of size, notch factors, and surface roughness effects are factored in the results.
S-N Fatigue is a general purpose fatigue analysis program any engineer, student to practicing, can use. The enhancements in loading options as well as the material definition outnumber the user interface shortcomings. If the next Windows release has the enhancements in the latest DOS version, S-N Fatigue will increase in value as a engineering and educational tool.
A similar product:
WaveMaker - Instron Corp.; 100 Royall St., Canton, MA 02121 fax: (617) 575-5751.