Counting method
A counting method is a statistical evaluation and reduction method of the operational strength within the field of materials engineering . Counting methods are used to convert a random load-time curve, for example measurement data from a test drive, into a collective load . This load spectrum forms the basis for a theoretical strength assessment by comparison with a corresponding Wöhler curve .
A characteristic of counting is always the conversion of a stress-time sequence into a sequence of classified reversal points and the subsequent counting of a specified feature as a fundamental damage event. A distinction is made between one-parameter and two-parameter counting methods. The latter allow a clear inverse transformation of mean values and amplitudes. Sequence and frequency information - mostly of subordinate interest from the point of view of operational stability anyway - are lost, however.
For example, a counting process records one or more of the following characteristics of the stress-time curve:
- the reversal points of the stress, i.e. either a maximum or a minimum,
- the size of the area between a minimum and the following maximum of the stress or vice versa,
- the stress exceeds a specified class limit in a positive or negative direction.
Procedures in which a feature is recorded are called single-parameter. Frequently used single-parameter counting methods are:
- Peak counting . The maxima above and the minima below the basic load are counted.
- Peak count between base stress cycles . Only the largest maxima or the smallest minima between two basic load cycles are counted, so small intermediate oscillations are neglected.
- Area counting . The size of the class transitions between two turning points is counted.
- Area pair count . The frequency of full swing cycles is determined.
- Class crossing count . Any given class limits are exceeded.
- Class limit violation counting with reset widths . This procedure is a modification of the class boundary crossing count. The exceeding of the specified class limit is only counted when the load falls below a second associated class limit. The difference between the first and the second associated class limit is the reset width. As a result, smaller intermediate oscillations are filtered out of the counting result.
Processes in which two features are recorded are called two-parameter. Frequently used two-parameter counting methods are:
literature
- Otto Buxbaum: Fatigue , publishing Stahleisen mbH, 1986
- Erwin Haibach: Fatigue , VDI Verlag, 1989
- Dieter Radaj: Fatigue strength: Fundamentals for engineers , Springer Verlag, 2007
Remarks
- ↑ A sequence effect may, however, have a significant influence on service life - a sudden transition from high to low stress amplitude due to mechanical crack phenomena can have a more favorable effect on service life than vice versa.