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Fatigue and Time Dependent Crack Growth
Load Definition
Fatigue and Time Dependent Crack Growth
The load system methodology in Zencrack allows general definition of time history and fatigue loading. The crack growth integration uses two theories to calculate contribution to crack growth from these two types of load history:
- fatigue crack growth in which the growth increment is based on stress intensity factor range and cyclic load
- time-dependent or sustained load crack growth in which the growth increment is based on stress intensity factor and applied time varying load
Fatigue-only and time-only analyses can be carried out or both effects can be considered in a combined fatigue and time analysis.
Combined Fatigue and Time Dependent Crack Growth
In the most general case of an analysis with fatigue and time dependent effects, the total growth over a load cycle is the combination of the time independent (fatigue) and time dependent growth:
This type of analysis is most easily performed using the "full cycle" load system approach in which a time history of results data is extracted from the f.e. analysis.
For simpler cases, it is possible to provide input in the form of consistent raw load time history data and a counted spectrum with time information indicating when each cycle occurs. Normally there is no requirement for such time information in a fatigue analysis. However, it is essential when a combined growth analysis is performed. The supplied pre-processing program, rainflow, is capable of generating the required counted spectrum with time information.
The crack growth analysis is performed based on the time dependent load history and the da/dt vs K growth data. Whenever the time integration reaches the time at which a fatigue cycle is deemed to occur, the da/dn vs deltaK integration for that cycle is carried out and the total da is incremented.
Raw load time history
Counted spectrum with cycles at known time points
Fatigue Crack Growth
In this type of analysis the load history is a sequence of load cycles with each being defined by a minimum and maximum load level. For general cases and complex cycles the "full cycle" method may be used. For simpler cases, multiple similar cycles may be grouped into blocks of cycles. A general form of this input is supplied via a spectrum file with the simplest form of constant amplitude loading defined by minimum and maximum load levels. The crack growth is dependent on the range of stress intensity factor:
- da/dn = f(deltaK)
This crack growth relationship may take a simple standard form or can be input using a user subroutine.
Time Dependent Crack Growth
In this type of analysis the load level may be constant or changing with time. An input of load level vs time is required. For the "full cycle" approach the variation is included in the f.e. analysis and time dependent results are extracted from the f.e. analysis. For simpler cases, the time variation of load can be defined as a series of (load, time) data points. The crack growth is dependent on the instantaneous stress intensity factor:
- da/dt = f(K)
This crack growth relationship may take a simple standard form or can be input using a user subroutine.
Technical
Load Definition
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