Fatigue & sustained load crack growth

Crack growth prediction is possible in Zencrack using two theories:

• 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 crack growth

In this type of analysis the load history is a counted spectrum of load cycles with each being defined by a minimum and maximum load level. 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.

Zencrack relates a finite element analysis at a known load level to the load level for the current position in the cyclic history. Integration through the cycle history provies a range of da vectors for each node along the crack front for a give consistent increment of cycles, dn.

Sustained load 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. 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.

Zencrack relates a finite element analysis at a known load level to the load level for the current position in the time history. Integration through the time history provies a range of da vectors for each node along the crack front for a give consistent increment of time, dt.

Combined fatigue and sustained load crack growth

In a combined fatigue and sustained load crack growth analysis both the da/dn and da/dt effects contribute to the crack growth. The input must consist 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 incremented.