Equipment Integrity & Fracture Mechanics

Fracture mechanics is often thought of as being related to the imminent failure of equipment containing cracks through brittle fracture. But the information that a fracture mechanics assessment can yield is a whole lot more than just a critical load, or a critical crack size, that is predicted to cause catastrophic failure.

A fracture mechanics assessment can provide different information to the asset or integrity engineer, depending on whether or not flaws have been detected. These two scenarios, and a general discussion on fracture mechanics assessment techniques, are discussed in more detail below.

No flaws detected

This scenario often applies at the equipment design phase. A fracture mechanics assessment is carried out on the assumption that flaws exist from the moment the equipment is put into service. The assumed flaw size for the assessment is the largest flaw that can be missed by the selected NDT method. The assessment is then able to provide the following information

  • growth rates of the flaw
  • time to reach the critical size
  • inspection intervals and locations

Another approach at this phase of equipment life is to carry out a “defect scan” using a software package like BEASY (more on BEASY later) where a contour plot of critical crack sizes can be drawn over the surface of a component for a given operational load.

Flaws detected

If flaws have been detected, a fracture mechanics assessment can yield extremely valuable information, which can be used by the asset or integrity engineer to make an informed decision about the operation or repair of the equipment. The following information can be generated for the equipment:

  • growth rates of the flaw
  • time to reach the critical size
  • a new value of operational load to slow crack growth to a predetermined rate
  • a new value of operational load to stop any crack growth at all
  • the effectiveness of repair methods that don’t completely eliminate the presence of the cracks
  • a sensitivity study of the effect of errors in the measurement of the flaw size using NDT, if different NDT methods are giving different results
Assessment techniques

A fracture mechanics assessment is generally carried out by either using stress intensity factor solutions from engineering critical and fitness-for-service codes, or from numerical analysis. Certain situations, such as when flaws are located in flat plates or tubes (pipes), are readily assessed using solutions contained in assessment codes such as BS 7910 or API 579. For geometry or load configurations that fall outside of the validity of the codes, numerical modelling is required to calculate the fracture parameters.

Certain modern software packages have been designed to accurately calculate fracture parameters and model crack propagation in both 2D and 3D. One of the most advance of these is BEASY. BEASY is based on the boundary element method (BEM), as opposed to the finite element method. BEASY requires only the surface of the component to be discretised (meshed), making crack propagation through the volume of the component far simpler from a computational perspective. Thanks to the special formulation used in the dual BEM crack elements, the user is only required to model one of the crack faces, while the other one is automatically generated by the software during the solve. The user is able to insert multiple seed cracks from a library of corner, surface or embedded flaws without modelling or meshing the crack geometry, and the software automatically propagates the cracks until user defined criteria for halting the analysis are met. All the parameters are output for direct calculation of failure assessment parameters and generation of FAD diagrams.

Software like BEASY also includes a Defect Scanner module, which creates a plot of critical crack sizes on the modelled geometry. Areas with small critical crack sizes indicate higher sensitivity to the presence of flaws, and highlight regions requiring higher inspection frequencies. The crack growth threshold properties of a given material can also be used as inputs to the assessment to plot minimum crack sizes on the component geometry before fatigue crack growth is expected to occur.

Summary

Equipment failure, unnecessary maintenance shuts and badly designed repairs can cost operators large sums, costs that can be avoided by the application of predictive assessments. Modern numerical techniques and software enhances fracture mechanics assessments, and makes them more affordable. The information that these assessments provide is essential for critical asset management and maintenance strategy considerations, resulting in cost savings, less equipment downtime and accident prevention.

Dr Matt Rudas

About the author

Matt has extensive experience in the numerical modelling of crack propagation and engineering critical assessments having carried out fracture studies on offshore platform welds, rail head checks, flaws in industrial rotating equipment, gearing, bridge structures and gas turbine engines. Contact Mechsafe for further information.

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