Linear Elastic Fracture Mechanics

It’s essential that you understand which failure mode, or modes, may be applicable to your application, so you can mitigate the risk of those failure modes when you’re designing your product. It’s also essential that you understand the criticality of an actual material failure, namely a crack or a crack-like defect, when you find one in a product that’s already in service.

Material failure is generally classified into two categories, namely ‘deformation’ failure or ‘fracture’ failure.  Fracture failure can occur suddenly, for example at the end of a tensile test, but this is associated, in ductile materials at least, with gross plastic deformation.  A more common occurrence of fracture failure is where cyclic loads, including repetitive loading and unloading, cause the initiation and then the propagation of a crack, or cracks, over a period of time.  This failure mode is generally known as ‘fatigue’ and depending upon the number cycles involved, it’s then further defined as being either Low Cycle Fatigue (LCF) or High Cycle Fatigue (HCF).

In general, a crack will initiate from either a known defect (identified from your Non-Destructive Testing (NDT) regime) in the base material or a notch in the component and it will grow over time as the cyclic load is applied; this is where fracture mechanics comes into play.

Fracture mechanics can be broken down into two categories; Linear Elastic Fracture Mechanics (LEFM) and Elastic Plastic Fracture Mechanics (EPFM). Of the two, LEFM is the most widely used.

There are two main approaches to LEFM which are geometry dependent:

• If the geometry of your component falls in line with the geometric forms stipulated in the likes of BS 7910, or API 579-1 / ASME FFS-1, then we can go down a traditional hand calculation route with the support of a spreadsheet to provide the iterative loops for each small increment of crack growth. This approach is typically coupled with the development of a ‘failure assessment diagram’ which provides an understanding of how the development of a crack affects the likelihood of a gross plastic deformation type failure mode in the material ahead of the crack front.
• If the geometry of your component falls outside of the geometric forms stipulated in the likes of BS 7910, or API 579-1 / ASME FFS-1, then we need to follow a finite element (FE) analysis based route. This is where the actual crack geometry is built into the FE model and stress intensity factors are calculated through the J-Integral method; typically this is performed in a number of steps with steadily increasing crack sizes.

We’re experts in both of these LEFM approaches, so we’ll follow the best route that gives you the right result in terms of predicting the remaining fatigue life of your component.