Fracture mechanics

Increasing the reliability of parts with fracture mechanic methods

Individual components of a structural part are often bonded with an adhesive agent. Although this type of bonding is quite easy to do, it is nevertheless a complex structural aspect of stability assessment:

  • The stiffness of the part in question can change abruptly along the bondline edge. This can lead to significant increases in the effective tension and may cause the bonding to fail.
  • Thick bondlines can lead to inhomogeneities, e.g. cavities, in the adhesive layer. The edges of these cavities will, in turn, be subjected to a local increase of tension, possibly resulting in a cohesive breakdown within the bondline.
  • Should a cavity occur at the contact point with the part, the bonding may fail there.

The failure mechanisms described are triggered by local effects. As such, fracture mechanics are particularly suitable for describing the stability of bondings.

Leichtwerk AG applies the following approach to bondings analysis:

  1. All bondings are generally considered to be predamaged; i.e. some type of flaw is assumed to be present.
  2. Application of an in-house procedure for experimental determination of fracture mechanic failure properties.
  3. Application of an in-house procedure to calculate the fracture mechanic conditions to which complex parts are subjected.

Computational analysis

Often, the local effects of fracture mechanic strains can only be analyzed for very small or simple structures. Leichtwerk AG has a number of in-house procedures for performing fracture mechanic analyses on large complete structures like, for example, the rotor blades of offshore wind turbines with a blade length of over 60m. These procedures have lead to significant improvements in the structural reliability, and subsequently the economic efficiency, of the turbines.

Method I:

The analysis of delaminations between the layers of fiber reinforced structures requires a method which can be used in the post-processing stage of a standard FE analysis. The analysis involves the sectional forces that branch out during delamination.

Method II:

A special bondline element that considers both branched structures as well as thick bondlines was developed for the ANSYS FE system. Furthermore, it allows a simple variation of the geometric crack front for crack propagation analysis.

Experimental determination of fracture mechanic failure properties

Fracture mechanic properties are determined by using optimized standard specimen:

  • Peeling strength of Double Cantilever Beam (DCB) specimen
  • Shear strength via End-Notched Flexure Specimen (ENF)

The specimen geometry (of the part in question) can be adapted with regard to the substrate material, the manufacturing process, type and thickness of bonding, as well as the dimensions.

A special testing machine with a customized control program was developed to carry out the specimen trials.