Fracture Mechanics

Fracture mechanics is the science of predicting the behaviour of material in the presence of flaws or cracks. The use of fracture mechanics techniques in the assessment of performance and reliability of structures is on the increase and the prediction of crack propagation of an existing or postulated flaw in a structure plays an important part. Such flaws may be the result, for example, of manufacturing processes or they may initiate over time as the result of in-service loading. Once present, cracks may propagate as a result of time-dependent load effects or cyclic loading.

Zentech has developed a 3D crack analysis tool called Zencrack which reads in an uncracked finite element model and produces a cracked finite element model. Stress intensity factors or the energy release rate are calculated automatically from the results of the cracked finite element analysis. Furthermore, crack growth can be undertaken by extending the crack position. An updated finite element model is then created and run to simulate crack growth.

Structures designed "correctly" from static load considerations may not reach design life because of the effect of crack propagation. This is a major issue in a variety of industries in which damage tolerance is a key part of the design process. Aerospace structures, gas turbine engines, pressure vessels and pipelines are obvious examples where failure could lead to catastrophic consequences and loss of life.

Increasing demands of reliability and performance mean that the often over-conservative empirical methods in codes of practice can produce less than optimised designs. On-going safety assessment for in-service components requires the prediction of future crack growth rates.

In addition to these well recognised areas for crack growth simulation, the on-going software and hardware developments make possible new areas of investigation. These include:

• non-linear fracture mechanics and crack tip plasticity
• consideration of the effect of residual stress
• design of experiments
• combining numerical analysis with experimental testing and material development
• studying the effect of surface treatment
• studying interfacial cracks
• assessment of durability of additive manufactured components
• assessment of adhesively bonded joints and composite patch repairs.

Zencrack was first released in 1990 following a consultancy project with the (then named) Royal Aerospace Establishment at Farnborough, UK. It can be applied in a variety of commercially important applications. These include design of laboratory experiments, testing and developing advanced materials, assessing the effects of surface treatments and the study of component repairs.