National Physical Laboratory

Fracture Toughness

Although fracture toughness values for relatively brittle materials cannot be used as a design parameters in the same way as for ductile materials, primarily because of the small size of critical defects, the crack initiation and propagation behaviour is critical for understanding their usability.  Ceramics and hardmetals require different approaches because the latter are suitable for methods based on hardness indentations where for ceramics the uncertainties in calibration of such methods are too great.

Advanced Technical Ceramics

Advanced technical ceramics have a number of CEN, ASTM and ISO standards for the determination of fast fracture toughness, to which NPL has contributed to most of the VAMAS TWA3 round-robins set up to validate the methods which include:

  • single-edge pre-cracked beam (SEPB)  
  • chevron notch (CNB)
  • surface crack in flexure (SCF)
  • single-edge vee-notch beam (SEVNB)

Not all methods are suitable for all materials and NPL can advise users on the best choice for a given material or application.


For hardmetals, there are no equivalent standards, primarily because of difficulties in pre-cracking the test-pieces. The short rod method has been in use for some time but its requirement for the use of mercury in the test means that alternative methods have been sought.  The Palmqvist indentation cracking method has been the subject of an international round robin within VAMAS TWA21 run by NPL. Results indicate that there is a good degree of alignment between the short rod method and the Palmqvist total crack length method for most materials provided that proper care and attention is given to surface preparation and sample annealing.

Crack Propagation

The propagation of cracks is another aspect of our current programme. NPL is developing a stiff cracking device which allows improved control for the process of introducing pre-cracks into brittle materials of all types. The stiffness is a key element which will minimise the spring-back of the machine and hence better control on crack growth. With this device we hope to improve the quality of pre-cracking for fracture toughness measurement, as well as having an ability to study directly slow crack growth.

Edge Chipping

Many components made from hard materials suffer from edge chipping. Loading near edges allows the development of cracks which run out at the side surface producing shell-shaped flakes. Research at NPL has shown that the size of the flake and the load required to produce it are related to the distance of loading from the edge, and the material’s fracture toughness. This has implications for the sizing of chamfers, which have the effect of removing the ability to load close to the edge: the higher the toughness, the smaller the necessary chamfer. More recently we have shown that the ranking of materials when subject to impact edge loading, is similar to that produced by applying a steadily increasing load.

For more information: Roger Morrell

Last Updated: 25 Apr 2012
Created: 25 Jul 2007


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