Fracture toughness techniques

T. D. Burchell, I. Oku and M. Eto, A Comparison of Fracture Toughness Measurement Techniques as Applied to Nuclear Graphites, Extended Abstracts and Program, International Carbon Conference, 1990, pp 278 279... 

T.D. Burchell and. T. Oku and M. Eto. A comparison of fracture toughness measurement techniques as applied to nuclear graphite. In Proc. Carbone 90, Paris, France. Pub. Groupe Francais D etude des Carbones, 1990. pp. 278 279. 

Since the early 1980s, the study of mechanical properties of materials on the nanometre scale has received much attention, as these properties are size dependent. The nanoindentation and nanoscratch are the important techniques for probing mechanical properties of materials in small volumes. Indentation load-displacement data contain a wealth of information. From the load-displacement data, many mechanical properties such as hardness and elastic modulus can be determined. The nanoindenter has also been used to measure the fracture toughness and fatigue properties of ul-... 

The nano indentation technique can also be used to measure fracture toughness at small scales, by using the radial crack, which occurs when brittle materials are indented by a sharp indenter [72,73]. Lawn et al. [74] have shown that a simple relationship exists between the fracture toughness, Ag, and the lengths of the radial cracks, c, in the form of ... 

Cutwater, J. O., Murphy, M. C., Kumble, R. G. Berry, J. T. (1974). Double torsion techniques as a universal fracture toughness test method. Fracture Toughness and Slow-Stable Cracking, ASTM Special Technical Publication 559, pp. 121-31, American Society for Testing and Materials. 

Further details of the above techniques to improve the transverse fracture toughness of composites by controlled interfaces of various nature and modifying materials are discussed in the following sections. The effectiveness of these toughening methods on transverse fracture toughness and strength in relation to the controls are summarized in Table 7.1. 

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

In addition, it has been shown (Boccaccini etal., 2001 Chlup etal., 2001) that the chevron-notched specimen flexural technique (the CN-technique) can be a reliable method of assessing fracture properties (fracture toughness, work of fracture) in thermally shocked brittle matrix composites reinforced by brittle fibres. 

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