Indentation fracture test

Schwartz MM (1992) Handbook of structural ceramics, McGraw-HiU Publishers, USA Guillou MO, HenshaU JL, Hooper RM et al. (1992) Indentation fracture testing and analysis, and its application to zirconia, silicon carbide and silicon nitride ceramics. J of Hard Matls 3 421 34... 

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. 

However E must be obtained at the same temperature and time conditions as the fracture test because of the viscoelastic nature of polymers. In order to avoid the associated uncertainties, it is considered preferable to determine Gy or G. (subject to the validity criteria of AV) directly from the load versus deflection curve up to the same load point as used for Ay or A l. determination (see Fig. 3). The energy should be corrected for speeimen indentation (suffered under loading and support pins) by deducing the energy of indentation. ( j from t y. Therefore ... 

Lac Each, R., Gyurova, L. A.,Grellmann, W. Application of indentation fracture mechanics approach for determination of fracture toughness of brittle polymer systems. Polym. Test. 26 (2007) 51-59. 08Pin Pinter, G. Slow crack growth in PE-HD under static and cyclic loads. Habilitation thesis, Montanuniversitat Leoben, 2008. 

Lawn, B.R. and Marshall, D.B., Indentation fracture and strength degradation in ceramics. Fracture Mechanics of Ceramics, Vol. 3 Flaws and Testing, Bradt, R.C., Hasselman, D.P.H. and Lange, F.F., Eds., Plenum Press, 1978. 

In order to overcome many of the difficulties associated with the indentation fracture toughness measurement as noted above, the single-edged notched beam (SENB) test - which often is used for metallic and polymeric materials - has been adapted to suit ceramics [24]. For this, a bar of ceramic, usually in the order of 3 x 4 mm cross-section and 25-45 mm length, has a notch introduced on one side. The bar is then loaded in four-point bending and the fiacture toughness determined from the failure load, Pm> as per ... 

Determining fracture toughness by indentation hardness testing... 

The indentation fracture (IF) test is simple and requires only small volumes of material. Cracks develop around a Vickers indentation in a flat ceramic surface in proportion to its fracture toughness. Kj(- can be estimated from crack lengths. The ratio of force to crack length is independent of load and material hardness, and so is a useful parameter. The test is suitable for dense materials (97% theoretical) at ambient temperature. 

Interfacial fracture toughness can be estimated using a number of qualitative methods such as the adhesion tape test, or scratch test. In addition to DCB tests, more quantitative measurements can be made using indentation delamination tests, blister tests, or various interfacial fracture specimens (Section 4). It is important to note that, in the case of organic (viscoelastic) coatings, the tests should be performed at the proper ambient conditions and strain rate. 

Since polymers are viscoelastic materials it is necessaiy to specify both the temperature and the time scale under which the result was obtained. This protocol is meant for low speed ( quasi-static ) displacement contrd mode of loading and as a bade test condition, it is recommended that 23°C and a load-point displacement rate of 10 mm/min be used, but other conditions are possible. Li all cases, the time-D-ffacture should be quoted and all related determinations (yield stress and indentation correction, see below) should be obtained at comparable loading times as for the fracture test... 

As with the low-rate method [2] both load and displacement are to be measured in the fracture test in order to derive energy from integrating the load versus load-point displacement diagram and the method must include careful measurements and corrections for machine compliance and specimen indentation (unless an external displacement measuring device is used, e.g. optical, but at high speeds this would be impractical). 

Hardness. Glass hardness tests usually measure the resistance to abrasion by grinding or grit-blasting, resistance to scratching, or penetration by an indenter. The method to be used depends on expected service conditions. Knoop hardness (Table 4) is commonly used, because other methods usually fracture the glass. 

Figure 10.6. (a) Indentation nanohardness of silver/chromium multilayers and single films of the constituent metals, as a function of depth affected by plastic deformation, (b) Charpy impact energies, a measure of fracture toughness, of three materials, as a function of test temperature they are mild steel, ultrahigh-carbon steel and a composite of the two kinds of steel (courtesy Dr. J. Wadsworth) (Fig. 10.6(b) is from Kum et at. (1983)). 

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