Indentation fracture

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)). 

Fig. 4. Model of local plastic deformation of lamellae beneath the stress field of the indenter. The mosaic block structure introduces a weakness element allowing faster slip at block boundaries leading to fracture (right)...

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 ... 

Flowever, in order to probe the fracture toughness of thin films or small volumes using ultra low load indentation, it is necessary to use special indenters with cracking thresholds lower than those observed with the Vickers or Berkovich indenters (for Vickers and Berkovich indenters, cracking... 

HIPS is an immiscible blend that is used in many applications and was employed as material for many of the automotive bumpers. The PS portion is strong and inflexible, whereas the polybutadiene particles are flexible allowing an impact to be distributed over a larger area. The polybutadiene rubbery portion allows the bumper to bend and indent and protects the PS from fracturing while the PS phase resists further deformation. This combination gives a strong flexible material. 

Compression stress (MPa) Dynamic indentation hardness (MPa) Tensile strength (MPa) Bonding index Brittle fracture index ... 

Mohs hardness is a measure of the relative hardness and resistance to scratching between minerals. Other hardness scales rely on the ability to create an indentation into the tested mineral (such as the Rockwell, Vickers, and Brinell hardness - these are used mainly to determine hardness in metals and metal alloys). The scratch hardness is related to the breaking of the chemical bonds in the material, creation of micro fractures on the surface, or displacing atoms in the metals of the mineral. Generally, minerals with covalent bonds are the hardest while minerals with ionic, metallic, or van der Waals bonding are much softer. 

Lawn B. R., Marshall D. B., 1978, Indentation fracture and strength degradation in... 

Niihara K., 1983, A fracture mechanics analysis of indentation-induced Palmquist crack in ceramics, J. Mater. Sci., Lett. 2, 221. 

Peters C. T., 1979, The relationship between Palmqvist indentation toughness and bulk fracture toughness for some WC-Co cemented carbides, J. Mater. Sci., 14, 1619-1623. 

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. 

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