Measurement of Fracture Toughness

There are two methods generally adopted for the measurement of fracture toughness (K, J. Both methods depend on using specimens of specific geometry and the contain known initial crack length. 

The criterion for fracture given in Equation 15.54 can be written in a general form as  

/ is a dimensionless constant that depends on the sample shape, crack geometry, and crack size relative to sample dimensions. The crack should be atomically sharp. The problem lies in producing such a minutely sharp crack in a macroscopic specimen. The value of / for different specimen and crack geometries can be obtained from fracture mechanics handbooks. 

The second common test uses a chevron notch specimen. In this specimen, the initial crack is chevron-shaped. As the crack grows by the increasing load, the crack front constantly widens. This causes the crack growth to be stable prior to fracture. Because the crack grows with the increasing load, the growth is the result of the atomically sharp crack formation. Thus, there is no need to precrack the specimen. The fracture toughness [5] is obtained from  

Schematic of single-edge notched beam specimen. 

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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)). 

Pharr, G.M., Harding, D.S. and Oliver, W.C., Measurement of fracture toughness in thin films and small volumes using nanoindentation methods. MRS, 1995, pp. 663-675. 

Harding, D. S., Oiive, W. C., and Pharr, G. M., Cracking During Nanoin Denation and Its Use in the Measurement of Fracture Toughness," Materials Research Society Symposium Proceedings,Wo. 356,1995,pp. 663-668. 

ASTM El820-99 Standard Test Method for Measurement of Fracture Toughness , 1999 Annual Book of ASTM Standard Volume 3.01 Metals-Mechanical Testing Elevated and Low-Temperature Tests Metallography, American Society for Testing and Materials, 1999. 

A.G. Atkins and J.F.V. Vincent, An Instrumented Microtome for Improved Histological Sections and the Measurement of Fracture Toughness, J. Mater. Sci. Lett., 3 310-312 (1984). 

ASTM E813-89, Standard Test Method forfc, a Measure of Fracture Toughness, 1989 Annual Book of ASTM standards. Part 10 (American Society for Testing and Materials, Philadelphia, Pennsylvania, p.700. 

In Chapters 2 and 3, the restrictions in the use of linear elastic fracture mechanics (LEFM) were discussed in terms of the dimensions of the crack and the body (specimen, component, or structure) relative to the size of the crack-tip plastic zone. Simple estimates of the plastic zone sizes were given in Section 3.6. A more detailed examination of the role of constraint (plane strain versus plane stress) and the variations in plastic zone size from the surface to the interior of a body would help in understanding fracture behavior and the design of practical specimens for measurements of fracture toughness. Note that the plastic zone size in actual materials... 

The multi-specimen J-integral technique [ASTM E813] also provides a method for determination of Jj, a measure of fracture toughness. A critical evaluation of ASTM E813-81 and E813-87 has been published [Narisawa and Takemori, 1989 Huang, 1989, 1990, 1991]. 

There are some conditions about the size of the specimen and the crack length, a. Atkinson (1979b) experimentally indicated that W should be greater than 12. Pletka et al. (1979) suggested that L should be greater than 2W from measurements of fracture toughness. Hence, the following condition is necessary for the size of the DT specimen l2d[Pg.530]

Figure 13.8). Another test that is emerging as a substitute for the impact test is the measurement of fracture toughness, which in essence measures the resistance to failure of a material with a predetermined crack. 

Wells [11] first proposed the crack opening displacement as a measure of fracture toughness for tough metals such as the structural steels where LEFM is not valid. The concept became a standard in 1979 in the form of British Standard (BS) 5762 [12], which is now-superseded by BS 7448 Part 1. 1991. Method of determination of Afi. critical CTOD and critical J values of metallic materials [13], More recently. ASTM E1290-9,3, Crack-tip opening displacement (CTOD) fracture toughness measurement [14] was issued. It is stated that this method may be used to characterize the toughness of materials that arc (a) too ductile or fail the size conditions to be tested for or (b) likely to produce unstable crack extension that would invalidate Ji tests in accordance with the requirements of ASTM E81.3 [15]. 

Figure 19.4 Schematic diagrams showing common fracture test geometries. Single edge-notched (a) three-point-bend, (b) tensile and (c) compact tensiem tests, each using specimens with razor-sharpened notches, are employed for fracture mechanics measurements of fracture toughness and fracture energy. The Izod (d) arxl Chatpy (e) tests are widely practised in industry to evaluate toughness, but use bluntly notched (or un tched) specimens and do not give fracture mechanics data...

Kriese, M.D., Gerberich, W.W. and Moody, N.R., Quantitative adhesion measures of multilayer films Part 1. Indentation mechanics. J. Mater. Res., 14(7), 3007-3018 (1999). Pharr, G.M., Harding, D.S. and Oliver, W.C., Measurement of fracture toughness in thin films and small volumes using nanoindentation methods. MRS, 1995, pp. 663-675. Pethica, J.B. and Oliver, W.C., Tip surface interactions in STM and AFM. Phys. Scripta, T19A, 61-66 (1987). 

VI.20] AMERICAN SOCIETY FOR TESTING AND MATERIALS, Standard Test Method for Jj, A Measure of Fracture Toughness, ASTM E813, Annual Book of ASTM Standards, Vol 03.01, ASTM, Philadelphia, PA (1991). 

ASTM E813-89, Standard Test Method for Ja Measure of Fracture Toughness (Philadelphia, 1990)... 

ASTM E 1820, Standard Test Method for Measurement of Fracture Toughness , American Society for Testing and Materials. 

A large volume of experimental CNIS data was analyzed for a wide range of polypropylene-based materials to determine their usefulness as a comparative measure of fracture toughness and as a parameter for materials selection and design. LEFM was used as a tool for the comparison of the CNIS with the fracture... 

As it is well-known, the ultimate objective in the development of ceramic and glass matrix composites is to increase the fracture toughness of the brittle matrix. In order to analyse the toughening achieved, most researchers have used conventional measurement of fracture toughness by indentation. The fracture toughness (Kic) from the micro-hardness indentation method can be calculated by means of the following equation [73] ... 

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