Three-point bending specimen

Fig. 3.18. (a) Shear stress eontours and (b) shear stress distributions aeross the thickness of a three-point bending specimen in a short beam shear test. After Cui and Wisnom (1992). Reproduced by permission of... 

The third example of time effects in elastic fracture concerns accelerated crack growth. Up to the early 80 s it has been assumed that a unique relation exists between the stress intensity factor K and the rate of crack growth da/dt. This contention had been confirmed in many steady crack growth experiments. Using three point bending specimens Chan and Williams [33] obtained for HDPE in water the following relation between da/dt, and the acting stress intensity factor Kc ... 

Figure 9. TEM images of stained sections showing deformation in the vicinity of a crack subcritically grown under impact in a double edge-notched, three-point bend specimen of 3LE31 (a) low-magnification micrograph showing the presence of planar deformation zones adjacent to the crack and (b) the zones in (a) shown at a higher magnification.

In general, it is not possible to measure CTOD, but rather the crack-opening displacement (COD). The latter quantity can be determined at the outer end of the notch with a suitable clip gauge. Thus, for a notched three-point-bend specimen, it was shown that a plastic hinge can form around the tip of the crack [Brostow and Comeliussen, 1986]. If the center of rotation is known, the CTOD can be calculated from the measured COD. A Standard has been published [BS 5762]. 

Manhanty D.K., Savent G.Y. 1992. Experimental investigations on stable crack growth in three-point bend specimens under mode I and mixed mode (I and II) loading[J. Engineering Fracture Mechanics, (43) 41-53. 

The DT test [ ] is similar in concept to the CVN test. The simplicity of a three-point bend specimen has been combined with an energy criterion for fracture resistance. In this test, a 16 mmx38 mmx 181 mm specimen with a sharp notch is fractured on a large pendulum or drop-weight impact machine, and the fracture energy is measured. 

Fig. 2. Three point bend specimen for fatigue crack growth tests. All dimensions are in millimeters (not drawn to scale).

The K calibration for compact specimens is given in the ASTM Method E 399-74 [ ]. The K calibration for three-point bend specimens [ ] is given by... 

S = span dimension for three point bend specimen TL = designation for crack growth in the rolling direction TS = designation for crack growth in the thickness direction... 

The figure also shows a schematic diagram of load vs displacement plots describing the procedure for determination of the critical load in a linear-elastic fracture mechanics (LEFM) test (see Fig. 10.1c). The determination of Ki is dependent on testing the material under conditions in which it exhibits essentially linear-elastic behavior indicative of a plastic zone that is very small relative to flaw size and specimen dimensions, the domain of LEFM. The equations for the compact specimen and three-point bend specimen are as follows ... 

The two most common specimen types used to measure fracture toughness are (a) the three-point bend specimen shown schematically, and (b) the compact specimen shown mounted in grips with a clip gage extensometer attached. Panel (c) shows a schematic diagram of various types of load vs displacement plots describing the determination of an LEFM test (ASTM, 2013c). 

Nanstad R K, Sokolov M A, Merkle J G and McCabe D E (2007), Experimental evaluation of deformation and constraint characteristics in precracked Charpy and other three-point bend specimens, pp. 259-267 in Proceedings ofASME 2007 Pressure Vessel and Piping Conference and Eighth International Conference on CREEP and Fatigue at Elevated Temperatures, American Society of Mechanical Engineers, New York. 

FIG. 22—Change in stress intensity factor with crack extension for a three-point bend specimen under load control compared to displacement control [28]. 

Mode 1 Fracture toughness from notched three point bending specimens... 

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