Notched beam test specimen

Figure 7.10 Specimens for toughness measurement and ioading conditions. CT, NBT and DT for compact tension, notch beam test and doubie torsion respectiveiy.

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. 

Three-Point Bending Test of Notched-Beam Specimens... 

Pendulum-Type Instruments. Pendulum-type machines are used for notched or imnotched specimens that may be of different sizes and supported as a cantilever (Izod) or as a bar supported at its ends (Charpy). These tests are also referred to as flexed-beam impact tests. 

The impact toughness of a material, as measured by a flexed-beam test on a notched specimen, depends on the thickness of the specimen and the radius of the notch, and therefore cannot be considered a material property. Furthermore, the transition from predominantly plane-stress to predominantly plane-strain is unpredictable when complex geometries are encountered in structural components. Because this transition can create imexpected and catastrophic failures, some polymers normally considered to be very tough and ductile may benefit from impact modification. The modified versions are less sensitive to geometric variations. 

ASTM D5045 has been accepted for the determination of the critical stress intensity factor, KIC. Although KIC is temperature- and rate-dependent and the dependence is specific for any material, it is relatively independent of specimen geometry. Single-edge-notched beams are typically used for the test, but other... 

Category 2 tests are fracture mechanics tests that use notched and precracked specimens that are statically or dynamically loaded, such as cantilever beam, compact tension, and double cantilever beam specimens, and are conducted per ASTM E 399, Test Method for Plane-Strain Fracture Toughness of Metallic Materials. 

The Single Edge Notched Beam (SENB) test can be used for rapid rates of loading. It is simple. A narrow notch cut across the width of a long specimen is easier to make and measure than a sharp crack. The test is performed in 4-point bend, with the inner pair of forces applied to the opposite face to the notched one. It is applicable to non-porous materials at both ambient or high temperature. The results are sensitive to notch size, and they are improved by precracking the specimen at the end of the notch. 

Examples of flexed-beam impact are the Izod and Charpy impact test, in which a small bar of polymer is struck with a heavy pendulum. In the Izod test the bar is held vertically by gripping one end in a vice and the other free end is struck by the pendulum. In the Charpy test the bar is supported near its ends in a horizontal plane and struck either by a single-pronged or two-pronged hammer so as to simulate a rapid three-point or four-point bend test, respectively (Figure 12.33(a)), It is customary to introduce a centre notch into the specimen so as to add to the severity of the test, as discussed in Section 12.5.1 above. The standard Charpy impact specimen has a 90° V-notch with a tip radius of 0.25 mm. For polymers a very much sharper notch is often adopted by tapping a razor blade into a machined crack tip, which has important consequences for interpretation of the subsequent impact test. 

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