Flexed-beam impact

The interpretation of impact tests is not straightforward and it is necessary to consider several alternatives, as follows  

It was proposed independently by Brown [104], and by Marshall, Williams and Turner [105], that Charpy impact tests on sharply notched specimens can 

It is conventional to follow Williams and co-workers [106] and express Equation (12.33) as 

Vincent [110] and others have recognized that the impact strength depends on the geometry of the notch, which led Fraser and Ward [111] to propose that for comparatively blunt notches (i.e. those not introduced by a razor blade or a sharp cutting tool) failure occurs when the stress at the root of the notch reaches a critical value. This stress, which in a glassy polymer marks the stress required to initiate a craze, can be calculated by assuming that the deformation is elastic. On this hypothesis, the Charpy test, as undertaken in 

For pure bending, the nominal stress at the root of the notch (7n is given by (7n = M/I)y, where I is the second moment of area (= Bt / 2 for a rectangular beam) andy is the distance to the neutral axis. 

It was proposed independently by Brown [115], and by Marshall, Williams and Turner [116], that Charpy impact tests on sharply notched specimens can be analysed quantitatively in terms of linear elastic fracture mechanics. It is assumed that the polymer deforms in a linear elastic fashion up to the point of failure, which occurs when the change in stored elastic energy due to crack growth satisfies the Irwin-Kies relationship (Equation (13.10) above). So that 

Using the linear stress assumption, the maximum stress at the root of the notch is the product of the nominal stress and the stress concentfation factor Calculations of ak for general shapes of notch are available in the literature, but when the crack length c is much greater than the notch tip radius p, reduces to the simple expression 

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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. 

Fracture Toughness from Flexed-Beam Impact Tests. As stated above, the so-called impact toughness values obtained from Izod and Charpy tests are not material properties because they depend on specimen thickness, notch depth, notch radius, and other factors imrelated to material properties. These... 

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. 

Fracture Toughness from Flexed-Beam Impact Tests. As stated above, the so-called impact toughness values obtained from Izod and Charpy tests are not material properties because they depend on specimen thickness, notch depth, notch radius, and other factors unrelated to material properties. These values are therefore useful only for the comparison of different materials, and are useless for design calculations. By contrast, fracture toughness (ASTM D5045-99), which is a material property, is useful in design calculations. Consequently it is desirable to have fracture toughness values that are taken under impact conditions, and especially if they could be obtained from the Izod or Charpy test machines. 

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