Notch-impact tests provide information on the resistance of a material to sudden fracture where a sharp stress raiser or flaw is present. In addition to providing information not available from any other simple mechanical test, these tests are quick and inexpensive, so they are frequently employed.
Types of Notch-impact Test
In various standard impact tests, notched beams are broken by a swinging pendulum or a falling weight. The most common tests of this type are the Charpy V-notch and the Izod tests. Specimens and loading configurations for these are shown in Fig. 1.
A swinging pendulum arrangement is used for applying the impact load in both cases; a device for Charpy tests is shown in Fig. 2. The energy required to break the sample is determined from an indicator that measures how high the pendulum swings after breaking the sample. Some broken Charpy specimens are shown in Fig. 3. The impact resistance of polymers (plastics) is often evaluated with the use of the Izod test.
Another test that is used fairly often is the dynamic tear test. Specimens for this test have a center notch, as for the Charpy specimen, and they are impacted in three-point bending, but by a falling weight. These specimens are quite large, 180 mm long, 40 mm wide, and 16 mm thick. An even larger size, 430 mm long, 120 mm wide, and 25 mm thick, is also used.
In notch-impact tests, the energies obtained depend on the details of the specimen size and geometry, including the notch-tip radius. The support and loading configuration used are also important, as are the mass and velocity of the pendulum or weight. Hence, results from one type of test cannot be directly compared with those from another. In addition, all such details of the test must be kept constant, as specified in the published standards, such as those of ASTM.
Trends in Impact Behavior, and Discussion
Polymers, metals, and other materials with low notch-impact energy are generally prone to brittle behavior and typically have low ductility and low toughness in a tension test.
However, the correlation with tensile properties is only a general trend, as the results of impact fracture tests are special due to both the high rate of loading and the presence of a notch.
Many materials exhibit marked changes in impact energy with temperature. For example, for plain carbon steels of various carbon contents, Charpy energy is plotted versus temperature in Fig. 4.
However, even for the same carbon content, and for heat treatment to the same hardness (ultimate strength), there are still differences in the impact behavior of steels due to the influence of different percentages of minor alloying elements. This behavior is illustrated by Fig. 5.
In Figs. 4 and 5, there tends to be a region of temperatures over which the impact energy increases rapidly from a lower level that may be relatively constant to an upper level that may also be relatively constant. Such a temperature-transition behavior is common in various materials.
The fracture surfaces for low-energy (brittle) impact failures are generally relatively smooth, and in metals have a crystalline appearance. But those for high-energy (ductile) fractures have regions of shear where the fracture surface is inclined about 45◦ to the tensile stress, and they have, in general, a rougher, more highly deformed appearance, called fibrous fracture. These differences can be seen in Fig. 3.
The temperature-transition behavior is of some engineering significance, as it aids in comparing materials for use at various temperatures. In general, a material should not be severely loaded at temperatures where it has a low impact energy.
However, some caution is needed in attaching too much significance to the exact position of the temperature transition. This is because the transition shifts even for different types of impact tests, as discussed in the book by Barsom (1999).
Notch impact test results can be quantitatively related to engineering situations of interest only in an indirect manner through empirical correlations, with this situation applying both to the energies and to the temperature transition.
By the use of fracture mechanics, materials containing cracks and sharp notches can be analyzed in a more specific way. In particular, the fracture toughness can be quantitatively related to the behavior of an engineering component, and loading-rate effects can be included in the analysis.
However, these advantages are achieved at the sacrifice of simplicity and economy. Notch-impact tests have thus remained popular despite their shortcomings, as they serve a useful purpose in quickly comparing materials and obtaining general information on their behavior.
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