charpie impact test the elongation at break-in reduction in area obtained by the tensile test can give an impression of the toughness of a material but this only applies to a quasi-static load and only at room temperature in many cases however components are also subjected to a shock load and not always at room temperature this applies for example to shock absorbers in cars and their bearings these components must withstand shock loads both in summer at high temperatures and in winter at extremely cool temperatures the ideal conditions of the tensile test cannot meet reality components with good toughness behavior in the tensile test become brittle at low temperatures and lead to premature material failure for this reason the so-called sharpie impact test or sharpie v-notch test is used to test the toughness of a material under an impact-like load as a function of temperature in the charpi impact test a notched specimen is abruptly subjected to bending stress the specimen is usually 55 millimeters long and has a square cross section with an edge length of 10 millimeters the notch in the middle has a v-shaped geometry in special cases the notch can also be u-shaped the notch provides a defined predetermined breaking point which generates a triaxial stress state in the notch base the notched specimen is placed in the support of a pendulum impact testing machine a pendulum hammer is then released from a certain height at the lowest point of the circular trajectory the striker of the hammer hits the opposite notch-facing side of the specimen the impact speed is usually between 5 and 5.5 meters per second the sample is fractured by the striker and absorbs part of the kinetic energy of the hammer with the remaining residual energy the hammer swings to a certain height due to the absorbed kinetic energy by the sample however it does not reach its initial height again the deformation energy and thus the final height achieved depends on the toughness of the specimen the tougher the material the more it has to be deformed until it breaks the required deformation energies are correspondingly high and the pendulum energy is strongly absorbed the hammer then only reaches a low final height after fracturing the specimen in the present case the deformation energy absorbed by the specimen is 130 joules very brittle specimens on the other hand break almost without deformation and therefore require only a low deformation energy the pendulum hammer swings almost at the initial level in this case the absorbed deformation energy is only 23 joules note that a comparison between a tough and a brittle material based on deformation energy is only possible if identical specimen geometries are used the deformation energy required for fracturing the specimen is called notch impact energy k more precisely kv for specimens with v-shaped notch or k-u for specimens with u-shaped notch the notch impact energy can be determined from the difference between the potential energy of the pendulum hammer immediately upon release and at the final height reached afterwards at a given initial height and mass of the pendulum hammer the notch impact energy depends only on the final height the notch impact energy can be read off directly from a dial gauge by a drag indicator which is carried along from the lowest point as soon as the pendulum hammer hits the specimen the notch impact energy determined in this way strongly depends on the cross-sectional area of the specimen large cross-sections always require higher deformation energies than smaller ones even if under certain circumstances a more brittle behavior is present comparisons in toughness by the notch impact energies are therefore only possible if they were obtained from identical specimen geometries if at all a comparison with different geometries is only possible if the notch impact energy is related to the cross-section of the specimen this quotient of notch impact energy and cross-sectional area is often referred to as notch toughness alpha although in most cases this term is used identically to that of notch impact energy note that even notch toughness is not a pure material parameter as it is not dependent on the material alone the notch impact energy and thus the notch toughness is also influenced by the shape of the specimen cross-section and in particular by the shape of the notch and the speed at which the hammer hits the specimen thus notch impact energy and notch toughness are purely technological parameters that are not taken into account in any design calculations the standard compliant indication of the toughness of a material must include not only the notch impact energy but also the notch shape and the energy capacity of the testing machine the energy capacity can be omitted if the energy capacity corresponds to the standard value of 300 joules for example the indication kv-150 equals 40 joules means that the notch impact energy was 40 joules when using a pendulum impact tester with a maximum of 150 joules and a v-shaped notched specimen if the notch impact energy had been obtained on a specimen with a u-shaped notch and a standard pendulum impact tester of 300 joules the indication would have been ku equals 40 joules however the mentioned influences on notch impact energy such as fracture speed temperature and not shape are only of minor significance with regard to the actual objective of the charpi impact test this is because the v-notch test serves less to compare different materials with each other than to qualitatively compare the toughness of a single material at different temperatures this can be illustrated in a diagram by plotting the notch impact energy for several identical specimens against the temperature in this way for example the temperatures at which a material tends to become brittle can be determined in order to define the limits of use of the material especially materials with a body centered cubic lattice structure such as phoridic steels and materials with hexagonal lattice structures show a particularly strong dependence of toughness on temperature while these materials have high toughness at high temperatures they become brittle at low temperatures many plastics show such a behavior as well which also begin to become brittle at low temperatures while they are relatively tough at high temperatures the temperature range at which the material has low notch impact energy values and thus behaves brittle is referred to as lower shelf accordingly the upper shelf indicates the temperature range at which the material behaves relatively tough between the lower and the upper shelf there is a transition range which is characterized by strongly scattering values the reason for the large scattering in the transition area lies in small microstructural differences between the individual samples which cause the material to become brittle at slightly higher or lower temperatures therefore the toughness scatters very strongly despite identical temperatures due to the steeply sloping curve from upper shelf to lower shelf this transition range is also referred to as steep front due to the continuous curve from the upper to the lower shelf no specific temperature can be assigned to this transition nevertheless different approaches are used to define such a transition temperature in order to identify the temperature below which embrittlement of the material is to be expected the transition temperature is frequently defined by the notch impact energy itself the transition temperature is often defined as the temperature at which the specimen has an average notch impact energy of 27 joules however values of 40 or 60 joules can also be used to define the transition temperature it is also possible to define the transition temperature as the temperature at which the notch impact energy corresponds to 50 percent of the upper shelf in comparison to materials with body centered cubic lattice structures the temperature has hardly any influence on the toughness for materials with face centered cubic lattice structures with such materials there is no distinct lower or upper shelf and therefore no steep front some materials behave relatively tough over the entire temperature range such as aluminium or show relatively brittle behavior such as hardened steels or lamellar graphite castings toughness is not only influenced by temperature but also by the structural state of the material quenched in tempered steels and fine-grained structural steels for example are characterized by their outstanding toughness this toughness is also present at lower temperatures compared to normalized steels therefore for quenched and tempered steels the steep front shifts to lower temperature values in this way the sharpie impact test can also be used to evaluate the success of heat treatments or to check microstructural states the reverse effect on the position of the steep front in steels is caused by aging aging leads to embrittlement and consequently shifts the transition temperature to higher values thus the influence of aging effects can also be examined in the charpi impact test hardened steels also show a shift in transition temperature to higher values due to their low toughness in summary the charpi impact test may have the following objectives determination of the transition temperature which means the onset of possible embrittlement verification of heat treatments examination of aging effects the fracture behavior of the specimens used can not only be assessed on the basis of the notch impact energy even the type of the fracture provides information about the toughness or embrittlement of the specimen a very tough behavior can be seen by a strongly deformed fracture surface often the ductal sample is not even divided into two parts but only pull through the two supports in a strongly deformed state such a fracture in the area of the upper shelf is therefore also called deformation fracture or sliding fracture the fracture surface of steels often appears in a matte gray color under the microscope the fracture surface shows a dimple-like structure for this reason a deformation fracture is also called a dimple fracture in the area of the lower shelf however there is hardly any deformation the sample is usually separated in two halves when the hammer strikes such brittle fracture is also referred to as cleavage fracture the fracture surface appears shiny whitish in the transition temperature range the fracture surface often shows characteristics of both types of fracture this means a strongly deformed area followed by an area with less deformation this type of fracture is then also referred to as mixed fracture the deformation speed also has a major influence on the fracture behavior if the pendulum hammer hits the specimen at higher speeds brittle fracture is favored and the notch impact energies decrease conversely lower deformation speeds are more likely to lead to a deformation fracture with higher notch impact energy values due to high impact speeds the stress in the material increases so rapidly that the bond strength of the atomic planes is exceeded before the dislocations could have moved through the material to a significant extent note that dislocations do not move infinitely fast but can only move at the speed of sound a plastic deformation which is ultimately based on dislocation movements therefore does not take place at very high deformation speeds the material breaks almost without deformation by tearing apart the atomic planes this leads to the so-called cleavage fracture this happens preferably in those atomic layers which are relatively loosely packed at slow deformation speeds however the dislocations can move over long distances and deform the material when the critical shear stress is reached the material is then plastically deformed before it fractures this leads to the already mentioned deformation fracture one should therefore be aware of the influence of the deformation rates when it comes to assessing the toughness of materials this is because deformation speeds in practice usually differ from those in the charpi impact test therefore the results cannot be easily transferred to real situations we hope you enjoyed the video and found it helpful thanks for watching