Impact Testing of Metals
Abstract
Impact testing of metal experiment was conducted by performing Charpy V-notch impact tests and the ductile-brittle transition temperature of the three metallic samples was established. Three samples of metals were studied ( 1018 hot-rolled steel, brass and aluminum) to assess the Charpy V-notch impact energy as a function of temperature and the ductile-brittle transition temperature. The DBTT for the sample tested were recorded as follows: -109F, 32F, 71F, 140F and 205F respectively. From the results obtained, it is evident that the fracture energy varies with the original temperature of the specimen under investigation. For Steel, the highest fracture energy was recorded at ice water temperature, followed by room temperature. All the samples recorded the lowest fracture energy at dry ice temperature.
Introduction
The impact testing of metal experiment was conducted by performing Charpy V-notch impact tests and the ductile-brittle transition temperature of the three metallic samples was established. Impact testing of metals is significant to determine the amount of force a metal object can withstand before it is used in a specific application. The aim of the experiment was to conduct Charpy V-notch impact tests and to establish the ductile-brittle transition temperature for Steel, Aluminum and Brass metallic samples.
The notched-bar impact test of metals is used to provide information about failure mode under high velocity environments; usually ductile-brittle mode. There is a relationship between the energy absorbed before metal failure mode and the area under the stress-strain curve (toughness). It is imperative to note that brittle metals demonstrate a small area under the stress-strain curve. As a consequence, these materials absorb little energy during the impact failure. In other words, materials with limited toughness absorb little energy during impact failure. An exponential increase is noted in the area under the curve as well as the energy absorbed and toughness as plastic deformation capability increases (Barsom & Rolfe, 1970). The ductility of a material directly determines the amount of energy it absorbs as well as the area under the curve.
It is worth noting that the impact test presupposes that the material’s resistance to shock loading relies on the material’s ability to equalize concentrated stresses rapidly and safely. As a result, the data from impact test can only be assessed and recorded only as an expression of energy absorbed in causing fracture. This form of assessment and recording of impact test data is due to the fact that the distribution and intensity of the stress is too sophisticated to be analyzed satisfactorily.
Charpy V-notch test has gained popularity over the years as one of the best forms of impact testing besides the Izod beam. Charpy V-notch impact testing entails a falling pendulum that strikes with a single blow. The amount of energy at the position on the specimen where the pendulum strike (at the buck point of travel) is quite reproducible given that the pendulum have a definite weight, specific fixed height of release and its swing towards a vertical plane (Barsom & Rolfe, 1970). The remaining energy after the pendulum strike is used to relay it to a specific distance on the measuring scale. As a result, the energy absorbed in the process of breaking the specimen can be established by measuring the difference in angle of rise when the pendulum swings free and after the pendulum has broken the specimen. It is crucial to note that notched-bar impact test outcomes are affected by specimen heat treatment, temperature and degree of strain. Ductile-brittle temperature is the temperature at which a sharp decrease occurs.
Apparatus/ Equipment Used
The experiment utilized Charpy testing machine and a digital temperature measuring device. It also employed the use of three standard AISI Charpy V-notch impact test specimens of aluminum, steel and brass.
Experimental Procedure
Three type of metals: 018 hot-rolled (HR) steel, aluminum, and brass were studied for impact test. The Charpy V-notch impact energy as a function of temperature was measured and the ductile-brittle transition temperature (DBTT) was determined.
One set of specimens was immersed into ice water until the specimens attained the temperature of ice water (32°F). The temperature of the specimens was assessed using the digital temperature measuring device. Another set of specimens were immersed into dry ice until they attained the temperature of dry ice (-109°F). On other hand, a different set of specimens was kept in a furnace which was maintained at a temperature of 140°F and a different set of specimens kept at another furnace maintained at 205 °F. The remaining set of specimens was kept at room temperature (70 °F).
The temperatures of the specimens were measured and recorded immediately prior to the testing of each test specimen to ensure that they possess the same temperature as earlier recorded. The safety latch was then disengaged and the hammer released and the impact energy was recorded. The break was used to slow down the pendulum once the harmer started to sing back from its highest position in the forward direction. The fracture energy for each set of specimen was noted and the test was recorded as a function of specimen temperature.
Results and Discussion
Impact Energy Versus Temperature for tested samples
The DBTT for the sample tested were recorded as follows: -109F, 32F, 71F, 140F and 205F respectively. From the results obtained, it is evident that the fracture energy varies with the original temperature of the specimen under investigation. For Steel, the highest fracture energy was recorded at ice water temperature, followed by room temperature. All the samples recorded the lowest fracture energy at dry ice temperature. It is rational to conclude that dry ice temperature made the materials more brittle thus easily breakable (Barsom & Rolfe, 1970). On the other hand, the materials kept at high temperature were more ductile thus more energy was absorbed. Also, it is apparent that the type of material determines the amount of fracture energy given that the different material demonstrates different fracture energy. From the results, steel recorded the highest fracture energy as compared to the other samples in all the temperatures. It was followed by brass then aluminum as indicated by the results.
Conclusion
Temperature play a significant role in determining the amount of energy a material is capable of absorbing during impact. High temperature increases the ductility of the materials, thus high fracture temperature and energy absorbed. On the other hand, low temperatures increases the brittleness of the materials, thus low energy is absorbed during impact.
References
Barsom, J. M., & Rolfe, S. T. (1970). Correlations between K IC and Charpy V-notch test results
in the transition-temperature range. In Impact testing of metals. ASTM International.
