EXPERIMENT 6 IMPACT TEST

Transcription

1 Salman Bin Abdul-Aziz University College of Engineering Mechanical Engineering Department جامعة سلمان بن عبد العزيز آلية الهندسة قسم الهندسة الميكانيكية 2591 همك معمل هندسة وميكانيكا المواد ME 2591 Materials Engineering & Mechanics of Materials Lab EXPERIMENT 6 IMPACT TEST Course Coordinator Dr. Ashraf A. Ali Associate Professor Mechanical Engineering Department د./ أشرف عبد الفتاح على حسانين أستاذ مشارك- قسم الهندسة الميكانيكية H / G

2 Impact Test 1. Background 1.1 General Definition A test designed to give information on how a specimen of a known material will respond to a suddenly applied stress, e.g. shock. The test ascertains whether the material is tough or brittle. A notched test piece is normally employed and the two methods in general use are either the Izod or Charpy test. The result is usually reported as the energy in ft.lbs. or KJ. required to fracture the test piece. Impact is a very important phenomenon in governing the life of a structure. In the case of aircraft, impact can take place by the bird hitting the plane while it is cruising, during take off and landing there is impact by the debris present on the runway. An arm held at a specific height (constant potential energy) is released. The arm hits the sample and breaks it. From the energy absorbed by the sample, its impact strength is determined. The Izod impact test differs from the Charpy impact test in that the sample is held in a cantilevered beam configuration as opposed to a three point bending configuration as shown in figure 1. This test can also be used to determine the notch sensitivity. 1.2 Charpy Impact Test The Charpy impact test, also known as the Charpy v-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's toughness and acts as a tool to study 1

3 temperature-dependent brittle-ductile transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. But a major disadvantage is that all results are only comparative. The test was developed in 1905 by the French scientist Georges Charpy. It was pivotal in understanding the fracture problems of ships during the second World War. Today it is used in many industries for testing building and construction materials used in the construction of pressure vessels, bridges and to see how storms will affect materials used in building. 1.3 Izod Impact Test Izod impact test is an ASTM standard method of determining impact strength. A notched sample is generally used to determine impact strength. The test is named after the English engineer Edwin Gilbert Izod ( ), who described it in his 1903 address to the British Association, subsequently published in Engineering Journal. 2

4 Charpy Impact Test 1. Objectives 1.1- Determining the influence of the notch shape on notched bar impact work 1.2- Determining the influence of the specimen material on notched bar impact work 1.3- Determining the Ductile Brittle Transition Temperature (DBTT) 1.4- Visual evaluation of the fracture surface 2. EQUIPMENT 2.1 WP 400 Pendulum Impact Tester, 25 N (Figure 2) 1 Anvil (footing) 7 Bottom hand lever to release the hammer 2 Support 8 Brake 3 Protective ring 9 Notched bar specimen 4 Maximum indicator 10 Safety hook for holding the hammer 5 Scale 11 Hammer lock 6 Hammer Figure 2. Device name according to DIN of G.U.N.T. WP 400 Pendulum impact tester, 25 Nm 3

5 2.2 Principle of the Charpy Test The Charpy test is carried out with a pendulum impact tester according to DIN After it is released, the hammer affixed to a pendulum arm describes a circular arc and transmits some of its kinetic energy to the specimen at the lowest point of the hammer path. In the area of the specimen close to the notch base, the impact generates stress directed along multiple axes. The hammer executes its swing and carries along a maximum indicator that displays the notched bar impact work (A k ) on the scale. Since the level of the notched bar impact depends on the shape of the specimen, the shape must always be indicated, for example A k(iso V) = 10 J The symbols have the following meaning: m : Mass of the pendulum hammer in kg g : Gravitational acceleration in m/s 2 H : Starting height of the pendulum hammer in m h : Height of the pendulum hammer after impact in m ISO V: DIN specimen shape with a V notch The quotient of the notched bar impact and the nominal cross-section (the residual surface of the specimen in the notch base) is the notched bar impact strength: The symbols have the following meaning: a k : notched bar impact strength in Nm/cm2 A k : notched bar impact work in Nm S o : Cross-sectional area of the specimen before fracture in cm2 The notched bar impact strength also strongly depends on the specimen shape. Consequently, the values that are measured from samples with different geometries cannot be compared with each other. 4

6 2.3 Type K thermocouple and digital readout unit 2.4 Beakers of room-temperature water, and boiling water 2.5 Cryo-beakers of salted iced water and super cold liquids 5

7 3. Materials Two different materials, Steel and Brass, have been used in different V-notch and specimen dimensions as shown in the following table: Material Specifications Specimen Dimensions Machining steel, 9 SMn 28 Brass, CuZn40Pb2 Machining steel, 9 SMn 28 Brass, CuZn40Pb2 6

8 4. PROCEDURE CAUTION: - When using the Charpy testing machine, stand well clear of the swinging area of the pendulum both when the arm is cocked and for some time after the arm is released for a test while it is still swinging. Serious injury will result from a swinging pendulum arm. - The hammer can cause serious injury when falling such as broken bones, severed fingers, etc. The test area must therefore be kept free of people. Hazards to the Unit and Operation - Only use the specified notched bar samples and properly orient them in the counter bearings. If other specimens are used, there is the danger that the hammer peen or counter bearing may become damaged. For each material repeat the following steps Designate a person as the "operator" of the Charpy test machine: all other persons must stand clear during testing Designate a person as the "monitor and recorder" of temperatures and impact energies Designate a person as the "test specimen loader" who will remove test specimens from the liquid bath, quickly placing them on the test fixture of the Charpy testing machine Designate a person as the "test specimen retriever" who will retrieve the broken halves of the test specimens, will bind the halves together and will mark the test temperature on each pair of specimen halves for later examination and inspection. Use the following procedure to conduct tests in the order shown after exposure to the preconditions to give the approximate test Place the thermocouple probe in the appropriate liquid being sure to allow both the test specimens and the thermocouple to equilibrate for at least five minutes prior to testing. Record the indicated temperature "Cock" the pendulum by activating the "raise" mechanism and stand clear while the pendulum is held in the "cocked" position. 7

9 Using the tongs, quickly remove the test specimen from the bath and place it on the test fixture with the notch opening facing away from the direction of the cocked pendulum Stand clear Release the pendulum Secure the pendulum in its rest position (i.e., hanging vertically) and retrieve the fractured specimen halves. Record the impact energy (read directly from the dial on the Charpy testing machine) Repeat these steps for the each temperature and each material. 5. RESULTS AND DISCUSSION 5.1 Fill in the following table: Impact Energy (J) Materials 9 SMn 28 9 SMn 28 CuZn40Pb2 CuZn40Pb2 sample specifications 45 degree 90 degree 5x7 mm 10x7 mm Hot Temperature ( o C) Room Temperature ( o C) Cold Temperature ( o C) 8

10 5.2 Plot the impact energy versus temperature for each material on the same graph to be similar to that shown in Figure 3, use suitable scale. 9

11 5.3 Compare these impact results for each metal/sample specifications to tabulated values from a source such as the ASM Metals Handbook. Comment on differences and similarities. 5.4 Examine the type and degree of deformation of each fracture surface, Correlate this information with the corresponding impact energies. Comment on the correlations. Evaluating the Fracture Surface The shear fracture portion Z (ductile fracture) is the portion of an unpolished, fibrous, or rugged fracture surface in reference to the entire surface. It can be mathematically calculated. It is characterized by compressions and necking at the edges, a sign of ductility. The shear fracture portion Z is indicated in percent by the following equation: The symbols have the following meaning: Z : Shear fraction portion in % S : Initial specimen cross-section before 10

12 testing in cm S : smallest specimen cross-section after the specimen is fractured in cm (separation fracture) A separation fracture defines a nearly flat fracture surface with non-deformed and smooth edges which is a sign of brittleness. The specimen cross-section after fracturing is measured and then calculated by the following equation: K : Notch V : Ductile fracture S : Brittle fracture surface A : Fracture width Figure 4. Percent reduction of area at fracture 11