Laboratory 3 Impact Testing of Metals

Transcription

1 Department of Materials and Metallurgical Engineering Bangladesh University of Engineering Technology, Dhaka MME 222 Materials Testing Sessional 1.50 Credits Laboratory 3 Impact Testing of Metals 1. Objective In manufacturing locomotive wheels, coins, connecting rods, etc., the components are subjected to impact (shock) loads. These loads are applied suddenly. The stresses induced in these components are many times more than the stresses produced by gradual loading. Impact tests are, therefore, performed to assess the shock absorbing capacity of materials subjected to suddenly applied loads. After completion of this experiment, students should be able to 1.1 conduct Charpy V-notched impact test, and 1.2 determine ductile-to-brittle transition temperature of different metals and understand the fracture behaviour of metals having different crystal structure. 2. Materials and Equipment 2.1 Charpy V-notched impact test standard samples of mild steel and copper 2.2 Impact testing machine 2.3 Coolants (ice cubes, dry ice) 2.4 Heating furnace 3. Experimental Procedure 3.1 Check the zero calibration of the impact tester. Lift the pendulum (or Charpy striker) till it gets latched in its position and release the hatch to swing it freely without placing any sample. Apply the break to the pendulum slowly by operating break lever. Note the reading directly on the dial as indicated by the indicating pointer. It should read ZERO indicating that the machine is error-free. If not, mark the reading. 3.2 Now place one Charpy test specimen on support. Make sure that the notch of the sample is facing the pendulum. 3.3 Fix the pendulum in its position and note the initial energy reading. Allow the pendulum to swing freely and break the specimen. Note down the final energy reading. 3.4 Before proceeding for next test, remove the broken piece of the tested specimen and bring indicating pointer and the pendulum to their respective original positions. 3.5 Measure the depth d below the notch and the breadth b of the broken specimen to calculate the effective cross-sectional area A of the specimen below the notch (A = bd mm 2 ) 3.6 Repeat steps 3.2 to 3.5 conduct impact tests of different specimens using three different temperatures, viz., 100 C (boiling water), 0 C (ice water) and 40 C (dry ice). 3.7 Complete Data Sheet, Table

2 4. Results 4.1 Display the test data in the Table. 4.2 Plot impact energy versus temperature and % brittleness versus temperature. 4.3 Find Ductile-to-Brittle Transition Temperature (DBTT) of the tested materials. 5. Discussion 5.1 Answer the following questions: 1. What are the main uses of the Charpy test? 2. List the ASTM Standard specifications for the Charpy impact test with title. 3. What is the necessity of making a notch in impact test specimen? 4. If the sharpness of V-notch is more in one specimen than the other, what will be its effect on the test result? 5. In which way the values of impact energy will be influenced in the impact tests are conducted on two specimens, one having smooth surface and the other having scratches on the surface? 6. What is the effect of temperature on the values of rupture energy and notch impact strength? 7. Give the estimated values of DBTT for your steel. Suggest two ways in which DBTT can be lowered. 8. What are the three basic factors which contribute to brittle fracture of steels? Do all three have to be present for brittle fracture to occur? 9. Why do aluminium alloys do not show DBTT? 2

3 Table 3.1: Data Sheet for Charpy V-notch Test Impact Testing Machine s Correction Factor = J This value should be added to the initial and final reading obtained from the indicator reading to obtain the actual energy data. Thus, for example, Initial reading = Initial indicator reading + Correction factor. Material Used Initial Reading E 1 Final Reading E 2 Absorbed Energy E = E 1 E 2 Effective Area Below Notch A = b d Notch Impact Strength i = E/A Average Impact Energy Average Impact Strength 1 J J J mm 2 kj/m 2 kj kj/m

4 6.0 Theoretical Background 6.1 Introduction The static properties of materials and their attendant mechanical behaviour are dependent on their intrinsic material properties (chemical composition and structure), the type of heat treatment the material may have received and the design factors such as stress concentrations. The behaviour of a material is also dependent on the rate at which the load is applied. Polymeric materials and metals which show delayed yielding are most sensitive to load application rate. Low-carbon steel, for example, shows a considerable increase in yield strength with increasing rate of strain. In addition, increased work hardening occurs at high-strain rates. This results in reduced local necking, hence, a greater overall material ductility occurs. A practical application of these effects is apparent in the fabrication of parts by high-strain rate methods such as explosive forming. This method results in larger amounts of plastic deformation than conventional forming methods and, at the same time, imparts increased strength and dimensional stability to the part. In design applications, impact situations are frequently encountered, such as cylinder head bolts, in which it is necessary for the part to absorb a certain amount of energy without failure. In the static test, this energy absorption ability is called toughness and is indicated by the modulus of rupture. As it was realized that the results of laboratory tensile tests (at low loading rates) could not be extrapolated to predict fracture behaviour of materials at high loading rates, impact test conditions were chosen to represent those most severe relative to the potential for fracture namely, 1. deformation at a relatively low temperature 2. a high strain rate (i.e., rate of deformation), and 3. a triaxial stress state (which may be introduced by the presence of a notch). A similar toughness measurement is required for such dynamic loadings; this measurement is made with a standard ASTM impact test known as the Izod or Charpy test. 6.2 Impact Testing When using one of these impact tests, a small notched specimen is broken in flexure by a single blow from a swinging pendulum. With the Charpy test, the specimen is supported as a simple beam, while in the Izod it is held as a cantilever. A standard Charpy impact machine is used. This machine consists essentially of a rigid specimen holder and a swinging pendulum hammer for striking the impact blow. Impact energy is simply the difference in potential energies of the pendulum before and after striking the specimen. The machine is calibrated to read the fracture energy in N-m or J directly from a pointer which indicates the angular rotation of the pendulum after the specimen has been fractured. The apparatus for performing impact tests and the standard configurations for Izod (cantilever) and Charpy (three-point) impact tests are illustrated schematically in Fig Although two standardized tests, the Charpy and Izod, were designed and used extensively to measure the impact energy, Charpy v-notched impact tests are more common in practice. The load is applied as an impact blow from a weighted pendulum hammer that is released from a position at a fixed height h. The specimen is positioned at the base and with the release of pendulum, which has a knife edge, strikes and fractures the specimen at the notch. The pendulum continues its swing, rising a maximum height h which should be lower than h naturally. The energy absorbed at fracture E can be obtained by simply calculating the difference in potential energy of the pendulum before and after the test such as, ( ) (6.1) where m is the mass of pendulum and g is the gravitational acceleration. If the dimensions of specimens (10x10x55 mm) are maintained as indicated in standards, notched-bar impact test results are affected by the lattice type of materials, testing temperature, thermo-mechanical history, chemical composition of materials, degree of strain hardening, etc. 4

5 0.25 mm radius 45 2 mm Notch Details Figure 6.1: (a) Specimen used for Charpy and Izod impact tests. (b) A schematic drawing of an impact testing apparatus. The Charpy test does not simulate any particular design situation and data obtained from this test are not directly applicable to design work as are data such as yield strength. The test is useful, however, in comparing variations in the metallurgical structure of the metal and in determining environmental effects such as temperature. It is often used in acceptance specifications for materials used in impact situations, such as gears, shafts, or bolts. It can have useful applications to design when a correlation can be found between Charpy values and impact failures of actual parts. 5

6 6.3 Failure of Materials The failure of engineering materials is almost always an undesirable event for several reasons; these include putting human lives in jeopardy, causing economic losses, and interfering with the availability of products and services. Even though the causes of failure and the behaviour of materials may be known, prevention of failures is difficult to guarantee. The usual causes are improper materials selection and processing and inadequate design of the component or its misuse. Also, damage can occur to structural parts during service, and regular inspection and repair or replacement are critical to safe design. There are two general types of failure: 1. Fracture, through either internal or external cracking; fracture is further sub-classified into two general categories: ductile and brittle. This classification is based on the ability of a material to experience plastic deformation. Ductile metals typically exhibit substantial plastic deformation with high energy absorption before fracture. However, there is normally little or no plastic deformation with low energy absorption accompanying a brittle fracture. 2. Buckling. Although failure of materials is generally regarded as undesirable, some products are designed in such a way that failure is essential for their function. Typical examples are (a) food and beverage containers with tabs (or entire tops) which are removed by tearing the sheet metal along a prescribed path; (b) shear pins on shafts that prevent machinery damage in the case of overloads; (c) perforated paper or metal, as in packaging; and (d) metal or plastic screw caps for bottles Ductile fracture Ductile fracture surfaces have distinctive features on both macroscopic and microscopic levels. Figure 6.2 shows schematic representations for two characteristic macroscopic fracture profiles. The configuration shown in Fig. 6.2a is found for extremely soft metals, such as pure gold and lead at room temperature, and other metals, polymers, and inorganic glasses at elevated temperatures. These highly ductile materials neck down to a point fracture, showing virtually 100% reduction in area. The most common type of tensile fracture profile for ductile metals is that represented in Fig. 6.2b, where fracture is preceded by only a moderate amount of necking. Sometimes a fracture having this characteristic surface contour is termed a cup-and-cone fracture because one of the mating surfaces is in the form of a cup and the other like a cone. In this type of fractured specimen (Fig. 6.3a), the central interior region of the surface has an irregular and fibrous appearance, which is indicative of plastic deformation. Figure 6.2: (a) Highly ductile fracture in which the specimen necks down to a point. (b) Moderately ductile fracture after some necking. (c) Brittle fracture without any plastic deformation. 6

7 Figure 6.3: (a) Cup-and-cone fracture in aluminium. (b) Brittle fracture in mild steel. Much more detailed information regarding the mechanism of fracture is available from microscopic examination, normally using scanning electron microscopy. Studies of this type are termed fractography. The scanning electron microscope is preferred for fractographic examinations because it has a much better resolution and depth of field than does the optical microscope; these characteristics are necessary to reveal the topographical features of fracture surfaces. When the fibrous central region of a cup-and-cone fracture surface is examined with the electron microscope at a high magnification, it is found to consist of numerous spherical dimples (Fig. 6.4a); this structure is characteristic of fracture resulting from uniaxial tensile failure. Each dimple is one half of a microvoid that formed and then separated during the fracture process. Dimples also form on the 45 shear lip of the cup-andcone fracture. However, these will be elongated or C-shaped, as shown in Fig. 6.4b. This parabolic shape may be indicative of shear failure. Fractographs such as those shown in Fig. 6.4 provide valuable information in the analyses of fracture, such as the fracture mode, the stress state, and the site of crack initiation. Figure 6.4: (a) Scanning electron fractograph showing spherical dimples characteristic of ductile fracture resulting from uniaxial tensile loads. 3300x. (b) Scanning electron fractograph showing parabolic-shaped dimples characteristic of ductile fracture resulting from shear loading. 5000x. Because they are nucleation sites for voids, inclusions have an important influence on ductile fracture and, consequently, on the workability of materials. Inclusions may consist of impurities of various kinds and of second-phase particles, such as oxides, carbides, and sulphides. The extent of their influence depends on such factors as their shape, hardness, distribution, and fraction of total volume; the greater the volume fraction of inclusions, the lower will be the ductility of the material. Voids and porosity can also develop during processing of metals, such as the voids resulting from casting and metalworking processes such as drawing and extrusion. 7

8 6.3.2 Brittle Fracture Brittle fracture takes place without any appreciable deformation and by rapid crack propagation. The direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress and yields a relatively flat fracture surface, as indicated in Fig. 6.2c. Fracture surfaces of materials that fail in a brittle manner have distinctive patterns; any signs of gross plastic deformation are absent. For example, in some steel pieces, a series of V-shaped chevron markings may form near the centre of the fracture cross section that point back toward the crack initiation site (Fig. 6.5a). Other brittle fracture surfaces contain lines or ridges that radiate from the origin of the crack in a fanlike pattern (Fig.6.5b). Often, both of these marking patterns are sufficiently coarse to be discerned with the naked eye. For very hard and fine-grained metals, there is no discernible fracture pattern. Brittle fracture in amorphous materials, such as ceramic glasses, yields a relatively shiny and smooth surface. Figure 6.5: (a) Photograph showing V-shaped chevron markings characteristic of brittle fracture. Arrows indicate origin of crack. Approximate actual size. (b) Photograph of a brittle fracture surface showing radial fan-shaped ridges. Arrow indicates origin of crack. Approximately 2x. For most brittle crystalline materials, crack propagation corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes (Fig. 6.6a); such a process is termed cleavage. This type of fracture is said to be transgranular (or transcrystalline) because the fracture cracks pass through the grains. Macroscopically, the fracture surface may have a grainy or faceted texture (Fig. 6.3b) as a result of changes in orientation of the cleavage planes from grain to grain. This cleavage feature is shown at a higher magnification in the scanning electron micrograph of Fig. 6.6b. 8

9 Grains Path of crack propagation Figure 6.6: (a) Schematic cross-section profile showing crack propagation through the interior of grains for transgranular fracture. (b) Scanning electron fractograph of ductile cast iron showing a transgranular fracture surface. In some alloys, crack propagation is along grain boundaries (Fig. 6.7a); this fracture is termed intergranular. Figure 6.7b is a scanning electron micrograph showing a typical intergranular fracture, in which the threedimensional nature of the grains may be seen. This type of fracture normally occurs when the grain boundaries are soft, contain a brittle phase, or have been weakened by liquid- or solid-metal embrittlement. Grain boundaries Path of crack propagation Figure 6.7: (a) Schematic cross-section profile showing crack propagation along grain boundaries for intergranular fracture. (b) Scanning electron fractograph showing an intergranular fracture surface. 50x. 9

10 Impact energy 6.4 Ductile-to-Brittle Transition The energy to fracture as measured by a Charpy test decreases with increasing temperature, Fig At higher temperatures, the impact energy is relatively large, corresponding to a ductile mode of fracture. As the temperature is lowered, the impact energy drops suddenly over a relatively narrow temperature range, below which the energy has a constant but small value that is, the mode of fracture is brittle. One of the primary functions of the Charpy and the Izod tests is to determine whether a material experiences a ductile-to-brittle transition with decreasing temperature and, if so, the range of temperatures over which it occurs. The ductileto-brittle transition is related to the temperature dependence of the measured impact energy absorption. Many metals undergo a sharp change in ductility and toughness across a narrow temperature range. The temperature at which such sharp decrease in impact energy occurs where the material s behaviour changes from ductile to brittle is called the ductile-brittle transition temperature (DBTT) (Fig. 6.8b). Structures constructed from alloys that exhibit this ductile-to-brittle behaviour should be used only at temperatures above the transition temperature to avoid brittle and catastrophic failure. (a) Figure 6.8: (a) Schematic curves for the three general types of impact energy versus temperature behaviour. (b) Typical ductile-to-brittle transition curve for annealed low-carbon steel. DBTT Temperature (b) Most metals with body-centred cubic (bcc) structures (like low alloy steels) and some hexagonal close-packed (hcp) crystal structures show a sharp transition temperature and are brittle at low temperatures. The transition temperature depends on such factors as the composition (see Fig. 6.9), microstructure, grain size, surface finish, and shape of the specimen, and the deformation rate. High rates, abrupt changes in workpiece shape, and the presence of surface defects (notches, porosity, etc.) raise the transition temperature. Figure 6.9: Influence of carbon content on the Charpy V-notch energy versus temperature behaviour for steel. 10

11 Impact Energy % Brittleness In addition to the ductile-to-brittle transition represented in Fig. 6.8b, two other general types of impact energy versus temperature behaviour have been observed; these are represented schematically by the upper and lower curves of Fig. 6.8a. Here it may be noted that low-strength face-centred cubic (fcc) crystalline metals (some aluminium and copper alloys) and most HCP metals do not experience a ductile-to-brittle transition (corresponding to the upper curve of Fig. 6.8a). They have many slip systems and retain high impact energies (i.e., remain tough) with decreasing temperature. For high-strength materials (e.g., high-strength steels and titanium alloys), the impact energy is also relatively insensitive to temperature (the lower curve of Fig. 6.8a); however, these materials are also very brittle, as reflected by their low impact energies. Besides low carbon steels, most ceramics and polymers also experience a ductile-to-brittle transition. For ceramic materials, the transition occurs only at elevated temperatures, ordinarily in excess of 1000 C. Alloys showing a range of temperatures over which the ductile-to-brittle transition occurs present some difficulty in specifying a single ductile-to-brittle transition temperature. A number of criteria in impact testing are proposed to define the transition temperature, Fig. 6.10: 1. Some critical energy level This temperature is often defined as the temperature at which the impact energy assumes some value (e.g., 20 J), although the average energy concept (i.e., the temperature having the average is often used. 2. A given fracture surface appearance (e.g., 50% fibrous fracture) The brittle fracture surface has a crystalline appearance, while the portion of the specimen which fracture in a ductile manner will have a so-called fibrous appearance. The transition temperature is defined as the temperature at which the fracture surface appearance shows a given (e.g., 50% fibrous fracture) brittleness. For example, the temperature, at which the fracture surface consists of 50 per cent cleavage (crystalline) and 50 per cent ductile (fibrous) types of fracture, is often used and called fracture appearance transition temperature (FATT). Perhaps the most conservative transition temperature is that at which the fracture surface becomes 100% fibrous; on this basis, the transition temperature is approximately 110 C for low alloy steels. 100% Fractured surface appearance E/2 50% E E Impact Energy T 1 T 2 T 3 Figure 6.10: Various criteria of transition temperature obtained from Charpy tests. 0% Any of these criteria are usable. Perhaps the most direct criterion for a particular metal is to define the transition temperature as that temperature at which some minimum amount of energy is required to fracture (criterion 1). During World War II, allied Victory ships literally broke in two in conditions as mild as standing at the dock because of the use of steel with a high-transition temperature, coupled with high-stress concentrations. It was found that specimens cut from plates of these ships averaged only 9 J Charpy energy absorption at the service temperature. Ship plates were resistant to failure if the energy absorption value was raised to 20 J at the service temperature by proper control of impurities. 11