P A (1.1) load or stress. elongation or strain

Transcription

1 load or stress MEEN 3145 TENSION TEST - BACKGROUND The tension test is the most important and commonly used test in characterizing properties of engineering materials. This test gives information essential in the preliminary design of products and structures, such as tensile strength, yield strength, as well as many important secondary properties, such as the ability to absorb strain energy, and experience elastic and plastic deformation. The basic idea of the tensile test is simple. A bar of uniform cross section is subjected to a tensile, or pulling force applied to on end, while the other end is held fixed. Under the load, the bar stretches, and the applied load and the elongation are measured at different stages as the test progresses. From this data, a plot of the load versus deflection can be made which gives the desired information. Curves obtained this way have a variety of shapes which depend on the properties of the material being tested. Some typical curves are shown in Figure 1.1. medium carbon steel some non-ferrous metals some polymers elongation or strain Stress strain curves for various materials. Figure 1.1 Before proceeding with a discussion of the curves, it is necessary to make a few comments and definitions. Load versus displacement curves are almost never used. This curve is usually converted to an equivalent stress versus strain curve, which may be thought of as a normalized curve. Stress in a uniaxial tension loading is computed from the following relationship: P A (1.1) Where: = stress (psi, Pa) P = load (lbf, N) A = cross sectional area (in2,mm2) 1-1

2 Stress defined in this manner is called engineering stress. It should be distinguished from the true stress which is calculated by considering the fact that the cross sectional area of the bar is continually changing (becoming smaller) as a result of Poisson's effect, and the fact that a material deforms plastically at a constant volume. The quantity strain also must be defined. l2 l l 1 1 (1.2) where: = strain l 1 = original length of a specimen or attached gage (in, mm) l 2 = deformed length of a specimen or attached gage (in, mm) Again this definition is for engineering strain. When strains are small, the difference between engineering strain and true strain is insignificant, and no distinction is made is made between the true stress, true strain, and engineering stress and engineering strain. They are simply referred to as stress and strain. Load-displacement curves converted to stress-strain curves retain their original shapes, so the forms of the curves on Figure 1.1 may also be used as stress-strain curves if their coordinate axes are properly converted. As mentioned above, a number of characteristic properties of the tested material are defined on the stress-strain curve. The first portion of the curve is a straight line for most engineering materials. The point at which the line departs from this straight line, or linear relationship, is called the proportional limit, as shown on Figure 1.2. The slope of this line is Young's modulus, or Modulus of Elasticity, which is a measure of stiffness of the material being tested. For steels, this property is usually very close to 30,000,000 psi (206,900 Pa). The relationship between stress and strain in this region is linear and is expressed by: E (1.3) where: = strain = stress (psi, Pa) E = Young's modulus (psi, Pa) 1-2

3 Transition from linear region to non-linear region. Figure 1.2 After the straight-line portion, the curve usually starts to bend downward. However, when the load is released, the specimen will return to its original length. The material is still in its elastic region. The last point from which the material will return to its original length is called the elastic limit. This point cannot be established in terms of the geometry of the curve and it must be determined by a trial procedure. The load is removed and then a higher value of load is repeatedly applied. When some plastic, or permanent deformation is observed, it is said that the elastic limit has been reached. The elastic limit and the proportional limit are usually sufficiently close to one another that they are usually used as one property. Often this elastic limit is referred to as the yield point. After the elastic limit has been exceeded, further deformation results in permanent elongation of the specimen. In the case of mild steel, the curve will enter the region in which elongation will be produced with no additional load being applied. The stress at which this happens is call the yield strength, and the region in which the curves flattens is called the yield jog. This is also shown in Figure 1.2. For materials which do not display a yield jog, the yield strength is defined by considering the shape of the curve usually by a technique referred to as the offset method. In this technique, a strain of 0.2 percent is marked on the strain axis and a line parallel to the initial straight line of the curve is drawn. The point at which this line intersects the stress-strain curve is defined as the offset yield strength of the material. This procedure is illustrated in Figure

4 Determination of Yield Stress Using 0.2 % Offset Method Figure 1.3 In normal practice the yield strength is used as the elastic strength of the material. When the material is loaded beyond the yield jog, if it exists, the curve will start to rise again. The material is now entering a region in which it is said to be strain hardening. The curve rises to a maximum value, at which point the material is said to have reached its ultimate strength. Loading beyond this point results in a visible reduction in the cross sectional area of the specimen. This phenomenon is commonly referred to as necking. Further loading accelerates the necking process, and the specimen will break. The behavior described above is typical for ductile materials. Brittle materials behave somewhat differently, usually exhibiting an elastic region followed by little or permanent deflection before rupture. A curve for cast iron would show this type of behavior. It should be mentioned that the when the load is removed at any point during the test, the curve will drop down to the strain axis along a straight line parallel to the elastic line. If the specimen has been loaded beyond its yield point, it will retain a permanent deformation or elongation. Two additional items to be discussed are percent elongation and percent reduction in area. Both of these definitions pertain to the ductility of the material. Numerically, the higher these two values are, the more ductile the material. Elongation is measured after the specimen breaks, and is defined by: % elongation ( l f l o ) 100% l o (1.4) where: l o = original length(in, mm) l f = final length(in, mm) 1-4

5 Similarly, % reduction in area ( A A ) 100% o f (1.5) Ao where: A o = initial cross sectional area(in 2, mm 2 ) A f = final cross sectional area(in 2, mm 2 ). The final area is computed by measuring the diameter at the point of rupture of the broken specimen. 1-5

6 TENSILE TEST PROCEDURE OBJECTIVE: The objective is to understand the relation of stress vs strain when a uniaxial load is applied to a standardized specimen. EQUIPMENT: Universal Testing Machine, extensometer, micrometer, dial caliper, specimens. PROCEDURE: 1) After procuring the specimen from the instructor, determine material type, visually inspect for any flaws, and measure specimen diameter. Determine the expected maximum load and elongation for each material. 2) Make two marks two inches apart along the length of the specimen. These will be used to determine final length. Connect the two marks with a thin, straight line. Opposite to the line and the two marks, place marks using a marker every quarter (¼) inch. These marks will be used to compare elongation at different points on the specimen. Indicate on specimen which end is to be placed into the moving cross-head and which is to be placed into the fixed cross-head. 3) Follow procedure for installing specimen into MTS machine. Follow procedure for conducting test on specimen with the MTS machine. 4) Monitor and record load and elongation as the test proceeds. Be sure to observe any physical changes in specimen. 5) Record the following: a) Final diameter at neck. b) Final length as measured along the line. c) Measure final length of each division. d) Record type of fracture, include sketch. e) Record type of noise fracture made. REPORT: The report should cover the following: a) Stress/Strain curve, two graphs for specimen. One for the whole test, and one for the linear region to determine yield point and Young's Modulus. b) Young's modulus, yield point, % elongation, % area reduction, ultimate tensile strength. Determine these and compare with theoretical values. c) Graph showing percent elongation of each quarter inch division. Increment presented from fixed cross-head to moving cross-head. d) A table giving the type of material, yield strength, ultimate strength, percent elongation, and modulus of elasticity. Compare with published data. 1-6

7 TENSILE TESTING PROCEDURE FOR MTS UNIVERSAL TESTING MACHINE The following is the procedure for conducting a tensile test for the ME 3145 laboratory. I. START-UP 1. Turn on load frame by pressing the ON switch on the lower-right side of the frame. 2. Ensure load cell cable is connected below crosshead (not jumper used when using small load cell when testing above crosshead). 3. Open TWE software. 4. Select Custom Templates from the left edge. 1-7

8 5. Select MEEN3145 Tensile Test Tension (Extensometer-Crosshead). 6. Select Monitor from the top bar. 7. Press Interlock button. 1-8

9 8. Ensure that the machine is set for bottom testing by pressing the button for using the workspace below the crosshead. II. POSITIONING THE CROSSHEAD: BEFORE MOVEMENT OF THE CROSSHEAD, BE SURE THAT: 1. Limit stops are set. 2. All personnel are clear and alerted to machine Operation. INSERTING SPECIMEN 1. Raise crosshead to ensure necessary clearance, to about 375 mm mark, using the remote paddle-control. (press un-lock button on paddle first, looks like open padlock). 2. Thread specimen into TOP hex-shaped adapter. Do not force. Do not torque any threaded sections. Specimen should be threaded all the way, then backed off about a ¼ turn. 3. Remove retaining pin on bottom chuck. 4. Thread onto bottom of specimen as in step Lower head using remote paddle-control. LOWER HEAD SLOWLY so that the pin adapter will be able to slip into lower coupling. Make sure that fingers, sleeves, skin, and hair is away from pinch point. STOP head when holes for retaining pin line up in chuck and adapter. 6. Insert retaining pin. 1-9

10 PREPARING FOR TEST 7. Install Extensometer to specimen with line-up pin in extensometer until secured to specimen, thin removed before testing. 8. Press unlock button on paddle to return control to computer. TESTING 1. Before running test, check extensometer removal point in Define screen. 2. Return to Monitor screen. 3. Right click on Crosshead at bottom, Zero Signal. Repeat for Load and Extensometer. 4. Press green arrow. 5. Click OK on pop-up window, providing grip separation is and diameter is in. 6. Pause run and remove extensometer once the test clears the linear region. 7. File>Export>Raw Data to export data to.txt file. 1-10