Lab Test: Tensile Testing

The mechanistic properties of worldlys atomic number 18 deter mined by performing cargonfully designed science laboratory trys that replicate as nearly as possible the armed service conditions. In real life, there ar m both(prenominal) factors involved in the record in which committal be employ on a material. The following be some common examples of modes in which lade might be applied plastic, compressive, and shear. These properties be im mienant In materials selections for mechanical design. some other factors that often complicate the design butt on Include temperature and time factors.The conductic of this lab is confined to the pliant stead of polymers. general anatomy 1 shows a flexible nisus auto similar to the bingle drug ab design in this lab. This test is a deleterious manner, in which a exemplar of a commonplace shape and dimensions (prepared according to ASTM D 638 standard test method for tensile properties of plastics) is subjected to an axial load. During a ordinary tensile experiment, a dog- wad shaped pattern Is gripped at Its 2 quits and Is pulled to elongate at a determined vagabond to Its breakpoint a exceedingly ductile polymer may non clear up it its breakpoint.The tensile tester seed in this lab is manufactured by insertion (model 5569). It has a maximum load of 2 or 50 ink and a variable pulling roam. The setup of the experiment could be changed to accommodate different personas of mechanical testing, according to the ASTM standard (e. G. Compression test, etc). For analytic purposes, a plot of stock (o) versus extend (E) Is constructed during a tensile test experiment, which evict be d ace automatically on the software provided by the instrument manufacturer. Stress, in the metric system, is commonly mea certaind in N/ mm or Pa, much(prenominal) that 1 N/mm = 1 Pa.From the experiment, the value of stress is lactated by dividing the inwardness of force (F) applied by the cable car in the axial focus by its cross-sectioned country (A), which is mea authorizedd former to running the experiment. Mathematically, It Is expressed In par 1. The strain values, which have no units, stooge be reckon using par 2, where L Is the Instantaneous distance of the model and LO Is the initial space. (Equation 1) (Equation 2) A true stress-strain bending would look similar configuration 2. The stress-strain wind shown In Figure 2 Is a textbook example of a stress-strain bender.In reality, not all stress-strain crooks perfectly resemble the one shown In Figure 2. This stress-strain curve Is typical for ductile metallic elements. Another function to take bankers bill is that Figure 2 shows an engineering stress-strain curve. When a material r from separately onees its ultimate stress strength of the stress-strain curve, its cross-sectional area reduces dramatically, a term cognize as necking. When the computer software plots the stress-strain curve, it assumes that the cross sectional area stay constant throughout the experiment, even during necking, therefore causing the curve to huckster down.The consecutive stress- change in the cross sectional area of the specimen throughout the experiment. Theoretically, even without measuring the cross-sectional area of the specimen during the tensile experiment, the true stress-strain curve could still be constructed by assuming that the volume of the material stays the same. Using this concept, both the true stress (UT) and the true strain (ET) could be calculated using Equations 3 and 4, respectively. The derivation of these equations is beyond the scope of this lab report. Consult whatever standard mechanics textbook to instruct more astir(predicate) these equations.In these equations, LO refers to the initial length of the specimen, L refers o the instantaneous length and o refers to the instantaneous stress. (Equation 3) (Equation 4) Figure 2 also shows that a stress-strain curve is d ivided up into four realms bendable, yielding, strain clayeyening (commonly occurs in metallic materials), and necking. The area on a lower floor the curve represents the amount of nix required to fall upon each(prenominal) of these events. The total area low the curve (up to the point of fracture) is also cognize as the modulus of toughness.This represents the amount of dynamism needed to break the taste, which could be comparisond to the impact energy of the assay, determined from impact tests. The area under the linear surface area of the curve is known as the modulus of resilience. This represents the minimum amount of energy needed to filter the model. The linear region of the curve of Figure 2, which is called the elastic region (past this region, is called the plastic region), is the region where a material behaves elastically. The material get out run off to its pilot light shape when a force is released dapple the material is in its elastic region.The slope of the curve, which feces be calculated using Equation 5, is a constant and is an inwrought property of material known as the elastic modulus, E. In metric units, it is unremarkably expressed in Pascal (Pa). (Equation 5) Figure 3(a) shows typical stress-strain curves of polymers. The figure shows that materials that are hard and brittle do not deform very much before good luck and have very steep elastic modulo. The mechanical property of polymers generally depends on their degree of crystalline, molecular weights and glass change temperature, tugboat.Highly crystalline polymeric materials with a Tug above the room temperature are usually brittle, and vice versa. When a semi-crystalline polymer undergoes a tensile test, the amorphous chains, allow for fix aligned. This is usually evident for transparent and translucent materials, which become opaque upon turning crystalline. Figure 3(b) shows a diagram showing the mechanical property of some common polymers. Important touch on sure you yield safety glaze before scratch line any operation. Your eyeball could be hurt by a broken piece of polymer. withal wear gloves to protect against any residue on the automobile and samples. . 1 prototype provision The polymer specimens were injection-molded into dog-bone shapes. Their dimensions were determined according to the ASTM D 638 standard mentioned earlier in the introduction. (1) Measure the burdensomeness, breadth and gage length of polymer samples in mm. These dimensions should be approximately the same for each sample. (2) as well as make note of any sample defects (e. G. Impurities, air bubbles, etc. ). The following samples volition be tested 1) polypropene (UP), polystyrene (AS), polycyclic cutting (polymer), high density polyethylene (HIDE), and Dentally for psychoanalysis of mechanical properties. ) Polystyrene to compare effects of nutriment direction on mechanical properties. 3) Polypropylene to analyze effects of strain rate o n mechanical properties. . 2 bluebill Software Setup 1) Turn on the tensile test machine. The switch is locate on the right side of the machine. likewise turn on the video extensors. (2)Go to the background and double- pass over on the Bluebill icon. (3) On the main page, select rise to start a invigorated sample. Name your test and click Browse to select the folder you would like to save it in. snarl next. (4) Choose which method you would like to use.Create and save a new method if needed. (5) Method set up Save after any changes are made. General utilize for display purposes Specimen specifies sample dimensions and parameters. A doggone sample is used for tensile testing. Select rectangular, and specify the width, thickness and think length of the sample. The gauge length is the distance between the clamps before starting the test. Control describes the actual test. Select point of reference for mode of displacement, then specify the rate of accompaniment. Most use 5 m m/min or 50 min/mm, depending on if you want a slack or fast test. residual of Test identifies the criteria for the end of the test. A large load draw is experienced when sample failure occurs. For this test, when the sample load drops by a plastered percentage of the peak load, he machine will baulk. Data specifies if the data is acquired manually or automatically, while the strain chit recognizes whether the strain is measured from the video exterminates or the book of facts. Results and Graphs select what data is shown and how it is displayed. (1) Make sure the proper load jail mobile phone is installed, every 2 ink or 50 ink depending on the load stray and sensitivity of the sample.To switch load cells, make sure the machine is off. Unscrew the bolts and charter using the go pastle. Make sure to political hack the new load cell into the port behind the machine. (2) Calibrate the load cell by licking on the sacking in the upper berth right hand corner. Make sure a ll loads are removed from the load cell and click calibrate. (3) Install the correct type of clamps for the testing. For tensile testing, non or ink samples can be used. Install the clamps using the pins. likewise install height brackets if needed. Zero the load once the clamps are installed. 4) Press the up and down arrows on the controller until the clamps are Just touching. Press the reset gauge length button at the top of the screen to zero the position of the clamps. (5) recitation the up and down arrows until the clamps are about 100 mm apart. This is a typical gauge length for the dog bone samples. (6) Place the polymer sample between the grips of both the tensile test machine. While retentiveness the sample vertically with one hand, use another hand to turn the carry on of the top grip in the stop direction as tightly as possible. The specimen should be gripped such that the two ends of the specimen are covered by the grip, approximately 3 mm by from its gage-length. I t is important that the specimens are tightly gripped onto the specimen grips to prevent slipping, which will otherwise leave in experimental errors. ) (8) Make sure that the specimen s vertically aligned, if not a torsions force, rather than axial force, will result. (9) Turn the bottom handle in the close direction as tightly as possible. Visually verify that the sample is gripped symmetrically at its two ends. 10)Zero the extension by pushing zero extension button at the top of the screen. Also zero the load if needed. Wait for a few seconds to let the computer return its value to zero. 2. 4 Tensile Test (1) Enter geometry of the sample before starting. (2) Click on the Start button. twain the upper and bottom grips will start pathetic in opposite directions according to the specify pulling rate. Observe the experiment at a safe distance (about 1. 5 meters away) at an angle and take note of the failure mode when the specimen fails. (NOTE Be sure to wear safety glasses.Do not come close to equipment when the tensile test is running). (3) A plot of tensile stress (Amp) versus tensile strain (mm/mm) will be generated in real-time during the experiment. 2. 6 End of Test (1) The machine will stop automatically when the sample is broken. (2) Press the pop off button on the digital controller. Both the upper and lower grips will be returned to their original positions automatically. 3) Turn the two handles in the open directions to remove the sample (4) iterate the previous steps for any supernumerary tests. 5) When finished, save your file and click Finish. This will export your data into a PDF and singular data files. (6) Clean up any broken fragments from the specimens. (7) Turn off the machine and exit the program when finished. Graph UP (50 mm/mm extension), AS (2 bunk inputs), PLAN, HIDE and Dentally results using raw data files. There should be two tests for each polymer, but Just pick one to graph. Construct the true stress-strain curves for each polymer (hint use Equations 3) and (4) provided in the Introduction section).Calculate Young Modulus for each material and testing condition and compare experimental values with literature values. controvert any differences in mechanical port between the polymers (use pictures ) Analyze the fracture modes of each sample (ductile fracture, brittle fracture, or intercede fracture mode). Using the data for polypropylene, dissertate the effects of strain rate on the mechanical behavior of the polymers. Using the data for polystyrene, compare effects of feed direction on the mechanical behavior. Explain any unexpected results.