Lab 9 Tensile Testing Iowa State University

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Stress in metric system is usually measured in N m2 or Pa such that 1 N m2 1 Pa. From the experiment the value of stress is calculated by dividing the amount of force F. applied by the machine in the axial direction by its cross sectional area A which is. measured prior to running the experiment Mathematically it is expressed in Equation 6 1. The strain values which have no units can be calculated using Equation 6 2 In the. equation L is the instantaneous length of the specimen and L0 is the initial length. Equation 6 1,Equation 6 2, A typical stress strain curve would look like Figure 6 2 The stress strain curve. shown in Figure 6 2 is an example of a text book stress strain curve In reality not all. stress strain curves perfectly resemble the one shown in Figure 6 2 This stress strain. curve is typical for ductile metallic elements Another thing to take note is that Figure 6 2. shows an engineering stress strain curve Once a material reaches its ultimate stress. strength of the stress strain curve its cross sectional area would reduce dramatically a. term known as necking,When the computer,software plots the stress. strain curve it assumes that,the cross sectional area. stays constant throughout,the experiment even during. necking therefore causing,the curve to slope down,The real stress strain.
curve could be constructed, directly by installing a gauge which measures the change in the cross sectional area of. the specimen throughout, the experiment Figure 6 2 Various regions and points on the stress strain. 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 T and the true strain. T could be calculated using Equation 3 and Equation 4 respectively The derivation of. these equations is beyond the scope of this lab report Consult any standard mechanics. textbook to learn more about these equations In these equations L0 refers to the initial. length of the specimen L refers to the instantaneous length and refers to the. instantaneous stress,T Equation 6 3,T ln Equation 6 4. Figure 6 2 also shows that a stress strain curve is divided into four regions which. are as follows elastic yielding strain hardening commonly occurs in metallic materials. and necking The area under the curve represents the amount of energy needed to. accomplish each of the events The total area under the curve up to the point of. fracture is also known as the modulus of toughness This represents the amount of. energy needed to break the sample which could be compared to the impact energy of the. sample determined using Impact test The area under the linear region of the curve is. known as the modulus of resilience This represents the minimum amount of energy. needed to deform the sample, The linear region of the curve of Figure 6 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 will return to its original shape when a force is released while the. material is in its elastic region The slope of the curve which could be calculated using. Equation 6 5 is a constant and is an intrinsic property of a material is known as the. elastic modulus E In metric unit it is usually expressed in Pascal Pa. E Equation 6 5, Figure 6 3 a shows the typical stress strain curves of polymers 2 The figure. shows that materials that are hard and brittle do not deform very much before breaking It. has a very steep modulus of elasticity and a short stress strain curve. The mechanical property of polymers generally depends on their degree of. crystallinity molecular weights and glass transition temperature Tg Highly crystalline. polymeric materials with a Tg above the room temperature are usually brittle and vice. versa When a semi crystalline polymer undergoes a tensile test the amorphous chains. will become aligned This is usually evident for transparent and translucent materials. which become opaque upon turning crystalline Fibers are often added to polymers a. term known as composite materials to improve its mechanical properties In addition to. providing extra strength to a polymer fibers help prevent crack propagation Moreover. the presence of the fibers prevent the amorphous portion of the polymer chains from. aligning themselves when subjected to a tensile force therefore in most cases making. them brittle Figure 6 3 b shows a diagram showing the mechanical property of some. common polymers 3, Figure 6 3 a A plot of stress strain curves of typical polymeric materials b A summary diagram of. the properties of common polymers,2 Experimental Procedure. A website of An Introduction to Stress Strain Curve. http www shodor org zbrewer weave2 tutorial node4 html. A website of An Introduction to Polymer Processing. http islnotes cps msu edu trp back l tensil html, Make sure you wear protecting glasses before starting any operation Your eyes. could be hurt by a broken piece of polymer,2 1 Specimen Preparation.
1 The polymer specimens are dog bone shaped They were injection molded and its. dimensions were determined according to the ASTMD 638 mentioned earlier in the. introduction, 2 Measure the thickness width and gage length of polymer samples using a pair. vernier calipers These dimensions should be approximately the same for each sample. Note HDPE LDPE GFPP and nylon will be used in the Lab. 2 2 TRAPEZIUM2 Software setting, 1 Go to desktop and double click on the TRAPEZIUM2 icon A Login window. will appear Go to the Login box and type user in Username box and then type. user in the Password box, 2 The main window will be displayed on the computer screen Figure 6 4. Figure 6 4 Main Window of TRAPEZIUM2 Software, 3 Click on New icon that is located on the top left side of the main. window The Test Wizard window will be displayed Figure 6 5. Figure 6 5 Test Wizard Window of TRAPEZIUM2 Software. 4 Go to the Test Wizard window and click on the Method data option of the test. wizard toolbar located on the left hand side of the test wizard window The method. data window will appear Select on the appropriate testing method determined by your. instructor, Note Ask your instructor for the appropriate testing method slow medium or.
fast pulling rate since every polymer samples have different testing conditions. 5 In order for the computer to calculate the stress applied on the sample the cross. sectional dimension of the specimen must be entered into the software To do so click. on the Specimen option of the test wizard toolbar Then enter the measured width. and thickness of the specimen this step is optional for automatic stress strain curve. generation,2 3 Instrumental Setting, 1 Go to the tensile testing instrument Press on the Return button on the digital. controller for a few seconds until a beeping sound is heard The sample grips both the. top and bottom grips will be returned automatically to its starting position. 2 Place the polymer sample at the bottom grip While still holding it vertically with one. hand use another hand to turn its handle in the closing direction as tightly as possible. Note The specimen should be gripped such that the two ends of the. specimen are covered by the grip approximately 3 mm away from its gage length It. is important that the specimens are tightly gripped onto the specimen grips to. prevent slipping which will otherwise result in experimental errors The Open and. Close direction of the handle is noted on the grip. 3 Use the Up or and Down buttons which are located next to the Return button. to adjust the position of the upper grip, Note Make sure that the specimen is vertically aligned if not a torsional. force rather than axial force will result, 4 Turn the upper handle to close direction as tightly as possible Visually verify if the. sample is gripped symmetrically equidistant at its two ends. 2 4 Starting Tensile Test, 1 Go to the computer At the top of the main window right click on the mouse while. placing the mouse cursor on the Force button located at the top of the main window. and select zero option Wait for the machine to, return the force to zero This will be indicated by.
a beeping sound,2 Similarly place the mouse cursor on the. Stroke button which is next to the Force,button and right click to select the zero option. Again wait for a few seconds to let the computer,return its value to zero. 3 Click on the Start icon that is, located at the top of the main window The Start Testing window will appear Figure. 4 Click on the Begin Test button found on the Start Testing window Both the upper. and bottom grips will start moving in opposite directions according to the specified. Figure 6 6 A picture of the Start,Testing window, 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, 5 A plot of Force kN versus Stroke mm will be generated in real time during the. experiment,2 5 Finishing, 1 The machine will stop automatically when the sample is broken Click the icon. Export and type a file name in the box TXT, 2 Turn the two handles to their OPEN direction one at a time to remove the sample. 3 Press the Return button on the digital controller Both the upper and lower grips. will be returned to their original positions automatically. 4 Repeat step 4 of section 2 2 of the procedure to run more samples Otherwise let. your teaching assistant to print out each of the graphs obtained in the experiments as well. as the file of the generated force and stroke table. 5 Clean up any broken fragments from the specimens. 3 Assignments,Do the following for each of the polymer sample. 3 1 Construct the true stress strain curve hint use the equations 3 and 4 provided in. the Introduction section, 3 2 Calculate the Young s Modulus for each curve and compare with the literature values.
3 3 Analyze the fracture modes of each sample ductile fracture brittle fracture or. The topic of this lab is confined to the tensile property of polymers Figure 6 1 shows a tensile testing machine which looks similar to the one used in this lab This test is a destructive method in which a specimen of a standard shape and dimensions prepared according to ASTM D 638tandard test method for tensile properties of s

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