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Idea Transcript
Tensile Properties & the Tension Test Christoph Leser
Tension Test Take a sample of the material and increase the load until it fails. Simple enough BUT… For this test to be repeatable in labs around the world, the sources of variability in the test results must be minimized. e.g. » If one lab uses 1/4 inch diameter specimens, a lab using 1 inch diameter specimens might get a different result. » If one lab applies load at 500 MPa/min, another lab using 1000 MPa/min might get a different result.
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Stress - Strain We, however, are interested in the stress-strain diagram. Load σ = engineering stress = A0 Where AO is the original area of the specimen L-L0
ε = engineering strain = L0 X 100% Where LO is the original length of the specimen A typical stress – strain graph is shown in the figure below. SUT Sy Unstable deformation “necking” 0 Page 10
0.2%
Homework Tension Test 1. Using the TestWorks software in simulator mode, run a tension test and collect load-extension data for the Acetal specimen. Complete a post-test analysis on the test data and determine the following properties: a. Modulus of Elasticity b. Offset Yield Strength (0.2%) c. Ultimate Tensile Strength (UTS) d. elongation at fracture 2. Comment on the use of an extensometer for this test. Is it required? What precautions must be taken during an actual test? Why? 3. A connector link in a clutch mechanism is a two-force member which will experience a 4.5 kN tensile load in service. It is currently made of a mild steel. Would the Acetal tested in Problem 1 make a good replacement for the steel? (It's less expensive and easier to fabricate.) Calculate link dimensions that provide a safety factor of 5 guarding against static tensile overload. 4. Repeat Problem 1 for the mild steel specimen. 5. Report the three different yield strengths that are evident in the data for the mild steel tension test simulation. Which would you choose as the "official" yield strength? Why? Page 17
Lab Notes Tension Test
Objective Determine the tensile properties for samples of candidate design materials. The two materials under consideration are mild steel and an acetal copolymer. Tensile properties of interest are the yield stength, ultimate tensile strength, modulus of elasticity and elongation at fracture. Apparatus and Materials 1. MTS C42 electromechanical test system with TestSuite software 2. Mild steel specimens (standard round geometry; ref. ASTM E8) 3. Acetal specimens (standard "dogbone" geometry; ref. ASTM D638) 4. Micrometer or dial caliper Principles and Background The tension test is one of the oldest and most useful of the material property tests. The properties it determines (elastic modulus, yield strength, elongation at fracture, to name a few) are very useful to the design engineer. The electromechanical test system uses an electric motor and a gear train to drive one, two or four power screws and move a crosshead up or down. The specimen is gripped between a stationary reference and the moveable crosshead, and stretched or compressed as the crosshead moves up or down. A control system and software can implement various control modes such as load, strain or displacement control. Historically, before the advent of control systems, the crosshead was driven by the motor at a constant rate of displacement. While load and strain control modes see extensive use in modern testing systems, a constant rate of displacement is still widely used and will be the control mode used for this exercise. Page 18