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1 MFGT104 Materials and Quality Chap 14: Tensile Testing Viscosity and Melt Index Professor Joe Greene CSU, CHICO MFGT 104

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2 Chap 14: Tensile Testing Objectives –Recognize and define common terms related to tensile testing. –Describe the terms stress and strain and the effects of both in tensile loading. –List the equipment necessary to conduct a tensile test. –Describe the operation of various equipment related to tensile testing. –Relate the general procedures used in conducting a tensile test. –Perform the necessary calculations related to tensile testing. –Recognize expected tensile test results. –Describe common variations in standard tensile test procedures including creep testing.

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3 Introduction Tensile tests are conducted on standard tensile specimen “dog- bone” shape, but any rectangular cross section will work. Tensile tests are used to determine –elastic limit and percent elongation –tensile strength and modulus of elasticity –proportional limit –yield point and yield strength Tensile test are conducted with ASTM standards –metals (Section E8) –plastics (Section D 638 –fibers (Section D 2343) –adhesives (Section D 897) –paper (Section D 987) –rubber (Section 412)

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4 Tensile Testing Principles Tensile loads are those that tend to pull a sample apart Tensile loads produce deformations. Deformation is a change in the form of a specimen that is produced by the applied load. Tensile stress is load per unit area. For rectangular bars For circular specimens

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5 Tensile Test Examples Example What is the stress developed in a rectangular specimen that is 0.5in x 0.5in at 1000lb tensile load? What is the stress developed in a round specimen with a 0.505 in. diameter at 1000lb tensile load?

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6 Elongation and Strain When the tensile test begins and the load is increased the specimens grows longer as it is pulled. Initial cross sectional area is used. A two point gage punch is typically used. A extensometer or strain gage can be used. Elongation is the final length minus the original length. Strain is the elongation divided by the initial length. Example, Given the final length of a specimen as 1.005 in and the original length of 1.000 in, what is the strain?

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7 Poisson’s Ratio Poisson’s ratio is the ratio of lateral strain to axial strain Deformation occurs in two directions during tensile testing –Lateral direction- at right angles to the test –Axial direction- at the ends of the tensile bar –Example If the lateral strain is 0.005 and the axial strain is 0.010, what is the Poisson’s ratio? Most engineering materials, the values for Poisson’s ratio (P.R.) range from 0.25 to 0.7. Approx. 0.3 for plastics

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8 Testing Procedure Tensile tests yield a tensile strain, yield strength, and a yield stress Tensile modulus or Young’s modulus or modulus of elasticity –Slope of stress/strain Yield stress –point where plastic deformation occurs –Some materials do not have a distinct yield point so an offset method is used Stress Strain 0.002 in/in 1000 psi Yield stress Yield strength Slope=modulus

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9 Tensile Calculation Examples Suppose we want to use the length of a cable car to pull a cable car weighing 1200 lb plus 6 passengers at 175 lbs each. What would be the minimum diameter of the cable have to be if the cable’s yield strength was 32,000 psi?

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10 Test Specimens Tensile test specimens –Standard shape is a tensile bar with narrow midsection. Smaller midsection ensures proper necking at the center and not necking at the grips or off centered. –Ends can be flat or threaded –Alternative bar shapes Straight bar round square

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11 Testing Machines Testing machines are employed to apply measurable loads –portable hand-held devices with capacities 2,000 to 5,000 lb –stationary machines 300,000 lb or more Tensile test is used to determine the tensile properties of a material, tensile strength, elongation, modulus Procedure –measure specimen’s length, width, cross sectional area –place specimen is grippers –apply a steady load on the sample, ie. Pull sample at 0.05in per min –Note: very fast pulling rates can lead to different results. –measure the deformation or length of the specimen –continue test until fracture

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12 Expected Results Stress is measured load / original cross-sectional area. True stress is load / actual area. True stress is impractical to use since area is changing. Engineering stress or stress is most common. Strain is elongation / original length. Modulus of elasticity is stress / strain in the linear region Note: the nominal stress (engineering) stress equals true stress, except where large plastic deformation occurs. Ductile materials can endure a large strain before rupture Brittle materials endure a small strain before rupture Toughness is the area under a stress strain curve

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13 Creep Testing Creep –Measures the effects of long-term application of loads that are below the elastic limit if the material being tested. –Creep is the plastic deformation resulting from the application of a long-term load. –Creep is affected by temperature Creep procedure –Hold a specimen at a constant elevated temperature under a fixed applied stress and observe the strain produced. –Test that extend beyond 10% of the life expectancy of the material in service are preferred. –Mark the sample in two locations for a length dimension. –Apply a load –Measure the marks over a time period and record deformation.

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14 Creep Results Creep versus time Creep (in/in) Time (hours) Primary Creep Secondary Creep Tertiary Creep l0l0 lFlF Constant Load Fixed

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15 Physical Testing Melt Flow Index

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16 Melt Index Melt index test measure the ease of flow for material Procedure (Figure 3.6 from MFGT041 book) –Heat cylinder to desired temperature (melt temp) –Add plastic pellets to cylinder and pack with rod –Add test weight or mass to end of rod (5kg) –Wait for plastic extrudate to flow at constant rate –Start stop watch (10 minute duration) –Record amount of resin flowing on pan during time limit –Repeat as necessary at different temperatures and weights

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17 Melt Index and Viscosity Melt index is similar to viscosity Viscosity is a measure of the materials resistance to flow. –Viscosity is measured at several temperatures and shear rates –Melt index is measured at one temperature and one weight. High melt index = high flow = low viscosity Low melt index = slow flow = high viscosity Example, (flow in 10 minutes) Polymer Temp Mass –HDPE190C 10kg –Nylon235C 1.0kg –PS200C 5.0Kg

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18 Viscosity Kinematic viscosity, , is the ratio of viscosity and density Viscosities of many liquids vary exponentially with temperature and are independent of pressure where, T is absolute T, a and b units are in centipoise, cP Ln shear rate, Ln 0.010.1110100 T=400 T=300 T=200

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19 Summary Tensile test provide important data, which is often used to identify the physical and mechanical properties of materials. Tensile test can be used to construct stress-strain curves. Important information gathered from tensile testing are –tensile strength, yield, ultimate –tensile elongation, ultimate –tensile modulus or Young’s modulus –Poisson’s ratio During tensile testing materials exhibit elastic and plastic deformations Creep testing is a long term test which can be conducted at elevated temperatures.

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