MFGT104 Materials and Quality Chap 14: Tensile Testing Viscosity and Melt Index

1. MFGT104Materials and Quality Chap 14: Tensile TestingViscosity and Melt Index Professor Joe Greene CSU, CHICO MFGT 104

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.

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)

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

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?

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?

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

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 Yield stress 1000 psi Stress Yield strength Slope=modulus 0.002 in/in Strain

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?

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

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

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

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.

14. Creep Results Fixed lF l0 Tertiary Creep Creep (in/in) Secondary Creep Constant Load Primary Creep Time (hours) • Creep versus time

15. Physical Testing • Melt Flow Index

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

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) PolymerTempMass • HDPE 190C 10kg • Nylon 235C 1.0kg • PS 200C 5.0Kg

18. Viscosity T=200 T=300 Ln T=400 0.01 0.1 1 10 100 Ln shear rate, • 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

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.