Dynamic and Thermal Effects

1. Dynamic and Thermal Effects UAA School of Engineering CE 334 - Properties of Materials Lecture # 19 Dynamics and Thermal Effects

2. Dynamic, Thermal and Triaxiality Effects Objectives: • Dynamic Effects • Thermal Effects • Triaxiality Effects • Testing Methods: for Combined Effects • Transition between ductile and brittle. Dynamics and Thermal Effects

3. What are Dynamic Effects • Most materials behavior varies with changes in load rates. • Important to consider when materials are subjected to vibration or impact loads. Dynamics and Thermal Effects

4. Strain-rate Sensitive Materials Most metals as well as virtually any polymer will have a hardness that is dependent upon the strain-rate of the test. Dynamics and Thermal Effects

5. Yieldand ultimatestrengths tend to increase with increased load rate. Ductility tends to decrease slightly with increased load rate. Fracture strength tends to be relatively unaffected.  Strain-Rate Effects on Stress vs. Strain Diagram Higher load rate Lower load rate  Dynamics and Thermal Effects

6. Effects of Strain Rate The ratio of yield strength to ultimate strength approaches 1 as the strain rate increases. Slow loading Fast loading (10% strain)/0.001 sec. lo=10 in, 5 in/0.005 sec. Steel tension test: (10% strain)/200 sec. lo=10 in, 5 in/1000 sec. Dynamics and Thermal Effects (10% strain)/0.1 sec. lo=10 in, 5 in/0.5 sec.

7. Elastic Aftereffect Elastic aftereffect is a delay in which the instantaneous strain,1, reaches only a fraction of the strain, o, which corresponds to a stress o. Dynamics and Thermal Effects

8. Thermal Effects • Temperature effects are reflected in changes in such properties as yield strength, viscous flow, and ultimate strength. • Yield strength of a material decreases with increases in temperature. • Fracture strength may decrease slightly with increases in temperature. Dynamics and Thermal Effects

9. Review Thermal Dynamics-Isothermal Expansion • An isothermal expansion is one that occurs slow enough that a material absorbs heat from its surroundings and maintains a thermal equilibrium. • When expansion is so fast that there is no time for temperature compensation, the process is termed adiabatic. • If a member is loaded adiabatically to a certain stress and that stress is maintained, further expansion will occur as the material absorbs heat until thermal equilibrium is again obtained. Dynamics and Thermal Effects

10. Isothermal Expansion The resistance to sudden loading is called internal frictionand energy is lost in the process. Adiabatic Process Isothermal Process Energy lost Thermal equilibrium point Dynamics and Thermal Effects

11. Effects of Temperature and Strain Rate This graph show the effect of strain rate on the ultimatestrength of an aluminum specimen at various temperatures. Slow loading Fast loading Dynamics and Thermal Effects

12. Triaxiality Effects - What is Traxiality Triaxiality increases as the three principle stresses become equal. Dynamics and Thermal Effects

13. Testing MethodsDynamic Effects-Impact Testing • The impact test is a method for evaluating the toughness and notch sensitivity of engineering materials. • Charpy, ASTM E23 - A test specimen is machined to a 10mm x 10mm (full size) cross-section, with either a “V” or “U” notch. Specimens can be tested to different temperatures. Dynamics and Thermal Effects

14. Charpy Impact Test • An arbitrary test that measures the energy absorbed by a material during impact loading. • Useful in determining the transitiontemperature of a material. • A number of specimens are created from the material being tested. Each specimen is tested at a different temperature. • Commonly specified test for steels used in cold regions. Dynamics and Thermal Effects

15. Temperature Chamber • Mechanical cooling • to -30°C and heating to 100°C allows long term unattended testing. Dynamics and Thermal Effects

16. Charpy Impact Machine Dynamics and Thermal Effects

17. The Principal Features of a Charpy Impact Machine • Single-blow pendulum • An anvil and a support on which the specimen is placed to receive the blow • A moving mass whosekinetic energyis great enough to cause rupture of the test specimen placed in its path • A means for measuring theresidual energyof the moving mass after the specimen has been broken Dynamics and Thermal Effects

18. Specimen: after failure Dynamics and Thermal Effects

19. Combined EffectsDuctile-Brittle Transition The ductility of a given material will change with variations in triaxiality, load rate, and temperature. The range in which the behavior of a material changes from ductile tobrittle is called the transition range. Dynamics and Thermal Effects

20. Combined EffectsEffects on Yield & Fracture Dynamics and Thermal Effects

21. References • Davis, Troxell, Hauck, The Testing of Engineering Materials, Fourth Edition, McGraw-Hill Book Company, New York, 1982. • Durrant, Holladay, An Introduction to the Properties of Materials, Brigham Young University, 1980. Dynamics and Thermal Effects