Fatigue and Creep

1. Fatigue and Creep UAA School of Engineering CE 334 - Properties of Materials Lecture # 10 Fatigure and Creep

2. Fatigue • What is fatigue ? ASTM D 671: “A general term used to describe the behavior of materials under repeated cycles of stress or strain which cause a deterioration of the material that results in a progressive failure.” • When fatigue should be considered? Whenever a material is subjected torepeated loadingconditions. Fatigure and Creep

3. Fatigue Loading (a) (b) (c) (a) Pulsating Stress (b) Partly Reversed (c) Completely reversed Fatigure and Creep

4. Low Force Torsional Fatigue Testing This system is designed for performing torsional monotonic failure and fatigue tests It is also ideal for determining torsional shear strength and torsional modulus of medical bone screws. Fatigure and Creep

5. Critical Factors of Fatigue Loadings The critical factors of fatigue loadings are: • the StressRange • thenumberof loadcycles. Fatigure and Creep

6. Characteristics of Fatigue • Loss of strength. • Loss of ductility. • Increaseduncertainty of both strength and service life. • Principal cause of these characteristics is the imperfections in the materials. Fatigure and Creep

7. Steps Leading to Fatigue Fracture Nucleation - formation of macroscopic cracks and production of stress concentrations. Crack growth- The propagation of microscopic cracks. Fracture • Fatigue fracture is neither sudden norhidden. It is progressive and discernable. Fatigure and Creep

8. Fatigue Fracture Fatigure and Creep

9. Factors Influencing Premature Fatigue Failure • Design Featuressuch as notches, holes, fillets, uneven surface roughness, or any other feature that tends to create stress concentrations. • Fabrication Cracks. • Temperature. • Speed of Loading. • Corrosive environments. • Detection methods include visual inspection, ultrasonic and X-ray technology. Fatigure and Creep

10. Statistical Nature of Fatigue Fracture I • Cycles endured, n: The number of cycles that a specimen can withstand without fracture. • Fatigue life, N:The numberof cycles required to cause fractureunder a given stresscondition. • Probability Fatigue life, Np: The fatigue life for which p percent of the population will survive. Fatigure and Creep

11. Statistical Nature of Fatigue Fracture II • Fatigue strength: The highest stress that a material can withstand for Ncycles. • Fatigue limit:The higheststress that a material will withstand if it is subjected to aninfinite amountof stress cycles. • S-N diagram: A means of representing the results of fatigue tests or the predicted fatigue life of a material. Fatigure and Creep

12. S-N Diagrams 0.01 0.10 0.50 0.90 0.99 Fatigure and Creep

13. Types of StructuresSubject to Fatigue • Tanks • Pressure vessels • Bridges • Ships • Airplanes • Vehicles Fatigure and Creep

14. Classic Fatigue Failures • Molasses Tank, Boston, Jan. 1919 • 2,300,000 gals of molasses spilt. • 12 people drown or died, 40 injured, several horses drowned. • Houses damaged, part of elevated railway knocked down. • Engineers could not agree on the cause of the failure. • Failures of European welded bridges prior to WW II. • Most failures occurred suddenly at low temperatures and while lightly loaded. • Investigations showed fractures began at welds. Fatigure and Creep

15. More Failures • World War II ship failures • By 1946, 1000 of 5000 merchant ships built developed cracks of considerable size. • By 1952 at least 9 T-2 tankers and 7 Liberty ships had broken completely in two as the result of brittle fatigue fractures. • Studies showed fractures initiated at hatch covers, square cutouts, defective welds, and poor steel quality. • Aircraft Failures • In mid 1950s two Comet aircraft failed catastrophically. • Investigation showed fracture initiated from rivet holes near openings in fuselage. Fatigure and Creep

16. Creep • What is Creep: In a comparatively slow process, the progressively increasing strain causes failure in a stress often much less than required strength to cause failure. • Creep Failure: rupture or excessive distortion • Examples: Many materials at high temperatures Concrete and wood under long termloading Frozen soil under long term loading Fatigure and Creep

17. Creep and Stress Relaxation Tests that characterize material performance under constantstrainor stress conditions fall into the category of creep and stress relaxation. These tests can provide important information as to material or component properties under longer term conditions. Fatigure and Creep

18. Creep Tests • Creep tests are usually performed under constantload or stress conditions. These type of tests are performed by going to a load or stress point, then holding the load or stress value. The resulting increase in strain is recorded over time. • Long term creep tests utilize a special test frame designed specifically for that purpose. Fatigure and Creep

19. Stress Relaxation • Stress relaxation tests are usually performed under constant strain conditions. • This usually involves going to a specific load or strain point then holding the strain value. The resulting decrease in load or stress values is recorded over time. Fatigure and Creep

20. Creep due to Elevated Temperature Strain-Time Curves at Room and at Elevated Temperature Fatigure and Creep

21. Recovery of Strain after Unloading After unloading, some of the strain is recovered, but an appreciable plastic strain has become permanent. Fatigure and Creep

22. Stress Relaxation If the material is subjected to a fixed strain at an initial stress, the load required to maintain the strain is progressively decreasing with time. Relaxation of 0.35% Carbon Steel at 455oC Fatigure and Creep

23. Creep Comparing with Fatigue Unlike repeated loads of fatigue, creep affects the entire body ofthe material under stress instead of producing a localized rupture. Fatigure and Creep

24. The END • No Lab for Fatigue and Creep! • Hardness testing this week! • Have fun!! Fatigure and Creep