Design

1. Design

2. Design • Learning Activities • Read Handbook pp126-136 • View Slides; • Read Notes, • Listen to lecture • Do on-line workbook • Do homework • Lesson Objectives • When you finish this lesson you will understand: • Mechanical and Physical Properties (structure sensitive and structure insensitive) Keywords Structure Sensitive Properties, Structure Insensitive Properties, Stress, Strain, Elastic Modulus, Yield Strength, Tensile Strength, Ductility, Elongation, Proportional Limit, Fatigue, Stress Range, Stress Ratio, Endurance Limit, Toughness, Charpy, Ductile, Brittle, Hardness, Creep

3. Introduction Welding Design • Welding design involves consideration of strength requirements, cost, and service conditions • Mechanical & Physical properties • Joint Design • Welding stress and distortion 0.1.1.3.0.T1.95.12

5. Mechanical Properties

6. Mechanical Properties Stress and Strain • Stress is defined as force per unit area • Pounds per square inch, psi • Megapascals (Newtons/mm2), MPa • Strain is defined as change in dimension divided by original dimension • Expressed as percent (%) 0.1.1.3.1.T2.95.12

7. Tensile test provides a plot of stress versus strain Elastic Modulus (E) Yield strength Tensile Strength Ductility Mechanical Properties Tensile Test Tensile Strength Stress, psi or MPa Yield Strength Slope = Modulus Ductility Strain 0.1.1.3.1.T3.95.12

8. A = Proportional (Elastic) Limit B = 0.2% Offset Yield Strength C = Ultimate Tensile Strength Slope = Elastic Modulus

10. Questions? • Turn to the person sitting next to you and discuss (1 min.): • The elastic portion of the stress-strain curves for steel, rubber, aluminum and tungsten are plotted here. Which material is which?

11. Fatigue Design Fatigue • Fatigue is material failure due to cyclic loading • Cyclic rather than static loading • Tension - compression • Tension - tension • Occurs at stress levels below the tensile strength Tension - Tension Stress 0 Tension - Compression

12. Stress Range Stress Ratio Linnert, Welding Metallurgy AWS, 1994

13. Linnert, Welding Metallurgy AWS, 1994

14. Mechanical Properties Fatigue Appearance • Distinct fracture surface has a characteristic texture • Concentric line pattern • Smooth portion referred to as clamshell texture • Sources of fatigue • Cracks • Notches • Sharp corners Initiation site 0.1.1.3.1.T10.95.12

15. Endurance Limit Linnert, Welding Metallurgy AWS, 1994

16. Fatigue Design Factors Affecting Fatigue • Welds have pre-existing stress risers or initiation sites from which fatigue cracks can grow • Slag intrusions • Undercut • Weld toe radius • Other factors • Butt joints vs. lap joints • Sharp corners, notches Undercut Intrusion Smooth weld toe

17. Mechanical Properties Fatigue of Welds • In general, welds have pre-existing stress risers or initiation sites from which fatigue cracks can grow • Slag intrusions • Undercut • Hardness variations • Design considerations • Butt joints rather than lap joints Undercut Intrusion Butt joint Lap joint 0.1.1.3.1.T11.95.12

18. Questions? • Turn to the person sitting next to you and discuss (1 min.): • In the previous discussion we looked at stress cycles where both the min. and max. stress were positive. What do you think might happen if the minimum stress were compressive like the bottom curve?

19. Questions? • Turn to the person sitting next to you and discuss (1 min.): • In the previous discussion we looked at stress cycles where both the min. and max. stress were positive. What do you think might happen if the minimum stress were compressive like the bottom curve? Goodman Diagram

20. Toughness Ability of a metal to resist fracture in the presence of a notch, and to accommodate loads by plastic deformation Conditions Influencing Behavior • Rate of Straining • Nature of Load - Uniaxial or Multiaxial • Temperature

21. Mechanical Properties Toughness Scale • Toughness is a measure of the ability of a material to absorb energy prior to failure • Impact energy measured by the Charpy test Hammer Specimen Charpy V-Notch specimen 0.1.1.3.1.T6.95.12

22. Mechanical Properties Ductile to Brittle Transition • Steels have greatly reduced toughness at lower temperatures Energy absorbed for fracture Temperature 0.1.1.3.1.T7.95.12

23. Linnert, Welding Metallurgy AWS, 1994

25. Linnert, Welding Metallurgy AWS, 1994

26. Mechanical Properties Effect of Discontinuities on Properties • Fracture mechanics - analysis of the failure of structural materials with pre-existing flaws • Fracture toughness testing is used for brittle materials or thick sections • Strain rate • Temperature • Thickness 0.1.1.3.1.T8.95.12

27. Questions? • Turn to the person sitting next to you and discuss (1 min.): • Consider the two beams. Which will experience less impact stress?

28. Questions? • Turn to the person sitting next to you and discuss (1 min.): • Consider the two beams. Which will experience less impact stress? • For a steady load, doubling the length of the beam will double the resulting bending stress • For an impact load, doubling the length of the beam will reduce the resulting impact stress to 70.7% of the original. • See “Design of Weldments” p 3.1-6 Lincoln Arc Welding Foundation

29. Mechanical Properties Hardness • Hardness - resistance to indentation • Measured by pushing an indenter into the surface of a material • Wear resistant materials have high hardness • Hardness can be correlated to tensile strength 0.1.1.3.1.T5.95.12

30. Mechanical Properties Measures of Ductility • % Elongation at failure • % Reduction in area 0.1.1.3.1.T4.95.12

31. Creep Linnert, Welding Metallurgy AWS, 1994

34. Homework