welding design 1998/mj1/matjoin2/1 design. lesson objectives when you finish this lesson you will...
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Welding Design
1998/MJ1/MatJoin2/1
Design
Lesson ObjectivesWhen you finish this lesson you will understand:• Mechanical and Physical Properties (structure sensitive and structure insensitive)
Learning Activities1. Read Handbook
pp126-1362. View Slides; 3. Read Notes, 4. Listen to lecture5. Do on-line
workbook6. Do homework
KeywordsStructure 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
Design
Welding Design
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Welding Design
Welding design involves consideration of strength requirements, cost, and service conditions Mechanical & Physical properties Joint Design Welding stress and distortion
Introduction
0.1.1.3.0.T1.95.12
Welding Design
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Welding Design
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Mechanical Properties
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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 (%)
Mechanical Properties
0.1.1.3.1.T2.95.12
Welding Design
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Tensile Test
Tensile test provides a plot of stress versus strain
Elastic Modulus (E) Yield strength Tensile Strength Ductility
Strain
Str
ess,
psi
or
MP
a
Yield Strength
Tensile Strength
Slope = Modulus
Ductility
Mechanical Properties
0.1.1.3.1.T3.95.12
Welding Design
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A = Proportional (Elastic) Limit B = 0.2% Offset Yield StrengthC = Ultimate Tensile Strength Slope = Elastic Modulus
E
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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?
Welding Design
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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
Tension - Compression
Str
es
s
0
Fatigue Design
Welding Design
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Stress Range
Stress Ratio
Welding Design
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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
Initiationsite
Mechanical Properties
0.1.1.3.1.T10.95.12
Welding Design
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Endurance Limit
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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
Fatigue Design
Welding Design
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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
Lap joint
Butt joint
Undercut Intrusion
Mechanical Properties
0.1.1.3.1.T11.95.12
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?
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
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ToughnessAbility of a metal to resist fracture in the presence of
a notch, and to accommodate loads by plastic deformation
• Rate of Straining• Nature of Load - Uniaxial or Multiaxial• Temperature
Conditions Influencing Behavior
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Toughness
Toughness is a measure of the ability of a material to absorb energy prior to failure
Impact energy measured by the Charpy test
Scale
Hammer
Specimen
Charpy V-Notch specimen
Mechanical Properties
0.1.1.3.1.T6.95.12
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Ductile to Brittle Transition
Steels have greatly reduced toughness at lower temperatures
Temperature
En
erg
y ab
sorb
ed f
or
frac
ture
Mechanical Properties
0.1.1.3.1.T7.95.12
Welding Design
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Welding Design
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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
Mechanical Properties
0.1.1.3.1.T8.95.12
Turn to the person sitting next to you and discuss (1 min.):• Consider the two beams. Which will experience less impact stress?
Turn to the person sitting next to you and discuss (1 min.):• Consider the two beams. Which will experience less impact stress?
(1) For a steady load, doubling the length of the beam will double the resulting bending stress
(2) 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
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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
Mechanical Properties
0.1.1.3.1.T5.95.12
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Measures of Ductility
% Elongation at failure % Reduction in area
Mechanical Properties
0.1.1.3.1.T4.95.12
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Linnert, Welding MetallurgyAWS, 1994
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