ge oil & gas maps measuring total strain stress in...
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GE Oil & Gas MAPS
Measuring total strain stress in components John McCarthy
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Contents
Stress
Residual stress
Measuring stress
Residual stresses in engineering components
Summary
Background
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Why measure stress….?
• Stress is the metric most commonly used by engineers to
judge:
– how hard a material is working
– how close a material is to failure
– how long a material might last
– how severe the effects of an environment might be on a material
– whether there may be some distortion in a material under load
• Strain can be the key metric in
some circumstances
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Applied stress is a result of applied loading
Residual stress is a result of strain during forming
Total stress is the sum of applied and residual stresses
Total vs applied vs residual stress
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Considered in design and test
• “easy” to calculate
• “easy” to measure
• controlled in testing and design
Applied stress
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Usually ignored in design and test
• difficult to calculate
• difficult to measure
• un-controlled in testing and design
• reduced by shake-down
• changes through manufacture
Residual stress
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Creation of residual stresses in a simple beam
Stress
Strain
Stress
Strain
Stress
Strain
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Where do residual stresses come from?
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Influences fatigue life
• Tensile residuals reduce fatigue life
• Compressive residual increase fatigue life
Residual stress – why worry?
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Can promote fracture
• Tensile residuals drive crack growth
• Compressive residuals inhibit crack growth
Residual stress – why worry?
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Can promote plastic collapse
• Residual stress acting with applied stress will promote
• Residual stress acting against applied stress will inhibit
Residual stress – why worry?
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Causes distortion
• “Random” distortion, bad fit, deformed panels, poor quality
• But we can use distortion to understand, eg
Residual stress – why worry?
Almen strips
Deformation from welding
Residual stress
deformation during
pipe manufacture
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How do we measure stress?
Finite element methods (difficult for residual stresses)
Strain gauging (changes in stress only)
Hole drilling (total stress; semi-destructive)
Neutron diffraction (absolute stress in small components; no in-situ measurement)
X-ray diffraction (absolute stress; limited portability; near surface only i.e. depths ~10mm)
Ultrasonic methods (stress has a very small response; swamped by microstructural and temperature effects in industrial applications)
Magnetic methods (total stress; ferromagnetic materials only)
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MAPS Stress Measurement Technology
• Industrially applicable method
of stress measurement – ferromagnetic materials
– absolute biaxial stress
– stress magnitude and direction
– stress depth profiling
– accurate to a few % of material yield
• Portable, rapid, non-
contacting, non-destructive – measurement through most
engineering coatings
– dynamic with potential velocities >7ms-1
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MAPS Validation • Neutron diffraction
– weldments and surface treatments
• X-ray diffraction
– aerospace bearings and rail heads
• Hole drilling
– weldments and rail
• Strain gauging
– engineering components
Centre of Plate
-250
-200
-150
-100
-50
0
50
100
150
200
0 2 4 6 8 10 12
Depth / mm
Stress /M
Pa
s11 MAPS
s11 XRD
Upper plate surface Lower plate surface
-45
0
45
90
0 100 200 300 400 500
X / mm
Mo
st
Te
ns
ile
Ax
is /
°
Hole Drilling 'near'
Hole Drilling 'far'
MAPS
W
e
l
d
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Blind trial on 300M Blind Trial Applied Stresses and MAPS Stresses at 850 Hz by load case
-800
-600
-400
-200
0
200
400
600
800
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13
Load Case
Ap
plie
d S
tre
ss (
MP
a)
Stress from Strain Gauges
MAPS Stress
High strength steel 300M
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Residual stresses in a rolled steel plate
Centre of Plate
-250
-200
-150
-100
-50
0
50
100
150
200
0 2 4 6 8 10 12
Depth / mm
Str
ess /M
Pa
s11 MAPS
s11 XRD
Upper plate surface Lower plate surface
E d g e o f P late
-250
-200
-150
-100
-50
0
50
100
150
200
0 2 4 6 8 10 12
D ep th / m m
Str
es
s /
MP
a
s11 M AP S
s11 X R D
Uppe r pla te surface Low er pla te surface
Stress depth profiles through 12 mm rolled steel plate by MAPS and then XRD with layer removal.
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Residual stresses on welded plate
-300
-200
-100
0
100
200
300
0 100 200 300 400 500
s11
/ M
Pa
X / mm
Hole Drilling 'near'
Hole Drilling 'far'
MAPS
W
e
l
d
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Residual stresses in a butt welded plate
Weld Neutron Diffraction Magnetic Methods
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Residual stresses in a laser shock-peened spring steel (heavy & light)
MAPS measurements
Neutron measurements
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NPL Residual Stress Measurement Project: Ring & Plug sample
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NPL Residual Stress Measurement Project: Ring & Plug sample
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NPL Residual Stress Measurement Project: Ring & Plug sample
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Residual stresses in saw blades
Tight Blade Loose Blade
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Post Weld Heat Treatment in pipes
Potential SCC problem
Comparative measure for measurement of PWHT
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Residual stresses in pipelines
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Residual stresses in Coiled Tubing
Crack
New Fatigued
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Residual stresses in train wheels
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-300
-250
-200
-150
-100
-50
0
50
0 1 2 3 4 5 6
Pri
ncip
al S
tress / M
Pa
Depth / mm
Visit 1 Principal stress 1
Visit 3 Principal stress 1
Residual stresses in train wheels
Visit 1 – 20,000
miles service
Visit 3 – 200,000 miles service
Onset of tensile stress with wheel service
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Residual stresses in bearing races
• Aerospace bearing inspection – surface compressive stresses
desirable to improve wear
– all manufactured components require verification
– MAPS dynamic measurement at 7m/s
– non-intrusive depth profiling
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Residual stresses in bearing races
Hoop stress Cross-track stress
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Alteration of residual stresses from shot peening
Peened: Stresses to 0.15mm Peened: Stresses to 0.5mm
Unpeened: Stresses to 0.15mm
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Changes in residual stress during fatigue
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Effects of variation in residual stress
Three components:
• Steel from same supplier and factory
• One prototype assembly
• One assembly from production at Factory 1 (–50% of reference life)
• One assembly from production in Factory 2 (+50% of reference life)
Rear suspension arm
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Prototype
Factory 1 (–50% life)
Factory 2 (+50% life)
Effects of variation in residual stress
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Flexible riser construction
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Critical asset for deep (to 2km) water oil & gas production
High pressure (typical design pressure of 20MPa)
Up to 400mm bore
25 year design life
Complex unbonded composite structure
Design life of high tensile steel “armour wires” dominated by fatigue generated by pressure, tension and flexure
New API mandates consideration of residual stress
Flexible Risers
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Stress and strain in a bent bar Steel bar deformed by 4 point bend
Compressive stress
Tensile stress
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Stress in a bent bar
0 100 200 300 400 500 600 700 800 900 1000
Axial position (mm)
2947Hz
885Hz
434Hz
226Hz
166Hz
126Hz
70Hz
56Hz
42Hz
33Hz
21Hz
Str
ess
Go
ing
de
ep
er
Scan along here
Compressive stress
Tensile stress
Co
mp
ress
ive
str
ess
Te
nsi
le s
tre
ss
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Strain in a bent bar
0 100 200 300 400 500 600 700 800 900 1000
Axial position (mm)
2947Hz
885Hz
434Hz
226Hz
166Hz
126Hz
70Hz
56Hz
42Hz
33Hz
21Hz
Go
ing d
eeper
Inc
rea
sin
g p
last
icit
y
Scan along here
Compressive stress
Tensile stress
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Measuring stress gives us some useful information about what is going on in a component
Calculating & measuring applied stress is not too difficult
Calculating & measuring residual stress is difficult and so it is ignored
Residual stress distributions can be complex
Summary
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Understanding residual stresses can be important if we are to determine some aspects of component behaviour
Controlling residual stresses can improve component performance
Summary
