Measurement of residual stresses in weldments

1. MEASUREMENT OF
RESIDUAL STESS IN
WELDMENTS
N PRAKASAN
ME METALLURGY
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2. Measurement of Residual Stress in weldments
INTRODUCTION
Residual
Stress
 Residual
stress refers to the stress that exists in a
weldment after all external loads have been removed.
 Other terms: internal stress, initial stress, inherent
stress, reaction stress and locked-in stress.
 The residual stress that occurs when a structure is
subjected to nonuniform temperature change is usually
called thermal stress.
 The main causes are
• Differential plastic flow,
• Differential cooling rates,
• Phase transformations with volume changes,
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3. Measurement of Residual Stresses in weldments
INTRODUCTION
Residual
Stress
 Tensile
stress : Weld metal and adjacent base metal
 Compressive stress: Areas further away from weld
metal. Weld is stretched by plates.
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4. Measurement of Residual Stresses in weldments
INTRODUCTION
 Residual
Stress distribution in the weldment
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5. Measurement of Residual Stresses in weldments
Classification
of Techniques
Measurement of Residual Stress:
Stress relaxation techniques,
•
•
•
•
for
the
Sectioning using electric-resistance strain gauges
Mathar-Soete drilling
Gunnert drilling
Rosenthal-Norton sectioning
Diffraction
techniques (NDT),
• X-ray diffraction technique
• Neutron diffraction technique
Ultrasonic
techniques (NDT),
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6. Measurement of Residual Stresses in weldments
MEASUREMENT
BY STRESS RELAXATION
The
residual stress is determined by
measuring the elastic strain release.
The
determination of residual stress takes
places when the stress is relaxed by cutting
the specimen into pieces or by removing a
piece from the specimen.
Electric
or mechanical strain gauges are used
to measure the strain release.
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7. Measurement by stress relaxation
Sectioning
using electric-resistance strain
gauges:


Electric-resistance strain gauges are
mounted on the surface of the test
specimen. A small piece of metal
containing the gauges is removed
from the specimen.
The strain that takes place during
the removal of the piece is
determined.

The strain measured is elastic strain
that resulted from the residual stress
distribution in the specimen.
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8. Measurement by stress relaxation
Sectioning
using electric-resistance strain
gauges:

The Residual stresses can be determined by substituting
measured values of the strain components
, into
the following equations:
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9. Measurement by stress relaxation
Mathar-Soete drilling technique:




When a small circular hole is drilled
in a plate containing residual stress,
the stress in the areas around the
hole is partially relaxed.
The residual stress that existed in
the drilled area is determined by
measuring the stress relaxation.
Three strain gauges are placed at
120O from each other, and then a
hole is drilled in the center.
The magnitudes and directions of
the principle stresses are calculated
by measuring strain changes at the
three gauges.
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10. Measurement by stress relaxation
 Gunnert




drilling technique:
This technique involves, the drilling
of four (3 mm) holes through the
plate perpendicular to the surface at
the measuring point in a circle 9 mm
in diameter.
A groove 0.63 in. (16 mm) from the
measuring point is then trepanned
around the holes in steps, and
measurements are made.
The diametrical distance between
these holes at different levels below
the surface of the plate is measured
by means of a specially designed
mechanical gauge. The perpendicular
distance between the plate surface
and the gauge location at different
levels below the surface is also measured.
The residual stress present in the interior of the specimen can be
calculated based on the measurement data.
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11. Measurement by stress relaxation
Rosenthal-Norton sectioning technique:




This technique is used for determining residual stress in a thick
weldment.
Two narrow thin blocks, one parallel to the weld and the other
transverse to the weld are sectioned
with full thickness of plate.
Strain gauges are mounted on the
walls of the blocks and strain
relaxation is measured that results
from slicing the blocks into small
pieces.
Two blocks, one longitudinal and
one transverse, must be cut in
order to determine the
three-dimensional stress distribution.
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12. Measurement of Residual Stresses in weldments
MEASUREMENT
BY DIFFRACTION :
Elastic strains in metals can be determined by
measuring the strain on lattice planes using
diffraction techniques.
When a polycrystalline metal is placed under
stress, the elastic strains in the metal are
manifested in the crystal lattice of the individual
grains of the metal.
The diffraction techniques are capable of
measuring the inter-atomic spacings, which are
indicative of elastic strain.
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13. Measurement by Diffraction
X-ray diffraction: Principle:


The basic principle is the usage of the Debye-Scherrer method
measuring the shift of the reflections or peaks, in the region of low
or high 2θ (Bragg’s angle).
The lattice strain will be measured in directions that are oblique and
inclined to the surface of the specimen in a plane which includes
the direction of the stress to be measured.
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14. Measurement by Diffraction

Recommended X-ray tube and {hkl} plane for some common
materials
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15. Measurement by Diffraction
X-ray diffraction:
 Position
sensitive scintillation detector (PSSD) based Xray Diffractometer and Stress X-ray diffractometers are
widely used for residual stress measurements.
techniques measure stresses from 2 µm to 50 µm
into the surface, however, subsurface and internal
stress cannot be measured nondestructively.
 This
 To
measure internal stress with X-ray techniques, the
weldment must be systematically sectioned, and the
surfaces to be measured must be electropolished.
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16. Measurement by Diffraction
Neutron diffraction:
 Neutron
sources:
 Fission
• predominantly employed in steady-state nuclear reactors.
• Continuous sources,
• a monochromatic beam of neutrons is produced by using a
monochromating device to select a given neutron wavelength.
 Spallation
• usually in pulsed sources
• the neutron beam usually consists of a series of short pulses each
containing a spectrum of wavelengths.
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17. Measurement by Diffraction
Neutron diffraction:
 Principle:




When a monochromatic radiations fall on a sample, the diffraction
from a Brag plane results in the form of a cone (Debye Scherrer
cone), with angle 2θ.
The diffracted beam is shaped using suitable optical devices
The intensity profile is recorded as a circle on a two dimensional
detector.
Diffraction from polycrystalline sample in a Debye Scherrer cone :
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18. Measurement by Diffraction
Neutron diffraction:
 This
technique is capable of measuring internal stress
without destroying the weldment.
 Neutrons penetrate several inches of metal and can be
diffracted from the internal volume of a metal weldment
to measure internal residual stresses.
 Optimal resolution and 3 Dimensional maps can be
obtained.
 The
volume resolution of the neutron stress
measurements is 10 mm3.
 The equipment is expensive
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19. Measurement by Ultrasonic method
Ultrasonic method
 Ultrasonic
stress measurement techniques are based
on the acoustic-elasticity effect, according to this, the
velocity of elastic wave propagation in solids is
dependent on the mechanical stress.
 In this method, waves are transmitted by a transducer,
propagate through a region of the material and are
detected by a receiving transducer.
 Ultrasonic measurement configurations:
Throughthickness
pulse-echo
throughthickness
pitch-catch
surface pitchcatch
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20. Measurement by Ultrasonic method
 Ultrasonic
Equipment and Software for Residual
Stress Measurement.

The Ultrasonic Computerized Complex (UCC) with an Expert
System (ES) software developed for analysis of the influence of
residual stresses on the fatigue life of welded components.
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21. Measurement by Ultrasonic method
Ultrasonic method
 This
technique is effective for the analysis of residual
stresses in the interior of the material.
 Advantages:
•
•
•
•
•
Depth is higher than that obtained by X-ray method
Quick method
inexpensive
Portable
Free of radiation
 Disadvantages:
• Low resolution
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22. Measurement by stress relaxation
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23. Measurement of Residual Stresses in weldments
 References
:
1. AWS Residual stress and Distortion.
2. N.S.Rossini, M.Dassisti, K.Y. Benyounis and A.G.Olabi, Methods
of Measuring Residual Stresses in Components.
3. Determination of Residual Stresses by X-ray Diffraction - Issue 2,
National physical laboratory.
4. Y. Kudryavtsev and J. Kleiman, Measurement of Residual
Stresses in Welded Elements and Structures by Ultrasonic
Method, IIW Document XIII-2339-10
5. Structural and Residual Stress Analysis by Non-destructive
Methods, Evaluation, Application, Assessment. Viktor Hauk,
Elsevier, 1997.
6. CEN TC 138, Non Destructive testing – Test Method for Residual
Stress by X-Ray Diffraction.
7. Ceramic Data Supplied by Stresstech, Tikkutehtaantie 1, FIN40800, VAAJAKOSKI, Finland.
8. IAEA-TECDOC-1457, Measurement of residual stress in materials
using neutrons
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