Ultrasonic Measurement of
Residual Stresses
Ultrasonic Method of Residual Stress Measurement
One of the
promising directions in development of non-destructive techniques for residual
stress measurement is application of ultrasound. Ultrasonic stress measurement
techniques are based on the acoustic-elasticity effect, according to which the
velocity of elastic wave propagation in solids is dependent on the mechanical
stress. The relationships between the changes of the velocities of longitudinal
ultrasonic waves and shear waves of orthogonal polarization under the action of
tensile and compressive external loads in steel and aluminum alloys are
presented in Figure 1. As can be seen from Fig. 1, the intensity and character
of such changes could be different, depending on material properties.
Different
configurations of ultrasonic equipment can be used for residual stress
measurements. In each case, waves are launched by a transmitting transducer,
propagate through a region of the material and are detected by a receiving
transducer as shown in Figure 2 [6]. The technique when the same transducer is
used for excitation and receiving of ultrasonic waves is often called
pulse-echo method (Figure 2a). This method is effective for analysis of
residual stresses in the interior of material. In this case the
through-thickness average of residual stresses is measured. In the
configuration shown in Figure 2c, the residual stress in a surface/subsurface
layer is determined.
∆C/C ·10-3
σ, MPa
1 2 3
Figure 1.
Change of ultrasonic longitudinal wave velocity (C L) and shear waves velocities
of orthogonal
polarization (C SX3; C SX2) depending on the mechanical stress σ in steel A
(1), steel B (2) and aluminum alloy (3) [10]:
● - C SX3; ○
- C SX2; x - C L
Figure 2.
Schematic view of ultrasonic measurement configurations:
(a)
through-thickness pulse-echo, (b) through-thickness pitch-catch and (c) surface
pitch-catch
The depth of
this layer is related to the ultrasonic wavelength, often exceeding a few
millimeters, and hence is much greater than that obtained by X-ray method.
Other advantages of the ultrasonic technique are the facts that the
instrumentation is convenient to use, quickly to set up, portable, inexpensive
and free of radiation hazards.
In the
proposed in [7, 9, 10] technique, the velocities of longitudinal ultrasonic
wave and shear waves of orthogonal polarization are measured at a considered
point to determine the uni- and biaxial residual stresses. The bulk waves in
this approach are used to determine the stresses averaged over the thickness of
the investigated elements. Surface waves are used to determine the uni- and
biaxial stresses at the surface of the material. The mechanical properties of
the material are represented by the proportionality coefficients, which can be
calculated or determined experimentally under an external loading of a sample
of considered material.
In general, the change in the ultrasonic wave
velocity in structural materials under mechanical stress amounts only to tenths
of a percentage point. Therefore the equipment for practical application of
ultrasonic technique for residual stress measurement should be of high
resolution, reliable and fully computerized.
Ultrasonic Equipment and
Software for Residual Stress Measurement
Examples of Residual Stress
Measurements Using the Ultrasonic Method
Specimen for Fatigue
Testing
Compound Pipes and Pipes
with Surfacing
Measurement of Residual
Stresses in Welded Samples
Measurement of Residual
Stresses in Welded Structures
References
Ultrasonic Equipment and Software for Residual Stress Measurement UltraMARS
Figure 3.
Using of UltraMARS System for Residual Stress Measurement in Laboratory
The developed equipment allows one to determine the magnitudes and signs of uni- and biaxial residual and applied stresses for a
wide range of materials as well as stress, strain and force in various size
fasteners. The sensors, using quartz plates measuring from 3*3 mm to 10*10 mm
as ultrasonic transducers, are attached to the object of investigation by
special clamping straps (see Figure 3) and/or electromagnets.
The main technical characteristics of the
measurement unit:
- stress can be measured in materials with thickness 2 - 150 mm;
- weight of unit with sensors: 6 kg
- error of stress determination (from external load): 5 -10 MPa;
- error of residual stress determination: 0.1 sy (yield strength) MPa;
- stress, strain and force measurement in fasteners (pins) 25-1000 mm
long;
- independent power supply
(accumulator battery 12 V);
- overall dimensions of measurement device: 300x200x150 mm;
The
supporting software allows controlling the measurement process, storing the
measured and other data and calculating and plotting the distribution of
residual stresses. The software also allows an easy connection with standard
PC’s.
An example of
presentation of the residual stress measurement data, using the developed
software, is shown in Figure 4. The software allows comparing different data on
residual stress measurement and transferring selected data for further fatigue
analysis. In Figure 4, the left side of the screen displays information on the
measured ultrasonic wave velocities as well as other technical information on
the sample. The right side of the screen displays the distribution of
calculated residual stresses.
In the
example of residual stress measurement presented in Figure 4, a plate made from
low carbon steel, with yield strength of 296 MPa, was heated locally, with the
focal point of heating located approximately 50 mm from the left side of the
plate. The distribution of both components of residual stresses in the
specimen, as a result of this local heating are shown in the right side of Figure 4. As
can be seen, in the heating zone, both residual stress components are tensile
and reach the yield strength of the considered material. In the compression
zone, located between the edge of the plate and the centre of the heating zone,
the longitudinal component of residual stresses reaches minus 140 MPa.
Figure 4. Distribution of residual stresses in a
low carbon steel plate
after local
heating [10].
Examples of Residual Stress Measurements Using the Ultrasonic Method
One of the
main advantages of the developed technique and equipment is the possibility to
measure the residual and applied stresses in samples and real structure
elements. Such measurements were performed for a wide range of materials, parts
and structures. A few examples of the practical application of the developed
technique and equipment for residual stress measurement based on using of the
ultrasonic technique are presented below.
Specimen for Fatigue Testing
The residual
stresses were measured in a 500x160x3 mm specimen made of an aluminum alloy (σy = 256 MPa, σu = 471 MPa)
with a fatigue crack. The residual stresses were induced by local heating at a
distance of 30 mm from the centre of the specimen.As can be seen from Figure 5, in the heating zone,
both components of the residual stress are tensile. In the compression zones,
the longitudinal component of residual stresses reaches minus 130 MPa.
Figure 5. Distribution of residual stresses induced
by local heating in a specimen made of an aluminum alloy with a fatigue crack:
L – distance from the center of specimen [10]
Compound Pipes and Pipes with Surfacing
Another
example of measuring the residual stresses by ultrasonic method is associated
with compound pipes. Compound pipes are used in various applications and they
are made by fitting under pressure one pipe with an outer diameter into a pipe
with approximately the same inner diameter. For residual stress measurement,
rings were cut-off from a number of compound pipes of different diameters. The
width of the rings was 16 mm. Residual stresses were measured across the
prepared cross-sections in three different locations at 120 degrees to each
other with a subsequent averaging of the measurement results. Depending on the
differences between the inner diameter D1 of the outer pipe and the outer
diameter D2 of the inner pipe, the measurements were made in 3 to 5 points
along the radius. The distribution of residual stresses as measured across the
wall thickness of the compound pipe is presented in Figure 6.
Figure 6.
Residual stress distribution in a compound ring with the following dimensions
[10]:
inner ring:
D1 = 160mm and D2 = 180mm; outer ring: D1 = 180mm and D2 = 220mm (D1- inner diameter, D2- outer diameter,
width of the ring – 16 mm)
The results
of the residual stress measurement by using ultrasonic method in rings cut-off
from the pipes with inner surfacing are presented in Figure 7.
a) b)
Figure 7.
Residual stress distribution in rings with inner surfacing [10]:
a) ring withD1 = 150mm andD2= 180mm;
b) ring with D1 = 180mm and D2= 220mm.
(D1- inner
diameter, D2- outer diameter, width of the rings - 16 mm)
The residual stresses were
measured in a specimen measuring 1000x500x36 mm, representing a butt-welded
element of a wind tunnel. The distribution of biaxial residual stresses was
investigated in X (along the weld) and Y directions after welding and in the process
of cyclic loading of the specimen [7]. Figure 8 represents the distribution of
longitudinal (along the weld) and transverse components of residual stresses
along the weld toe. Both components of the residual stress reached their
maximum levels in the central part of the specimen: longitudinal -195 MPa,
transverse - 110 MPa.
Figure 8.
Distribution of longitudinal (along the weld) and transverse
components of
residual stresses along the butt weld toe [7]
The
ultrasonic method was applied also for residual stress measurement in a
specimen measuring 900x140x70 mm and made of low-alloyed steel, representing
the butt weld of a structure [8]. The distribution of residual stress
components in X3 (along the weld) and X2 (perpendicular to the weld) directions
as well as through the thickness of the specimen near the weld (X1 direction)
are presented on Figure 9.
Measurement of Residual Stresses in Welded Structures
The developed
ultrasonic equipment could be used for RS measurement for both laboratory/factory
and field conditions.
The residual
stresses were measured by the ultrasonic method in large-scale welded panels in as-welded condition and during the fatigue
loading of the panels [11]. The objectives of the study were to identify the
residual stress distribution and relaxation in specimens with welded
longitudinal attachment and welded panel that represent large scale models of
ship structural detail, and compare the results of experimental and numerical
analyses. During the fatigue testing the residual stresses were measured after 1, 2, 10 and 2010 cycles of loading. Figure
10 shows the process of residual stress measurement after certain number of
cycles of loading. Figure 11 illustrates the distributions of the residual
stress in large-scale welded panel near the weld that is critical from the
fatigue point of view in as-welded condition and after 2010 cycles of loading.
A B
C D
Figure 9.
Welded specimen (A) and distribution of the residual stresses along the butt
weld I-I (B), perpendicular to the weld II-II (C) and through the thickness
near the weld III-III (D) [8]: ● – σ22 ; ○ - σ33; ∆- σ11
Figure 10.
Measurement of residual stresses using UltraMARS system
in
large-scale welded panel in as-welded condition and during the fatigue loading
of the panel
Figure 11.
The distributions of residual stress in large-scale welded panel near the weld
that is
critical from the fatigue point of view in as-welded condition
and after
2010 cycles of loading [11]
The process
and some of the results of ultrasonic measurement of residual stresses in
welded elements of a bridge are shown in Figures 12 and 13. The residual
stresses were measured by ultrasonic method in the main wall of the bridge span
near the end of one of welded vertical attachments. In the vicinity of the weld
the measured levels of harmful tensile residual stresses reached 240 MPa. Such
high tensile residual stresses are the result of thermo-plastic deformations
during the welding process and are one of the main factors leading to the
origination and propagation of the fatigue cracks in welded elements.
Figure 12. Installing the ultrasonic gage for
residual stress measurement
Figure 13.
Distribution of longitudinal (oriented along the weld) residual stresses
near the
fillet weld in bridge span: x – distance from the weld toe
Based on the
ultrasonic method the stresses were measured in the bridge both in conditions
of no traffic on the bridge as well as in condition when a few heavy loaded
trucks were put in certain locations to determine the total stress.
Summary
1. Residual stresses play an important role in operating
performance of materials, parts and structural elements. Their effect on the
engineering properties of materials such as fatigue and fracture, corrosion
resistance and dimensional stability can be considerable. The residual
stresses, therefore, should be taken into account during design, fatigue
assessment and manufacturing of parts and welded elements.
2. Certain progress has been achieved during the past few
years in improvement of traditional techniques and development of new methods
for residual stress measurement. The developed advanced ultrasonic method,
based on it portable instrument and the supporting software can be used for
non-destructive measurement of applied and residual stresses in laboratory
samples and real parts and structural elements in many applications for a wide
range of materials. The developed ultrasonic technique was successfully applied
in construction industry, shipbuilding, railway and highway bridges, nuclear
reactors, aerospace industry, oil and gas engineering and in other areas during
manufacturing, in service inspection and repair of welded element s and
structures.
References
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Mechanics. 2005. 417 p.
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Trufyakov, P. Mikheev and Y. Kudryavtsev. Fatigue Strength of Welded
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Kudryavtsev and J. Kleiman. Fatigue Improvement of Welded Elements and
Structures by Ultrasonic Impact Treatment (UIT/UP). International Institute of
Welding. IIW Document XIII-2276-09. 2009.
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Kudryavtsev. Residual Stress. Springer Handbook on Experimental Solid
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Analysis and Beneficial Redistribution. X International Congress and Exposition
on Experimental and Applied Mechanics. Costa Mesa, California USA, June 7-10,
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[6]. Handbook
of Measurement of Residual Stresses. Society for Experimental Mechanics. Edited
by J. Lu. 1996. 238 p.
[7]. Y.
Kudryavtsev. Application of the ultrasonic method for residual stress
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Kudryavtsev, J. Kleiman and O. Gushcha. Residual Stress Measurement in Welded
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Proceedings of the 20th International Offshore (Ocean) and Polar Engineering
Conference ISOPE-2010, June 20-26, 2010, Beijing, China.
