A Study on Approaches for Measuring Outstanding Stress
Basavaraj Kawdi
Mechanical Engineering Department
BKIT Bhalki ,India
E-mail:[email protected]
Vrushabha C M
Gogte Institute of Technology
Belagavi ,India
E-mail:[email protected]
Abstract
Enduring stresses are one of the key factors in
shaping the engineering belongings of parts and structural constituents. This
fact plays a momentous role, for example, in fatigue of welded elements. Many
different procedures and distinctions of means for measuring residual stresses
have been advanced to ensemble various specimen geometries and measurement
purposes. The several specific methods have grew over several decades and their
practical applications have greatly benefited from the development of
complementary technologies, notably in material cutting, full-field deformation
measurement techniques, numerical methods and computing power. These matching
technologies have stimulated advances not only in measurement accuracy and
reliability, but also in range of application; much greater detail in residual
stresses measurement is now available. The purpose of this review is to
classify the different methods and to provide an overview of some of the recent
advances in the area of residual stress measurement and act as a summary
document to aid technique selection between destructive, semi destructive and
non- destructive techniques for residual stresses. For each method scope,
physical limitation, advantages and disadvantages are summarized. In the end
this paper indicates some promising directions for future developments.
Keywords: Residual Stresses,
Destructive Methods, Semi Critical Methods, Non-Destructive Methods.������������������������
The
engineering properties of materials and structural components, notably fatigue
life, distortion, dimensional stability, corrosion resistance, and brittle
fracture can be considerably influenced by residual stresses. Such effects
usually bring to considerable expenditure in repairs and restoration of parts,
equipment, and structures. Accordingly, residual stresses analysis is a
compulsory stage in the design of parts and structural elements and in the
estimation of their reliability under real service conditions. Systematic
studies had shown that, for instance, welding residual stresses might lead to a
drastic reduction in the fatigue strength of welded elements. In multicycle
fatigue (N > 106 cycles), the effect of residual stresses can be comparable
to the effect of stress concentration. Surprisingly significant are the effect
of residual stresses on the fatigue life of welded elements as regards
relieving harmful tensile residual stresses and introducing beneficial
compressive residual stresses in the weld toe zones. Currently, the residual
stresses are one of the main factors determining the engineering properties of
materials, pats, and welded elements, and should be taken into account during
the design and manufacturing of different products. Although successful
progress has been achieved in the development of techniques for residual
stresses management, considerable effort is still required to develop efficient
and cost-effective methods of residual stress measurement and analysis as well
as technologies for the beneficial redistribution of residual stresses.
Residual
stresses can be defined as the stresses that remain within a material or body
after manufacture and material processing in the absence of external forces or
thermal gradients. They can also be produced by service loading, leading to
inhomogeneous plastic deformation in the part or specimen.
Residual
stresses are generated during most manufacturing processes involving material
deformation, heat treatment, machining or processing operations that transform
the shape or change the properties of a material. They are originated from a
number of sources and can be present in the unprocessed raw material,
introduced during manufacturing or arise from in-service loading.
Classification
of Residual Stress Measuring Techniques During the past years many different
methods for measuring the residual stresses in different types of components
have been developed. Techniques to measure Type I (except for techniques such
as diffraction, which selectively sample �special� grains, i.e. those correctly
oriented for diffraction) residual stresses may be classified as either
destructive or semi destructive or non-destructive as shown in Figure 1.
The
destructive and semi destructive techniques are dependent on inferring the
original stress from the displacement incurred by completely or partially
relieving the stress by removing material. These methods rely on the
measurement of deformations due to the release of residual stresses upon
removal of material from the specimen. Sectioning, contour, hole drilling,
ring-core and deep-hole are the principals destructive and semi destructive
techniques used to measure residual stresses in structural members.
Non-destructive methods include X-ray or neutron diffraction, ultrasonic
methods and magnetic methods. These techniques usually measure some parameter
that is related to the stress. They for the assessment of fatigue-related
damage become increasingly important since many structural components, e.g.
bridges, aircraft structures or offshore platforms, need to be inspected
periodically to prevent major damage or even failure.
For inspection in the field or on large
constructions, small, mobile and easy to handle devices are essential. Additionally,
cost minimizing requires short measuring times without time- consuming
preparation of the part prior to the test.
Figure 1: Residual
stresses measuring techniques
The hole-drilling method, which is relatively
simple and fast, is one of the most popularly used semi destructive methods of
residual stress evaluation which can provide the measurement of residual stress
distribution across the thickness in magnitude, direction and sense. It has the
advantages of good accuracy and reliability, standardized test procedures, and
convenient practical implementation. The damage caused to the specimen is
localized to the small, drilled hole, and is often tolerable or repairable. The
principle involves introduction of a small hole (of about 1.8 mm diameter and
up to about 2.0 mm deep) at the location where residual stresses are to be
measured.
Figure 2: Schematic
illustrations of the application of hole-drilling methods for residual stress
measurement
The
hole-drilling method is, in comparison to other residual stresses measuring
techniques, a common, cheap, fast and popular method. It is applicable in
general to all groups of materials. Firstly, the materials should be isotropic
and the elastic parameters should be known. Secondly, the analyzed materials
should be machine-able, i.e. the boring of the hole should not prejudice the
measured strain. The method determines macro residual stresses. Most of
in-depth evaluation algorithms provide a solution to determine an elastic plane
stress state. However, to avoid local yielding because of the stress
concentration due to the hole, the maximal magnitude of measured residual
stress should not exceed 60-70% of local yield stress. The local resolution of
the method is dependent on the equipment used. Laterally, the resolution ranges
in the area of produced hole diameter. The minimal analyzable depth of the hole
does not exceed 0.5 x d0 (hole diameter).
The
deep hole method is a further variant procedure that combines elements of both
the hole-drilling and ring-core methods. In the deep-hole method, a hole is
first drilled through the thickness of the component. The diameter of the hole
is measured accurately and then a core of material around the hole is trepanned
out, relaxing the residual stresses in the core. The diameter of the hole is
re-measured allowing finally the residual stresses to be calculated from the
change in diameter of the hole. The deep-hole method is classified as a semi
destructive method of residual stresses measurement since although a hole is
left in the component, the diameter of the hole can be quite small and could
coincide with a hole that needs to be machined subsequently. The main feature
of the method is that it enables the measurement of deep interior stresses. The
specimens can be quite large, for example, steel and aluminum castings weighing
several tons.
Sectioning
technique is a destructive method that relies on the measurement of deformation
due to the release of residual stress upon removal of material from the
specimen. It has been used extensively to analyze residual stresses in
structural carbon steel, aluminum and stainless steel sections. The sectioning
method consists in making a cut on an instrumented plate in order to release
the residual stresses that were present on the cutting line. For this, the
cutting process used should not introduce plasticity or heat, so that the
original residual stress can be measured without the influence of plasticity
effects on the cutting planes� surface. The strains released during the cutting
process are generally measured using electrical or mechanical strain gauges. In
general, the strips of material released by the sectioning process may exhibit
both axial deformation and curvature, corresponding to membrane and bending
(through thickness) residual stresses, respectively.
The
contour method provides higher spatial resolution, while the sectioning
technique is easier to apply since almost no calculations are needed. The
method has found a number of applications: for example, carbon steel Tee-join
welded, quenched and impacted thick plates, cold-expanded hole and aluminum
alloy forging. It offers improvements over conventional relaxation methods of
measuring residual stresses. The theory of the contour method is based on a
variation of Bueckners elastic superposition principle.
The
method was first published in detail in 2001, where the contour method was
numerically verified by 2D finite element (FE) simulation and experimentally
validated on a bent steel beam having a known residual stress distribution. The
potential of the contour method was later demonstrated on a 12-pass TIG BS4360
steel weld to measure a complex 2D stress
variation across the weld section. The result obtained from the contour method
was in excellent quantitative agreement with the outcome measured by a
completely different technique non-destructive neutron diffraction. A high
stress component, over the initial yield stress of the material, was measured
in that case.
The
X-ray method is a non-destructive technique for the measurement of residual
stresses on the surface of materials. X-ray diffraction techniques exploit the
fact that when a metal is under stress, applied or residual stress, the
resulting elastic strains cause the atomic planes in the metallic crystal
structure to change their spacings. X-ray diffraction can directly measure this
inter-planar atomic spacing; from this quantity, the total stress on the metal
can then be obtained. Since metals are composed of atoms arranged in a regular
three- dimensional array to form a crystal, most metal components of practical
concern consist of many tiny crystallites (grains), randomly oriented with
respect to their crystalline arrangement and fused together to make a bulk
solid. When such a polycrystalline metal is placed under stress, elastic
strains are produced in the crystal lattice of the individual crystallites. In
other words, an externally applied stress or one residual within the material,
when bellow the yield strength of the material, is taken up by inter-atomic
strains in the crystals by knowing the elastic constants of the material and
assuming that stress is proportional to strain, a reasonable assumption for
most metals and alloys of practical concern.
Moreover,
in the case of a nanostructured material, it is not easy to use diffraction
techniques because of the difficulty involved in analyzing the shape of the
nanomaterial diffraction peak. It is difficult to pinpoint the peak location or
to determine the peak shift in order to study the macroscopic stress due to
severe plastic deformation for many materials. For this reason, mechanical
methods are the only techniques known for the study of residual stresses in all
kinds of surface nanostructured materials without the effect of nanostructure.
The speed of measurement depends on a number of factors, including the type of
material being examined, the Xray source, and the degree of accuracy required.
The gauge volume is a trade-off between the need for spatial resolution within
the expected strain field and the time available for data collection. With
careful selection of the X-ray source and test set-up speed of measurement can
be minimized. New detector technology has also greatly reduced the measurement time.
Neutron
diffractions method is very similar to the X-ray method as it relies on elastic
deformations within a polycrystalline material that cause changes in the
spacing of the lattice planes from their stress-free condition. The application
of neutron diffraction in solving engineering relevant problems has become
widespread over the past two decades. The advantage of the neutron diffraction
methods in comparison with the X-ray technique is its lager penetration depth.
In fact the X-ray diffraction technique has limits in measuring residual
stresses through the thickness of a welded structure. On the other hand, a
neutron is able to penetrate a few centimeters into the inside of a material,
thus it can be applied widely to evaluate an internal residual stress of
materials. It enables the measurement of residual stresses at near-surface�������� depths��� around�� 0.2mm�� down
to bulk measurement of up to 100mm in
aluminum or 25 mm in steel.
The
non-destructive residual stresses measurement methods have the obvious
advantage of specimen preservation, and they are particularly useful for
production quality control and for measurement of valuable specimens. However,
these methods commonly require detailed calibrations on representative specimen
material to give required computational data. The diffraction methods such as
X-ray and neutron diffraction can be applied for the polycrystalline and fine
grained materials as well as metallic or ceramic. However, they cannot be used
for large welds because the limited space available on most beam lines or X-ray
diffractometers or for nanostructured materials because of the difficulty
involved in analyzing the shape of the nanomaterial diffraction peak. The
advantage of the neutron diffraction method in comparison with the X-ray
technique is its lager penetration depth as x-ray method is limited for the
measurement of residual stresses on the surface of materials. However, the
relative cost of application of neutron diffraction method, is much higher,
mainly because of the equipment cost and it is not recommended to be used for
routine process quality control in engineering
applications.
Anawa EM, Olabi AG.(2008). Control of welding
residual stress for dissimilar laser welded materials. Journal of materials processing technology,204:22-33.
JLu. (2013).Handbook
of Measurement of Residual stresses. SEM, Bethel.ISBN: 978- 0132557382; 1:319-322.
Kandil FA, Lord JD.(2011).A review of residual stress measurement methods, a guide to technique
selection. NPL Report MAT; (A):04.
Kiel S.(2012). Experimental
determination of residual stresses with the ring-core method and an online measuring
system. Exp Tech. .16(5):17-24.
Lachmann C, Nitschke-Pagel Th, Wohlfahrt H.(2012).Non-
destructive characterization of fatigue processes in cyclically loaded welded
joints by the Barkhausen noise method. Stanford University: 2nd International
Workshop on Structural Health Monitoring;
Milbradt KP. (2011).Ring-method determination of residual stresses. Proc SESA.9(1):63-74.
Macherauch E, Kloos KH. (2013).Origin, Measurements
and Evaluation of Residual Stresses. Residual
Stresses in Science and Technology, 3-26.
Olabi AG, Hashmi MSJ.(1996). Stress relief
procedures for low carbon steel welded components, Journal of Materials Processing Technology, 56: 552-562.
Totten G. (2012).Handbook
on residual stress. SEM, Bethel .1:417. ISBN: 978-0871707291
Trufyakov V, Mikheev P, Kudryavtsev Y. (2009).Fatigue Strength of Welded Structures.
London: Harwood Academic, 100.
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