Inspection of equipment’ and structures’ stress-strain state during the
residual lifetime assessment on the objects of industry and transport
Modern diagnostics of structural materials’ state, possessing the large
arsenal of various physical methods and means, is not already limited by flaw
detection tasks and is more and more widely used for solution of the tasks of
the materials’ mechanical characteristics determination. And methods and means
of residual and working internal stresses measurement occupy the major place
In connection with this methods of engineering diagnostics, combining the
fracture mechanics, physical metallurgy and non-destructive testing, come to the
forefront. Methods of stress-strain state (SSS) assessment are primarily
referred to such methods.
All leading diagnostic centers of the world are occupied nowadays by the
problem of mechanical stresses measurement in operating structures in order to
assess their state. However, till date the effectiveness of various methods and
means for stress control remains low at their application directly on metal
The analysis of capabilities of the known stresses and strains control
methods and means in the base metal and welded joints of metal components and
structures allows naming the following their sufficient drawbacks:
unsuitability for inspection of long pipelines and structures, large-sized
products, metal components and vessels;
impossibility to use most of the methods in the plastic strain area;
variation of the metal structure is not considered;
impossibility to assess the depth layers of the metal for most inspection
the need to make graduated diagrams based on the preliminarily prepared
samples, which, as a rule, do not reflect the actual energy state of metal
the need for inspection surface and inspection objects preparation
(dressing, active magnetization, sensors adhesion, etc.);
complexity of testing sensors location determination related to the
direction of the action of maximum stresses and strains, determining the
Besides, the traditional methods and means of stress NDT, based on active
interaction of the instrument signal with the structure’s metal, obtain indirect
information on the stressed state of the inspection object, i.e. they have
insufficient self-descriptiveness of physical fields used at inspection. Indeed,
the field introduced in the investigated material, while interacting with
self-magnetic fields of the material, changes its properties and the inspection
object’s SSS characteristics. The nature, amount and lifetime of changes are
determined by the dynamic ratio of the energies of the interacting fields. In
practice such changes are simply neglected during carrying out the
This note refers, first of all, to the following methods:
the coercive force method (various modifications of coercive force
the magnetic anisotropy method (the Complex 2.05 and 2.06 Type
the methods using the Barkhausen effect (the Stresscan, Intromat,
Pion and other Type instruments).
It should be kept in mind that any physical field introduced
artificially from the instrument in the inspection object, being in a
stress-strain state (even after working loads relief), will by all means
interact with the material’s proper physical fields (for example,
electromagnetic) formed at the lattice level. Neglecting the actual energy state
of inspection object (for example, the "magnetic memory of the metal" indicates
this) is a gross mistake for all methods of SSS inspection! It especially
concerns stress concentration zones (SCZs) concentrated, as a rule, at the depth
and in the volume, and coming to the product’s surface in the form of glide
lines (sites of cracks formation!) with the width of several microns (and even
of several submicrons!). Besides, inspection is carried out, as a rule, on
equipment shut down for repairs after working loads relief in conditions of the
residual SSS, when stresses and strains have opposite signs and different values
as compared to the working ones. In these objective conditions the
above-mentioned inspection methods turn out to be ineffective for the inspection
object’s actual SSS assessment both by their physics and by the metrological
conditions (instrument sensors, as a rule, are sufficiently larger than the area
of SCZs), and what is important - it is not known, to what depth the metal
should be artificially magnetized, where and how to install the sensor, when
zones of maximum stresses (working or residual) are unknown.
Thus, the above-listed drawbacks of the well-known methods of SSS inspection
are conditioned by these methods’ physics and are regular. The lack of the
metrological basis for materials’ SSS characteristics measuring means
certification and calibration (till date there are no unified standards and
samples in Russia and other countries) leads to ambiguity of requirements and
wrong methodical approach to the developed inspection means. Besides, at present
no country in the world has any programs and centers for experts training in
non-destructive testing of metal components’ and structures’ SSS. There are no
standards 1), specifying the
general requirements to methods and means of stresses and strains NDT in
1) First such
standard was prepared in Russia by "Energodiagnostika" Co. Ltd. specialists and
presented at the IIW Commission V meeting on July 14, 2004, in Osaka (Japan) for
discussion and obtaining a resume. The standard GOST R 52330-2005 is entitled
"Non-destructive testing. Stress-strained state tests on industrial objects and
transport. General requirements".
As it is known, determining of the actual stress-strain state with detecting
of stress concentration zones (SCZs)2) - the main sources of damages
development - based on 100% inspection of the entire metal volume is an
indispensable condition at equipment life assessment. Exactly SCZs, and not the
design average values of working stresses, determine the operability of any
2) One should
distinguish the traditional concept "stress concentrator" influenced by the
product configuration from the materials science concept "stress concentration"
occurring in zones of stable dislocation slipbands due to the action of working
loads. A SCZ is a local product zone, in which large strain occurred as compared
to the average strain across the entire product volume due to unfavorable
combination of structure features, material structure inhomogeneities and
It is known that under the influence of operating loads the work of the metal
components’ metal is mainly determined by dislocations glide and shear strain.
And metal fatigue damageability accumulation in many cases occurs in conditions
of a low- and a high-cycle working load. It is obvious that the traditional
methods of stress control cannot assess the actual SSS of a structure since in a
general case SCZs due to shear strain are unknown. In the course of industrial
investigations it was established that only "passive" methods of SSS diagnostics
can answer the questions set and are the most suitable for practical
Passive NDT methods using the measurements of proper physical fields of
structures, first of all, are:
These two methods are nowadays widely spread in practice for early
diagnostics of damages in metal components and structures. Besides, exactly
these two methods allow ensuring the 100% inspection of metal components in the
quick control mode.
As it was demonstrated in practice, the MMM method, as compared to the AE
method, gives additionally the information on the inspection object’s actual
SSS, which allows the more objective determination of not only a SCZ, but also
the reason of this zone formation. Application of the MMM method on the
inspection object does not require execution of any preparatory works.
Let us consider some examples of metal components’ SSS assessment using the
Fig.1 shows the distribution of the Hp field’s normal component along the extended and
compressed sides of a ?25х3мм vertical pipe, clamped between the two horizontal
pipes ?42х4mm. The pipes are made of steel 3. The visual bend of a ?25х3mm pipe
segment occurred during heating of the pipe heating system with hot water from
the room temperature ~20oС to 50÷55oС. Let us make quality and quantitative
characteristic of this pipe segment’s SSS by the pattern of the Hp field distribution without carrying out special
calculations of bend stress level.
Fig.1. Distribution of the Hp field normal component along the extended and compressed
sides of a ?25х3mm vertical pipe, clamped between two horizontal pipes ?42х4mm:
1 - pipe, st.3, ?25х3mm; 2 - pipe, st.3, ?42х4mm; 3 - T-joint; 4 - Нр=0 lines; dн - external diameter.
The Hp field value in the
maximum flexure area along the entire pipe perimeter (compressed, extended and
neutral) is practically the same and equal to 400A/m. The measurement results
correspond to the design regularities obtained in the course of special methodic
investigations. The Hp field
distribution with sign alternation in zones of the pipe strain sign alternation
(see unit A in Fig.1) should be specially considered. As it is seen in
the figure, the Hp=0 line location
has a regular nature. The many-years experience in magnetic fields investigation
on pipelines and various metal components revealed presence of stable lines of
sign alternation of the Hp magnetic field intensity normal component in areas of developing metal damages.
This very diagnostic parameter (the Hp=0 line) was taken as a basis of practical
techniques for inspection of metal components using the magnetic memory of
metal. Papers [1, 2] showed that Hp=0 lines, recorded on the pipe surface,
correspond to dislocations glide planes along the pipe section. The direct
experimental confirmation of the Hp=0 line coincidence with the stress
concentration line (SCL)3) and
with the maximum dislocation density was obtained in paper .
3) In later investigations it was established that SCL in
a general case correspond to the line of the maximum Нр field
Fig.2 shows basic regularities characterizing the SSS of the pure iron during
the mechanical exposure, which were obtained in paper  as a result of design
investigations. It is seen in fig.2, b, that the glide plane angle
relative to the normal tensile stresses has much lower values as compared to
compressive stresses. For example, at tensile stresses of 12 kgг/mm2 for the pure iron the glide plane angle is equal to ~45o , and at compressive
stresses of the save value of 12 kgг/mm2 this angle is equal to ~70o.
Based on the results of the Hp field measurements, presented in fig.1, simple calculations of glide plane
angles indicate the following. The angle between the Hp=0 line and the pipe axis on the side of tensile
stresses is equal to:
where c - is the glide line length (Hp=0).
The angle between the Hp=0 line
and the pipe axis on the compressive stress side is equal to (90o-26,5o)=63,5o.
Thus, by means of simple geometric calculation of the angle of the Hp=0 lines arrangement relative to
the pipe axis, which were detected during the inspection by the MMM method, the
fairness of regularities established in the paper  was confirmed. In presence
of the dependence α(σ) for steel 3, being similar for the pure iron (see
fig.2,b), based on the data of the inspection by the MMM method, it is
possible to determine the value and the sign of residual stresses directly on
the pipeline by the angle α.
Рис.2. Basic dependencies characterizing the
SSS of the pure iron during the mechanical exposure: a - Poisson’s
ratio; b - the glide plane angle; c - shear strain; d - density of dislocations N=10nd; e – shear strain; f - density of dislocations N=10nd, σ - mechanical tensile and compressive
stresses, kG/mm2; σpc=0,95;
σy=4,3; σt=17; εt=0,14.
Fig.3 shows inspection results of a live steam pipeline section (branch B)
obtained at power unit #1 at Konakovo TPS. It is seen in Fig.3,b (the
bottom part of the diagram) that the field normal component gradient
(dH/dx) when intersecting with a crack (Cr1) with the length
of 60mm and the larger opening, has the value being 1,5 times less than at
intersecting with a crack (Cr2) with the length of 7 mm and the less opening. It
is also seen in Fig.3,b that the field gradient in the SC zone turned
out to be approximately equal to the field gradient obtained on the crack (Cr2),
being at the stage of initial development.
Fig.3a. Results of a live steam
pipeline section inspection: 1 - direction of the inspection; 2 - suspension; 3
- SC zone; 4 - crack 1; 5 - crack 2; 6 - MSV.
Fig.3b. Results of a live steam
pipeline section inspection: 1 - crack 1; 2 - crack 2; 3 – SC zone.
Assessment of absolute values of the residual stresses level in the SC zone
(see fig.3), carried out according to the technique by Energodiagnostika Co.
Ltd., shows that in this case this value (assessment was carried out by the
normal component of the Hp field)
sufficiently exceeds the ultimate strength of the steam pipeline’s metal (steel
15Cr1Mo1V, σt=500-700 MPa). It is well agreed with the
well-known provision that stress concentration level at the crack top may meet
its critical value.
As it was pointed out above, according to the MMM method, the gradient of the
magnetic leakage field Hp (dH/dx), recorded during scanning with the sensor of the
TSC-type instrument along the surface of metal components, is a basic diagnostic
SSS parameter. It was established that exactly this diagnostic parameter, by
virtue of magnetometric effect, directly reflects the energy state of metal
surface and depth layers in SCZs. The maximum value of the field gradient
determined on the metal surface with accuracy of up to one millimeter
corresponds to the source of crack initiation. In the area of the most intensive
process of strain and, finally, failure the domain structure is subject to
sufficient changes. Dimensions of domains, directions of which correspond to
glide direction, meet critical sizes. Design researches in paper  showed that
the critical size of an iron domain might have the volume covering up to ten
grains. The domain with maximum dimensions finally "breaks" - a crack forms. At
present Energodiagnostika Co. Ltd. possesses quantitative criteria
characterizing the limiting state of the metal by strength conditions and the
initial development of cracks.
Paper  presents various examples from practice illustrating capabilities
of the MMM method at inspection of SSS not only of pipelines but also of other
various units of metal components and structures.
For example, the available experience of the 100% inspection of K-300 turbine
rotors at Konakovo TPS, of K-200 at Cherepovets TPS and Zainskaya TPS, of T-100
at Severodvinskaya TPS-2, of PT-60 and T-100 at Petrozavodskaya TPS and others
(more than 100 various-type turbines were inspection in total) allows drawing
the following conclusion: SCZs - the sources of damages development (as a rule,
in the form of cracks) - make not more than 3-5% of the entire surface and
volume of the rotors’ metal. The rest 95% of the metal volume of turbine rotors
after their long-term operation remain in the satisfactory state! Thus, the
problem of turbine rotors’ lifetime assessment is solved by means of timely
detection of maximum stress concentration zones and their removal by ordinary
grinding in the course of the repairs. Energodiagnostika Co. Ltd. uses the
similar approach during the lifetime assessment with the 100% inspection by the
MMM method on all types of metal components: turbines, boilers, steam and water
pipelines, gas and oil pipelines, vessels and other inspection objects.
Based on the 20-years experience of the MMM method practical application a
conclusion can be made about its unique capabilities for detecting local SCZs,
determining the actual SSS, reliability and lifetime of metal components.
Besides, it should be once more pointed out that the absolute value of stresses
in the local SCZ, characterized by glide lines with maximum values of the
magnetic field gradient (the width of these lines of dislocations cluster is
~0,1÷10 micrometers) before initiation of a microcrack development is by order
greater than the metal conventional ultimate strengt σt. It
is obvious that in these conditions the traditional methods of stresses NDT,
intended for determining of average stress values σ on long segments, turn out
to be unsuitable for their practical application.
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