On the Problem of Stress-Strained State Characteristics Measurement of
Structural Materials on Complex Engineering Objects. Energy Concept of
Materials' Stress-Strained State (SSS) Diagnostics
Dr. V.T. Vlasov, Dr., Professor A.A. Dubov
Foreword
The ideological basis of the energy concept of SSS diagnostics was determined
by investigation results of objective processes of the material’s proper energy
re-distribution and establishment of regularities describing the objectively
existing relations of the material’s macro characteristics with external impact
parameters and impact response.
In the course of this concept development first the necessity and then the
opportunity occurred for creation of a tool for carrying out further
investigations and theory development – a new seven-dimensional dynamic
self-regulating material model considering interaction of normal and shear
stresses and strains, the model that varies its parameters depending on
amplitude (up to breaking) and frequency (from statics and infrasonic to
ultrasonic) characteristics of external impact.
V.T. Vlasov presented the energy concept of materials’ SSS
diagnostics and its most important consequences at Scientific-Technical Councils
of the State Institute of Physical-Technical Problems (The STC Chairman is
Academician L.N. Lupichev) and of the International Institute of Complex
Engineering Systems Safety at the Russian Academy of Sciences (RAS) Institute of
Engineering Science (The STC Chairman is a RAS Corresponding Member N.A.
Makhutov) and they were highly appreciated.
1. Internal stresses, classification and effect on materials’ strength
Internal residual mechanical stresses, occurring in a part,
welded joint or structure in general, are the subtlest reason of unexpected
failures of objects. These stresses in steels may approach the yield strength,
and in aluminum and titanium alloys - 70-80% of the yield strength and they
often turn out to be more dangerous in terms of strength reduction than some
types of defects.
Stresses existing and getting balanced inside a solid, a rigid aggregate of
materials, a fabricated or welded structure after removing the reasons causing
their occurrence are accepted to be called residual stresses. These stresses are
always internal and their occurrence is always associated with inhomogeneous
linear or volume strains in the adjacent volumes of a material, an aggregate or
a structure.
Residual stresses are divided in three types classified by the length of the
force field created by them:
-
the first type - balancing 1) in macroscopic volumes (в пределах
детали или конструкции);
-
the second type - balancing in microvolumes (within the
metal structure crystallites);
-
the third type - balancing in ultra microscopic volumes
(within the lattice). Such definitions of residual stresses were first given
by N.N. Davidenkov in 1935.
1) The term
"balancing" is not quite correct, and it is better to use another term, for
example, "developing" or "occurring". The point is that stresses of all the
three types are interrelated with each other and each of the stresses is the
reason or a consequence of "adjacent"-type stresses, and in the case of
"balancing" within the limits of their volumes we would have all-sufficient
stresses not related to each other.
In general the study of residual stresses started very long ago.
V.I. Rodman in 1857 and then I.A. Umov in 1871 conducted the first serious
investigations. N.V. Kalakutsky, who first developed the method of residual
stresses calculation and first suggested experimental methods of their
measurement, started systematic investigations in 1887. During the following
years the methods of residual stresses investigation were reduced mainly to
development of their measurement methods – an important practical task within
the problem of structures reliability assessment.
According to the above-said, residual stresses belong to internal stresses of
the material. Internal stresses represent the demonstration of the material’s
proper internal energy interaction with the energy of the external field (force,
thermal, etc.) influencing the material fabricated as a specific part or
structure. Therefore stresses occurring in the operated part or structure
material under the influence of external fields and determining the material’s
resistance to external effects - its strength - also belong to internal
stresses. And variation and re-distribution of the material’s internal energy
among its components under the influence of operating load causes occurrence of
"new" residual stresses. To avoid confusion it is appropriate to
introduce the following classification of internal stresses:
-
process residual stresses - are stresses being the
consequence of physical and physical-chemical processes starting in the
material at a part or a structure2) fabrication and continuing after
fabrication;
-
load stresses - are stresses occurring in the operated
part or structure material as an elastic reaction of the material to external
load, load stresses disappear at removing the external effect;
-
operating residual stresses - are stresses being the
consequence of processes of proper internal energy interaction of a part or a
structure with external field energy, occurring and accumulating in the
material during the entire period of a part of a structure operation;
-
working stresses - are a vector sum of process, load and
operating stresses;
-
actual stresses - are a vector sum of process and
operating stresses formed by the moment of measurements.
2) Every process
operation of the entire cycle of a part or a structure fabrication introduces
sequentially its residual stresses with their characteristic features. Residual
process stress will present the result of their dynamic vector interaction.
Thus, strength, reliability and suitability degree of welded
structures for application according to their operational designation are in
many respects determined by presence, nature and amount of working and
actual internal stresses. Material degradation during the process of
the long-term operation in many respects, but not in all of them, contributes to
this.
2. Material degradation and its role in the material’s strength
Indeed, at the stage of objects design and construction the mechanical
properties of structural materials used are known with the required accuracy,
and at the possibility of experimental determining of residual stresses the
initial life of an object’s strength can be estimated as well. And the accuracy
and authenticity of an object’s life assessment at the stage of its erection
does not seem to be a serious characteristic since there are pre-operational
tests, and 15 or 20 years of life are not so important - it is still so far
away!
But when the time of the assumed physical wear of equipment and structures is
approaching, and in some cases it has already expired, the accuracy and
authenticity of residual life assessment become vitally critical in the direct
sense. And here methods of residual life estimation of critical objects and
methods of their safe operation periods prolongation taking into account real
conditions, which often lead to unpredictable variations of material’s
properties and its degradation, gain acute actuality. And the final stage of the
material degradation is the newly appeared defects, the "growth" process of
which in conditions of operation of a structure made of degrading material is
poorly investigated and often develops avalanche-like, so the time left till the
structure failure is unknown and often too little to prevent the disaster.
Therefore to obtain authentic results of strength residual life calculation
of objects operated for a long time it is necessary to know first of all
the actual mechanical characteristics of the material3) and characteristics of its
stress-strained state formed by the present time as a result of an
object operation.
3) It should be
noted that senseless to require obtaining absolute values of internal stresses
without the knowledge of actual mechanical characteristics of the material
formed during the process of an object’s long-term operation, - there is nothing
to compare them with! In these cases qualitative variations of the stress field
are more useful.
This task has become the major not only in investigation and assessment of
objects’ static strength, it becomes decisive in investigation and assessment of
fatigue strength due to the local nature of fatigue failure and its strong
dependence on the actual material’s stress-strained state.
So, the following tasks sequentially occurred at solution of the problem of
critical objects reliability:
-
determination of residual stresses;
-
determining the nature of internal stresses and components
values;
-
determining the actual mechanical characteristics of the material
and its stress-strained state characteristics.
It is quite obvious that non-destructive methods of structural materials’
state diagnostics should provide such a possibility. But are they ready to solve
such tasks?
According to the authors’ opinion, at present the metal magnetic memory (MMM)
method meets to a greater extent the ideology of the energy concept of SSS
diagnostics.
The principal novelty of the MMM method consists in the use of the
objectively existing but not studied before phenomenon of
"magnetoplastics". Investigation of complex processes of the material’s
proper energy re-distribution under the influence of external force and/or
magnetic fields required the knowledge not only from the filed of metal physics,
elasticity, plasticity and strength theories, fracture mechanics, basics of
radio engineering and even thermodynamics, but it also required addressing such
fields of science as quantum physics, solid-state physics, theory of
dislocations, electromagnetic field theory, which seem to be quite remote from
the practical problems solved. But the obtained results surpassed all
expectations: not only the functional correlation of various internal energy
fields with each other and with external fields was established, which ensures
development of well-known active diagnostic methods like the coercive force
method, the residual magnetization method, the Barkhausen noise method and
others, but also to reveal quantitative criteria for determination of strong and
weak magnetic fields, energy ratios of force and magnetic fields determining the
magnetoelasticity boundaries and of the newly introduced in practical
application phenomenon of magnetoplastics.
Indeed, some results of joint work in the field of experimental and
theoretical investigations of magnetic phenomena physics are beyond the
classical idea of magnetism and domain structure. However, at the same time
there is not only absence of conflict between them, but they also erase "white"
spots in the theory of magnetism, of which specialists working in this field
have been aware for a long time.
It should be noted that we obtained not a system of separately
established facts, confirmed by results of experimental investigations
carried out by A.A. Dubov and by experiments obtained before, of course,
independently from him by the well-known national and foreign researchers of
magnetic phenomena, but a domain structure theory logically built on the
example of iron was developed.
Theses of the obtained results were presented in 2002 in
St.-Petersburg at the XVI-th All-Russian Diagnostics Conference and the more
detailed presentation was made in 2003 at the III-d International Conference
"Equipment and structures diagnostics using the MMM method". Specialists working
actively in the field of materials’ SSS diagnostics by magnetic methods
demonstrated their interest to this work. However, unfortunately, we did not see
any well-known national magnetic scientists at any of these our
presentations.
A book presenting the detailed contents of the work performed is being
prepared for publication at present.
3. Classification and analysis of physical methods of structural materials
diagnostics
Analysis of the trends of current non-destructive inspection methods and
means4) development allowed
approaching the answer on this question. Let us consider the dynamics of
scientists’ efforts distribution in the field of diagnostics methods and means
development by uniting the topics of allied investigations in directions.
4) The analysis was
carried out by the materials of international conferences, symposiums and by
special periodic literature for the periods from 1966 till 1974 (125
publications were selected) and from 1987 till 1994 (over 1000 reports and
articles were analyzed here).
Table 1. Dynamics of scientific efforts distribution by
directions
Direction index |
Characteristic of direction |
1966-1974 |
1987-1994 |
I |
Development of new means realizing the traditional approach to
diagnostics |
70% |
26% |
II |
Improvement of sorting norms based on statistic investigations |
15% |
28% |
III |
Search for new approaches to materials and structures diagnostics
(stress measurement and the acoustic emission (AE) method) |
10% |
36% |
It should be noted that since the beginning of the с 90-s the search
for new approaches to materials diagnostics has become the major trend of
diagnostic means development. And it is worth saying that the observed nowadays
increased intensity of works on searching of new approaches to diagnostics is
the third, stronger raise of interest towards this direction, which appeared in
the late 50-s and had its first peak in the mid 80-s and the second - in early
90-s. The conclusion drawn is confirmed by the more and more noticeable
re-orientation of the thematic orientation of presentations and exposition of
not only Russian but also International scientific-technical conferences
"Non-destructive testing and diagnostics" starting from 1997.
Growth of scientific interest towards new approaches to diagnostics is
obvious. But one can not help drawing attention to the fact that the scope of
works by the II-nd direction - improvement of sorting norms based on
statistic investigations - has grown sufficiently as well. And this, to
the authors’ opinion, indicates not only the will to improve the authenticity of
flaw detection results but also the more and more noticeable insufficiency of
information obtained at objects diagnostics for assessment of their state.
The analysis of works presenting scientific directions demonstrates that, in
fact, the final goals of some works belonging to different directions are the
same. Indeed, the actual goal of works dedicated to improvement of sorting norms
and investigation of defects influence on structures strength is the search for
new informative characteristics of defects determining the degree of their
danger at a structure operation. And topics associated with stress waves
emission investigation and development of materials’ stressed state detection
methods and means are an attempt to solve the problem of structures reliability
assessment by new ways.
The correctness of determination of diagnostic means development trends
revealed in early 90-s, when the world applied science has accumulated large
experience in the field of diagnostic methods and means development, is of no
doubt because, in fact, it is nothing but statistics. However, the
perspectiveness of directions in the aspect of usefulness of their results in
solution of the task of complex engineering objects residual life assessment is
not doubtless.
The deeper analysis of works by national and foreign researchers has drawn
the author to the following two preliminary conclusions:
Firstly, in no way trying to humiliate the importance of the
I-st and the II-nd directions and the significance of success achieved there,
the author considers that from the viewpoint of the possibility to enter
the qualitatively new, in the principal aspect, level of object reliability determination these two directions have no
future since they are restricted to each other: new instruments allow
improving inspection norms and new norms stimulate instruments improvement.
Secondly, as the analysis of works by the III-d direction
demonstrated, despite the inflow of new intellectual forces and modern computer
means a "breakthrough" to the qualitatively new level is not so far
foreseen.
The point is that the III-d direction develops two different non-intersecting
concepts, which have not suffered any variations since late 50-s (from the
moment of the AE method appearance), though, in fact, both methods of stress
state measurement and AE methods have as a test object different phases of the
same process - the material reactions to loading and environment factors
effect.
Besides, capabilities of modern macroelectronics and computer engineering led
many of western specialists away from solution of merely physical tasks, and the
searched answer is hidden exactly there, in the physics of processes. Many
national specialists, trying to catch up with foreign colleagues in the
direction of inspection means improvement, "drove" into the same, but already
broken track5).
5)Lately a number of
private national companies has occupied advanced positions in terms of program
software development for diagnostics, leaving the well-known foreign companies
behind. The most interesting results were obtained at Intellect Co. Ltd. in
Nizhny Novgorod (the Head is A.L.Uglov).
So, the analysis results may be formulated as follows:
-
the major direction of materials diagnostics means development is
the search for possibilities to determine certain mechanical characteristics
of the material, associated with its stressed state by parameters of
physical fields used for diagnostics;
-
perspectiveness of current concepts, forming the basis of important
and interesting investigations by the major direction, raises serious
doubts.
Of course, doubts in the perspectiveness of concepts lying in basis of the
major direction of the material state diagnostics means development, in the
aspect of sufficient improvement of structures reliability assessment
authenticity, required serious proof.
Modern diagnostics possesses large arsenal of various methods and means for
measurement of mechanical characteristics of materials. Methods and means of
residual and elastic internal stresses measurement are presented most
widely.
There is a standard classification of non-destructive
diagnostics methods dividing them by the nature of physical fields interaction
and by the ways of obtaining of primary information in nine types:
magnetic, electric, eddy-current, radio-wave, thermal, optic, infra-red, acoustic and capillary. Each type, in turn, is
divided in various groups.
This classification, introduced for flaw detection methods and means and
applied nowadays for classification of materials’ stressed state diagnostics
methods and means, has a formal nature, dividing all the
variety of non-destructive diagnostic methods rather by the way of the
used effect selection than by the type of physical fields.
However, at solving the tasks of the next, higher level of complexity - the
tasks of materials’ properties determination, and, in particular, of mechanical
characteristics - more distinct division of methods exactly by the type
of physical fields need to be done.
In fact, determination of material’s properties is reduced to measuring of
variations of certain used physical fields parameters. In other words, if a test
object with certain known beforehand abilities to resist external effects is
influenced by a physical field with known or specified parameters6), the used field parameters variations
caused by the object’s reaction will represent an "imprint" of its properties in
the area specified by the type of the physical field. And the reactions "echoes"
will be seen also in spaces of other fields but as indirect "imprints" or a
secondary reaction. Thus, for example, in case of a thermal field influence, the
direct characteristics will be thermal ones and indirect characteristics –
mechanical, electromagnetic and others. If an object is influenced by a
mechanical force field the direct reaction characteristics will belong to
mechanical characteristics, and indirect demonstrations can be observes in
thermal, electromagnetic and other fields.
6) "Known" and
"specified" do not always mean the same. Generally speaking, "specified"
parameters are known but they often belong to external conditions of the field
excitation in the investigated material, and the parameters of actually excited
field remain partially or completely unknown.
Sorting the known methods of materials’ state diagnostics by the type of
physical fields, we obtain the following types:
-
electric;
-
magnetic;
-
electromagnetic;
-
thermal;
-
mechanical.
And the well-known and widely used methods like optic, radio-wave, X-ray,
acoustic, holographic, capillary, electric resistance methods, strain gage as
well as moire, grid, photoelasticity and other methods did not disappear but
occupied their places within these five types.
Keeping in mind that classification of diagnostic methods is not an end in
itself but it is only a means in the search for the reasons of low authenticity
of their results, let us consider in more detail just some most characteristic
types of diagnostics.
Electromagnetic methods, which are often divided depending
on the frequency range in the following groups or subtypes (by the increase of
the excited field frequency): radio-wave, microwave methods, infra-red,
optic (the visible range), ultraviolet, X-ray and gamma-methods are the
most widely represented in investigations of materials’ properties. All these
varieties are in this or that way based on interaction of the exciting
electromagnetic fields with proper electromagnetic fields of the investigated
material created by its molecules, atoms or their electron shells. And the
greatest effect is displayed when frequencies of the exciting and the proper
fields are close to each other, which in fact follows from the molecular
thermodynamics and confirms its conclusions. And frequencies of proper
electromagnetic fields being in sufficiently different ranges, of course, depend
on the stressed state of the material. This explains the occurrence of such a
variety of subtypes of electromagnetic methods.
The most widely spread in practice X-ray method uses variation of the
reflected rays spectrum caused by variation of the lattice units oscillation
frequency and by change of the distance between the units and crystallographic
planes. The informative parameters of the X-ray method are: intensity,
position and width of spectrum diffraction peaks determined by the
lattice strain.
Mechanical methods7) of material properties diagnostics include various types of static and dynamic measurement methods of hardness and other mechanical material
characteristics using the results of contact interaction of the test
object – indenter with the investigated material8). ЭThis has been known for a long time
and is absolutely obvious.
7) The most
widespread mechanical method of diagnostics - materials’ hardness measurement -
is conventionally non-destructive since an object surface quality still changes.
Operating requirements to the surface quality restricts application of this
method.
8) V.A. Rudnitsky’s
doctoral thesis gives the analysis of current methods of materials’
characteristics determination by contact strain parameters and the vast
bibliography.
As for referring of the acoustic, including ultrasound methods to mechanical methods - it looks, to put it
mildly, somewhat unusual. But this is, in fact, fair since the acoustic field is
a mechanical stress field created in this or that way in the restricted volume
of the investigated material and causing oscillatory or aperiodic displacements
of material particles, i.e. local material strains. In fact, this limited
strained material volume is an indenter, whose remarkable feature is that it can
move inside the investigated material. And the strained area dimensions are
determined not by the lattice parameters (in case of metals and other
crystalline or polycrystalline materials) and dimensions of molecules (in case
of amorphous materials), but by the length of the excited field inside
the material, and they make from fractions to tens of mm.
Now, comparing the two considered methods, one can understand why the results
of internal stresses measurement by the X-ray and acoustic methods simply have
to be different since in the first case the determining factor is strain at the
microlevel, creating the III-d type stresses, and in the second case - an
aggregate of the I-st and the II-nd type stresses. And all these three types of
stresses, at all the integrity of correlation between them, have not only
sufficiently different values but also the different nature and very often
different signs. Moreover, while calibrating the X-ray method reacting to
microstrains, determining the III-d type stresses, on specimens by tensile or
compression efforts, i.e. actually by the I-st type stresses, a gross principal
error is made, which is often not even suspected of.
As we can see, the suggested classification of physical methods of
diagnostics, while allowing looking at diagnostics methods from
another, less usual side, gives the grounds to think about the mechanism
of parameters correlation of physical fields used for diagnostics with the
measured material characteristics and the material properties in
general, as well as demonstrates the degree of closeness of the used
for diagnostics physical method to the measured characteristics of the
investigated material.
In other words, classification of physical methods gains a principal
nature in the aspect of the task of the material’s stressed state
determination, specifying the way of establishing the reasons of very
low authenticity9) of materials’
stressed state characteristics measurement results.
9) Here it is
appropriate to remind of comparative testing results of various physical methods
at residual stresses measurement when the measured values differed not only
quantitatively but also by their sign: some methods indicated the compressed
state of the materials, others - the extended state.
Thus, classification and analysis of physical methods of materials’ stressed
state diagnostics allow drawing the first, not quite sensational but important
conclusion: mechanical methods of diagnostics are direct research
methods, and all other methods (according to the suggested classification) are
indirect.
4. Assessment of materials’ state diagnostics results authenticity
So, practically all methods of materials’ stressed state diagnostics are
either indirect or are used as indirect ones.
Ideological basis of indirect methods is application of certain approximating
functions more often obtained experimentally and sometimes theoretically and
reflecting the objectively existing correlation of the recorded variation of the
used field parameters with the actually occurred variations of material’s state
usually expressed by separate mechanical characteristics or a certain aggregate
of its characteristics. But since this correlation, being the consequence of
secondary phenomena of the internal material’s energy transformation
accompanying the process of its state variation, is determined by many factors, the area of rightful application of indirect methods is restricted by
adequacy of the approximating functions used by the investigated process. And
the boundaries of this area can be determined, is possible at all, only
qualitatively.
Energy parameters and, first of all, intensity and instantaneous
power10) are principally
important parameters of fields introduced in the material in order to
investigate its properties. The point is that the introduced in the investigated
material field, interacting with proper material’s field, changes its
properties. And the nature, amount and lifetime11) of variations are determined by the
dynamic ratio of interacting fields’ energies. Most often variations of material
properties in the process of carrying out diagnostics are simply not noticed or,
either not assuming the possibility of such variations or being aware of them,
neglected on purpose considering the intensity of fields used for diagnostics to
be small. But in both cases we have another source of methodical error at
material’s characteristics measurement by indirect methods. And the value of
this error can be very high.
10) Power is energy
transmitted by the introduced field through the considered surface per time
unit. Intensity is a time-average energy transmitted by the introduced field
through a unitary pad perpendicular to the energy propagation direction, i.e.
intensity is an average specific power. Instantaneous power is a field power at
a specific moment of time.
11) Lifetime is a
conventional time period during which the amount of variations caused by
external effect decreases to the pre-specified value. Variations lifetime is
determined by the ratio of relaxation and retardation (aftereffect) rates.
Besides, most of the methods pretending to the quantitative
evaluation of the measured material’s characteristics are
relative since they are based on measurement of used physical field
informative parameter variations in the loaded and unloaded states of the
material. This is achieved either by relieving the load from the test object
(which is seldom practicable) or by the use of reference specimens compared to
the test object. It is clear that both alternatives introduce additional
error of known value: in the first case - due to relaxation-retardation
processes flow, in the second case – due to non-identity both of measurement
conditions and of the very materials of the specimen and the object, having not
only different pre-histories but most frequently different shapes.
Consequently, these not taken into account before methodical
mistakes12) in
determination of mechanical characteristics by indirect methods, being a basic
component of the resulting measurement error cannot be expressed
quantitatively. And this means that at such an approach it is
not correct to speak about the authenticity of quantitative results of
mechanical characteristics measurement by indirect methods.
12) Methodical
mistakes are traditionally considered the mistakes associated with correctness
of the measurement process performing - the measurement technique, which leads,
as it follows from the above-said, to principal errors.
The last comment is also fair because there is no sufficiently
convincing expert method of assessing the material’s stressed state
determination correctness and authenticity.
Indeed, one of the most widespread methods of stress measurement -
the method using strain sensors, being the most trusted by specialists, though
it may seem strange, is also indirect and belongs to electric methods since it
uses the dependence of a sensitive element’s electric resistance on its
geometric dimensions. I.e., this is actually an indirect method of strain
measurement, which is, of course associated with the mechanical stress value via
the elastic modulus but, unfortunately, not only with it. Therefore the
application scope of the strain-gage method of stress measurement is restricted
to elastic area, and the less we know about the investigated materials’
properties, the less we can say about the stress, and besides, not inside the
material but only on its surface.
Even destructive methods like the method of holes, the method of
columns, trepanation method and others, in fact, cannot be standard since they
introduce their own residual stresses due to material machining at hole drilling
or columns cutting.
And, finally, the main and the most unpleasant drawback of all
non-destructive methods is that, while allowing assessing with this or that
(even high) error the amount of stress, they do not provide the opportunity to
determine the nature of strains caused by stresses actually existing in the
material, i.e. to determine the material’s state (brittle or plastic) and to
assess the degree of its closeness to the material’s critical states (creep or
failure). The reason is in limited informative capabilities of the
methods traditionally using for measurements not more than 4
independent informative parameters of physical fields used for diagnostics.
5. Conclusions
Thus, while noting the highest development level of modern non-destructive
methods and means of materials and structures diagnostics, one has to state not
only the lack of means for authentic determination of materials’ SSS
characteristics in structures of operated objects but also impossibility to
assess the very authenticity of the obtained results.
Generalizing the results of the carried out analysis, the following
conclusions can be drawn:
-
all the currently known methods, except for mechanical ones, are
indirect and relative;
-
the variety of ultrasound methods indicates their potentially high
self-descriptiveness, however, the currently available means use not more than
4 independent informative parameters;
-
ultrasound methods realized by the well-known technical
means, at all their variety, being integral spectral or integral
amplitude-phase, are used as indirect methods;
-
all currently known diagnostic means measure only certain
parameters of the sued physical fields associated in a general case not with
mechanical stresses but with a certain aggregate of the material’s SSS
characteristics, by the way correlated by insufficiently studied and not
always monotonous and unambiguous regularities;
-
it is impossible to determine the nature and amount of the
methodical error of the material’s stressed state characteristics
measurement;
-
authenticity and, moreover, accuracy of the material’s stressed
state characteristics measurement by non-destructive physical
methods, described by diagnostic means developers, raise serious
doubts;
-
there is no sufficiently convincing expert method for assessing the
correctness of the material’s stressed state characteristics determination by
non-destructive physical methods..
6. Analysis and systematization of the reasons for low effectiveness of
non-destructive methods application for SSS diagnostics
The obvious reason for such a long lack of vitally necessary improvement of authenticity of assessment and predicting of terms and conditions of
critical objects safe operation is dissociation of strength specialists and
diagnostic methods and means developers. This dissociation is the
reason that strength specialists, due to the lack of objective characteristics
reflecting the currently formed material’s properties, develop various
calculation techniques based on any available characteristics, which at least
qualitatively and at least partially provide the idea of the current material’s
state. And diagnostic methods and means developers, being in the proud solitude,
"became thoroughly engrossed" in the search for methods and means of residual
stresses determination sometimes not thinking about the authenticity of
measurement results.
This obvious reason for insufficient application effectiveness of structural
materials’ SSS diagnostic means at objects life assessment cane be formulated
more strictly: the lack of scientifically grounded concept of materials’
stress-strained state (SSS) diagnostics and of the general concept of complex
diagnostics. Such formulation is so far of a private nature, so to say,
not concerning the state of things with strength specialists, but it already
brings in some elements of constructivism as it points out the direction of
actions and requires deeper analysis of the situation formed.
The results of further analysis demonstrate that the true depth reasons of
"stagnation" in solution of the main problem are more complex and form two
problems, which are common for strength sciences and diagnostic methods
sciences:
-
ideological: the lack of clear idea of the determining
role of a certain number of basic independent characteristics of the material
and about their functional-determining interrelation with the material’s
stress-strained state (SSS) characteristics and, as a consequence, the
lack of scientifically grounded methodology determining the goals,
tasks and criteria of structural materials’ SSS diagnostics;
Indeed, the lack of requirements to the measured SSS
characteristics, the lack of metrological basis for certification and
calibration of materials’ SSS characteristics measurement means lead to
ambiguity of initial requirements and wrong methodical approach to developed
means, which results in not only inadmissibly low authenticity of measurement
results but very often impossibility of correct identification of the measured
parameter of the physical field used and of the measured physical
characteristic of the investigated material as well. Besides, it is
practically impossible to assess the authenticity of results (if, as it was
note above, it can be spoken about at all) due to the lack of methodical and
metrological recommendations and norms.
-
physical: insufficient understanding, and in a number of
cases unstudied physical processes of interaction of fields used for
material’s properties diagnostics with its proper fields and, as a
consequence, no idea of insufficient self-descriptiveness of non-destructive
diagnostic methods and means used for investigation of complex physical
processes of the material’s internal energy re-distribution in the form of
re-distribution of the I-st, the II-nd and the III-d type stresses determined
by basic characteristics of the material and, at the same time, determining
its SSS.
It should be especially noted that dangerous trends of simplified
approach to residual life assessment of complex objects appeared during the
last years. Some developers of residual stress measurement means, carrying out
tests on samples in conditions of uniaxial loading, obtain good correlation of
measurement results for one, or in the best case, two parameters of the used
physical fields with the value of the load being variable right up to failure.
Not troubling themselves by studying the processes of material’s resistance to
external loads, not trying to understand fracture mechanics, they transfer the
obtained results to real objects thinking that a unique means for measurement
of the test object’s residual life was developed. This, at the least,
discredits the new interesting solutions, but the main point is that the price
of such an approach to wards the most complex problem of residual life
estimation may be terrible.
The conducted analysis if the reasons for insufficient application
effectiveness of structural materials’ SSS diagnostic means at life assessment
of complex engineering constructions demonstrates their objectiveness, the most
important consequence of which in the moral aspect should be the fair shared
responsibility for the lack of the required means for materials’ properties
diagnostics among the strength specialists and the developers of diagnostic
methods and means. Realizing the equal responsibility will, of course, bring
together positions of both parties solving, in fact, the same problem -
providing acceptable guarantees of objects safety, but the efforts can be united
only on condition of constructive approach.
The main thing is that analytically grouped reasons already gain another,
constructive nature specifying the way of solving of the most actual problem of
assurance of complex engineering objects safe operation.
7. Suggestions
To the authors’ opinion, in order to solve the problem of authentic
measurement of structural materials’ and welded joints’ stress-strained state
characteristics the following particular measures need to be performed:
7.1. Development of unified scientifically grounded
requirements to methods and means for the material’s SSS measurement. These requirements should:
-
be based on clear idea of the determining role and of interrelation of
independent basic characteristics of the material - this is an ideological
basis;
-
have a new classification of methods and means for stress-strained
state characteristics measurement of materials in general and of
welded joints in particular;
-
contain classification, list and criteria for assessment of the
material’s basic characteristics and of its SSS characteristics, and
these characteristics should, on the one hand, be subject to obligatory
measurement at diagnostics of the material’s state and, on the other hand,
they should be subject to obligatory application as basic characteristics at
calculations of the actual or predicted life. This will, of course, require
correction of the life estimation techniques, but only in this way, by
creating conditions for bringing together strength sciences and diagnostics
sciences, the problem of achieving the required level of objects safety can be
solved.
7.2. Development of the technique and means for
metrological calibration and qualification of SSS parameters measurement means allowing assessing objectively the effectiveness and accuracy of the
developed means. Creating of authentic expert method for diagnostic means
calibration, of course, seems to be a rather difficult task, the solution of
which may be delayed. Nevertheless, a unified system of standard calibration
means (for example, of samples and techniques) need to be introduced urgently,
at least conventionally. Such a unified system will allow not only comparing
correctly various methods of diagnostics but it may become in future a certain
prototype of diagnostic results assessment criteria.
7.3. It is necessary to start the
development of normative documents regulating measurement of materials’ SSS
parameters at object diagnostics depending on the category of
their potential danger for man and environment.
In 2003 under the authors’ initiative and jointly with Gosstandard ТC-132
"Engineering diagnostics" the draft standard "Non-destructive testing.
Stress-strained state tests on industrial objects and transport. General
requirements" was developed. Concerned organizations and private persons have
discussed this draft standard.
It should be noted in conclusion that investigation of complex processes of
the materials’ proper energy re-distribution under the influence of external
force, magnetic and other fields will require knowledge from the fields of
science, which seem to be far away from practical problems solved: quantum
physics, solid-state physics, metal physics, dislocations theory, elasticity,
plasticity and strength theories, fracture mechanics, electromagnetic field
theory and even radio engineering basics. This, of course, determines the high level of requirements to
specialists developing various SSS inspection methods. It should be noted
that structural materials’ SSS diagnostics represents the next
after flaw detection, higher level of diagnostics and requires
a new ideology, a new concept. Only the new concept is able not only to
reconcile various physical methods of non-destructive testing, which excellently
got together and supplemented each other within flaw detection but "conflicting"
with each other at present within this new type of diagnostics, but also, taking
into account specificity of their physical "interrelations", to unite them in a
unified system able to sufficiently accelerate the solution of the problem of
authenticity improvement of complex engineering objects’ residual life
assessment. |