On the problem of stress-strain state characteristics measurement of structural materials on complex engineering objects. Energy concept of materials stress-strain state (SSS) diagnostics
Dr. V.T. Vlasov, Dr., Professor A.A. Dubov
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.
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.
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.
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.
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.
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.
Table 1. Dynamics of scientific efforts distribution by directions
||Characteristic of direction
||Development of new means realizing the traditional approach to diagnostics
||Improvement of sorting norms based on statistic investigations
||Search for new approaches to materials and structures diagnostics (stress measurement and the acoustic emission (AE) method)
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).
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.
Sorting the known methods of materials’ state diagnostics by the type of physical fields, we obtain the following types:
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.
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.
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.
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.
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.
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.
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;
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.
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.
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.