Method of measuring duration of serviceability of metals

FIELD: testing of strength properties of metals.

SUBSTANCE: method can be used for estimation of deformation-strength properties due to applying load as well as for determining damages by means of X-ray diffraction analysis. Values of structural-sensitive parameter of crystal lattice of tested material are determined by X-ray diffraction analysis in initial and post-deformation states. Deformation-strength characteristics of metal are determined by calculation from changes in structural-sensitive parameter. Serviceability is judged by comparing really achieved characteristics with admissible ones. Width of X-ray line β is used as structural-sensitive parameters. Strength of deformation P, deformation Δl provided by the deformation and corresponding values of structural-sensitive parameter β are registered during testing. Dependence of true stresses S and structural-sensitive parameter β on degree of relative residual deformation δ are calculated on the basis of P and Δl. Destruction diagram (S-δ½) and linearized diagram (β½-δ½) are built to show inflection points. Deformation-strength characteristics S_D and δ_D corresponding to inflection point at destruction diagram (destruction point D) is taken as criterion of admissible surface strength which provides maximal serviceability of metal. Factor of merit η and factor of destruction Δ can be also taken as criteria of serviceability of metal.

EFFECT: improved precision of estimation.

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The proposed solution relates to the field of study of strength properties of metals and for the evaluation of their deformation and strength characteristics by application thereto of tensile (compressive) static loads and determine the resulting damage by x-ray structure analysis.

The problem of accurate determination of resource efficiency of technical products in General and metallic materials in particular is becoming increasingly important, given the constant expansion of the range of these materials, tightening their mode of operation, the combined effect on them of the power, process, chemical and other factors.

In 60-ies widely developed fracture mechanics of materials and structures, leading back to the classical works Griffiths (1920). The traditional calculation of the strength of the particular structure or component is based on that design (detail) is destroyed when its dangerous section is reached the voltage level equal to the tensile strength of the material (or some ultimate strain). However, practice shows that the breaking load is often much less than theory predicts. This is due to the presence of the structural elements of technological or operational defects. The most dangerous and which of them cracks. Using the criteria of fracture mechanics, it is possible to estimate the size of the cracks that are allowed in different zones of the structures under service loads. Such calculations are based norms defects governing permissible sizes of cracks and crack-like defects for specific types of structures under static, cyclic and dynamic loading.

Criteria of strength and workability of the metal for these types of loading are different. However, it is still in design and engineering calculations, estimates durability and reliability are used only traditional statistical data on the elastic limit, yield point, and a temporary resistance. This is mainly due to the ease of use of well-designed laws of continuum mechanics. When considering plasticity and finite deformation theory of mechanics normally comes from the fact that the effects of history are not important, and the presence of microdefects in the surface layers and uniformly dispersed throughout the volume of the material are not taken into account. Currently, it is well known that the fracture of solids, the performance of machine parts and structures (life) depend, and in some cases are determined by the properties of the surface layers is the presence of microcracks. Special effect comprising their surface layers has on the fatigue processes, which accompany the operation of all moving into the land, air and water environments, objects, stationary power plants, machine tools, products, organizations and home appliances. The leading role belongs to the superficial layers of frictional interaction of the metals, when the surface layers are the main working volume and are exposed to different environments (lubricants).

According to the requirements of modern technology, it is obvious that without assessing the physical state of the surface layers, especially from the point of view of their destruction (the degree of micro destructions), it is impossible to assess the quality of the material, its structural and technological capabilities, as well as the resource of his health.

When solving problems on the definition of resource efficiency of metallic materials, there are three kinetic stage of the process of destruction, each of which corresponds to a scale determined by the thickness of the metal layer, in which the localized deformation processes (see, for example, Nthalire and other Forecasting wear resistance and durability of materials and machine parts, Novosibirsk, 1997, p.7). The 1st stage is the damaging stage of accumulation of defects and micro destructions has the microscale of the order of 0.01-0.3 μm. 2nd - stage development of cracks occurs at the scale of 0.1 to 20 μm, which allows emanate provisions and criteria linear mechanics. 3-I - fracture stage is the macro-scale of the order of 10...10⁴um.

Because the process of destruction begins (usually) from the surface, of course, that research involved evaluation methods structure-sensitive lattice parameters of the investigated metal, in particular, the method of x-ray analysis, one advantage of which is the fact that the recorded parameters of the structural condition are averaged over the volume and provide a good correlation with the physical properties of materials. The experimental basis of radiographic detection of structural changes of the investigated metal options diffraction pattern on the radiograph or the diffraction pattern. Registered parameter may be the distance between adjacent parallel planes of the crystal lattice (d), which carries information about the elastic stresses of the 1st kind, associated with the elastic deformation of the investigated metal. Another parameter is the width of the interference lines on the radiograph (β), which is a criterion for evaluating the stresses of the 2nd kind, due not only to the micro-deformation of the crystal lattice, but also the density of dislocations, describing the destruction of the crystals (bending, compression, fragmentation) and the presence of dispersed ELEH the clients substructure of metal.

One of the known variants of the use of radiographic techniques in relation to the objectives of the study the deformation of metals is a method of moving the x-ray beam, which gives the opportunity to receive information about their condition at different depths in the range of 0.1 to 20 μm (depending on the wavelength and angle of incidence of the x-ray beam to the sample surface), i.e. within the first two above-mentioned stages of destruction, which are of the greatest practical interest.

The solution closest to the proposed by its technical essence and adopted for the prototype is implemented in the known device (A.S. of the Russian Federation No. 2115901, CL G 01 L 1/25) method of determination of residual stresses in real structures, in particular, in the pipeline, to change structure-sensitive parameter of the crystal lattice of the investigated metal.

The method is that the x-ray structure analysis determines the value of the structure-sensitive parameter of the crystal lattice of the metal pipe in its original and postdeformation (after welding) States. As structure-sensitive parameter of the metal used, the distance between the planes of the crystal, d is obtained on the metal in the initial state (d_oand after various the CSO type of effects (deformation, strengthening, technology, etc.) - d.

Comparing the obtained values of d and d_o, by calculation, determine the deformation-strength characteristics of the metal, due to power or technological factors. In particular, knowing d and d_oyou can determine the elongation Δd=d-d_oin the direction perpendicular to the reflective surface of the crystal. Then, in accordance with the law of Hooke determine residual stresses σ_OSTin metal according to.

Knowing σ_OSTcompare it σ_Tr- yield strength tensile given metal and thereby determine a margin of safety, i.e. the resource of his health.

The disadvantage of this method is that it is not possible to estimate the resource dependency of the strength and workability of the metal from the degree of defectiveness (micro destructions) in the surface layers that inevitably arise while processing technological methods and conditions.

Thus, the task is to eliminate this drawback by considering the degree of damage to the surface layers.

In accordance with the task proposed method of determining resource efficiency of metal to change the structure-sensitive parameter to its crystal lattice, it is to the prototype, is that the x-ray structure analysis determines the values of the structure-sensitive parameter of the crystal lattice of the studied metal in its original and postdeformation States, by changing this parameter raschetnym determined by the deformation-strength characteristics of the metal, due to operational loads and/or konkretnimi its process, and the resource of his health is judged by comparing the actually obtained deformation and strength characteristics valid.

The method differs from the prototype in that as a structure-sensitive parameter, use the width of the x-ray line (β), during the test, record the deformation force (P), the resulting deformation (Δl) and the corresponding values of the structure-sensitive parameter (β), which then determine by calculation according to the true stress (S) and structure-sensitive parameter (β) the degree of relative residual deformation (δ), build the decomposition chart (S-δ^1/2) and linearized chart (β^1/2-δ^1/2with registration at points of inflection, caused by the transition of elastic-plastic deformation stages in plastically-destruction phase, and formazione-strength characteristics of S _Dand δ_Dcorresponding to the inflection point on the decomposition chart (point of destruction (D)is taken as the criterion allowable surface durability, ensuring maximum availability of the material.

While the quality of the material can be judged by the coefficient of q η=δ_p/δ or by the coefficient of destruction Δ=δ_d/δ_pwhere δ - relative residual deformation of the material; δ_p- elasto-plastic component in the total relative residual deformation; δ_d- plastically-destructive component in the total relative residual deformation.

The proposed method is illustrated by the following drawings.

Figure 1(a, b) presents a typical diagram expansion of the metal in the coordinates of the "load - extension" (P-Δ (l)(a) and true stress - plastic strain (S-δ^1/2)(b), so D is the point of destruction.

Figure 2 - typical linearized diagram of the width of the x-ray lines from the full relative residual deformation (β^1/2-δ^1/2).

Figure 3 - experimental dependence of the width of the x-ray line (β^1/2from the size of a full relative residual deformation (δ^1/2) sample of aluminium alloy AMg6.

It should replace the IC, what to linearize presents dependencies and other known methods, in particular, logarithmic or semi-logarithmic coordinates, or presenting them respectively in tabulated form. The location of the point D remains unchanged.

The method is implemented as follows.

Mechanical tests conducted by the deformation of the sample of studied metal in a known manner on a tensile testing machine, for example, type "Instron", with a record of traditional charts stretching (compression) in the coordinates of the "load - extension" (P-Δl (see figa). The data and determine the true stress (S=P/F_i) and the corresponding value of the relative residual deformation (δ), with which you can build now the decomposition chart (S-δ^1/2) (figb).

On the diagram explicitly detected inflection point (t.D.), which, as research has shown, is located between the yield stress (S_T) and tensile strength (S_In) and corresponds to the transition of elastic-plastic deformation stages in plastically-destruction phase, which is caused by the appearance and accumulation of micro-damage in the material.

Deformation-strength characteristics corresponding to the inflection point on the decomposition chart, respectively, S_Dand δ_D accept criterion for allowable surface durability, ensure maximum performance of the investigated metal.

In parallel, on the same or another sample that is identical to the subjects (i.e. with the same technological heredity) and subjected to a predetermined strain, conduct x-ray diffraction analysis of the surface layers with the registration of the width of the interference line (β) and determine its dependence on the degree of relative residual deformation of the sample βf(δ). Communication parameter (β) with relative residual deformation is also present in the linearized form, for example, in the form of

Then after or during operation of the investigated material (if it is part of the natural site) or material processing technology methods (rolling, forging, turning, grinding, heat treatment and so on) defined in the surface layer of the investigated material is structure-sensitive parameter β_iwith the help of which you can judge the degree of the actual defects in the analyzed layer.

Having a specific value β_iand using the previously obtained for this material dependence β^1/2-δ^1/2(see figure 2)find a specific value degree relative residual deformation of the surface layer δ_idue to previous exp is acacia or any kind of treatment. Then compare the obtained values δ_i- coordinate δ on the decomposition chart of the material (see figb). While the actual deformation δ_iin the surface layer did not exceed the deformation characteristic of the point of destruction D, you can be sure that the material is fully functional, since the degree of damage is small and close to its original state.

When specified or actually achieved full relative residual deformation δthat is always equal to the sum of elastic-plastic δ_pand destruction δ_dcomponents (δ=δ_p+δ_d), degree expected or actual destruction of a material can be estimated by the so-called "coefficient of destruction" Δ=δ_d/δ_por "coefficient of q" η=δ_p/δthat can be easily located by linear dependencies - decomposition diagram S-δ^1/2(figb). Between them they related expressions η=1/1+Δ or Δ=1/η-1. It is obvious that until the current deformation δ_idoes not exceed the value of the characteristic point D, the entire deformation is mainly determined by its elastic-plastic component δ=δ_p, the decomposition component δ_dclose to 0.

As operation details (the h is) or material processing technology, the actual deformation in the surface layer δ _ieventually exceed δ_dand then the corresponding coefficient of q (η) becomes less than 1, and the ratio degradation (Δ) not equal to zero.

Strictly speaking, after the point of destruction D material cannot be considered continuous. This suggests that the material has plastically-destruction stage. From the point of view of the concept of safe damage (Strength, resource and safety of machines and structures /edited Nagahata and Ann, Moscow, 2000, s) further operation details (node, items), although still possible, but it should be borne in mind that it is associated with loss of reliability, with the risk level is higher, the more detail and the more actual deformation of the surface layer.

Thus, the elastic-plastic-destructive analysis of the behaviour of metallic material and its surface layers during deformation, based on the decomposition charts uniaxial tension (compression), is a sensitive method of evaluating the mechanical condition - degree of destruction (microdamage) of the material and its surface layers (coatings) in the exploitation, processing technology and so on, which in mechanics of deformed solids can improve the accuracy of forecasting RA is topolinski industrial products, evaluation of the quality of the material used, as well as to prevent possible emergency - sudden collapse, breakage of machine parts and structures.

With regard to the processing of metallic materials technological methods proposed criterion allows reasonably assign processing modes, which result in an optimum state of the surface layer from the point of view of its durability.

The advantage of the analysis of the material using the decomposition of the diagram, i.e. from the standpoint of the development and accumulation of micro-damage in the deformation process, is illustrated by the results of the development of the technology of forming of aluminium alloy AMg6. The alloy was subjected to stamping static and dynamic (pulsed magnetic field - PULSE) methods with the same degrees precipitation - 7,15 and 25%. If traditional mechanical characteristics HB, σ_tthat σ_insubsequent testing of specimens for tensile strength was almost the same (the scatter of the experimental data for these indicators were in the range of 0.3-4%), the degree of degradation of the alloys is significantly different. It was established experimentally that the transition in plastically-destruction region during subsequent tensile came to statically extruded sample before (δ=6.9 per cent)than for dynamically stanova the CSO (δ =10,8%). The coefficients of q if this were respectively equal to η_stat=0.6 and η_Dean=0.8, which resulted in an increase of two orders of magnitude durability under cyclic test - 2·10³up to 1·10⁵cycles at σ_max=1 MPa.

The ratio q of the alloy after the PULSE even when the thinning parts of 25% has a quite high value (η=0,7), while under static stamping the same item is destroyed. This suggests that when the deformation of the PULSE in the alloy structure is formed significantly fewer micro-damage and, consequently, it is possible to reduce the number of transitions in the manufacture of parts is a multistage process static punching replace one-, two-step process dynamic forming and thereby significantly improve the productivity and quality of the final product.

1. The method for determining the resource efficiency of metal to change the structure-sensitive parameter to its crystal lattice, consisting in the fact that the x-ray structure analysis determines the values of the structure-sensitive parameter of the crystal lattice of the studied metal in its original and postdeformation States, by changing this parameter by calculation, determine the stress-strain ha is acteristic metal, due to operational loads and/or specific types of technological processing, and the resource of his health is judged by comparing the actually received the deformation-strength characteristics with a valid, characterized in that as a structure-sensitive parameter, use the width of the x-ray line (β), in the process of testing register the deformation force (P), the resulting deformation (Δl) and the corresponding values of the structure-sensitive parameter (β), which then determine by calculation according to the true stress (S) and structure-sensitive parameter (β) the degree of relative residual deformation (δ), build the decomposition chart (S-δ^1/2) and linearized chart (β^1/2-δ^1/2with registration at points of inflection, due to the elastic-plastic transition stage in plastically deforming the decomposition stage, and deformation-strength characteristics of S_Dand δ_Dcorresponding to the inflection point on the decomposition chart (point of destruction (D)is taken as the criterion allowable surface durability, ensuring maximum availability of the material.

2. The method according to claim 1, characterized in that as the criteria working the ti metal is used, the ratio of the quality factor (η )is defined from the ratio

η=δ_p/δ,

where δ - relative residual deformation of a test specimen;

δ_p- elasto-plastic component in the total relative residual deformation of the sample.

3. The method according to claim 1, characterized in that as the health criteria of the metal using the factor decomposition (Δ), determined from the ratio

Δ=δ_d/δ_p,

where δ_d- destruction component in the total relative residual deformation of the sample.