CN110044703B - Method for measuring creep mechanical property of material by adopting clamped C-shaped ring small sample - Google Patents
Method for measuring creep mechanical property of material by adopting clamped C-shaped ring small sample Download PDFInfo
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Abstract
A method for measuring the creep mechanical property of a material by adopting a clamped-supported C-shaped ring small sample belongs to the field of measurement. The method comprises designing and manufacturing a clamped C-shaped ring small sample and a clamp; establishing a conversion relation between a small deformation stage and uniaxial creep of a clamped C-shaped ring small sample; establishing a finite element model, and obtaining a conversion formula coefficient according to a reference stress method; carrying out a plurality of groups of creep tests with the same temperature and different loads on the clamped C-shaped ring small sample; deriving a plurality of groups of loading point-time curves obtained by a creep test; carrying out differential processing on the loading point-time curve to obtain steady-state displacement rates under different load conditions; and placing the fixed-branch C-shaped ring in a log-log coordinate to perform linear fitting, so as to obtain the uniaxial creep parameters obtained by inversion of the fixed-branch C-shaped ring. The method has the advantages of simple sample stress, simple test equipment, capability of obtaining fracture data, independence of a conversion formula between single shafts and materials, large sample deformation and high measurement precision. Can be widely used in the field of measuring the creep mechanical property of materials.
Description
Technical Field
The invention belongs to the field of material creep measurement, and particularly relates to a method for measuring material creep mechanical property by adopting a clamped C-shaped ring small sample.
Background
In order to achieve higher energy utilization, the working temperature and working pressure of more and more equipment and components are continuously increased, which puts higher requirements on the performance of in-service equipment materials.
For high temperature equipment, creep is its most dominant failure mode. Creep is a concern for many engineering problems.
Creep, also known as creep, is a phenomenon in which a solid material undergoes a slow plastic deformation with increasing strain over time while maintaining a constant stress.
Creep often increases with increasing temperature. The rate of creep is related to material properties, loading time, loading temperature, and loading structural stress.
The creep property of the high-temperature equipment material, particularly the creep property of the in-service equipment material, is measured, and the method has important significance for predicting the residual service life of the equipment and making safety evaluation.
Creep test, i.e. a material mechanical property test for measuring the slow plastic deformation of a metal material under the action of constant temperature and constant stress for a long time.
In general, the creep performance of a material can be tested by a traditional uniaxial creep test, but the standard sample has larger volume and more required materials, so that the application of the standard sample in the material test of a service component is limited, and the development of a small sample creep method is promoted for these reasons.
As a new technology for measuring the creep property of a material, the small sample testing technology is an important method for evaluating the creep mechanical property of the material due to the characteristics of small sample size, small damage to the structural integrity of in-service equipment, simple and convenient processing and the like.
At present, small samples for measuring the creep mechanical property of in-service equipment materials mainly comprise plate samples represented by small punch samples, three-point bent straight rod small samples represented by fixed-support straight rod small samples, cantilever beam small samples and simply-supported three-point bent small samples, and annular small samples represented by rings.
The small plunger sample is difficult to obtain a conversion relation formula between the small plunger sample and a single shaft theoretically due to complex stress. The small sample of the solid support straight rod has small deformation and low test precision. Creep rupture data cannot be obtained by the cantilever beam small sample, the simply supported three-point bending small sample and the circular ring small sample. For the components such as pipelines in the plate-fin heat exchanger, flow channels of a brazing microcell and the like, the sampling volume is too small, and the sampling of a small punch bar sample and a three-point bent bar sample cannot be completed from the structure.
Disclosure of Invention
The invention aims to solve the technical problem of providing a method for measuring the creep mechanical property of a material by adopting a clamped C-shaped ring small sample. The method combines the advantages of simple theoretical analysis of the three-point bending small sample, large deformation of the circular ring small sample and high measurement precision, and provides a novel solid support C-shaped ring small sample. The sample is simple in stress, simple and easy in test equipment, capable of obtaining fracture data, independent of a conversion formula between single shafts and materials, large in sample deformation and high in measurement precision, and various defects of various small samples at present are overcome.
The technical scheme of the invention is as follows: the method for measuring the creep mechanical property of the material by adopting the clamped-supported C-shaped ring small sample is characterized by at least comprising the following steps of:
designing and manufacturing a clamped C-shaped ring small sample and a clamp, and assembling the sample and the clamp according to design requirements;
step (2), establishing a conversion relation between a small deformation stage and uniaxial creep of a clamped C-shaped ring small sample according to a beam bending model and a Norton creep constitutive equation;
step (3), establishing a finite element model, and obtaining a conversion formula coefficient according to a reference stress method;
step (4), carrying out a plurality of groups of creep tests with the same temperature but different loads on the clamped C-shaped ring small samples under the actually set test conditions;
step (5), deriving a plurality of groups of load point displacement-time curves with the same temperature but different loads obtained in the creep test in the step (4) from a computer matched with the creep test bed;
step (6), the displacement-time curve of the loading point obtained in the step (5) is subjected to differential processing, and when the displacement rate is not changed or changes little for a long period of time, the deformation of the sample can be considered to enter a steady-state stage, so that the steady-state displacement rates under different loading conditions are obtained;
and (7) substituting different loads and corresponding steady state displacement rates into the conversion relational expression between the clamped C-shaped ring and the uniaxial creep obtained in the steps (2) and (3), calculating to obtain corresponding load and steady state creep strain rate, and performing linear fitting on the load and steady state creep strain rate in a double logarithmic coordinate to obtain uniaxial creep parameters obtained by clamped C-shaped ring inversion.
Specifically, the cross section of the C-shaped ring small sample is a rectangular section;
the radius R of the neutral layer of the C-shaped ring small sample and the thickness H of the sample must meet the condition that R is H ≥ 5.
The clamp consists of an upper plate, a lower plate, a left baffle, a right baffle and a disc;
wherein, the upper plate, the lower plate and the disc are connected and fixed into a whole through screws;
a longitudinal guide groove is formed in the middle of the upper plate of the clamp, so that the centering loading of a pressure head is ensured;
the left baffle and the right baffle are arranged between the upper plate and the lower plate;
rectangular grooves are respectively formed in the left baffle and the right baffle;
the left baffle and the right baffle are connected through a bolt, and fixed end restraint of the sample is achieved.
Further, in step (1), a small sample of the clamped C-ring is manufactured in the size shown, and the sample and the jig are assembled as required.
Further, in step (2), the Norton creep constitutive equation is:
wherein,
the creep strain rate in the steady state phase, B is the creep constant, σ is the stress, and n is the creep stress index.
Further, in the step (2), the steady-state displacement rate equation of the loading point in the small deformation stage of the clamped-supported C-shaped ring small sample is as follows:
C(θ)＝-sinθ+0.308+0.904(1-cosθ)
wherein R is the radius of a neutral layer of the solid-supported C-shaped ring small sample, b is the thickness of the sample, P is the test load, H is the thickness of the sample, and alpha is a coefficient value introduced by a reference stress method. Theta is the included angle between the cross section of the curved bar and the horizontal plane, and when the bending moment at a certain position in the curved bar is zero, namely theta is equal to theta1，θ＝θ2。
Further, in the step (2), the conversion relation between the clamped-supported C-shaped ring small sample and the uniaxial creep is as follows:
wherein σeqIn order to be the equivalent stress,
in order to achieve an equivalent steady state creep strain rate,
in order to be a steady-state displacement rate,
further, before the step (3) is executed, checking whether a conversion coefficient between the clamped C-shaped ring and the uniaxial creep is obtained through finite elements or not, and if so, directly entering the step (4); if not, go to step (3).
Compared with the prior art, the invention has the advantages that:
(1) according to the technical scheme, the advantages of simplicity and convenience in theoretical analysis of the three-point bending small sample, large deformation of the circular ring small sample and high measurement precision are combined, and the novel solid support C-shaped ring small sample is provided;
(2) the solid-supported C-shaped ring small sample in the technical scheme of the invention has simple stress and simple test equipment, can obtain fracture data, is irrelevant to a conversion formula between single shafts and a material, and has large deformation of the sample and high measurement precision;
(3) the solid support C-shaped ring small sample in the technical scheme of the invention belongs to a ring small sample, and for ring members such as pipelines and the like, due to the special structures, the sampling is very convenient, and the structure of the member can not be damaged;
(4) in the technical scheme of the invention, the two ends of the solid-supported C-shaped ring small sample are restrained by fixed ends, so that the data fluctuation is small; meanwhile, the device is simple in stress, capable of obtaining fracture data, independent of a conversion formula and materials among single shafts, large in deformation and high in measurement accuracy.
Drawings
FIG. 1 is a flow chart of a method for measuring creep mechanical properties of a material according to the present invention;
FIG. 2a is a schematic cross-sectional view of a mounting structure of a clamped C-ring small sample according to the present invention;
FIG. 2b is a schematic view of a mounting structure of a clamped C-ring small sample according to the present invention;
FIG. 3 is a schematic view of a finite element model of a clamped C-ring small sample according to the present invention;
FIG. 4 is a graph showing the relationship between log β and creep stress index n;
FIG. 5 is a schematic structural view of a creep test apparatus;
FIG. 6 is a graph of creep test load point displacement versus time;
FIG. 7 is a graph of steady state strain rate versus stress.
In the figure, 1 is a screw, 2 is an upper plate, 3 is a left baffle, 4 is a lower plate, 5 is a pressure head, 6 is a right baffle, 7 is a threaded hole, 8 is a sample, 9 is a disc, 10 is a guide groove, 51 is a heating furnace, 52 is a linear displacement transducer LVDT, 53 is a weight, and 54 is a computer.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples.
As shown in fig. 1, the method of the present invention is as follows:
(1) designing and manufacturing a clamped C-shaped ring small sample and a clamp, wherein the cross section of the C-shaped ring small sample is a rectangular section. The radius R of the neutral layer of the C-shaped ring small sample and the thickness H of the sample must meet the condition that R is H ≥ 5. Rectangular grooves are formed in the left baffle and the right baffle of the clamp respectively, and the left baffle and the right baffle are connected through bolts to achieve fixed end restraint of a sample. A guide groove is formed in the middle of the upper plate, and centering loading of a pressure head is guaranteed. The upper plate and the lower plate are connected with a disc at the bottom through a hexagon screw;
(2) establishing a conversion relation between a small deformation stage and uniaxial creep of the clamped C-shaped ring small sample according to a beam bending model and a Norton creep constitutive equation;
(3) firstly, judging whether a conversion coefficient between a clamped C-shaped ring and uniaxial creep is obtained through a finite element or not, and if so, directly entering the step (4); if not, entering the step (3) to establish a finite element model, and obtaining a conversion formula coefficient according to a reference stress method;
(4) under the actually set test conditions, carrying out a plurality of groups of creep tests with the same temperature and different loads on the clamped C-shaped ring small samples;
(5) deriving a plurality of groups of loading point displacement-time curves (loading point-time curves for short, the same below) with the same temperature and different loads obtained in the creep test in the step (4) from a computer matched with the creep test bed;
(6) carrying out differential processing on the loading point displacement-time curve obtained in the step (5), and when the displacement rate does not change for a long period of time or changes little, determining that the deformation of the sample enters a steady-state stage to obtain the steady-state displacement rate under different load conditions;
(7) and (3) substituting different loads and corresponding steady state displacement rates into the conversion relation between the clamped C-shaped ring and the uniaxial creep obtained in the steps (2) and (3), calculating to obtain corresponding load and steady state creep strain rate, and putting the strain rate into a log-log coordinate to perform linear fitting to obtain uniaxial creep parameters obtained by the clamped C-shaped ring inversion.
Example (b):
the material is 1.25Cr0.5MoSi, four groups of load values of a clamped C-type ring test are respectively 40N, 43N, 46N and 49N, the test temperature is 550 ℃, the cross section of a sample is a rectangular section, the thickness is 1.2mm, the inner radius is 4.6mm, the outer radius is 5.4mm, the length of a straight section of the sample is 2mm, and the creep mechanical property of the material is measured by using the method of the technical scheme of the invention:
1. the cross section of the C-shaped ring small sample is a rectangular section, the thickness of the cross section is 1.2mm, the inner radius of the cross section is 4.6mm, the outer radius of the cross section is 5.4mm, and the length of a straight edge section of the sample is 2 mm.
As shown in fig. 2a and 2b, the jig is composed of an upper plate 2, a lower plate 4, a left baffle 3, a right baffle 6, and a circular disk 9.
Wherein, the upper plate 2, the lower plate 4 and the disc 9 are fixedly connected into a whole through a screw 1;
a longitudinal guide groove 10 is formed in the middle of the upper plate of the clamp, so that the centering loading of the pressure head 5 is ensured;
the left baffle 3 and the right baffle 6 are arranged between the upper plate and the lower plate;
rectangular grooves are respectively formed in the left baffle and the right baffle;
and the left baffle and the right baffle are connected through a bolt positioned in the threaded hole 7, so that the fixed end of the sample 8 is restrained.
2. Establishing a steady-state displacement rate equation at the loading point of the clamped C-shaped ring small sample according to a beam bending model and a Norton creep constitutive equation:
C(θ)＝-sinθ+0.308+0.904(1-cosθ)
wherein R is the radius of a neutral layer of the solid-supported C-shaped ring small sample, b is the thickness of the sample, P is the test load, H is the thickness of the sample, and alpha is a coefficient value introduced by a reference stress method. Theta isThe included angle between the cross section of the curved bar and the horizontal plane, when the bending moment is zero at a certain position in the curved bar, namely theta is equal to theta1，θ＝θ2。
The conversion relation between the clamped C-shaped ring small sample and the uniaxial creep is as follows:
wherein σeqIn order to be the equivalent stress,
in order to achieve an equivalent steady state creep strain rate,
in order to be a steady-state displacement rate,
3. since the coefficients of the clamped C-ring and the uniaxial transformation formula were not obtained, a finite element model of a small sample of the clamped C-ring was built in the ABAQUS finite element analysis software, as shown in fig. 3.
The straight section of the sample is fixed in four planes in the axial direction (U1 ═ U2 ═ U3 ═ 0). A constant load P is applied to the sample center by the cylindrical indenter, constraining the indenter to all degrees of freedom except in the z-direction. The contact form between the pressure head and the sample is surface-surface contact, the friction coefficient is 0.3, and the sample grid is of a three-dimensional eight-node uncoordinated integral unit type (C3D 8I). And (3) carrying out numerical simulation under eight groups of conditions with n values from 1 to 8 to obtain steady-state displacement rates under different conditions, substituting the steady-state displacement rates into the displacement rate equation at the loading point in the step (2), wherein the relation between log beta and the n value is shown in figure 4.
As shown in fig. 4, when α ═ η, log β and n are approximate straight lines, and β can be obtained from the intersection of the straight lines and the vertical axes, that is, η ═ 0.38, and β ═ 0.5.
4. Four sets of tests with clamped-supported C-ring small sample loads of 40N, 43N, 46N and 49N, respectively, were performed at a test temperature of 550 ℃.
The structural configuration of the creep test apparatus for carrying out the test is shown in fig. 5.
Specifically, the creep test bed comprises four parts, namely a heating furnace 51, a linear displacement sensor LVDT 52, a weight 53 and a computer 54, wherein the heating furnace is used for generating the temperature required by the test, the weight is used for applying pressure to the sample, the linear displacement sensor LVDT is used for detecting the displacement of the sample, and the computer is used for recording and storing the deformation data of the sample in the test process to obtain the creep displacement of the indenter.
Since creep testing is prior art, the specific testing procedure is not described in detail herein.
5. The multiple sets of load point-time curves with the same temperature but different loads obtained from the creep test in step (4) were derived from a computer associated with the creep test stand, and are shown in fig. 6.
6. And (3) carrying out differential processing on the loading point-time curve obtained in the step (5), and when the displacement rate does not change or changes little for a long period of time, determining that the deformation of the sample enters a steady-state stage to obtain the steady-state displacement rates under different loading conditions, wherein the result is shown in fig. 7.
7. And (3) substituting different loads and corresponding steady state displacement rates into the conversion relation between the clamped C-type ring and the uniaxial creep obtained in the steps (2) and (3), calculating to obtain corresponding loads and steady state creep strain rates, putting the loads and the steady state creep strain rates into a log-log coordinate to perform linear fitting, and obtaining the uniaxial creep parameters obtained by the clamped C-type ring inversion, wherein the material parameters B obtained by the uniaxial test fitting are 6.69E-19, n is 6.38, the material creep parameters B obtained by the clamped C-type ring inversion are 5.58E-19, and n is 6.45. The creep parameters of the material obtained by the clamped C-shaped ring small sample are very close to those of a single shaft, and the data is accurate.
According to the technical scheme, a clamped C-shaped ring small sample and a clamp are designed and manufactured firstly, and then a conversion formula coefficient is obtained through a finite element simulation method; if the conversion formula coefficient between the clamped C-shaped ring and the uniaxial creep is obtained, the conversion formula coefficient obtained before can be directly used without repeating the step of finite element simulation; then obtaining a plurality of groups of loading point-time curves with the same temperature but different loads through a creep test, carrying out differential processing on the curves to obtain steady-state displacement rates under different load conditions, and substituting the loads and the steady-state displacement rates into a conversion formula to calculate corresponding stress and steady-state strain rates; and finally, performing linear fitting on the calculated stress and steady-state creep strain rate in a log-log coordinate to obtain the creep parameters of the material.
In the technical scheme of the invention, the conversion formula of the clamped C-shaped ring small sample and the uniaxial creep is independent of materials, the measurement precision is high, and the result is reliable.
The technical scheme of the invention solves the problems of complex data processing, imperfect theoretical analysis, large external influence factors and the like of various small samples at present, and provides the creep test method which is simple, convenient, high in precision and low in cost. The test method for obtaining the material creep performance parameters is provided for evaluating the creep life of equipment under a high-temperature condition and detecting the creep mechanical property of a new material in long-term service.
The invention can be widely applied to the field of measurement of material creep mechanical properties.
Claims (6)
1. A method for measuring the creep mechanical property of a material by adopting a clamped-supported C-shaped ring small sample is characterized by at least comprising the following steps:
designing and manufacturing a clamped C-shaped ring small sample and a clamp, and assembling the sample and the clamp according to design requirements; two ends of the C-shaped ring small sample are fixed in the clamp in a fixed end restraining manner;
step (2), establishing a conversion relation between a small deformation stage and uniaxial creep of a clamped C-shaped ring small sample according to a beam bending model and a Norton creep constitutive equation;
step (3), establishing a finite element model, and obtaining a conversion formula coefficient according to a reference stress method;
step (4), carrying out a plurality of groups of creep tests with the same temperature but different loads on the clamped C-shaped ring small samples under the actually set test conditions;
step (5), deriving a plurality of groups of loading point-time curves with the same temperature but different loads obtained in the creep test in the step (4) from a computer matched with the creep test bed;
step (6), the displacement-time curve of the loading point obtained in the step (5) is subjected to differential processing, and when the displacement rate is not changed or changes little for a long period of time, the deformation of the sample can be considered to enter a steady-state stage, so that the steady-state displacement rates under different loading conditions are obtained;
and (7) substituting different loads and corresponding steady state displacement rates into the conversion relational expression between the clamped C-shaped ring and the uniaxial creep obtained in the steps (2) and (3), calculating to obtain corresponding load and steady state creep strain rate, and performing linear fitting on the load and steady state creep strain rate in a double logarithmic coordinate to obtain uniaxial creep parameters obtained by clamped C-shaped ring inversion.
2. The method for measuring the creep mechanical property of a material by using a clamped C-shaped ring small sample according to claim 1, wherein the cross section of the C-shaped ring small sample is a rectangular section;
the radius R of the neutral layer of the C-shaped ring small sample and the thickness H of the sample must meet the condition that R is H ≥ 5.
3. The method for measuring the creep mechanical property of a material by using a clamped C-shaped ring small sample as claimed in claim 1, wherein the fixture is composed of an upper plate, a lower plate, a left baffle, a right baffle and a disc;
wherein, the upper plate, the lower plate and the disc are connected and fixed into a whole through screws;
a longitudinal guide groove is formed in the middle of the upper plate, so that the centering loading of a pressure head is ensured;
the left baffle and the right baffle are arranged between the upper plate and the lower plate;
rectangular grooves are respectively formed in the left baffle and the right baffle;
the left baffle and the right baffle are connected through a bolt, and fixed end restraint of the sample is achieved.
4. The method for measuring the creep mechanical property of the material by using the clamped C-type ring small sample according to the claim 1, wherein in the step (2), the steady state displacement rate equation of the loading point at the small deformation stage of the clamped C-type ring small sample is as follows:
C(θ)＝-sinθ+0.308+0.904(1-cosθ)
wherein R is the radius of a neutral layer of the solid-supported C-shaped ring small sample, b is the width of the sample, P is the test load, H is the thickness of the sample, and alpha is a coefficient value introduced by a reference stress method; theta is an included angle between the cross section of the clamped C-shaped ring small sample and the horizontal plane, and when the bending moment at a certain position in the clamped C-shaped ring small sample is zero, namely theta is equal to theta1，θ＝θ2(ii) a B is a creep constant, n is a creep stress index,
the steady-state displacement rate of the loading point is obtained in the small deformation stage of the clamped C-shaped ring small sample.
5. The method for measuring the creep mechanical property of a material by using a small specimen of a clamped C-type ring as claimed in claim 4, wherein in the step (2), the conversion relation between the small specimen of the clamped C-type ring and the uniaxial creep is as follows:
wherein σeqIn order to be the equivalent stress,
in order to achieve an equivalent steady state creep strain rate,for the steady-state displacement rate of the loading point in the small deformation stage of the clamped C-shaped ring small sample,
eta is the stress coefficient value, P is the test load, R is the neutral layer radius of the clamped C-shaped ring small sample, b is the width of the sample, H is the thickness of the sample, n is the creep stress index, and alpha is a coefficient value introduced by a reference stress method.
6. The method for measuring the creep mechanical property of the material by using the clamped C-type ring small sample is characterized in that before the step (3) is executed, whether the conversion coefficient between the clamped C-type ring small sample and the uniaxial creep is obtained through finite elements or not is checked, and if the conversion coefficient exists, the step (4) is directly carried out; if not, go to step (3).
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| GB0821168D0 (en) * | 2008-11-19 | 2008-12-24 | Univ Nottingham | Specimen creep test and methods of using such tests |
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| JP2007085778A (en) * | 2005-09-20 | 2007-04-05 | Fujitsu Ltd | Creep physical property testing method, device, and program |
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Non-Patent Citations (2)
| Title |
|---|
| 《A new approach to evaluate material creep properties by C-shape ring specimen with fxed constraints》;Hai-Yang Yu et al.;《International Journal of Pressure Vessels and Piping》;20210303;第191卷;第104357-1~10页 * |
| 《Determination of creep parameters from three-point bending test with C-shape specimen》;Guo-Yan Zhou et al.;《International Journal of Pressure Vessels and Piping》;20131031;第110卷;第57-60页 * |
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