CN102564844B - Method for measuring compact tension specimen fracture parameters by using double extensometers - Google Patents

Abstract

The invention discloses a novel method for measuring compact tension specimen fracture parameters by using double extensometers. The novel method comprises the following steps of: measuring two opening displacements by using the double extensometers; obtaining a crack tip opening displacement (CTOD) of a compact tension test through calculation by using a triangular relation; and calculating a CTOD value under each load and a nominal stress corresponding to the CTOD value, and obtaining a relation between the nominal stress and the CTOD, thereby obtaining a CTOD design curve of the compact tension test. The fracture parameter test method disclosed by the invention has the characteristics of simpleness, easy operation, simpleness for processing data, high precision of a critical CTOD value, comprehensive data, large information quantity, freeness from test precision influences of other parameters, such as strength, Young modulus, rotary factor and load, capabilities of obtaining the CTOD data of the whole test loading process and obtaining the CTOD design curve of the compact tension test under an actual board thickness condition.

Description

A kind of method that adopts two extensometers measurement compact tensile specimen fracture parameters

Technical field

The present invention relates to a kind of metal material fracture toughness technical field of measurement and test technology, particularly a kind of method that adopts two extensometers measurement compact tensile specimen fracture parameters.

Background technology

Crack tip opening displacement (hereinafter referred " CTOD " is expressed as " δ ") and CTOD design curve are the important parameters in the elastic-plastic fracture mechanics, are widely used in the safe reliability assessment of hardware.

At present, the measuring method at the critical CTOD value of compact tensile specimen mainly contains: flexibility method, profile method, the microscope direct method of measurement.The flexibility method test period is short, obtained using widely in practical operation, but measuring accuracy is lower; Profile method and microscope direct method of measurement precision height, but because the making complexity of section causes the test period long, be not widely used.

(1) flexibility method

" Unified method of test for the determination of quasistatic fracture toughness " and " the uniform tests method of GB/T 21143-2007 metal material quasistatic fracture toughness " regulation adopts flexibility method to measure the CTOD value.Flexibility method is according to feature load p, opening displacement plasticity component V _p, crack length a and material parameter (yield strength R _P0.2, elastic modulus E), the computing formula of utilizing flexibility method to derive to obtain is calculated the CTOD value.This method is simple to operation, test period is short, but because twiddle factor r has adopted unified recommendation, does not consider the influence of the strength of materials, specimen size and crack length, can have influence on the computational accuracy of critical CTOD value, computational accuracy also is subjected to yield strength R simultaneously _P0.2Influence with the elastic modulus E test error.

(2) profile method

Recommend among the appendix J of standard " the uniform tests method of GB/T 21143-2007 metal material quasistatic fracture toughness " to adopt profile method to measure the CTOD value.Profile method is with the section of the sample after the unloading, in enlargement factor is the opening displacement that 30 ~ 50 times microscopically is directly measured crack tip, this method principle is simple, measuring accuracy is high, but owing to be the crack tip opening displacement of testing after unloading, measured value is the plasticity part δ of CTOD value _p, and section preparation process complicated operation, the test period is long, is not suitable for large-scale application.

(3) the microscope direct method of measurement

Document " A method for the metallographical measurement of the CTOD at cracking initiation and the role of reverse plasticity on unloading " has proposed a kind of method that adopts microscope directly to measure crack tip opening displacement, the operation of this method is consistent with profile method, just the position of test is distinguished to some extent, this method measuring accuracy height, but have the deficiency identical with profile method.

At present, the CTOD design curve mainly contains the design curve that records based on test in the standards such as design curve, CVDA-84 and JWES2805 that propose based on people such as D-B Model Design curve, Wells and Burdekin, the EPRI method design curve that EPRI-Electric Power Research Institute proposes.D-B modelling curve is based on certain hypothesis, the scope of application less (σ＜0.6 σ s); Wells, Burdekin, CVDA-84 and JWES2805 design curve are based on the design curve of test experience, and precision is relatively poor; The design curve that the EPRI method proposes is based on Finite Element Method and calculates, and precision is higher, but difficulty in computation is big, and does not consider the influence of actual plate thickness, can not provide the CTOD design curve under the actual (real) thickness.

(1) D-B modelling curve

D-B modelling curve be with real crack length and plastic zone radius as effective crack length, find the solution by Castigliano and to obtain crack tip opening displacement.D-B modelling curve has been ignored the material hardening effect when theory is found the solution, the scope of application is less, and only is applicable to the central burst flat board that is stretched.

(2) Wells, Burdekin, CVDA-84 and JWES2805 design curve

People such as Wells, Burdekin uses foil gauge to test the relation of norminal strain and CTOD, and has drawn different design curves.People's such as Wells design curve is based on that a large amount of tests obtain, in theory analytically and unsuccessful.Standard such as CVDA-84 and JWES2805 has adopted the comparatively conservative result who obtains in people such as the Wells test to make design curve.These design curves are crossed all based on test, lack theoretical the support, and result of calculation is too conservative and only be applicable to the central burst flat board that is stretched.

(3) EPRI design curve

The EPRI design curve adopts Finite Element Method to calculate the J integration, and calculates the CTOD design curve according to the relation of J integration and CTOD.EPRI design curve precision height be applicable to the compact tension specimen test, but difficulty in computation is big, and is not considered the influence of actual plate thickness can not provide the design curve under the actual plate thickness.

At compact tensile specimen, the open report that can accurately measure the CTOD design curve method of testing under critical CTOD value and the actual (real) thickness condition is not simultaneously also arranged, there is not corresponding patent to announce yet.

Summary of the invention

Technical matters to be solved by this invention provides a kind of method that adopts two extensometers to measure the compact tensile specimen fracture parameters, and is simple to operate and can accurately measure CTOD design curve under critical CTOD value and the actual (real) thickness condition.

For the purpose that realizes solving the problems of the technologies described above, the present invention has adopted following technical scheme:

A kind of method that adopts two extensometers measurement compact tensile specimen fracture parameters of the present invention, described parameter is critical CTOD value and CTOD design curve, CTOD is that crackle Free Surface each point actual measurement opening displacement curve cathetus partly is extrapolated to the resulting opening displacement of crack tip, be characterised in that: adopt two extensometers to measure 2 different displacements of locating of plane, crackle place respectively simultaneously, this two places displacement is expressed as V respectively ₁, V ₂Utilize the permanent principle of setting up of whole loading procedure intermediate cam shape relation, calculate CTOD value and corresponding nominal stress under each load, obtained the relation curve of nominal stress σ and crack opening displacement δ, it is the CTOD design curve, the ratio of the plus load of σ representation unit thickness and the tough bandwidth that do not ftracture, concrete measuring method is:

1) adopting two extensometers to measure the P-V curve is two corresponding opening displacement V of maximum load point on load-displacement curve ₁, V ₂, utilize triangular relationship, calculate the CTOD critical value of compact tension specimen test, the CTOD critical value is the corresponding CTOD value of maximum load point on the P-V curve:

（1）

In the formula, δ is crack opening displacement, also is CTOD, and a is a crack length, and h is the distance of second extensometer apart from specimen surface, V ₁, V ₂Be respectively the opening displacement that two extensometers record; First extends counts apart from the nearer extensometer of crack tip, extends for second and counts apart from crack tip extensometer far away;

2) the two extensometers of employing record the V under each load ₁, V ₂Value, and (1) formula of bringing into is calculated CTOD value and corresponding nominal stress σ under each load, the σ calculating formula as the formula (2), obtained the series data point of nominal stress σ and corresponding C TOD, data point is adopted least square fitting, remove parameter c, the d of the exponential function in match (3) formula, promptly obtain the CTOD design curve of compact tension specimen test;

(2)

In the formula, σ is a nominal stress, and P is a load, and B is the thickness of sample, and W is a specimen width, and a is a crack length;

（3）

In the formula, δ is the CTOD value, and σ is a nominal stress, and c and d are the exponential function parameter.

The test philosophy of this patent is: adopt two extensometers opening displacement V of two diverse locations in plane, real time record crackle place respectively simultaneously in the process of the test of the two extensometer test of compact tension specimen test CTOD ₁, V ₂Variation with load; Utilize the similar calculating crack tip opening displacement of triangle CTOD, can obtain according to Similar Principle of Triangle:

Derivation draws the expression formula of crack tip opening displacement CTOD:

Wherein: δ---crack tip opening displacement

V ₁, V ₂---two opening displacements that extensometer records respectively

A---crack length

H---second extensometer is apart from the distance of specimen surface

The width of W---sample

R(W-a)---plastic hinge mechanism center of rotation is apart from the distance of crack tip

By adopting technique scheme, the present invention has following beneficial effect:

Test gained critical CTOD value is compared with flexibility method, and measuring accuracy height, data processing are simple; Compare with the microscope direct method of measurement with profile method, the test period is short, and is simple to operate.Test gained CTOD design curve calculates between gained plane stress and the plane strain design curve at EPRI, has obtained the design curve under the actual (real) thickness.

Fracture parameter method of testing of the present invention is simple, data processing is simple; Critical CTOD value precision height is not influenced by the measuring accuracy of other parameter such as intensity, Young modulus, twiddle factor, load etc.; Data are comprehensive, can obtain testing the CTOD data that load whole process; Contain much information, can obtain the CTOD design curve of the compact tension specimen test under the actual plate thickness condition.

Description of drawings

Fig. 1 is the two extensometer test of compact tension specimen test CTOD synoptic diagram.

Among the figure, 1-first extensometer, 2-second extensometer.Adopt two extensometers opening displacement V of two diverse locations in plane, real time record crackle place respectively in the process of the test simultaneously ₁, V ₂Variation with load.

Fig. 2 is the schematic diagram that utilizes the similar calculating crack tip opening displacement of triangle CTOD.

Fig. 3 is the P-V curve of compact tension specimen test, i.e. load-displacement curve.

Fig. 4 is the CTOD design curve of compact tension specimen test, i.e. σ-δ curve.

Fig. 5 is the comparison of the design curve of actual measurement design curve and the calculating of EPRI method.

Embodiment

Embodiment 1

Adopt two extensometer methods to test the fracture parameter of hull structural steel A36 steel plate, compact tensile specimen is of a size of 20mm * 50mm * 48mm, prefabricating fatigue crack length is 27.86mm, and two extensometers are clamped in specimen surface respectively and apart from the position of specimen surface 25mm.The mechanical property of this steel plate is as follows: yield strength 380MPa, tensile strength 530MPa, reduction of area 74.5%, elongation after fracture 33%, elastic modulus 217GPa.

Two extensometers write down the opening displacement V of 2 of tests simultaneously ₁, V ₂, to shut down when being loaded on maximum load, the load-displacement curve that measures is as shown in Figure 3.Two extensometers record maximum load respectively and put pairing opening displacement V ₁, V ₂Be respectively 2.23mm and 3.72mm.Bring test value into formula (1) and calculate critical CTOD value for 0.570mm.Record the V under each load ₁, V ₂Value is also calculated corresponding C TOD value and the series data point of corresponding nominal stress σ, the CTOD design curve that series data point line gets compact tension specimen test as shown in Figure 4, typical data point to the curve among Fig. 4 adopts least square method to carry out match, the parameter c, the d that obtain exponential function are respectively 1.09 and 15.6, (3) formula that parameter is brought into obtains CTOD design curve formula, shown in (3-2) formula:

（3-2）

Table 1 is a CTOD design curve typical data point.

Table 1 CTOD design curve typical data point

σ（Mpa）	80	83	85	88	90	93	95
								δ（mm）	0.0053	0.0094	0.01369	0.0235	0.0334	0.0557	0.0776
σ（Mpa）	98	100	103	105	106	108	110
								δ（mm）	0.1261	0.1728	0.2739	0.3700	0.4287	0.5739	0.7641

Calculate the CTOD value according to flexibility method and be 0.608mm, microscope direct method of measurement actual measurement CTOD value is 0.577, and result of calculation and flexibility method, microscopic method test result contrast as shown in table 2.The deviation of flexibility method and microscopic method measured value is 6.6%, and two extensometer method test value and microscopic method measured value deviation reduce to 1.2%.

Table 2 pair extensometer method, National Standard Method and the contrast of microscope mensuration δ test result

Calculate the compact tension specimen that obtains respectively under plane strain and the plane stress condition according to the EPRI method and test the CTOD design curve suc as formula shown in (3-3) and the formula (3-4).For compact tensile specimen, compare CTOD design curve and pair CTOD design curve that the test of extensometer methods obtains that the EPRI method calculates, as shown in Figure 5: the CTOD design curve that test obtains is between plane stress that is calculated by the EPRI method and plane strain design curve.

(plane strain) (3-3)

(plane stress) (3-4)

Claims (1)

1. one kind is adopted two extensometers to measure the method for compact tensile specimen fracture parameter, it is characterized in that: described parameter is critical CTOD value and CTOD design curve, CTOD is that crackle Free Surface each point actual measurement opening displacement curve cathetus partly is extrapolated to the resulting opening displacement of crack tip, be characterised in that: adopt two extensometers to measure 2 different opening displacements of locating of plane, crackle place respectively simultaneously, these 2 opening displacements of locating are expressed as V respectively ₁, V ₂Utilize the permanent principle of setting up of whole loading procedure intermediate cam shape relation, calculate CTOD value and corresponding nominal stress under each load, obtained the relation curve of nominal stress σ and crack opening displacement δ, it is the CTOD design curve, the ratio of the plus load of σ representation unit thickness and the tough bandwidth that do not ftracture, concrete measuring method is:

1) adopting two extensometers to measure the P-V curve is two corresponding opening displacement V of maximum load point on load-displacement curve ₁, V ₂, utilize triangular relationship, calculate the critical CTOD value of compact tensile specimen, critical CTOD value is the corresponding CTOD value of maximum load point on the P-V curve:

（1）

In the formula, δ is crack opening displacement, also is CTOD, and α is a crack length, and h is the distance of second extensometer apart from specimen surface, V ₁, V ₂Be respectively the opening displacement that two extensometers record; First extends counts apart from the nearer extensometer of crack tip, and second extends counts apart from crack tip extensometer far away;

2) the two extensometers of employing record the V under each load ₁, V ₂Value, and (1) formula of bringing into is calculated CTOD value and corresponding nominal stress σ under each load, the σ calculating formula as the formula (2), obtained the series data point of nominal stress σ and corresponding C TOD, data point is adopted least square fitting, remove parameter c, the d of the exponential function in match (3) formula, promptly obtain the CTOD design curve of compact tensile specimen;

（2）

In the formula, σ is a nominal stress, and P is a load, and B is the thickness of sample, and W is a specimen width, and α is a crack length;

（3）

In the formula, δ is the CTOD value, and σ is a nominal stress, and c and d are the exponential function parameter.

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