A yield strength correction calculation method based on instrumented indentation method

Abstract

The application discloses a yield strength correction calculation method for instrumented indentation method, which comprises the following steps: a target material is subjected to cyclic loading test by using an instrumented indentation tester to obtain a load-displacement curve, and true stress and true strain are obtained by the load-displacement curve, and the true stress and true strain measuring points have all entered a plastic hardening stage, and the yield strength cannot be accurately extrapolated. The instrumented indentation method can very accurately obtain the tensile strength, and it can be considered that no matter whether there is a yield platform, the constitutive curve of the plastic hardening stage of the material constitutive relation remains unchanged, that is, the strengthening coefficient K and the strain hardening index n obtained by fitting the Holloman equation in this part remain unchanged, and due to the existence of the yield platform, the plastic hardening stage of the constitutive curve is translated as a whole to obtain a new constitutive equation, and the influence of the yield platform can be considered. The application improves the precision of the instrumented indentation method in on-site detection of the yield strength of steel structure materials.

Description

Technical Field

[0001] This invention belongs to the field of material mechanical property testing methods, and relates to a method for correcting and calculating yield strength based on instrumented indentation method. Background Technology

[0002] Instrumented indentation, as a novel method for testing the mechanical properties of materials, has been gradually applied in various fields. According to relevant experimental studies, the instrumented indentation method has a relatively small error in tensile strength testing, generally within 5%, but a relatively large error in yield strength testing, approximately 10%. Analysis of the causes of this error reveals that the instrumented indentation method calculates yield strength using the nominal yield point, while the true stress-strain point corresponding to the first unloading has already entered the plastic hardening stage. In the theoretical calculation model of the instrumented indentation method, the constitutive equation used for fitting the true stress-strain point is the Holloman equation. This equation is generally applicable to materials with a power-law constitutive law and does not consider the existence of a yield plateau. Therefore, the true stress-strain curve obtained by the instrumented indentation method cannot fully represent the material's own variation law, i.e., it is not suitable for calculating the yield strength of metallic materials with a yield plateau. Based on the above analysis, this patent proposes a corrected yield strength calculation method for the instrumented indentation method. Summary of the Invention

[0003] The purpose of this invention is to improve the accuracy of on-site testing of the yield strength of steel structure materials using the instrumented indentation method, and to propose a yield strength correction algorithm for the instrumented indentation method.

[0004] The technical solution adopted in this invention is: a method for correcting the yield strength calculation using the instrumented indentation method, comprising the following steps:

[0005] First, the target material is subjected to cyclic loading test using an instrumented indentation tester to obtain the load-displacement curve. Since the true stress and true strain measurement points obtained by the instrumented indentation method have entered the plastic hardening stage, the instrumented indentation method can accurately determine the tensile strength. It can be assumed that the curve law of entering the plastic hardening stage remains unchanged, that is, the strengthening coefficient K and strain hardening exponent n obtained by fitting the Holloman equation in this part remain unchanged. Due to the existence of the yield plateau, the plastic hardening stage is translated as a whole to obtain a new constitutive equation as shown in equation (1).

[0006]

[0007] In the formula, E is the elastic modulus; σ is the total stress; ε is the total strain; σ y ε is the yield strength; y ε represents the initial yield strain; stdenoted as the strain at the end of the yield plateau; K is the strength coefficient; n is the strain hardening exponent; and C is an empirical constant.

[0008] Then, the initial value of the yield strain ε y1 and ε st It can be calculated using the following formulas (2) and (3):

[0009] Eε y1 =K(βε y1 -C) n (2)

[0010]

[0011] In the formula, the yield plateau coefficient β is an important parameter characterizing the yield plateau length, which can be obtained by instrumented indentation test; according to relevant test verification, for commonly used materials of steel structure bridges with yield plateau, the constant C value can be taken in the range of 0.0151-0.0152.

[0012] Finally, ε y1 Substituting into equation (4), the corrected yield strength value σ of the material is obtained. y Thus, the corrected constitutive equation is obtained.

[0013] σ y1 =Eε y1 (4)

[0014] The beneficial effects of this invention are as follows: For materials with a significant yield plateau, the Holloman constitutive equation does not consider the existence of the yield plateau, which leads to errors in the yield strength test results. By modifying the Holloman equation, setting a constant C to shift the original plastic hardening stage curve as a whole, and introducing a yield plateau coefficient β, the value of constant C is calculated to be in the range of 0.0151-0.0152 based on the completed test results, thus obtaining the final modified constitutive equation.

[0015] Verification has shown that the modified constitutive equation can effectively improve the accuracy of yield strength calculation. The yield strength test error of Q235 angle steel has been reduced from 10.8% to 4.6%, and the yield strength test error of Q345 steel plate has been reduced from 10.1% to 4.3%. Based on the instrumented indentation method, this patent further considers the accuracy of on-site testing for steel with a yield plateau, and can realize accurate on-site detection of the mechanical properties of materials in in-service structures. Attached Figure Description

[0016] Figure 1 This is a theoretical diagram of the constitutive equation of this invention. Detailed Implementation

[0017] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0018] A method for correcting yield strength using instrumented indentation method, comprising the following steps:

[0019] First, an instrumented indentation tester was used to perform cyclic loading tests on the target material to obtain load-displacement curves, such as... Figure 1 The dotted line is shown. Since the true stress and true strain measurement points obtained by the instrumented indentation method have all entered the plastic hardening stage, the tensile strength can be calculated very accurately by the instrumented indentation method. It can be assumed that the curve law of entering the plastic hardening stage remains unchanged, that is, the strengthening coefficient K and strain hardening exponent n obtained by fitting the Holloman equation in this part remain unchanged. Due to the existence of the yield plateau, the plastic hardening stage is translated as a whole, and a new constitutive equation is obtained as shown in equation (1). Figure 1 The solid line shown.

[0020]

[0021] In the formula, E is the elastic modulus; σ is the total stress; ε is the total strain; σ y ε is the yield strength; y ε represents the initial yield strain; st denoted as the strain at the end of the yield plateau; K is the strength coefficient; n is the strain hardening exponent; and C is an empirical constant.

[0022] Then, the initial value of the yield strain ε y1 and ε st It can be calculated using the following formulas (2) and (3):

[0023] Eε y1 =K(βε y1 -C) n (2)

[0024]

[0025] In the formula, the yield plateau coefficient β is an important parameter characterizing the yield plateau length, which can be obtained by instrumented indentation test; according to relevant test verification, for commonly used materials of steel structure bridges with yield plateau, the constant C value can be taken in the range of 0.0151-0.0152.

[0026] Finally, ε y1 Substituting into equation (4), the corrected yield strength value σ of the material is obtained. y Thus, the corrected constitutive equation is obtained.

[0027] σ y1 =Eε y1 (4)

[0028] The embodiments of this invention have been described in detail above with reference to the accompanying drawings, but this invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations of these embodiments within the scope of the principles and technical concept of this invention will still fall within the protection scope of this invention.

Claims

1. A method for correcting the yield strength calculation using the instrumented indentation method, characterized in that: Includes the following steps: First, cyclic loading tests were performed on the target material using an instrumented indentation tester to obtain load-displacement curves. True stress and true strain were then calculated from these curves. Since the true stress and true strain measurements obtained by the instrumented indentation method all occurred during the plastic hardening stage, the yield strength could not be accurately extrapolated. The constitutive curves in the plastic hardening stage remained unchanged. The strengthening coefficient was obtained by fitting the Holloman equation. K With strain hardening index n Remaining unchanged, due to the existence of the yield plateau, the entire plastic hardening stage is translated, resulting in the constitutive equation as shown in equation (1): （1） In the formula, E It is the elastic modulus; The total stress; For total strain; Yield strength; The initial strain for yielding; The strain at the end of the yield plateau; K The strength coefficient; n The strain hardening index; C These are empirical constants; Then the initial value of yield strain as well as The results are obtained from equations (2) and (3): （2） （3） In the formula, the yield plateau coefficient The parameter characterizing the yield plateau length was obtained by instrumented indentation testing; for commonly used materials in steel bridge structures with yield plateaus, the empirical constant C value ranges from 0.0151 to 0.0152. Finally Substituting into equation (4), the modified yield strength value of the material is obtained. Thus, the corrected constitutive equation is obtained: （4）。

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