KR100491969B1 - Evaluation method for the amount of adiabatic shear band formation in the same materials by dynamic torsional tests - Google Patents

Abstract

The present invention provides a predetermined dynamic torsion test equipment (eg, dynamic Kolsky bar) test for Ti-6Al-4V alloys used in strategic industries such as defense, aerospace, precision machinery, and automotive industries. Equipment to investigate the deformation and fracture behavior under dynamic loads and to evaluate the degree of formation of adiabatic shear bands that are critical to the dynamic deformation of materials. The present invention for this purpose is to change the colony size and α-lamellar spacing for Widmanstatten tissues having excellent creep strength, fracture toughness, resistance to crack propagation, and thermal insulation shear band through dynamic torsion test. Evaluate the formation. In the present invention, the major microhistological factors such as α lamellar spacing and colony size are correlated with not only quasi-static deformation behavior but also dynamic deformation behavior as well as the degree of formation of adiabatic shear bands. Relate and analyze. In addition, the theoretical critical shear conditions for the formation of adiabatic shear bands are used to support the validity of the analytical results. The present invention provides an advantage that the degree of formation of the adiabatic shear band which appears when a high strain rate is applied to the same material having different microstructure factors can be simply evaluated through a dynamic torsion test.

Description

Evaluation method for the amount of adiabatic shear band formation in the same materials by dynamic torsional tests}

The present invention relates to a method for evaluating the degree of insulation shear band formation in the same material using a dynamic torsion test, and more specifically, Ti- which is used in the strategic industries such as defense industry, aerospace industry, precision machinery industry and automobile industry. Investigation of deformation and fracture behavior under dynamic loading for 6Al-4V alloys is used to evaluate the degree of formation of adiabatic shear band critical to the dynamic deformation properties of materials.

Meanwhile, the present invention changes the colony size and α-lamellar spacing for the Widmanstatten tissue, which has excellent creep strength, fracture toughness, and resistance to crack propagation. It relates to a method for evaluating the degree of formation.

On the other hand, the present invention provides a dynamic torsion test that directly contributes to the fundamental understanding of the role of microstructure factors and deformation mechanisms by directly examining deformation and fracture behavior under dynamic loads for various factors that determine the dynamic deformation characteristics of materials. The present invention relates to a method for evaluating the degree of thermal insulation shear band formation in the same material.

On the other hand, the present invention is useful data for analyzing the ballistic resistance and penetration performance by simply evaluating the degree of formation of the insulating shear band when the high strain rate is applied to the same material having different microstructure factors by dynamic torsion test. The present invention relates to a method for evaluating the degree of insulation shear band formation in the same material using a dynamic torsion test.

As is well known to those skilled in the art, Ti-6Al-4V alloys have attracted attention as an advanced structural material because they have characteristics such as low density, high specific strength, excellent high temperature properties, and corrosion resistance. The mechanical properties of this alloy are highly dependent on the microstructure, which is divided into Widmanstatten, equiaxed and bimodal depending on the processing and heat treatment conditions. Among them, the Widmanstatten tissue is known to have excellent creep strength, fracture toughness, and resistance to crack propagation, and lamellars having alternating α and β phases arranged in a similar direction alternately. It is common to form shaped colony structures. The major microhistological factors affecting the mechanical properties of the tissues are the sphere grains and colony size, the thickness of the grain boundary α phase, the fractions of the α and β phases, the α lamellar spacing, and the like. Through processing heat treatment, these microstructural factors can be controlled and used to study the required mechanical and dynamic strain characteristics.

On the other hand, in general, under dynamic loads, the resistance to deformation and fracture is lower than under quasi-static loads, so dynamic deformation and fracture phenomena should be closely investigated and evaluated before the material is applied. In addition, under dynamic loads, areas where plastic deformation is locally concentrated may be generated, which is called an adiabatic shear band. In these bands, the material's ability to withstand loads drastically deteriorates, leading to final failure, or in parts processing defects in ultrafast machining processes, which can be a processing constraint. Therefore, the study on the dynamic deformation behavior of materials is essential for alloy design, microstructure improvement, and control of high-speed processing process. In addition, it is necessary to predict the degree of formation of the insulating shear band through a test.

However, until now, there has been no research method for the dynamic deformation behavior of the material as described above and a method for predicting the formation of the adiabatic shear band through a test, and also a method for relatively simple evaluation through the dynamic twist test. It doesn't exist.

Therefore, the technical problem to be achieved by the present invention is to investigate the deformation and fracture behavior under dynamic load by using a predetermined dynamic torsion test equipment (eg, dynamic Kolsky bar test equipment) to determine the degree of thermal insulation shear band formation. The purpose is to provide a method of evaluation.

Another object of the present invention is to perform a dynamic torsion test in the same material having different microstructural factors and to analyze the correlation between the microstructural factors and the dynamic deformation behavior as well as the degree of formation of the insulating shear band. To improve the evaluation technique of -6Al-4V alloy and its validity It is to provide a method for evaluating the degree of insulation shear band formation in the same material using dynamic torsion tests.

In order to achieve the above object, the method of evaluating the degree of insulation insulation band formation in the same material using the dynamic torsion test according to the present invention is based on the material appearing under the dynamic load of high strain rate for the same material having different microstructure factors. Testing the material specimen with predetermined dynamic torsion testing equipment to obtain data on the degree of formation of adiabatic shear bands critical to deformation and fracture properties; An analysis step of applying the critical shear strain obtained through the following shear stress formula and critical shear strain formula to the degree of thermal insulation band formation; And

Shear stress ) Formula;

Critical shear deformation ) Formula;

( ; Shear Deformation, Shear strain rate, ; Temperature, β; Thermal conversion ratio, ρ; Density, c ; Specific heat, -∂τ / ∂θ; Thermal softening rate, ∂τ / ∂γ; Work hardening rate)

And analyzing the shear stress-shear strain curve obtained through the dynamic torsion test in association with the critical shear strain. In the shear stress-shear curve, as a reference point ( d τ = 0) of adiabatic shear band formation. The shear strain at the maximum shear stress point is defined as the critical shear strain, and the smaller the critical shear strain is, the greater the thermal softening rate is.

In a preferred embodiment of the present invention, the material specimen is a titanium alloy of Ti-6Al-4V, the chemical composition of this alloy is shown in the table below.

element Al V Fe O C N H Ti Content (% by weight) 6.19 4.05 0.19 0.12 0.02 0.01 0.004 Bal.

In a preferred embodiment of the present invention, the dynamic torsion tester is a dynamic Kolsky bar tester.

In a preferred embodiment of the present invention, the material specimen for the dynamic torsion test is in the form of a hexagonal columnar tube.

In a preferred embodiment of the present invention, the point at which the thermal softening of the material starts to be larger than the strain hardening and the strain rate hardening is taken as the maximum shear stress point.

In a preferred embodiment of the present invention, the reference point for the formation of the insulating shear band that can be caught by the shear deformation at the point of maximum shear stress is dτ = 0.

In a preferred embodiment of the present invention, the smaller the critical shear strain, the higher the thermal softening rate-∂τ / ∂θ, it can be seen that the formation of the heat insulating shear band is increased.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the method for evaluating the degree of thermal insulation shear band formation in the same material using a dynamic torsion test according to the present invention. In the following description of the present invention, when it is determined that detailed descriptions of related well-known technologies or configurations may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted. In addition, terms to be described below are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of a user or an operator. Therefore, the definition should be made based on the contents throughout the specification.

For reference, the related documents related to the present invention are as follows, and the author and title are written in the original language for clarity of the related documents.

-M.A. Meyers: Dynamic Behavior of Materials, John Wiley & Sons, NY (1994) p. 448;

-Y. Bai and B. Dodd: Adiabatic Shear Localization-Occurrence, Theories and Applications, Pergamon Press, NY (1992) p. 125;

Culver, R. S .: In Metallurgical Effects at High Strain Rates, Plenum Press, NY (1973) p. 519;

-D-G. Lee, S. Kim, S. Lee, and C.S. Lee: "Effects of Microstructural Morphology on Quasi-Static and Dynamic Deformation Behavior of Ti-6Al-4V Alloy", Metall. Mater.Trans. A, Vol. 32A (2001) p. 315.

Referring to FIG. 1, the present invention accumulates a certain amount of torque between a clamp 2 and a dynamic loading pulley 4, and then destroys the clamp 2 so that an elastic shear wave is instantaneous. It is transmitted to the specimen 10 to deform the specimen 10, in which the incident wave, reflected wave, and transmitted wave is sensed by the strain gauge 12, 14 and LVDT (Linear Variable Differential Transformer) 20, respectively. A dynamic torsion tester 100 having devices recorded on the oscilloscope 32 and the recorder 34 is used. In FIG. 1, reference numeral 6 denotes a gauge for measuring the amount of torque accumulated between the fastener 2 and the dynamic loading pulley 4, 16 is a power supply, and 36 is a personal computer (PC). , 41 is a bar for transmitting torsional force to the specimen 10, 42 is a hydraulic pump, 43 and 44 are compressors, 45 is a static loading pulley, 46 is an electric motor, 47 Is a reducer.

The dynamic torsion test equipment 100 is a device designed to perform a torsion test unlike conventional compression and impact equipment. Specimen 10 used in this dynamic torsion testing equipment 10 is a tube-shaped hexagonal column shape with a thickness of preferably 0.28 mm, as shown in FIG. 2, which is carefully and precisely processed using a predetermined electric discharge machine. To be required.

In order to evaluate the degree of formation of the adiabatic shear band with the data obtained through the dynamic torsion testing equipment 100 described above, an analysis tool for this is required, which may be expressed as follows.

According to Bay et al., Ignoring phenomena such as elastic deformation, strain rate, temperature history effect, and phase transformation of materials, shear stress is Shear deformation ), Shear strain rate ( ) And temperature (θ).

The maximum shear stress point is the point at which thermal softening starts to become larger than strain hardening and strain rate hardening, so it can be regarded as the reference point for forming the adiabatic shear band ( d τ = 0). . In addition, the thermal shear band forming conditions may be expressed by heat transfer fraction (β), density (ρ), specific heat ( c ), etc. of the plastic strain energy, as suggested by Culver et al.

Formula (1)

From the formula (1), the higher thermal softening rate (-∂τ / ∂θ) and heat conversion ratio (β), the lower the work hardening rate (∂τ / ∂γ), density (ρ), and specific heat ( c ), It can be seen that the maximum shear stress for forming the shear band is small. Insulation shear bands can be formed if they meet the criteria as given in formula (1), but in order to form insulated shear bands, sufficient thermal softening that can cause plastic instability must be preceded. Therefore, fast deformation rate is required to give sufficient heat and insulation effect converted by shear plastic deformation. The shear plastic deformation that satisfies these conditions can be defined as a critical shear strain (γc), and on the dynamic shear stress-shear curve, the reference point for forming the adiabatic shear band ( d τ = 0), that is, the maximum shear stress It can be caught by shear deformation at the point. Therefore, as shown in the following formula (2), it can represent the critical shear deformation required to form the adiabatic shear band.

Formula (2)

It can be seen from Equation (2) that the smaller the critical shear strain, the higher the thermal softening rate (-∂τ / ∂θ), and thus the formation degree of the adiabatic shear band increases. This means that the concentration of local plastic deformation increases, and the microstructure shows that the width of the adiabatic shear band is small.

The primary data obtained from the dynamic torsion testing equipment 100 as shown in FIG. 1 is converted into a predetermined program (eg VP dir program) and converted into a shear stress-shear strain curve.

Hereinafter, the embodiment of the present invention will be described in more detail. The examples described below are merely exemplary and should not be construed as limiting the invention.

<Example 1>

The titanium alloy used in this example is a Ti-6Al-4V alloy and its chemical composition is shown in Table (1).

Table (1)

element Al V Fe O C N H Ti Content (% by weight) 6.19 4.05 0.19 0.12 0.02 0.01 0.004 Bal.

Using the Ti-6Al-4V alloy shown in Table (1), heat treatment was performed under the conditions as shown in Table (2) to obtain three different W1, W3, and W5 specimens of Widmanstatten structure. . In Table (2), in order to clarify the heat treatment contents for the specimen 10 (FIG. 2), the heat treatment contents were used as the original English.

Table (2)

Specimen Heat Treatment Microstructure Colony Size α-lamellas facing W1 Solution Treat (1050 ℃, 30min) + Furnace Cooling to 750 ℃ (14 ℃ / min) + Air Cooling to Room Temp. Small Small W3 Solution Treat (1050 ℃, 30min) + Furnace Cooling to 960 ℃ (14 ℃ / min) + Furnace Cooling to 750 ℃ (4 ℃ / min) + Air Cooling to Room Temp. Small Large W5 Solution Treat (1050 ℃, 3hours) + Furnace Cooling to 750 ℃ (4 ℃ / min) + Air Cooling to Room Temp. Large Large

As a result of examining the microstructural factors of the three specimens (W1, W3, W5) tissue, as shown in Table (3), the W1 specimen has a small microstructure of both colony size and α lamellar spacing, and W3 specimen. The silver colony size was similar to that of the W1 specimen, but the α lamella spacing showed a large microstructure, and the W5 specimen showed both the colony size and the α lamella spacing.

Table (3)

Psalter α lamellar spacing (μm) Colony Size (μm) W1 2.2 ± 0.5 472 ± 74 W3 6.0 ± 1.5 483 ± 77 W5 6.3 ± 1.8 737 ± 114

As shown in FIG. 2, a dynamic torsion test was performed on the W1 specimen by using a dynamic torsion test apparatus (or dynamic Kolsky bar equipment) 100 as shown in FIG. 1 by discharge machining the specimen 10. The shear stress-shear curves converted using the data obtained in this example are shown in FIG. 3.

<Example 2>

The dynamic torsion test was conducted in the same manner as in Example 1 except that the dynamic torsion specimen W3 was used, and the shear stress-shear curve obtained from the data is shown in FIG. 3.

<Example 3>

The dynamic torsion test was performed in the same manner as in Example 1 except that the dynamic torsion specimen W5 was used, and the shear stress-shear curve obtained from the data is shown in FIG. 3.

Comparative Example

The maximum shear stress and critical shear strain of the W1 specimens were measured and shown in Table (4). The maximum shear stress and critical shear strain of the W3 specimens were measured and shown in Table (4). The maximum shear stress and critical shear strain of the W5 specimens were measured and shown in Table (4).

Table (4)

Psalter Maximum shear stress; (MPa) Shear strain at maximum shear stress point; (%) W1 694 15.9 W3 679 13.2 W5 655 11.2

As shown in Table (4), the maximum shear stress and critical shear strain decreased with increasing colony size and α lamellar spacing. According to Equation (2), the W5 specimen with the smallest critical shear strain formed an insulating shear band. It can be predicted that the most likely to be, and also the most dense, insulating shear band will be produced.

In order to confirm this, as shown in Figs. 4a to 4c, the strained side of the fracture specimens tested were examined by scanning electron microscopy to examine the degree of formation of the adiabatic shear bands. W1 (Fig. 4a), W3 (Fig. 4b), In the order of W5 (FIG. 4C) specimens, it was confirmed that a thermally insulative shear band was formed in 15 μm, 12 μm, and 8 μm widths (both 30 μm, 24 μm, and 16 μm). In other words, in the case of the W5 specimen, the shear strain was concentrated in a specific region, so that the formation of the adiabatic shear band was easy, which is in agreement with the result predicted and evaluated by the shear stress-shear curve.

As described above, the present invention relates to a method for evaluating the degree of formation of adiabatic shear band critical to the dynamic deformation characteristics of a material by investigating deformation and fracture behavior under dynamic load using a dynamic Kolsky bar test equipment. In this regard, it is very easy to evaluate from the shear stress-shear curve obtained from the dynamic torsion test, providing the advantage of investigating and evaluating the properties before applying the material to dynamic deformation and fracture under dynamic loading. do.

In addition, the present invention can be used as a useful data for analyzing the ballistic performance and penetration performance by simply evaluating the formation of the adiabatic shear band formed by applying a high strain rate to the same material having different microstructure factors through a dynamic torsion test. It provides the benefits of doing so.

Although a preferred embodiment of the present invention has been described in detail above, those skilled in the art to which the present invention pertains may make various changes without departing from the spirit and scope of the invention as defined in the appended claims. It will be appreciated that modifications or variations may be made. Therefore, changes in the future embodiments of the present invention will not be able to escape the technology of the present invention.

1 is an embodiment of a dynamic torsion testing equipment for the method of the present invention.

2 is a schematic diagram of a specimen used for the method of the present invention.

3 is a comparative graph of embodiments of the method of the present invention.

Figures 4a to 4c is an observation of the degree of thermal insulation shear band formation for embodiments of the method of the present invention by scanning electron microscopy (SEM).

<Description of the symbols for the main parts of the drawings>

2: Clamp 4: Dynamic Loading Pulley

10: Psalm 20: LVDT

32: oscilloscope

Claims (7)

In the method for evaluating the formation of the insulating shear band in the same material using a dynamic torsion test, In order to obtain data on the degree of formation of adiabatic shear bands critical to the deformation and fracture properties of materials under high strain rate dynamic loads for the same material with different microstructure factors, the material specimens were subjected to a predetermined dynamic torsion tester. Testing through; An analysis step of applying the critical shear strain obtained through the following shear stress formula and critical shear strain formula to the degree of thermal insulation band formation; Shear stress ) Formula; Critical shear deformation ) Formula; ( ; Shear Deformation, Shear strain rate, ; Temperature, β; Thermal conversion ratio, ρ; Density, c ; Specific heat, -∂τ / ∂θ; Thermal softening rate, ∂τ / ∂γ; Work hardening rate) Analyzing the shear stress-shear strain curve obtained through the dynamic torsion test and the critical shear strain in association; In the shear stress-shear strain curve, the shear strain at the maximum shear stress point is defined as the critical shear strain as a reference point ( d τ = 0) of the insulation shear band formation, and the critical shear strain is defined as the degree of formation of the insulation shear band. Applying to the evaluation; The dynamic torsion test equipment is a dynamic Kolsky bar test equipment, characterized in that the evaluation of the insulation shear band formation degree in the same material using a dynamic torsion test. The method according to claim 1, wherein the material specimen is a titanium alloy of Ti-6Al-4V. delete The method according to claim 1 or 2, wherein the material specimen for the dynamic torsion test is in the form of a hexagonal columnar tube in the same material using the dynamic torsion test. 2. The point of claim 1, wherein the time point at which thermal softening of the material starts to be greater than strain hardening and strain rate hardening, Evaluation method for forming the insulating shear band in the same material using a dynamic torsion test, characterized in that the reference point of the formation of the insulating shear band that can be caught by the shear deformation at the maximum shear stress point is dτ = 0. delete The method according to claim 1, wherein the smaller the critical shear strain, the greater the thermal softening rate -∂τ / ∂θ, so that the formation degree of the adiabatic shear band increases. Evaluation Method of Formation of Insulating Shear Band in.