JPH06201551A - Method for measuring fracture toughness value of brittle material - Google Patents

Abstract

PURPOSE:To measure the fracture toughness value of brittle material by using a relatively small test piece of indefinite form. CONSTITUTION:This method is comprising the following steps. A part from a cut part located at the outer periphery of a test piece to the vicinity of the tip of the cut part is locally heated, and a crack is generated and grown in the first step. The vicinity of the tip of the crack is locally heated again by using laser and the like when the temperature distribution of the test piece becomes uniform, and the time from the start of the heating to the start of the crack growing is measured in the second step. The thermal physical property of the material, the heating area, the heating amount and the size of the test piece are substituted into a conversion expression in the third step.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the fracture toughness value of brittle materials for structures such as ceramics.

[0002]

2. Description of the Related Art Conventionally, the Japanese Industrial Standard "Fracture toughness test method for fine ceramics" is mainly JIS-R16.
Fracture toughness value K according to the method specified in 07-1990
Seeking _1C .

[0003]

However, when the fracture toughness value is obtained by the conventional method, there are the following problems. It was necessary to introduce a Vickers indentation (or Knoop indentation) or a notch at the starting point of the pre-crack and to generate the pre-crack by using a special pre-crack introducing jig. Also, a specially processed special test piece was required. In addition, it is practically impossible to collect a test piece specified in JIS-R1607-1990 from a film-shaped material or a ceramic member of various machines, for example, a small turbofan and a test piece such as a bearing thereof. Met. Therefore, there was a problem that the fracture toughness value could not be measured. The present invention has been made in view of the above problems, and an object of the present invention is to provide a measuring method capable of measuring a fracture toughness value by collecting a relatively small amorphous test piece.

[0004]

According to the present invention, in order to measure the fracture toughness value of a ceramic member, for example, an unfixed test piece of about 10 mm × 10 mm having a plate thickness of about 3 mm is usually used for 1 to 2 pieces. Collect individual pieces, make a simple notch on the outer periphery,
The first step of locally heating the vicinity of the notch tip to generate and develop a pre-crack, and after the temperature distribution of the test piece becomes uniform, output the vicinity of the pre-crack tip again and laser with known irradiation area, etc. A second step of locally heating by means of, for example, the time from the start of heating to the start of crack growth from the laser irradiation start time and the time when a crack growth start signal from an acoustic emission sensor or the like attached to the test piece is received. The measurement values of specific heat, thermal conductivity, Young's modulus, density, and laser absorptivity obtained by collecting a test piece from the specimen by a known method, and the heating calorie required until the crack growth start obtained from the second step and And a third step of substituting the heating time into the conversion formula, which is a fracture toughness value measuring method.

[0005]

The present invention has the above-described construction and operates as follows. In advance, the physical properties of the test piece such as the plate thickness, the shape such as the outer dimension, the specific heat, the thermal conductivity, the density, the Young's modulus, and the laser absorptivity are obtained by known methods. Further, a correlation curve between the thermal stress intensity factor and the heating calorific value when the vicinity of the tip of the crack is locally heated is obtained in advance. Next, a notch is introduced into the test piece, and the vicinity of the tip thereof is locally heated by, for example, a laser or the like to propagate the crack to an appropriate length to form a precrack. Next, irradiate a laser near the tip of the pre-crack, start heating, and measure the time until the crack starts to propagate by experiment, and test from the laser output and laser irradiation area at that time, laser irradiation time, etc. Calculate the heating value of the piece. The thermal stress intensity factor is obtained from the above correlation curve based on this heating heat quantity, and the fracture toughness value is obtained by regarding this value as the fracture toughness value of the material.

[0006]

Example: Nominal plate thickness of 2,
A test piece was obtained by cutting a plate glass of 4,8 mm into 50 mm × 100 mm. The test piece was cracked 10 mm from the center notch in the longitudinal direction by a laser at a right angle to the side.
I made progress. The thermal diffusivity, specific heat, density, linear expansion coefficient, and longitudinal elastic coefficient of the test material glass were each 0.68.
× 10 ^-6 m ² / s, 1.06 × 10 ³ J / kgK, 2.49
× 10 ³ kg / m ³ , 7.71 × 10 ^-6 1 / k, 34.6G
I asked for Pa. The fracture toughness value was determined to be 0.25 to 0.28 MPam ^1/2 by a three-point bending test for comparison. Therefore, the value of K _1C / E is 0.7 to 0.
It is 8 × 10 ⁻⁵ m ^1/2 . In the experiment, the ratio of the laser beam radius R and the distance D between the crack tip and the irradiation center is R /
D was kept constant at 0.5, D was changed to 4 to 15 mm, laser output was changed to 3 to 10 w, and heating was performed, and the time until the crack started to grow was measured. The output mode of the laser is
Since the normal TEM00 mode is close to the Gaussian distribution, the mirror of the resonator is changed to use the TEM01 mode in order to approach the assumption of the heat conduction analysis that the heating energy is uniformly distributed in the plane. When the amount of reflected heat from the glass surface of the carbon dioxide laser was measured using a laser power meter, it was 8.5% of the irradiation laser output, so it was considered that the remaining 91.5% was absorbed by the glass. The heating amount was used. Here, the conversion formula of the thermal stress intensity factor when the vicinity of the crack tip is heated by local heating such as a laser is obtained, for example, as in formula (1).

[Equation 1] Here, α, E, a, c, and ρ are the linear expansion coefficient, longitudinal elastic modulus, thermal diffusivity, specific heat, and density of the material, respectively, and the heat sources are evenly distributed at radius R with energy density 90. I'm assuming. D is the distance from the crack tip to the center of the heat source, S
Is a variable that represents the distance from the crack tip to any point on the crack, ξ is S / D, L is (S + D), r is the distance from any point on the crack to any point within the heat source radius. Variable to represent, r
₁ is Lcos θ− (R ² −L ² sin θ) ^1/2 , r ₂ is Lc
os θ + (R ² −L ² sin θ) ^1/2 , θ ₁ is sin ⁻¹ (R
/ L), θ ₂ is sin ⁻¹ (−R / L), t is heating time,
z is the integral exponential function of _{^{r 2 / 4at, E 1 (}} z) Hatsugi.

[Equation 2] Here, r = r ₁ and r = r ₂ attached to [] represent increments of values in [] from r = r ₁ to r = r ₂ . In this conversion formula,
Since the heating heat quantity q ₀ and the time t until the crack growth start are given by the previous experiment, the thermal stress intensity factor at the crack growth start can be obtained. This stress intensity factor is the fracture toughness value K _{1C of the} material.
The fracture toughness value can be obtained by assuming that Using the value obtained by dividing the laser heating heat quantity by the plate thickness b as the heating amount, and arranging by the value K _1C / E obtained by dividing the plate thickness average laser heating heat quantity and the stress intensity coefficient by the longitudinal elastic modulus E, FIG. 2.3. It becomes like 4. When the plate thickness is 1.75 mm, K _1C / determined by three-point bending test
It can be seen that it is close to the E value. By the way, in this experiment, since the energy of the carbon dioxide laser is almost absorbed near the glass surface, it is said that there is no temperature change in the plate thickness direction used when deriving equation (1) as the plate thickness increases. The assumption no longer holds, and the K _1C / E value is measured undersized. Therefore, when the fracture toughness value is measured using the conversion formula (1), it is suitable when the plate thickness is relatively thin. Also, K _1C / E depending on the distance D from the crack tip to the heating center
The difference in the values does not appear so much and it is convenient for the measurement because it is not necessary to precisely control the heating position when measuring the fracture toughness value.

[0007]

INDUSTRIAL APPLICABILITY The present invention can measure the fracture toughness value of hard and brittle materials such as ceramics with a comparatively small amount of test pieces that cannot be cut out according to the standard and contribute to the development of industry.

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention and shows the configurations of a test piece, a notch, a pre-crack, a laser device, an acoustic emission sensor, and a crack growth start time measuring device.

[Fig.2] Example of fracture toughness measurement of 1.75 mm thick glass

[Fig. 3] Example of measuring fracture toughness of glass with a plate thickness of 3.90 mm

[Fig. 4] Example of measuring fracture toughness of glass with a thickness of 7.65 mm

[Explanation of symbols]

 1. Fracture toughness value measuring device 2. Mobile platform 2a. Mobile platform control circuit 3. Heating device 3a. Laser oscillator 3b. Power supply circuit 3c. Output controller device 3d. Mirror 3e. Lens 3f. Mirror drive motor 4. AE sensor 4a. Amplifier 4b. Discriminator 4c. Transmission frequency coefficient device 4d. Amplitude distribution measuring device 4e. Relative energy measuring device 4f. Buzzer 5. Crack propagation start time measuring device 5a. Brittle material

Claims (1)

[Claims] 1. The first step of locally heating the vicinity of the notch to the vicinity of the tip of the notch on the outer peripheral edge of the plate to develop a crack and after the temperature distribution of the test piece becomes uniform, A second step in which the vicinity of the tip is locally reheated using a laser or the like, and the time from the start of heating to the start of crack growth is measured,
Method of measuring fracture toughness value comprising thermophysical properties of material, heating area and heating amount, and third step of substituting test piece dimensions into conversion formula