CN106404553B - Three-point bending sample ductile fracture toughness JIC auxiliary test device and test method - Google Patents

Abstract

A JIC auxiliary test device for ductile fracture toughness of a three-point bending sample and a test method are provided, wherein the method obtains COD (chemical oxygen demand) gauge opening displacement with different heights by means of a knife edge auxiliary device, calculates the force application point displacement of the three-point bending sample, and further obtains the ductile fracture toughness of the three-point bending sampleJ _IC The safety evaluation work of high strength and high toughness can be effectively carried out.

Description

JIC auxiliary test device and test method for ductility and fracture toughness of three-point bending specimens

technical field

The invention belongs to the technical field of detection of fracture mechanical _{properties of metal materials, and in particular relates to a JIC} test device for the ductility fracture toughness of a three-point bending sample and a test method thereof.

Background technique

With the improvement of metallurgical quality, high-strength and high-toughness alloys are used more and more in engineering as structural materials, and safety evaluations are often required. The plane strain fracture toughness K _ІC is an important safety evaluation index and structural part design index, so it is particularly important to obtain the plane strain fracture toughness K _ІC of high-strength and high-toughness alloys through testing. However, for high-strength and high-toughness alloys, it is usually not feasible _to directly test the plane strain fracture toughness K _ІC , because the judgment condition B ≥ 2.5( K _ІC / R _p0.2 ) ² and P _max / P _Q ≤ 1.1; if a larger size fracture toughness sample is used in the test, the capacity of the testing machine is usually not reached. At present, the plane strain fracture toughness K _ІC of high-strength and high-toughness alloys is tested by measuring the ductile fracture toughness J _IC , and K _ІC is obtained from the conversion formula. The three-point bending sample has become the first choice for the sample form in the field of fracture mechanical performance testing of metal materials due to its advantages of simple form, fast processing, and strong representativeness. In particular, the notch of the three-point bending sample is located on the upper surface, so that the COD gauge can be placed The COD gauge can be effectively protected outside the low-temperature liquid, which makes the three-point bending specimen the only choice for the specimen form in _{the JIC} test of low-temperature ductility fracture toughness. During the ductile fracture toughness J _IC test, it is necessary to obtain the displacement V -load F curve of the force application point, and for the three-point bending specimen, because the COD gauge can only measure the notch opening displacement in the horizontal direction, it cannot directly obtain the displacement in the vertical direction. The force application point displacement, as shown in Figure 1, and then the test cannot obtain the effective ductile fracture toughness J _IC . At present, most testers directly use the displacement of the testing machine frame to approximate the displacement of the force application point. Since the displacement of the test machine frame includes the displacement of the force application point, equipment displacement, beam displacement, sample elastic displacement and test fixture elastic displacement, this The displacement of the testing machine frame obtained by the test is far greater than the displacement of the force application point, thereby causing test errors.

Contents of the invention

In order to solve the problem that _the displacement of the application point of the three-point bending specimen cannot be tested in the ductile fracture toughness JIC test, the invention provides a test device and a testing method for the ductility fracture toughness JIC of the three _- point bending specimen. This method obtains the opening displacement of COD gauges at different heights by means of the knife-edge auxiliary device, calculates the displacement of the force application point of the three-point bending specimen, and then obtains the ductile fracture toughness J _IC of the three-point bending specimen, making the safety evaluation of high strength and high toughness Work can be carried out effectively.

In order to achieve the above technical purpose, the technical solution adopted is: the JIC auxiliary test device for the ductility and fracture toughness of the three-point bending sample is equipped with two sets of knife-edge sets that clamp the two COD gauges respectively, and the two knife-edge sets are different. The height is set, the two blades of the lower blade group are symmetrically arranged on both sides of the notch of the three-point bending sample, and are set on the three-point bending sample, and the two blades of the upper blade group are respectively arranged on two symmetrical On the knife-edge auxiliary devices on both sides of the notch of the three-point bending sample, the clamping arms on both sides of the knife-edge auxiliary device are respectively clamped on the three-point bending sample by bolts. Before the test, the two knife-edges of the upper knife-edge group The two edge edges of the edge and the lower edge edge group are aligned in the horizontal direction, and the two edge edges of the two edge edge groups on the same side are aligned in the vertical direction.

Gaskets are provided between the bolts of the present invention and the three-point bending sample.

The bolt head of the bolt described in the present invention is an arc surface, and the center of the washer is provided with a groove matching the bolt head.

The test method of the JIC auxiliary test device for the ductility and fracture toughness of three-point bending specimens comprises the following steps:

Step 1. Install two knife-edge auxiliary devices symmetrically on both sides of the notch of the three-point bending sample, and set up symmetrical knife-edge groups on both sides of the three-point bending sample notch of the three-point bending sample. The knife edge group composed of auxiliary devices is higher than that of the three-point bending specimen, the two knife edge edges of the upper knife edge group and the two knife edge edges of the lower knife edge group are aligned in the horizontal direction, and the two knife edge edges on the same side The two blades of the blade group are aligned in the vertical direction, and the two blade groups clamp a COD gauge respectively;

（2）Step 2. Use the three-point bending test machine to apply load to the three-point bending sample. Two COD gauges apply tension to the two knife-edge blade groups so that the knife-edge auxiliary device does not slide relative to the three-point bending sample. Test the upper knife-edge blade group. The opening displacement V ₁ of the cutting edge and the opening displacement V ₂ of the lower cutting edge group are determined by the formula

(2)

In the formula:

q - the displacement of the force application point;

S - span of three-point bending specimen;

V ₁ - the opening displacement of the two cutting edges of the upper cutting edge group;

V ₂ - the opening displacement of the two cutting edges of the lower cutting edge group;

z ₁ - the distance between the lower knife edge and the surface of the three-point bending specimen;

z2 - the distance between the upper knife edge and the surface _of the three-point bending specimen;

The force application point displacement q of the three-point bending sample can be obtained, and the Δ a - J resistance curve, ductile fracture toughness J _IC and plane strain fracture toughness K _IC of the three-point bending sample can be obtained by using the multi-sample method.

The beneficial effects of the present invention are:

With the aid of the knife-edge auxiliary device, by testing the opening displacement of COD gauges at different heights, the displacement of the force application point of the three-point bending specimen is calculated by using geometric principles, and then the ductile fracture toughness J _IC of the specimen is obtained, and the obtained data are highly accurate , Low error. This enables the safety evaluation of high-strength and high-toughness alloys to be carried out effectively. At the same time, the device has the advantages of convenient disassembly and reusability.

Description of drawings

Figure 1 is a schematic diagram of the opening displacement of the COD measurement gap in the prior art;

Fig. 2 is the schematic diagram of force application point displacement test of the present invention;

Fig. 3 is a schematic diagram of the assembly plane of the knife-edge auxiliary device of the present invention;

Fig. 4 is a side view structural schematic diagram of the knife-edge auxiliary device of the present invention;

Fig. 5 is a three-dimensional schematic diagram of the assembly of the knife-edge auxiliary device of the present invention;

Fig. 6 is a three-dimensional structural schematic diagram of the knife-edge auxiliary device of the present invention;

Fig. 7 is the front view form and size of the fracture toughness J _IC sample of X70 pipeline steel according to the embodiment of the present invention;

Fig. 8 is the top view form and size of the fracture toughness J _IC sample of X70 pipeline steel according to the embodiment of the present invention;

Fig. 9 is the opening displacement V ₁ / V ₂ - load F curve of the 1# sample COD gauge of the present invention;

Fig. 10 is the displacement q -load F curve of 1# sample force application point of the present invention;

Fig. 11 is a graph showing the relationship between J integral and crack growth Δ a of X70 pipeline steel of the present invention;

Fig. 12 is the structural representation of bolt of the present invention;

Fig. 13 is a schematic diagram of the three-dimensional structure of the gasket of the present invention;

In the figure: 1. Knife edge auxiliary device, 101, clamp arm, 102, bolt, 1021, bolt head, 2, upper knife edge group, 3, lower knife edge group, 4, three-point bending specimen notch, 5, three points Bending specimen, 6, COD gauge, 7, spacer, 701, groove, 8, knife edge group.

Detailed ways

Three-point bending specimen ductility fracture toughness JIC auxiliary test device and its testing method, by testing the opening displacement of COD gauges of different heights, using geometric principles to calculate the displacement of the force point of the three-point bending specimen, the test principle is shown in Figure 2, the application The force point displacement q can be calculated by formula (2).

（1）

(1)

（2）

(2)

In the formula:

q - the displacement of the force application point;

S - span of three-point bending specimen;

V ₁ , V ₂ - the opening displacement of the two blades;

z ₁ , z ₂ - the distance between the two knife edges and the specimen surface;

The test of the displacement of the point of application of force is realized by means of the knife-edge auxiliary device. The knife-edge auxiliary device includes a pair of knife-edge blades, gaskets and bolts. A pair of knife-edge blades are symmetrically placed on both sides of the notch of the three-point bending JIC specimen. Four _gaskets and four bolts are used to fix the knife-edge auxiliary device from the front, back, left, and right respectively. The other pair of knife-edge blades are set on On the three-point bending sample, ensure that the knife edges on both sides of the notch of the sample are on the same horizontal line, as shown in Figure 3, 4, and 5, and when the COD gauge applies tension to the two knife edge groups, ensure that they are not opposite to the sample. Sliding occurs.

The following problems exist when the auxiliary device is used to indirectly test the displacement of the force application point of the three-point bending specimen: 1. The two cutting edges on each side of the notch of the three-point bending specimen must be aligned in the vertical direction, as shown in Figure 2, A1 It needs to be on the same line with B1, and A2 and B2 also need to be on the same line, so as to ensure that the displacement of the applied force point obtained according to the transformation of similar triangles is correct. 2. The knife edges on both sides of the notch of the three-point bending specimen must be aligned in the horizontal direction. As shown in Figure 2, A1 and A2 must be on the same horizontal line, and B1 and B2 must also be on the same horizontal line. The force application point displacement obtained by similar triangle transformation is correct. 3. When the COD gauge applies tension to both sides of the notch on the knife edge, the knife edge can not slide relative to the sample, so as to ensure that all the data collected during the test, the displacement of the force point converted by the similar triangle is valid. To sum up, 1. The knife-edge auxiliary device in the present invention adopts wire cutting to form at one time, so as to ensure that the two knife-edge edges of the two symmetrically arranged knife-edge auxiliary devices are on the same line; 2. The two knife-edge auxiliary devices adopt a left-right matching design, corresponding to The size is the same, and the bolt positioning size is also the same, so that the blades on both sides of the notch can be guaranteed to be on the same horizontal line; The knife-edge auxiliary device and the sample do not slide relative to each other.

As shown in Figures 6, 12, and 13, the JIC auxiliary test device for ductility and fracture toughness of three-point bending specimens. The edge aids used include the upper edge set for mounting a COD gauge, and spacers with center grooves and bolts with arcuate bolt heads for locating and installing the edge aids.

As shown in Figure 5, the installation sequence of the knife-edge auxiliary device is: install the left knife-edge auxiliary device and the right knife-edge auxiliary device on the left and right sides of the gap 5mm away from the three-point bending specimen, and install them on the front left, front right, rear left, Add spacers with central grooves to the rear right four positions, and install bolts from four positions at the same time, press the bolt heads on the spacers, and ensure that the knife-edge auxiliary devices on the left and right sides remain horizontal during the installation process.

Example 1

After the successful invention _of a JIC test device for ductility fracture toughness of three-point bending specimens, _JIC tests of ductility fracture toughness in air and seawater environments were carried out for various grades and pipeline steel materials with different thicknesses. The test of a pipeline steel material is taken as an example below.

1. Test material: X70 pipeline steel

2. Heat treatment state: Quenched and tempered

3. Test temperature: 23~25℃ Relative humidity: 40~50%

4. Sample type: SE(B) sample, its shape and size are shown in Figures 7 and 8.

5. Test item: ductility fracture toughness J _IC test

6. Test method: Install the knife edge auxiliary device on the X70 pipeline steel SE(B) sample, set up the lower knife edge groups on both sides of the gap, and use two COD gauges to test the opening displacement V ₁ and V ₂ , the force application point displacement q of the sample can be obtained from the formula (2), and the Δ a - J resistance curve, ductile fracture toughness J _IC and plane strain fracture toughness K _IC of X70 pipeline steel are obtained by using the multi-sample method.

7. Test results:

7.1. Displacement q of force application point

Since the methods for obtaining the displacement q of the application point of each sample are the same, only the test results of the displacement q of the application point of the 1# sample are given below. The opening displacement V ₁ / V ₂ -load F curve of the 1# sample COD gauge automatically collected by the testing machine is shown in Figure 9, and the displacement q -load F curve of the 1# sample force application point obtained through the processing of formula (1) is shown in Figure 10 .

7.2. Test data

The load F _s and plastic component U _p of each sample can be obtained from the displacement q -load F curve of the force application point, and the thickness B , width W , initial crack length a ₀ and crack growth Δ a of each sample can be obtained through measurement , the J value of each sample can be obtained by formula calculation. See Table 1 for test data.

Table 1 Test data

7.3. X70 pipeline steel Δ a - J resistance curve (see Figure 11)

7.4. X70 pipeline steel Δ a - J resistance curve equation

J =312.07×Δ a ^0.5784 ( R =0.9893)

7.5. Test results (see Table 2)

Table 2 Test results

。

.

Claims (3)

1. A testing method of a three-point bending sample ductile fracture toughness (JIC) auxiliary testing device is characterized by comprising the following steps of: the auxiliary test device is provided with two groups of knife edge groups (8) which are respectively clamped by two COD gauges, the two knife edge groups (8) are arranged at different heights, the two knife edges of the lower knife edge group (3) are symmetrically arranged at two sides of a three-point bending sample notch (4) and are arranged on a three-point bending sample (5), the two knife edges of the upper knife edge group (2) are respectively arranged on knife edge auxiliary devices (1) at two sides of the two symmetrically arranged three-point bending sample notch, clamping arms (101) at two sides of the knife edge auxiliary devices (1) are respectively clamped on the three-point bending sample (5) through bolts (102), and before the test, the two knife edges of the upper knife edge group (2) and the two knife edges of the lower knife edge group (3) are aligned in the horizontal direction, and the two knife edges of the two knife edge groups (8) at the same side are aligned in the vertical direction;

the test method comprises the following steps:

the method comprises the steps that firstly, two knife edge auxiliary devices are symmetrically arranged on two sides of a three-point bending sample notch respectively, symmetrical knife edge groups are arranged on two sides of the three-point bending sample notch of the three-point bending sample, the knife edge groups formed by the two knife edge auxiliary devices are higher than the knife edge groups of the three-point bending sample, two knife edges of an upper knife edge group and two knife edges of a lower knife edge group are aligned in the horizontal direction, two knife edges of the two knife edge groups on the same side are aligned in the vertical direction, and the two knife edge groups respectively clamp one COD gauge;

step two, a three-point bending test machine is used for applying load to the three-point bending test sample, two COD gauges apply tension to two knife edge groups, the knife edge auxiliary device does not slide relative to the three-point bending test sample, and the opening displacement V of the upper knife edge group is tested ₁ And the opening displacement V of the lower knife edge group ₂ From the formula

Wherein:

q-point of application displacement;

s-three-point bending sample span;

v1-opening displacement of two knife edges of the upper knife edge group;

v2-opening displacement of two knife edges of the lower knife edge group;

z1-the distance between the lower knife edge and the surface of the three-point curved sample;

z2-the distance between the upper knife edge and the surface of the three-point curved sample;

the force application point displacement q of the three-point bending sample can be obtained, and the delta a-J resistance curve and the ductile fracture toughness J of the three-point bending sample can be obtained by adopting a multi-sample method _IC And plane strain fracture toughness K _IC 。

2. The method of testing a three point bend specimen ductile fracture toughness JIC-assisted test apparatus according to claim 1, wherein: a gasket (7) is arranged between the bolt (102) and the three-point bending sample (5).

3. The method of testing a three point bend specimen ductile fracture toughness JIC-assisted test apparatus according to claim 2, wherein: the bolt head (1021) of the bolt (102) is an arc surface, and a groove (701) matched with the bolt head (1021) is arranged in the center of the gasket (7).

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