CN102679931B - Novel method for measuring fatigue crack propagation length in situ - Google Patents

Abstract

The invention relates to a method for measuring the extension length of fatigue cracks in situ. The advantages and characteristics of the present invention are: a) 100 times metallographic microscope, capacitive sensor, signal processing system, CCD camera, computer, etc. are integrated to realize the integration of data acquisition and processing, and realize the in-situ and real-time measurement of fatigue Crack growth length; b) Solve the shortcomings of the common reading method that usually takes a long time to measure, the operation is cumbersome, and it is easy to have an adverse effect on subsequent measurements; c) Using a capacitive sensor, the acquired displacement signal is converted into an electrical signal, and through the signal The transmission and conversion equipment obtains the final crack length, which realizes semi-automatic measurement; d) transmits the real-time image collected by the CCD camera through the metallographic microscope to the computer, realizing the visualization of the entire testing process, and solving the problem that traditional measurement can only observe the results, The problem of not being able to directly observe the process.

Description

A new method for in situ measurement of fatigue crack growth length

technical field

The invention relates to a new method capable of measuring fatigue crack extension length in situ during fatigue performance testing, which can realize real-time on-line measurement of fatigue crack extension length. The invention belongs to the field of testing mechanical properties of materials.

Background technique

Fatigue refers to the phenomenon that materials, parts and components produce local permanent damage at a certain point or certain points under cyclic loading, and form cracks after a certain number of cycles or further expand the cracks until they are completely broken. Quoting the definition made by the American Society for Testing and Materials (ASTM) in "Standard Definitions of Terms Related to Fatigue Tests and Statistical Analysis of Data" (EZ06-72): A deflection at a certain point or points The development of localized permanent structural changes that occur in a material when cracks form or complete fracture occurs after sufficient cyclic flexing is called "fatigue".

The phenomenon that a material fails under cyclic loading far below its strength limit is called fatigue failure. Fatigue damage mainly has the following characteristics:

1) Fatigue occurs only when the material is subjected to cyclic loading.

2) Fatigue is a "developmental process" that occurs over a period of time. "Crack formation" and "fracture" are the result of the accumulation of damage that continues to form during this development.

3) Fatigue usually occurs in a local area rather than controlled by the entire structure or component.

4) The fatigue process is the cumulative process of damage caused by permanent changes inside the structure.

In all aspects of modern engineering, most of the main parts and components work under the action of cyclic load, and fatigue damage is the main failure mode. With the development of modern industry, the requirements for the fatigue performance of materials are also gradually increasing. Therefore, it is of obvious social and economic value to study the fatigue failure mechanism of materials and to find ways to improve the fatigue performance of materials.

At present, the commonly used fatigue resistance indicators of materials include fatigue strength, overload endurance value, fatigue notch sensitivity, fatigue crack growth rate, and fatigue crack growth threshold value. Among them, the most commonly used indicators to evaluate the fatigue performance of materials are the fatigue crack growth rate (da/dN) and the fatigue crack growth threshold (ΔK _th ), especially ΔK _th has become the most important indicator to measure the fatigue performance of materials and important components. According to the relevant regulations in GB/T 6398-2000, the range value of the stress intensity factor corresponding to the fatigue crack growth rate equal to 1×10 ^-7 mm/cycle is defined as ΔK _th .

There are many factors that affect the fatigue threshold of a material. The following is a brief introduction to the relevant parameters that may be involved in the fatigue test process (taking the standard compact tension (Compact Tension, CT) specimen specified in GB/T6398-2000 as an example):

1) Number of cycles N: the number of force cycles.

2) Fatigue crack growth rate da/dN: The amount of fatigue crack growth after one force cycle, expressed in the national standard as a function of the stress intensity factor ΔK at the tip of the fatigue crack.

3) Stress intensity factor K: the magnitude of the elastic stress field strength in the region near the crack tip.

4) Crack length a: The length of the straight-edge crack equivalent to the actual crack. For CT samples, a is calculated from the afterburner line.

5) Maximum load P _max : the maximum algebraic value of the cyclic force.

6) Minimum load P _min : minimum algebraic value of cyclic force.

7) Range of force value ΔP: the difference between the maximum and minimum force, that is, ΔP=P _max -P _min .

8) Stress ratio R: the ratio of the minimum force to the maximum force, that is, R= P _min /P _max .

9) Maximum stress intensity factor K _max : the stress intensity factor corresponding to the maximum force, and it changes with the growth of the crack length.

10) Maximum stress intensity factor K _min : the stress intensity factor corresponding to the minimum force.

11) Stress intensity factor range ΔK: the difference between the maximum and minimum stress intensity factor values, that is, ΔK=K _max －K _min .

For CT specimens,

(1)

In the formula: α = a/W (W is the distance from the line of force action to the edge of the CT sample); B is the thickness of the CT sample.

From the formula (1), it can be seen that during the whole test process, the accurate measurement of the fatigue crack growth length is one of the key factors. The methods currently used to measure fatigue cracks include ordinary reading method, strain gauge method, equivalent potential method, etc. Among them, the ordinary reading method has relatively high accuracy.

The operation steps of the common reading method are to remove the fatigue sample from the fatigue testing machine after several cycles and place it under a metallographic microscope to measure its length. This method has high accuracy, but has the following defects:

1) When measuring the crack growth length, it is necessary to unload the load, disassemble the sample, and measure the crack length under a microscope; after the measurement, the sample needs to be reinstalled and reloaded. The entire operation process is cumbersome, time-consuming and laborious, and may cause changes in the clamping and alignment parameters of the CT sample, affecting the repeatability of the test results.

2) The process of disassembling the sample interrupts the action of the load, which has a greater impact on the subsequent fatigue crack growth rate da/dN, causing the measured da/dN to deviate from the true value.

3) Relying on the da/dN value of the upper level to estimate the number of cycles required, it is impossible to observe the real-time crack growth length, which reduces the experimental efficiency. In addition, the entire operation process has relatively high requirements for the experience of the operator.

Therefore, seeking a more accurate measurement method will undoubtedly help to improve the accuracy and repeatability of the material fatigue threshold test.

The present invention aims at the shortcomings of the ordinary reading method for fatigue crack testing, and proposes the combination of metallographic microscope and capacitance sensor and other equipment to achieve the purpose of improving the accuracy of fatigue crack growth length measurement and realizing the visualization of fatigue crack growth process, and proposes specific implementations.

The inventor of the present invention used fatigue crack + in-situ measurement (fatigue crack + in-situ measurement) as keywords to search the United States Patent Abstracts (USPTO), European Patent Abstracts (EP-PCT), "China Patent Information Network" and "China Patent Search by the State Intellectual Property Office of the People's Republic of China, no similar patents were found.

Contents of the invention

The purpose of the present invention is to provide a new method and device for in situ measuring fatigue crack growth length. More specifically, the purpose of the present invention is to provide a method that can measure the fatigue crack length in situ and observe the fatigue crack growth in real time during the fatigue test, so that various fatigue stress indicators can be determined more accurately, so as to better measure The fatigue performance of various materials under different process conditions has practical significance for the design and manufacture of many important components.

The present invention is a new method for in-situ measurement of fatigue crack growth length, which can be used for real-time and accurate measurement of fatigue crack growth length during fatigue performance testing; the method is characterized in that metallographic microscope and Charge Coupled Device (Charge Couple Device) are used , CCD) camera to track the propagation path of the fatigue crack tip in real time, and convert the mechanical displacement into the phase change of the electrical signal immediately through the capacitive sensor, and then obtain the accurate fatigue crack length after processing by the measurement circuit; the brief measurement method is as follows : (a) Clamp the fixture and the polished standard CT sample on the fatigue testing machine, and place the base and bracket of the measuring equipment in an appropriate position; (b) Adjust the positioning rack so that the objective lens of the metallographic microscope is aligned with the machined The prefabricated crack tip, the machined crack tip is observed in the center of the field of view (the intersection of the horizontal and vertical scale lines) in the metallographic microscope. Zero the crack length reading on the monitor; (c) connect the CCD camera so that the image of the sample surface can be clearly displayed on the monitor; (d) start the fatigue testing machine, and change the position of the metallographic microscope by adjusting the rack so that The tip of the fatigue crack is always in the picture collected by the CCD camera; through the reading on the electronic display and the crack length in the photo collected by the metallographic microscope, the length of the real fatigue crack can be accurately determined; at the same time, the length of the real fatigue crack can also be determined according to the CCD camera The collected photos are used to analyze the reasons for the deviation of the crack propagation direction during the crack propagation process; combined with the observation of the microstructure of the sample after the fatigue test by optical microscope and scanning electron microscope, the fatigue fracture mechanism of the tested material can be further analyzed .

The present invention is a special device used in the method for in-situ measuring fatigue crack extension length, which is characterized in that it includes: standard CT sample, metallographic microscope, CCD camera, cushioning base, capacitance sensor, fixture, pin, data transmission System, adjustment knob, positioning rack and computer; the layout of each component is as follows: the standard CT sample and the pin are connected to the fatigue testing machine with a fixture, while the metallographic microscope and the positioning rack are synchronized and directly installed on the cushioning base , adjust its position through the positioning knob; the CCD camera is connected to the metallographic microscope, and the collected data is transmitted to the computer through the data transmission system for processing, and the final fatigue crack length and fatigue crack surface topography photos are obtained.

Features of the invention

The method of the present invention combines metallographic microscope, positioning rack, capacitive sensor, signal conversion system, CCD camera, computer and other equipment to propose a new method for in-situ measurement of fatigue crack length, and the displacement signal is converted into an electrical signal by the capacitive sensor , enabling in-situ measurement and visualization of the fatigue crack growth process throughout the fatigue experiment. Not only can the fatigue crack length be accurately measured at any time, but also the propagation path of the fatigue crack tip can be observed and collected in real time.

The working principle of the in-situ measurement method is to collect crack tip images through a 100x metallographic microscope (adjusting the position through a synchronous positioning rack) and a CCD camera, and connect the collected images to a computer to realize the visualization of the fatigue crack growth process. At the same time, the capacitive sensor synchronized with the metallographic microscope converts the displacement signal into an electrical signal, and then converts it into a digital signal through a signal converter, and finally obtains the fatigue crack length after computer processing.

Description of drawings

Figure 1 is a schematic diagram of the equipment for in-situ measurement of fatigue crack growth length. The equipment is mainly composed of a metallographic microscope, a capacitive sensor, a CCD camera, a positioning rack, a data acquisition and transmission system, and a cushioning base.

Fig. 2 is a side view of the equipment for in-situ observation of fatigue crack growth length.

Figure 3 is a standard CT sample diagram, which is processed according to the size regulations in the national standard (GB/T 6398-2000) by wire cutting.

Fig. 4 is a graph showing the relationship between the fatigue crack growth rate da/dN and the stress intensity factor range ΔK at the crack tip of a material under various stress ratios.

Fig. 5 is the fitting diagram of lg(da/dN) and lgΔK under various stress ratios of the material.

Fig. 6 is a graph showing the relationship between the crack growth threshold ΔK _th and the stress ratio of the material.

Detailed ways

Below in conjunction with embodiment the present invention is described in detail:

Embodiment one

Specific steps are as follows:

The test of fatigue crack growth threshold (ΔK _th ) is measured by the falling K method specified in ASTM E647-08 and GB/T 6398-2000. The detailed measurement process is as follows:

1) Cut a blank of a certain size from the test material, and use wire cutting to process the standard CT and special fixtures that meet the requirements of GB/T 6398-2000. The specific size of the CT sample is shown in Figure 3.

2) Install the processed fixture on the load fatigue testing machine, adjust the distance between the pull bars of the fatigue testing machine to a suitable position, and place the standard CT sample in the fixture to ensure that the testing machine is loaded on the CT sample. Force has good centering.

3) Put the base and bracket of the measuring device in an appropriate position so that the microscope objective lens is facing the CT sample surface, and adjust the focus knob of the metallographic microscope to the eyepiece to clearly observe the sample surface.

4) Adjust the positioning rack so that the tip of the machined crack can be clearly displayed on the display and adjusted to the center of the field of view, and the crack length in the display is zeroed at this time.

5) Select an appropriate stress ratio R and load P for pre-cracking. When the reading on the display is about 3 mm, stop loading and the crack preparation ends. Accurately record the fatigue crack length a and cycle number N.

6) Adopt a pre-designed specific R and select a suitable maximum load P _max , keep R constant, reduce the load step by step for data collection, and accurately record with the new method of in-situ measurement of fatigue crack growth length of the present invention The value of a and N under each level of force, and calculate the ΔK, K _max and other parameters at this time according to the formula (1). In the process of load reduction, each level of force makes the expansion increment Δa larger than the plastic zone γ _y corresponding to the previous level K _max (maximum stress intensity factor) 4~6 times, until the average crack growth rate Δa/ΔN is close to 1 ×10 ^-7 mm/cycle, the K-drop experiment ends. In order to ensure that the change of collected data is within the allowable range, the load reduction range of each stage does not exceed 10%. When da/dN is within the range of 10 ^-6 ~10 ^-7 mm/cycle, the load reduction range can be appropriately reduced to ensure There are enough data points for the fit.

According to the pairs formula:

(2)

Among them, C and m are constants related to the properties of the material itself.

After logarithmic processing, lg(da/dN)=m·lgΔK+lgC can be obtained. From this, it can be seen that lg(da/dN) and lgΔK have a linear relationship. According to the da/dN-ΔK correspondence relationship obtained in the range of da/dN in the range of 10 ^-6 ~10 ^-7 mm/cycle, draw the corresponding points in the lg(da/dN)-lgΔK coordinate system, and use the method of linear fitting Get C and m, put them into the formula (2), and set da/dN=1×10 ^-7 mm/cycle to find ΔK, which is the required fatigue crack growth threshold ΔK _th .

Claims (1)

1. A method for in-situ measurement of fatigue crack growth length, used for real-time and accurate measurement of fatigue crack growth length during fatigue performance testing, characterized in that metallographic microscope and charge coupled (Charge Couple Device, CCD) are used The camera tracks the propagation path of the fatigue crack tip in real time, and converts the mechanical displacement into the phase change of the electrical signal immediately through the capacitive sensor, and then obtains the accurate fatigue crack length after processing by the measurement circuit; the brief measurement method is as follows: (a ) Clamp the fixture and the polished standard CT sample on the fatigue testing machine, and place the base and bracket of the measuring equipment in an appropriate position; (b) Adjust the positioning rack so that the objective lens of the metallographic microscope is aligned with the machined prefabricated crack The tip of the machined crack is observed in the metallographic microscope at the very center of the field of view, that is, the intersection of the horizontal and vertical scale lines; the crack length reading on the display is zeroed; (c) the base and support of the measuring device are placed on the in a proper position, so that the microscope objective lens is facing the surface of the CT sample, adjust the focus knob of the metallographic microscope to the eyepiece to clearly observe the sample surface; (d) adjust the positioning rack, so that the monitor can clearly display Machine the tip of the crack and adjust it to the center of the field of view, and adjust the crack length in the monitor to zero; (e) Select the appropriate stress ratio R and load P to prefabricate the crack; when the reading on the monitor is about When the value is 3 mm, the loading is stopped, the crack prefabrication ends, and the fatigue crack length a and the number of cycles N are accurately recorded; (f) adopt a pre-designed specific R and select a suitable maximum load P _max , keep R constant, Reduce the load step by step for data collection, accurately record the values of a and N under each level of force, and calculate the ΔK and K _max parameters at this time according to the formula (1); Make the expansion increment Δa larger than 4~6 times of the plastic zone γ _y corresponding to the upper level K _max , K _max is the maximum stress intensity factor, until the average crack growth rate Δa/ΔN is close to 1×10 ^-7 mm/cycle, The K reduction experiment is over; in order to ensure that ^the change of the collected data is within the allowable range, the load reduction ^range of each level shall not exceed 10%. Loading range to ensure that there are enough fitting data points; (1) In the formula: α = a/W , W is the distance from the line of force action to the edge of the CT sample; B is the thickness of the CT sample; According to the pairs formula: (2) Among them, C and m are constants related to the properties of the material itself; After logarithmic processing, lg(da/dN)=m·lgΔK+lgC can be obtained. From this, it can be seen that lg(da/dN) and lgΔK have a linear relationship; according to da/dN in the range of 10 ^-6 ~10 ^-7 mm/cycle The da/dN—ΔK corresponding relationship obtained in the internal, make its corresponding point in the lg(da/dN)—lgΔK coordinate system, obtain C and m by linear fitting method, bring it into the formula (2), and Set da/dN=1×10 ^-7 mm/cycle to find ΔK, which is the required fatigue crack growth threshold ΔK _th .