Vibration aging blind hole detection method Jinan Rio Tinto Information Technology Co., Ltd. (
1
Stress conceptGenerally speaking, an object, in the absence of external force and external moment action, temperature equilibrium, phase change has been terminated, the stress that still exists inside and maintains its own balance is called internal stress.
According to the classification method proposed by the German scholar Mahluh, internal stress is divided into three categories:
First
I
The intra-class stress is an internal stress that exists in a large area of ​​the material (many grains) and maintains equilibrium across the entire section of the object. When the number of an objectI
When the intra-class stress balance and the internal moment balance are destroyed, the object will produce a macroscopic dimensional change.First
II
Internal stress is an internal stress that exists in a small range (a region inside a grain or grain).Class III internal stresses are internal stresses that exist in a very small range (several atomic distances).
The residual stress usually referred to in engineering is the first
I
Internal stress. So far, the firstI
The internal stress measurement techniques are the most complete, and their effects on material properties and component quality are also studied most thoroughly.In addition to such classification methods, the engineering community is also accustomed to categorizing and naming according to the process of generating residual stress, such as casting stress, welding stress, heat treatment stress, grinding stress, shot peening stress, etc., and generally refers to Is the first
I
Internal stress.(
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StressResidual stresses in mechanical components and large mechanical components have a significant impact on fatigue strength, stress corrosion resistance, dimensional stability and service life. Appropriate and well-distributed residual compressive stress may be a factor that increases fatigue strength and improves stress corrosion resistance, thereby prolonging the service life of parts and components. Inappropriate residual stress reduces fatigue strength, produces stress corrosion, and loses dimensional accuracy. Even leading to early failures such as deformation and cracking.
(
3
) the generation of stressIn mechanical manufacturing, various processes often produce residual stresses. However, if, in essence, the cause of residual stress can be attributed to:
1.
Uneven plastic deformation;
2.
Uneven temperature change;
3.
Uneven phase transition
(
4
) adjustment of stressAiming at the specific service conditions of the workpiece, certain process measures are taken to eliminate or reduce the residual tensile stress unfavorable to its performance, and sometimes a beneficial residual compressive stress distribution can be introduced, which is the adjustment problem of residual stress.
The usual methods for adjusting residual stress are:
1
Natural aging
The components are placed outdoors, and the residual stress is relaxed and the dimensional accuracy is stabilized under repeated temperature stresses through repeated changes in climate and temperature. It is generally believed that after one year of natural aging, the residual stress only drops.
2%
~10%
However, the relaxation stiffness of the workpiece is greatly improved, and the dimensional stability of the workpiece is good. However, due to the long aging time, it is generally not used.2
Hot aging
Thermal aging is a traditional aging method that uses an annealing technique in a heat treatment to heat the workpiece to
500
~650
°C is kept for a long time and then slowly cooled to room temperature. The internal stress is removed by atomic diffusion and plastic deformation under the action of heat. In theory, thermal aging is used, and the stress can be completely eliminated as long as the annealing temperature and time are appropriate. However, in actual production, residual stress can usually be eliminated.70
~80%
However, it has defects such as surface oxidation, hardness and mechanical properties of the workpiece material.3 vibration aging
The vibration aging is to cause the workpiece to resonate under the cyclic external force applied by the exciter, relax the residual stress, and obtain dimensional accuracy stability. That is to say, under the action of the machine, the component is locally plastically deformed, so that the residual stress is released, so as to reduce and adjust the residual stress. It is characterized by short processing time, wide application range, low energy consumption, small equipment investment and easy operation, so the vibration aging is
70
After being introduced from developed countries, it was promoted in China.4 static overload method
The static overload method is temporarily applied to the member in the form of static or static torque and held under this load for a period of time, so that the dimensional accuracy of the part obtains a stable aging method. When used in welded parts, the load needs to be increased so that the sum of the original stress and the additional stress is close to the yield limit of the material to eliminate the residual stress. The accuracy and stability effect of the static overload method depends on the magnitude of the additional stress and the holding time under stress.
In particular, it is noted that the residual stress is still maintained in the component after the static overload method.
5 thermal shock aging method
1970
A novel aging method for stabilizing the dimensional accuracy of workpieces that occurred around the year. The essence is to rapidly heat the workpiece, so that the thermal stress caused by the heating process is superimposed on the residual stress, and the plastic deformation is caused by exceeding the yield limit of the material, so that the original residual stress is quickly relaxed and stabilized.
6 ultrasonic aging method
Ultrasonic aging was first developed in the former Soviet Union and promoted in developed countries. The method was first applied to military, nuclear submarines, aerospace and other military fields where stress relief is very strict.
(
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Stress detection methodThe effect of detecting vibration aging is actually to verify whether the residual stress in the workpiece is eliminated and homogenized. At present, the test methods for residual stress are generally divided into two categories. One type is quantitative measurement: such as blind hole method,
X
Ray method, magnetic measurement method, sandblasting method, cutting method, collar method, etc.; the other type is qualitative test: such as vibration parameter curve method, dimensional accuracy stability method, etc.1 vibration parameter curve method
Whether a vibratory aging process is successful or not, the final test method should be the test of the rate of change of residual stress and the retention of dimensional accuracy. However, it is impossible to use the above two parameters in the vibration process, which requires a long and complicated test process. Usually, in the control process of actual production and application, the amplitude-frequency characteristic parameter curve and the amplitude-time parameter curve test method before and after vibration aging are often used, and
JB/T5926-91
Standard number4.1
Terms orJB/T10375-2002
In the standard6.2
The terms are accepted for acceptance.A
Amplitude-frequency characteristic curve scanning method
In the vibration process, as the residual stress decreases, the internal damping of the member decreases. Therefore, the amplitude-frequency characteristic curve shows a decrease in the natural frequency, an increase in the formant, and a narrowing of the frequency band.
B
,amplitude
-
Time curve monitoringWidth
-
The frequency characteristic curve is performed before and after the vibration processing, and the frequency is constantly changing. Sometimes it is necessary to minimize the excitation force (the lowest eccentricity) in order to obtain a better curve. The curve of amplitude versus time is plotted while the frequency is constant. This method can control the effective time of the vibration treatment by the change of the amplitude, and can detect the change of the residual stress by the amount of change of the amplitude.2 blind hole method
A widely used residual stress test method is the blind hole drilling method. It is to drill a small hole in the measured point, so that the stress of the measured point is partially or completely released, and the released strain is measured by a strain gauge attached to the small hole in advance, and then the residual stress is calculated according to the principle of elastic mechanics. Come. This method has good precision, so it has become a widely used residual stress test method.
3 dimensional accuracy stability method
The dimensional accuracy stabilization method is based on periodic measurement of component dimensional accuracy. It includes two aspects: on the one hand, the amount of change in the dimensional accuracy of the observed component over time, compared with the thermal aging or precision tolerance; on the other hand, it is necessary to observe the dimensional accuracy variation of the component after the static and dynamic loads. It is also comparable to the traditional process (thermal aging) to identify the feasibility of the vibration aging process.
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