Ultrasonic nonlinear static parameter monitoring method for creep damage of high-temperature alloy component

Abstract

The invention provides an ultrasonic nonlinear static parameter monitoring method for creep damage of a high-temperature alloy component, and particularly relates to the technical field of material monitoring. The method comprises the following steps: S1, arranging an ultrasonic probe; s2, determining the emission frequency of the excitation end; s3, respectively calculating an incident angle and an emergent angle of the signal; s4, exciting fundamental frequency ultrasonic waves; s5, receiving the signal and uploading the collected signal to a cloud processing system; s6, carrying out fast Fourier transform and calculating a nonlinear parameter of a static component; s7, establishing a linear relation with creep time, and constructing a creep damage linear quantitative model; s8, calculating a static component nonlinear parameter of the healthy sample; and S9, completing quantitative analysis of the damage by using nonlinear parameters. The method solves the problems that a traditional nonlinear ultrasonic method depends on second harmonic nonlinear parameters, phase velocity matching conditions in the middle and later periods are seriously damaged, response shows a nonlinear trend, unique creep time cannot be determined according to the parameters, and then the unique damage state cannot be determined.

Description

Technical Field

[0001] This invention patent relates to the field of materials monitoring technology, specifically to an ultrasonic nonlinear static parameter monitoring method for creep damage of high-temperature alloy components. Background Technology

[0002] The creep damage evolution process of high-temperature alloys can be divided into three typical stages: the initial deceleration stage is dominated by dislocation slip, and the strain rate gradually decreases; the steady-state stage forms a dynamic balance between hardening and recovery, characterized by linear strain accumulation; and the final acceleration stage is characterized by an exponential increase in strain rate due to the aggregation of grain boundary voids until fracture. During this process, the microstructure gradually evolves from dislocation tangles to an intergranular crack network, posing a significant challenge to material lifetime prediction. Traditional nonlinear ultrasonic methods rely on second-harmonic nonlinear parameters (…). The measurement of the phase velocity was performed, but due to the severe disruption of the phase velocity matching conditions in the middle and later stages, its response exhibited a non-monotonic characteristic of first increasing and then decreasing, forming an underdetermined inverse problem—that is... The creep time exhibits a non-linear trend, making it impossible to determine a unique creep time based on this parameter, and thus to determine the damage state.

[0003] Research shows that using static component nonlinear parameters ( It does not rely on strict phase velocity matching conditions; even in the presence of group velocity mismatch, its linear relationship with dislocation density remains intact. This characteristic stems from the long-range interaction mechanism between the stress field and the defect. The creep exhibits monotonically increasing growth throughout its entire lifespan, and continues to respond, especially during the accelerated crack propagation phase. This characteristic provides a physical basis for online assessment of the entire lifespan of high-temperature components and is expected to solve the challenges of creep monitoring and remaining life prediction for critical engineering components such as aero-engine blades. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, this invention provides an ultrasonic nonlinear static parameter monitoring method for creep damage of high-temperature alloy components, the specific technical solution of which is as follows: An ultrasonic nonlinear static parameter monitoring method for creep damage in high-temperature alloy components, specifically including the following steps: S1. Determine the positions of the signal excitation end and the signal receiving end on the surface of the component to be monitored, and arrange the ultrasonic probes accordingly; S2. Based on the dispersion curve of the component to be monitored, select the frequency on the dispersion curve that matches the group velocity under the zero frequency of mode S0 as the transmission frequency of the signal excitation end. S3. Calculate the signal incident angle at the signal excitation end and the signal emission angle at the signal receiving end according to Schnell's law; S4. The signal generator is used to excite the fundamental frequency ultrasonic wave in the monitored component through the ultrasonic probe of the signal excitation end via the attenuator; S5. The signal received by the signal receiving end is transmitted to the oscilloscope after being amplified, and the oscilloscope collects the signal and uploads it to the cloud processing system; S6. The cloud processing system performs fast Fourier transform on the received signal, extracts the static component amplitude and the fundamental amplitude , and calculates the static component nonlinear parameter , wherein ; S7. The linear relationship between the static component nonlinear parameter and the creep time is established, and a creep damage linear quantification model is constructed; S8. The static component nonlinear parameter of the healthy sample is calculated as a reference, and the normalization processing is performed on to calculate the nonlinear parameter ; S9. Based on the linear relationship between the static component nonlinear parameter and the creep time , the nonlinear parameter is used to judge the degree of creep damage and the life stage, and the damage quantification analysis is completed.

[0005] Preferably, in S1, the high-temperature alloy material used in the monitored component satisfies the dislocation proliferation rate of 10³-10 4 / m²·s in the environment of 550-900℃; the signal excitation end uses an industrial probe with a center frequency of 5MHz and a bandwidth≥3MHz to emit a Hanning window modulated wave; the signal receiving end uses a low-frequency probe with a center frequency of 0.5MHz±5%; the installation distance between the signal excitation end and the signal receiving end satisfies ; wherein the wavelength , is the phase velocity of the signal excitation end emission frequency, and the signal excitation end emission frequency is f.

[0006] Preferably, in S3, the signal incidence angle of the signal excitation end is calculated using the phase velocity of the signal excitation end emission frequency; and the signal emission angle of the signal receiving end is calculated using the phase velocity when the S0 mode frequency is zero.

[0007] Preferably, in S4, the signal generator uses a RAM-5000SNAP nonlinear high-energy ultrasonic test system; and the adjustable attenuation range of the attenuator is 10-60dB, and the SNR≥20dB.

[0008] Preferably, in S5, the amplifier is a low-noise amplifier with a fixed gain of 60dB; the cloud processing system includes a fast Fourier transform module, a parameter extraction module, a data processing module, and a wireless communication module; the signal excitation end, signal receiving end, signal generator, attenuator, amplifier, and oscilloscope are all electrically connected to the cloud processing system.

[0009] Preferably, in S7, the static component nonlinear parameter With creep time The linear relationship is: ; In the formula, , It is a linear fitting constant; Among them, at each creep time point, The signal excitation and signal reception yielded the following results. The nonlinear parameters of each static component are calculated and their variances are determined to ensure that the nonlinear parameters of the static components at each creep time point are consistent. The error is controlled within ±5%, and the time resolution reaches 0.1 hours.

[0010] More preferably, in S8, a healthy sample made of the same material as the component to be monitored is selected, and the operations in S1-S6 are repeated to calculate the static component nonlinear parameters of the healthy sample. As a benchmark.

[0011] More preferably, in S9, the nonlinear parameters obtained in S8 are used. The determination of the degree of creep damage and its lifespan stage is specifically divided into the following four stages: Phase I: When 1 < When the value is ≤1.2, the component under monitoring is in the early stage of creep, at which time dislocation slip is dominant; Phase II: When 1.2 < When the value is ≤1.4~1.5, the component under monitoring is in the middle stage of creep, and the precipitated phase coarsens. Phase III: When When the value is >1.4 to 1.5, the component under monitoring is in the late stage of creep, where micropores coalesce, and microcracks form and propagate. End of Stage III: When the static component nonlinear parameter The parameter begins to decrease, i.e., the nonlinear parameter. As the descent begins, macroscopic cracks form, and the component begins to fracture and fail. At this point, the component under monitoring is in the final stage of Phase III.

[0012] The beneficial effects of this invention are: Compared to traditional nonlinear ultrasound methods that rely on second harmonic nonlinear parameters The application utilizes static component nonlinear parameter , which is independent of strict phase velocity matching condition, and its linear relationship with dislocation density is maintained even in the presence of group velocity mismatch, which is derived from the long-range interaction mechanism between stress field and defects, so that The linear trend of the parameter is monotonously increasing throughout the whole creep period, and the parameter still responds continuously when the crack expands in the later stage of accelerated creep, so that the unique creep time and the corresponding damage state can be determined according to the parameter, thereby providing a basis for online evaluation of the whole life of the high-temperature alloy component. BRIEF DESCRIPTION OF DRAWINGS

[0013] The drawings constituting the specification of the application are used to provide further understanding of the application and do not constitute improper limitation on the application.

[0014] Figure 1 The system for the monitoring method of the application is mainly composed of the structure shown in the figure. Figure 2 The monitoring method provided by the application is shown in the figure. Figure 3 The trend graph of the HP40 alloy component to be monitored in the embodiment with respect to the change of creep time is shown in the figure. Figure 3 (a) in the figure is the trend graph of the conventional second harmonic nonlinear parameter with respect to the change of creep time. Figure 3 (b) in the figure is the trend graph of the static component nonlinear parameter provided by the application with respect to the change of creep time. Figure 4 The trend graph of the P92 alloy component to be monitored in the embodiment with respect to the change of creep time is shown in the figure. Figure 4 (a) in the figure is the trend graph of the conventional second harmonic nonlinear parameter with respect to the change of creep time. Figure 4 (b) in the figure is the trend graph of the static component nonlinear parameter provided by the application with respect to the change of creep time. Figure 5 The linear relationship graph of the nonlinear parameter and the creep time in the embodiment is shown in the figure. Figure 5 (a) in the figure is the linear relationship graph of the nonlinear parameter of the HP40 alloy component and the creep time. Figure 5 (b) in the figure is the linear relationship graph of the nonlinear parameter of the P92 alloy component and the creep time. DETAILED DESCRIPTION

[0015] The specific implementation of the ultrasonic nonlinear static parameter monitoring method for creep damage of high-temperature alloy components provided by the present invention will be further described with reference to the accompanying drawings and embodiments.

[0016] like Figures 1-2 As shown, an ultrasonic nonlinear static parameter monitoring method for creep damage in high-temperature alloy components specifically includes the following steps: S1. Determine the positions of the signal excitation end and the signal receiving end on the surface of the component to be monitored, and arrange the ultrasonic probes accordingly; Preferably, in S1, the high-temperature alloy material used in the component to be monitored meets the requirement that the dislocation multiplication rate is 10³ to 10⁻⁶ in an environment of 550℃ to 900℃. 4 / m²·s; the signal excitation end uses an industrial probe with a center frequency of 5MHz and a bandwidth ≥3MHz to transmit Hanning window modulated waves; the signal receiving end uses a low-frequency probe with a center frequency of 0.5MHz±5%; the installation distance between the signal excitation end and the signal receiving end... satisfy Among them, wavelength , The phase velocity is the frequency of the signal excitation terminal. This is the transmission frequency of the signal excitation terminal.

[0017] S2. Based on the dispersion curve of the component to be monitored, select the frequency on the curve that matches the group velocity under the zero frequency of mode S0 as the transmission frequency of the signal excitation end. S3. According to Schnell's Law: Calculate the incident angle and the exit angle of the signal respectively; in, The longitudinal wave velocity in the wedge is... It is the phase velocity (signal incident angle) at the transmission frequency of the signal excitation end in the material medium or the phase velocity (signal emission angle) at the zero frequency of the S0 mode.

[0018] S4. The signal is generated by a signal generator and passed through an attenuator to excite the fundamental frequency ultrasonic waves in the component to be monitored through the ultrasonic probe at the signal excitation end; Preferably, in S4, the signal generator adopts the RAM-5000SNAP nonlinear high-energy ultrasonic testing system. This signal generator has microsecond-level triggering accuracy and, in conjunction with the Hanning window modulation wave, can achieve high time resolution excitation. The adjustable attenuation range of the attenuator is 10-60dB, and the SNR is ≥20dB. In addition, the attenuator also has the following functions: (1) It accurately reduces the signal power through the resistor network, prevents the receiving circuit from overload or saturation, protects sensitive electronic components, and thus realizes dynamic adjustment of the input signal amplitude; (2) The calibrated attenuator can be used to compensate for system errors (such as cable loss), ensure the uniformity of amplitude measurement benchmark, and optimize measurement accuracy; (3) Noise suppression function.

[0019] S5. The signal is received by the signal receiver and amplified by the amplifier before being transmitted to the oscilloscope. The oscilloscope collects the signal and uploads it to the cloud processing system. Preferably, in S5, the amplifier is a low-noise amplifier with a fixed gain of 60dB, which effectively reduces the impact of transmission loss on the results; at the same time, the amplifier can also perform automatic gain control (AGC) to compensate for signal attenuation caused by increased sound path.

[0020] Preferably, the cloud processing system includes a fast Fourier transform module, a parameter extraction module, a data processing module, and a wireless communication module; It is worth emphasizing that the signal excitation end, signal receiving end, signal generator, attenuator, amplifier, and oscilloscope are all electrically connected to the cloud processing system.

[0021] S6. The cloud processing system performs a Fast Fourier Transform on the received signal to extract the amplitude of the static component. With fundamental amplitude And calculate the static component nonlinear parameters. ,in, ; S7. Establish static component nonlinear parameters With creep time linear relationship ,pass The monotonically increasing characteristic is used to construct a linear quantification model of creep damage; where, , It is a linear fitting constant; It is worth noting here that, in order to ensure the nonlinearity of the static component parameters at each creep time point... Errors are controlled within ±5%, with a time resolution of 0.1 hours, and are performed at each creep time point. The signal excitation and signal reception yielded the following results. The nonlinear parameter values ​​of each static component are calculated, and the variance is solved.

[0022] S8. Another health sample of the same material as the monitored component is selected, and the operations in S1-S6 are repeated to calculate the static component nonlinear parameter of the health sample As a reference, the static component nonlinear parameter of the health sample is taken as a reference value The static component nonlinear parameter of the health sample is normalized to calculate the nonlinear parameter .

[0023] S9. Based on the linear relationship between the static component nonlinear parameter and the creep time, it can be known that The static component nonlinear parameter is also a nonlinear parameter linearly related to the creep time . The nonlinear parameter is used to judge the degree of creep damage and the life stage, and complete the damage quantification analysis.

[0024] Preferably, in S9, the creep damage degree and the life stage are specifically divided into the following four stages: Stage I: when 1 ≤1.2, the monitored component is in the early stage of creep, and dislocation slip is dominant at this time; Stage II: when 1.2 ≤1.4-1.5, the monitored component is in the middle stage of creep, and the precipitated phase is coarsened; Stage III: when >1.4-1.5, the monitored component is in the late stage of creep, and micro-holes are aggregated, micro-cracks are formed and expanded; it is worth noting that In the creep acceleration period (stage III), the change rate per hour exceeds 3%, and the 0.1h resolution can capture the critical damage jump; End of stage III: when the static component nonlinear parameter starts to decrease, i.e., the nonlinear parameter starts to decrease, macroscopic cracks are formed, and the component starts to appear fracture failure, at this time, the monitored component is in the end of stage III. When the parameter is monitored to decrease from increase, at this time, the monitored component has entered the fracture countdown, and needs to be warned and immediately stopped.

[0025] In order to better understand the monitoring method provided by the present application, the following will be described in combination with specific embodiments: ​​Taking typical high-temperature alloys HP40 (HP40 is a nickel-based cast high-temperature alloy commonly used in equipment such as ethylene cracking furnace tubes and hydrogen conversion furnace tubes; in the aerospace field, HP40 is also used to manufacture high-temperature engine components, such as combustion chamber liners) and P92 (P92 steel is a representative of third-generation martensitic heat-resistant steel, specifically designed for ultra-supercritical thermal power units; this material has been applied in 85% of domestic megawatt-class ultra-supercritical units, especially in the nuclear power field, where P92 is commonly used in fourth-generation sodium-cooled reactor pressure vessels) as examples, according to GB / T 2039-2012 "Metallic Materials Uniaxial Tensile Creep Test Method", a time-sharing loading creep method was used to prepare specimens with different degrees of damage to verify the accuracy of the monitoring structure proposed in this invention. The creep time setting covers the entire creep life cycle.

[0026] On the damaged HP40 sample, the frequency that matches the group velocity at zero frequency of S0 mode in S4 mode is selected as the excitation mode. In this example, the frequency is 4.8MHz, the number of cycles is 5, a 5MHz transducer is used at the signal excitation end, a 0.5MHz transducer is used at the signal receiving end, and the corresponding wedge incident angle is 27.7°.

[0027] On the damaged P92 sample, the frequency that matches the group velocity at zero frequency of S0 mode in S3 mode is selected as the excitation mode. In this example, the frequency is 5.6MHz, the number of cycles is 10, a 5MHz transducer is used at the signal excitation end, a 0.5MHz transducer is used at the signal receiving end, and the corresponding wedge incident angle is 27°.

[0028] Calculate the creep time corresponding to each of the two types of samples above. Values ​​(obtained after multiple variance control errors) and establish static component nonlinear parameters. With creep time Linear relationship, such as Figure 3 (b) and Figure 4 As shown in (b); To more intuitively relate to the nonlinear parameters dependent on the second harmonic... Compared with traditional nonlinear ultrasonic methods, this embodiment also includes the second harmonic nonlinear parameters of the two types of samples. With creep time Linear relationship, such as Figure 3 (a) and Figure 4 As shown in (a); Will Figure 3 (a) and Figure 3 (b) Figure 4 (a) and Figure 4 A comparison in (b) shows that the second harmonic nonlinear parameter is dependent on... Traditional nonlinear ultrasound methods exhibit response that varies with creep time. The changes exhibit obvious nonlinear characteristics, especially in the early stages of creep ( ), The creep intensity gradually increases with damage accumulation, exhibiting good monotonicity and sensitivity; however, when creep enters the middle and late stages (after 600 h, fracture occurs around 960 h), due to the proliferation of microcracks, pore connections, and degradation of the elastic modulus within the material, the nonlinear acoustic response exhibits "oversaturation" or scattering enhancement effects, leading to... The signal no longer rises continuously with increasing damage; instead, it exhibits a trend of decline, attenuation, and even fluctuation. In contrast, the signal based on the static component nonlinear parameters... It maintains good monotonic growth characteristics within the same creep stage, that is... The signal increases almost linearly throughout the creep process, and even in the middle and late stages when material properties deteriorate significantly and the macroscopic creep rate accelerates, it can still continuously reflect the cumulative damage.

[0029] Therefore, in A curve will appear with "a Two corresponding The non-unique mapping phenomenon of "" leads to the use of There are ambiguities and potential for misjudgment when assessing creep damage; The curves always maintain a monotonically reversible relationship, and there is no problem of multiple values ​​corresponding, which significantly improves the certainty and accuracy of damage assessment.

[0030] Furthermore, this phenomenon also confirms the difference between the two types of nonlinear parameters from a mechanistic perspective: It is more sensitive to local elastic distortion and modulation effects in materials. When damage develops to a certain critical level, local scattering, interface slip, and inter-crack interactions can cause second harmonic signal distortion. This more directly reflects the overall nonlinear cumulative effect and has a stronger ability to resist local modal disturbances and structural degradation. Therefore, the static component nonlinear parameters... As a key criterion, it is more helpful to achieve stable and reliable creep damage characterization throughout the entire life cycle.

[0031] Select healthy HP40 and P92 samples, and calculate the static component nonlinear parameters of the HP40 and P92 healthy samples respectively. ,right Normalization is performed to obtain nonlinear parameters. Linear relationship with creep time (e.g.) Figure 5 (a) and Figure 5The monitoring results of the damage monitoring member HP40 and P92 and the conventional nonlinear monitoring method are compared and explained in the following Table 1 and Table 2, respectively.

[0032] Table 1 Comparison table of monitoring damage results of HP40 alloy (900 DEG C, 33 MPa)

[0033] Table 2 Comparison table of monitoring damage results of P92 (650 DEG C, 115 MPa)

[0034] In summary, the prediction structure error between the results obtained by the monitoring method and the two samples prepared in the examples with different known damage degrees is less than 8%, that is, the underdetermined inverse problem formed by the conventional nonlinear monitoring method is effectively avoided by using the monitoring method provided by the present application, and the unique creep time and the corresponding damage degree and life stage can be accurately determined, which provides a basis for online evaluation of the creep whole cycle damage state of the high-temperature alloy member.

[0035] In the present application, the terms such as "upper", "lower", "bottom", "top" and the like indicate the orientation or positional relationship shown in the drawings, which is only a relationship word determined for the purpose of describing the structural relationship of the components or elements of the present application, and cannot be understood as a limitation of the present application. The terms such as "connected", "connected" and the like should be understood broadly, which means that it can be fixedly connected, integrally connected or detachably connected; it can be directly connected or indirectly connected through an intermediate medium. For related researchers or technicians in the field, the specific meaning of the above terms in the present application can be determined according to the specific circumstances, and cannot be understood as a limitation of the present application.

[0036] Of course, the above description is not a limitation of the present application, and the present application is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the essential scope of the present application should also be within the protection scope of the present application.

Claims

1. A method of ultrasonic nonlinear static parameter monitoring of creep damage in a high-temperature alloy component, characterized in that, Specifically comprising the following steps: S1. Determine the positions of the signal excitation end and the signal receiving end on the surface of the component to be monitored and arrange the ultrasonic probes respectively; S2. Select a frequency on the dispersion curve of the component to be monitored that matches the group velocity at the zero frequency of the S0 mode as the transmission frequency of the signal excitation end based on the dispersion curve of the component to be monitored; S3. Calculate the signal incidence angle of the signal excitation end and the signal emission angle of the signal receiving end respectively according to Snell's law; S4. Use the signal generator to excite the fundamental frequency ultrasonic wave in the component to be monitored through the ultrasonic probe of the signal excitation end via the attenuator; S5. Use the signal receiving end to receive the signal and transmit it to the oscilloscope after passing through the amplifier, and the oscilloscope collects the signal and uploads it to the cloud processing system; S6. The cloud processing system performs a fast Fourier transform on the received signal, extracts the static component amplitude and the fundamental amplitude and calculates the static component nonlinear parameter wherein ; S7. Establishing static component nonlinear parameters with the linear relationship of creep time , and constructing a creep damage linear quantification model; S8. Calculate static component nonlinear parameter of healthy sample As a reference, the nonlinear parameter of the healthy sample is calculated The nonlinear parameter is calculated after normalization ; S9. Nonlinear parameter based on static component With the linear relationship of the creep time , the degree of creep damage and the life stage are judged by using the nonlinear parameter , and the damage quantification analysis is completed.

2. The method of claim 1, wherein the method is used to monitor the creep damage of a superalloy component. In S1, the high-temperature alloy material adopted by the component to be monitored satisfies the dislocation proliferation rate of 10 3 ~ 10 5 / m 2 ·s in the environment of 550℃~900℃ 4 / m²·s; The signal excitation end uses an industrial probe with a center frequency of 5 MHz and a bandwidth of ≥3 MHz to transmit Hanning window modulated wave; The signal receiving end uses a low-frequency probe with a center frequency of 0.5 MHz±5%; The installation distance between the signal excitation end and the signal receiving end Satisfies ; wherein the wavelength , is the phase velocity of the signal excitation end emission frequency, is the emission frequency of the signal excitation end.

3. The method of claim 2, wherein the method is used to monitor the creep damage of a superalloy component. In S3, the signal incidence angle of the signal excitation end is calculated using the phase velocity of the signal excitation end transmission frequency; The signal emission angle of the signal receiving end is calculated using the phase velocity at the zero frequency of the S0 mode.

4. The method of claim 3, wherein the method is used to monitor the creep damage of a superalloy component. In S4, the signal generator uses a RAM-5000SNAP nonlinear high-energy ultrasonic test system; The adjustable attenuation range of the attenuator is 10-60 dB, and the SNR is ≥20 dB.

5. The method of claim 4, wherein the method further comprises: In S5, the amplifier uses a low-noise amplifier with a fixed gain of 60 dB; The cloud processing system includes a fast Fourier transform module, a parameter extraction module, a data processing module, and a wireless communication module; The signal excitation end, the signal receiving end, the signal generator, the attenuator, the amplifier, and the oscilloscope are all electrically connected to the cloud processing system.

6. The method of claim 5, wherein the method further comprises: In S7, the static component nonlinear parameter With the linear relationship of creep time is: ； wherein , is a linear fit constant; Wherein, at each creep time node The secondary signal excitation and signal receiving are carried out The static component nonlinear parameter value is obtained and the variance is solved, and the static component nonlinear parameter at each creep time node is ensured The error is controlled within ±5%, and the time resolution reaches 0.1 hours.

7. The method of claim 6, wherein the method further comprises: In S8, a healthy sample of the same material as the component to be monitored is selected, and the operations in S1-S6 are repeated to calculate the static component nonlinear parameter of the healthy sample As a reference.

8. The method of claim 7, wherein the method is used to monitor the creep damage of a superalloy component. In S9, the nonlinear parameters obtained from S8 are used. The determination of the degree of creep damage and its lifespan stage is specifically divided into the following four stages: Stage I: When 1 ≤ 1.2, the member to be monitored is in the early stage of creep, when dislocation slip dominates; Stage II: when 1.2 < t / t0 < 1.4, the monitored member is in the middle of creep, and the precipitated phase is coarsened; when 1.4 ~ 1.5, the monitored member is in the middle of creep, and the precipitated phase is coarsened; Stage III: When >1.4-1.5, the monitored member is in the later stage of creep, micro holes are aggregated, and micro cracks are formed and expanded. End of stage III: when the static component nonlinear parameter starts to decrease, i.e. the nonlinear parameter starts to decrease, macro-cracks are formed and the component starts to fail, i.e. the component to be monitored is at the end of stage III.

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