CN112129830B - Airplane metal structure burn detection method based on eddy current conductivity - Google Patents
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Abstract
The application discloses an aircraft metal structure burn detection method based on eddy current conductivity, which comprises the steps of obtaining the corresponding relation between the conductivity and the structural strength and hardness of a metal structure material through a burn test simulated by heat treatment of the metal structure material, establishing a burn evaluation database based on eddy current conductivity and developing burn detection software through summarizing burn degree evaluation standards, realizing rapid evaluation of the burn degree of the aircraft metal structure, and providing basis for aircraft burn emergency repair scheme formulation; the application improves the technologies of temperature compensation, lift-off compensation, phase amplitude detection, filtering and the like of the traditional eddy current signals to form the conductivity detection device, provides burn grade standards of the airplane metal structure, establishes a burn evaluation database and develops a burn evaluation client, thereby realizing rapid evaluation of the burn degree of the airplane metal structure.
Description
Technical Field
The application relates to the technical field of burn evaluation of aircraft metal structures, in particular to an aircraft metal structure burn detection method based on eddy current conductivity.
Background
The burning of the metal structure of the airplane can be caused by the impact of the enemy or the oil leakage and the fire of the oil tank, and the like, is the most common damage form of the airplane, and the existing metal structure burning detection system of the airplane in China has the problems of lag detection technology, low detection precision, lack of experimental data support, no burning evaluation standard and the like, and seriously affects the burning emergency repair of the metal structure of the airplane.
The conductivity is an important parameter for representing the conductivity of the metal material, is closely related to the microstructure, mechanical property and heat treatment state of the material, and the eddy current method is a common measurement method for detecting the conductivity of the metal material, has the advantages of simple structure, low power consumption, sensitive measurement, rapidness and no damage, and is widely applied to the heat treatment state identification and the heat treatment quality detection of aluminum and titanium alloys.
Disclosure of Invention
The technical problems to be solved by the application are as follows: the method for detecting the burn of the metal structure of the airplane overcomes the defects of the prior art, improves the technologies of temperature compensation, lift-off compensation, phase amplitude detection, filtering and the like of the traditional eddy current signals, provides the burn grade standard of the metal structure of the airplane, establishes a burn evaluation database and develops a burn evaluation client, thereby realizing the rapid evaluation of the burn degree of the metal structure of the airplane.
The technical scheme adopted by the application for solving the technical problems is as follows: according to the aircraft metal structure burn detection method based on eddy current conductivity, a burn experiment is simulated through heat treatment of a metal structure material, a corresponding relation between the conductivity and the structural strength of the metal structure material is obtained, a burn evaluation database based on eddy current conductivity is established and burn detection software is developed through summarizing burn degree evaluation standards, so that quick evaluation of the burn degree of the aircraft metal structure is realized, and a basis is provided for aircraft burn emergency repair scheme establishment; the method comprises the following specific steps:
firstly, selecting a typical aircraft structural material, and performing burn simulation experiments under corresponding conditions to obtain a burned metal material;
step two, placing the metal material obtained in the step one near a coil magnetic field Hp of an eddy current conductivity detection device, inducing eddy current on the surface of the material under the action of an alternating magnetic field, generating an alternating counter magnetic field Hs by the eddy current, and measuring the conductivity of a conductor by measuring the impedance change of the detection coil; the method comprises the following steps in the measurement process:
1) Temperature drift compensation is carried out on the eddy current sensor through a compensation technology;
2) Nonlinear compensation is carried out on the exponential characteristic of the lift-off effect of the eddy current sensor by adopting an exponential operation circuit;
3) According to the basic principle of eddy current detection, adopting ANSYS analysis software of a finite element method to establish a geometric model of a cylindrical eddy current sensor probe coil, analyzing the influence of the probe coil parameters and performing simulation analysis on the model to realize finite element analysis of the eddy current sensor;
4) Performing software denoising by Matlab software by adopting a wavelet grading denoising method so as to obtain an accurate real signal and complete wavelet denoising of the eddy current detection signal;
step three, researching the corresponding relation between the conductivity, the strength and the hardness of the metal material in different simulated burn states according to the conductivity obtained in the step three, and drawing up four different burn degree standards;
step four, based on the metal material performance test results in the step one and the step three, establishing a burn evaluation mathematical model based on conductivity, establishing a burn evaluation database based on conductivity on an SQL Server 2012, and developing and analyzing a diagnosis client by utilizing a Visual Studio 2012 on a Windows platform;
fifthly, inputting initial information such as the characteristics of the metal materials of the airplane to be detected, inputting the initial information into the client, setting a detection path and a detection method, performing corresponding detection calculation, comparing the calculation result with the same type of data in the burn evaluation database, judging whether burn exists or not, and determining a detection report.
In the first step, the typical aircraft structural material is 7050 aluminum alloy or TC4 titanium alloy, and the corresponding conditions are that the temperature is 250-1150 ℃ and the time is 1-10 min.
In the second step, the impedance change caused by measurement is converted into a voltage change differential output through a Z/V converter, the voltage change differential output is detected and filtered to adjust the output DC voltage for display, according to the principle of a single-frequency impedance method, an alternating signal is generated by adopting a DDS technology from the phase analysis to be supplied to a bridge and a detection coil, the signal is amplified, phase-sensitive detected and filtered to become a DC signal containing phase information and amplitude information of the coil impedance change, and the DC signal is input into a computer after being processed.
In the second step, the geometric model of the cylindrical eddy current sensor probe coil is modeled by adopting a two-dimensional model, grid division is performed by adopting a mapping network, a regular shape is mapped onto an irregular area, and a PLANE53 unit is selected for grid division of PLanE and axisymmetric magnetic field problems in analysis so as to obtain higher analysis precision.
Further, in the second step, a boundary condition is set, and a load and a solution are applied, where the load refers to the boundary condition and an external or internal acting force function, where the sensor housing and a pipeline connected thereto need to apply the boundary condition, apply a VOLT constraint, set to a zero potential, apply a magnetic signal as an excitation element on grid nodes divided by an air domain, and use a wavefront solver to accurately obtain an analysis result.
Further, in the second step, the noise elimination process is processed according to the following method: the signal is first subjected to wavelet multiple-resolution, and the noise part is usually contained in three noise frequency bands of CD1, CD2 and CD 3; then processing the wavelet coefficient according to the forms of threshold value and the like; and then reconstructing to achieve the purpose of noise elimination.
In the second step, the eddy current conductivity detection device comprises a computer, an excitation signal source, an eddy current sensor, a Z/V transmitter, a voltage follower, an isolation measurement circuit and a subsequent circuit, wherein the excitation signal source, the eddy current sensor, the Z/V transmitter, the voltage follower, the isolation measurement circuit and the subsequent circuit are connected with the computer;
the excitation signal source is a sine wave generator, which adopts a 10 KHz-1 MHz highly stable sine wave signal generated by a special DDS digital frequency synthesis chip AD9850 and adopts an AGC feedback network to improve the stability of the signal amplitude;
the electric vortex sensor is of a cylindrical structure, an upper coil and a lower coil are adopted, the lower coil is a detection element, and the upper coil is a compensation coil;
the Z/V transmitter is an amplifier and a differential circuit.
In the third step, the variation of intensity and conductivity in the burn process of the airplane metal structure is used for defining, and a TC4 titanium alloy standard test piece with four different burn grades of I-IV is provided, wherein the grade I is a non-burn, the grade II is a mild burn, the grade III is a medium burn and the grade IV is a severe burn.
Further, in the fifth step, the detection report includes component information, material information, a history of the detection component, display of a detection result, and a diagnosis conclusion.
The beneficial effects of the application are as follows:
1. the application improves the technologies of temperature compensation, lift-off compensation, phase amplitude detection, filtering and the like of the traditional eddy current signals to form the conductivity detection device, provides burn grade standards of the airplane metal structure, establishes a burn evaluation database and develops a burn evaluation client, thereby realizing rapid evaluation of the burn degree of the airplane metal structure.
2. The eddy current conductivity detection device has the advantages of simple structure, low power consumption, automatic temperature drift compensation and lift-off compensation functions, high detection precision, good stability, accurate detection and high intelligent degree, and can provide basis for the establishment of an aircraft burn emergency repair scheme.
Description of the drawings:
FIG. 1 is a schematic diagram of eddy current testing of metal conductivity in the method of the present application.
FIG. 2 is a schematic diagram of an eddy current sensor output signal measurement system in the method of the present application.
FIG. 3 is a schematic diagram of Z/V transformation temperature compensation technique in the method of the application.
FIG. 4 is a schematic diagram showing the effect of temperature compensation on the sensor output voltage in the method of the present application.
FIG. 5 is a block diagram of an experimental verification platform for sensor structure optimization in the method of the present application.
Fig. 6 is a front view of an eddy current microsensor structure in the method of the present application.
FIG. 7 is a schematic diagram of the effect of lift-off on sensor output voltage and phase in the method of the present application.
Fig. 8 is a schematic circuit diagram of the principle of the index compensation technique in the method of the application.
Fig. 9 shows the relationship between the pull-out distance and the output voltage after the exponential characteristic compensation.
Fig. 10 is a top view of an eddy current microsensor structure in the method of the present application.
Fig. 11 is a schematic of the overall software flow of burn test analysis software in the method of the present application.
Fig. 12 is a schematic diagram of a burn test procedure in the method of the present application.
Fig. 13 is a graph of conductivity versus intensity.
FIG. 14 is a graph of conductivity versus hardness.
Detailed Description
Examples: referring to fig. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 and 14, in fig. 6, 1 a-detecting element, 1 b-compensating coil, 1 c-frame, 1 d-core.
The application is described in detail below with reference to the drawings and examples.
Step one: the 7050 aluminum alloy or TC4 titanium alloy is subjected to burn simulation experiments at the temperature of 250-1150 ℃ for 1-10 min.
Step two: providing an aircraft metal structure eddy current conductivity detection device, wherein the measurement range reaches 0.3-63MS/m, the measurement precision reaches about +/-1%, and the error is controlled within 0.5%;
the principle of eddy current conductivity detection is shown in fig. 1, if a metal material is placed near a coil magnetic field Hp, eddy current is induced on the surface of the material under the action of an alternating magnetic field, which is called eddy current, and meanwhile, the eddy current generates an alternating counter magnetic field Hs, and the conductivity is the only parameter for determining the size of the eddy current magnetic field, so that the conductivity of a conductor can be measured by measuring the impedance change of the detection coil.
FIG. 2 is a schematic block diagram of an eddy current measuring system, wherein impedance change caused by measurement is converted into voltage change differential output through a Z/V converter, and the voltage change differential output is subjected to detection and filtering adjustment to output direct current voltage for display;
the excitation signal source is a sine wave generator with high stability, the item adopts a sine wave signal with high stability of 10 KHz-1 MHz generated by a special DDS digital frequency synthesis chip AD9850, the stability of the frequency can reach 10 < -7 > within the temperature range of-20-60 ℃, and the stability of the signal amplitude can be improved by 10 < -4 > by adopting an AGC feedback network;
aiming at the problem of large temperature drift in the existing burn detection technology, as shown in fig. 3, an eddy current sensor with a temperature drift compensation function is provided, wherein a Z/V transmitter in the figure comprises amplifiers U1B and R3, L1 and L2 to form a differential circuit, Z/V conversion is realized, U1A, U C forms a voltage follower, a measuring circuit and a subsequent circuit are isolated, L1 is a measuring coil of the sensor, L2 is a compensation coil, the characteristic parameters of the L2 are completely the same as L1, and because the compensation coil L2 and the detection coil L1 are in the same temperature field, the L2 automatically compensates the temperature drift caused by the detection coil.
Experiment verification one:
fig. 4 is sensor output voltage measurement data for the early no load and load (with temperature compensation) conditions for a 2mm thick duralumin alloy sheet. It can be seen that under the condition of load (with temperature compensation), the output voltage is a horizontal curve along with the temperature change and basically does not change along with the temperature change;
because the lift-off effect is mainly reflected on the amplitude of the impedance and has small influence on the phase, the project starts from the phase analysis according to the single-frequency impedance method principle, an alternating signal is generated by adopting a DDS technology and is supplied to an electric bridge and a detection coil, the signal is amplified, phase-sensitive detected and filtered and then becomes a direct current signal containing phase information and amplitude information of the impedance change of the coil, and the direct current signal is input into a computer system after being processed, as shown in figure 5;
to achieve the maximum sensitivity of the sensor, the sensor should have a cylindrical structure, a dual coil structure, a lower coil structure, a sensing element, and an upper coil structure, as shown in fig. 6 and 10.
Experiment verification II:
in the early experiments, an aluminum alloy with the conductivity of 35.2MS/m is selected as a tested piece to verify the sensor. The results indicate that increasing the core size can improve the sensor sensitivity to some extent. The aluminum alloy is used as a tested piece, the distance between the sensor and the test piece is adjusted, meanwhile, amplitude information and phase information are collected, the experimental result is shown in fig. 7, the influence of lift-off in lmm on the phase of the impedance information of the sensor is small, the lift-off mainly reacts in the amplitude information, and the influence of lift-off effect on the sensor can be reduced by using the phase;
as can be seen from fig. 7, the lift-off distance and the voltage signal output by the sensor are in a nonlinear relationship, and the nonlinear section of the sensor is compensated by adopting an exponential operation circuit, so that the sensitivity and anti-interference performance of the sensor can be furthest expanded on the premise of ensuring the measurement accuracy requirement;
the index compensation circuit is designed based on the index characteristics of the transistor as shown in fig. 8. The output voltage is used as a nonlinear compensation link, so that the influence generated by the lift-off effect can be effectively corrected. In order to solve the negative effect that the exponential operation circuit has attenuation on a relatively close distance, the output and the input of the exponential operation circuit pass through the integrated operational amplifier subtracting circuit.
Experiment verification three:
fig. 9 is a graph showing the relationship between the output voltage and the lift-off distance in the early detection experiment of the eddy current lift-off effect on duralumin alloy. After being corrected by the exponential characteristic, the output voltage and the lifting distance basically form a linear relation, and the lifting distance is obviously improved;
aiming at the defects of the existing burn detection technology, the software technology to be researched comprises a finite element analysis technology of an eddy current sensor and an eddy current detection signal wavelet denoising technology based on Matlab;
the finite element analysis technology of the eddy current sensor is characterized in that according to the basic principle of eddy current detection, ANSYS analysis software of a finite element method is adopted to build a geometric model of a cylindrical eddy current sensor probe coil, the influence of the probe coil parameters is analyzed, and simulation analysis is carried out on the model;
the sensor model is modeled by adopting a two-dimensional model, grid division of the sensor model adopts a mapping network division, a regular shape is mapped onto an irregular area, and PLANE53 units are selected for grid division of PLanE and axisymmetric magnetic field problems in analysis so as to obtain higher analysis precision;
boundary conditions are then set, and loads are applied and solved. The load refers to the boundary condition and external or internal force function where the sensor housing and the pipeline to which it is connected need to impose boundary conditions, impose a VOLT constraint, and set to zero potential. Applying magnetic signals on grid nodes divided by an air domain as excitation elements, and utilizing a wave front solver to accurately obtain an analysis result;
the wavelet analysis denoising study of the electric vortex detecting signal based on Matlab is to perform software denoising by means of higher-level Matlab software by adopting a wavelet grading denoising method so as to obtain an accurate real signal and eliminate various interferences;
the noise elimination process can be processed as follows: the noise part is usually contained in three noise frequency bands of CD1, CD2 and CD3 when the signal is subjected to wavelet multi-resolution, then the wavelet coefficient can be processed according to the form of threshold value and the like, and then the reconstruction can be carried out to achieve the purpose of noise elimination.
And experimental verification is four:
the satisfactory result is obtained by filtering high-frequency measurement noise by adopting a one-dimensional wavelet reconstruction method through the comparison of the original eddy current test signal accompanied with high-frequency noise and the signal subjected to noise elimination treatment by using a given soft threshold value noise elimination, and the aim of filtering the measurement noise and keeping the useful impedance eddy current signal is achieved.
Step three: performing burn simulation experiments on TC4 titanium alloy at the temperature of 250-1150 ℃ for 1-10 min, and then respectively testing the conductivity, strength and hardness of the test piece;
according to reference data analysis, the relationship between the conductivity and the strength of the material is more visual, so that the acceptance criterion is defined by the strength and the conductivity variation in the burn process of the metal structure of the airplane, and a TC4 titanium alloy standard test piece with four different burn grades of I-IV is provided, wherein the grade I is an unburnt burn, the grade II is a mild burn, the grade III is a moderate burn and the grade IV is a severe burn.
When the degree of conductivity decrease is less than 5% and the intensity decrease is controlled within 10%, the burn is not burnt in class I; when the conductivity is reduced by 5% -10%, the strength is reduced by 10% -30%, and the burn is a grade II mild burn; when the conductivity is reduced by 10% -20%, the strength is reduced by 20-50%, which is a III grade moderate burn; when the conductivity is reduced by more than 20%, the intensity is reduced by more than 50%, which is a serious burn of grade IV.
Step four: a conductivity-based burn evaluation mathematical model (σb= 181.53 κ -1746.3; h= -15.67 κ+561.87, where σb is tensile strength, κ is conductivity, H is brinell hardness) was built, and an expert database was relied upon in a control computer to develop analytical diagnostic software on a Windows platform using Visual Studio 2012, the database being developed using SQL Server 2012. The specific functionality comprises hardware parameter configuration and control command issuing, measurement data acquisition and analysis processing, data management and a good human-computer interface.
The hardware parameter configuration and control command issuing is to issue control commands and modify hardware configuration parameters, such as changing excitation source frequency, amplitude, gain control, amplification factor and the like, through a PCI interface; the measurement data acquisition and analysis processing is based on the frequency value of the PCI interface acquisition analog signal and the count value N measured by the counter, calculates the phase difference pulse width, and finally obtains the impedance phase of the probe coil and the conductivity value of the tested piece by adopting a related numerical calculation method; in order to facilitate the inquiry and processing of later experimental data, the calculation results of measured phase values, conductivity values and the like are stored in real time, and the whole software is required to have efficient data response and processing capacity so as to meet the real-time and precision requirements of the conductivity measurement of a test piece.
The general flow of the specific analysis software is shown in fig. 11, after a user logs in the system, initial information such as characteristics of aluminum alloy 7050 (experiment is not performed), TC4 titanium alloy materials and the like of the airplane component is input, a detection path, a detection method and the like are set, corresponding detection calculation is performed, after the calculation result is compared with data of the same type in an expert database, whether burn exists or not is judged, and the type of burn, the degree of burn and guidance comments are determined.
The specific burn detection flow is shown in fig. 12, and the detection mainly comprises three modules of detection start, defect detection and detection report generation and output, wherein the detection start module mainly comprises a detection method and path selection; burn detection includes burn type identification and determination of burn extent; the detection report mainly comprises component information, material information, a history of the detection component, display of a detection result, diagnosis conclusion and the like.
The above description is only of the preferred embodiments of the present application, and is not intended to limit the present application in any way, and any simple modification, equivalent variation and modification made to the above embodiments according to the technical principles of the present application still fall within the scope of the technical solutions of the present application.
Claims (4)
1. According to the aircraft metal structure burn detection method based on eddy current conductivity, a burn experiment is simulated through heat treatment of a metal structure material, a corresponding relation between the conductivity and the structural strength of the metal structure material is obtained, a burn evaluation database based on eddy current conductivity is established and burn detection software is developed through summarizing burn degree evaluation standards, so that quick evaluation of the burn degree of the aircraft metal structure is realized, and a basis is provided for aircraft burn emergency repair scheme establishment; the method comprises the following specific steps:
step one, selecting a typical aircraft structural material, and performing burn simulation experiments under corresponding conditions to obtain a burned metal material;
step two, placing the metal material obtained in the step one near a coil magnetic field Hp of an eddy current conductivity detection device, inducing eddy current on the surface of the material under the action of an alternating magnetic field, generating an alternating counter magnetic field Hs by the eddy current, and measuring the conductivity of a conductor by measuring the impedance change of the detection coil; the method comprises the following steps in the measurement process:
1) Temperature drift compensation is carried out on the eddy current sensor through a compensation technology;
2) Nonlinear compensation is carried out on the exponential characteristic of the lift-off effect of the eddy current sensor by adopting an exponential operation circuit;
3) According to the basic principle of eddy current detection, adopting ANSYS analysis software of a finite element method to establish a geometric model of a cylindrical eddy current sensor probe coil, analyzing the influence of the probe coil parameters and performing simulation analysis on the model to realize finite element analysis of the eddy current sensor;
4) Performing software denoising by Matlab software by adopting a wavelet grading denoising method so as to obtain an accurate real signal and complete wavelet denoising of the eddy current detection signal;
the impedance change caused by measurement is converted into a voltage change differential output through a Z/V converter, the voltage change differential output is subjected to detection and filtering to regulate and output direct current voltage, the direct current voltage is displayed, according to the principle of a single-frequency impedance method, an alternating signal is generated by adopting a DDS technology and is supplied to a bridge and a detection coil from the beginning of phase analysis, the signal is amplified, subjected to phase-sensitive detection and filtering to become a direct current signal containing phase information and amplitude information of the coil impedance change, and the direct current signal is input into a computer after being processed;
modeling a geometric model of a cylindrical eddy current sensor probe coil by adopting a two-dimensional model, dividing grids by adopting a mapping network, mapping a regular shape onto an irregular area, and selecting PLANE53 units for grid division of PLanE and axisymmetric magnetic field problems in analysis to obtain higher analysis precision;
setting boundary conditions, applying loads and solving, wherein the loads refer to boundary conditions and external or internal acting force functions, wherein boundary conditions need to be applied to a sensor shell and a pipeline connected with the sensor shell, VOLT constraints are applied, zero potential is set, magnetic signals are applied to grid nodes divided by an air domain as excitation elements, and a wavefront solver is utilized to accurately obtain analysis results;
the noise elimination process is processed according to the following method: the signal is first subjected to wavelet multiple-resolution, and the noise part is usually contained in three noise frequency bands of CD1, CD2 and CD 3; then processing the wavelet coefficient according to a threshold value form; then reconstructing to achieve the purpose of noise elimination;
the eddy current conductivity detection device comprises a computer, an excitation signal source, an eddy current sensor, a Z/V transmitter, a voltage follower, an isolation measurement circuit and a follow-up circuit, wherein the excitation signal source, the eddy current sensor, the Z/V transmitter, the voltage follower, the isolation measurement circuit and the follow-up circuit are connected with the computer; the excitation signal source is a sine wave generator, which adopts a 10 KHz-1 MHz highly stable sine wave signal generated by a special DDS digital frequency synthesis chip AD9850 and adopts an AGC feedback network to improve the stability of the signal amplitude; the electric vortex sensor is of a cylindrical structure, an upper coil and a lower coil are adopted, the lower coil is a detection element, and the upper coil is a compensation coil; the Z/V transmitter is an amplifier and a differential circuit;
step three, according to the conductivity obtained in the step two, obtaining the corresponding relation between the conductivity, the strength and the hardness of the metal material in different simulated burn states, and drawing up four different burn degree standards;
step four, based on the metal material performance test results in the step one and the step three, establishing a burn evaluation mathematical model based on conductivity, establishing a burn evaluation database based on conductivity on an SQL Server 2012, and developing and analyzing a diagnosis client by utilizing a Visual Studio 2012 on a Windows platform;
fifthly, inputting initial information of the characteristics of the metal materials of the airplane to be detected, inputting the client, setting a detection path and a detection method, performing corresponding detection calculation, comparing the calculation result with the same type of data in the burn evaluation database, judging whether burn exists or not, and determining a detection report.
2. The method for detecting burn in aircraft metal structures based on eddy current conductivity according to claim 1, wherein: in the first step, the typical aircraft structural material is 7050 aluminum alloy or TC4 titanium alloy, and the corresponding conditions are that the temperature is 250-1150 ℃ and the time is 1-10 min.
3. The method for detecting burn in aircraft metal structures based on eddy current conductivity according to claim 1, wherein: in the third step, the method is defined by the variation of strength and conductivity in the burn process of the airplane metal structure, and TC4 titanium alloy standard test pieces of I-IV levels under four different burn grades are provided, wherein I level is an unburnt burn, II level is a mild burn, III level is a moderate burn and IV level is a severe burn.
4. The method for detecting burn in aircraft metal structures based on eddy current conductivity according to claim 1, wherein: in the fifth step, the detection report includes component information, material information, a history of the detection component, display of a detection result, and a diagnosis conclusion.
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| CN112966354B (en) * | 2021-03-29 | 2022-05-13 | 湖北三江航天江北机械工程有限公司 | Anti-scouring capability assessment method for gas flow drainage device |
| CN113740506B (en) * | 2021-08-26 | 2022-08-12 | 成都飞机工业（集团）有限责任公司 | In-situ evaluation method for titanium alloy burned tissue |
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