Real-time monitoring method for optimized layout strain of loop-type measuring point of aircraft composite material key structure
Technical Field
The invention provides a real-time monitoring method for optimized layout strain of a loop-shaped measuring point of a key structure of an aircraft composite material, and particularly relates to a layout method of an intelligent monitoring system for a weak position of a service structure of an aircraft, which can effectively realize real-time monitoring of strain of a structural part of the aircraft and belongs to the technical field of surface strain monitoring of the structural part of the aircraft.
Background
The aircraft structure is required to bear the effects of complex cyclic fatigue load, accidental impact load and the like, and is also required to bear the tests of severe external environmental factors such as temperature, humidity and the like, which seriously affect the structural performance of the aircraft. Monitoring the key structure of the airplane is an important means for avoiding the failure of the weak part of the structure, avoiding causing sudden catastrophic accidents, reducing the maintenance cost and prolonging the service life. The composite material is widely applied to airplane structural parts, but the damage modes are complex and various, and are generally divided into fiber fracture, matrix failure, fiber-matrix interface debonding and the like, so that direct observation from the surface is difficult, and the detection is difficult. Therefore, it is necessary to monitor the fatigue life of the aircraft structure, and to make maintenance strategies and maintenance solutions to ensure the safe service of the aircraft and to obtain the maximum economic benefits. The fatigue crack monitoring sensor is an important component of a structure health monitoring system, is easy to integrate with a structure, has high sensitivity, and is a key point for designing and researching the damage tolerance of the structure health monitoring technology based on the sensor which can reliably work in a severe environment.
At present, a real-time monitoring scheme for the strain of a key structure of an airplane is generally to directly install a strain sensor at a position to be measured and then monitor the strain state of a weak part of the key structure in the service process of the airplane in real time. For the prediction of the strain at the position where the strain sensor cannot be directly installed, a scheme of randomly arranging strain gauges is mostly adopted, and the actual strain condition of the aircraft structural member cannot be accurately reflected.
Based on the situation, the invention designs the loop type measuring point optimization layout model, which can effectively save the number of strain sensors and simultaneously monitor the strain of the weak part of the key structure of the airplane in real time. The main strain state of the position to be predicted can be reasonably predicted by using data measured by each measuring point, and meanwhile, the strain conditions of the key parts of the mechanical parts along all directions can be predicted by adopting the layout method, so that the safe and reliable operation of mechanical equipment is ensured.
Disclosure of Invention
The invention provides a real-time monitoring method for strain of key parts of airplane parts, which monitors data of each measuring point based on a point location method loop type epoxy carbon nano composite film sensor, reasonably predicts the strain state of the key parts of the airplane parts, and feeds back the strain state to operators in time, thereby preventing accidents and ensuring the safe and reliable work of the airplane parts.
The technical scheme of the invention is as follows:
a real-time monitoring method for the optimized layout strain of a loop-shaped measuring point of a key structure of an aircraft composite material is used for monitoring the actual strain of the key part of an aircraft part and predicting the fatigue life, acquiring monitoring data of each measuring point through each epoxy carbon nano composite film sensor arranged on the surface of the aircraft part, and transmitting the monitoring data to a computer based on a wireless network protocol to realize the real-time monitoring of the key part of the aircraft part;
the method comprises the following specific steps:
(1) preparation of loop type epoxy carbon nano composite film sensor
The epoxy carbon nano composite film sensor comprises a carbon nano tube protective layer and a loop type epoxy carbon nano film layer, wherein the carbon nano tube protective layer is wrapped on the upper surface and the lower surface of the loop type epoxy carbon nano film layer; the loop type epoxy carbon nano film layer comprises a copper wire, a carbon nano tube loading layer on the outer surface of the copper wire and a plurality of loop type or loop type film induction channels which are arranged inside and outside, wherein a film ground source and a film current source are respectively arranged at two ends of each channel, and a potential difference exists between the two ends;
the total resistance of the epoxy carbon nano composite film sensor is formed by intrinsic resistance R of the carbon nano tube1Contact resistance R generated by contact2Collectively, the total resistance R is expressed in the form:
R=αR1+βR2
wherein: alpha and beta are proportionality factors and constants;
alpha is a scale factor, and the value range is 0.099-0.199;
beta is a scale factor, and the value range is 0.901-0.801;
R1is the intrinsic resistance of the carbon nanotube;
R2the carbon nanotube contacts the resistance resistor;
wherein: ρ is the resistivity; l is the length of the resistance wire; s is the sectional area of the resistor;
according to the theory of Neugebauer-webb, the contact resistance R of the carbon nanotube2Comprises the following steps:
wherein: h is the thickness; k is a height coefficient and has a value range of 1.1-1.9; d is the center distance of the carbon nano tube;
detecting a rate of change of resistance of the resistor with respect to an initial resistance
Comprises the following steps:
wherein: r0Is the initial resistance; riContact resistance between different tubes;
in order to more intuitively embody the change condition of the output voltage of the sensor along with the crack propagation, the output signal V of the loop type epoxy carbon nano composite film sensorCIs defined as:
wherein: v is the real-time output voltage value of the epoxy carbon nano composite film sensor, V0The initial output voltage value of the epoxy carbon nano composite film sensor is obtained;
the monitoring of the epoxy carbon nano composite film sensor on the composite material structure crack mainly depends on the consistency of the epoxy carbon nano composite film sensor and the structure damage; when the composite material is subjected to fatigue damage, the epoxy carbon nano composite film sensor tightly combined with the structure is cracked at the same position; the crack of the epoxy carbon nano composite film sensor continuously expands along with the crack of the structure to cause the change of the resistance of the epoxy carbon nano composite film sensor, which is expressed as the change of the output voltage of the epoxy carbon nano composite film sensor; therefore, the crack damage condition of the structure is reflected by monitoring and analyzing the change of the output voltage of the epoxy carbon nano composite film sensor; from the analysis of the sensor mechanism, the loop type epoxy carbon nano composite film sensor can keep stable output of potential difference under constant current before crack monitoring;
(2) building structure surface measuring point optimized arrangement model
The structure is divided into two types, namely a structure surface and a hole surface, wherein a circular epoxy carbon nano composite film sensor is arranged on the structure surface, and a circular epoxy carbon nano composite film sensor is arranged on the hole surface;
in the measuring range, a monitoring sensing network consisting of a plurality of loop type epoxy carbon nano composite film sensors or ring type epoxy carbon nano composite film sensors is constructed, the monitoring sensing network is in a coordinate axis, a transverse axis is an X axis, a longitudinal axis is a Y axis, and the X axis and the Y axis are respectively a branch; the number of the measuring points of the epoxy carbon nano composite film sensor on each branch is the same, the measuring points of the two branches are distributed in the same rule, and the epoxy carbon nano composite film sensors are communicated through a lead; the high potential and the low potential of two adjacent epoxy carbon nano composite film sensors are communicated; when the epoxy carbon nano composite film sensor monitors cracks, the output signal begins to change; carrying out quantitative monitoring on the crack length, wherein the crack length starts to expand from 0; the voltage distribution of the epoxy carbon nano composite film sensor changes when the crack expands, and the stepped rise is generated along with the increase of the length of the crack; and establishing a corresponding relation between the crack change length and the output voltage change, and judging the generation of the crack and the crack length by analyzing the output voltage of the grating type film sensor.
The invention has the beneficial effects that: a method for a loop-back type epoxy carbon nano composite film sensor and optimizing the distribution of measuring points is provided for airplane parts, so that the precision is improved, the number of the sensors is saved, and the safety monitoring of airplane equipment is guaranteed. In addition, a monitoring model of the key parts of the aircraft parts is provided, the fatigue life of the critical parts of the aircraft parts at risk points can be reasonably predicted by using the model on the basis of measured data, and the crack initiation and expansion conditions of the critical parts of the aircraft parts along all directions can be predicted by using the established sensing network, so that the safe and reliable work of aircraft equipment is ensured.
Drawings
FIG. 1 is a non-porous loop type epoxy carbon nanocomposite film sensor.
FIG. 2 is a ring-type epoxy carbon nanocomposite film sensor with holes.
FIG. 3 is a model of a pore-free replica-type point distribution.
FIG. 4 is a model of the distribution of measuring points in the form of rings with holes.
In the figure: 1, a hole-free loop type thin film induction channel; 2 a hole-free clip type film connecting channel; 3 non-porous return type film ground source; 4 a non-porous loop type thin film current source; 5, an annular thin film induction channel with holes; 6 ring-type film connecting channel with holes; 7 conducting wires.
Detailed Description
The following detailed description of the embodiments of the invention refers to the accompanying drawings and accompanying claims.
A real-time monitoring method for the optimized layout strain of a loop-shaped measuring point of a key structure of an aircraft composite material is used for monitoring the actual strain of the key part of an aircraft part and predicting the fatigue life, acquiring monitoring data of each measuring point through each epoxy carbon nano composite film sensor arranged on the surface of the aircraft part, and transmitting the monitoring data to a computer based on a wireless network protocol to realize the real-time monitoring of the key part of the aircraft part;
the method comprises the following specific steps:
(1) preparation of loop type epoxy carbon nano composite film sensor
The epoxy carbon nano composite film sensor comprises a carbon nano tube protective layer and a loop type epoxy carbon nano film layer, wherein the carbon nano tube protective layer is wrapped on the upper surface and the lower surface of the loop type epoxy carbon nano film layer; the loop type epoxy carbon nano film layer comprises a copper wire, a carbon nano tube loading layer on the outer surface of the copper wire and a plurality of loop type or loop type film induction channels which are arranged inside and outside, wherein a film ground source and a film current source are respectively arranged at two ends of each channel, and a potential difference exists between the two ends;
the carbon nano tube film is prepared according to a vacuum filtration method, and the process flow comprises the process flows of grinding, stirring, ultrasound, filtration, drying and the like. A loop-back type thin film sensor is prepared as shown in fig. 1 and fig. 2.
1) The carbon nano tube is weighed by an electronic weighing instrument and poured into the triton X-100 reagent for grinding, and then the deionized water solution is poured for even mixing and stirring.
2) And putting the carbon nano tube mixed aqueous solution test tube into an ultrasonic crusher for ultrasonic treatment.
3) And (3) placing the carbon nanotube mixed liquid in the ultrasonic pulverizer into a table centrifuge for centrifugal treatment.
4) And putting the solution after the centrifugal treatment into a vacuum suction filter, and performing suction filtration to obtain the carbon nanotube film.
5) And (3) putting the carbon nanotube film subjected to vacuum filtration treatment into a vacuum drying oven, and adjusting the temperature for heating.
The carbon nanotube film prepared by the vacuum filtration method is black and flaky overall, has smooth surface without obvious cracks, has certain adhesiveness and higher flexibility, and can bear bending deformation at high temperature.
In carbon nanotube films, the total resistance of the film is due to the intrinsic resistance R of the carbon nanotubes themselves1Contact resistance R generated by contact2Collectively, the total resistance R is expressed in the form:
R=αR1+βR2
wherein: alpha and beta are scaling factors (constants);
alpha is a scale factor, and the value range is 0.099-0.199;
beta is a scale factor, and the value range is 0.901-0.801;
R1is the intrinsic resistance of the carbon nanotube;
R2the carbon nanotube contacts the resistance resistor.
Wherein: ρ is the resistivity; l is the length of the resistance wire; s is the sectional area of the resistor.
According to the theory of Neugebauer-webb, the contact resistance R of the carbon nanotube2Comprises the following steps:
wherein: h is the thickness; k is a height coefficient and has a value range of 1.1-1.9; d is the center distance of the conductive carbon nanotube.
Detecting a rate of change of resistance of the resistor with respect to an initial resistance
Comprises the following steps:
wherein: r0Is the initial resistance; riThe contact resistance between different tubes.
In order to more intuitively embody the change of the output voltage of the sensor along with the crack propagation, the output signal V of the loop type carbon nano composite film sensor is usedCIs defined as:
wherein: v is the real-time output voltage value of the sensor, V0Is the initial output voltage value of the sensor.
The monitoring of the epoxy carbon nano composite film sensor on the composite material structure crack mainly depends on the damage consistency of the epoxy carbon nano composite film sensor and the matrix structure. When the composite material is subjected to fatigue damage, the carbon nano film sensing layer tightly combined with the substrate is cracked at the same position. The cracks on the sensor continuously expand along with the cracks of the matrix, so that the resistance of the sensor changes, and the resistance is expressed as the change of the output voltage of the sensor. Therefore, the crack damage condition of the matrix structure is reflected by monitoring and analyzing the change of the output voltage of the sensor. From the analysis of sensor mechanism, the loop type epoxy carbon nano composite film sensor can keep stable output of potential difference under constant current before crack monitoring.
(2) Building structure surface measuring point optimized arrangement model
The structure is divided into two types, namely a structure surface and a hole surface, wherein a circular epoxy carbon nano composite film sensor is arranged on the structure surface, and a circular epoxy carbon nano composite film sensor is arranged on the hole surface;
in the measuring range, a monitoring sensing network consisting of a plurality of loop type epoxy carbon nano composite film sensors or ring type epoxy carbon nano composite film sensors is constructed, the monitoring sensing network is in a coordinate axis, a transverse axis is an X axis, a longitudinal axis is a Y axis, and the X axis and the Y axis are respectively a branch; the number of the measuring points of the epoxy carbon nano composite film sensor on each branch is the same, the measuring points of the two branches are distributed in the same rule, and the epoxy carbon nano composite film sensors are communicated through a lead; the high potential and the low potential of two adjacent epoxy carbon nano composite film sensors are communicated; when the epoxy carbon nano composite film sensor monitors cracks, the output signal begins to change; carrying out quantitative monitoring on the crack length, wherein the crack length starts to expand from 0; the voltage distribution of the epoxy carbon nano composite film sensor changes when the crack expands, and the stepped rise is generated along with the increase of the length of the crack; and establishing a corresponding relation between the crack change length and the output voltage change, and judging the generation of the crack and the crack length by analyzing the output voltage of the grating type film sensor.
Description of the model:
(1) and calibrating the sensor, and carrying out grading treatment on each channel.
(2) The indirect prediction model indirectly predicts the equivalent relation between the voltage of the point to be measured and the crack length through the output signal of each channel, and then processes the prediction result of each channel.
(3) Because the actual working environment and the structure of the aircraft parts may be complex and changeable, a certain error may exist in the indirect prediction model within a reasonable range when prediction is performed.
(4) The prediction accuracy can be significantly improved by appropriately increasing the number of channels under the structure allowing conditions.