Fireproofing performance detection system of composite material for fireproof cable based on temperature detection

Abstract

The present application belongs to the field of physical measurement technology, and relates to a composite material fire resistance performance detection system for fireproof cable based on temperature detection. A special high-temperature-resistant distributed optical fiber is pre-embedded between the insulation layer and the shielding layer of a sample. Time series data of the temperature of each spatial node is obtained by demodulating back Raman scattering light signals. A radial heat conduction partial differential equation set containing a multilayer structure is constructed. The time series data of the temperature is used as an observation constraint and a boundary condition. An inverse time series iteration is solved by using an adjoint equation method. The internal temperature distribution gradient of an area without the optical fiber is inversed. The time node at which each layer of material reaches a critical thermal decomposition threshold value is calculated according to the internal temperature distribution gradient, and an internal thermal breakdown evolution track is output. The present application breaks through the external temperature measurement limitation, eliminates the internal temperature measurement blind area, and realizes quantitative monitoring of the thermal breakdown physical process between the composite material layers.

Description

Technical Field

[0001] This invention belongs to the field of physical measurement technology and relates to a fire resistance performance testing system for composite materials used in fire-resistant cables based on temperature detection. Background Technology

[0002] Current methods for testing the fire resistance of composite materials used in fire-resistant cables typically involve collecting temperature data by arranging an array of thermocouples on the outer sheath or the fire-exposed surface of the sample. During testing, the combustion source continuously heats the sample, and the thermocouples transmit the collected surface temperatures to a data processing terminal in real time. The terminal then assesses the fire resistance limit of the composite material based on the rate of change of surface temperature and the temperature rise on the unexposed surface of the sample, combined with pre-defined criteria. This testing method relies on the visible temperature change characteristics of the sample's exterior. Throughout the entire testing cycle, the temperature acquisition elements remain fixed at the outermost physical boundary of the composite material, without any intrusive multi-point placement within the composite's internal structure.

[0003] Fire-resistant cable composite materials have a multi-layered physical structure, with differences in thermal conductivity and specific heat capacity between different material layers. During combustion, heat is conducted from the outside in, creating complex temperature gradients within each layer. The aforementioned testing method, which involves placing thermocouples on the sample surface, can only obtain temperature data for the outermost layer of the composite material and cannot penetrate the outer sheath to obtain the true temperature distribution of each internal layer. This results in existing fire resistance testing systems only recording the final macroscopic manifestation of overall material failure, failing to quantify the evolution of temperature gradients between internal layers. In other words, there is a technical problem in being unable to obtain the internal temperature distribution of the composite material to reveal the interlayer thermal runaway mechanism. Summary of the Invention

[0004] The purpose of this invention is to provide a temperature-based testing system for the fire resistance performance of composite materials used in fire-resistant cables, which can solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A temperature-based fire resistance performance testing system for fire-resistant cable composite materials includes a combustion application device, an embedded optical fiber assembly, an optical signal demodulation device, a heat conduction inverse calculation device, and a thermal breakdown trajectory output device. The embedded optical fiber assembly comprises special high-temperature resistant distributed optical fibers distributed along the radial cross-section of the fire-resistant cable composite material sample and embedded between the insulation layer and the shielding layer. The combustion application device is located on the outside of the composite material sample to provide a standard fire source environment. The optical signal demodulation device is connected to the special high-temperature resistant distributed optical fiber, injects laser pulses into the fiber at a set frequency, and receives the returned backscattered Raman light signal. The temperature time-series data of each spatial node of the special high-temperature resistant distributed optical fiber is demodulated; the inverse thermal conductivity calculation device receives the temperature time-series data, establishes a set of radial thermal conductivity partial differential equations including the multi-layer structure of the composite material sample, uses the temperature time-series data as observation constraints and boundary conditions, and performs inverse time-series iterative solution of the radial thermal conductivity partial differential equations using the adjoint equation method to obtain the internal temperature distribution gradient of the region where the special high-temperature resistant distributed optical fiber is not deployed; the thermal breakdown trajectory output device is connected to the inverse thermal conductivity calculation device, calculates the time node when each layer of material reaches the critical thermal decomposition threshold based on the internal temperature distribution gradient, and outputs the internal thermal breakdown evolution trajectory.

[0007] Preferably, the fiber optic pre-embedded assembly further includes a miniature polytetrafluoroethylene (PTFE) positioning slot disposed at the junction of the insulation layer and the shielding layer. The special high-temperature resistant distributed optical fiber is fixed to the radial section of the composite material sample through the miniature PTFE positioning slot. The special high-temperature resistant distributed optical fiber includes an inner fiber core, a quartz cladding wrapped around the outer side of the inner fiber core, and a high-temperature resistant polyimide coating coated on the outer side of the quartz cladding. The cross-sectional diameter of the miniature PTFE positioning slot matches the outer diameter of the special high-temperature resistant distributed optical fiber coated with the high-temperature resistant polyimide coating.

[0008] Preferably, the optical signal demodulation device includes a pulsed laser source, a wavelength division multiplexer, a photodetector, and a data acquisition card. The pulsed laser source emits the laser pulse. The common end of the wavelength division multiplexer is connected to the special high-temperature resistant distributed optical fiber. The reflecting end of the wavelength division multiplexer is connected to the photodetector. The photodetector converts the Stokes light signal and the anti-Stokes light signal in the received backscattered Raman light signal into electrical signals. The data acquisition card performs analog-to-digital conversion on the electrical signals at the set frequency to generate the temperature time series data.

[0009] Preferably, the specific method for constructing the radial heat conduction partial differential equation system by the heat conduction inverse calculation device is as follows: obtain the initial thermal conductivity and initial specific heat capacity of each layer of the composite material sample; establish the radial heat conduction partial differential equation system with radial distance and time as independent variables based on the cylindrical coordinate system; set the outer boundary condition of the radial heat conduction partial differential equation system as the temperature function of the fire-receiving surface generated by the combustion application device; and set the inner boundary condition of the radial heat conduction partial differential equation system as the adiabatic condition at the central axis of the composite material sample.

[0010] Preferably, the process of the heat conduction inverse calculation device performing inverse time-series iterative solution using the adjoint equation method includes: constructing a corresponding adjoint state equation based on the radial heat conduction partial differential equation set; using the difference between the temperature time-series data of the spatial node where the special high-temperature resistant distributed optical fiber is located and the calculated temperature as the source term of the adjoint state equation; discretizing the adjoint state equation in the inverse time direction to obtain the gradient descent direction; updating the initial temperature distribution of the area where the special high-temperature resistant distributed optical fiber is not deployed based on the gradient descent direction and performing forward heat conduction calculation until the iteration termination condition is met.

[0011] Preferably, the thermal breakdown trajectory output device extracts the extreme temperature sequence of each layer of material corresponding to the spatial position in the internal temperature distribution gradient, compares the extreme temperature sequence with the preset critical thermal decomposition threshold temperature of each layer of material, records the specific timestamp of each data point in the extreme temperature sequence reaching the critical thermal decomposition threshold temperature as the time node, and arranges the time nodes in chronological order to generate the internal thermal breakdown evolution trajectory containing time and spatial dimensions.

[0012] Preferably, the inner wall of the miniature PTFE positioning slot is provided with a compensation filling groove that extends along the axis of the special high-temperature resistant distributed optical fiber. The compensation filling groove is filled with high thermal conductivity silicone grease. The high thermal conductivity silicone grease tightly wraps around the outer surface of the special high-temperature resistant distributed optical fiber coated with the high-temperature resistant polyimide coating. The outer edge of the miniature PTFE positioning slot is provided with a serrated interlocking structure that matches the physical interface of the insulation layer and the shielding layer. The tooth height and tooth pitch of the serrated interlocking structure are configured according to the difference in elastic modulus between the insulation layer and the shielding layer materials.

[0013] Preferably, the optical signal demodulation device further includes a reference fiber channel and a dual-channel differential amplifier. The reference fiber channel is placed in a constant temperature environment away from the combustion application device. The transmission end of the wavelength division multiplexer is connected to the reference fiber channel. The output end of the reference fiber channel is connected to the other input end of the photodetector. The two input ends of the dual-channel differential amplifier are respectively connected to the output channel of the photodetector corresponding to the Stokes optical signal and the output channel corresponding to the anti-Stokes optical signal.

[0014] Preferably, when establishing the radial heat conduction partial differential equation system, the heat conduction inverse calculation device sets the initial thermal conductivity and the initial specific heat capacity as piecewise linear functions that change with temperature, based on the different temperature ranges of each layer of the composite material sample. In each execution of the forward heat conduction calculation, the device extracts the temperature values ​​corresponding to each spatial node at the current time step and updates the local thermal conductivity and local specific heat capacity of each spatial node according to the piecewise linear function. The updated local thermal conductivity and local specific heat capacity are then substituted into the radial heat conduction partial differential equation system.

[0015] Preferably, when the heat conduction inverse calculation device performs the inverse time-series iterative solution, it calculates the L2 norm of the error matrix between the temperature time-series data and the calculated temperature at the current iteration number, and determines whether the L2 norm is less than a preset stopping threshold. If the L2 norm is less than the preset stopping threshold, the iteration is terminated and the initial temperature distribution corresponding to the current iteration number is output as the internal temperature distribution gradient. If the L2 norm is greater than or equal to the preset stopping threshold, the initial temperature distribution is updated according to the gradient descent direction combined with a preset step size factor, and the current iteration number is increased.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] 1. This invention utilizes a special high-temperature resistant distributed optical fiber pre-embedded between the insulation and shielding layers. The temperature time-series data of each spatial node of the optical fiber is used as observation constraints and boundary conditions. Combined with a constructed set of radial heat conduction partial differential equations for a multilayer structure, the adjoint equation method is employed for inverse time-series iterative solution. This technique overcomes the physical limitations of surface temperature measurement, enabling the inversion of the internal temperature distribution gradient in areas without optical fiber. Based on the internal temperature distribution gradient, the time nodes when each layer of material reaches the critical thermal decomposition threshold are calculated, and the internal thermal breakdown evolution trajectory is output. This eliminates the blind zone in temperature measurement within the composite material, achieving quantitative monitoring of the interlayer thermal breakdown physical process within multilayer composite materials.

[0018] 2. A reference fiber channel placed in a constant-temperature environment is introduced during the optical signal demodulation process to eliminate drift interference caused by ambient temperature fluctuations in the demodulation of backscattered Raman light signals. In the reverse-time iterative solution, the thermal conductivity and specific heat capacity of each layer of material are set as piecewise linear functions that change with temperature, and the local thermal property parameters are updated during each forward heat conduction calculation, so that the inversion result of the internal temperature distribution gradient closely matches the changes in the physical properties of the material at high temperatures. The serrated interlocking structure on the outside of the micro PTFE positioning slot ensures the positional stability of the special high-temperature resistant distributed optical fiber during the thermal expansion process of the composite material. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the overall workflow of the fire-resistant cable composite material fire resistance testing system of the present invention.

[0020] Figure 2 This is a flowchart illustrating the assembly and positioning process of the fiber optic pre-embedded component structure of the present invention.

[0021] Figure 3 This is a flowchart of the signal processing of the optical signal demodulation device of the present invention;

[0022] Figure 4 This is a flowchart illustrating the construction of the radial heat conduction partial differential equation system of the present invention.

[0023] Figure 5 This is a flowchart of the reverse-time iterative solution of the adjoint equation method of the present invention;

[0024] Figure 6 This is a flowchart of the thermal breakdown evolution trajectory generation process of the present invention. Detailed Implementation

[0025] In one embodiment, a temperature-based fire-resistant cable composite material fire resistance performance testing system is applied to the fire resistance performance testing of multilayer composite material samples for fire-resistant cables. The testing system includes a combustion application device, an embedded optical fiber assembly, an optical signal demodulation device, a heat conduction inverse calculation device, and a thermal breakdown trajectory output device. The combustion application device is located on the outside of the composite material sample to provide a standard fire source environment for the fire resistance performance testing of the sample. The temperature change of the standard fire source environment conforms to the temperature rise curve specified in the corresponding fire resistance test standard, maintaining stable fire conditions throughout the testing cycle. The embedded optical fiber assembly includes special high-temperature resistant distributed optical fibers. These fibers are distributed along the radial cross-section of the fire-resistant cable composite material sample and embedded between the insulation layer and the shielding layer of the sample. The fiber layout is completed during sample preparation, forming a stable physical bond with the interlayer structure of the sample. Each spatial node of the optical fiber corresponds to a fixed spatial position on the radial cross-section of the sample, used to collect temperature-related optical signals at the corresponding position. The optical signal demodulation device is connected to the special high-temperature resistant distributed optical fiber via an optical transmission link. The optical signal demodulation device injects laser pulses into the special high-temperature resistant distributed optical fiber at a set frequency, and at the same time receives the back Raman scattered light signal returned from the optical fiber. The received back Raman scattered light signal is split, photoelectrically converted and demodulated, and outputs the temperature time series data corresponding to each spatial node of the special high-temperature resistant distributed optical fiber. The temperature time series data includes the location identifier of the corresponding spatial node, the acquisition timestamp and the demodulated temperature value. The input end of the inverse thermal conductivity calculation device is connected to the output end of the optical signal demodulation device to receive the temperature time series data output by the optical signal demodulation device. The inverse thermal conductivity calculation device first obtains the multilayer structure information of the composite material sample, establishes a radial thermal conductivity partial differential equation system containing all material layers of the sample, uses the received temperature time series data as the observation constraint and boundary condition of the radial thermal conductivity partial differential equation system, and uses the adjoint equation method to perform inverse time series iterative solution of the radial thermal conductivity partial differential equation system to invert and obtain the internal temperature distribution gradient of the region in the sample where no special high temperature resistant distributed optical fiber is deployed. The internal temperature distribution gradient is a continuous temperature field data containing the radial spatial dimension and time dimension of the sample. The input end of the thermal breakdown trajectory output device is connected to the output end of the inverse thermal conduction calculation device. It receives the internal temperature distribution gradient output by the inverse thermal conduction calculation device. Based on the internal temperature distribution gradient, the thermal breakdown trajectory output device extracts the temperature extreme value sequence in the corresponding spatial region of each layer of material in the sample. It compares the temperature extreme value sequence with the critical thermal decomposition threshold of the corresponding material, calculates the time node when each layer of material reaches the critical thermal decomposition threshold, and generates and outputs the thermal breakdown evolution trajectory inside the sample based on the time node and the corresponding spatial position.

[0026] refer to Figure 1Specifically, the combustion application device includes a ignition source supply unit, a sample clamping fixture, and a temperature calibration unit. The sample clamping fixture is used to fix the composite material sample, ensuring that the sample's flame-receiving surface and the flame output end of the ignition source supply unit maintain a fixed relative position, preventing sample displacement during testing and thus avoiding changes in the flame-receiving conditions. The temperature calibration unit is used to monitor the temperature changes of the flame-receiving surface in real time and feeds the monitored temperature data back to the ignition source supply unit. The ignition source supply unit adjusts the fuel supply based on the feedback temperature data to ensure that the temperature change of the flame-receiving surface conforms to the preset standard fire resistance curve, maintaining a stable standard ignition source environment.

[0027] Furthermore, multiple sets of radial sensing units are arranged along the axial direction of the composite material sample using special high-temperature resistant distributed optical fibers. Each set of radial sensing units corresponds to a radial cross-section of the sample. The optical fibers within the same radial cross-section are continuously arranged along the circumference of the sample, covering the entire circumferential range of the radial cross-section. This ensures that the spatial nodes of the optical fibers are uniformly distributed at the interface between the insulation layer and the shielding layer, guaranteeing that the collected temperature time-series data can reflect the circumferential and axial temperature distribution at the interface. The spatial resolution of the optical fiber matches the sampling capability of the optical signal demodulation device, and the axial distance between adjacent spatial nodes is consistent with the circumferential distance, forming a uniformly distributed spatial sampling grid.

[0028] The demodulation process of backscattered Raman light signals in an optical signal demodulation device is based on the temperature-sensitive characteristics of Raman scattering. The backscattered Raman light generated when a laser pulse propagates within an optical fiber contains Stokes and anti-Stokes signals. The intensity of the anti-Stokes signal is highly sensitive to temperature changes, while the intensity of the Stokes signal is less sensitive and can be used as a reference signal. The temperature demodulation calculation process satisfies the following relationship:

[0029]

[0030] in, For temperature The intensity of the anti-Stokes light signal under the current, The intensity of the Stokes light signal at the same temperature, It is a constant related to the fiber material and the wavelength of the incident light. It is Planck's constant. The speed of light in a vacuum. For Raman frequency shift, For Boltzmann constant, This represents the thermodynamic temperature of the space node corresponding to the optical fiber.

[0031] Based on the above relationship, the optical signal demodulation device can calculate the thermodynamic temperature of each spatial node of the optical fiber according to the intensity ratio of the collected Stokes optical signal and the anti-Stokes optical signal. Combined with the collection timestamp and the location identifier of the spatial node, continuous temperature time series data can be generated.

[0032] refer to Figure 4 In the process of establishing the radial heat conduction partial differential equations using the inverse thermal conductivity calculation device, based on the cylindrical structure of the composite material sample, a cylindrical coordinate system is used to describe the spatial structure of the sample. The axis of the cylindrical coordinate system coincides with the central axis of the sample, and the radial coordinates point from the central axis to the outer surface along the radial direction of the sample. Due to the axisymmetry of the sample's structure and the fire conditions along the circumference and axial directions, the circumferential and axial heat conduction fluxes can be neglected, and the heat conduction process is simplified to a one-dimensional unsteady-state heat conduction process along the radial direction. For the sample's... For a layered material, the corresponding partial differential equation for radial heat conduction satisfies the following relationship:

[0033]

[0034] in, For the first Density of layer material, For the first The specific heat capacity under constant pressure of the layer material For the first Layer material in radial position ,time The corresponding thermodynamic temperature, For the first Thermal conductivity of the layer material Radial coordinates, It is a time variable.

[0035] The boundary conditions of the radial heat conduction partial differential equations include outer boundary conditions, inner boundary conditions, and interlayer interface continuity conditions. The outer boundary is set on the outer surface of the sample, corresponding to the maximum value of the radial coordinate. The outer boundary condition is set as a function of the temperature of the flame-receiving surface generated by the combustion application device, satisfying the following relationship:

[0036]

[0037] in, The radial coordinates are those corresponding to the outer surface of the sample. This is a function of the temperature of the exposed surface as a function of time. The inner boundary is located at the central axis of the sample, corresponding to a radial coordinate of 0. The inner boundary condition is set as adiabatic, satisfying the following relationship:

[0038] A continuity condition is set at the interface between adjacent material layers, including a temperature continuity condition and a heat flow continuity condition, wherein the temperature continuity condition satisfies the following relationship:

[0039]

[0040] The heat flow continuity condition satisfies the following relationship:

[0041]

[0042] in, For the first Layer material and the first The radial coordinates corresponding to the interface of the layer material.

[0043] refer to Figure 5 The inverse thermal conductivity calculation device employs the adjoint equation method to perform inverse-time iterative solutions to the radial thermal conductivity partial differential equations. The solution process uses the temperature time-series data of the spatial nodes where the special high-temperature resistant distributed optical fiber is located as the observation constraint, with the goal of inverting the temperature field distribution across the entire radial region of the sample. Specifically, the inverse thermal conductivity calculation device first discretizes the radial spatial region and time interval, dividing the radial coordinate interval into several uniformly distributed grid nodes. The distance between adjacent grid nodes is the spatial step size, and each interface layer of the sample and the optical fiber deployment position correspond to an independent grid node. The time interval corresponding to the detection period is divided into several uniformly distributed time steps, with the interval between adjacent time steps being the time step size, matched to the sampling frequency of the optical signal demodulation device. After discretization, the radial thermal conductivity partial differential equations are transformed into a linear equation system, which is then numerically solved using the finite difference method.

[0044] Based on this, the inverse heat conduction calculation device constructs the corresponding adjoint state equation according to the discretized radial heat conduction partial differential equations. The difference between the time-series temperature data of the spatial node where the special high-temperature resistant distributed optical fiber is located and the calculated temperature of the corresponding node obtained from the forward heat conduction calculation is used as the source term of the adjoint state equation. The adjoint state equation is then discretized and solved in a time-step manner in the inverse time direction to obtain the gradient descent direction of the objective function with respect to the initial temperature distribution. Based on the gradient descent direction, the inverse heat conduction calculation device updates the initial temperature distribution in the region without the special high-temperature resistant distributed optical fiber. Using the updated initial temperature distribution as the initial condition, forward heat conduction calculation is performed to obtain the temperature field distribution of the entire radial region. This inverse time-series solution and forward calculation process is repeated until the iteration termination condition is met, outputting the converged temperature field distribution of the entire radial region, i.e., the internal temperature distribution gradient of the region without optical fiber.

[0045] refer to Figure 6After receiving the internal temperature distribution gradient, the thermal breakdown trajectory output device first divides the internal temperature distribution gradient into spatial regions, matching the temperature field data with the radial intervals corresponding to each layer of the sample, thus obtaining sub-regions of the temperature field corresponding to each layer. For each sub-region of the temperature field corresponding to a layer, the thermal breakdown trajectory output device extracts the extreme temperature values ​​within that sub-region at each time step, forming an extreme temperature sequence corresponding to the time step. The thermal breakdown trajectory output device compares the extreme temperature sequence with the preset critical thermal decomposition threshold temperature of that layer of material point by point, recording the timestamp corresponding to the data point in the extreme temperature sequence that first reaches the critical thermal decomposition threshold temperature, as the time node when that layer of material reaches the critical thermal decomposition threshold. The thermal breakdown trajectory output device arranges the time nodes corresponding to all material layers in chronological order, and combines the radial spatial position of the material layer corresponding to each time node to generate an internal thermal breakdown evolution trajectory containing both time and spatial dimensions. The trajectory data can intuitively present the time and spatial location of thermal decomposition failure of each layer of material during the heat conduction from the outer surface of the sample to the inside.

[0046] In this embodiment, the multilayer structural parameters and thermophysical parameters required to construct the radial heat conduction partial differential equation system are defined as shown in Table 1. The parameters in the table are the basic input terms for solving the heat conduction equation and can be adapted and adjusted according to the structure and material type of different samples.

[0047] Table 1. Definition of parameters for multilayer structure and thermal conduction equation of composite material specimens.

[0048] In Table 1, the fiber optic deployment interface corresponds to the interface between the insulation layer and the shielding layer, which is the deployment location of the special high-temperature resistant distributed optical fiber. The grid node temperature data corresponding to this location is the observation constraint for the inverse heat conduction calculation process.

[0049] This embodiment fully covers all the core components and workflows of the detection system. By pre-embedding special high-temperature resistant distributed optical fibers between the insulation layer and the shielding layer, temperature time-series data at fixed positions inside the sample is acquired. Using the temperature time-series data as an observation constraint, combined with the radial heat conduction partial differential equations of the multilayer structure, the adjoint equation method is used for inverse time-series iterative solution to obtain the internal temperature distribution gradient of the entire radial region of the sample. Finally, based on the internal temperature distribution gradient, the thermal breakdown evolution trajectory inside the sample is generated and output, eliminating the temperature measurement blind zone inside the composite material and realizing the quantitative monitoring of the interlayer heat conduction process inside the multilayer composite material.

[0050] In a preferred embodiment, the fiber optic pre-embedded assembly further includes a miniature PTFE positioning slot. This slot is positioned at the junction of the insulation and shielding layers of the composite material sample. The special high-temperature resistant distributed optical fiber is fixed to the radial section of the sample via the miniature PTFE positioning slot. The special high-temperature resistant distributed optical fiber includes an inner core, a quartz cladding surrounding the inner core, and a high-temperature resistant polyimide coating applied to the outside of the quartz cladding. The inner core transmits laser pulses and backscattered light signals. The quartz cladding and the inner core create a refractive index difference, constraining the light signal to undergo total internal reflection within the core. The high-temperature resistant polyimide coating protects the quartz cladding and the inner core, preventing optical performance degradation under high-temperature conditions. The inner diameter of the miniature PTFE positioning slot matches the outer diameter of the special high-temperature resistant distributed optical fiber coated with the high-temperature resistant polyimide coating, allowing the fiber to be tightly embedded within the positioning slot. This prevents radial displacement of the fiber during sample preparation and testing, ensuring a stable correspondence between the fiber's spatial nodes and the sample's radial position.

[0051] refer to Figure 2 Furthermore, the inner wall of the miniature PTFE positioning slot has a compensation filling groove that runs through the axis of the special high-temperature resistant distributed optical fiber. The compensation filling groove is filled with high thermal conductivity silicone grease, which, after filling, tightly wraps around the outer surface of the special high-temperature resistant distributed optical fiber coated with a high-temperature resistant polyimide layer. The compensation filling groove is used to accommodate the high thermal conductivity silicone grease, eliminating air gaps between the outer surface of the optical fiber and the inner wall of the positioning slot. This prevents the additional thermal resistance caused by air gaps from affecting the accuracy of the optical fiber's temperature acquisition. The high thermal conductivity silicone grease enables rapid heat flow conduction between the optical fiber and the positioning slot, ensuring that the temperature of the optical fiber remains consistent with the true temperature of the interface between the insulation layer and the shielding layer.

[0052] The outer edge of the miniature PTFE positioning slot is equipped with a serrated interlocking structure. This serrated interlocking structure physically interlocks with the inner surface of the insulating layer and the outer surface of the shielding layer. The tooth height and tooth pitch of the serrated interlocking structure are configured based on the difference in elastic modulus between the insulating and shielding layer materials. When the sample undergoes thermal expansion during combustion, the difference in thermal expansion coefficients between the insulating and shielding layer materials will cause relative displacement between the layers. The serrated interlocking structure can counteract the influence of this relative displacement on the positioning slot through physical interlocking, preventing the positioning slot from causing axial or circumferential displacement of the optical fiber. This ensures the positional stability of the optical fiber throughout the entire detection cycle and guarantees that the spatial position corresponding to the temperature time series data does not shift.

[0053] In this embodiment, the structural parameters of the micro polytetrafluoroethylene positioning slot are adapted and adjusted according to the interlayer material characteristics of the sample. The specific parameter adaptation relationship is shown in Table 2.

[0054] Table 2 Compatibility Table of Structural Parameters for Miniature PTFE Positioning Slots

[0055] In Table 2, the matching relationships of interference fit, transition fit and clearance fit are adjusted according to the temperature change range during the testing process to ensure that the optical fiber can maintain a stable physical bond with the positioning slot in environments ranging from room temperature to high temperature.

[0056] This embodiment refines the structure of the fiber optic pre-embedded component. A micro polytetrafluoroethylene positioning slot is used to stably fix the special high-temperature resistant distributed optical fiber between sample layers. The compensation filling groove on the inner wall of the positioning slot and the high thermal conductivity silicone grease eliminate interlayer thermal resistance and improve the accuracy of temperature acquisition. The sawtooth interlocking structure on the outer side of the positioning slot counteracts the influence of interlayer relative displacement during sample heating, ensuring the stability of the optical fiber position and providing an accurate spatial position and temperature data basis for subsequent inverse thermal conduction calculation.

[0057] refer to Figure 3 In a preferred embodiment, the optical signal demodulation device includes a pulsed laser source, a wavelength division multiplexer (WDM), a photodetector, and a data acquisition card. The pulsed laser source generates laser pulses with a set pulse width and repetition frequency. The center wavelength of the laser pulse matches the low-loss transmission window of the special high-temperature resistant distributed optical fiber, ensuring stable light intensity during long-distance transmission within the fiber. The common end of the WDM is connected to the special high-temperature resistant distributed optical fiber via an optical transmission link. The reflector end of the WDM is connected to the photodetector. The WDM is used to split the backscattered Raman light signal returning from the fiber, separating the Stokes and anti-Stokes light signals, and transmitting the two separated optical signals to their respective inputs at the photodetector. The photodetector converts the received Stokes and anti-Stokes light signals into corresponding analog electrical signals. The amplitude of the analog electrical signal is linearly related to the intensity of the corresponding optical signal. The input terminal of the data acquisition card is connected to the output terminal of the photodetector. The data acquisition card performs analog-to-digital conversion on the analog electrical signal output by the photodetector at a set frequency that is consistent with the repetition frequency of the laser pulse, generating a corresponding digital signal sequence. After demodulation processing, the digital signal sequence generates temperature time series data.

[0058] Furthermore, the optical signal demodulation device also includes a reference fiber optic channel and a dual-channel differential amplifier. The reference fiber optic channel is placed in a constant-temperature environment far from the combustion application device, and the temperature of this environment remains constant, unaffected by temperature fluctuations in the combustion environment during detection. The transmission end of the wavelength division multiplexer is connected to the reference fiber optic channel, and the output end of the reference fiber optic channel is connected to the other input end of the photodetector. The reference fiber optic channel is used to acquire the reference Raman scattered light signal under constant-temperature conditions. The reference light signal is used to eliminate systematic errors in the demodulation results caused by factors such as laser source power fluctuations, photodetector dark current drift, and ambient temperature changes. The two input ends of the dual-channel differential amplifier are respectively connected to the output channels of the photodetector corresponding to the Stokes light signal and the corresponding anti-Stokes light signal. The dual-channel differential amplifier is used to differentially amplify the two analog electrical signals, suppress common-mode noise in the two signals, improve the signal-to-noise ratio of the electrical signal corresponding to the weak optical signal, and provide a high-quality signal foundation for subsequent analog-to-digital conversion and demodulation processing.

[0059] Specifically, in this embodiment, the temperature demodulation process introduces a isothermal reference signal from the reference fiber channel to correct the temperature demodulation formula, eliminating errors caused by system drift. The corrected temperature demodulation relationship is as follows:

[0060]

[0061] in, The reference temperature is the thermodynamic temperature of the isothermal environment in which the fiber optic channel is located. The intensity of the anti-Stokes optical signal in the reference fiber optic channel under constant temperature conditions. The Stokes light signal intensity of the reference fiber optic channel under constant temperature conditions. These are system constants related to the reference fiber channel; the physical meanings of the remaining parameters are consistent with those in the aforementioned temperature demodulation formula.

[0062] By using the modified temperature demodulation formula, the optical signal demodulation device can eliminate the influence of systematic errors such as light source power fluctuations and device performance drift on the temperature demodulation results, thereby improving the accuracy and stability of temperature time series data. The digital signal sequence generated by the data acquisition card is first processed by digital filtering to remove high-frequency noise and random interference. Then, the peak amplitude of the filtered signal is extracted and substituted into the modified temperature demodulation formula to calculate the temperature value corresponding to each spatial node of the optical fiber. Combined with the acquisition timestamp and the location identifier of the spatial node, temperature time series data is generated.

[0063] In this embodiment, the core device functions and compatibility relationships of the optical signal demodulation device are shown in Table 3. The performance parameters of the device are matched with the sampling frequency, temperature range, and spatial resolution requirements of the detection system.

[0064] Table 3. Functions and Compatibility of Core Components in Optical Signal Demodulation Device

[0065] In Table 3, the adaptation requirements of each device can be adjusted according to the temperature measurement accuracy, sampling frequency, spatial resolution and other indicators of the detection system to ensure the stability and accuracy of the optical signal demodulation process.

[0066] This embodiment details the structure and demodulation process of the optical signal demodulation device. It uses a wavelength division multiplexer to split the backscattered Raman light signal, a photodetector to perform linear conversion of the optical signal to an electrical signal, a dual-channel differential amplifier to suppress common-mode noise and improve the signal-to-noise ratio, and a reference fiber channel to provide a constant-temperature reference signal to eliminate the influence of system drift on the demodulation results, thus providing high-precision temperature time-series data for subsequent inverse thermal conductivity calculation.

[0067] In a preferred embodiment, when establishing the radial heat conduction partial differential equation system, the heat conduction inverse calculation device first obtains the initial thermal conductivity and initial specific heat capacity of each layer of the composite material sample. Based on the different temperature ranges of each layer of the sample, the initial thermal conductivity and initial specific heat capacity are set as piecewise linear functions that change with temperature. During each forward heat conduction calculation, the temperature value corresponding to each spatial node at the current time step is extracted, and the local thermal conductivity and local specific heat capacity of each spatial node are updated according to the piecewise linear function. The updated local thermal conductivity and local specific heat capacity are substituted into the radial heat conduction partial differential equation system, so that the heat conduction calculation process conforms to the change law of the thermophysical parameters of the material at different temperatures.

[0068] Specifically, the The piecewise linear function of the thermal conductivity of the layer material with temperature satisfies the following relationship:

[0069]

[0070] in, For the first Layer material in the first The linear coefficient of thermal conductivity within a temperature range For the first Layer material in the first The thermal conductivity constant term within a temperature range, and The first The lower and upper thermodynamic temperatures of a temperature range.

[0071] No. The piecewise linear function of the isobaric specific heat capacity of the layered material with temperature satisfies the following relationship:

[0072]

[0073] in, For the first Layer material in the first Linear coefficient of specific heat capacity within a temperature range For the first Layer material in the first The specific heat capacity constant term within a temperature range, and the physical meaning of the other parameters are consistent with the parameters in the aforementioned piecewise linear function of thermal conductivity.

[0074] When performing forward heat conduction calculations, the inverse heat conduction calculation device first extracts the temperature values ​​of each spatial node obtained from the previous time step for each time step. Based on the temperature range to which each node's temperature value belongs, it calls the corresponding piecewise linear function to calculate the current local thermal conductivity and local specific heat capacity of each node. The updated local thermal conductivity and local specific heat capacity are then substituted into the discretized radial heat conduction partial differential equations to solve for the temperature values ​​of each spatial node at the current time step, thus completing the forward calculation for that time step. After completing the calculations for all time steps in sequence, the temperature field distribution in the entire time domain and the entire radial spatial domain is obtained.

[0075] Furthermore, in the process of inverse heat conduction calculation using the adjoint equation method for inverse time-series iterative solution, the objective function of the inverse heat conduction problem is first constructed. The objective function is used to quantify the error between the temperature value at the fiber optic deployment location obtained from the forward calculation and the measured temperature time-series data. The objective function satisfies the following relationship:

[0076]

[0077] in, Let be the objective function. Initial time Temperature distribution in the radial region of the sample at that time. This is the end time of the detection cycle. The radial coordinates corresponding to the deployment location of the special high-temperature resistant distributed optical fiber. The fiber optic deployment location obtained from the forward calculation at time... Temperature value, This refers to the temperature value at the corresponding location and time in the measured temperature time series data.

[0078] Based on this, the inverse heat conduction calculation device constructs the corresponding adjoint state equation according to the radial heat conduction partial differential equations and the objective function. The independent variable of the adjoint state equation is in the inverse time direction, that is, from the termination time. To the initial time The stepping motion, accompanied by the state equations, satisfies the following relationship:

[0079]

[0080] in, For the first The accompanying state variables of the layer material, The Dirac function is used to limit the source term of the adjoint equation to act only at the radial coordinate corresponding to the fiber deployment location. The physical meaning of the other parameters is the same as that of the parameters in the aforementioned formula.

[0081] The inverse heat conduction calculation device discretizes the adjoint state equation and performs a step-by-step solution in the inverse time direction to obtain the distribution of the adjoint state variables in the full time domain and the full radial spatial domain. The gradient of the objective function with respect to the initial temperature distribution satisfies the following relationship:

[0082]

[0083] in, Let the gradient of the objective function be the initial temperature distribution. Initial time The corresponding distribution of accompanying state variables.

[0084] The inverse heat conduction calculation device uses the gradient descent method to update the initial temperature distribution based on the calculated gradient descent direction. The update process satisfies the following relationship:

[0085]

[0086] in, This represents the current iteration number. For the first The initial temperature distribution corresponding to the next iteration. For the first The initial temperature distribution corresponding to the next iteration. This is the preset gradient descent step size factor.

[0087] When performing inverse-time iterative solutions, the heat conduction inverse calculation device calculates the L2 norm of the error matrix between the calculated temperature and the measured temperature time series data at the fiber optic deployment location at the current iteration number. It then checks if the L2 norm is less than a preset shutdown threshold. If the L2 norm is less than the preset shutdown threshold, the iteration terminates, and the full-time and full-radial spatial domain temperature field distribution corresponding to the current iteration number is output as the internal temperature distribution gradient. If the L2 norm is greater than or equal to the preset shutdown threshold, the initial temperature distribution is updated based on the gradient descent direction and a preset step size factor. The current iteration number is increased, and the process of forward heat conduction calculation, inverse-time solution of the adjoint equation, gradient calculation, and initial temperature distribution update is repeated. The L2 norm of the error matrix satisfies the following relationship:

[0088]

[0089] in, The second norm of the error matrix is ​​given by , and the physical meanings of the other parameters are consistent with those in the aforementioned objective function.

[0090] After receiving the internal temperature distribution gradient, the thermal breakdown trajectory output device extracts the extreme temperature sequence corresponding to the spatial location of each material layer in the internal temperature distribution gradient. It compares the extreme temperature sequence with the preset critical thermal decomposition threshold temperature of each material layer, records the specific timestamps of each data point in the extreme temperature sequence reaching the critical thermal decomposition threshold temperature as time nodes, and arranges the time nodes in chronological order to generate an internal thermal breakdown evolution trajectory containing both time and spatial dimensions. Specifically, the first... The time point at which the layer material reaches the critical thermal decomposition threshold satisfies the following relationship:

[0091]

[0092] in, For the first The time point at which the layer material reaches the critical thermal decomposition threshold. For the first The radial range corresponding to the layer material, For the first The critical thermal decomposition threshold temperature of the layer material.

[0093] After calculating the time nodes corresponding to all material layers, the thermal breakdown trajectory output device arranges the time nodes in ascending order and combines them with the radial interval of the material layer corresponding to each time node to generate discrete data points of the thermal breakdown evolution trajectory. The discrete data points contain timestamps and corresponding radial spatial positions. Through interpolation, a continuous thermal breakdown evolution trajectory can be generated, which intuitively presents the change law of the spatial position of the thermal breakdown front over time during the process of heat conduction from the outer surface of the sample to the inside.

[0094] In this embodiment, the piecewise linear function coefficients of the thermal property parameters of each layer of material are defined as shown in Table 4. The coefficients can be adjusted according to the high-temperature thermal property test results of the material to ensure that the heat conduction calculation process conforms to the actual thermal property change law of the material.

[0095] Table 4 Definition of Coefficients for Piecewise Linear Functions of Material Thermophysical Parameters

[0096] In Table 4, the threshold values ​​of the temperature ranges are divided according to the thermal decomposition temperature, glass transition temperature and other thermophysical characteristic temperatures of the corresponding materials, to ensure that the thermal conductivity and specific heat capacity in each temperature range change with temperature in a linear manner.

[0097] This embodiment details the working process of the inverse thermal conduction calculation device and the thermal breakdown trajectory output device. By setting the thermal conductivity and specific heat capacity of the material as piecewise linear functions that change with temperature, the thermal conduction calculation process closely matches the thermal property changes of the material under high temperature conditions. The adjoint equation method is used to construct a solution framework for the inverse thermal conduction problem, achieving accurate inversion of the temperature distribution inside the region without fiber optic cables. The L2 norm of the error matrix controls the iteration termination condition to ensure the accuracy of the inversion results. By calculating the time nodes when each layer of material reaches the critical thermal decomposition threshold, a thermal breakdown evolution trajectory containing time and space dimensions is generated, realizing a quantitative presentation of the internal thermal breakdown process of the composite material.

Claims

1. A fire resistance performance testing system for fire-resistant cables based on temperature detection, characterized in that, This includes a combustion application device, an optical fiber pre-embedded component, an optical signal demodulation device, a heat conduction inverse calculation device, and a thermal breakdown trajectory output device; The fiber optic pre-embedded assembly includes a special high-temperature resistant distributed optical fiber distributed along the radial cross section of the fireproof cable composite material sample and pre-embedded between the insulation layer and the shielding layer. The combustion application device is disposed on the outside of the composite material sample to provide a standard ignition source environment; The optical signal demodulation device is connected to the special high-temperature resistant distributed optical fiber, injects laser pulses into the special high-temperature resistant distributed optical fiber at a set frequency and receives the returned back Raman scattered light signal, and demodulates the back Raman scattered light signal into temperature time series data of each spatial node of the special high-temperature resistant distributed optical fiber. The inverse thermal conductivity calculation device receives the temperature time series data, establishes a set of radial thermal conductivity partial differential equations including the multilayer structure of the composite material sample, uses the temperature time series data as observation constraints and boundary conditions, and uses the adjoint equation method to perform inverse time series iterative solution of the radial thermal conductivity partial differential equations to invert and obtain the internal temperature distribution gradient of the region where the special high temperature resistant distributed optical fiber is not deployed. The thermal breakdown trajectory output device is connected to the heat conduction inverse calculation device, and calculates the time node when each layer of material reaches the critical thermal decomposition threshold based on the internal temperature distribution gradient and outputs the internal thermal breakdown evolution trajectory. The specific method by which the inverse thermal conductivity calculation device constructs the radial thermal conductivity partial differential equation system is as follows: the initial thermal conductivity and initial specific heat capacity of each layer of the composite material sample are obtained, and the radial thermal conductivity partial differential equation system with radial distance and time as independent variables is established based on the cylindrical coordinate system. The outer boundary condition of the radial thermal conductivity partial differential equation system is set as the temperature function of the fire-receiving surface generated by the combustion application device, and the inner boundary condition of the radial thermal conductivity partial differential equation system is set as the adiabatic condition at the central axis of the composite material sample. The process of the heat conduction inverse calculation device using the adjoint equation method for inverse time-series iterative solution includes: constructing a corresponding adjoint state equation based on the radial heat conduction partial differential equation set; using the difference between the temperature time-series data of the spatial node where the special high-temperature resistant distributed optical fiber is located and the calculated temperature as the source term of the adjoint state equation; discretizing the adjoint state equation in the inverse time direction to obtain the gradient descent direction; updating the initial temperature distribution of the area where the special high-temperature resistant distributed optical fiber is not deployed based on the gradient descent direction; and performing forward heat conduction calculation until the iteration termination condition is met. The thermal breakdown trajectory output device extracts the extreme temperature sequence of each layer of material in the internal temperature distribution gradient, compares the extreme temperature sequence with the preset critical thermal decomposition threshold temperature of each layer of material, records the specific timestamp of each data point in the extreme temperature sequence reaching the critical thermal decomposition threshold temperature as the time node, and arranges the time nodes in chronological order to generate the internal thermal breakdown evolution trajectory containing time and spatial dimensions.

2. The fire resistance performance testing system for fire-resistant cables based on temperature detection according to claim 1, characterized in that, The fiber optic pre-embedded assembly also includes a miniature polytetrafluoroethylene (PTFE) positioning slot disposed at the junction of the insulation layer and the shielding layer. The special high-temperature resistant distributed optical fiber is fixed to the radial section of the composite material sample through the miniature PTFE positioning slot. The special high-temperature resistant distributed optical fiber includes an inner core, a quartz cladding wrapped around the outer side of the inner core, and a high-temperature resistant polyimide coating coated on the outer side of the quartz cladding. The cross-sectional diameter of the miniature PTFE positioning slot matches the outer diameter of the special high-temperature resistant distributed optical fiber coated with the high-temperature resistant polyimide coating.

3. The fire resistance performance testing system for fire-resistant cables based on temperature detection according to claim 1, characterized in that, The optical signal demodulation device includes a pulsed laser source, a wavelength division multiplexer, a photodetector, and a data acquisition card. The pulsed laser source emits the laser pulse. The common terminal of the wavelength division multiplexer is connected to the special high-temperature resistant distributed optical fiber. The reflective terminal of the wavelength division multiplexer is connected to the photodetector. The photodetector converts the Stokes light signal and the anti-Stokes light signal in the received backscattered Raman light signal into electrical signals. The data acquisition card performs analog-to-digital conversion on the electrical signals at the set frequency to generate the temperature time series data.

4. The fire resistance performance testing system for fire-resistant cables based on temperature detection according to claim 2, characterized in that, The inner wall of the miniature PTFE positioning slot is provided with a compensation filling groove that runs through the axis of the special high-temperature resistant distributed optical fiber. The compensation filling groove is filled with high thermal conductivity silicone grease. The high thermal conductivity silicone grease tightly wraps the outer surface of the special high-temperature resistant distributed optical fiber coated with the high-temperature resistant polyimide coating. The outer edge of the miniature PTFE positioning slot is provided with a serrated interlocking structure that matches the physical interface of the insulation layer and the shielding layer. The tooth height and tooth pitch of the serrated interlocking structure are configured according to the difference in elastic modulus between the insulation layer and the shielding layer materials.

5. The fire resistance performance testing system for fire-resistant cables based on temperature detection according to claim 3, characterized in that, The optical signal demodulation device further includes a reference fiber channel and a dual-channel differential amplifier. The reference fiber channel is placed in a constant temperature environment away from the combustion application device. The transmission end of the wavelength division multiplexer is connected to the reference fiber channel. The output end of the reference fiber channel is connected to the other input end of the photodetector. The two input ends of the dual-channel differential amplifier are respectively connected to the output channel of the photodetector corresponding to the Stokes optical signal and the output channel corresponding to the anti-Stokes optical signal.

6. The fire resistance performance testing system for fire-resistant cables based on temperature detection according to claim 1, characterized in that, When establishing the radial heat conduction partial differential equation system, the heat conduction inverse calculation device sets the initial thermal conductivity and the initial specific heat capacity as piecewise linear functions that change with temperature, based on the different temperature ranges of each layer of the composite material sample. During each execution of the forward heat conduction calculation, it extracts the temperature values ​​corresponding to each spatial node at the current time step and updates the local thermal conductivity and local specific heat capacity of each spatial node according to the piecewise linear function. The updated local thermal conductivity and local specific heat capacity are then substituted into the radial heat conduction partial differential equation system.

7. The fire resistance performance testing system for fire-resistant cables based on temperature detection according to claim 1, characterized in that, When performing the inverse time-series iterative solution, the heat conduction inverse calculation device calculates the L2 norm of the error matrix between the temperature time-series data and the calculated temperature at the current iteration number, and determines whether the L2 norm is less than a preset stopping threshold. If the L2 norm is less than the preset stopping threshold, the iteration is terminated and the initial temperature distribution corresponding to the current iteration number is output as the internal temperature distribution gradient. If the L2 norm is greater than or equal to the preset stopping threshold, the initial temperature distribution is updated according to the gradient descent direction combined with a preset step size factor, and the current iteration number is increased.

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