CN120869801B - Electro-assisted biaxial tensile forming limit test system and method for coated sheet materials - Google Patents
Electro-assisted biaxial tensile forming limit test system and method for coated sheet materialsInfo
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Abstract
The invention relates to a plating plate electrically-assisted biaxial stretching forming limit test system and a method, which belong to the technical field of bipolar plate test, wherein the test method comprises the following steps: the invention captures the layered initiation and expansion of the interface in real time through multi-physical field detection, can synchronously track the dynamic evolution of the necking initiation, expansion and interface failure in the stretching process, can be closer to the actual working condition requirement to avoid the thermal mismatch of the interface aggravated by Joule heat, and can make up the influence of insufficient sensitivity of a general ultrasonic algorithm, redefine the failure criterion as a plating layer-substrate interface-separated critical state and further reduce the error of the critical failure point predicted by the shaping limit curve.
Description
Technical Field
The invention belongs to the technical field of bipolar plate testing, and particularly relates to an electrically-assisted biaxial stretching forming limit testing system and method for a coated plate.
Background
In the production process of the ultrathin titanium alloy bipolar plate, in order to improve the corrosion resistance and the conductivity of the bipolar plate, an electroplating nickel plating process is required after stamping forming. Because the deep plating capability and the uniform plating capability of electroplating are poor, the condition of plating leakage of the micro flow channel structure of the bipolar plate is easy to occur, and the service life requirement of the bipolar plate cannot be ensured. In order to effectively solve the problems, the plating process is advanced, the plating process is utilized to plate nickel on the surface of the ultrathin titanium substrate, and then stamping is performed, so that the situation that the micro flow channel structure of the bipolar plate is not plated is avoided, and the bipolar plate after stamping can be directly used. However, in order to realize the forming-service integrated manufacturing of the ultrathin nickel-titanium alloy plated bipolar plate, a forming limit curve is also required to be constructed by carrying out a forming limit test on the preplating coating plate, and the forming performance of the ultrathin nickel-titanium alloy plated plate is evaluated by utilizing the forming limit curve, so that the defect that interface delamination cannot be generated between a base material and a coating after the ultrathin nickel-titanium alloy plated plate is stamped and formed is ensured.
When the pre-coated plate is subjected to forming limit test, the bipolar plate is generally stretched in a mechanical property test mode, then the failure morphology of the base material and the coating is observed by combining metallographic analysis or a scanning electron microscope, and a forming limit curve is established based on a toughness fracture criterion. However, the test methods by combining mechanical property test with metallographic analysis or scanning electron microscope observation are based on the defect that interface layering is generated between the substrate and the coating, so that the defects of destructiveness, hysteresis and the like exist, the real-time capturing and early warning of a failure mechanism in the dynamic loading process are difficult to realize, and interface layering failure and critical strain cannot be associated in real time. Meanwhile, a forming limit curve established based on a toughness fracture criterion is formed by taking a material tensile fracture as a failure criterion, the core assumption is that the uniformity and the integrity of the material are adopted, the delamination failure of a coating and substrate interface of a bipolar plate is always earlier than the fracture of the substrate, once the interface of the coating and the substrate is separated, the key functions such as conductivity, corrosion resistance and the like of the bipolar plate are completely lost even if the substrate is not broken, so that the bipolar plate cannot be normally used, the bipolar plate has a unique failure mechanism, the established forming limit curve cannot capture the critical point of the interfacial delamination failure, the output forming limit predicted value is virtually high, and the technological parameters (such as current density and strain path) for obtaining the fracture criterion based on the forming limit curve cannot effectively inhibit the interfacial delamination expansion, so that the forming technological parameters are generally excessively reduced in order to avoid the bipolar plate failure, and the production efficiency is sacrificed. Therefore, the forming limit curve established based on the ductile fracture criteria is not suitable for evaluating the forming performance of the ultra-thin nickel-plated titanium plate.
At present, an ultrasonic detection technology is mostly adopted in the prior art to identify layering defects in the bipolar plate, and the detection technology can avoid the defects of damage to the bipolar plate and the like. However, static characterization of internal defects of the bipolar plate can only be obtained through ultrasonic detection, dynamic evolution of necking initiation, expansion and interface failure in the stretching process cannot be synchronously tracked, dynamic influence of layering expansion on forming limit (such as local necking threshold) is difficult to quantify, in addition, in the electric auxiliary stretching process of the bipolar plate, a local temperature field of a material can be obviously changed by a current-induced joule heating effect, strain localization and interface layering of a necking area are accelerated, an existing forming limit test method focuses on a single physical field more, failure mechanisms under actual working conditions are difficult to reproduce, the difference of bonding strength between a preplating layer and a substrate interface is obvious, the sensitivity of a general ultrasonic algorithm is insufficient, and an obtained forming limit curve is difficult to predict a critical failure point.
Disclosure of Invention
In view of the above, the invention provides an electrically assisted biaxial stretching forming limit test system and method for a coated sheet material, so as to solve the defects in the prior art, and the invention can synchronously track the dynamic evolution of the initiation and the expansion of necking and the failure of an interface in the stretching process, and quantifying the dynamic influence of layering expansion on the forming limit, redefining a failure criterion as a critical state of separation of a coating-substrate interface, and further reducing the error of the forming limit curve in predicting the critical failure point.
The technical scheme of the invention is that the electrically assisted biaxial stretching forming limit test method of the coating plate comprises the following steps:
heating the bipolar plate sample by using pulse current to enable the bipolar plate sample to reach a preset temperature, stretching the bipolar plate sample, and collecting temperature, displacement, load and surface deformation images and performing ultrasonic detection when the bipolar plate sample is stretched;
Processing the acquired temperature, displacement, load, surface deformation image and ultrasonic signals to obtain a temperature gradient curve, a load-displacement curve, a reflection wave attenuation rate, a strain cloud picture, harmonic distortion degree, layering area occupation ratio and an energy accumulation curve of the ultrasonic signals;
positioning a layered sprouting area on a bipolar plate sample by using a temperature gradient curve, a load-displacement curve and a strain cloud picture;
obtaining crack expansion rate of the layered sprouting area according to reflection wave attenuation rate, harmonic distortion degree and energy accumulation curve of ultrasonic signals of the layered sprouting area;
And establishing a basic forming limit curve based on a plastic instability criterion, dynamically correcting the basic forming limit curve by using a crack expansion rate and a layering area occupation ratio to obtain a forming limit curve FLC, and evaluating the stamping forming performance of the bipolar plate by using the forming limit curve FLC.
Preferably, the step of obtaining the reflection wave attenuation rate, the harmonic distortion degree, and the layered area ratio includes:
preprocessing the acquired ultrasonic signals through wavelet noise reduction and EMD decomposition, removing noise, separating out interface reflected wave signals and transmitted wave signals, and further obtaining reflected wave attenuation rate and harmonic distortion;
threshold segmentation is carried out on the interface reflected wave signals to obtain a layered region boundary;
and calculating the proportion of the number of pixels of the layering region to the total number of pixels to obtain the proportion of the area of the layering region to the total area of the bipolar plate sample.
Preferably, using the temperature gradient profile, the load-displacement profile and the strain cloud, locating the stratified initiation region on the bipolar plate specimen comprises:
and aligning the temperature gradient curve with the load-displacement curve, when the abrupt change point on the temperature gradient curve is overlapped with the abrupt change point on the load-displacement curve, comparing the abrupt change point on the temperature gradient curve with the strain localization area on the strain cloud picture, and when the strain localization area is overlapped with the position of the abrupt change point on the temperature gradient curve, determining the area as a layering sprouting area.
Preferably, establishing the base forming limit curve based on the plastic destabilization criterion comprises:
Selecting a plastic instability criterion:
Calibrating parameters in a plastic instability criterion according to historical test data;
and establishing an FLD prediction model in stages according to the plastic instability criterion to obtain a model in the uniform deformation, local instability and critical failure stage so as to form a basic forming limit curve.
Preferably, the plastic destabilization criteria are selected by the interfacial thermal expansion coefficient difference, bonding strength of the bipolar plate coupon:
The right FLD adopts a correction maximum force criterion;
the left FLD uses Hill'48 yield criterion.
Preferably, establishing the FLD predictive model in stages according to the plastic destabilization criteria includes:
Predicting a diffusion necking starting point based on a Swift hardening criterion, and triggering initial early warning by utilizing the slope change of a load-displacement curve;
When the temperature and the intensity of the ultrasonic signal rise synchronously, switching to an M-K model, introducing an equivalent initial defect, and simulating strain localization caused by interface layering;
And (3) critical failure, namely judging the formation of a shear band through ultrasonic signal energy spectrum mutation by combining with a bifurcation theory, and outputting critical fracture strain.
Preferably, dynamically modifying the base forming limit curve using crack growth rate, delamination area occupancy comprises:
The crack growth rate and the delamination area ratio are introduced into the following formula:
;
Wherein FLC base is a basic forming curve, FLC new is a modified forming limit curve FLC, k 1 is a layered area ratio weight coefficient, k 2 is a crack growth rate weight coefficient, A d is a layered area ratio, V c is a crack growth rate in mm/s, and V crit is a critical crack growth rate in mm/s.
Preferably, evaluating the performance of the bipolar plate press forming using the forming limit curve FLC includes:
superposing a forming limit curve FLC to the biaxial stretching strain field;
generating a safety-transition-dangerous area three-dimensional thermodynamic diagram through an interpolation algorithm, and identifying critical fracture position probability:
safety region ε 1<0.8 FLC new;
A transition region of 0.8 FLC new<ε1<FLCnew;
Dangerous area epsilon 1> FLC new;
where ε 1 is the maximum principal strain.
The test system comprises a double-pull test machine, wherein the end parts of a double-pull test machine are fixedly connected with the clamping ends of the double-pull test machine in a one-to-one correspondence manner so as to carry out double-pull on the double-pull test machine, a pulse current element is respectively connected with the end parts of the double-pull test machine through wires so as to heat the double-pull test machine, a plurality of thermocouples are respectively fixedly arranged on one side of the double-pull test machine, which is close to the clamping ends, so as to collect the temperature of the double-pull test machine, a plurality of ultrasonic detection elements are respectively fixedly arranged on one side of the double-pull test machine, which is close to the clamping ends, so as to carry out ultrasonic detection on the double-pull test machine, a plurality of displacement sensors are respectively fixedly arranged on the end parts of the double-pull test machine, so as to collect the displacement applied to the double-pull test machine, so as to collect the load applied to the double-pull test machine, a DIC high-speed camera is arranged right above the middle of the double-pull test machine so as to collect the surface deformation image of the double-pull test machine, and the performance of the double-pull test machine is evaluated by utilizing a test method.
Preferably, the bipolar plate sample is cross-shaped.
Compared with the prior art, the system and the method for testing the electrically assisted biaxial stretching forming limit of the plating plate provided by the invention have the advantages that pulse current is utilized to heat a bipolar plate sample and then biaxially stretch the bipolar plate sample, meanwhile, temperature, displacement, load and surface deformation images are collected and ultrasonic detection is carried out, further, information such as a temperature gradient curve, a load-displacement curve, reflection wave attenuation rate, strain cloud images, harmonic distortion degree, layering area occupation ratio and the like is obtained, a layering forming region can be positioned, crack expansion rate of the region is obtained, layering forming and expansion of an interface are captured in real time through multi-physical field detection, dynamic influences of necking forming, expansion and interface failure in the stretching process can be synchronously tracked, dynamic monitoring of interface failure is quantized and layered expansion is realized, the effect of being close to actual requirements is avoided, the influence of thermal mismatch of a joule heating exacerbation interface can be made up, a basic forming limit curve is established and dynamically corrected according to a plastic instability criterion, a quantitative relationship between the layering forming region and the bipolar plate failure characteristic is established through dynamic capturing interface, the critical failure condition is further defined, and the critical failure state of the bipolar plate failure condition is further predicted, and the critical failure state is further defined by the multi-physical field detection is further defined.
Drawings
FIG. 1 is a schematic diagram of a test system of the present invention;
FIG. 2 is a flow chart of the forming limit prediction of the present invention.
Reference numerals illustrate:
1. Clamping end, ultrasonic detection element, load detection element, temperature sensing system, bipolar plate sample, pulse current element, ultrasonic detection system, upper support seat, lower support seat, left support seat, right support seat, insulating spacer, thermocouple, insulating sleeve, displacement sensor, DIC high-speed camera, DIC analysis system, and DIC analysis system.
Detailed Description
The invention provides an electrically assisted biaxial stretching forming limit test system and method for a coated plate, and the invention is described below with reference to the structural schematic diagrams of fig. 1 to 2.
In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings are merely for convenience in describing the technical solutions of the present invention and to simplify the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and therefore should not be construed as limiting the present invention.
An electrically assisted biaxial stretching forming limit test method for a coated plate comprises the following steps:
heating the bipolar plate sample by using pulse current to enable the bipolar plate sample to reach a preset temperature, stretching the bipolar plate sample, and collecting temperature, displacement, load and surface deformation images and performing ultrasonic detection when the bipolar plate sample is stretched;
Processing the acquired temperature, displacement, load, surface deformation image and ultrasonic signals to obtain a temperature gradient curve, a load-displacement curve, a reflection wave attenuation rate, a strain cloud picture, harmonic distortion degree, layering area occupation ratio and an energy accumulation curve of the ultrasonic signals;
positioning a layered sprouting area on a bipolar plate sample by using a temperature gradient curve, a load-displacement curve and a strain cloud picture;
obtaining crack expansion rate of the layered sprouting area according to reflection wave attenuation rate, harmonic distortion degree and energy accumulation curve of ultrasonic signals of the layered sprouting area;
And establishing a basic forming limit curve based on a plastic instability criterion, dynamically correcting the basic forming limit curve by using a crack expansion rate and a layering area occupation ratio to obtain a forming limit curve FLC, and evaluating the stamping forming performance of the bipolar plate by using the forming limit curve FLC.
According to the method for testing the electric auxiliary biaxial stretching forming limit of the plating plate, firstly, pulse current is utilized to heat a bipolar plate sample, the temperature of the bipolar plate sample is detected to reach the preset temperature and current density, the temperature of the bipolar plate sample meets the stretching requirement, then the bipolar plate sample is subjected to biaxial stretching, temperature, displacement, load, surface deformation images and ultrasonic detection during stretching of the bipolar plate sample are collected, temperature, displacement, load, surface deformation images and ultrasonic signals during stretching of the bipolar plate sample are obtained, dynamic monitoring of multi-physical-field interface failure under the condition of thermal-electric-force coupling is achieved, the multi-physical-field detection can compensate the influence of insufficient sensitivity of a general ultrasonic algorithm, the dynamic evolution of interface layering initiation and expansion can be captured in real time, the dynamic influence of necking initiation, expansion and interface failure in the stretching process can be synchronously tracked, the dynamic evolution of the layering expansion on forming limit is quantified, the information such as temperature gradient curve, load-displacement curve, reflection wave attenuation rate, strain map, harmonic degree, layering proportion and the like is collected, the dynamic failure rate of the bipolar plate can be further predicted according to the critical failure rate of the dynamic failure stability threshold, the dynamic failure condition of the dynamic failure condition can be further defined by the threshold failure condition of the dynamic failure condition of the bipolar plate, and the dynamic failure condition can be further predicted, and the critical failure condition of the dynamic failure condition can be further defined, and the critical failure condition of the dynamic failure condition can be further predicted, and the threshold and the dynamic failure condition of the dynamic failure condition can be further defined, and the threshold has a threshold failure limit can be predicted.
The method for testing the electric auxiliary biaxial stretching forming limit of the coated plate in the embodiment is based on a pulse current field auxiliary Nakajima forming limit test method, and can acquire an interface integrity dynamic damage evolution rule under the action of an electro-plastic coupling size effect, so that an electric auxiliary stamping forming limit curve based on substrate/coating interface layering failure is established, and further, the stamping forming performance of the ultrathin nickel-plated titanium plate is accurately evaluated.
Because the traditional detection method has the defects of destructiveness, hysteresis and the like, when the interface defect is observed, the coating is failed, and the forming limit curve obtained by the detection method in the embodiment can be predicted before the interface fails.
The electrically assisted biaxial stretching forming limit test method for the coated plate has the following advantages:
The failure criterion is accurate, namely, the forming limit evaluation is changed from material fracture to interface function loss, and the actual failure mode of the pre-plated coating plate is attached;
The resource efficiency is improved, namely the production efficiency loss caused by excessively conserved process parameters is avoided, and the productivity is improved;
the method has industrial application value of providing theoretical support for the optimization of the forming process of the preplating coating parts such as the bipolar plate of the new energy automobile, the aerospace composite laminate and the like.
According to the method for testing the forming limit of the electric auxiliary biaxial stretching of the coated plate, in the embodiment, for a bipolar plate electric auxiliary stretching scene, quantitative relation between the electric auxiliary biaxial stretching and the forming limit of the plate (such as critical fracture strain and local necking threshold) is established by utilizing the delamination failure characteristics of a dynamic capture interface, classical plastic instability theory is fused with a multi-physical-field dynamic detection technology, and an interface delamination damage-forming limit dynamic mapping model is provided, so that a forming limit curve FLC is obtained, the performance of stamping forming of the bipolar plate is evaluated by utilizing the forming limit curve FLC, and important basis can be provided for optimizing a coating process and forming parameters.
In the embodiment, the reflection wave attenuation rate, the harmonic distortion degree and the energy accumulation curve of the ultrasonic signal of the layered sprouting area are input into the 1D-CNN+LSTM model, so that the crack expansion rate is output, the unit is mm/s, the prediction of the crack expansion rate is realized, the crack expansion direction can be obtained, and the quantitative evaluation and early warning of the crack expansion are realized by utilizing the dynamic change rule of the characteristic parameters and combining a physical model or a data driving algorithm.
For example, the crack growth rate V c =0.1 mm/s is determined when the energy value increases from 100 mv2 x s to 500 mv2 x s in 1 second based on the energy accumulation curve of the ultrasonic signal.
And judging a crack expansion rate threshold, namely judging layering initiation if the attenuation rate of the reflected wave exceeds a preset threshold (for example, the attenuation rate is more than or equal to 20 dB/mu s), and judging crack acceleration expansion if the harmonic distortion degree continuously rises (for example, THD is increased from 3% to 8%).
The reflection wave attenuation rate in the embodiment can reflect the energy loss of ultrasonic waves at a layering interface, the geometric dimensions (such as crack length and depth) of the layering are directly related, and The Harmonic Distortion (THD) can represent the nonlinear distortion degree of the signal and is related to the change of the dynamically-expanded crack tip stress field.
As a further optimization scheme, the steps of obtaining the reflection wave attenuation rate, the harmonic distortion degree and the layered area ratio in the embodiment include:
preprocessing the acquired ultrasonic signals through wavelet noise reduction and EMD decomposition, removing noise, separating out interface reflected wave signals and transmitted wave signals, and further obtaining reflected wave attenuation rate and harmonic distortion;
threshold segmentation is carried out on the interface reflected wave signals to obtain a layered region boundary;
and calculating the proportion of the number of pixels of the layering region to the total number of pixels to obtain the proportion of the area of the layering region to the total area of the bipolar plate sample.
In the embodiment, the collected ultrasonic signals are preprocessed through wavelet noise reduction and EMD decomposition, so that physical quantities (such as attenuation rate and THD) directly related to layering can be separated from complex acoustic signals, and the signal-to-noise ratio problem in a dynamic environment is solved.
The specific implementation form of obtaining the layered area ratio is as follows:
performing ultrasonic image segmentation, namely performing threshold segmentation on the reflected wave signals, and identifying the boundary of a layering region;
Calculating the proportion of the number of pixels of the layering region to the total number of pixels;
And outputting in real time, namely dynamically updating the numerical value of the layered area ratio through a multichannel signal acquisition system, namely calculating the proportion of the layered area to the total detection area in real time through an ultrasonic signal processing algorithm, and judging the area as a layered damage area when the attenuation rate of the detected reflected waves exceeds 20 dB/mu s.
As a further optimization scheme, in this embodiment, using a temperature gradient curve, a load-displacement curve, and a strain cloud chart to locate a layered initiation region on a bipolar plate sample includes:
and aligning the temperature gradient curve with the load-displacement curve, when the abrupt change point on the temperature gradient curve is overlapped with the abrupt change point on the load-displacement curve, comparing the abrupt change point on the temperature gradient curve with the strain localization area on the strain cloud picture, and when the strain localization area is overlapped with the position of the abrupt change point on the temperature gradient curve, determining the area as a layering sprouting area.
In the electric auxiliary stretching process, local thermodynamic behavior mutation is caused when an interface is layered and started, and the electric auxiliary stretching process is concretely characterized in that friction heating is generated, friction is generated in a layered area due to interface dislocation or microcrack expansion, local temperature rise is caused, strain localization is carried out, layering and starting are carried out, strain concentration is carried out in a necking area, the Joule heating effect is intensified, temperature gradient mutation is formed, heat conduction is blocked, material heat conduction paths are interrupted due to layering, and heat is accumulated near a layered interface, so that a temperature abnormal point is formed.
Thus, the present embodiment uses multiple physical fields to detect spatio-temporal synchronization:
time synchronization, namely aligning the time stamp of the temperature gradient abrupt change point with the inflection point of the load-displacement curve, and eliminating temperature fluctuation (such as environmental interference) caused by non-layering;
space mapping, namely acquiring a real-time strain field through a DIC high-speed camera system, and confirming a layered sprouting area if a temperature abrupt change point coincides with a strain localization area.
The strain localization area is an area with strain greater than or equal to a threshold value in the strain cloud image.
And (3) the inflection point of the load-displacement curve characteristic is identified, namely the load-displacement curve is subjected to slope reduction or fluctuation during layering initiation, the rigidity of the material is prompted to be reduced, and the temperature data are synchronously analyzed in combination with the time point.
And energy dissipation, namely, energy release is carried out in a layering process, and energy mutation points are calculated through load integration and are matched with temperature gradient change time.
Temperature gradient curve abrupt point location-monitoring the temperature gradient slope (st= fatter/. Cndot.) in real time by a thermocouple array, when a region ST suddenly increases (e.g., from 2 ℃ per mm to 8 ℃ per mm), it is identified as a potential delamination initiation point.
Additionally, thermal imaging assistance, such as a thermal infrared imager, may be used to supplement thermocouple data to capture the spatial non-uniformities of the temperature field.
As a further optimization, establishing the basic forming limit curve based on the plastic instability criterion in this embodiment includes:
Selecting a plastic instability criterion:
Calibrating parameters in a plastic instability criterion according to historical test data;
and establishing an FLD prediction model in stages according to the plastic instability criterion to obtain a model in the uniform deformation, local instability and critical failure stage so as to form a basic forming limit curve.
As a further optimization scheme, in this embodiment, the plastic instability criterion is selected by the interfacial thermal expansion coefficient difference and the bonding strength of the bipolar plate sample:
The right FLD adopts a correction maximum force criterion;
the left FLD uses Hill'48 yield criterion.
In the embodiment, a maximum force criterion (Modified Maximum Force Criterion) is corrected, the influence of current-induced thermal stress on a necking threshold is quantified through harmonic distortion extracted by an ultrasonic signal, and a Hill'48 yield criterion is utilized to combine with a temperature gradient abrupt point positioning interface layering initiation region to correct a local strain path.
In the embodiment, parameters of criteria, such as a strain path sensitivity coefficient in a maximum force correction criterion and an anisotropy coefficient in a Hill'48 yield criterion, are calibrated through historical test data, so that a mapping relation with ultrasonic characteristics (reflection wave attenuation rate and layering area occupation ratio) is established.
As a further optimization scheme, the step of establishing the FLD prediction model according to the plastic destabilization criterion in stages in this embodiment includes:
predicting a diffusion necking starting point based on a Swift hardening criterion, and triggering initial early warning by utilizing the slope change (dF/depsilon=0) of a load-displacement curve;
Local instability, namely switching to an M-K model when the temperature and the intensity of an ultrasonic signal synchronously rise, introducing an equivalent initial defect (defect factor f 0 =0.99), and simulating strain localization caused by interface layering;
And (3) critical failure, namely judging shear band formation through ultrasonic signal energy spectrum mutation by combining bifurcation theory (Loss of Ellipticity criterion), and outputting critical fracture strain.
In this embodiment, the FLD prediction model is built in stages according to the plastic instability criterion, and the layering area ratio and the temperature gradient slope can be input into a Random Forest (Random Forest) model to output layering, necking and microcrack probability, so as to implement static damage classification.
The slope of the temperature gradient in this embodiment outputs the temperature distribution at each location on the bipolar plate sample by acquiring temperature data in real time. For example, thermocouples with a distance of 2mm can be arranged in the region where the bipolar plate sample is easy to be necked, when the temperature difference of the adjacent thermocouples reaches 5 ℃, gradient slope ST=2.5 ℃ per mm is calculated, and the abrupt point is also an interfacial delamination initiation point.
As a further optimization scheme, the dynamic modification of the basic forming limit curve by using the crack growth rate and the layering area occupation ratio in the embodiment includes:
The crack growth rate and the delamination area ratio are introduced into the following formula:
;
The FLC base is a basic forming curve, namely a theoretical curve without considering interface layering damage, the FLC new is a corrected forming limit curve FLC, the interface layering damage parameters are dynamically adjusted to be more fit with actual working conditions, k 1 is a layering area occupation ratio weight coefficient, k 2 is a crack growth rate weight coefficient, A d is a layering area occupation ratio, V c is a crack growth rate, the unit is mm/s, V crit is a critical crack growth rate, the crack growth rate when the interface layering reaches a failure threshold is represented, and the unit is mm/s through test or historical data calibration.
In the embodiment, the basic forming limit curve is dynamically corrected, and the crack expansion rate V c, the layering area ratio A d and the second derivative of the load curve are led into a XGBoost model to obtain the FLC new safety threshold, so that accurate regression of the threshold is realized.
In addition, k 1 (stratification area duty weight) is fitted by historical test data, for example, comparing the deviation of the measured FLD from the theoretical FLD at different interface stratification levels, and the regression analysis is used to determine the value. k 1 can reflect the influence of static damage (such as layering area) on the forming limit, and the weakening effect of the interface layering area on the FLD is more remarkable when the weight is larger;
k 2 (crack growth rate weight), and is calibrated by a fatigue test or a dynamic tensile test in combination with the proportional relation between the crack growth rate and the critical value. For example, monitoring the sensitivity of crack growth to FLD, the physical meaning of k 2 is to reflect the impact of dynamic damage (e.g., crack growth rate), and higher weights indicate that rapid crack growth significantly reduces the critical strain threshold.
In this embodiment, the damage weights k1 and k2 are trained through the pre-plating coating historical test data, so that the interface thermal-force coupling characteristics are adapted.
As a further optimization, the evaluation of the performance of stamping forming of the bipolar plate by using the forming limit curve FLC in this embodiment includes:
superposing the forming limit curve FLC to a biaxial stretching strain field (plane);
generating a safety-transition-dangerous area three-dimensional thermodynamic diagram through an interpolation algorithm, and identifying critical fracture position probability:
safety region ε 1<0.8 FLC new;
A transition region of 0.8 FLC new<ε1<FLCnew;
Dangerous area epsilon 1> FLC new;
where ε 1 is the maximum principal strain.
In this embodiment, the process parameters can be fed back in real time through the three-dimensional thermodynamic diagram:
Triggering grading early warning when detecting that the dangerous area duty ratio exceeds a threshold value:
First-stage early warning, namely reducing the electric pulse intensity to a preset lower limit (such as current density J=50A/mm 2) and slowing down the accumulation of Joule heat;
Switching a strain path to a plane strain mode (beta=0) to inhibit layered expansion of an interface;
The optimized process parameters are automatically stored in a database for initial parameter setting of a subsequent test.
Note that the trigger condition and threshold:
Primary early warning:
triggering a trigger mark when detecting that the dangerous area proportion reaches 15% of a threshold value;
Responding measures, namely reducing the intensity of pulse current to a preset safety value (such as the current density is reduced from 100A/mm < 2 > -50A/mm < 2 >), and delaying the expansion of the damage by reducing the accumulation of joule heat;
and an intervention target is to slow down damage accumulation and strive for process adjustment time.
Second-level early warning:
Triggering the mark when the dangerous area ratio is not reduced or continuously increased to 25% of the threshold value or when the dynamic parameter abnormality is detected (such as crack expansion rate exceeds a critical value Vcrit and temperature gradient mutation rate is more than or equal to 10 ℃ per second) after primary early warning;
Switching a strain path to a plane strain mode, and inhibiting strain localization and interface layering expansion;
and the intervention target is to forcedly terminate the high risk state and avoid failure.
In the embodiment, the closed loop feedback of the technological parameters can be realized, and parameters such as current intensity, strain path and the like are automatically adjusted according to the interface damage threshold detected in real time, so that the sheet forming performance is improved.
The method for testing the electrically assisted biaxial stretching forming limit of the coated plate breaks through the fracture failure assumption of the traditional FLC, proposes an interface layering failure-forming limit association model, redefines a failure criterion as a critical state of the separation of a coating-substrate interface, and comprises the following steps:
1. and (5) reconstructing failure standard:
the layered initiation and expansion of the interface are captured in real time through multi-physical-field detection (the ultrasonic reflection wave attenuation rate and the abrupt change point on the temperature gradient curve);
and establishing a double-threshold failure criterion of the interfacial delamination area occupation ratio (A d is more than or equal to 15%) and the crack growth rate (V c≥Vcrit).
2. Dynamic FLC correction:
The FLC curve is dynamically modified based on the interface damage parameters.
3. And (3) specificity optimization:
Based on the pre-plating layer-substrate interface characteristics (such as thermal expansion coefficient difference and bonding strength), the ultrasonic detection frequency (such as 20 MHz) and the characteristic selection of a machine learning algorithm are optimized, and the sensitivity is improved.
Referring to fig. 1, fig. 1 is a schematic diagram of a test system of the embodiment, which is an electrically assisted biaxial stretching forming limit test system of a plating sheet, and includes a biaxial stretching tester, wherein the ends of the bipolar plate sample 9 are fixedly connected with the clamping ends 1 of the biaxial stretching tester in a one-to-one correspondence manner to biaxially stretch the bipolar plate sample 9, a pulse current element 10 is respectively connected with the ends of the bipolar plate sample 9 through wires to heat the bipolar plate sample 9, a plurality of thermocouples 17 are respectively fixed on one side of the bipolar plate sample 9 near the clamping ends 1 to collect the temperature of the bipolar plate sample 9, a plurality of ultrasonic detection elements 5 are respectively fixed on one side of the bipolar plate sample 9 near the clamping ends 1 to ultrasonically detect the bipolar plate sample 9, a plurality of displacement sensors 19 are respectively fixed on the ends of the bipolar plate sample 9 to collect displacement during biaxial stretching, a load detection element 6 is respectively fixed on a loading shaft connected with the clamping ends 1 of the biaxial stretching tester to collect load applied during biaxial stretching of the bipolar plate sample 9, a high-speed camera 20 is respectively arranged above a middle plate 9 to collect a positive side of the bipolar plate sample 9, and an image of the bipolar plate is formed by using a deformation image, and the surface deformation performance of the bipolar plate is evaluated by using the method of the deformation image forming test method.
Referring to fig. 2, fig. 2 is a flowchart of forming limit prediction in this embodiment, and the specific operation process of the electrically assisted biaxial stretching forming limit test system for a coated sheet in this embodiment is as follows:
step 1, an upper supporting seat 12, a lower supporting seat 13, a left supporting seat 14 and a right supporting seat 15 are arranged on a double-pull testing machine;
Step 2, in order to prevent the pulse current from damaging equipment such as a double-pull testing machine, and the like, insulating gaskets 16 are respectively arranged on the clamping ends, and the clamping ends 1 are fixedly connected with the upper supporting seat 12, the lower supporting seat 13, the left supporting seat 14 and the right supporting seat 15 through fastening screws and insulating sleeves 18;
step 3, preparing a bipolar plate sample 9, and adopting a digital image related speckle strain measurement system to perform multi-energy field assisted double-pull orthogonal loading real-time strain measurement;
step 4, installing an ultrasonic detection element 5, a displacement sensor 19 and a thermocouple 17 at a preset position of the bipolar plate sample 9, wherein the ultrasonic detection element 5 is connected with an ultrasonic detection system 11, the displacement sensor 19 and the load detection element are connected with a load-displacement signal detection system together, and the thermocouple 17 is connected with a temperature sensing system 8;
Step 5, before the test starts, pulse current is introduced by using a pulse current element 10 to enable the bipolar plate sample 9 to reach preset temperature and current density;
step 6, performing an electric auxiliary biaxial stretching test, controlling the upper support seat 12, the lower support seat 13, the left support seat 14 and the right support seat 15 to move simultaneously through a numerical control system on a double-pull testing machine, performing biaxial stretching at a set strain rate, and simulating an electric-thermal-force coupling failure scene under actual working conditions;
Step 7, outputting a load-displacement curve and a temperature gradient curve in the load-displacement signal detection system and the temperature sensing system 8, and combining abrupt points on the temperature gradient curve to position a layering sprouting area;
Step 8, decomposing the ultrasonic signals obtained in the ultrasonic detection system 11 through wavelet noise reduction and EMD, extracting characteristic parameters (such as reflection wave attenuation rate and harmonic distortion degree) related to layering in the ultrasonic signals, and predicting the crack growth of an interface according to the acoustic signals detected in real time;
Step 9, training a machine learning model based on historical test data, wherein the input parameters comprise a layering area ratio, a crack expansion rate, a temperature gradient curve slope and the like, and the Random Forest (Random Forest), the 1D-CNN+LSTM and XGBoost;
Step 10, outputting static damage classification, crack expansion rate and FLCnew safety threshold values by using a trained machine learning model to the layered area occupation ratio, the temperature gradient curve slope, the reflection wave attenuation rate, the harmonic distortion degree, the load curve second derivative and the like, so as to obtain a forming limit curve FLC under the current loading condition, superposing the forming limit curve FLC to a biaxial stretching strain field, generating a safety-danger area thermodynamic diagram, and marking a critical fracture area;
step 11, when the duty ratio of the dangerous area exceeds a threshold value, automatically reducing the electric pulse intensity or switching the strain path, delaying failure and recording the optimized forming limit parameters;
and 12, taking out the four clamping ends 1 and disassembling the bipolar plate sample 9 after the test is finished, and carrying out surface observation and microstructure observation.
The digital image related speckle strain measurement system in this embodiment comprises a DIC high-speed camera 20 and a DIC analysis system 21, wherein the DIC high-speed camera 20 is arranged right above the middle of the bipolar plate sample 9, the frame rate is not less than 500 fps, the resolution is not less than 2K, and black and white speckle patterns (with the spot size of 0.1-0.3 mm) are required to be sprayed on the surface of the bipolar plate sample 9.
Specifically, a high-contrast speckle pattern is sprayed on the surface of the bipolar plate sample 9, matte black primer and white specks (diameter is 0.2 mm) are adopted, so that the speckle coverage rate is ensured to be more than 60%, a DIC high-speed camera 20 acquires a surface deformation image at 500 fps during a biaxial stretching test, and the surface deformation image is transmitted to a DIC analysis system 21 in real time.
The DIC analysis system 21 incorporates a digital image correlation algorithm to calculate full-field strain distribution (e.g., green-Lagrange strain tensor) in real time, and outputs strain cloud images and high risk region coordinates.
In the embodiment, sub-pixel level analysis (precision + -0.05%) of the strain field on the surface of the sample is realized through the DIC analysis system 21, and interface layering positioning errors are reduced from + -1 mm to + -0.2 mm in the conventional method by combining thermocouple arrays and ultrasonic detection.
In this embodiment, the displacement sensor 19 may be a laser displacement meter or an extensometer, the ultrasonic detection element 5 is a solid couplant to realize stable connection with the bipolar plate sample 9, and the ultrasonic detection element 5, the load detection element 6, the displacement sensor 19 and the thermocouple 17 collect dynamic data at a sampling rate of 1 kHz.
As a further optimization, the bipolar plate sample 9 in this embodiment is cross-shaped.
The bipolar plate sample 9 in the embodiment is cross-shaped, multi-axis stress state simulation can be realized by adopting a linear cutting mode, a four-arm structure of the cross-shaped sample allows bidirectional tensile load to be synchronously applied on an X/Y axis, complex stress states (such as plane strain and bidirectional tensile combined loading during bipolar plate stamping) in actual stamping forming are reproduced, controllable excitation of interface layering failure is realized, a center area of the cross-shaped sample is designed to be a necking sensitive area, interface layering is artificially induced to be initiated in a preset area through geometrical transition and Joule heat effect coupling of pulse current, accurate monitoring of an ultrasonic detection element and a thermocouple is facilitated, stress distribution of a traditional sample (such as rectangle) is dispersed, and an interface failure starting point is difficult to position.
Specifically, when the bipolar plate sample 9 is cross-shaped, a plurality of thermocouples 17 are respectively arranged in the necking sensitive areas of the four extension sections of the bipolar plate sample 9, the thermocouples 17 of each necking sensitive area are arranged at equal intervals along the length direction of the extension section, and the thermocouples 17 can be fixed on the bipolar plate sample 9 by electric welding.
The foregoing disclosure is merely illustrative of preferred embodiments of the present invention, but the embodiments of the present invention are not limited thereto, and any variations within the scope of the present invention will be within the scope of those skilled in the art.
Claims (7)
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN102749253A (en) * | 2012-07-10 | 2012-10-24 | 首钢总公司 | Testing system and method of plate high temperature forming limit diagram |
| CN118243496A (en) * | 2024-03-14 | 2024-06-25 | 西北工业大学 | A metal sheet pulse current coupled ultrasonic multi-energy field double pulling experimental device and experimental method |
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| JP3905821B2 (en) * | 2001-11-08 | 2007-04-18 | 住友化学株式会社 | Biaxial tensile tester for film stretch observation |
| MX2008009816A (en) * | 2006-02-01 | 2008-09-11 | Nippon Steel Corp | Breaking prediction method. |
| JP2009236757A (en) * | 2008-03-27 | 2009-10-15 | Nippon Steel Corp | High temperature tensile testing method of bar steel material, fire-proof performance predicting method of non-fire-proof coated steel material, and steel material superior in high temperature strength |
| JP2013217693A (en) * | 2012-04-05 | 2013-10-24 | Shimadzu Corp | High temperature testing device |
| JP7217322B1 (en) * | 2021-09-01 | 2023-02-02 | Ｊｆｅスチール株式会社 | METHOD FOR DETERMINING FORMING LIMIT FOR SHEET METAL SHEET, FORMING LIMIT DETERMINATION SYSTEM AND COMPUTER PROGRAM |
| CN117330404A (en) * | 2022-06-23 | 2024-01-02 | 宝山钢铁股份有限公司 | Accurate prediction method of plane strain specimen width and simplified testing method of forming limit |
| CN119985079A (en) * | 2025-02-14 | 2025-05-13 | 江苏理工学院 | A local strain measurement method and system suitable for tension, compression and fatigue deformation of brittle metal materials |
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| CN118243496A (en) * | 2024-03-14 | 2024-06-25 | 西北工业大学 | A metal sheet pulse current coupled ultrasonic multi-energy field double pulling experimental device and experimental method |
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