Known cure monitoring techniques include differential scanning calorimetry (DSC), rheology, dynamic mechanical analysis (DMA), and thermal gravimetric analysis (TGA). These material characterization techniques may be utilized to monitor thermo-physical and mechanical properties of a resin during cure. Typically, the techniques must be conducted on a small sample in a controlled laboratory instrument.
Another known cure monitoring technique, dielectric analysis (DEA), utilizes a loss factor, ε″, to monitor the different phases of cure (e.g., in glass-fiber reinforced epoxy). The degree of cure measured from DEA correlates well with DSC and Raman spectroscopy for isothermal cure of in epoxy resins. During the resin transfer molding process, the maximum of the ionic conductivity indicated minimum viscosity. After viscosity reaches minimum, the first zero slope of the derivative of log of the ionic conductivity with respect to the cure time (DLIC) demarks the onset of gelation. DLIC also loosely correlated with degree of cure from onset of gelation to full cure and the DLIC plateau estimated vitrification when compared with DSC. Although these tests may be performed in a laboratory environment, DEA may be implemented to provide in-situ cure monitoring at production scale.
Conventional bulk wave ultrasound has also been implemented as a cure monitoring technique. For thermoset resins, ultrasonic velocity has been used to infer the degree of cure because of its association with the modulus of the resin. Bulk wave ultrasound, in pulse-echo mode, can monitor the completion of resin cure by measuring when the time delay plateaus (i.e., the ultrasonic velocity becomes a fixed value) in graphite/epoxy composites and epoxy matrices. Other ultrasonic phenomena have also been used for monitoring degree of cure including, attenuation (i.e., amplitude of signal), instantaneous phase, and the mean value of each frequency curve weighted by the maximum corresponding spectral amplitude.” Both contact and air-coupled ultrasonic techniques have been employed for cure monitoring, with the latter negating the need for resin-transducer contact with the composite being monitored. Air-coupled ultrasonic cure monitoring, however, must account for the exothermal behavior of the cure process as well as the alteration of acoustic air path. Because the temperature varies greatly throughout the cure process, the speed of the sound wave in the air changes accordingly. This change in the speed of sound in air impacts the measured data because the transducers are not in direct contact with the composite, and changes in sound velocity must be accounted for to determine the correct time of flight in the composite.
A guided wave approach in carbon fiber reinforced polymer (CFRP) laminates has been used to demonstrate that the group velocity of guided waves propagating normal to the carbon fiber increased as final degree of cure of the composite increased, and the expected porosity level decreased (wave propagation in this case was normal to the carbon fiber through the panel and not through the thickness). However, this work was done using three separate composite panels that had already completed the cure process.
One aspect of the present disclosure is a method of controlling a curing process for materials such as fiber reinforced composite materials. The method may include forming an uncured composite part from a plurality of layers of material having an uncured matrix and reinforcing fibers. An actuator is operably connected to a surface of the composite part at a first location, and at least one sensor is operably connected to a surface of the composite part at a second location that is spaced apart from the first location. The actuator and sensor are operably connected to a surface of the part directly or indirectly such that waves can be transmitted to (excited in) the part by the actuator, and waves propagating through the part can be detected by the sensor. The actuator excites waves in the composite part at the first location, and the waves propagate through the composite part due to internal reflection. At least one wave metric is measured at the second location by the sensor. At least one parameter of the curing process (e.g. cycle time, oven temperature, or pressure) is adjusted. The adjustment may be made before the composite part is fully cured. Alternatively, if a series of parts are being produced, the parameter may be adjusted after cure of a part is completed, during the cure cycle of the next part. The adjustment is based, at least in part, on a wave metric measured by the sensor. The part may comprise a composite part having a polymer matrix, or the part may comprise a polymer material that is not a composite.
Another aspect of the present invention is a method of in-process cure monitoring and control of fiber reinforced polymer matrix composite materials. The method includes exciting waves into a fiber reinforced polymer matrix material during curing of the polymer matrix material at a first location. At least one wave metric is measured at a second location that is spaced apart from the first location. The fiber reinforced polymer matrix material has acoustic boundaries that cause internal reflection of the wave to form guided waves that propagate through the fiber reinforced polymer matrix material from the first location to the second location. The method includes adjusting at least one parameter of the curing process based, at least in part, on a wave metric measured at the second location.
The present disclosure involves a “guided wave” approach to monitor curing of composite materials such as carbon fiber reinforced polymer (CFRP) composites. At least one piezoelectric actuator (transducer) is utilized to introduce/excite sound (vibration) waves into composite part at a first location (e.g. a first point on a surface of the part). The physical boundaries of the part cause the waves to reflect internally and propagate through the part to a second location that is spaced apart from the first location. Internally reflected waves are generally referred to herein as guided waves. During the curing process a piezoelectric sensor at the second location is utilized to measure the amplitude, time of arrival (“TOA”) or velocity, and other metrics of the guided waves. Additional sensors at other locations may be utilized to sense/measure wave propagation through other regions of the composite part. In general, the material properties of the composite part (e.g. a polymer matrix) change as the matrix cures, changing the propagation of the guided waves as the part cures. Data from the sensor(s) can be utilized to determine if the composite material is curing as expected (i.e. within an acceptable range), and the parameters of the curing process (e.g. time and/or temperature) can be adjusted before the curing process is completed to thereby provide improved consistency in the parts. The measured wave data may also be utilized to detect defects (e.g. voids in the matrix material). The curing process may be actively adjusted during a cure cycle of a part or prior to curing of a part if the wave data for a prior part shows that adjustment is required to minimize the formation of defects and/or to prevent formation of additional defects. If the detected defects exceed predefined acceptance criteria, the parts may be discarded. In this way, the need to conduct additional testing after a part is completed can be eliminated.
Key variables including composite material properties (viscosity and degree of cure) and phase transitions (gelation and vitrification) during a cure cycle may be identified from the amplitude and group velocity of guided waves. An automated cure monitoring system according to the present disclosure may include high-temperature piezoelectric transducers to excite guided waves through composite panels (e.g. Hexcel® IM7/8552 prepreg) during cure in an oven. Average normalized peak voltage, which is directly proportional to wave amplitude and inversely proportional to wave attenuation, may be selected as a metric to describe the guided waves phenomena throughout the entire cure cycle. The group velocity of the guided waves can be utilized to detect the transition of the matrix material from a rubbery state to a glassy state.
A guided wave system according to the present disclosure may compliment known cure monitoring technologies without completely replacing them. For example, dielectric cure monitoring has been used for cure monitoring in composite manufacturing of carbon-fiber reinforced epoxy composites. The guided wave system of the present disclosure may operate at higher frequencies than dielectric analysis, which may provide additional benefits such as potential defect detection.
The guided wave system of the present disclosure can be utilized in-situ in connection with known curing equipment and techniques. The guided wave system of the present disclosure is scalable from producing flat composite panels to full-scale complex structures utilized in aerospace and space applications (e.g., cylinders/barrels, wing skins, etc.). The guided wave system may include a robust linear array of high temperature sensors (piezoelectric transducers) incorporating modifications identified during development. Such modifications include the addition of five more sensors (original system had three), running with optimized equipment settings (e.g., actuation volts peak-to-peak and frequency), and more efficient data processing and display. The linear array of high temperature piezoelectric transducers may be replaced by a single multiplexed optical fiber with phase-shifted fiber Bragg gratings (PS-FBGs) which can sense wave signals during cure and in-service, enabling life cycle monitoring as the sensors remain embedded in the composite.
In contrast to known bulk wave ultrasound techniques that only provide information about a part directly underneath (or near) the ultrasonic transducer, a guided wave system/method according to the present disclosure interrogates a continuous wave path through the thickness of the panel along the line or path from the actuator to the sensor. The overall response of a large portion of the composite can be monitored instantaneously using a pair of piezoelectric transducers utilizing a guided wave approach.
The guided wave system/method of the present disclosure measures/estimates one or more variables such as viscosity, degree of cure, Tg, gelation, vitrification, and porosity during cure, and the process parameters (e.g. oven temperature, cure time, pressure) can be dynamically tuned/adjusted based on the measurements.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in
With reference to
With reference to
Referring again to
Upper and lower surfaces 24 and 25 of composite part 10 form acoustic boundaries that cause internal reflection of waves generated by actuator 30. Due to the internal reflection of the waves, the waves are guided from the first location (actuator 30) through the composite part 10 to the locations of the sensors 1-8. In the illustrated example, the composite part comprises 24 layers of prepreg composite material in which all of the fibers are oriented in the direction of the arrow 14. However, it will be understood that the composite part 10 may include numerous plies oriented in different directions as required for a particular application. Also, it will be understood that the tool plate and composite part 10 could have a variety of configurations, and the present invention is not limited to a flat or planar configuration as shown in
Wave metrics such as the amplitude and group velocity of guided waves in a CFRP panel 10 (Size: 610 mm×178 mm) during cure in real-time may be utilized to control the curing process. These metrics may be utilized to identify key variables such as composite material properties (viscosity and degree of cure) and phase transitions (gelation and vitrification) during a cure cycle. The transformation of the resin from the liquid to rubbery state is typically referred to as gelation. The subsequent transformation from the rubbery state to the glass state is defined as vitrification. A generalized time-temperature-transformation (TTT) diagram for an epoxy resin (e.g., Hexcel® 8552) is shown in
Testing/experimentation was performed to verify the process/control of the present disclosure. For comparison with experimental results, a semi-empirical cure process model was simulated with the specific cure parameters used in the experiment. The modeling flow chart (
The experimental setup for the automated guided wave system for in-process cure monitoring is shown in
Both panels 10 were interrogated during the entire cure period utilizing guided waves generated by actuator 30. A guided wave was excited into the plate 10 using a five-cycle Hanning windowed sinusoidal toneburst signal emitted from a waveform generator 32 (Agilent Technologies: 81150A) to an amplifier 34 (Krohn-Hite Corporation: Model 7602M) to a piezoelectric transducer 30 (Physical Acoustics Corporation: Nano-30 (ø7.9 mm, height: 7.1 mm, frequency range: 150-750 kHz, resonant frequency: 300 kHz)) that is rated for use up to 177° C. The amplifier 34 magnified the input signal to a peak-to-peak voltage of approximately 120 V. The plate response along the fiber direction was recorded by identical piezoelectric transducers/sensors 1-8 in a pitch-catch configuration on two oscilloscopes 36, 38 (Agilent Technologies: MSO9064A and Tektronix: MSO3014).
Fluorinated ethylene propylene (FEP) release film 20, 22 was placed on the top and bottom 24, 25 of the composite panel 10. Sensors 1-8 were bonded to a thin (0.1 mm) sheet of steel 23 (“caul plate”) that was placed on top 24 of the panel 10. The caul plate 23 prevents sensors 1-8 from being pressed into upper surface 24 of panel 10 during cure, while still allowing the guided wave to propagate in the composite to be measured. Sensors 1-8 were located at distances, x, from the actuator 30 of 57, 76, 95, 114, 133, 152, 171, and 190 mm, respectively. A vacuum bag 26 and breather cloth 28 covered the panel 10 and full vacuum was applied using a vacuum pump 40.
For testing purposes, an oven 12 was used to cure the panel. However, it will be understood that the guided wave system of the present disclosure may be utilized in connection with suitable devices such as an autoclave to provide the proper pressure (e.g. 690 kPa). A port in the back of the test oven (not shown) may be utilized for ingress/egress of cables (not shown). It will be understood that the example described herein demonstrates use of a guided wave system and process at elevated temperatures during cure, and shows that meaningful information can be derived from the results. However, the goal of this example was not to produce a finished part in a production setting. Thus, some of the conditions (e.g. pressure) that are typically used for production of finished components were not duplicated in this example.
The automation code for the sensing system of the test was written in MATLAB® and utilized the Instrument Control Toolbox to control both the waveform generator 32 and the oscilloscopes 36, 38. The algorithm is outlined in
The plate response at sensor 4 (
Referring to
In addition to the experimental study, a zero-dimensional (material only) simulation of the cure response (
In Equation 1.0 above, σ is degree of cure, {dot over (q)} is specific heat flow, {dot over (q)}baseline is the baseline heat flow, and HT is total reaction heat. Equation 2.0 is a time derivative of Equation 1. In Equation 3.0, η is the viscosity, R is the universal gas constant (8.314 J/K mol), T is temperature, σ is degree of cure, and the remaining variables are model fit parameters to rheology tests (η01=7.5×10−11 Pa s, η02=4.81×10−2 Pa s, E1=81908 J/mol, E1=13228 J/mol, σg=0.545, A=2.466, B=0.0, C=0.0, ηmax=1.0×106 Pa s).
A zero-dimensional analysis in RAVEN®, which assumes constant temperature through the thickness, was shown to have insignificant difference in the composite curing response with a one-dimensional (drill-through) analysis, which did account for temperature variation throughout the thickness. This is primarily because the composite panel is thin and the temperature gradient across the thickness is therefore negligible. For the one-dimensional comparison analysis, the recorded air temperature of the oven 12 and the average temperature of the two part thermocouples 27A, 27B (modeled as the temperature of the vacuum bag) were utilized as inputs to the model. Heat transfer coefficients were applied at the boundaries of the model which included each material layer listed in the materials and methods section. A heat transfer coefficient of 30 W/m2K was used throughout all simulations.
The glass transition temperature (Tg) was calculated using the DeBenedetto equation:
where σ is the degree of cure and λ=0.78, Tg0=−7° C., and Tg∞=250° C. which are model parameters for Hexcel® 8552 resin that are fit to experimental data during material characterization. Vitrification is the process in which the curing composite transitions to a glassy state. The onset of vitrification occurs as the Tg approaches the temperature of the part being cured (
V
peak(OTi,xj,ƒc
where OTi, xj, and ƒck denote the discrete oven time, location (i.e., sensor), and center frequency of actuation, respectively, at which the measurement was taken. For reference, the plate response in
This normalized the peak voltage values to a number between zero and one. The maximum value of one occurs at the peak voltage, and this maximum is not necessarily at the end of a cure cycle.
All of the sensors 1-8 measure the guided waves at an identical oven time in one measurement. However, because the system loops through the actuation center frequencies, measurements at different actuation center frequencies occur at slightly different oven times (approximately 30 second increments).
Vnorm,peak was first interpolated at defined oven times. Vnorm,peak was then averaged across sensor and center actuation frequency at defined oven times according to Equation 7.0:
The average normalized peak voltages, Vavg,norm,peak, are the averaged results from every sensor and actuation frequency throughout the entire cure cycle. This was done to remove sensor and frequency variation in the results allowing for cleaner interpretation and communication of the effect of curing on the amplitude of the guided waves.
x=c
g×TOA+B 8.0
where B is a constant of the linear fit.
The group velocity was then averaged across center actuation frequency (Equation 9.0) at the defined oven times and then normalized (Equation 10.0).
As with peak voltage, the group velocity values were first interpolated at defined oven times because measurements at different actuation center frequencies occur at slightly different oven times (about 30 second increments).
The key transition points during cure cycle can be identified utilizing a cure monitoring system and process according to the present disclosure. First, during the initial temperature ramp the viscosity of the resin is reduced (drops), and the resin begins to flow and the composite begins to consolidate. During this time, an increase in the average normalized peak voltage of the guided waves was observed (
When the resin begins to gel (during the ramp to 177° C.), the average normalized peak voltage starts dropping and continues to drop throughout gelation (
Also, the average normalized peak voltage increases throughout vitrification (transition from rubbery to glassy state) before reaching a plateau just prior to the end of cure (
The vitrification point denoted in both the simulation and experimental results was calculated by simulation from the semi-empirical model. In general, identifying the vitrification point solely from experimental results would be difficult. However, one can readily determine if vitrification is occurring or has occurred by the increase in amplitude and subsequent plateau during the transition from rubbery to glassy state. Similar to gelation, vitrification is a process and the vitrification “point” is the time prior to which one would describe the resin as rubbery and after which one would describe the resin as a glassy. It does not occur instantaneously at a single point in time.
These trends were observed in both cure cycles. One difference between the two cure cycles is the ratio of the average normalized peak voltage when the resin was in the liquid state (including at minimum viscosity) compared with the glassy state near the end of cure. The average normalized peak voltage was higher in the liquid state than the glassy state for the two-stage cure cycle (
Also, as the degree of cure (
It will be understood that the goal of testing described herein was not to produce a high quality composite panel, but rather to develop and test the system and quality of the signals recorded during cure at elevated temperatures inside an oven 12. Additional modifications may be made to ensure the parts fabricated utilizing the system/process of the present disclosure have no sign of mark-off in the regions where the sensors were placed above the part 10 on the caul plate 23.
Dispersion curves for the composite laminate during cure may be developed to account for the wavenumber, phase velocity, and group velocity frequency dependence throughout the cure cycle. Denser spacing of the sensors may be utilized to measure higher wavenumbers (smaller wavelengths) as with the A0 wave mode.
The measured wave metrics (amplitude, TOA, velocity) may be utilized to adjust a cure cycle parameter such as cure cycle time or temperature or pressure. The adjustment may be made during the cycle of a part. This adjustment to the cure recipe (cycle) may also be after a part is cured for a subsequent part. For example, if a large number of components are being produced in a production environment, the wave metrics can be measured during initial test runs of the parts, and the parts can be tested to determine the wave metrics that correspond to finished parts having acceptable physical properties. Also, testing may be done to determine the effect of variations in cycle parameters (e.g. curing cycle time, oven temperature, and pressure) on the wave metrics and the physical properties of the final parts. This can be utilized to determine acceptable/allowable ranges for the wave metrics such that the process will produce acceptable parts if the measured wave metrics fall within the allowable ranges. The allowable wave metrics may correspond to acceptable ranges for cure variables. However, it will be understood that empirical testing may be conducted to determine allowable wave metrics without determining the cure state (or other variables) corresponding to the allowable wave metrics.
During production, parts may be cured according to baseline cure cycle parameters and the wave metrics can be monitored during the cure cycles of each part. If the measured wave metrics for a particular part indicate that the part is not curing properly according to predefined criteria, the cure cycle parameters can be adjusted during the cure cycle. For example, the cure cycle time, temperature, and/or pressure can be increased from the baseline if the measured wave metrics indicate that the part is curing too slow such that the part is unlikely to achieve a complete cure or to have defects using baseline cure cycle parameters. Conversely, the cure cycle time and/or temperature can be reduced from the baseline if the measured wave metrics indicate that the part is likely to “over cure” or cause defects absent adjustment from the baseline.
Also, if the measured wave metrics show a trend during production, the cure cycle parameters can be adjusted to prevent the measured wave metrics from falling outside the acceptable range. For example, if the measured amplitude of the guided waves at a specified time during a cure cycle is within an allowable range for a series of parts (e.g. part numbers 490-500 of a production run), but the measured amplitude is steadily decreasing for each part 490-500, one or more cure cycle parameters may be adjusted for the next part (e.g. part number 501) to compensate for the trend and increase the measured amplitude of the guided waves. As discussed above, the effects (correlation) between cure cycle parameters and wave metrics (which may be correlated with material cure states and/or properties) may be determined empirically (or utilizing computer modeling). This information can be utilized to determine the required adjustments to the cure cycle parameters to correct the trends in the measured wave metrics.
The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein.
All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.
All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. Each range disclosed herein constitutes a disclosure of any point or sub-range lying within the disclosed range.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. As also used herein, the term “combinations thereof” includes combinations having at least one of the associated listed items, wherein the combination can further include additional, like non-listed items. Further, the terms “first,” “second,” and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity).
Reference throughout the specification to “another embodiment”, “an embodiment”, “exemplary embodiments”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and can or cannot be present in other embodiments. In addition, it is to be understood that the described elements can be combined in any suitable manner in the various embodiments and are not limited to the specific combination in which they are discussed.
It is to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present invention, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.
This patent application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/397,180 filed on Sep. 20, 2016, titled, “Automated Wave Guide System for In-Process Monitoring of CFRP Composite Laminates,” and to U.S. Provisional Patent Application No. 62/442,708 filed on Jan. 5, 2017, titled, “Automated Wave Guide System for In-Process Monitoring of CFRP Composite Laminates,” the contents of each application is hereby incorporated by reference in its entirety.
The invention described herein was made in the performance of work under a NASA contract and by an employee of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. § 202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. § 202, the contractor elected not to retain title.
Number | Date | Country | |
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62397180 | Sep 2016 | US | |
62442708 | Jan 2017 | US |