Title of Invention

A METHOD OF MAKING COMPOSITES WITH PASSIVE VIBRATION DAMPING EFFECT

Abstract

"A method for passive damping on composite materials" This invention relates to a method of passive vibration damping composite materials such as Kevlar & Graphite composites with embedded Kevlar flexcore at ambient & varying temperatures using surface activation technique of Plasma etching along with thin hybrid layers (on one side only) of high sensitivity ferro-electrically soft & hard piezoelectric ceramic material layers. The surface activation is done using Plasma etching technique for getting better adhesion of piezoceramic material with composites. Hydrophilic polymers such as KFRP (Kevlar Fiber Reinforced Plastic) & Hydrophobic Graphite Composites were treated with Radio Frequency (RF) plasma to modify the surface properties such that they get reflected in the adhesion enhancement between plasma treated polymer surfaces and the thin piezoceramic material coating at elevated temperatures. It has been found that there is significant passive damping contribution at resonant frequencies from the thin hybrid piezoelectric coatings on one side of the substrate composite materials like Graphite & Kevlar fiber with embedded Kevlar flexcore at elevated space domain temperatures. (Fig 1)

Full Text

This invention relates to a method of making composites having passive vibration damping effect, composites made thereby and articles made therewith such as, but not restricted to, spacecraft reflectors. This invention is particularly directed to passive vibration damping of composites made of hydrophilic polymers and hydrophobic graphite. Background of the Invention Hydrophilic polymers that are subjected to vibration damping include fiber reinforced plastics such as Kevlar Fiber Reinforced Plastic hereinafter referred as KFRP and Graphite laminated composite materials. These composites are used in the development of small size spacecraft reflectors. These materials have high specific stiffness and low Coefficient of Thermal Expansion (CTE) and they are presently being used for spacecraft reflectors having a diameter of up to say 2m. However, these materials have certain limitations with respect to their usage in the development of ultra-light weight large size reconfigurable composite reflectors due to the high out of plane stiffness of laminates. More recently, it has been observed in literatures such as "Vibration Suppression of Laminated Composite Beams Using Embedded Magnetostrictive Layers" by A V Krishnamurthy et al, IE (I) Journal-AS, Vol. 78, March 1998, pp 38-44, that materials like piezoceramics, magnetostrictive materials & shape memory alloys have been investigated for active vibration suppression. Magnetostrictive materials like Terfenol-D rods have displayed good potential for active vibration damping purpose. Active vibration damping, per se may produce system instability for thin flexible reflectors. In most of these applications, mini actuators built using Terfenol-D rods are used as point actuators on the host structure for vibration control. With the advent of particulate piezoceramic / magnetostrictive composites, it is shown that such materials could also be used as an intelligent distributed layer over the host substrate to introduce distributed control of vibration. However, in general, these materials do not possess good inherent structural damping. Hence use of such reflectors having active control may lead to instability, if there is an error in the feed back control. It is expected that a thin layer of passive damping coating could enhance the stability of the system in such circumsta without any significant increase in weight. Thin hybrid layers of viscoelastic magnetostrictive powders can provide vibration damping for structural systems. Althc these materials can be used for damping of thin flexible structural systems, they 1 practical limitations of usage in the domain of microwave antenna applications du Electro-Magnetic Interference (EMI) & Electro-Magnetic Coupling (EMC) probleir higher transmit & receive radio frequencies. Efforts have been put in the direction oi of piezoceramic material coatings in lieu of magnetostrictive materials on the compo; to avoid EMI / EMC problems for microwave antenna applications. Moreover, elev damping levels of piezo coated specimens at higher temperatures also are desir qualities for space domain temperatures from the micro vibration control point of for the space structures. It has been observed that some ceramic and polymeric coatings yield signifi structural damping and there is a renewed interest in using them for passive vibra control. These materials dissipate vibrational energy mainly through magneto / elec elastic coupling. During vibration, the coating of such materials would undergo a ch. of strain initiating as movement of magnetic domains thereby dissipating the mechar energy through hysteresis. This damping capacity is dependent on the strain induce the material. It is observed that the ferromagnetic material has high damping at low st levels (around 50-100 µ strain). However, for structural vibration control, induced str are often to the tune of at least one order higher in magnitude. Hence, if , ferromagnetic material is used, the extent of damping achieved maybe negligible. Although these ferromagnetic materials can be used for damping of thin flex structural systems, they have sometimes the limitations, in that, such materials are suitable for microwave antenna applications due to EMI / EMC interference problerr higher transmit & receive frequencies. Passive damping of structural systems using hybrid layer(s) of soft viscoelastic or 1 ceramics having high damping characteristics has attracted attention of lot of research; Brief Summary of the Invention Managing micro vibrations in the process of development of thin light weight high frequency reconfigurable spacecraft reflectors subjected to milli 'g' vibrations is one of the key potential issues faced in this field due to the movement of the three axis, namely, spacecraft gyros, altitude correction exercises of spacecrafts and differential thermal gradients, which can adversely affect the performance of such spacecraft reflectors. The method of making composites having passive vibration damping effect according to this invention is thus far unexplored in this field. The invention discloses a novel method for providing passive damping on composite materials, such as KFRP and Graphite, which finds use in spacecraft reflectors. The surface of the composite material is activated by plasma etching, said etched surface is subsequently coated with a thin piezoceramic coating to damp these perpetual micro vibrations. The thin hybrid layer(s) of the piezoceramic materials when coated on a KFRP / Graphite substrates have resulted in significant passive structural damping at elevated temperatures. Further, the contribution of the thin hybrid piezoceramic layers toward the overall weight is almost negligible thereby maintaining the light weight requirements demanded by spacecraft reflectors. A preferred method of surface activation is the Plasma Corona surface etching treatment on the surface of the composite materials to provide better adhesion of the thin piezoceramic coat with the composite substrate thereby making the piezo coated KFRP & Graphite composites ideal for micro vibration control in the futuristic ultra light weight spacecraft reflectors. As significant damping has been observed at elevated temperatures in the tests carried out on piezo coated specimen, vis-a-vis baseline specimen, the thickness of the flexcore presently in use for the baseline specimens can be reduced or it can be totally eliminated for the same given design specifications. When the proposed piezoceramic coating is layered on bare composite skin per se it will eventually lead to the development of ultra lightweight small size spacecraft reflectors for space domain. This invention paves the way for futuristic ultra lightweight small size reconfigurable spacecraft reflectors capable of handling micro vibrations. The vibration damping characteristics of piezo-coated skins would get enhanced many folds, vis-a-vis, the bare or baseline reflector skins. Brief Description of the Drawings Fig la and lb shows Cross-sectional details of the Graphite & Kevlar specimen respectively according to the invention. Fig. 2a- 2h shows FRF Measurements on baseline & coated specimen at varying temperatures. Fig 3a-3h shows Composite Loss Factor calculations for baseline & coated specimen at varying temperatures Detailed Description of the Invention Keeping microwave reflector applications in mind, the instant invention provides a novel method for passive vibration on composite materials, such as Kevlar Fiber Reinforced Plastic (KFRP) and Hydrophilic Graphite composite materials but not necessarily restricted to the same, for their application in spacecraft reflectors. Thin high sensitivity ferroelectrically soft piezoelectric layers & hybrid piezo ceramic powder coatings (which are independent of Electromagnetic problems) are used in lieu of magnetostrictive powder coatings for vibration damping effects on a wide gamut of composite materials at varying temperatures. A combined hybrid approach of active-passive piezo layers system on composites, however, enhances the vibration damping. At elevated temperatures the adhesion of thin layers of piezoceramic materials on composites become a potential issue, which needs to be addressed at length. Surface Activation Of Composites By Plasma: Figure la shows the cross sectional dimensions of the Graphite specimen. Reference numeral (10) denotes the graphite skin and numeral (100) denote the Kevlar flexcore. Figure lb shows the cross sectional dimensions of the Kevlar specimen. Reference numeral (20) denotes the Kevlar skin and the numeral (200) denotes the Kevlar flexcore. The details of the adhesion technique adopted are briefly mentioned as follows for the Kevlar and Graphite laminates with embedded Kevlar flexcore specimens. Plasma etching is used to remove the unwanted materials at the molecular level. It provides uniform etching and is proven to be a superior technique compared to wet chemical processes. Plasma is an environmentally safe method for organic removal and surface modification. Ablation roughens the surface, increasing the total contact area between the adhesive and the substrate surface. Adding polar functional groups to the composite surface structure results in surface chemical restructuring which greatly increase the surface energy and associated adhesion to other materials. Applying an electric field using one of following three sources at different frequencies can generate plasma used for surface modification. •Low frequency: less than 100 KHz. • Radio frequency: 13.56 MHz • Microwave frequency: 2.45 GHz Frequency selection is one of the key parameters in this process. Low frequency is the least expensive method but unfortunately it is also the least efficient method for cleaning action and surface activation. MW source plasmas are generated downstream or in a secondary environment. Downstream is defined as plasma generated in one chamber and drawn by a vacuum differential into the work area or another chamber. This method produces a less homogeneous process, which results in the non-uniformity across the work area. In surface modification, the effective depth of the modification is tens of nanometers so the uniformity of the process becomes the most important. Higher concentrations of electronically charged particles are found in RF plasmas. RF plasmas have also been noted to be more homogeneous which helps in treating irregularly shaped and overly large objects. RF Plasma interaction with the surface of the work specimen causes several resultant effects, each of which has a reaction to the adhesion process. These effects are: • Organic removal • Cross-linking via activated species of inert gases • Ablation or Etching • Surface chemistry restructuring The details of the these effects are as follows: 1. Organic Contamination Removal The presence of organic contamination on the composite surface is a major problem that prevents adequate adhesion. Contamination may exist in the form of residues, mold release agents, anti-oxidants, carbon residues or other organic compounds. Oxygen plasma is excellent for removing organics and is commonly used for this purpose. Oxygen plasma causes a chemical reaction using surface contamination resulting in their volatilization and removal from the plasma chamber. Care must be taken in selection of cleaning process parameters to ensure that organics are completely removed. Critical parameters may include sufficient power density to remove but not polymerize the organics. In RF plasma, oxygen (O2) is fragmented into monatomic oxygen (O), O+ and O\O at the pressure around 0.1 torr. The reactive plasma species readily combine with any organic hydrocarbon and produce water vapor, CO and CO2, which is carried away in the vacuum stream. Whether or not the organic removal is completed can be confirmed through the contact angle measurement. 2. Cross-linking via activated species of inert gases Inert gas (helium or argon) plasma break C-C or C-H bonds by ion and vacuum ultra violet photon bombardment and create free radicals on the polymer surface. These free radicals in turn recombine and make crosslink bonds on the surface, which forms a barrier layer at the surface, thus, restrict the orientation of polymer chains. The cross linked layer also works as barrier layer and prevents the migration of polymer ingredients from matrix to the surface. 3. Ablation Ablation can be done with either active or inert gases. In this process energetic ions and photons break polymeric chains at the surface, which results in the removal of a few mono-layers from the surface. Roughening of the surface plays a significant role in adhesion by increasing the total contact area between the adhesive and the substrate surface. 4. Surface chemical restructuring By grafting of polar groups to the polymer chains at the surface, one can greatly increase the surface energy and thus aid the adhesion to other substrate materials. The plasma treatment eliminates the primer coatings for paint adhesion by surface preparation. In addition to providing the hydrophilic surfaces for adhesion, plasma treatment can also provide hydrophobic surfaces. Plasma Etching Process According to this invention, plasma polymers such as KFRP & Graphite Composites are treated with RF plasma to modify the surface properties such that they facilitate in the adhesion enhancement between the plasma treated polymer surfaces and the piezoceramic material coating. Within the stipulated 48 hours of the surface activation by RF plasma etching, the specimens are coated with the hybrid layer of piezo ceramic material (P) as shown in figures la and lb, the coating having a total thickness of 300 microns on one side of the specimen only, using boat grade epoxy resin. (Hybrid layer of piezoceramic material, PZT-5A 150 microns thickness + PZT-4 150 microns thickness). Vibration Test Procedure Each cantilever beam specimen was mounted in a clamp apparatus, which provides a firm boundary condition at the root end of the beam. The composite test specimens were bonded on edge with a very stiff structural adhesive to steel blocks, which were then clamped in the beam test fixture. Depending on the thickness and the stiffness of the particular specimen, data was acquired for bending modes in the frequency range of 20 Hz to 3500 Hz. Basic data acquisition technique used is as per ASTM E-756-98 and SAE J1637 {20] & [21] which are both vibrating beam technique methodologies. Excitation is provided at the free end of the beam using a non-contacting magnetic exciter. The response of the beam is measured with a piezoelectric crystal, which is mounted near the root of the beam. It is absolutely essential to minimize all extraneous sources of damping apart from damping introduced to the beam from the material that is being tested. Consequently, transducers that are extremely light and/or of the non-contacting variety may also be used to measure the response of the beam. The piezoelectric crystals have negligible mass and the electrical connection to them is made via very thin foil leads making them ideal to measure beam response. The test fixture is mounted in an environmental chamber so that the effects of temperature on the modal composite properties can be defined. The modal loss factor is estimated using the half-power bandwidth technique. For experimental estimation of modal loss factor, the specimens were tested for the "composite" modal loss factor (CLF) and resonance frequency across a temperature range of (- 70 °C) to (150 °C) in different temperature steps. Data Acquisition Resonance frequency and loss factor values are then collected from zoom transform measurements at each bending mode of interest. The modal loss factor is estimated using the half-power bandwidth technique. To acquire the test data, random noise is applied to the magnetic exciter via a power amplifier. The piezo electric crystal is used to monitor the response of the composite beam. The force imparted to the beam by the non-contacting magnetic exciter is approximated by monitoring the current in a resistor mounted in parallel to the magnetic exciter coil. The current across this resistor is approximately proportional to the force in the exciter coil as a function of frequency. The frequency response is measured by taking the ratio of the signal from the piezo-electric crystal (response of beam) and dividing it by the signal that is proportional to the force imparted to the beam. The frequency response of the beam is monitored over a range, which includes as many bending modes of the beam as possible. Piezoelectric Properties Following properties of the piezo materials as shown in Table 3 have been used in the Finite Element Modeling. Observation of the tests At very high temperatures, the piezoelectric coating applied to the graphite beam test article is observed to de-laminate near the end of the test. The piezoelectric coating applied to the KFRP test article also causes significant bending (curling) of the test article as temperature decreased from about -4 °C. However, unlike the surface treatment using normal epoxies and chemical etching processes, the piezoelectric coating applied by this inventive method does not peel off from the KFRP test article. The piezoelectric coating induces only minor bending (curling) of the beam test article for cold temperatures in the graphite beam. Data in the form of "composite" modal loss factor (CLF), was acquired for bending modes in the 100 Hz to 3000 Hz frequency range for the KFRP test specimen & graphite test specimens. A beam free length of 355.6 mm was utilized for all cases in order to maximize the signal quality of the Frequency Response Functions (FRF) utilized to estimate the modal damping in the bending modes of the test articles. The graphs shown in figures 2a to 2h shows the FRF measurements on baseline and coated specimens at varying temperature. This 355.6 mm free length was established via theoretical iterations on FEA software for modeling structures as mentioned above. In basic terms, the modal composite resonance frequency data describes the frequency content of the measurements and is also a general indicator of the stiffness of the composite beam, though the mass is also a factor affecting resonance frequency. The modal composite loss factor data describes the damping performance of the material as a function of temperature for the particular beam geometry. Estimation of CLF Figures 3a to 3h show the Composite Loss Factor (CLF) graph and calculations for baseline and coated specimen at varying temperatures. For all types of test specimen the calculation of the damping material properties theoretically requires the resonant frequency of each mode, the half-power bandwidth (3 dB down points) or modal loss factor of each mode, the geometric properties of the beam, and the densities of the materials comprising the specimen. Unless the specimen material is self-supporting, the calculation begins with determination of the frequency response of the uniform baseline beam. The results of the uniform beam calculations serve as input to the calculation of damping material properties. If the specimen material is self-supporting, the calculation ends with the results of the uniform baseline beam. The damping material's modulus (either shear or Young's) and loss factor can be estimated as per ASTM Standards with a single beam composite specimen vibrating in its several modes thus determining the properties as a function of frequency. By conducting the experimental tests at several temperatures, the properties are determined as a function of temperature. Observations and Results The method for passive vibration damping according to this invention proved that the surface activation done on the Graphite composite specimens by plasma Corona before carrying out the thin hybrid piezoceramic coating on the composites was effective up to a test temperature of 150°C after which the piezoelectric coating is de-laminated from the graphite test article at or near the conclusion of the test. The observations of the experimental investigation are that the composites having the thin piezoelectric material coating exhibit elevated damping levels for the high temperatures around 70°C (159° F). It is also seen that the damping is higher in the graphite beam than in the KFRP via the thin piezoelectric coating. Table-4 shows CLF calculations for the Graphite- Piezo coated ( Hot case) Experimental damping studies have shown effective increase in modal damping at resonant frequencies. The invention has shown that the damping in the baseline specimen is around a factor of 10 lower than the damping measured in the specimen with the piezoelectric coating at elevated temperatures. The modal loss factor / composite loss factor of 0.1 has been obtained at 70 degree centigrade. This is good damping contribution from the piezoelectric coating to the substrate materials. This invention has shown that Plasma Corona surface etching treatment for better adhesion of the piezo coat with the KFRP & Graphite composites substrate is ideal for futuristic spacecraft reflectors. Moreover, elevated damping levels of piezo coated specimens at higher temperatures also are desirable qualities for space domain temperatures (about 150°C) from the micro vibration control point of view for the space structures. We Claim: 1. A method of making composites with passive vibration damping effect comprising the steps of: subjecting composite materials such as polymeric and graphite composites to plasma etching; and coating the thus etched surface with a layer of piezoelectric material. 2. The method as claimed in claim 1, wherein the composite materials are etched by Plasma Corona surface etching treatment and the composites treated are hydrophilic polymers and hydrophobic graphite. 3. The method as claimed in claim 1, wherein the piezoelectric coating is ferro- electrically soft piezoelectric layer and piezoceramic powder coating. 4. The method as claimed in claim 1 or 3, wherein the coating thickness is about 300 microns on one surface of the composite material. 5. The method as claimed in claim 1 or 2, wherein the electric field required to produce the plasma is generated by a radio frequency power supply. 6. The method as claimed in claim 5, wherein the radio frequency is about 13,56 MHz. 7. The method as claimed in claim 3 or 4, wherein the said layer of hybrid piezoceramic material comprises 150 microns thickness of PZT-5A type ceramic and 150 microns thickness of PZT-4 type ceramic. 8. The method as claimed in any one of the preceding claims, wherein the said composite materials are fiber reinforced plastic and/or Graphite composites. 9. The method as claimed in claim 1 to 8, wherein said composite is graphite composite having a flexcore of fiber reinforced plastic. 10. Composites having passive vibration damping effect made by a method as claimed in claims 1 to 9. 11. Reconfigurable spacecraft reflectors made from composites having passive vibration damping effect as claimed in claims 1 to 9.

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1434-CHE-2006 AMENDED PAGES OF SPECIFICATION 31-08-2012.pdf

1434-CHE-2006 AMENDED CLAIMS 04-10-2012.pdf

1434-CHE-2006 AMENDED CLAIMS 17-09-2012.pdf

1434-CHE-2006 AMENDED CLAIMS 31-08-2012.pdf

1434-CHE-2006 CORRESPONDENCE OTHERS 17-09-2012.pdf

1434-CHE-2006 CORRESPONDENCE OTHERS 09-12-2011.pdf

1434-CHE-2006 EXAMINATION REPORT REPLY RECEIVED 04-10-2012.pdf

1434-CHE-2006 EXAMINATION REPORT REPLY RECEIVED 31-08-2012.pdf

1434-CHE-2006 FORM-1 31-08-2012.pdf

1434-CHE-2006 CORRESPONDENCE OTHERS.pdf

1434-CHE-2006 FORM-18.pdf

1434-CHE-2006 FORM-8.pdf

1434-che-2006-abstract.pdf

1434-che-2006-claims.pdf

1434-che-2006-correspondnece-others.pdf

1434-che-2006-description(complete).pdf

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1434-che-2006-form 1.pdf

1434-che-2006-form 26.pdf

1434-che-2006-form 3.pdf