Recommendations and Guidance for the Development of a Process Specification for the Fabrication of Carbon Fiber Reinforced Epoxy Composite Structures

Transcription

1 DOT/FAA/AR-02/XX Office of Aviation Research Washington, D.C Recommendations and Guidance for the Development of a Process Specification for the Fabrication of Carbon Fiber Reinforced Epoxy Composite Structures Draft dated 22 July 2002 Final Report This document is available to the U.S. public through the National Technical Information Service (NTIS), Springfield, Virginia U.S. Department of Transportation Federal Aviation Administration other validation efforts. This is a review document only and is intended to be used by the WSU / FAA designated review team for commentary.

2 NOTICE This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The United States Government assumes no liability for the contents or use thereof. The United States Government does not endorse products or manufacturers. Trade or manufacturer's names appear herein solely because they are considered essential to the objective of this report. other validation efforts. This is a review document only and is intended to be used by the WSU / FAA designated review team for commentary.

3 1. Report No. DOT/FAA/AR-02/XX 4. Title and Subtitle 2. Government Accession No. 3. Recipient's Catalog No. Technical Report Documentation Page 5. Report Date Recommendations and Guidance for the Development of a Process Specification for the Fabrication of Carbon Fiber Reinforced Epoxy Composite Structures 6. Performing Organization Code 7. Author(s) Gregg Bogucki, William McCarvill, Stephen Ward, Norman Johnson, Richard Moulton, John Tomblin 9. Performing Organization Name and Address National Institute for Aviation Research Wichita State University 1845 North Fairmount Wichita, KS Performing Organization Report No. 10. Work Unit No. (TRAIS) 11. Contract or Grant No. 12. Sponsoring Agency Name and Address U.S. Department of Transportation Federal Aviation Administration Office of Aviation Research Washington, DC Type of Report and Period Covered Final Report 14. Sponsoring Agency Code 15. Supplementary Notes. 16. Abstract 17. Key Words 18. Distribution Statement 19. Security Classif. (of this report) Unclassified 20. Security Classif. (of this page) Unclassified This document is available to the public through the National Technical Information Service (NTIS), Springfield, Virginia No. of Pages 22. Price other validation efforts. This is a review document only and is intended to be used by the WSU / FAA designated review

4 ACKNOWLEDGEMENTS The following document was assembled with help from many individuals across many areas of expertise. The following FAA personnel are acknowledged for their individual contributions to this document: Curtis Davies Larry Ilcewicz Peter Shyprykevich David Swartz - 4

5 TABLE OF CONTENTS Page 1.0 INTRODUCTION Objective Background Certification Process Role of the Process Specification FABRICATION PRODUCIBILITY DEMONSTRATION GUIDELINES Discriminator Panel First Article Inspection Destructive Inspection FABRICATOR QUALIFICATION RECOMMENDATIONS SPECIFICATION RECOMMENDATIONS SCOPE APPLICABLE DOCUMENTS REQUIREMENTS Personnel Required Materials Required Equipment Facilities Tooling Required Procedures Process Instructions Tool Preparation Material Preparation Lay Up Procedures Cure Cycle Panel Identification Inspection Machining QUALITY ASSURANCE Responsibility for Inspection Inspection Monitoring Procedures Equipment Monitoring Procedures Materials Monitoring Procedures Facilities Monitoring Procedures Tooling Documentation Test Methods NOTES CONCLUDING REMARKS AND FUTURE NEEDS REFERENCES Appendix A Example Process Specification - 5

6 - 6

7 LIST OF FIGURES Figure Page 1 Building Block Approach LIST OF TABLES Table Page 1 Minimum Recommended Items to be Addressed for Personnel 2 Minimum Recommended Items to be Addressed for Materials 3 Minimum Recommended Equipment Control Items 4 Minimum Recommended Items to be Addressed for Facilities 5 Minimum Items to be Addressed in Tooling Control 6 Minimum Recommended Items to be Addressed in Procedures 7 Minimum Recommended Items to be Addressed for Machining 8 Minimum Recommended Items to be Addressed for Parts 9 Minimum Recommended Items to be Addressed for Laboratories - 7

8 EXECUTIVE SUMMARY To be added. - 8

9 1.0 INTRODUCTION 1.1 Objective This document recommends guidance and criteria for the development of process specifications for carbon fiber reinforced materials to be used on aircraft structures. These recommendations were prepared by a team of industry experts that have extensive experience with material specifications, part processing, qualification programs and design allowables. The purpose of this report is to establish a definitive set of recommendations to guide the development of new and revised composite prepreg process specifications for use in fabricating test panel laminates. Recommendations are given based on state-of-the-art processes currently used within the aerospace industry. This is intended to advance the work that has been done through previous FAA programs such as Advanced General Aviation Transport Experiment [AGATE]. These programs have established methodology for developing design allowable data, control of the establishment of them and sharing the database developed. This document can also be used to develop an industry approach such that the following goals can be achieved: Greatly reduce the number of material and process specifications for identical composite material systems. Develop property databases that uniquely define a given material Establish material batch testing and process monitoring sufficient to minimize variability and preclude property changes over time. Reduce costs through common documentation and shared databases of basic material properties This document complements the recommendations and guidance for composite prepreg material specifications in Reference 1. Section 5.0 of this document list recommendations in a format that parallels the typical sections found in a process specification. A list of areas needing improvement and enhancement is given in Section 6.0. Appendix A contains a template for a process specification. 1.2 Background Unlike structural parts that use metallic materials in the manufacturing process, the material properties of a composite are manufactured into the structure as part of the fabrication process. Therefore, it is essential that material and process specifications used to produce composite materials contain sufficient information to ensure that critical parameters in the fabrication process are identified to facilitate production and adherence to standards in the final engineered part. Due to the wide variety of composite aircraft structures now emerging for certification at the same time composite - 9

10 applications are expanding for other industries, control of the materials is rapidly becoming a vital issue with respect to the overall assurance of safety. In recent years, NASA, Industry and the Federal Aviation Administration [FAA] have worked together to develop a cost-effective method of qualifying composite material systems by sharing central material qualification databases such as Mil-Handbook-17 and AGATE. Through these shared databases a manufacturer can select an approved composite material system to fabricate parts and perform a smaller subset of testing to a specific application. For materials to be accepted into these shared databases, it is required that the raw materials be manufactured in accordance with a material specification which impose control of the key physical, chemical and mechanical properties, and processed in accordance with a process specification that controls key processing parameters. Steady growth in the use of composites has continued in transport aircraft and rotorcraft. General aviation has emerged recently with the growth of new composite aircraft and composite material applications in primary structures. Several new composite aircraft are undergoing the certification process and many more aircraft are currently undergoing the design and development processes that take advantage of composite materials for primary structural applications. With this growth of composite applications, certifications issues have emerged with respect to the exact philosophy of quality control and quality assurance methods needed to guarantee a safe and consistent material supply. 1.3 Certification Process The objective of the composite aircraft structure certification process is to validate that the design meets the intended use requirements and application. In this context, the design validation process (to establish by proof) is accomplished through verification (to prove by evidence) and qualification (to define attributes or characteristics) of the materials, processes, and analysis tools. A widely acknowledged validation process used within the composite aircraft industry for the substantiation of composite structure is called the Building Block Approach. The Building Block Approach, Figure 1, is a process using analysis and associated tests of increasing structural complexity. The Building Block Approach is integrated with supporting technologies and design considerations. Refer to MIL-HDBK-17F, Volume 3, Chapter 4 for a complete description of the Building Block Approach. A key technology supporting the Building Block Approach is material and process specifications. The material and process specifications are inter-woven through out the certification validation process. Material specifications are used to define the material s attributes and define the qualification characterization tests (see Ref. 1). Materials used within the building block tests are purchased in accordance with the material specification. The material specification is then used for the procurement of production material to ensure the delivered aircraft use materials that are of the same quality and performance standards used in the certification validation process. Process specifications define and control the processes used for the conversion of materials into structural components. It is widely accepted that the performance properties of composite laminates are directly determined by the specific processes used for their fabrication. It is critical that the test - 10

11 specimens fabricated through the various levels of the Building Block Approach utilize the same process, and that this process is representative of the process that will be used in the fabrication of production aircraft/rotorcraft. Figure 1 - Building Block Approach [Ref: MIL-HDBK-17F, Volume 3, Chapter 4] A key element of the validation process during the coupon level of the Building Block Approach is material qualification. It is during qualification that the composite material is fully defined and characterized. Qualification tests are planned and conducted to: Define material attributes, Establish material performance properties, and Verify material characteristics comply with the intended application. The objective in defining material attributes is to establish the material property limits. Examples of attributes in which limits are set include: Resin content Fiber areal weight Cured per ply thickness Fiber volume. These attributes define the material and its resulting performance properties. Other attributes which are often over-looked are related to the physical structure of the material which affects processing characteristics. Example attributes of this type include: Fiber sizing level and type Level of impregnation Resin impregnation method (hot melt film or solution) Width tolerance Backing material selection. - 11

12 Performance properties are established, or made stable, through statistically significant testing. It is imperative that the materials natural variability is captured at this time. The objective is not to meet a desired level of performance, but rather establish the true performance of the material. Mechanical properties are typically thought of as the only performance properties. But, there are other performance related properties that have a direct bearing on the more familiar mechanical properties. These properties would include: tack or handling characteristics kinetic behavior, rheological behavior, sensitivity to ambient moisture and temperature (out-time effects), and resistance to fluids and solvents. Multiple material batches (typically a minimum of three to five) are tested to establish the true material variability. Properties results obtained from these sets of tests are used in the establishment of requirement values within the material specification (see Ref 1). Two often over-looked parts of the material qualification process are (1) verifying that the full scale production processes yield laminates (components) that comply with the performance requirements as established through allowable generation and design analysis, and (2) verifying that the facility chosen to fabricate production components uses processes that yield performance properties equivalent to those developed during the material qualification. This is accomplished by a thorough evaluation of the first full scale production component, process validation trials, and the performance of fabricator qualification tests. 1.4 Role of the Process Specification The objective of the process specification is to provide a means by which engineering requirements can be documented and communicated to the various organizations involved in the fabrication of composite laminates. These engineering requirements are specific to the manufacture of high quality and consistent composite laminates. It is imperative that the process specification be clear and complete to ensure that the resulting laminates are consistent in quality. All process specifications contain requirements and procedures to be used in the fabrication process. This is the intent of specifications, to flow down and specify any regulatory (or engineering) requirements or procedures that are necessary for the fabrication process. The level of detail contained in these requirements and procedures is a matter of style. Specifications can be organized in the following styles: (1) detailed process requirements and procedures, (2) end item requirements only, or (3) a mixture of process requirements and procedures, and end-item requirements. In the detailed process requirements and procedure style the specification specifies detail fabrication procedures which when followed will produce components that fully meet the end-item requirements. In the case of the end-item requirements style, no detail procedures are defined. The fabricator may use any process as long as the final component complies with the end item requirements. This usually requires extensive testing or inspection of the produced component to ensure compliance with the end-item - 12

13 requirements. This style is not recommended for the fabrication of laminates to be used in test programs. The last style is a mixture of the first two styles. Detailed process requirements are used in conjunction with general procedures, followed by end-item requirements. Recommendations presented in this document support the last style. In some cases, specific procedures will not be defined, but rather require that an internal procedural document be generated. An example of this would be an inspection plan. Appendix A contains an example composite fabrication process specification based on the recommendations presented. A process specification for fabrication of production hardware will contain additional requirements beyond the example given in Appendix A to address issues related to complex contoured parts, acceptance limits for defects (all defect sizes are usually unacceptable for test panels), and part tooling, bagging and cure procedures. 2.0 FABRICATION The fabrication of consistent test panels during the certification process is critical. Consistency must be maintained with respect to the various physical characteristics (thickness, ply orientation, and flatness), but also with respect to cure state and defect level. Data obtained from the test panels is used to establish the mean values and statistical parameters for the material properties. Therefore it is critical that the fabrication process used to fabricate the test panels is stable and consistent; however it is equally critical that it is equivalent to the full-scale production processes. The establishment of and adherence to a process specification helps ensure consistent test panel fabrication. In addition, as changes are introduced to the material manufacturing process or to the component fabrication process, test panels are fabricated and tested to assess the impact of the change on performance properties (see the Material Changes section of Reference 1). Example changes include: cure cycle modifications, out-time evaluations, and impregnation process parameters. The test panels are used to assess the impact of environmental effects, such as: fluid exposure, moisture, and thermal cycles. While the test panel process must be consistent, it must also capture as much as possible the process variables found in the component fabrication process. Cure heatup rate is an example where the test panel process will not necessarily match the component rate due to differences in part and tool masses, unless deliberate steps are taken to match them. A key role of the test panel fabrication process specification is to require complete documentation of the specific process parameters used to fabricate the test panels. This allows consistency for panels fabricated years after the original qualification test panels art fabricated. - 13

14 3.0 PRODUCIBILITY DEMONSTRATION GUIDELINES 3.1 Discriminator Panel As a rule thin flat constant thickness test panels are used to develop material performance properties. These small constant thickness flat panels do not discern material attributes that can impact ability to fabricate large-scale laminates. To fully evaluate a composite prepreg material, scale up effects must be addressed. Processing scale up includes laminate thickness, part area, and internal ply drop off features. Mechanical properties obtained from flat test panels will not assess the impact of processing scale up effects. But these scale up effects have a major impact on producibility of large-scale components. Tack is an attribute difficult to measure and will not necessarily impact the fabrication of test panels. The effect of tack differences can only be assessed through the fabrication of large full-scale contoured laminates. The need exists for a panel design that will discriminate these material changes, but it must be cost effective to fabricate, so that it can become part of any material and process evaluation. The objective of the Discriminator Panel is to distinguish one material from another similar (or like) material by exposing the differences related to fabrication processes. The difference in material attributes is often quantified by the amount of porosity within the discriminator panel. Material that has been found to yield porosity free test panels will sometimes yield significant porosity when the material is used to fabricate complex, contoured components. Sandwich configuration discriminator panels are often used to determine the propensity of a material to exhibit core crush during the cure cycle. The discriminator panel must be representative in thickness and contour, and contain ply drop off features representative of the proposed component designs. It must fabricated with representative of production methods (e.g. do not use hand collation for the panel when the production method is automated tape laying). The discriminator panel can be used in conjunction with test panels to asses the impact of processing or material changes. 3.2 First Article Inspection The objective of the First Article Inspection (FAI) is to verify that everything has come together (specifications, tooling, process instructions, process parameters, and design details) to produce an acceptable part. The FAI is performed to verify form, fit, and function. This is normally accomplished by nondestructive inspection in conjunction with an expanded dimensional inspection. Measurement of Key Characteristics is included in the FAI. The FAI should be performed on all components. 3.3 Destructive Inspection The objectives for performing a destructive inspection on a part are to: Verify the performance properties establish during coupon and element level testing (qualification and allowables) are the same in the component. Quantify internal (hidden) defects or indications detected by nondestructive inspection, i.e. validate nondestructive inspection methods. Validate laminate physical properties (resin content and thickness). - 14

15 Verify fiber path continuity within joints and complicated geometries (typically features that can not be verified through a Discriminator Panel or by nondestructive inspection). Geometric features that can only be evaluated by a destructive inspection are cocure joints. Cocure joint quality is strongly dependent on the tooling approach. It is only through the fabrication of full-scale hardware with the actual production tooling that cocure joint strength can be evaluated. The destructive inspection process includes: Full dimensional inspection. Nondestructive inspection. Section cuts through areas indicted by the nondestructive inspection. Excising coupons for mechanical property testing. Coupons typically excised include: glass transition temperature, degree-of-cure by DSC, resin content, short beam strength, compression strength, open hole tension or compression, and flexure. If other fabrication techniques, such as bonding or mechanical fastening, are used they should also be verified. The development of requirements for those additional fabrication techniques is beyond the scope of this document but should be included in the destructive inspection process. Destructive inspections should be performed on Part Families. While an FAI should be performed on each part, a destructive inspection is typically performed on one part that is representative of a given process, design, and tooling approach. The destructive inspection is repeated for major changes with tooling, design, fabricator location (or company) or process. A Discriminator Panel can be used to assess material changes and minor process changes. The destructive inspection does not have to be performed on the first assembly (component) produced, but it should be performed such that the results are evaluated prior to the assembly of the component on the first aircraft assembled. 4.0 FABRICATOR QUALIFICATION RECOMMENDATIONS During most certification efforts the initial test panels, elements, and components fabrication is performed at a different location or fabricator than the production components. This difference in fabricator or facility could result in material property values that are not representative of those produced by the full-scale production facility. It is recommended that the fabricators be qualified to validate that their processes yield properties that are from the same statistical population as the qualification and allowables data. The fabricator validates their processes through fabrication and testing of test coupons, discriminator panels, FAI, and destruct articles. The process of validating the material properties for an alternate process and/or facility is termed equivalency testing (see Reference 2 and the Development of Material Controls section of Reference 1). An audit to the process specification requirements is also recommended as part of the fabricator qualification process. - 15

16 5.0 SPECIFICATION RECOMMENDATIONS The recommended Process Specification outline is as follows. 1. Scope 2. Applicable Documents 3. Requirements 3.1 Personnel 3.2 Required Materials 3.3 Required Equipment 3.4 Facilities 3.5 Tooling 3.6 Required Procedures 4. Quality Assurance 4.1 Responsibility for Inspection 4.2 Inspection 4.3 Documentation 4.4 Test Methods 5. Notes 5.1 SCOPE The scope section defines the purpose or application of the specification. Guidelines presented herein are requirements and procedures specific for the fabrication and acceptance of carbon fiber reinforced composite laminates. Laminates processed in accordance with this specification are used in the qualification of new materials or establishment of mechanical property equivalency. 5.2 APPLICABLE DOCUMENTS This section of the process specification lists all the supporting documents, reports, specifications, or standards referenced within the process specification. In addition to listing the documents, sources for the documents are specified. 5.3 REQUIREMENTS The requirements section defines the process procedures and end item requirements Personnel In this section personnel training requirements are specified. Highly skilled technicians are necessary for the fabrication of quality laminates. Within some companies, only a small highly trained group of technicians are allowed to bag laminates to ensure that there are no bag failures during cure. This practice highlights that skilled craftsmen are required for the fabrication of composite laminates. Fabricators are encouraged to establish a comprehensive composites training program. Technicians should be required to pass both a written and practical proficiency test to validate they have the necessary skills and knowledge to fabricate quality laminates. Technicians should be qualified for each of the major fabrication processes (layup, bagging, cure, NDI/NDT, machining) that they will be performing. - 16

17 It is strongly recommended that a mentoring relationship be established between the company s technicians and engineering personnel. This mentoring relationship is best if the flow of knowledge is in both directions. Table 1: Minimum Recommended Items to be Addressed for Personnel Cleanliness, eating, drinking, smoking requirements Clothing, type of Experience Inspection personnel, ratio to manufacturing personnel Level of training Personnel status identified, qualified or unqualified Required Materials Requirements specific to the materials used in the fabrication process are defined in this section. All materials (and their sources) required for the fabrication procedures are listed within this section. The listing includes both consumable and structural materials. In cases where equivalent materials may be used, it should be stated. Materials that come in contact with the composite prepreg should be evaluated to verify they do not contaminate the prepreg material. It should be verified that all materials that have the potential to become a foreign object within the laminate can be detected by nondestructive methods that will be used on production parts. Prepreg materials are to be inspected upon receipt by the purchaser. Test methods, types of tests required, sampling requirements, criteria (acceptance value), and retest provisions are to be clearly defined. The prepreg inspection requirements can reference the applicable material specification. Supplier certification records should be reviewed and maintained with the laminate process instructions. Material that does not meet the established quality requirements should not be processed. Prepreg material freezer storage conditions (temperature) and shelf life are to be defined. Procedures for the disposition of out of date material also need to be defined. Experience has shown the extending shelf life by testing the material is not cost effective. In effect, material is released for use that has a shelf life in excess of the tested material. The shelf life of current materials exceeds 12 months, a time period sufficient for any component fabrication shop to manage their inventory. Ambient working life (or sometime called out time) should be defined. The time period should be associated with a defined temperature and relative humidity range. This time period should have been determined during the material qualification test program. Definition of this environment is critical. C-staged polymer resins are perishable at ambient temperatures, i.e. they continue to react. This continuing reaction could in time reduce the kinetics and flow characteristics of the material, thus affecting processability - 17

18 and properties. The time limit within which the material will produce quality laminates is a necessary requirement. Prepreg handling properties are a function of temperature. Too high a temperature and the material is sticky, making it difficult to position and handle the plies. Too low a temperature and the material is stiff and again difficult to work with. Some classes of resins are susceptible to moisture in that the water inhibits cure kinetics. Therefore controlling the relative humidity is important to ensuring the resulting laminate is representative of the material. High humidity will also increase the tack of the material. Prepregs with high tack have been shown to trap air and moisture between the plies, resulting in porosity in the laminates. Complete records should be maintained that document traceability of the fiber, resin, prepreg and laminates. These records should also document the total shelf life and out time of the prepreg materials up to the time of panel cure. Table 2: Minimum Recommended Items to be Addressed for Materials Kitting Materials Bagging of kitted materials Environmental conditions for kitting, cutting area Kit status identified [kit approved, unapproved] Method of cutting [hand, ultrasonic, Gerber cutter] Method of identification of material, plies, out times, storage conditions Unapproved kits, segregated Storage of Material Issuing materials Storage conditions [temperature, humidity, times] Storage location Storage monitoring system Rotation of materials Status of materials[approved or rejected] Transportation of Material Method of transportation Transportation containers/padding/protection Transportation environment Transportation personnel training Required Equipment The equipment necessary to perform the process specified within the specification are listed in this section. Sources for the equipment are also listed. Calibration and certification requirements are defined. Equipment requiring calibration and certification - 18

19 include: ovens, autoclaves, thermocouples, vacuum gages, and ply warming devises (e.g. hot air guns). Table 3: Minimum Recommended Equipment Control Items Access by other personnel not associated with autoclave or lay-up Autoclave status identified [approved, unapproved] Charts, type, recording of times, heat, pressure, vacuum of identification of nonconformance, back to part Control, location [heat, time, vacuum, vent] Controls, increments [heat, time, vacuum, vent] Controls, number of[heat, time, vacuum, vent] Controls, type of [heat, time, vacuum, vent] Cover gas, purity Fixtures [holding, tool] Heat survey intervals Heat surveys Length of time to cool temperature [load dependent] Length of time to reach cure temperature [load dependent] Mass, tools & parts Vs autoclave size Mass, tools Vs autoclave size Operating pressures [max, min] Operating temperatures [max, min] Overall configuration Proximity to lay-up area and staging area Rate of cool down Rate of heating Size of autoclave Thermocouples, type of, connections, number of Vacuum source Vacuum hoses Vacuum ports Venting, method of Facilities Collation (layup) of plies should be performed in a clean and controlled environment. Good house keeping procedures should be followed along with controlling the room temperature and relative humidity. The temperature and humidity requirements should align with the ambient out time requirements. Work areas are to be cleaned on a regular - 19

20 basis and inspected for potential foreign objects. Mold releases or other silicone containing materials should not be allowed into the room. Sanding, machining, or any other operation that generates dirt, dust or other debris is not allowed in the room. The inclusion of a positive pressure ventilation system in the layup room is an effective means of maintaining a clean environment. Table 4: Minimum Recommended Items to be Addressed for Facilities Lay-Up Area Access by other equipment [gas powered fork lifts] Access by other personnel [non-lay up] Air flow Environmental control, equipment for monitoring Contamination by other processes [chemical processing, painting, sanding] Floors [treatments, cleanliness] Humidity [max, min] Isolation from other contaminates Lay up area status [approved, unapproved] Lighting [lumens] Particulate count Pressure [positive] Proximity to staging area and autoclaves Temperature [max, min] Vacuum hose status [approved, unapproved] Walls [treatment, cleanliness] Staging Area Before Cure Access by other personnel not associated with autoclave or lay-up Equipment such as hoses, port status [approved, unapproved] Proximity to lay-up area and autoclaves Staging area status identified [approved, unapproved] Temperature [max, min] before cure Time [max, min] before cure Vacuum [max, min] Tooling Tools used in the fabrication process should be designed for the panel process conditions (e.g. cure temperature and pressure). Surface finish and flatness requirements should be defined (as applicable). All tools are to be clearly identified. Tool storage conditions need to be defined that ensure the tools are not damaged with - 20

21 time. A method for accurately positioning the plies is required. It is imperative that the ply orientation is within the engineering requirements, as strength and modulus properties are sensitive to orientation. A method must be used that allows for transfer of the tool zero direction to the panel and then to the machining equipment. Scribe lines on the tool and thin metal strips embedded along one edge of the panel are two methods that have been successfully used in the past. Table 5: Minimum Items to be Addressed in Tooling Control Method of cleaning, solvents, cleaning cloths Mold release agents Scribe marking Template inspection intervals Template surface conditions Templates, made from Templates, number of Tool heat up rate Tool inspection intervals Tool surface conditions Tool, method of moving, transportation Tooling condition [mold release applied, no mold release] Tooling configuration [flat, vertical, deep-v s] Tooling status identified [approved, unapproved] Tooling storage conditions and locations Tooling, expansion & contraction rate Vs composite materials Tooling, made from Vacuum ports, location, number of Warp clock Required Procedures This section defines the procedures required to fabricate quality laminates. These procedures need to be detail in nature to ensure consistency. - 21

22 Table 6: Minimum Recommended Items to be Addressed in Procedures Bagging configuration, detailed Bagging materials defined Bleeding materials defined Breathing materials defined Bridging techniques Check valves Creating channels leading to vacuum ports Cure cycle [time, temperatures, venting] Damming materials Damming techniques Environmental conditions of LAYUP AREA [see above] Gloves, type, use of Handcreams, foreign contamination Handling and transportation of parts, type of, techniques Inspection, pprocess [NDT, test coupons, tap] Kitting [see above] Lay-up techniques [hand, winding, machine] Location of vacuum ports Mandrel materials Mandrels, use of Method for marking ply location Method for using heat guns Method of check vacuum Method of thawing raw materials [time, temperature, humidity] Mixing of materials [suppliers, types, lots/batches] Number of layers of bagging, breathing and bleeding materials required Out time for raw materials Part traceability to drawing, specifications, revisions, materials, personnel, equipment, part status [acceptable, pending, unacceptable] Personnel [see above] Process specification, current revisions, method of updating, schedule of release Process specifications, available to all personnel Release agents [type, application of, curing, removing] Retest intervals for raw material Shop air, in line, method of checking contamination and moisture - 22

23 Storage conditions of raw material Storage time [max] for raw material Test sample configuration [lay-up] Test sample location [separate coupon, part of part] Testing methods for processed materials/parts Testing of process materials/parts Thermocouples, type of, connections, number of Type of heat guns [max temperature] Types of release materials and proper use [tedlar, perforated tedlar, bondable tedlar] Vacuum port status Vacuum pressure Process Instructions Detailed step-by-step planning instructions are to be prepared for each panel fabricated Tool Preparation Detail tool preparation procedures are defined in this section. Areas to cover include: tool inspection, verification that all tooling details are available and in good working condition, mold release application procedure (acceptable mold releases should be listed in the materials section), and tool clean-up procedures Material Preparation Procedures in this section cover the warming of the prepreg, ply cutting procedures, and ply handling procedures. Frozen prepreg in sealed containers must be warmed to room temperature prior to opening to prevent the condensation of moisture onto the prepreg. Plies are to be cut on surfaces specifically dedicated for the cutting of plies. Plies should not be trimmed or cut on the tool to ensure the tool or underlying plies are not damaged. If trimming of plies on the tool is required, a metal shim should be placed between the prepreg and tool surface. Individual plies are to be identified at the time of cutting Lay Up Procedures The plies are collated onto the tool as defined in the detail process instructions. Care should be taken to accurately align the plies with respect to the tool zero direction reference mark. Prior to collation the plies are inspected for visual defects. Damaged plies are repaired or replaced as applicable. Plies are debulked (compacted with vacuum pressure) as applicable. After each ply is collated, it s surface is inspected for foreign objects. The panel is bagged for cure as specified in the detail processing instructions. The panel is clearly identified to ensure traceability. Thermocouples are placed such that the panel temperature can be directly measured. - 23

24 Cure Cycle The panel is cured to the applicable cure cycle. Prior to cure a leak check should be performed on the vacuum bag. All leaks should be repaired before performing the cure cycle. The cure cycle should define heat-up rate, temperature range, time, vacuum, pressure, and cool down rate. The cure time and temperature are to be based on the slowest thermocouple in the cure run. Temperature, vacuum, and pressure are to be recorded as a function of time for the complete cure cycle Panel Identification Each panel is to be identified with a unique identification number. This identification number will provide traceability to the requesting document, prepreg batch number, cure cycle, and test type. Lines are also drawn at an angle across the laminate surface to aid in identifying specimen location within the panel Inspection After cure it is imperative that the panel is inspected to ensure that it meets all engineering requirements prior to machining of test specimens. Inspections or tests to be performed on each panel to verify they are acceptable for progression to the machining step include: panel thickness, surface flatness, completeness of cure, and void content. The testing of panels, which fail to meet these requirements will result in data that is not representative of the actual material properties. The panel should be visually inspected for surface defects that could be sources for premature test failures. The panel density and void content should also be determined by water displacement. Void content (or porosity) is also determined by nondestructive ultrasonic methods, or image analysis of polished edges. Nondestructive inspection standards should be fabricated as early as possible within the certification process. These nondestructive inspection standards are to contain known defects so that the nondestructive methods and criteria can be calibrated to defect severity. It is imperative that nondestructive inspection indications (such as sound loss or attenuation for ultrasonic through transmission) are tied to a known physical defect type, location, and size. Panel thickness is verified by measuring the panel thickness at a minimum of ten locations. The actual thickness is converted to a per ply thickness by dividing the measured thickness by the number of plies. The panel should not be tested if it fails to meet the required per ply thickness. The thickness data can also be interrogated to determine degree of flatness. Again, failure to meet engineering requirements, results in the panel not being tested. Completeness of cure is verified by measuring the glass transition temperature (Tg) and residual heat of reaction. Tg is measured by a variety of methods (TMA, DMA, RDA, and DSC). DSC is used to measures residual heat of reaction. Ply lay-up orientation is verified by polishing an edge for examination under a microscope (20 to 50X). - 24

25 Also, all records should to be reviewed to verify the panels were fabricated in compliance with the engineering requirements. The records should include: material traceability, panel identification, cure cycle parameters, and ply count Machining Machining diagrams are to be developed for each panel and included in the detail processing instructions. Type of machining equipment used to machine the panels into specimens is recorded along with cutting tool type, speeds, and feed speeds. After machining the specimens are inspected for damaged edges and dimensionally inspected. Specimens failing to meet engineering requirements are to be replaced. Table 7: Minimum Recommended Items to be Addressed for Machining Coolants, type of, use of Marking of trim lines Part storage Tools for trimming, hole drilling, type of [router,waterjet] Tools, control of cutting surfaces Tools, inspection intervals Table 8: Minimum Recommended Items to be Addressed for Parts Storage of Parts Issuing parts Storage conditions [temperature, humidity, times] Storage location Storage monitoring system Rotation of parts Status of parts [approved or rejected] Transportation of Parts Method of transportation Transportation containers/padding/protection Transportation environment Transportation personnel training - 25

26 Table 9: Minimum Recommended Items to be Addressed for Laboratories Laboratory approval method Laboratory equipment calibration, intervals Laboratory equipment [capabilities] Laboratory location [on site, off site, vendor] Laboratory personnel [training] 5.4 QUALITY ASSURANCE The Quality Assurance section defines all the examinations, inspections, and tests to be performed in order to verify that the processes, as well as the equipment, specified in the Requirements section are followed. Each inspection or examination required in this section should be tied directly to a requirement specified in the Requirements section. Inspections should not to be performed against requirements that are not specified in the Requirements section Responsibility for Inspection Organizations or personnel responsible for the performance of the quality assurance examinations are identified in this section. When personnel other than quality assurance inspectors are given the responsibility to perform inspection, they must be trained in the performance of quality assurance tasks and must be supported with complete documentation on the required inspection process Inspection The actual examinations or inspections (monitoring procedures) to be performed are listed in this section by categories Monitoring Procedures Equipment All equipment requirements requiring verification are to be listed in this section for inspection. Calibration frequency is also defined Monitoring Procedures Materials All material requirements requiring verification are to be listed in this section for inspection. This would include: freezer storage temperature, storage life, out time life at time of collation and cure, performance of receiving inspection tests, use of approved consumable materials, and batch numbers are recorded Monitoring Procedures Facilities All facility requirements requiring verification are to be listed in this section for inspection. This would include: clean room temperature and humidity, and cleaning schedule. - 26

27 Monitoring Procedures Tooling All tooling requirements requiring verification are to be listed in this section for inspection. This would include: performance of heat surveys to document temperature uniformity during the cure cycle, performance of tool visual inspections, and monitoring of storage conditions Documentation The documentation section lists all information reported. The time span all process records are to be retained is also specified in this section. A period of seven years is recommended Test Methods This section is to contain all test methods specified within the specification. Preferred practice is to reference industry test method specifications or standards such as ASTM. 5.5 NOTES The Notes section contains definitions and relevant information. 6.0 CONCLUDING REMARKS AND FUTURE NEEDS [TBD] 7.0 REFERENCES 1. DOT/FAA/AR-02/XX, Recommended Criteria and Guidelines for the Development of a Material Specification For Carbon Fiber/Epoxy Unidirectional Prepregs to be Used on FAA Certified Structures,??? DOT/FAA/AR-00/47, Material Qualification and Equivalency for Polymer Matrix Composite Material Systems. 3. AIR 4938, Composite and Bonded Structure Technician/Specialist Training Document, SAE. 4. AIR 52788, Composite and Bonded Structure Engineer: Training Document, SAE. 5. AIR 5279, Composite and Bonded Structure Inspector: Training Document, SAE. 6. ARP 5089, Composite Repair NDT/NDI Handbook, SAE. - 27

28 APPENDIX A EXAMPLE PROCESS SPECIFICATION - 28

29 Fabrication of Carbon Fiber Reinforced Epoxy Composite Parts 1.0 SCOPE 1.1 Application - This specification establishes the requirements and procedures for the fabrication and acceptance of carbon fiber reinforced composite laminates. Laminates processed in accordance with this specification are used in the qualification of new materials or establishment of mechanical property equivalency or batch acceptance. 2.0 APPLICABLE DOCUMENTS 2.1 The following documents form a part of this specification to the extent specified. [TBD] 3.0 REQUIREMENTS 3.1 Required Materials This section lists the approved specific materials needed to perform the operations specified within this specification along with the specific trade names and sources Material Listing a) Prepreg Test Material b) Film Adhesive c) Mold Release Agents Bulk Liquid Wipe-On d) Release Film Nonporous Fluorinated Ethylene Propylene (FEP) e) Release Film Nonporous Teflon Coated Glass Fabric f) Peel Ply Nylon Fabric g) Peel Ply Polyester Fabric h) Dam Materials Sealant Tape i) Dam Materials Cork j) Pressure Sensitive Tape General Purpose k) Pressure Sensitive Tape Teflon Film l) Pressure Sensitive Tape Nonporous Teflon Coated Glass Fabric m) Pressure Sensitive Tape Double Sided n) Bleeder Glass Fabric (style 120 and 7781) o) Breather Polyester Nonwoven p) Nylon Vacuum Bag Film q) Vacuum Bag Sealant Tape r) Wiping Materials Cotton Cloth Wipers s) Wiping Materials Cheesecloth, Cotton, Bleached t) Wiping Materials Paper and Synthetic Nonwoven Wipers - 29

30 u) Synthetic Gloves v) Barrier Hand Creams Material Requirements All prepreg and adhesive materials are required to have passed their applicable receiving inspection requirements as defined in the material specification and have been released for use Prepreg and adhesive materials that are frozen shall be warmed to ambient temperature for a minimum of two hours. The materials are considered to be at ambient temperature when there are no traces of moisture or condensation on the outside of the polyethylene storage bag. During ply collation the prepreg and adhesive material shall be stored at ambient temperature in the environmentally controlled lay-up room in sealed polyethylene storage bags Splices are prohibited unless authorized by the requesting engineering documents Gaps between adjacent pieces of prepreg in the same layer and parallel to the fibers shall be kept to a minimum and shall not exceed inches. Edge splice overlaps shall not exceed inches. Edge splices of consecutive plies of the same orientation shall not be located directly over each other. They shall be staggered by at least 2.0 inches Prepreg plies containing defects shall be replaced The prepreg storage life and ambient exposure time shall be recorded and monitored to ensure compliance with the ambient working life requirements established in the applicable material specification Prepreg plies shall be handled in a manner to prevent damage Mold releases are not to be applied within the ply collation room or area. 3.2 Required Equipment Equipment Listing a) Vacuum source and supply (minimum of 22 inches of mercury) b) Tool plate (also called base plate) c) Caul Plate d) Thermocouples e) Reference bar (metal) f) Cutting tools (knives or automated cutting system) g) Work tables h) Vacuum Ports Through The Bag Fittings i) Autoclave - 30