Composite Material Qualification Process

Transcription

1 Composite Material Qualification Analysis alone is generally not considered adequate for substantiation of composite structural designs. Instead the building block approach to design development testing is used in concert with analysis (first applied to aircraft qualification in the 1950s.) This approach is often considered essential to the qualification/certification of composite structures due to the sensitivity of composites to out-of-plane loads, the multiplicity of composite failure modes and the lack of standard analytical methods.

2 Composite Material Qualification Design substantiation is often called qualification in DoD applications and certification in civilian applications including FAA. Composite Materials Qualification Design with structural metals is a mature technology (too comfortable?) The designer can take materials properties from a handbook, specify the alloy and heat treatment and, with little or no testing, move on to initial designs for the metallic component. Tests are performed to verify that the alloy, process and resulting material properties are as specified; design, manufacture and qualification of the prototype component follow. The Building block approach has evolved for composites because of their unique characteristics.

3 Composite Material Qualification Building Block Approach Generate material basis values and preliminary design allowables. Based on the design/analysis of the structure, select critical areas for subsequent test verification. Determine the most strength-critical failure mode for each design feature. Select the test environment that will produce the strength-critical failure mode. Matrix-sensitive failure modes (compression, out-of-plane shear, and bondlines) Potential hot-spots caused by out-of-plane loads Composite Material Qualification Building Block Approach Design and test a series of test specimens, each one of which simulates a single selected loading condition and failure mode, compare to analytical predictions, and adjust analytical models or design allowables as necessary. Design and conduct increasingly more complicated tests that evaluate more complicated loading situations with the possibility of failure from several potential failure modes. Compare to analytical predictions and adjust models as necessary.

4 Composite Material Qualification Building Block Approach Design and conduct, as required, full-scale component static and fatigue testing for final validation of internal loads. Compare to analysis. Component qualification is often complicated by the fact that critical design conditions include hot, wet environments. This is often accomplished by overloading a test article that is in ambient conditions, or by analysis of failure modes coupled with strain measurements related back to subcomponent hot/wet tests. Composite Material Qualification Schematic of building block approach

5 Composite Material Qualification Building Block Approach Major purpose of this approach is to reduce program cost and risk while meeting technical, regulatory and customer requirements. Cost-efficiency is achieved by testing greater numbers of less-expensive small specimens, (each level involves fewer test articles than the one below.) assessing technology risks early in the program and using analyses in place of tests where possible. Composite Material Qualification Test levels can be defined in two basic ways: Structural Complexity Level Data Application Category

6 Composite Material Qualification Five levels of Structural Complexity (each geometry or form-based) Constituent Lamina Laminate Structural element Structural subcomponent Composite Material Qualification The material form(s) to be tested, and the relative emphasis to be placed on each level, should be determined early in the material data development planning process and will likely depend upon many factors: Manufacturing process Structural application Corporate practices Procurement or certification agency

7 Composite Material Qualification Most applications require at least two levels It is common to use all five levels A single level may suffice in rare instances Regardless of the level selected, physical and chemical property characterization of the prepreg (or the matrix if RTM) is necessary to support physical and mechanical property test results. Composite Material Qualification Constituent testing Evaluates the individual properties of fibers, fiber forms, matrix materials, and fiber-matrix preforms. Key properties, for example, include fiber and matrix density, and fiber tensile strength and tensile modulus

8 Composite Material Qualification Lamina testing Evaluates the properties of the fiber and matrix together in the composite material form. Prepreg properties are often included in this level, sometimes broken out in a separate level. Key properties include fiber areal weight, matrix content, void content, cured ply thickness, lamina tensile strengths and moduli, lamina compressive strengths and moduli, and lamina shear strengths and moduli. Composite Material Qualification Laminate testing Characterizes the response of the composite material in a given laminate design. Key properties include tensile strengths and moduli, compressive strengths and moduli, shear strengths and moduli, interlaminar fracture toughness and fatigue resistance.

9 Composite Material Qualification Structural element testing Evaluates the ability of the material to withstand common laminate discontinuities. Key properties include open and filled hole tensile strengths, open and filled hole compressive strengths, compression after impact strength, joint bearing and bearing bypass strengths. Composite Material Qualification Subcomponent testing Evaluates the behavior and failure mode of increasingly more complex structural assemblies (application specific.)

10 Composite Material Qualification Material property testing can also be grouped by data application into one or more of the following categories: Screening Qualification Acceptance Equivalence Structural substantiation Reproduction Verification Full Scale Laboratory Material/ and Design Development Certification Tests EMD Aircraft Components Elements/ Subcomponents Material Properties Manufacturing Material Selection Building Block Test Program

11 Composite Material Qualification Material and Selection Must look at both material and process together Need to evaluate part requirements and cost goals Material and process maturity are a factor Material allowables and process reproducibility will be required Experience from other programs can be used Outcome: Candidate Materials and es Composite Material Qualification Development Define process limits Develop mechanical properties at upper/lower limits Demonstrate reproducibility within limits Define critical steps/tools/equipment Develop necessary inspection/qc tools/process Outcome: Specifications

12 Composite Material Qualification Development Often composite materials must receive limited requalification every time a process parameter s altered at least to show equivalency. Due to the statistical variability inherent in composites, even relatively smal changes during manufacturing can alter the load path or failure mode and produce statistically significant changes in composite properties. Composite Material Qualification Currently qualified processes Hand lay-up Filament winding Resin transfer molding Fiber placement Potential processes Resin film infusion Vacuum assisted RTM Sheet Forming Pultrusion

13 Composite Material Qualification Material Properties Physical and Chemical property determination Density Viscosity Cure kinetics Out time Tack Glass transition temperature Environmental sensitivity Moisture resistance Solvent attack Upper use temperature Composite Material Qualification Material Properties Mechanical Properties Strength, Modulus, Notch Sensitivity, Fatigue Resistance, Damage Tolerance, etc. Examine all critical modes and environments Develop design allowables B basis confidence Outcome: Material Specifications, Design Allowables

14 Composite Material Qualification Material Properties Defect/Damage Sensitivity Mechanical effect of defects Voids Delamination Wrinkles Damage tolerance Low velocity impact FOD Repair Develop repair materials and processes Demonstrate utility Outcome: Repair Procedures and Specifications Composite Material Qualification Currently Qualified Materials (partial list) Carbon fibers: AS4, IM6, IM7, both uni and woven Glass fibers: E and S Resins: , 977, 8552, 8551, E773 Core materials: Korex Potential Material Systems Resins: PR-500, Core: Syncore, Rohacell Adhesives

15 Composite Material Qualification Elements and Subcomponents Fabrication of design details Validation of analysis Refined definition of inspection and repair requirements Risk reduction for manufacturing and assembly Outcome: Reduced Risk, Selection of Final M&P Composite Material Qualification Components Fabrication of actual components Manufacturing demonstration Destructive evaluation Demonstration of repairs Demonstration of componentlevel mechanical performance Validation of analysis Demonstration of systems interfaces Demonstration of damage tolerance Outcome: Low risk materials, process design, and manufacturing process

16 Composite Material Qualification Certification Tests Static Dynamic Fatigue Flight Test Flight clearance given based on data package of building block test results. Rotorcraft The building block approach for rotorcraft differs from that for fixed-wing aircraft in several ways. Limited NDE must accommodate larger defect sizes Must consider complex set of dynamic components Static fatigue and damage-tolerance requirements are addressed separately in rotorcraft. As with fixed-wing aircraft, design allowables usu. generated at the coupon level As with military aircraft the number and types of tests dependent upon component criticality. Damage-tolerance requirements for the rotor and drive systems are handled only at the full-scale level. Tailboom and roof beams/pylon support are fatigue critical and require full-scale testing and qualification, as a results these components undergo less coupon and subcomponent testing. Airframe testing is similar to that for fixed wing aircraft, except that fatigue is of more concern and thus receives more compete characterization at a lower level.

17 General Aviation The situation for general aviation is very different from that of the large aerospace contractors. Take kit aircraft, they have already designed, built and flown their aircraft; now they want FAA certification. General aviation in general is equipped to produce aircraft quickly, in perhaps one year from project inception to production. As a result, large subcomponents can be produced sooner and much more economically argues for inverted building block approach. The concept is that one should test the largest structure that is economically viable, allows such concerns as manufacturing induced defects to be addressed early in the design process... General Aviation Large scale articles should be used to identify the areas of critical concern and to develop element or subcomponent test plans for determining design allowables for the critical conditions. Coupon tests would be used primarily to establish the equivalency of the material to existing data, for environmental evaluation and for QA. Except in the case of truly new materials with no pre-existing data, the test matrix for coupon testing should be limited in scope... Changes in processing should only be accepted if equivalency of properties can be established (approach taken in qualifying RTMprocessed parts for the F-22.) AGATE program for sharing design allowables changes the landscape for GA in a very positive manner...

18 References Composite Material Qualification, Roland Cochran Quantifying qualification: the building block approach to designing composite structures, High-Performance Composites, July/August 1999, pp Mil-Hdbk-17 Composite Materials Handbook, Material Qualification and Equivalency for Polymer Matrix Material Systems, FAA Technical Report, Tomblin, J.S., Ng, C.Y. and Raju, K.S., 2000.