Building Block Approach

Transcription

1 Design Allowables

2 Building Block Approach $ Fullscale tests Analysis verification Component tests $ $ Sub-component tests Structural elements tests $ Allowable development Material specification development $ Material screening and selection $ $ Design-value development Material property evaluation Design and Analysis of Aircraft Structures 12-2

3 Typical Allowables Test Program for Metallic Materials Tension (L) (LT) 10 Compression (L) (LT) Shear (L) (LT) Bearing, e/d = 1.5 (L) (LT) jbearing, e/d = 2.0 (L) 10 (LT) 10 Design and Analysis of Aircraft Structures 12-3

4 Typical Tensile Stress-Strain Diagrams i Yield stress Ultimate tensile stress Stress, ksi Proportional p limit (a) Material having a definite yield point (such as same steels) tress, ksi St Yield stress Proportional limit Ultimate tensile stress (b) Material not having a definite yeild point (such as aluminum alloys, some magnesium alloys, and some steels) Strain, in/in offset Strain, in/in Primary modulus line Secondary modulus line Stress, ksi Yield stress Ultimate tensile stress (c) Some clad aluminum alloys offset Strain, in/in Design and Analysis of Aircraft Structures 12-4

5 Typical Creep-Rupture Curve Initial stage Second stage Third stage Strain, pe ercent Transition point Rupture point Creep intercept Slope-minimum creep rate Time, hours Design and Analysis of Aircraft Structures 12-5

6 Allowables are Required by FAA to Certify all Aircraft Structures FAA requirements FAR Material strength and design values Design and Analysis of Aircraft Structures 12-6

7 Material Design Allowables Strength properties (reduced) F tu, F ty, F su Mechanical properties Elastic properties (typical) F, G, u Physical properties (typical) W, k Design and Analysis of Aircraft Structures 12-7

8 Allowable Classifications Term Description Sub Group ID Class Identifies level of approval associated with an I, II, III allowable Basis Defines an allowables statistical basis A, B, S Maturity Defines an allowables level of development Firm, Preliminary, Estimated Design and Analysis of Aircraft Structures 12-8

9 Class Descriptions Class I Description FAA Certified Allowables: Values have been developed using a process where the FAA has approved the test plan and conformance to the plan has been established. II Generic Allowables: Obtained by following MMPDS-01 and 17 guidelines, support approved analysis methods, and are applicable to a range of products. III Program Specific Point Design: A value established for a specific application and is valid only for that application. May be used for similar application if validated. Design and Analysis of Aircraft Structures 12-9

10 Classes of Allowables "A" Basis A value which 99% of the measured values will exceed, associated with a 95% confidence level (single loadpath structures) "B" Basis A value which 99% of the measured values will exceed, associated with a 95% confidence level (multiple loadpath structures) Point design Associated with a unique structure and valid only for that particular configuration "S" Basis A value associated with specification acceptance values. No statistical significance may be assumed (initial designs) Design and Analysis of Aircraft Structures 12-10

11 Material Property Population Distributioin Frequency A B Typical Property Design and Analysis of Aircraft Structures 12-11

12 Maturity Description Term Description Situation Firm Preliminary Estimated Analysis which is representative of production has been completed on material/structure. Analysis has been completed on material/structure which has not yet been brought under control per specifications. Also applies to values for which all testing requirements needed to obtain "Firm" values have not been completed. Values provided based on little or no information which represent best estimates of firm values. Material specification in place Consistent product from stable, repeatable process Preliminary specifications in place Product variability; process practices continue to be optimized. No specific requirements Design and Analysis of Aircraft Structures 12-12

13 The Allowable Need is Dependent Upon the End User Material controlled by- Vendor Preliminary specifications Firm specifications; AMS, MIL, QQ Allowables Estimated design values Preliminary allowables Firm allowables Design/analysis Trade studies initial design Preliminary design Firm design drawing release hardware fabrication Design and Analysis of Aircraft Structures 12-13

14 Detail Fatigue Rating (DFR) System Operating requirements Configuration capability Short, medium, long operating flight profiles Life goal, and desired reliability Standardized load conditions Damage analysis Requred quality Standardized damage model based on rating system Quantita ative scale Good detail Margin Actual quality 1. Catalog details based on test or service experience 2. Test data for new details 3. Modification of (1) and (2) using influence factors 4. Calculated detail quality based on standard influence factors Material Fastener fit Load transfer Stress concentration Etc. Design and Analysis of Aircraft Structures 12-14

15 Durability Coupon Specimens Basic material properties fatigue specimens DFR cutoff Open hole Structural properties fatigue specimens Sheet specimens 12 Fastener dogbone Plate/extrusion specimens Open hole D Fastener lap splice 4 Fastener dogbone 15 Fastener dogbone 8 Fastener double shear splice Design and Analysis of Aircraft Structures 12-15

16 Preferred DFR Utilization Most preferred Use service demonstrated DFR Test demonstrated DFR Adjust above DFRs for similar details with modification factors Least preferred Determine DFR analytically Design and Analysis of Aircraft Structures 12-16

17 DFR Calculation Test data Log (ma ax. stress, KSI) Log (DFR) Factor SN curve Log (10 5 ) 95/95 Log (N, cycles) Design and Analysis of Aircraft Structures 12-17

18 Damage Tolerance Coupon Specimens Test type Center cracked tension panel (CCT) Specimen 4 wide CCT 16 wide CCT 24: wide CCT Recommended material form Extrusions and forgings (all forms) Crack growth Application Residual strength Plate, sheet (all forms) Plate, sheet (all forms) Primary specimen usage Simulates forged and extruded structure Characterizes environmental crack growth properties Fits in standard test chamber 2xxx: spectrum crack growth 7xxx: constant amplitude and spectrum crack growth, residual strength Plate, sheet (2xxx) 2xxx: Reliable crack growth allowables 48 wide CCT Largest practical residual strength th test t for high toughness alloys 92 wide CCT Plate (2xxx) Selected residual strength verification test for high toughness alloys Edge crack tension panel 9: wide ECT Plate (all forms) Simulates short crack performance of plate material Built-up Skin/stringer panel model Extrusion forgings, plate, sheet (all forms) Verify interaction of material and geometry factors Design and Analysis of Aircraft Structures 12-18

19 Material Crack Growth Rating Material A Material B Growth rate dl dn Standard 1 p dl dn = n 10-4 K max Z p M M A M B Stress intensity, K M - Measures relative material resistance to crack growth Design and Analysis of Aircraft Structures 12-19