Determining Widespread Fatigue Life in Fastened Aircraft Structure. Simone Fevola New Piper Aircraft, Inc.

Transcription

1 Determining Widespread Fatigue Life in Fastened Aircraft Structure Simone Fevola New Piper Aircraft, Inc.

2 Problem Challenges Thin Metal Sheet Analysis Domain Areas of concern hard to recognize at a distance Fatigue Damage is driven by bearing and bypass stresses-at every attachment Areas of concern hard to recognize at a distance Counter-sunk hardware obscures crack initiation sites Modeling holes is labor & resource intensive

3 CAD GEOMETRY:.IGS,.X_T

4 CFD 3d Modeling

5 MSC.Nastran 3-D Global Finite Element Model

6 Case Study Rib to wing skin thickness connections Inputs Loads: Shear Flows & Axial Loads at Cuts

7 inputs P5 are from 3D Fem internal distribution for loads & flows Layer Analysis Determines Spring Forces: Fastener Loads Distribution P1 N11 Q1 N16 N1 N6 BR1 N2 BR2 N3 BR3 N4 BR4 QSKIN1 QSKIN2 QSKIN3 QSKIN4 BR5 BR6 BR7 BR8 N7 N8 N9 Q1 cquad1 Q1 N12 N17 Q2 Q3 Q4 cquad2 N13 N18 cquad3 N14 Q1 Q2 Q2 Q3 Q3 Q4 Q2 Q3 N19 cquad4 TYPICAL FASTENER REPRESENTATION a Na to Nb Kshear b Nb to Nc Kshear c Kshear Na to Nc (Gap Spring) Q4 N5 N10 N15 Q4 N20 layer gap skin layer rib cap doubler rib quad layer

8 StressCheck P-elements Typical acreage problem: First three skin holes at each rib S1 By-Pass Stresses S1,S2,Sxy with cosine bearing in holes Sxy S2 S2+ Sxy Free edge of skin Rib Sta. StressCheck Solution for Principle Stress

9 StressCheck solution:raised Stress (3.8 g maneuver) S1 Raised= psi Solution error.24%

10 StressCheck Solution: 17 % csink effect 1145 psi at interior 979 psi at inner surface

11 NUEBER-HEYWOOD Notch Sensitivity Correction Equation Kt = UNCORRECTED RAISED STRESS CONCENTRATION Ka = NOTCH SENSITIVITY CORRECTED RAISED STRESS R= RADIUS OF THE NOTCH A=MATERIAL CONSTANT (IN), HALF WIDTH OF ELEMENTARY BLOCKS FOR ALUMINUM SHEET: 3 3 A = K a = F 1+ TU 2 K = K t K t t A R =. 058

12 Accuracy of Notch Equation life prediction for sheet: Ref. Landers & Hardrath R d D TEST DATA VS PREDICTION +/- 10 % life prediction for aluminum sheet based on notch equation reduces required scatter.0312 Diameter Not A Practical size hole

13 APPARENT RAISED STRESS AND STRESS CONCENTRATION FOR CURRENT PROBLEM, APPARENT RAISED STRESS= x 1.17 csink factor Kt = APPARENT RAISED STRESS BY-PASS STRESS = = 8.59

14 NUEBER-HEYWOOD FACTOR K a = = 6.29 NUEBER HEYWOOD RAISED STRESS PER G: = 24622

15 Un-notched Mean Fatigue Data

16 Corrected Notched Life & Widespread Crack Initiation This Fatigue analysis is based upon DOT/FAA AFS Report Output from ACE100-1, Version 1.0, Mar Airplane Class/Usage and other relevant data is as follows: NOTE: USER INPUT USAGE DATA USED FOR THIS ANALYSIS Airplane: Run No.: 1 Airplane Total Fatigue Damage and Unfactored Fatigue Life is determined for the following Flight Condition: Average True Air Speed : Knots Summary of Damage Calculations: Flight Segment Damage/1000 Hrs. Taxi : 0.141E-25 Gust : 0.572E-01 Maneuver : 0.251E-01 Landing: Impact : 0.363E-06 Rebound : 0.156E-07 GAG Cycle : 0.227E-02 Positive Gust Limit Load Factor : 3.37 Negative Gust Limit Load Factor : Positive Maneuver Limit Load Factor : 3.80 Negative Maneuver Limit Load Factor : Flight Stress - f(g), psi : 0.0 (+) *g Ground Min. Stress - f(g), psi : 0.0 (+) *g Average Endurance per Flight : 1.10 Hours Endurance Limit for neglecting damage : Cycles Material S-N data from : User input Load spectrum data is : User input Appropriate Spectrum Un-notched Data Used TOTAL DAMAGE : 0.846E-01 Unfactored Fatigue Life : Hours WIDESPREAD FATIGUE CRACK INITIATION AT +- 10% OR +13,008 to HRS

17 Stress at a deemed critical point versus Time and Cost Stress at any critical point Analysis A Analysis B Analysis C Analysis D Most accurate Solution for stress distribution Time (Man hours + Machine Time) to adequate solution, Cost $$$

18 Stress at a deemed critical point versus Modeling Experience Stress at any critical point Analysis A Analysis B Analysis C Analysis D Most accurate Solution for stress distribution Redundant Analysis and Modeling Experience

19 What we have learned With MSC products, we are able to construct & model global gross stress type acreage structure in historical record breaking times on our desktops. Include fastener representations in the 3D FEM for critical areas. (avoid separate uncoupled analyses) Use model spacing characteristic of required fastener spacing. Set realistic design parameters, fastener spacing, pitch, diameter to address widespread fatigue requirements. Importance of maximizing bonded structure to eliminate stress concentrations. Train experienced FEM analysts for thin domain type structure