Kul Aircraft Structural Design (4 cr) Fatigue Analyses

Transcription

1 Kul Aircraft Structural Design (4 cr) M Kanerva 2016

2 Objective and Contents of the Module The objective of the module is to describe (1) how aircraft fatigue analyses are performed and (2) how fatigue is controlled during the aircraft service Contents Overview Lessons learned Load spectra Atmospheric turbulence Aircraft response to turbulence Aircraft fatigue loads Fatigue analyses Structural integrity programs Page 2

3 Overview Material Fatigue Systematic work for understanding material fatigue started in 1800 s when August Wöhler conducted series of experiments when trying to solve fatigue problems of axels in trains Aircraft industry has been active in the development of fatigue analysis techniques especially after the fatigue related Comet accidents in 1950 s Page 3

4 Overview Aircraft Fatigue Fatigue is a specific concern in aircraft design for the following reasons: High level of safety is required Lightweight structure must be achieved Actual loads may differ from design loads Aircraft with fail-safe / damage tolerant structures may be operated far beyond their original design service life Page 4

5 Overview Data on Aging Aircraft (1995) Page 5

6 Overview Safety Requirements Damage-Tolerant Structures Specified maintenance free life to be achieved Inspection intervals and inspection methods to reveal cracks before they danger safety: residual strength as required (structures are typically designed to withstand at least limit loads) no fracture Page 6

7 Overview Safety Requirements Notes on Damage-Tolerant Structures One miss of a detectable crack is normally assumed Inspection intervals can be adjusted during the operational use of an aircraft Analyses to be verified by testing as needed and required Page 7

8 Overview Safety Requirements Safe-Life Structures Specified fatigue life with no visible cracks to be achieved Scatter factor of the order n = 3 10 to be applied in analyses and testing Note! Possible defects after manufacture to be taken into account Analyses to be verified by testing as needed and required Page 8

9 Overview Safety Requirements Fatigue Evaluation CS-25 AMC describes acceptable means of compliance: Damage-tolerance (fail-safe) evaluation Fatigue (safe-life) evaluation Design features to be considered Evaluation process Readily detectable defects Scatter factor Identification of principal structural Replacement times elements Type design developments Extent of damage and changes Inaccessible areas Testing of principal structural elements Identification of general damage locations Damage-tolerance analyses and tests Inspection Page 9

10 Lessons Learned Summary Typical reasons for fatigue failures: Insufficient fatigue life (design) Insufficient damage tolerance (design) Insufficient knowledge of the load spectrum (design) Inadequate full-scale fatigue testing (design) Material/manufacturing defects (production) Insufficient corrosion protection (design, production) Insufficient/unsuccessful inspection (design, maintenance) Note! More than one reason is typically needed for a catastrophic failure Page 10

11 Fatigue analyses Overview Lessons learned Load spectra Atmospheric turbulence Aircraft response to turbulence Aircraft fatigue loads Fatigue analyses Structural integrity programs Page 11

12 Load Spectra Introduction 1/2 The term load spectrum is normally used to mean any representation of cyclic loads applied to a structure (1) in a specific use and (2) in a specific time A load spectrum may represent cyclic loading of the whole structure (e.g. g-loads applied to an aircraft) For fatigue analyses, load spectra must anyhow be determined separately for all (possibly) fatigue critical details since structure loads depend on many parameters structures respond differently to external loading V r T Mwb Lwb W F L ht D Page 12

13 Load Spectra Introduction 2/2 A load spectrum for a structural detail is derived using an applicable measure for the load cycles, e.g.: stress or strain force per unit width bending moment of an aircraft wing or fuselage cross-section A load spectrum can be determined (1) experimentally or (2) analytically The former method is naturally limited to existing structures Page 13

14 Measured Spectrum Load vs. time data measured for a structure detail during operation is the exact form of a load spectrum Quality of the spectrum highly depends on the measurement system, e.g. on the frequency of the data measurement Frequency must be high enough to catch short load peaks The measured data as such is not very applicable for fatigue analyses Page 14

15 Measured and Filtered Spectrum For analysis/testing purposes, small load cycles with (practically) no effect on fatigue life are normally filtered off from the measured data, e.g. in the figure: Deadband criterion defines a band around the initial state: only one of the subsequent measurement points inside the band is accounted for Rise/fall criterion defines the minimum value of the half-cycle to be recorded Page 15

16 Cycle Counting Measured and filtered load spectrum is often processed by counting different load cycles included into the spectrum Different techniques are applied in cycle counting Note! Information on the sequence of load cycles is lost in the cycle counting process Page 16

17 Cycle Counting Level Crossing Count The so-called level crossing count is the simplest method for counting load cycles from the measured load history (or for continuous recording of the load history) In the method, exceedances of pre-specified load levels are recorded The method is normally not acceptable since very different load histories simplify to the same cycle count with this method Different loading, same cycle count Page 17

18 Cycle Counting Restricted Level Crossing Count The restricted level crossing count is an improved version of the level crossing count In this method, exceedances of pre-specified load levels are not recorded before a pre-specified reference level is reached after the exceedance Though recording of small fluctuations is avoided, the method is normally not satisfactory, either Different loading, same cycle count Page 18

19 Cycle Counting Range Pair Range Count In the range pair range count method full load cycles are searched from the load history: In the first search, each detected cycle is recorded and removed from the history Another search is then performed. This is continued until all cycles (and the remaining half-cycles) have been recorded The sequence of loads is lost in the process, in other aspects, the result is an exact description of the history Page 19

20 Cycle Counting Rainflow In the rainflow method, the load history is thought of being in vertical direction A rainflow is initiated from each inner corner, the flow continuing until it meets a flow that has initiated from a higher load peak Each rainflow defines a load half-cycle The method gives an identical result with the range pair range count method (and is relatively easy to code) Page 20

21 Exceedance Curves 1/9 Exceedance curve represents how many load peaks of at least specified magnitude a structure or structure detail encounters in a specified time Load level.5 Aircraft gust spectrum LOG IEXCEEDANCESI Log exceedances / time Page 21

22 Exceedance Curves 2/9 Exceedance curves can be generated: from measured load data; analytically; or partly from measured statistical data (e.g. man-made loads) and partly analytically (e.g. nature induced loads) Exceedance curves are generally used in fatigue analyses since: exact load histories do not exist for a new structure to be designed exceedance curves are practical for design purposes Page 22

23 Exceedance Curves 3/9 Exceedance curves are generally presented in the form of semi-log diagrams Typically then: exceedance curves for nature induced loads are close to linear exceedance curves for mechanically induced loads are non-linear Load level Load level.5 Aircraft gust spectrum Log exceedances / time Fighter aircraft manoeuvre spectrum Log exceedances / time Page 23

24 Exceedance Curves 4/9 The example below gives load spectra for the Fokker F28 in terms of acceleration increment: load spectra measured in service, design load spectra Page 24

25 Exceedance Curves 5/9 Load level exceedances can further be computed from the exceedance curve, e.g. the curve beside indicates that: load level 4 will be exceeded 250 times per year load level 3 will be exceeded 80 times per year load level exceedances in between 3 and 4 sum up to 170 Page 25

26 Exceedance Curves 6/9 A stair approximation of the exceedance curve is normally satisfactory for fatigue analyses allowing to represent loads with the number of load occurrences of selected load levels Load level Exceedances Occurrences Page 26

27 Exceedance Curves 7/9 For aircraft structures exceedance curves must be created by considering separately (at least) air and ground load cases Page 27

28 Exceedance Curves 8/9 As needed, curves must be created separately for different flight stages since aircraft mass, speed etc. vary from stage so stage resulting in a varying: mean stress; and load response Page 28

29 Exceedance Curves 9/9 To be noted is that exceedance curves do not contain information on the sequence of load peaks (similarly to the result given by cycle counting) do not indicate how positive and negative occurrences should be combined to load cycles Page 29

30 Fatigue analyses Overview Lessons learned Load spectra Atmospheric turbulence Aircraft response to turbulence Aircraft fatigue loads Fatigue analyses Structural integrity programs O Saarela Aircraft structural design 2014 Page 30