FATIGUE ANALYSES OF AERONAUTICAL STRUCTURAL DETAILS OF DIFFERENT COMPLEXITY

Transcription

1 FATIGUE ANALYSES OF AERONAUTICAL STRUCTURAL DETAILS OF DIFFERENT COMPLEXITY Paulo M S T de Castro, Paulo F P de Matos, Pedro M G P Moreira, Lucas F M da Silva IDMEC and Faculdade de Engenharia, Universidade do Porto, Portugal 1

2 context - Comet de Havilland : Spitfire, Mosquito,..., Comet de Havilland Comet I : the first commercial jet airliner After several catastrophic accidents Comet was taken from commercial use Testing revealed that these planes had a major fatigue weakness... the designers insisted on square windows... 2

3 context - Comet Wreckage of the fuselage of the Comet: The primary cracks are shown propagating well beyond the fastener lines of several fuselage frames, without arrest 3

4 context - Comet..in order to save costs, the same fuselage that had been tested until the ultimate design load, was later used for the fatigue test. Residual compressive stresses, arising from the ultimate loading, retarded the development of the fatigue cracks As a result, there were no indications from the fatigue test that the fuselage design was inadequate Abe Brot, Innovation often leads to new problems, priv. com. Feb 23,

5 context - Aloha 737 Aloha flight 243, Boeing 737,

6 context - Aloha

7 Presentation outline 1. Cold-worked hole specimens 2. Single rivet lap-joint 3. Lap-joint panel crack growth modelling 4. Conclusions 7

8 Cold worked specimens - contents 1.1 What are the questions? 1.2 Geometry and cold working process 1.3 Fatigue tests 1.4 Residual stresses: cold working technique 1.5 Fractography analysis 1.6 Conclusions 8

9 1.1 What are the questions? What is the effect of residual stress on the fatigue life? 1) S-N How do experimental measurements and numerical modelling of the residual stresses agree? 1) X-Ray measurements 2) FEM modelling What is the effect of the residual stress on fatigue crack initiation? 1) SEM measurements 2) Reconstitution of the fatigue life Why do the specimens with residual stresses last longer? 1) Later crack initiation? 2) Braking effect on crack propagation? 9

10 1.2 Geometry and cold working process Cold working process: Entrance face Exit face Specimen geometry Material: 2024-T3 Alclad 10

11 1.3 Fatigue tests 11

12 1.4 X-ray measurements X-ray measurements for: θ=0º and θ=90º Clad thickness Al clad is used to decrease corrosion problems Al clad has to be removed because of its low yield strength 12

13 1.4.1 X-ray measurements - θ=0º Entrance face Exit face 13

14 1.4.2 Residual stress: 3D FEA / model D elements (8 nodes) mandrel sleeve Mesh details 14

15 1.4.2 Residual stress: 3D FEA / model 15

16 1.4.3 FEA 3D vs exp. meas. - θ=0º 16

17 1.4.3 FEA 3D vs exp. meas. - θ=0º 17

18 1.5 Fractography analysis Inspect the specimen fracture surfaces Measure the fatigue striation spacing Crack growth rate assessment 18

19 1.5.1 Fractography: SEM measurements σ max =140MPa 19 Cold worked hole specimen Normal hole specimen crack length=2.50mm ( ) crack length=2.71mm ( )

20 1.5.1 Fractographic analysis σ max =120MPa σ max =160MPa σ max =180MPa σ max =200MPa 20

21 1.5.2 Fractographic fatigue crack history Nedbal et al. 21

22 1.5.2 D-factor: fat. crack history recons. D reflects the interactions of different micromechanisms on the crack front linking micro and macro features of the fatigue process. D-factor depends essentially on three simultaneously interaction factors: the existence of idle cycles; spatial dispersion of local directions of crack growth; influence of fatigue crack growth mechanisms other than the striating one. 22

23 1.5.3 Surface crack measurements 23

24 1.5.4 Fractographic reconstitution and macroscopic surface measurements Normal hole Expanded hole 24

25 Fractographic reconstitution and macroscopic surface measurements 25

26 1.6 Conclusions 1. X-ray technique measurements Inevitably the stresses are averaged over the irradiated volume of material It is not possible to resolve the steep stress gradients in the vicinity of the hole The peak values predicted by FEM are averaged out in the X-ray measurements 2. FEM simulations FEM simulations are in general good agreement with experimental measurements 26

27 1.6 Conclusions 3. Fractographic measurements Striation spacing increase with increasing in crack length and with the stress level Lower striation spacing measurements show the crack growth rate decreases due to the cold working process. The effect of the hole cold expansion: 1) the main source of the life prolongation is not a longer crack initiation time, 2) but an intensive braking of early crack growth stage 27

28 2. Single rivet lap-joint contents 2.1 Geometry 2.2 SEM analysis 2.3 Stress intensity factor calibration 2.4 Load transfer 2.5 Conclusions 28

29 2.1 Specimens geometry specimen type: lap splice with 3 rivet rows and 1 rivet column material: 2024-T3 Alclad sheets sheet thickness: 1.2 mm specimen length: 260 mm, 60 mm overlap specimen width: 20 mm rivets: 2117-T4, 3.2 x 6.4 mm riveting force: according to typical production standards 29

30 2.2 SEM analysis specimens: 93, 95, 224 and 229 specimen

31 2.2 SEM analysis Fatigue striation - specimen striations transversal longitudinal 31

32 2.2 SEM analysis - remarks The transversal crack growth rates are almost constant The longitudinal crack growth rates are similar to the transversal crack growth rates for small cracks The longitudinal crack growth rate increases exponentially with increasing crack length 32

33 2.2 SEM analysis - comparison Assembled fatigue striations spacing The fatigue striation spacing was converted in crack growth rate da/dn 33

34 2.3 Stress intensity factor calibration using FEM 3D Mesh detail Plates 20-node brick isoparametric elements (C3D20) 7296 elements Rivets 8-node brick elements (C3D8) 7344 elements Boundary constraints 34

35 2.3 Stress intensity factor calibration using FEM - notation 35

36 2.3 Stress intensity factor calibration using FEM Non dimensional Keff / Symmetric crack and Asymmetric 36

37 2.4 Load transfer 37

38 2.5 Conclusions Striation spacing increase with increasing in crack length and approximately constant along the thickness the stress intensity factor calibration for a symmetric and asymmetric crack was presented the load transfer behaviour for different crack lengths, for the case of a symmetric and asymmetric crack, was presented 38

39 3. Lap-joint crack growth simulation 3.1 Geometry contents 3.2 Franc2D/L model 3.3 Crack growth 3.4 Crack growth results 3.5 Conclusions 39

40 3.1 Lap-joint - geometry Cavallini, G. and R. Lazzeri, Report on experimental fatigue crack growth in medium and complex components. ADMIRE project report: ADMIRE-TR /UP, 2003, Dipartimento di Ingegneria, Università di Pisa 40

41 3.2 Franc2D/L model Franc2D/L model Mesh 41

42 3.2 Franc2D/L mesh details Mesh rivets are modeled by spring elements (triangles) Crack scenario at cycles - mesh 42

43 3.3 Crack growth inspected sites crack shape experimentally observed 43

44 3.3 Crack growth crack shape cracks were modeled as through cracks - approximation 44

45 3.3 Crack growth overall scenario scenario at different stages of the fatigue test 45

46 3.3 Crack growth model 46

47 3.4 Crack growth results 47

50 other projects L3 L 555 eb L1 299,5 L2 299,5 L1 p ei L3 440 Lc 44 ei 3,2 L s Lc d eb 1,6 Number of rows 2 L2 Number of columns 20 p 22 s 22 d 4,8 Specimens tested, SMAAC project L F M Silva, J P M Gonçalves, F M F Oliveira, P M S T de Castro, 'Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction', International Journal of Fatigue, vol.22, (4), 2000, pp

51 Rivet hole 1 1 mm interval to compensate the cut Rivet hole Fracture surface (hole is only visible for columns 1-5 and see below crack path); specimen #1 1 mm interval to compensate the cut Rivet hole 1 mm interval to compensate the cut Rivet hole L F M Silva, F M F Oliveira, P M S T de Castro, Task 3.2: Individual report on the test results of lap joint specimens, SMAAC report SMAAC-TR /IDMEC, Sept mm interval to compensate the cut Rivet hole 1 mm interval to compensate the cut Rivet hole 1 mm interval to compensate the cut 1 20 Direction Crack path Samples for sem visualisation of Figure 40 Rivet hole 1 mm interval to compensate the cut 20 Ductile fracture Fatigue 51

52 Fracture surface; specimen #3 L F M Silva, J P M Gonçalves, F M F Oliveira, P M S T de Castro, 'Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction', International Journal of Fatigue, vol.22, (4), 2000, pp

53 L F M Silva, J P M Gonçalves, F M F Oliveira, P M S T de Castro, 'Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction', International Journal of Fatigue, vol.22, (4), 2000, pp

60 Swift criterion for link-up L F M Silva, J P M Gonçalves, F M F Oliveira, P M S T de Castro, 'Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction', International Journal of Fatigue, vol.22, (4), 2000, pp

63 modelling problems remain. left blank...others... J S Domingues, L F M Silva, P M S T de Castro, Results of Task 5 problems type II and type V of the round robin programme a comparison, SMAAC report SMAAC- WP-5-ZZ-1.3/IDMEC, Sept

64 3.5 Lap - joint conclusions Crack growth modelling was done in a lap-joint panel from to cycles; A good agreement was found between the experimentation and the prediction of FRANC2D/L based on the Paris Law; Limitations: Cracks grow with an elliptical shape and not as through cracks; Rivets were treated as spring elements. It is only a simplification that has some limitations 64

65 4. Conclusions open hole The improvement of the fatigue live due to the cold working process was quantified The assessment of the residual stresses using the X-ray diffraction and FEM modelling are in general good agreement Striation spacing increase with increasing in crack length and with the stress level Lower striation spacing meas. show the crack growth rate decreases due to cold expansion 65

66 4. Conclusions single rivet lap-joint Striation spacing increase with increasing in crack length and approximately constant along the thickness the stress intensity factor calibration for a symmetric and asymmetric crack was presented the load transfer behaviour for different crack lengths for the case of a symmetric and asymmetric crack was presented 66

67 4. Conclusions Lap-joint crack growth modelling Crack growth in a lap-joint panel was modeled A good agreement was found between the experimentation and the prediction of FRANC2D/L based on the Paris Law; Limitations: Cracks grow with an elliptical shape and not as through cracks; Rivets were treated as spring elements. It is only a simplification that has some limitations 67

68 Acknowledgments: The authors would like to acknowledge in particular the European Union projects: SMAAC project BRITE BE , leader: Dr Peter Horst (then at DASA - Hamburg) ADMIRE project contract no. G4RD-CT , leader: Dr Alfonso Apicella, ALENIA 68