1. Materials Engineering Research Laboratory Ltd Tamworth Road Hertford SG13 7DG Tel.+44 (0)1992 500120 Fax+44 (0)1992 586439 enquiries@merl-ltd.co.uk www.merl-ltd.co.uk BS EN ISO 9001 Mode I and Mode II Delamination Characterisation of UD and Woven Glass Fibre Vinyl Ester Composites

2. FRACTURE MECHANICS FOR COMPOSITE STRUCTURES Delaminations may be present in composite structures Stress based failure criteria do not account for stress singularities from geometric and material discontinuities STRESS-BASED CRITERION Predicts translaminar failures where no singularities are present FRACTURE-BASED CRITERION Predicts onset and growth of delaminations (cracks) from singularities SUITABLE FAILURE CRITERION Predicts final failure e.g. tensile fibre failure, buckling, etc.

3. In the life of a component under normal operating conditions, a crack in a composite laminate should not initiate. If such damage is already present, be it from a manufacturing flaw or other acceptable damage event, then the damage will not grow any further during normal service. NO DAMAGE GROWTH DESIGN Fatigue cycles Crack length Crack detected Inspection intervals Metal Composite

4. DCB INTERLAMINAR FRACTURE TEST SPECIMENS ELS (end loaded split ) ISO 15024:2000(E) European Structural Integrity Society draft protocol No rollers for ELS Translucent specimen

5. MERL MMS 13 TEST PROGRAMME VT/Halmatic Test Panels UD glass fibre vinyl ester composite (stitched unidirectional blanket) Woven glass fibre vinyl ester composite MaterialStaticFatigue Mode I (DCB)Mode II (ELS)Mode I (DCB)Mode II (ELS) UD55515 Woven55515

6. DCB STATIC LOAD-DISPLACEMENT Typical UD DCB Typical woven DCB ratchetting

7. DCB STATIC TEST RESULTS Compliance vs. Crack Length UD Woven Linear but some scatter

8. DCB STATIC TEST RESULTS R-Curve UD Woven Both materials show an increase in delamination growth resistance that demonstrates the presence of fibre bridging - more in UD than Woven Models fitted to R-Curves for use in fatigue analysis

9. UD INITIATION DATA FROM THE INSERT WOVEN INITIATION DATA FROM THE INSERT DCB STATIC TEST RESULTS Similar data from the precrack VIS data preferred

10. ELS STATIC LOAD-DISPLACEMENT UD Woven Offset after initial loading

11. ELS COMPLIANCE vs. DELAMINATION LENGTH UD Woven Linear but some scatter

12. ELS R-CURVE UD Woven Both materials show an increase in delamination growth resistance that demonstrates the presence of fibre bridging Models fitted to R-Curves for use in fatigue analysis

13. UD INITIATION DATA FROM THE INSERT WOVEN INITIATION DATA FROM THE INSERT ELS STATIC TEST RESULTS Similar data from the precrack VIS data preferred

14. SUMMARY OF MODE I AND MODE II FRACTURE SURFACES MODE I UD MODE II UD MODE I WOVEN MODE II WOVEN

15. 1.Measure Compliance vs. Cycles All testing carried out under displacement control loading with an R ratio 0.1 and frequency 5Hz. 2.Determine Crack Length vs. Cycles From Compliance Data Compliance and crack length measured at the start and end of each fatigue test are used to convert the compliance data to crack length data. Mode I C 1/3 vs. a Mode IIC vs. a 3 3.Determine Peak Cyclic G vs. Cycles Mode I CBTG I = Mode II ECM G II = 4.Obtain Peak Cyclic G vs. Crack Growth Rate Relationship Note, these data where provided require factoring for the influence of fibre bridging effects OVERVIEW OF FATIGUE DATA ANALYSIS 3P  2B(a +  ) 3P 2 ma 2 2B

16. MULTI-STATION TESTING Fatigue testing under displacement control with constant amplitude

17. CRACK GROWTH DURABILITY TESTING

18. MODE I UD FATIGUE DATA Compliance (mm/N) Cycles Delamination (mm) Peak Cyclic G

19. MODE I UD FATIGUE DATA To account for fibre bridging multiply the fatigue G data by GIc/GIR for the Mode I data CORRECTED da/dN vs. peak cyclic G da/dN (mm/cycle) Peak Cyclic G I (J/m 2 ) G IMAX x (G Ic(VIS) / G IR ) (J/m 2 )

20. MODE I WOVEN CORRECTED FATIGUE DATA

21. MODE II UD CORRECTED FATIGUE DATA

22. MODE II WOVEN CORRECTED FATIGUE DATA

23. R-CURVE SPREAD MODE I UD FATIGUE DATA

24. CONCLUSIONS UD and woven both display fibre bridging in Mode I and Mode II Fibre bridging must be accounted for in fatigue tests Normalisation by the R-Curve can provide fatigue relationships free from fibre bridging Upper and lower bounds can account for scatter in data