FATIGUE LIFE ANALYSIS OF TAPERED HYBRID
Gretchen B. Murri
U.S. Army Research Laboratory, Vehicle Technology Directorate
NASA Langley Research C enter, Hampton, VA
Jeffery R. Schaff
United Technologies Research Center, East Hartford, CT
Alan L. Dobyns
Sikorsky Aircraft, Stratford, CT
Nonlinear-tapered flexbeam laminates from a full-size composite helicopter rotor hub flexbeam were
tested under combined constant axial tension and cyclic bending loads.
The two different graphite/glass
hybrid configurations tested under cyclic loading failed by delamination in the tapered region. A 2-D
finite element model was developed which closely approximated the flexbeam geometry, boundary
conditions, and loading.
The analysis results from two geometrically nonlinear finite element codes,
ANSYS and ABAQUS, are presented and compared.
Strain energy release rates (G) obtained from the
above codes using the virtual crack closure technique (VCCT) at a resin crack location in the flexbeams
are presented for both hybrid material types.
These results compare well with each other and suggest that
the initial delamination growth from the resin crack toward the thick region of the flexbeam is strongly
The peak calculated G values were used with material characterization data to calculate fatigue
life curves and compared with test data. A curve relating maximum surface strain to number of loading
cycles at delamination onset compared reasonably well with the test results.
Key words: flexbeam, delamination, VCCT, fatigue, strain energy release rate, fracture mechanics
Polymeric composites are used to manufacture hingeless, bearingless composite rotor hubs for
These components are made of fewer parts and offer the advantages of reduced weight and
drag, compared to metal hubs.
However, unlike metals, there are no fracture mechanics based methods
for predicting strength and life of these structures with damage.
Typically, damage is simulated in
structural analytical models and fracture mechanics based techniques are used to calculate strain energy
release rates, as proposed by O'Brien in ref. 1 and summarized by Martin in ref. 2.
In ref. 3, the fatigue
life to onset of delamination was correctly calculated for coupon-size tapered flexbeams under tension-
bending cyclic loading.
An analysis and test methodology that incorporates damage, failure initiation and
growth, and failure probability is critical to demonstrate reliability for new and existing rotor hub designs.
In order to accommodate bending loads in the flapping flexure region, composite flexbeams use internal
ply-drops to create a non-linear taper.
These internal ply-drops create material and geometric
discontinuities, and are sources of delamination onset [3-8].
In ref. 9, the effect of combined tension-
bending loading on two different hybrid graphite/glass-epoxy laminates with a nonlinear taper was
Test specimens were cut from a full-size helicopter tail-rotor flexbeam and were tested in a
hydraulic load frame under combined constant axial-tension load and transverse cyclic bending loads.