Fatigue Life Methodology for Tapered Composite Flexbeam Laminates

Abstract

The viability of a method for determining the fatigue life of composite rotor hub flexbeam laminates using delaminafion fatigue characterization data and a geometric non-linear finite element (FE) analysis was studied. Combined tension and bending loading was applied to non-linear tapered flexbeam laminates with internal ply drops. These laminates, consisting of coupon specimens cut from a full-size S2/E7T1 glass-epoxy flexbeam were tested in a hydraulic load frame under combined axialtension and transverse cyclic bending. The magnitude of the axial load remained constant and the direction of the load rotated with the specimen as the cyclic bending load was applied. The first delamination damage observed in the specimens occurred at the area around the tip of the outermost ply-drop group. Subsequently, unstable delamination occurred by complete delamination along the length of the specimen.Continued cycling resulted in multiple delaminations.A 2D finite element model of the flexbeam was developed and a geometrically non-linear analysis was performed.The global responses of the model and test specimens agreed very well in terms of the transverse displacement.The FE model was used to calculate strain energy release rates (G) for delaminations initiating at the tip of the outer ply-drop area and growing toward the thick or thin regions of the flexbeam, as was observed in the specimens. The delamination growth toward the thick region was primarily mode II, whereas delamination growth toward the thin region was almost completely mode I. Material characterization data from cyclic double-cantilevered beam tests was used with the peak calculated G values to generate a curve predicting fatigue failure by unstable delamination as a function of the number of loading cycles. The calculated fatigue lives compared well with the test data.

Testing and life prediction for composite rotor hub flexbeams☆

“…The flexbeams were fabricated using a combination of S2/8552 glass/epoxy and IM7/8552 graphite/epoxy. In contrast to earlier studies, where the plies were dropped in pairs in a regular staircase pattern [13][14][15]; or in groups of multiple plies, in an irregular staircase pattern [18]; these ply terminations occurred in overlapping stepped pairs of either 45/-45 plies, or 4-ply groups of [0 2 /45/-45], with the shorter plies of the pairs nearer the midplane and the longest plies to the outside of the specimen. Fig.…”

“…Both the static and cyclic loads were applied in the same manner as in ref. 18, and described above. For these specimens, the maximum strain levels for fatigue testing were between 4500 and 8000 με.…”

“…In the following studies [18][19][20], a combination of experiments and FEA of nonlinear tapered laminates under combined loading was used to predict delamination onset in these laminates. A fracture mechanics methodology, using strain energy release rate values calculated from the FEA, and material characterization data, was used to calculate a curve relating maximum surface strain in the flexbeam to the number of loading cycles at delamination onset.…”

“…18, and described above, using eqs. 1-4, and the appropriate relationships from the analyses and tests.…”

Fatigue life methodology for tapered hybrid composite flexbeams

“…In ref. 6, this approach was used to correctly calculate the fatigue life to onset of delamination for coupon-size tapered flexbeams under tension-bending cyclic loading.…”