Fatigue behavior of a hybrid particles modified fiberglass/epoxy composite under a helicopter spectrum load sequence

ABSTRACT The fatigue life of a glass fiber reinforced plastic (GFRP) hybrid composite containing 9 wt.% of rubber microparticles and 10 wt.% of silica nanoparticles in the epoxy matrix, under a standard helicopter rotor spectrum load sequence was determined and observed to be about three times higher than that of GFRP with unmodified epoxy matrix. The underlying mechanisms for the observed improvements in spectrum fatigue life of GFRPhybrid composite are discussed. Recently, we have observed that the hybrid GFRP composite containing 9 wt. % of micron-rubber and 10 wt. %of nano-silica particles in the epoxy matrix exhibit enhanced constant amplitude fatigue life by about eight to ten times over that of GFRP composite with unmodified epoxy matrix INTRODUCTION EXPERIMENTAL Materials and Processing The complete details of materials used and the processing employed to manufacture the GFRP composites can be found in Manjunatha et al wt.% of CTBN rubber in the final resin, were all individually weighed, degassed and mixed together and a stoichiometric amount of curing agent. The atomic force microscope (AFM) phase image of the particles modified bulk epoxy polymer is shown in The silica particles of about 20 nm in diameter were somewhat agglomerated to give a 'necklace-type' structure with an average width of about 1 µm. The resin mixture was used to prepare the GFRP composite laminate by the 'Resin Infusion under Flexible Tooling' (RIFT) technique The mechanical properties of both GFRP-neat and GFRP-modified composites are shown in Fatigue Testing Fatigue tests on both the GFRP-neat and GFRP-modified composites were conducted under a standard helicopter rotor spectrum load sequence, HELIX-32 shown in Spectrum fatigue tests were performed on GFRP composites with different reference stress levels ranging from 125 MPa to 200 MPa. The spectrum load sequence block with specific reference stress was repeatedly applied to the test specimens until failure and the fatigue life, expressed as the number of blocks to failure, was determined. The test specimens of size 150 mm x 12 mm x 2.6 mm with end-tabs were employed for the spectrum fatigue tests. All the tests were conducted using a computer controlled 25 kN servo-hydraulic test machine. When the specimen failed in-between any block, the fraction of the block completed was determined as the ratio of the number of reversals applied until then to the total number of reversals in the block. 5 The stiffness variation of the specimen subjected to spectrum fatigue loads was determined during the test as a function of the number of applied load blocks. Whenever stiffness measurement data were required, the fatigue test was intermittently stopped, a load cycle with σ max = 0.5 σ ref and stress ratio R=σ min / σ max = 0 was applied, the load, displacement data was obtained and analyzed. Considering the large number of load cycles in one block, insertion of this one cycle was assumed not to alter the fatigue damage in the material significantly. For the purpose of comparison, the normalized stiffness of the specimen was defined as the ratio of measured stiffness at any given time to the initial stiffness (obtained before application of the first spectrum load block). For one particular test with σ ref =160 MPa, the specimens were dismounted at the end of the application of one complete load block and photographs showing matrix cracks were obtained, as explained in Manjunatha et al. RESULTS AND DISCUSSION The spectrum fatigue life determined for both the GFRP-neat and GFRP-modified composites under the HELIX-32 load sequence at various reference stresses is shown in The fatigue failure mechanisms under cyclic loads in polymer composites involve The stiffness loss in 'stage I' and 'stage II' results primarily from matrix cracking 'stage III' [25] which lead to an improvement in the spectrum fatigue life of the GFRP-modified composite. It is to be noted that the fatigue life enhancement is about eight to ten times under constant amplitude loads at stress ratio, R =0.1 CONCLUSIONS Based on the results obtained in this investigation the following conclusions may be drawn: 1. The addition of 9 wt.% rubber micro-particles and 10 wt.% of silica nanoparticles to the epoxy matrix of a GFRP composite (i.e. to give the GFRPmodified material) enhances the fatigue life under the HELIX-32 spectrum load sequence by about three times. 8 2. The stiffness degradation of the GFRP-neat composite is more severe than that of the GFRP-modified composite during the fatigue loading. The suppressed matrix cracking and reduced crack and delamination growth rate in the modified epoxy matrix of the GFRP-modified composite enhances the fatigue life under spectrum load sequence. ACKNOWLEDGEMENT

Fatigue Behavior of a Nanocomposite Under a Fighter Aircraft Spectrum Load Sequence

Two different E-glass fiber reinforced plastic (GFRP) composite laminates having quasi isotropic [(+45/-45/0/90)2]S layup sequence were fabricated viz., GFRP with neat epoxy matrix (GFRP-neat) and GFRP with modified epoxy matrix (GFRP-nano) containing 9 wt. % of CTBN rubber micro-particles and 10 wt.% of silica nano-particles. Standard fatigue test specimens were machined from the laminates and end-tabbed. Spectrum fatigue tests under a standard fighter aircraft load spectrum, mini-FALSTAFF, were conducted on both the composites at various reference stress levels and the experimental fatigue life expressed as number of blocks to fail, were determined. The stiffness of the specimen was determined from the load-displacement data acquired at regular intervals during the fatigue test. The matrix cracks development in the test specimens with fatigue cycling was determined through optical photographic images. The fatigue life of GFRP-nano composite under mini-FALSTAFF load sequence was observed to be enhanced by about four times when compared to that of GFRP-neat composite due to presence of micro- and nano particles in the matrix. The stiffness degradation rate and matrix crack density was considerably lower in GFRP-nano composite when compared to that of GFRP-neat composite. The underlying mechanisms for improved fatigue performance of GFRP-nano composite are discussed

Enhanced fatigue behavior of a glass fiber reinforced hybrid particles modified epoxy nanocomposite under WISPERX spectrum load sequence

Two types of glass fiber reinforced plastic (GFRP) composites were fabricated viz., GFRP with neat epoxy matrix (GFRP-neat) and GFRP with hybrid modified epoxy matrix (GFRP-hybrid) containing 9 wt.% of rubber microparticles and 10 wt.% of silica nanoparticles. Fatigue tests were conducted on both the composites under WISPERX load sequence. The fatigue life of the GFRP-hybrid composite was about 4–5 times higher than that of GFRP-neat composite. The underlying mechanisms for improved fatigue performance are discussed. A reasonably good correlation was observed between the experimental fatigue life and the fatigue life predicted under spectrum loads