FATIGUE ON NC2/RTM6 COMPOSITES
a
Maria Odila Hilário Cioffi, aDaniel Ribeiro Nogueira Porto aHerman Jacobus Cornelis Voorwald,
b
Mirabel Cerqueira Rezende, cLuigi Ambrosio
a
Fatigue and Aeronautic Materials Research Group, Department of Materials and Technology
State University of São Paulo – UNESP. Av. Ariberto Pereira da Cunha, 333 CEP 12516-410
Guaratinguetá/SP-Brazil
b
AMR/IAE/CTA Praça Marechal Eduardo Gomes, 50 Campus do CTA - Vila das Acácias 12228-901
São José dos Campos – SP
c
Institute of Composite and Biomedical Materials, IMCB-CNR, Piazzale Tecchio 80 - 80125 NapoliITALY
[email protected]
Abstract
The fibrous composites technology has an important role in the development where the weight is a parameter to be consider,
in particularly for the aeronautic field. To produce high performance structural composites according to safety criteria,
reproducibility and low cost, the national industry has shown interest to the resin transfer molding process (RTM).
In this process low viscosity resin under low pressure is used into the hermetically closed mold, where the reinforce is
previously preformed. It is adequate to produce polymer composites components with complex geometry characteristics,
which require high quality of finish without scale limitations.
In this work, fatigue specimens of NC2 multiaxial reinforcement/RTM6 Monocomponent system composite were produced
according to the ASTM 3039 D. Axial fatigue tests were carry out using an INSTRON 8800 universal testing machine and a
sinusoidal load of 10Hz frequency and load ratio R = 0.1 was applied. It was observe that the delamination at the 10000 cycles
occurred independent of the load applied. Scanning electron microscopy techniques (SEM) was used to observe fatigue
fractures specimens.
Keywords: fatigue, composites, RTM process, NC2, RTM6.
Introduction
The RTM process is one of more efficient techniques to produce advanced fibers reinforcing polymeric matrix composites [1].
Is associated to an effective low cost technology to produce great scale composites components. It was developed as an
economic method of high quality composites to produce more complexes components than that obtained from the traditional
methods [2], present excellent controlling of mechanical properties and shorter cycle of process [3].
During the process the resin in liquid state is passed into the closed mold until the saturation of preform is allowed when the
cure is started. The quality of impregnation is depending on the permeability of reinforce by the resin [4], which represent the
resistance of the fiber to the resin flow expressed by Darcy’s law [5].
A precise preform release from defects and distortion presents a permeability assumed to be uniform which could avoid
sections of the unsaturated mold as opposed to defects and preform distortions could produce residual stress and stress
concentration to the components submitted to the loadings [4].
These anomalies in the preform may affect the mechanical properties of composites, consequently a significant changes in
stiffness, strength and fatigue life is occurred, as a result components submitted to the fatigue load has a redistribution of
residual stress into the composites structures [6-8].
The fatigue mechanisms to fibrous composites occur in four stages, nucleation of local damage, stable propagating of crack
due to the cycling load, local crack propagation which is dependent of fibers orientation, the matrix ductility and the adhesion
interfacial. The propagation of last loading cycle is analogous to the tensile test fail [9].
The fatigue fail analyses are carry out by the difference of stress intensity factor between the maximum and minimum loads
(∆K), from that is dependent the energy dissipation cycle while the Kmax control the fractures velocity. The stress ration,
K
R =  min
 K máx

 ,

conferee magnificence to the K data. The materials strength in relationship to the dimension of crack is
determine as the growing velocity by load cycle dc/dN. For an intermediate ∆K, the growing velocity follows the Paris-Erdogan
equation [10, 11].
dc
n
≡ β (∆K )
dN
Growing velocity (m/cycle) versus ∆K curve in the logarithmic scale result in a street line in Paris system with gradient equal to
n. In low stress intensity there is a limit, ∆Kth, under that the crack growing do not occur. Crack growing velocity is accelerate
according to the fast level of fracture, Kc, is subsequently [10, 11].
In the S/Nf fatigue curve, could be observed many materials with a fast crack growing for a high stress amplitude; in the
middle of the, related to the S decreasing curve and the Nf increasing in correspondent to the region of Paris system and to the
fatigue limit, related to a stress intensity under the ∆Kth value [10, 11].
Reinforced materials with long fibers has more effective influence on the fatigue behavior on the polymeric matrix composites.
The crack propagation in the composite matrix and through the interface, has controlling on a material fatigue, however is
important to attend to the strong effect of fibers in the materials stress redistribution, been the fail deformation of matrix also an
important parameter [10].
Experimental
Material
The figure 1a shows the NC2/RTM6 composites laminate processed by RTM and the figure 1b shows the impregnation map
from ultra-son analysis. The material was produced and provided by Hexcel composites
a)
b)
Figure 1 – NC2/RTM6 Composite a) Laminate; b) Impregnation map
Tensile Test
Tensile strength tests were realized in an INSTRON 8801 according to ASTM D3039. Figure 2 represent the tensile
specimens used in the test.
Figure 2 – Composite tensile test
Fatigue
For axial fatigue tests, a sinusoidal load of 10 Hz frequency and load ratio R = 0.1 was applied throughout this study.
Specimens were also done according to ASTM D3039 and tested in the same INSTRON 8801 used to the tensile tests.
Results
The results obtained of the axial fatigue tests for NC2/RTM6 composite are shown in the SxN curve indicated in Figure 2. In
the graphic could be seen the black scatter representing a delamination process that occurs during the test. It can be seen that
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independently of stress level the specimens show this phenomenon around of the 10 cycles, which is associated to the low
tenacity present characteristic of the matrix.
A tenacious matrix to composites provide to the material a high resistance due to the higher capacity to absorb energy before
the rupture [12]
500
Delamination
Rupture
Stress (MPa)
480
460
440
420
400
0
200000
400000
600000
800000
1000000
N (cycles)
Figure 3 – Axial Fatigue of NC2/RTM6 Composite.
During the fatigue tests, when the energy exceed the matrix absorption capacity the delamination process occur to adjust to
the new system, which minimize the accumulated stress until the rupture of material.
The red scatters indicate the cycles of rupture. The standard behavior fatigue could be observed, which means that, increasing
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the applied stress a reduction in the cycles is obtained until a fatigue limit, which was considered in this article as 10 Cycles.
Figures 4a, 4b, 4c, 4d, 4e e 4f present the fractures of NC2/RTM6 composites specimen tested in axial fatigue. The figure 8a
show how the fibers delaminate in the matrix and, in view of the feature, a good adhesion can be observed. Scarps formation
can be also observed in the matrix due to the intense shear process [13].
Figures 4b e 4c present the carbon fiber fractures characteristics of specimens submitted in fatigue load, typical radial
topography on broken fiber ends indicating crack propagation for each individual fiber is also indicated [14]. Graphitic planes
could also be observed.
a)
b)
c)
d)
e)
f)
Figure 4 – Fatigue Specimens SEM imagies. a) 5000 x; b) 7500x; c) 15000x; d) 5000x; e) 15000; f) 3500
In the figure 4d, can be observed as delamination process than hackles formation, which is characteristics of polymer matrix
submitted through the fatigue load. Cusps on the hackles can be also observed and are associated to the shear during the
fracture process [13].
Figures 4e indicates the fiber delamination region and a formation of feather stamp characteristics of the process, indicating
also the fracture direction, specifically, from inferior part to the superior part of figure [13].
Conclusions
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The NC2/RTM 6 composites present the delamination process around to the 10 cycles when submitted to the fatigue load
independently of the applied stress.
A fatigue standard behavior could be recognized through the SxN curve.
The fractures surfaces analyses indicated typical fracture aspect of specimens submitted in the cyclic load.
Acknowledgements
The authors express their acknowledgements to CNPq, process 472570/2006-4 and 300233/2006-0, FAPESP 02121-6 and to
the Maria Lúcia Brison de Mattos to the SEM support.
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