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Influence of loading frequency on the fatigue behaviour of coir fibre reinforced PP composite
Polymer Testing 41 (2015) 184e190
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test method
Influence of loading frequency on the fatigue behaviour of coir fibre reinforced PP composite Dijan Vinicius Osti de Moraes a, Rodrigo Magnabosco a, *, Gustavo Henrique Bolognesi Donato a, Sílvia Helena Prado Bettini b, Marcela Caroline Antunes b a b
FEI University, Brazil UFSCar, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2014 Accepted 5 December 2014 Available online 13 December 2014
The influence of loading frequency on the fatigue behaviour of a coir fibre reinforced polypropylene (PP) composite was studied. The mechanical behaviour was assessed through monotonic tensile and flexural tests, followed by cyclic bending fatigue tests employing a new specimen geometry, with loading frequencies ranging from 5 to 35 Hz. Results revealed that higher strain rates during monotonic loading lead to higher flexural strength, and higher loading frequencies in cyclic tests promote reduction in fatigue life. Fractographic examination showed that one of the reasons for reduced fatigue life under higher loading frequencies might be related to increased heat generation by hysteresis, leading to a fatigue damage mechanism governed by temperature effects. The results, thus, encourage the development of good practices regarding test frequencies in order to be able to uncouple thermal and mechanical effects and provide relevant data for structural integrity assessments. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Composites Polypropylene Coir fibre Fatigue Loading frequency
1. Introduction One of the most relevant causes of failure of polymerbased composites is fatigue. Considering macroscopic aspects, the main fatigue failure mechanisms are nucleation and growth of cracks (known as mechanical failure, in which loading is cyclic and the response can also be affected by thermal softening of materials, possibly leading to localised deformation) and/or thermal (in which thermal loading is cyclic). The occurrence of any of those
* Corresponding author. FEI University, Av. H. A. C. Branco, 3972 e office ~o Bernardo do Campo, SP 09850-901, Brazil. Fax: þ55 11 K5-09, Sa 43532900x2051. E-mail addresses: [email protected] (D.V. Osti de Moraes), [email protected] (R. Magnabosco), [email protected] (G.H. Bolognesi Donato), [email protected] (S.H. Prado Bettini), marcela. [email protected] (M.C. Antunes). http://dx.doi.org/10.1016/j.polymertesting.2014.12.002 0142-9418/© 2014 Elsevier Ltd. All rights reserved.
mechanisms (or the combination of both) can be related to the type of polymeric matrix or reinforcement phase, compatibility between composite phases, loading frequency, working temperature or viscoelastic behaviour of the polymer matrix [1,2]. Mechanical failure occurs in three stages, the first being the nucleation of small cracks at high stress concentration regions. The second stage is characterized by crack growth at every loading cycle until a critical crack size for a specific loading is reached. A third stage follows, and is usually referred to as mechanical instability, since the crack presents fast growth, leading to catastrophic failure. Mechanical failure (in which thermal effects such as softening are negligible for damage) is generally associated with low stress levels and/or low loading frequencies in the case of polymers and polymer-based composites, due to their low thermal conductivity. However, for polymeric composites reinforced with natural fibres, results that clarify the major
D.V. Osti de Moraes et al. / Polymer Testing 41 (2015) 184e190
effects influencing fatigue life are very scarce in the literature [3e8]. Considering that polymeric materials present low thermal conductivity, dissipation of the heat generated during cyclic loading may be insufficient for the maintenance of the working (or testing) temperature. If the aforementioned heat cannot be totally dissipated, the component under cyclic loading will experience temperature increase, which can cause a localised thermal softening leading to failure including thermal effects [3,5,7e9]. This phenomenon compromises data transferability between laboratory samples and real components. Bettini et al. [10] presented recent results regarding the fatigue behaviour of neat polypropylene (PP) and a 30%wt coir fibre reinforced PP, with or without maleic anhydride grafted polypropylene (PP-g-MA), tested at 6 Hz. The results revealed that neat PP failure incorporates localised softening leading to plastic collapse in spite of mechanical crack propagation if maximum applied stress during cyclic loading reaches 30 MPa, but only mechanical crack growth if maximum applied stress is reduced to 20 MPa. This was associated with the low heat generation at lower stress levels. In the case of coir fibre reinforced PP, the main fatigue mechanism was mechanical failure, characterised by a macroscopically brittle appearance of fracture surfaces, without plastic collapse, since mechanical properties increased and polymeric chain mobility is reduced in the presence of the reinforcing fibres, reducing the heat generation associated with the mechanical hysteresis. An important factor affecting fatigue behaviour of composites is the compatibility between phases. Glass fibre reinforced PP with 5%wt additions of PP-g-MA presented fatigue life 2.5 times higher than the same composite without compatibilizing agent [11]. Another study [12] compared the fatigue life under cyclic loading (6 Hz) of different coir fibre reinforced PP (with 20%wt or 40%wt coir fibre). It was concluded that longer fatigue lives were achieved in the composites with higher fibre content, without any influence of the amount of compatibilizing agent, varied between 4%wt and 8%wt of PP-g-MA. In those cases, the major fatigue mechanism was mechanical failure. Considering that higher loading frequencies are capable of increasing the ratio between generated and dissipated heat during cyclic loading, and taking into account that it can elevate component temperature, loading frequency may influence the fatigue failure mechanism of coir fibre reinforced PP composites. This work aims to investigate the influence of loading frequency on the fatigue behaviour of a coir fibre reinforced PP composite, seeking enhanced understanding of the factors that can influence the fatigue failure mode and the fatigue life of such material. 2. Materials The material chosen for this study is a coir fibre reinforced PP that presents, from exploratory works of this research group, the best mechanical strength and longer fatigue life when compared to other formulations. It is composed of 56%wt PP, 40%wt coir fibre and 4%w PP-g-MA, re-stabilized with Irganox 1010® and Irgafos 168® antioxidants.
185
Polypropylene (PP) granules used to prepare the composite were provided by Quattor, under the code PP HP 550K, with melt index of 3.8 g/10min. The compatibilizing agent was maleic anhydride grafted polypropylene (PP-gMA), supplied by Crompton-Uniroyal Chemical as Polybond® 3200, with melt index 110g/10min. Coir fibre was donated by “Projeto Coco Verde” (Rio de Janeiro, Brazil), in natura, without any previous chemical treatment, having lengths between 12 and 14 cm. 3. Experimental procedures Coir fibres were ground in a DPC-4 knife mill from “Metalúgica Braspec Ltda”, resulting in an average fibre length of approximately 1 cm. The physical mixture of composite components was kept for 4 h at 70 C before extrusion in a co-rotational twin screw extruder HAAKE Rheomex PTW 16 OS equipment, (D ¼ 16 mm and L/ D ¼ 25). Extrusion conditions were 200 rpm and screw zone temperatures of 180 C, 185 C, 185 C, 190 C, 185 C and 180 C. Before injection of the specimens for mechanical tests, the extruded material, in pellets, was kept for 4 h at 80 C before been injected in a Battenfeld HB 60/350 machine. After injection, specimens were maintained at 23 ± 2 C and relative humidity of 50 ± 1% for at least 48 h before mechanical testing. Tension tests following ASTM D638 [13] were conducted on a 30 kN Instron 5567 universal testing machine at 23 ± 1 C and relative humidity of 50 ± 1%. Type I specimens recommended by ASTM D638 [13] were used with cross-head speed of 5 mm/min, resulting in a strain rate of 0.0006 s1 at yield stress. Elastic modulus, yield strength at 0.2% strain, tensile strength and total elongation at rupture were determined after tension tests. Three-point bending tests were also conducted on the same Instron 5567 universal testing machine, following ASTM D790 [14] with specimens of 127 12.7 3.2 mm, actuator speed of 1.3 mm/min, distance between supports of 50 mm, at 23 ± 1 C and relative humidity of 50 ± 1%; a strain rate of 0.00017 s1 at yield stress was achieved. Additional threepoint bending tests were conducted on a servo-hydraulic MTS 810.25 universal testing machine, varying actuator speed from 0.02 to 40 mm/s, in order to address the effects of strain rate in the flexural yield strength of the composite. Fully reversed bending fatigue tests were possible after the development of a specific specimen geometry, based on ASTM B593 [15], which is only applicable to metallic materials. The proposition of a specimen adapted to polymeric materials was possible through numerical simulations based on the finite element method (FEM), considering large strain theory and an elastic-plastic material model, supported by the stress-strain response obtained in the tension tests. The central objective was the design of a specimen for displacement-controlled loading with a single and smooth radius in the loading region. Final geometry of the specimen, approximately 3 mm thick, is presented in Fig. 1. The finite element simulations needed for the development of the fatigue test specimen used as pre- and postprocessor the MSC.Patran 2010 software, and as solver MSC.Marc 2010. 3-D numerical simulations were performed to consider both bending and shear stresses along
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Fig. 1. (a) Specimen for fatigue tests (thickness ¼ 3 mm). (b) Boundary conditions scheme for the finite element models, including displacement and fixture regions.
the thickness, using 8-node hexahedral elements governed by J2 plasticity theory. The applied boundary conditions in the fixture region included (Fig. 1b) restricted translations in X (longitudinal direction) and Z (vertical direction) axes, and free displacements in the Y (transverse) axis, consequently allowing free deformation in width. The models were loaded by 1,000 increments up to a total displacement of 12.7 mm (maximum of real laboratory equipment). Cyclic fatigue tests were conducted on a rod-crank system machine set up for fatigue tests in fully reversed bending (strain ratio Re ¼ 1) with displacementcontrolled loading, at 23 ± 2 C. A microprocesser counter was employed for cycle counting, and a security switch was configured to disarm the electric motor when final failure of specimen took place. Each test set was conducted at a fixed imposed displacement amplitude, measured at the fixture between the specimen and the rod (denoted “displacement region” in Fig. 1b). This imposed displacement was considered in the FEM models and provided the maximum strains in the specimen surface. At least three tests were conducted for every strain level and loading frequency (5, 10, 15, 20, 25, 30 or 35 Hz were investigated), in order to correlate the number of cycles to failure to the maximum strain and loading frequency. Specimen temperature was measured during fatigue tests by an infrared pyrometer in the maximum strain region before a visible crack could be found. The failure criterion adopted in the bending fatigue tests was the total separation of the specimen. Fatigue surfaces were sputter coated with gold and analysed in a CamScan CS 3200 LV scanning electron microscope (SEM). 4. Results and discussion The results of the tensile and three-point bending tests are presented in Table 1, and the reduced standard deviations of the measurements indicate good repeatability of
the tests. It can be noted that the three-point bending tests showed a 31.5% higher value of yield strength if compared to tensile tests, even considering the smaller strain rate of the bending tests. This can be attributed to the different loading modes, in accordance with other works [16e19]. The influence of different strain rates (calculated at yield strength) in the values of elastic modulus (E) and flexural yield strength (YS) during three-point bending tests is presented in Fig. 2. The increase in YS and E values with increasing strain rate was expected, since both coir fibre and PP exhibit viscoelastic behaviour. Extrapolating the YS vs. strain rate curve as a logarithmic function at a strain rate of 0.3599 s1, which is the minimum strain rate during fatigue testing at the lower test frequency (5Hz), one obtains a yield strength of 74 MPa. This extrapolation validates the fatigue tests as elastic, since the highest strains imposed in fatigue tests (0.01766) lead to a maximum stress of 38.1 MPa, as calculated in the FEM simulations. If only the monotonic bending tests were evaluated, the lower strain rate indicates a yield strength (36.9 MPa), smaller than the applied maximum stress (38.1 MPa). If the influence of strain rate is considered, however, the hypothesis of elastic straining during fatigue tests is confirmed. For all fatigue tests, it was confirmed that all specimens (regardless of which maximum strain or loading frequency was employed) presented fracture at the maximum strain position predicted in the FEM simulations, as illustrated in Fig. 3. The number of cycles to failure as a function of loading frequency is presented in Fig. 4 for the studied six maximum strain levels. It can be observed that the higher the imposed strain amplitude and/or loading frequency, the lower is the fatigue life, as expected. Fig. 5 presents the influence of maximum strain and loading frequency in the observed temperature at the region of maximum strain of the specimens, showing that increasing the imposed
Table 1 Monotonic mechanical properties. Tensile test (strain rate 0.0006 s1)
Three point bend test (strain rate 0.00017 s1)
Elastic Modulus (GPa)
Tensile yield strength at 0.2% (MPa)
Tensile strength (MPa)
Strain at break (%)
Elastic Modulus (GPa)
Flexural yield strength at 0.2% (MPa)
3.16 ± 0.09
28.40 ± 0,78
50.66 ± 1.00
4.6 ± 0.2
2.94 ± 0.07
36.90 ± 2.77
D.V. Osti de Moraes et al. / Polymer Testing 41 (2015) 184e190
Fig. 2. (a) Elastic modulus and (b) flexural yield strength as a function of strain rate for the three-point bending tests.
strain amplitude and/or loading frequency lead to increasing temperature of the specimens. However, as the fatigue tests were displacement-controlled, even if the resulting stress is reduced by softening caused by the temperature increase, the test control remains essentially unchanged.
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SEM fractographic observation of specimens tested at 35 Hz in all maximum applied strains (Fig. 6) revealed the effects of the increased temperature due to cyclic loading on damage micromechanism. It can be observed that generalized plastic deformation took place on the PP matrix throughout the fracture surface. A significant amount of the coir fibre was decoupled from the polymer matrix due to the generated heat and the plastic deformation of the matrix. Those observations are evidence of thermal effects on fatigue performance when loading frequency is 35 Hz. If fracture surfaces of the specimens loaded at the highest strain (0.01766) and lowest strain (0.01106) are compared, a reduction can be observed in the incidence of decoupled fibre with the reduction of loading. This effect may explain the increased life at strain amplitude of 0.01106, while all the other strain conditions showed very similar fatigue life at 35 Hz, as shown in Fig. 4. This indicates that the strain amplitude has a small contribution to the type of failure when test frequency in 35 Hz. After comparing Figs. 6 and 7, it can be concluded that the effect of heating is more intense in the central section of the specimens, since heat dissipation is facilitated at the surface, even considering the higher strain levels and higher heat generation at the surface. However, at 35 Hz, there is only evidence of thermal-affected failures, without initiation and growth of cracks, which characterizes typical mechanical failure micromechanisms. Fig. 8 presents the surfaces of the samples which fractured at the lower loading frequencies, where the evidence of plastic deformation of the matrix and decoupling of the fibre resulting from heat softening was not observed, characterizing the nucleation and growth of cracks without relevant thermal effects. Their comparison to the fracture surfaces at 35 Hz (Fig. 6) demonstrates that lower loading frequencies leads to a longer loading cycle, which means less intense heat generation and longer time for heat
Fig. 3. Comparison between fracture location and position of maximum strain amplitude predicted in numerical simulations.
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Fig. 4. Number of cycles to failure as a function of loading frequency and maximum strain amplitude applied during fatigue tests.
Fig. 5. Specimen temperature as a function of loading frequency and maximum strain amplitude applied during fatigue tests.
dissipation; the occurrence of crack nucleation and growth and, consequently, mechanical failure is thus possible. Other evidence of mechanical failure is the macroscopic aspect of the fracture (Fig. 9), the near-surface regions
suffered more evident mechanical failure, given the more efficient heat dissipation, corresponding to the darker region of the fracture (arising from the rupture of fibres close to the matrix).
Fig. 6. SEM fractographs from the central region of specimens after fatigue tests for loading frequency of 35 Hz. Analysed regions refer to the maximum applied strain amplitude.
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Fig. 7. Near surface SEM fractographs of specimens after fatigue tests for loading frequency of 35 Hz. Analysed regions refer to the maximum applied strain amplitude.
Fig. 8. SEM fractographs of specimens after fatigue tests at the lowest frequencies that cause fracture.
The fractography of the test specimen at the lower strain amplitude (0.01106 mm/mm) and lower loading frequency (20 Hz) that promoted total fracture (Fig. 10) exhibited a matrix without evidence of deformation by heating over the whole surface, leading to mechanical failure by crack propagation. The fibres in the specimen broke close to the
matrix, without pronounced decoupling. This shows the effect of loading on the amplitude of heat generation. By reducing the applied strain magnitude, the heat generation due to the viscoelasticity of the material is reduced and, consequently, even at higher frequencies, time is available to dissipate the heat, thereby preventing heating and
Fig. 9. Macroscopic aspect of fracture after fatigue test at 15Hz and maximum strain amplitude of 0.01430.
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most real applications different loading frequencies can take place, and the change of damage micromechanics can decrease the reliability (and transferability) of tests that do not take these effects into account. Acknowledgements The authors want to thank “Projeto Coco Verde” (Rio de Janeiro, Brazil) for the donation of the coir fibre used in this research, and FEI for the financial and laboratorial support. References
Fig. 10. SEM fractography of specimen tested at 20 Hz and maximum strain amplitude of 0.01106.
softening of PP, avoiding its failure biased by thermal effects. With the loading frequencies and strain amplitudes investigated here, it was possible to analyse the fatigue behaviour of the coir fibre reinforced PP, and how the mechanical and/or thermally affected failure mechanisms taking place affect fatigue performance (life). 5. Conclusions It is concluded that, for the specimen geometry and test conditions used, the loading frequency really impacts the fatigue life of the coir fibre reinforced PP composite. The tests showed that, depending on the loading frequency and strain amplitude, competition between the mechanisms of mechanical failure and thermally affected failure promoted the alteration of the fatigue life of the material. Increasing the test frequency from 5 Hz to 35 Hz led to reduction of fatigue life of the material, due to the higher heat generation by hysteresis and less time available to dissipate it. This led to changes in the predominant micromechanism of fatigue damage, from mechanical failure, or nucleation and growth of cracks, to thermally affected failure, with the heating effect favouring local plastic collapse of the matrix. By reducing the amplitude of the applied strain, heat generation due to the viscoelasticity of the material is reduced and, consequently, heat dissipation is possible even for higher frequencies, avoiding (or minimizing) heating and softening of the matrix. These conclusions are relevant as motivation and justify the need to develop accelerated laboratory tests able to predict or reproduce material performance in real applications. Fatigue testing of polymerbased composites proved to be challenging since the realism of failure predictions must take into account that in
[1] K.K. Chawla, Composite Materials: Science and Engineering, second ed, Springer-Verlag, New York, 1998. [2] H. Yang, P. Qiao, M.P. Wolcott, Fatigue characterization and reliability analysis of wood flour filled polypropylene composites, Polym. Compos. 31 (4) (2010) 553. [3] S. Suresh, Fatigue of Materials, second ed, Cambridge University, Cambridge, 1998, p. 679. , Fatigue crack propagation in polypropylene [4] A. Pegoretti, T. Ricco reinforced with short glass fibres, Compos. Sci. Technol. 59 (1999) 1055. [5] M.A. Meyers, K.K. Chawla, Mechanical Behavior of Materials, Prentice-Hall, 1999, p. 680, 0132628171. ^ncia e engenharia de materiais: uma introduça ~o, fifth [6] Jr Callister, Cie ed, 2002. Rio de Janeiro. [7] S.E. Kultural, I.B. Eryurek, Fatigue behavior of calcium carbonate filled polypropylene under high frequency loading, Mater. Des. 28 (3) (Sept. 2007) 816e823. Istambul. [8] J. Roesler, H. Harders, M. Baeker, Mechanical Behavior of Engineering Materials, Metals, Ceramics, Polymers, and Composites, Springer Verlag, Berlin, 2007. [9] R.W. Hertzberg, J.A. Manson, M. Skibo, Frequency sensitivity of fatigue processes in polymeric solids, Polym. Eng. Sci. 15 (4) (1975) 252. [10] S.H.P. Bettini, M. Antunes, R. Magnabosco, Investigation on the effect of a compatibilizer on the fatigue behavior of PP/Coir fiber composites, Polym. Eng. Sci. 51 (11) (2011) 2184. [11] M.C.A. Cruz, J.A. Sousa, Correlations between dynamic-mechanical properties and fatigue life of glass fiber-reinforced polypropylene composites compatibilized with maleated polypropylene, in: 24th Annual Meeting of Polymer Processing Society (PPS-24), 2008 (Salerno, Italy. Proceedings). [12] M.A. Antunes, D.V.O. Moraes, R. Magnabosco, B.C. Bonse, S.H.P. Bettini, Fatigue life of PP reinforced coir fiber composites: effect of compatibilizer and coir fiber contents, Polym. Eng. Sci. 53 (10) (2013) 2159e2165. [13] ASTM D638: “Standard Test Method for Tensile Properties of Plastics”, ASTM, West Conshohocken, 2008. [14] ASTM D790: “Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials”, ASTM, West Conshohocken, 2011. [15] ASTM B593: “Bending Fatigue Testing for Copper-Alloy Spring Materials”, ASTM, West Conshohocken, 2009. [16] L.U. Devi, S.S. Bhagawan, S. Thomas, Mechanical properties of pineapple leaf fiber-reinforced polyester composites, J. Appl. Polym. Sci. 63 (13) (1997) 1739e1748. New York. [17] J. Rout, et al., The influence of fibre treatment on the performance of coir-polyester composites, Compos. Sci. Technol. 61 (9) (2001) 1303e1310. [18] P.A. Santos, et al., Natural fibers plastic composites for automotive applications, in: Automotive Composites Conference & Exhibition, 8, 2008 (Michigan, proceedings). ^ncia dos teores de [19] L. Caranti, B.C. Bonse, R.A. Costa, Estudo da influe fibra, agente compatibilizante e tamanho de fibra nas propriedades sitos de polipropileno com fibra de bambu, in: Congresso de compo Brasileiro de Polímeros, 11, 2011 (proceedings).
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test method
Influence of loading frequency on the fatigue behaviour of coir fibre reinforced PP composite Dijan Vinicius Osti de Moraes a, Rodrigo Magnabosco a, *, Gustavo Henrique Bolognesi Donato a, Sílvia Helena Prado Bettini b, Marcela Caroline Antunes b a b
FEI University, Brazil UFSCar, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2014 Accepted 5 December 2014 Available online 13 December 2014
The influence of loading frequency on the fatigue behaviour of a coir fibre reinforced polypropylene (PP) composite was studied. The mechanical behaviour was assessed through monotonic tensile and flexural tests, followed by cyclic bending fatigue tests employing a new specimen geometry, with loading frequencies ranging from 5 to 35 Hz. Results revealed that higher strain rates during monotonic loading lead to higher flexural strength, and higher loading frequencies in cyclic tests promote reduction in fatigue life. Fractographic examination showed that one of the reasons for reduced fatigue life under higher loading frequencies might be related to increased heat generation by hysteresis, leading to a fatigue damage mechanism governed by temperature effects. The results, thus, encourage the development of good practices regarding test frequencies in order to be able to uncouple thermal and mechanical effects and provide relevant data for structural integrity assessments. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Composites Polypropylene Coir fibre Fatigue Loading frequency
1. Introduction One of the most relevant causes of failure of polymerbased composites is fatigue. Considering macroscopic aspects, the main fatigue failure mechanisms are nucleation and growth of cracks (known as mechanical failure, in which loading is cyclic and the response can also be affected by thermal softening of materials, possibly leading to localised deformation) and/or thermal (in which thermal loading is cyclic). The occurrence of any of those
* Corresponding author. FEI University, Av. H. A. C. Branco, 3972 e office ~o Bernardo do Campo, SP 09850-901, Brazil. Fax: þ55 11 K5-09, Sa 43532900x2051. E-mail addresses: [email protected] (D.V. Osti de Moraes), [email protected] (R. Magnabosco), [email protected] (G.H. Bolognesi Donato), [email protected] (S.H. Prado Bettini), marcela. [email protected] (M.C. Antunes). http://dx.doi.org/10.1016/j.polymertesting.2014.12.002 0142-9418/© 2014 Elsevier Ltd. All rights reserved.
mechanisms (or the combination of both) can be related to the type of polymeric matrix or reinforcement phase, compatibility between composite phases, loading frequency, working temperature or viscoelastic behaviour of the polymer matrix [1,2]. Mechanical failure occurs in three stages, the first being the nucleation of small cracks at high stress concentration regions. The second stage is characterized by crack growth at every loading cycle until a critical crack size for a specific loading is reached. A third stage follows, and is usually referred to as mechanical instability, since the crack presents fast growth, leading to catastrophic failure. Mechanical failure (in which thermal effects such as softening are negligible for damage) is generally associated with low stress levels and/or low loading frequencies in the case of polymers and polymer-based composites, due to their low thermal conductivity. However, for polymeric composites reinforced with natural fibres, results that clarify the major
D.V. Osti de Moraes et al. / Polymer Testing 41 (2015) 184e190
effects influencing fatigue life are very scarce in the literature [3e8]. Considering that polymeric materials present low thermal conductivity, dissipation of the heat generated during cyclic loading may be insufficient for the maintenance of the working (or testing) temperature. If the aforementioned heat cannot be totally dissipated, the component under cyclic loading will experience temperature increase, which can cause a localised thermal softening leading to failure including thermal effects [3,5,7e9]. This phenomenon compromises data transferability between laboratory samples and real components. Bettini et al. [10] presented recent results regarding the fatigue behaviour of neat polypropylene (PP) and a 30%wt coir fibre reinforced PP, with or without maleic anhydride grafted polypropylene (PP-g-MA), tested at 6 Hz. The results revealed that neat PP failure incorporates localised softening leading to plastic collapse in spite of mechanical crack propagation if maximum applied stress during cyclic loading reaches 30 MPa, but only mechanical crack growth if maximum applied stress is reduced to 20 MPa. This was associated with the low heat generation at lower stress levels. In the case of coir fibre reinforced PP, the main fatigue mechanism was mechanical failure, characterised by a macroscopically brittle appearance of fracture surfaces, without plastic collapse, since mechanical properties increased and polymeric chain mobility is reduced in the presence of the reinforcing fibres, reducing the heat generation associated with the mechanical hysteresis. An important factor affecting fatigue behaviour of composites is the compatibility between phases. Glass fibre reinforced PP with 5%wt additions of PP-g-MA presented fatigue life 2.5 times higher than the same composite without compatibilizing agent [11]. Another study [12] compared the fatigue life under cyclic loading (6 Hz) of different coir fibre reinforced PP (with 20%wt or 40%wt coir fibre). It was concluded that longer fatigue lives were achieved in the composites with higher fibre content, without any influence of the amount of compatibilizing agent, varied between 4%wt and 8%wt of PP-g-MA. In those cases, the major fatigue mechanism was mechanical failure. Considering that higher loading frequencies are capable of increasing the ratio between generated and dissipated heat during cyclic loading, and taking into account that it can elevate component temperature, loading frequency may influence the fatigue failure mechanism of coir fibre reinforced PP composites. This work aims to investigate the influence of loading frequency on the fatigue behaviour of a coir fibre reinforced PP composite, seeking enhanced understanding of the factors that can influence the fatigue failure mode and the fatigue life of such material. 2. Materials The material chosen for this study is a coir fibre reinforced PP that presents, from exploratory works of this research group, the best mechanical strength and longer fatigue life when compared to other formulations. It is composed of 56%wt PP, 40%wt coir fibre and 4%w PP-g-MA, re-stabilized with Irganox 1010® and Irgafos 168® antioxidants.
185
Polypropylene (PP) granules used to prepare the composite were provided by Quattor, under the code PP HP 550K, with melt index of 3.8 g/10min. The compatibilizing agent was maleic anhydride grafted polypropylene (PP-gMA), supplied by Crompton-Uniroyal Chemical as Polybond® 3200, with melt index 110g/10min. Coir fibre was donated by “Projeto Coco Verde” (Rio de Janeiro, Brazil), in natura, without any previous chemical treatment, having lengths between 12 and 14 cm. 3. Experimental procedures Coir fibres were ground in a DPC-4 knife mill from “Metalúgica Braspec Ltda”, resulting in an average fibre length of approximately 1 cm. The physical mixture of composite components was kept for 4 h at 70 C before extrusion in a co-rotational twin screw extruder HAAKE Rheomex PTW 16 OS equipment, (D ¼ 16 mm and L/ D ¼ 25). Extrusion conditions were 200 rpm and screw zone temperatures of 180 C, 185 C, 185 C, 190 C, 185 C and 180 C. Before injection of the specimens for mechanical tests, the extruded material, in pellets, was kept for 4 h at 80 C before been injected in a Battenfeld HB 60/350 machine. After injection, specimens were maintained at 23 ± 2 C and relative humidity of 50 ± 1% for at least 48 h before mechanical testing. Tension tests following ASTM D638 [13] were conducted on a 30 kN Instron 5567 universal testing machine at 23 ± 1 C and relative humidity of 50 ± 1%. Type I specimens recommended by ASTM D638 [13] were used with cross-head speed of 5 mm/min, resulting in a strain rate of 0.0006 s1 at yield stress. Elastic modulus, yield strength at 0.2% strain, tensile strength and total elongation at rupture were determined after tension tests. Three-point bending tests were also conducted on the same Instron 5567 universal testing machine, following ASTM D790 [14] with specimens of 127 12.7 3.2 mm, actuator speed of 1.3 mm/min, distance between supports of 50 mm, at 23 ± 1 C and relative humidity of 50 ± 1%; a strain rate of 0.00017 s1 at yield stress was achieved. Additional threepoint bending tests were conducted on a servo-hydraulic MTS 810.25 universal testing machine, varying actuator speed from 0.02 to 40 mm/s, in order to address the effects of strain rate in the flexural yield strength of the composite. Fully reversed bending fatigue tests were possible after the development of a specific specimen geometry, based on ASTM B593 [15], which is only applicable to metallic materials. The proposition of a specimen adapted to polymeric materials was possible through numerical simulations based on the finite element method (FEM), considering large strain theory and an elastic-plastic material model, supported by the stress-strain response obtained in the tension tests. The central objective was the design of a specimen for displacement-controlled loading with a single and smooth radius in the loading region. Final geometry of the specimen, approximately 3 mm thick, is presented in Fig. 1. The finite element simulations needed for the development of the fatigue test specimen used as pre- and postprocessor the MSC.Patran 2010 software, and as solver MSC.Marc 2010. 3-D numerical simulations were performed to consider both bending and shear stresses along
186
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Fig. 1. (a) Specimen for fatigue tests (thickness ¼ 3 mm). (b) Boundary conditions scheme for the finite element models, including displacement and fixture regions.
the thickness, using 8-node hexahedral elements governed by J2 plasticity theory. The applied boundary conditions in the fixture region included (Fig. 1b) restricted translations in X (longitudinal direction) and Z (vertical direction) axes, and free displacements in the Y (transverse) axis, consequently allowing free deformation in width. The models were loaded by 1,000 increments up to a total displacement of 12.7 mm (maximum of real laboratory equipment). Cyclic fatigue tests were conducted on a rod-crank system machine set up for fatigue tests in fully reversed bending (strain ratio Re ¼ 1) with displacementcontrolled loading, at 23 ± 2 C. A microprocesser counter was employed for cycle counting, and a security switch was configured to disarm the electric motor when final failure of specimen took place. Each test set was conducted at a fixed imposed displacement amplitude, measured at the fixture between the specimen and the rod (denoted “displacement region” in Fig. 1b). This imposed displacement was considered in the FEM models and provided the maximum strains in the specimen surface. At least three tests were conducted for every strain level and loading frequency (5, 10, 15, 20, 25, 30 or 35 Hz were investigated), in order to correlate the number of cycles to failure to the maximum strain and loading frequency. Specimen temperature was measured during fatigue tests by an infrared pyrometer in the maximum strain region before a visible crack could be found. The failure criterion adopted in the bending fatigue tests was the total separation of the specimen. Fatigue surfaces were sputter coated with gold and analysed in a CamScan CS 3200 LV scanning electron microscope (SEM). 4. Results and discussion The results of the tensile and three-point bending tests are presented in Table 1, and the reduced standard deviations of the measurements indicate good repeatability of
the tests. It can be noted that the three-point bending tests showed a 31.5% higher value of yield strength if compared to tensile tests, even considering the smaller strain rate of the bending tests. This can be attributed to the different loading modes, in accordance with other works [16e19]. The influence of different strain rates (calculated at yield strength) in the values of elastic modulus (E) and flexural yield strength (YS) during three-point bending tests is presented in Fig. 2. The increase in YS and E values with increasing strain rate was expected, since both coir fibre and PP exhibit viscoelastic behaviour. Extrapolating the YS vs. strain rate curve as a logarithmic function at a strain rate of 0.3599 s1, which is the minimum strain rate during fatigue testing at the lower test frequency (5Hz), one obtains a yield strength of 74 MPa. This extrapolation validates the fatigue tests as elastic, since the highest strains imposed in fatigue tests (0.01766) lead to a maximum stress of 38.1 MPa, as calculated in the FEM simulations. If only the monotonic bending tests were evaluated, the lower strain rate indicates a yield strength (36.9 MPa), smaller than the applied maximum stress (38.1 MPa). If the influence of strain rate is considered, however, the hypothesis of elastic straining during fatigue tests is confirmed. For all fatigue tests, it was confirmed that all specimens (regardless of which maximum strain or loading frequency was employed) presented fracture at the maximum strain position predicted in the FEM simulations, as illustrated in Fig. 3. The number of cycles to failure as a function of loading frequency is presented in Fig. 4 for the studied six maximum strain levels. It can be observed that the higher the imposed strain amplitude and/or loading frequency, the lower is the fatigue life, as expected. Fig. 5 presents the influence of maximum strain and loading frequency in the observed temperature at the region of maximum strain of the specimens, showing that increasing the imposed
Table 1 Monotonic mechanical properties. Tensile test (strain rate 0.0006 s1)
Three point bend test (strain rate 0.00017 s1)
Elastic Modulus (GPa)
Tensile yield strength at 0.2% (MPa)
Tensile strength (MPa)
Strain at break (%)
Elastic Modulus (GPa)
Flexural yield strength at 0.2% (MPa)
3.16 ± 0.09
28.40 ± 0,78
50.66 ± 1.00
4.6 ± 0.2
2.94 ± 0.07
36.90 ± 2.77
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Fig. 2. (a) Elastic modulus and (b) flexural yield strength as a function of strain rate for the three-point bending tests.
strain amplitude and/or loading frequency lead to increasing temperature of the specimens. However, as the fatigue tests were displacement-controlled, even if the resulting stress is reduced by softening caused by the temperature increase, the test control remains essentially unchanged.
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SEM fractographic observation of specimens tested at 35 Hz in all maximum applied strains (Fig. 6) revealed the effects of the increased temperature due to cyclic loading on damage micromechanism. It can be observed that generalized plastic deformation took place on the PP matrix throughout the fracture surface. A significant amount of the coir fibre was decoupled from the polymer matrix due to the generated heat and the plastic deformation of the matrix. Those observations are evidence of thermal effects on fatigue performance when loading frequency is 35 Hz. If fracture surfaces of the specimens loaded at the highest strain (0.01766) and lowest strain (0.01106) are compared, a reduction can be observed in the incidence of decoupled fibre with the reduction of loading. This effect may explain the increased life at strain amplitude of 0.01106, while all the other strain conditions showed very similar fatigue life at 35 Hz, as shown in Fig. 4. This indicates that the strain amplitude has a small contribution to the type of failure when test frequency in 35 Hz. After comparing Figs. 6 and 7, it can be concluded that the effect of heating is more intense in the central section of the specimens, since heat dissipation is facilitated at the surface, even considering the higher strain levels and higher heat generation at the surface. However, at 35 Hz, there is only evidence of thermal-affected failures, without initiation and growth of cracks, which characterizes typical mechanical failure micromechanisms. Fig. 8 presents the surfaces of the samples which fractured at the lower loading frequencies, where the evidence of plastic deformation of the matrix and decoupling of the fibre resulting from heat softening was not observed, characterizing the nucleation and growth of cracks without relevant thermal effects. Their comparison to the fracture surfaces at 35 Hz (Fig. 6) demonstrates that lower loading frequencies leads to a longer loading cycle, which means less intense heat generation and longer time for heat
Fig. 3. Comparison between fracture location and position of maximum strain amplitude predicted in numerical simulations.
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Fig. 4. Number of cycles to failure as a function of loading frequency and maximum strain amplitude applied during fatigue tests.
Fig. 5. Specimen temperature as a function of loading frequency and maximum strain amplitude applied during fatigue tests.
dissipation; the occurrence of crack nucleation and growth and, consequently, mechanical failure is thus possible. Other evidence of mechanical failure is the macroscopic aspect of the fracture (Fig. 9), the near-surface regions
suffered more evident mechanical failure, given the more efficient heat dissipation, corresponding to the darker region of the fracture (arising from the rupture of fibres close to the matrix).
Fig. 6. SEM fractographs from the central region of specimens after fatigue tests for loading frequency of 35 Hz. Analysed regions refer to the maximum applied strain amplitude.
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Fig. 7. Near surface SEM fractographs of specimens after fatigue tests for loading frequency of 35 Hz. Analysed regions refer to the maximum applied strain amplitude.
Fig. 8. SEM fractographs of specimens after fatigue tests at the lowest frequencies that cause fracture.
The fractography of the test specimen at the lower strain amplitude (0.01106 mm/mm) and lower loading frequency (20 Hz) that promoted total fracture (Fig. 10) exhibited a matrix without evidence of deformation by heating over the whole surface, leading to mechanical failure by crack propagation. The fibres in the specimen broke close to the
matrix, without pronounced decoupling. This shows the effect of loading on the amplitude of heat generation. By reducing the applied strain magnitude, the heat generation due to the viscoelasticity of the material is reduced and, consequently, even at higher frequencies, time is available to dissipate the heat, thereby preventing heating and
Fig. 9. Macroscopic aspect of fracture after fatigue test at 15Hz and maximum strain amplitude of 0.01430.
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most real applications different loading frequencies can take place, and the change of damage micromechanics can decrease the reliability (and transferability) of tests that do not take these effects into account. Acknowledgements The authors want to thank “Projeto Coco Verde” (Rio de Janeiro, Brazil) for the donation of the coir fibre used in this research, and FEI for the financial and laboratorial support. References
Fig. 10. SEM fractography of specimen tested at 20 Hz and maximum strain amplitude of 0.01106.
softening of PP, avoiding its failure biased by thermal effects. With the loading frequencies and strain amplitudes investigated here, it was possible to analyse the fatigue behaviour of the coir fibre reinforced PP, and how the mechanical and/or thermally affected failure mechanisms taking place affect fatigue performance (life). 5. Conclusions It is concluded that, for the specimen geometry and test conditions used, the loading frequency really impacts the fatigue life of the coir fibre reinforced PP composite. The tests showed that, depending on the loading frequency and strain amplitude, competition between the mechanisms of mechanical failure and thermally affected failure promoted the alteration of the fatigue life of the material. Increasing the test frequency from 5 Hz to 35 Hz led to reduction of fatigue life of the material, due to the higher heat generation by hysteresis and less time available to dissipate it. This led to changes in the predominant micromechanism of fatigue damage, from mechanical failure, or nucleation and growth of cracks, to thermally affected failure, with the heating effect favouring local plastic collapse of the matrix. By reducing the amplitude of the applied strain, heat generation due to the viscoelasticity of the material is reduced and, consequently, heat dissipation is possible even for higher frequencies, avoiding (or minimizing) heating and softening of the matrix. These conclusions are relevant as motivation and justify the need to develop accelerated laboratory tests able to predict or reproduce material performance in real applications. Fatigue testing of polymerbased composites proved to be challenging since the realism of failure predictions must take into account that in
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