Tensile and Fatigue Behavior of Glass Fiber-Reinforced (MAT-8)/Polyester Automotive Composite Abdulkadir Yasar, İlyas Kacar & Ali Keskin
Arabian Journal for Science and Engineering ISSN 1319-8025 Arab J Sci Eng DOI 10.1007/s13369-013-0897-2
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Author's personal copy Arab J Sci Eng DOI 10.1007/s13369-013-0897-2
RESEARCH ARTICLE - MECHANICAL ENGINEERING
Tensile and Fatigue Behavior of Glass Fiber-Reinforced (MAT-8)/Polyester Automotive Composite ˙ Abdulkadir Yasar · Ilyas Kacar · Ali Keskin
Received: 2 February 2011 / Accepted: 15 November 2012 © King Fahd University of Petroleum and Minerals 2013
Abstract In this study, tensile and fatigue behavior of glass fiber-reinforced (MAT-8)/polyester automotive composite was investigated experimentally. The composite used in this study consists of polyester, fiber and gelcoats. The fiber contents were kept constant at 37 wt% during the tests. But, polyester and gelcoat contents were changed. The tests were conducted at five different MAT-8 thicknesses. It is clearly shown that polyester content is the only determinative factor for fatigue strength, while both polyester and gelcoat contents are effective for tensile strength. According to the results, optimum tensile strength was achieved, while fiber ratio was constant in the specific mixing of polyester and gelcoats. It is thought that the combination of 52.14 wt% polyester and 9.75 wt% gelcoats can be a preferable way for designs requiring the best tensile and fatigue strength properties at the same time. Keywords Glass fiber-reinforced composites · Fatigue strength · Tensile strength
A. Yasar (B) Ceyhan Engineering Faculty, Cukurova University, 01160 Adana, Turkey e-mail:
[email protected] ˙I. Kacar Mechanical Engineering Department, Cukurova University, 01130 Adana, Turkey A. Keskin Tarsus Technical Education Faculty, Mersin University, 33500 Mersin, Turkey
1 Introduction The composites are used in many fields such as automotive parts, civil engineering, various consumer goods and aircraft components where failure is often critical [1]. In recent years, there has been an increasing interest in utilizing fiber reinforcement polymeric (FRP) composites in automotive engineering application. Recently, many researchers [1–7] have shown that it is possible to produce composite materials with superior fatigue and tensile performance over conventional materials. A number of investigations have been carried out to evaluate the behavior of fatigue and tensile properties. The studies conducted by some researches on composites are given below. The fiber content has an evident influence on the composite failure strain. It is generally stated that the composite failure strain decreases with higher fiber content [4,5]. Increase in fiber content often leads to increase the strength, modulus and toughness [6]. For composite structures subjected to cycling load, fatigue and tensile properties become
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a quite important limitation that needs to be considered by the designer [7]. There are some general results about the fatigue behavior characteristic of glass fiber-reinforced composites (GFRP). Alternating loading process leads to a shorter fatigue life than threshold–tension or threshold–compression type loadings. Multi-axial loading (e.g., tension–torsion) drastically reduces fatigue life. The fatigue behavior of the pure matrix is usually not reflected in the fatigue behavior of a composite. Under similar conditions, tensile fatigue strength of carbon reaches its highest level, followed by Kevlar and glass; while ceramic composites exhibit the lowest fatigue strength [8]. Pannkoke [9] determined that the degradation by fatigue cycling depended on the fiber type and arrangement, matrix type, interfacial bond, mode of loading. Yuanjian and Isaac [10] assessed two alternative manufacturing routes for producing hemp fiber-reinforced polyester composites to compare their mechanical properties with those of glass fiber composites, and in particular, to investigate the low velocity impact and fatigue behavior of these composites. The effects of the specimen width on the monotonic strength and fatigue behavior of angle-ply laminates were studied experimentally by Kujawski [11]. Dyer and Isaac [12] studied the fatigue behavior of glass fiber-reinforced polyester and a polyurethane-vinyl-ester by S–N testing studying damage accumulation mechanism. The effects of resin type and fiber lay-up on the degree of damage occurring during each stage were discussed. The tensile properties of polypropylene composites reinforced with short glass fibers and short carbon fibers were investigated considering the combined effect of fiber volume fraction and mean fiber length. An empirical relationship of composite failure strain with fiber volume fraction, fiber length and fiber radius was also established by Fu et al. [6]. The fatigue behavior of composites with different fiber types (carbon AS4; HTA, E-glass, ceramic Al2 O3 , and Kevlar 49) has been studied by Hartwig et al. [8]. They concluded that the fatigue behavior of fiber dominated properties depends strongly on the type of fiber. Even fiber-dominated properties of carbon fiber composites were strongly influenced by the type of matrix. FRP was studied to assist the recognition of fatigue potential of E-glass FRP composite as an independent structural material for infrastructure application by Demers [7]. It was concluded that the fatigue design data included the effects of various combinations of the test parameters: frequency of load application, R ratio, specimen shape, fiber reinforcement percent fiber, resin and failure criteria in fatigue testing. Echtermeyer et al. [13] observed the decay of tensile fatigue modulus with fatigue life on R = −1 axial fatigue tests of glass/neopentyl glycol/iso polyester for constant load at frequency 2–5 Hz. The studies [14,15] on failure mechanism showed that under loading of tensile stress, the cracks start at fiber
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ends and propagate along the fiber matrix interface or cross through the matrix, and finally the failure takes place. Dharan [16] noted that fatigue mechanisms varied with applied stress level. He also noted that at high applied stress, fatigue was characterized by matrix cracking and a large number of fiber failures. Fatigue strength was strongly dependent on fiber strength. On the other hand, at low stress levels, fatigue was initially characterized by matrix cracking and random fiber breaks, followed by increasing fiber breakage and delamination. In the latter case, matrix is expected to play a more significant role because fatigue damage mechanisms within the matrix have enough time to initiate and develop. While several studies are available on the mechanical behavior of high percent GFRP, there are few reports on the effect of tensile and fatigue on GFRP, especially fiber content of 37 wt % and the different content of polyester. This paper presents the results and discussion of the studies carried out on fiber-reinforced polymer composites for tensile and fatigue properties. The fiber contents were kept constant at 37 wt % during the tests. But, polyester and gelcoat contents were changed. 2 Experimental Procedure The material types, thickness and contents used during the experiments are given in Table 1. In this study, the fiber contents were kept constant at 37 wt% during the tests. But, polyester and gelcoat contents were changed. The specimens consist of polyester, fiber, gelcoat, butanox-M60 and cobalt octoate. The properties of specimens are given as follows: Polipol 344-T is a special formulation thixotropic unsaturated polyester resin with high reactivity and medium viscosity, designed for hand lay-up and spray-up applications. Because of high heat resistance, it is widely used in the automotive industry on motor cabin and hood which are open to the effects of heat [17]. MAT-8 Fiber is developed with the properties of quick and easy wetting, fiber structure due to low resin consumption, easy sleep patterns, easy process ability with a wide use field from “E” glass type of fiber for general purpose applications. Table 1 The material types, thickness and contents used during the experiments Specimens
Thickness of material (mm)
344-T Polyester (%)
206 Gelcoats (%) 13
A
1.6
48.9
B
2.2
52.14
9.75
C
3.2
54.4
7.5
D
3.8
55.9
6
E
4.75
56.9
5
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MAT-8 is produced with TSI certificate according to TSEN-ISO 9002. Gelcoat GC 206 is based on a vinyl ester resin. It is suitable for mould production. It has a good handle ability, high quality with very good mechanical properties, high brightness, high temperature and high chemical resistances. Methyl ethyl ketone peroxide (MEKP) is the catalyst added to polyester resins and vinyl ester resins. As the catalyst mixes with the resin, a chemical reaction occurs, and creates heat which cures (hardens) the resin. Cobalt octoate is a primary drier which is an oxidation catalyst and acts as surface drier. It accelerates the catalytic action of MEKP to polymerize unsaturated polyester resin. In this study, the tensile and low-cycle fatigue tests were carried out on a servo hydraulic test machine (INSTRON 8801) equipped with load cell. Fatigue and tensile test sample dimensions are shown in Fig. 1. The thicknesses of MAT-8 samples were indicated with S. The thicknesses of GFRP specimens were chosen as 1.6, 2.2, 3.2, 3.8 and 4.75 mm, respectively. The hand lay-up production process was used to produce the composites. The hand lay-up process produces parts from an open glass reinforced mold. The mold surface was treated with several layers of release wax and then spray coated with a pigmented polyester resin called as gelcoat. The surface of the mold was duplicated by the gelcoat. Over the gelcoat, glass fiber was placed. Each layer was saturated with polyester resin that was specifically formulated to cure at room temperature. The polyester resins were catalyzed using MEKP 1 % and cobalt octoate 0.1 % by weight. Each glass layer was pressed by hand with rollers to process the polyester resin into the glass fiber. All samples were cut using a diamond-edged saw from larger sheets. Each specimen was exposed to the 90 ◦ C for an hour. All tests were conducted in an ambient environment. There is no multi-axial, bi-axial, or mono-axial direction in fiber arrangement in this study. Fiber
arrangement is the chopped strand mat (CSM). The weight of the CSM backing is 450 g/m2 . The tensile tests were conducted in load control at 250 N/min and it was kept constant during experiments. Fatigue tests were conducted using the same machine, and the results of these tests were plotted in terms of nominal stress amplitudes, σa,n (MPa) versus cycles to rupture, N R . Fatigue tests were performed in the axial load control under fully reversed loading (stress or load) ratio R = σmin / σmax = −1 which is sine waveforms using 15 samples for the determination of each S–N (Wöhler) curve. Each sample data used in the tests are an average of the three tested samples. The R ratio is defined as minimum applied stress divided by maximum applied stress. Stress intensity factor was obtained as K t =1. A sinusoidal load waveform was used at a constant frequency of 2 Hz. All the tensile and fatigue experiments were conducted in the Material Laboratory of Department of Mechanical Engineering of Cukurova University in Adana/Turkey. One of the most important experimental studies of material is hardness test. RM 401/A type Brinell Hardness Tester was used during the tests. Tests were performed at seven different points and 10-mm intervals on longitudinal section using Brinell Steel Ball intender with 31.25 kgf load for 25 s as shown in Fig. 2. The procedure employed for hardness testing was conducted in accordance with ASTM E10. The hardness value of all specimens was observed between 28 and 30 kgf/mm2 . In addition, the Archimedes method by weight was used to determine the density of GFRP specimens. The specimen was simply weighted dry in air, and then weighed again while it was submerged in methanol liquid using sensitive scale with 0.0001 g sensitivity. The difference between the two weights is buoyancy force. Dividing the buoyancy force by the density of the liquid converts the buoyancy force to specimen volume. Finally, the specimen weight divided by its volume
Fig. 1 Dimensions of samples (dimensions are in mm)
Fig. 2 The variation of the hardness value of all specimens with displacement
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Stress σ [MPa]
is sample’s density. This method is included in ASTM D792 and ASTM D3800. The morphological evaluations of the GFRP specimens were performed in a SEM Zeiss Evo 40 model microscope. The SEM observations were performed at Ni˘gde University Mechanical Engineering Laboratory. Prior to scanning electron microscopy (SEM) observations, some fracture surface of tensile and fatigue specimens were sputter-coated with gold.
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E
100 C
80 60
B
D
40
A
20 0 0
0,005
0,01
0,015
0,02
0,025
0,03
0,035
Strain ε [mm/mm]
3 Results and Discussion
Fig. 3 Room temperature tensile curves for GFRP
3.1 Tensile Properties
170 165
Table 2 shows the tensile properties of the GFRP. Typical stress–strain curves for each composite material are shown in Fig. 3. Figure 4 shows the comparison of ultimate tensile strengths (UTS), Young’s Modulus, hardness and density values of the specimens. As can be seen from Figs. 3 and 4, the sample B has the maximum UTS. However, sample E has the minimum UTS. The difference between specimen C which has the highest elongation and specimen D which has the lowest elongation is 18.60 %. Specimen A, having the highest Young’s Modulus value, also has the highest density (1.54 g/cm3 ) compared to those of other specimens. The lowest density (1.40 g/cm3 ) was observed in sample C. From the obtained results, it is understood that both polyester and gelcoat contents are effective in tensile strength. Generally, small content of polyester ratio and high content of gelcoats ratio increase the tensile strength. However, the highest tensile strength was observed in sample B. In the event of providing the mixing content of specific polyester and gelcoats, optimum tensile strength was achieved, while fiber ratio was constant. Hence, sample B has the most consistent content for tensile strength. It is also observed that the average hardness value of the content of sample B depends on polyester and gelcoat contents in parallel with tensile strength. Sample B is the best combination from the aspect of tensile strength; in addition, it has also the best strength after sample E in fatigue strength. Therefore, preTable 2 Tensile test results—failure stress, strain and the density value of each specimen
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A
B
C
D
E
160 155 150 145 140 135 130 125 120
Hardness
Density -2
3
(x10 )[gr/cm ]
-1
(2x10 )[HBS]
Elastic Modulus Ultimate Tensile -2 Stress [MPa] (5x10 )[GPa]
Fig. 4 Comparison of UTS, Young’s Modulus, hardness and density values of the specimens
ferring sample B combination is the most suitable way for designs requiring the best tensile and fatigue properties. On the other hand, gelcoat is determinant for composite density. Specimen A, in which gelcoats are used in the highest ratio, has the highest density. 3.2 Fatigue Properties The S–N curves of the GFRP specimens are shown in Fig. 5. The endurable nominal stress amplitudes at 104 , 105 and 106 cycles are compared in Fig. 6. From S–N curves, the fatigue strengths at 104 , 105 and 106 are different for five different combinations. The
Samples
Density (ρ) g/cm 3
Young’s Modulus (E) GPa
Ultimate tensile strength (UTS) MPa
Strain % (mm/mm)
Maximum elongation (mm)
A
1.54
7.2
153.84
2.416
0.725
B
1.47
6.16
165.58
2.607
0.782
C
1.40
6.5
148.33
2.626
0.788
D
1.41
6.78
148.99
2.213
0.664
E
1.49
6.73
141.67
2.257
0.677
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120 Stress Amplitude, Sa [MPa]
D 100
B E
80
f =2 Hz K t =1.0
C 60 40
R =-1
A
1
20 0 1,00E+02
1,00E+03
1,00E+04
1,00E+05
1,00E+06
1,00E+07
1,00E+08
1,00E+09
1,00E+10
Cycles to Failure, Nf
Stress Amplitude, Sa [MPa]
90 80
A
70
B
C
D
E
to generally aggravate the lifetime and fatigue strength of composites. 3.3 SEM Observations
60 50 40 30 20 10 0
N=10 4
N=10 5
N=10 6
Number of Cycles, N
Fig. 6 Comparisons of endurable stress amplitudes
endurable stress at 105 cycles is higher by 18 % than at 106 cycles for specimen E, which has the highest stress value. In fatigue test, it is understood that the shield-aimed gelcoat is fractured in the first cycle, and the rest polyester content is active in fatigue strength. Increasing polyester ratio is provided with increasing fatigue strength. Therefore, the ratio of polyester plays an effective role in fatigue strength. Because of the fact that sample E has the highest polyester content it shows the highest fatigue strength. The difference in fatigue strength increases at longer lifetimes (lower loads). It is evident from these fatigue graphics that the high percent polyester ratio in combination has a superior fatigue performance. These results are in agreement with the earlier studies by Demers [7] and Hartwig et al. [8]. The fatigue strength is dependent on both fiber content and gelcoat value. When a lay-up is containing a large number of fiber layers, fatigue tends to be dominated by fiber properties; on the other hand, when at lower fiber layers, fatigue tends to be dominated by gelcoat value. Moreover, the gelcoat value as matrix material is effective on the modulus of toughness, the total area under the stress–strain curve, which was seen in this study
The photographs of typical failed specimens are presented in Fig. 7. Fractographic studies with SEM were carried out on the fracture surface for GFRP specimen C. Gelcoat, MAT-8 Fiber and polyester structures of GFRP specimen are shown in Fig. 8. SEM micrographs of the fractured test specimens of (a) tensile and (b) fatigue are shown in Fig. 9. The brittle fracture of the matrix is observed in both tensile and fatigue tests, consistent with the brittle nature of the tensile stress– strain curves. It can be seen that most fibers are pulled out from the matrix. The tensile specimens (a) exhibit extensive splitting and shearing between fiber layers. The delamination grows under tensile loading, that is, the delamination surfaces separate perpendicularly to the plane of delamina-
Fig. 7 Photograph of typical failed specimens of (a) tensile; (b) fatigue
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increasingly load application. It coalesces to form a macroscopic fracture shortly before catastrophic failure. Polyester
4 Conclusions MAT-8
Gelcoat
Fig. 8 The structure of GFRP specimen
The following conclusions can be drawn from the study investigating tensile and fatigue behavior of glass fiberreinforced (MAT-8)/polyester automotive composite. The results indicated that polyester and gelcoat contents in these GFRP hand lay-up production made tensile and fatigue mechanisms to vary and determined final properties of the composites. While the gelcoat has the determinative character for composite density, thickness variation is obtained using fiber arrangement of the CSM. It is determined that tensile properties increase with higher gelcoat values up to the appropriate combinations, while fatigue properties decrease with increasing gelcoat. In addition, the combination of 52.14 wt% polyester and 9.75 wt% gelcoats (sample B) can be a preferable way for designs requiring the best tensile and fatigue properties. Optimum tensile strength was achieved, while fiber ratio was constant in the specific mixing of polyester and gelcoats. While the fatigue strength is mainly dependent on fiber content, the higher percent polyester ratio in combination also gives a superior fatigue performance. This research suggests that in lay-ups containing a large number of fiber layers in the loading direction, fatigue tends to be dominated by fiber properties, while at lower fiber layers, fatigue tends to be dominated by gelcoat value. Toughness is of particular importance as a property of the gelcoat value as matrix material and it was seen in this study to generally aggravate the lifetime and fatigue strength of composites reinforced with the same glass fiber and with resins of similar chemistry. Fracture surfaces in tensile test SEM micrography exhibited significant visible damage accumulation, whereas fracture surfaces showed a relatively clean, and straight-across break with little evidence of splitting on the edges in the fatigue test. It is also possible to examine all possible fiber arrangements to determine the most suitable combination in the future works. This will provide specific decreases and increases of combination ratio to keep material strength high from the aspect of both tensile and fatigue strength.
References Fig. 9 SEM micrographs of the fractured test specimens of (a) tensile; (b) fatigue
tion. On the other hand, the fatigue specimens (b) generally exhibit relatively clean, and straight-across breaks, with little evidence of splitting on the edges. The damage is well distributed throughout the composite and progresses with an
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