Durability of Pultruded GFRP through Ten-Year Outdoor Exposure Test

Pultrusion is an easy molding method of fiber-reinforced polymer (FRP) to obtain a long composite material with a uniform cross-section at relatively low cost. In some cases, pultruded FRPs are now used in bridges and deck projects. Since the application of pultruded FRP as a structural material is increasing, a study on the durability of pultruded FRP under outdoor conditions is necessary in terms of safety. Some studies have shown that pultruded glass fiber-reinforced polymer (GFRP) exhibits a slight reduction in mechanical properties during outdoor exposure. Since pultruded GFRP consists of multi-layers, the change of mechanical properties in each layer is important to understand. In this study, an outdoor exposure test on pultruded GFRP for 10 years was conducted with three types of pultruded GFRP, which have different laminate systems, including surface-coated specimens of each type. Changes of strength and elastic modulus due to outdoor exposure were discussed with a focus on the contribution of each layer based on the rule of mixtures.


Introduction
Recently, the use of fiber-reinforced polymer (FRP) as a structural material has increased due to its high resistance to corrosion and low weight.Pultrusion of FRP is a particularly easy method that can produce long sections of material with a uniform cross-section at relatively low cost.This makes FRP a structural material which can be used for construction.Information on the durability of pultruded FRP is important for using FRP safely.There were some studies on the durability of pultruded FRP used for construction with mainly glass fibers [1][2][3][4][5], but many of them only focused on moist environments.When the durability of FRP in an ambient condition is considered, an outdoor exposure test is an important way to understand the durability.As for the use of glass fiber-reinforced polymer (GFRP) reinforcement, GFRP bars embedded in concrete for seven years were investigated to assess the structural reliability of a bridge deck reinforced with GFRP bars [6,7].Some studies on the durability of press-molded or hand lay-up GFRP investigated by outdoor exposure tests were also reported [8][9][10].
A six-year outdoor exposure test for pultruded GFRP was conducted by the authors [11], and it was shown that a surface coating has a protective effect and reduces the deterioration of mechanical properties and that uncoated GFRP showed a slight reduction of mechanical properties.However, in the six-year outdoor test, deteriorated layers of pultruded GFRP were not investigated although they consisted of multi-layers.
This study presents an outdoor exposure test on pultruded GFRP for 10 years to investigate layers of GFRP which can deteriorate during outdoor exposure.Three types of pultruded GFRP, with different laminate systems, were tested, including painted specimens of the GFRP.By calculating the contribution ratios of each layer based on the rule of mixtures, deteriorations of strength and elastic modulus of the pultruded GFRP are discussed.

Specimen
Pultruded GFRP plates made of E-glass fibers and vinylester resin, which is bisphenol A with styrene solvent, were used to prepare the specimens in this study.Each plate was 420 mm wide, 3.2 mm thick.The laminate systems consist of the combination of three fiber types: continuous strand mat with random fiber directions, plain-woven cloth with bidirectional fibers and unidirectional roving, as shown in Figure 1.Table 1 shows three laminate systems that were used in this study.Note that since the percentages of "Layer system and fiber type" are approximate values, the percentages in R26 do not add up to exactly 100%.These plates were cut into 620-mm-length specimens.In this study, the direction corresponding to the direction of pultrusion is defined as the 0 ˝direction, and the lateral direction of pultrusion is therefore defined as the 90 ˝direction.
Polymers 2015, 7, page-page 2 By calculating the contribution ratios of each layer based on the rule of mixtures, deteriorations of strength and elastic modulus of the pultruded GFRP are discussed.

Specimen
Pultruded GFRP plates made of E-glass fibers and vinylester resin, which is bisphenol A with styrene solvent, were used to prepare the specimens in this study.Each plate was 420 mm wide, 3.2 mm thick.The laminate systems consist of the combination of three fiber types: continuous strand mat with random fiber directions, plain-woven cloth with bidirectional fibers and unidirectional roving, as shown in Figure 1.Table 1 shows three laminate systems that were used in this study.Note that since the percentages of "Layer system and fiber type" are approximate values, the percentages in R26 do not add up to exactly 100%.These plates were cut into 620-mm-length specimens.In this study, the direction corresponding to the direction of pultrusion is defined as the 0° direction, and the lateral direction of pultrusion is therefore defined as the 90° direction.Coated and uncoated specimens were also prepared for all cases.Coated specimens were coated with an epoxy intermediate coating with a designed thickness of 30 μm and then coated with an acrylurethane resin top coating with a designed thickness of 30 μm by using a brush.The thicknesses of the intermediate and the top coating were determined based on manufacturer's specification and controlled by measuring their weights.The actual thicknesses of the intermediate and the top coating were measured by using a digital stereomicroscope, and it was confirmed that the actual thicknesses had a range from 70 to 75 μm.Two sets of specimens were prepared for all cases.
An exposure test was carried out for 10 years (from June 2003 to July 2013), in Tsukuba, which is located near Tokyo and has a moderate climate with an annual mean temperature of 14.3 °C and an annual rain fall of 1326 mm.The exposure test was first started to investigate the safety of pultruded GFRP for use as a structural material.Figure 2 shows the maximum, average and minimum temperatures at the Tsukuba exposure site.The temperatures were measured at intervals of one hour for 10 years, and they were averaged monthly.The maximum and minimum temperatures reached 31.5 and −3.1 °C, respectively, as shown in Figure 2.
The specimens were placed facing south on a 5° slope using steel exposure racks, and Figure 3 shows specimens placed by using the steel exposure racks.One set of specimens was retrieved after one year and the other set was retrieved after 10 years.Coated and uncoated specimens were also prepared for all cases.Coated specimens were coated with an epoxy intermediate coating with a designed thickness of 30 µm and then coated with an acrylurethane resin top coating with a designed thickness of 30 µm by using a brush.The thicknesses of the intermediate and the top coating were determined based on manufacturer's specification and controlled by measuring their weights.The actual thicknesses of the intermediate and the top coating were measured by using a digital stereomicroscope, and it was confirmed that the actual thicknesses had a range from 70 to 75 µm.Two sets of specimens were prepared for all cases.
An exposure test was carried out for 10 years (from June 2003 to July 2013), in Tsukuba, which is located near Tokyo and has a moderate climate with an annual mean temperature of 14.3 ˝C and an annual rain fall of 1326 mm.The exposure test was first started to investigate the safety of pultruded GFRP for use as a structural material.Figure 2 shows the maximum, average and minimum temperatures at the Tsukuba exposure site.The temperatures were measured at intervals of one hour for 10 years, and they were averaged monthly.The maximum and minimum temperatures reached 31.5 and ´3.1 ˝C, respectively, as shown in Figure 2. The specimens were placed facing south on a 5 ˝slope using steel exposure racks, and Figure 3 shows specimens placed by using the steel exposure racks.One set of specimens was retrieved after one year and the other set was retrieved after 10 years.
Polymers 2015, 7, page-page  Table 2 shows the tensile strength and modulus of the raw materials that are provided by the manufacturers and used to design the laminate systems.Despite the same fiber, the tensile modulus and strength of the continuous strand mat (CSM) are lower than those of the glass fiber because the CSM has random fiber directions.Tensile modulus and strength based on the rule of mixtures were calculated by Equations ( 1) and ( 2).The rule of mixtures is usually used for unidirectional FRP, but in this study it was used to calculate the contribution ratios of each layer in the laminates.The contribution of plain-woven cloth was divided by two because half of the fibers are oriented to the lateral direction of the testing direction.Upon calculating the tensile modulus and strength for the 90° direction, Vrov was assumed to be zero because the roving layer can contribute mainly to the 0° direction.In Equation ( 2), the empirical coefficient for tensile strength is usually obtained by a test and the previous studies [12] suggest that the coefficient for unidirectional GFRP is approximately 0.75.Each term in Equation (2) shows the contribution of each layer to the strength, hence the ratio of contribution of each layer was calculated by Equations ( 3)-( 6).
( ) where Et is the tensile modulus of the FRP (GPa), Ef is the tensile modulus of the glass fiber (GPa), Em is the tensile modulus of the matrix resin (GPa), ECSM is the tensile modulus of the CSM (GPa), Vrov is the volume fiber content ratio of the roving layer (for the tensing direction), Vcloth is the volume fiber content ratio of the cloth, VCSM is the volume fiber content ratio of the CSM.Table 2 shows the tensile strength and modulus of the raw materials that are provided by the manufacturers and used to design the laminate systems.Despite the same fiber, the tensile modulus and strength of the continuous strand mat (CSM) are lower than those of the glass fiber because the CSM has random fiber directions.Tensile modulus and strength based on the rule of mixtures were calculated by Equations ( 1) and ( 2).The rule of mixtures is usually used for unidirectional FRP, but in this study it was used to calculate the contribution ratios of each layer in the laminates.The contribution of plain-woven cloth was divided by two because half of the fibers are oriented to the lateral direction of the testing direction.Upon calculating the tensile modulus and strength for the 90° direction, Vrov was assumed to be zero because the roving layer can contribute mainly to the 0° direction.In Equation (2), the empirical coefficient for tensile strength is usually obtained by a test and the previous studies [12] suggest that the coefficient for unidirectional GFRP is approximately 0.75.Each term in Equation (2) shows the contribution of each layer to the strength, hence the ratio of contribution of each layer was calculated by Equations ( 3)-( 6).
( ) where Et is the tensile modulus of the FRP (GPa), Ef is the tensile modulus of the glass fiber (GPa), Em is the tensile modulus of the matrix resin (GPa), ECSM is the tensile modulus of the CSM (GPa), Vrov is the volume fiber content ratio of the roving layer (for the tensing direction), Vcloth is the volume  Table 2 shows the tensile strength and modulus of the raw materials that are provided by the manufacturers and used to design the laminate systems.Despite the same fiber, the tensile modulus and strength of the continuous strand mat (CSM) are lower than those of the glass fiber because the CSM has random fiber directions.Tensile modulus and strength based on the rule of mixtures were calculated by Equations ( 1) and (2).The rule of mixtures is usually used for unidirectional FRP, but in this study it was used to calculate the contribution ratios of each layer in the laminates.The contribution of plain-woven cloth was divided by two because half of the fibers are oriented to the lateral direction of the testing direction.Upon calculating the tensile modulus and strength for the 90 ˝direction, V rov was assumed to be zero because the roving layer can contribute mainly to the 0 ˝direction.In Equation (2), the empirical coefficient for tensile strength is usually obtained by a test and the previous studies [12] suggest that the coefficient for unidirectional GFRP is approximately 0.75.Each term in Equation (2) shows the contribution of each layer to the strength, hence the ratio of contribution of each layer was calculated by Equations ( 3)- (6).
Polymers 2015, 7, 2494-2503 where E t is the tensile modulus of the FRP (GPa), E f is the tensile modulus of the glass fiber (GPa), E m is the tensile modulus of the matrix resin (GPa), E CSM is the tensile modulus of the CSM (GPa), V rov is the volume fiber content ratio of the roving layer (for the tensing direction), V cloth is the volume fiber content ratio of the cloth, V CSM is the volume fiber content ratio of the CSM.
where F is the tensile strength of the FRP (MPa), K is the empirical coefficient for tensile strength, F f is the tensile strength of the glass fiber (MPa), F m is the tensile strength of the matrix resin (MPa), F CSM is the tensile strength of the CSM (MPa), V rov is the volume fiber content ratio of the roving layer (for the tensing direction), V cloth is the volume fiber content ratio of the cloth, V CSM is the volume fiber content ratio of the CSM.
where R rov is the contribution ratio of the roving layer, R cloth is the contribution ratio of the cloth layer, R CSM is the contribution ratio of the CSM layer, R m is the contribution ratio of the matrix resin.

Testing Method
Tensile tests in the 0 ˝and 90 ˝directions and an in-plane shear test (tensile test at a 45 ˝direction) were carried out, and Figure 4 shows the definition of the testing directions.The specimens retrieved from the exposure test were washed with water, and five coupons were taken for each test.Note that the portions of each plate which were within a 5 cm distance from the edges were not used for the tests to avoid disturbance from the edges.
Polymers 2015, 7, page-page ( ) where F is the tensile strength of the FRP (MPa), K is the empirical coefficient for tensile strength, Ff is the tensile strength of the glass fiber (MPa), Fm is the tensile strength of the matrix resin (MPa), FCSM is the tensile strength of the CSM (MPa), Vrov is the volume fiber content ratio of the roving layer (for the tensing direction), Vcloth is the volume fiber content ratio of the cloth, VCSM is the volume fiber content ratio of the CSM.
where Rrov is the contribution ratio of the roving layer, Rcloth is the contribution ratio of the cloth layer, RCSM is the contribution ratio of the CSM layer, Rm is the contribution ratio of the matrix resin.

Testing Method
Tensile tests in the 0° and 90° directions and an in-plane shear test (tensile test at a 45° direction) were carried out, and Figure 4 shows the definition of the testing directions.The specimens retrieved from the exposure test were washed with water, and five coupons were taken for each test.Note that the portions of each plate which were within a 5 cm distance from the edges were not used for the tests to avoid disturbance from the edges.The tensile tests were carried out based on ISO 3268.The coupons were 250 mm long and 25 mm wide, and the test speed was 1 mm/min.Equation ( 7) was used to calculate the tensile strength.For the coated specimens, the designed thickness of the paint layer (120 μm) was subtracted from the measured value.
where, σt is the tensile strength (MPa), Pt is the maximum tensile load (N), b is the width of the coupon (mm), h is the thickness of the coupon (mm).
Equations ( 8) and ( 9) were used to calculate the coefficient of variation (CV) based on the results

Specimen
Testing direction Direction of pultrusion (direction of roving fibers)

Testing direction
Coupon for tensile test, 0°Coupon for tensile test, 90°C oupon for in-plane shear test, 45°T he portion of 5cm from the edges was not used for the counpos The tensile tests were carried out based on ISO 3268.The coupons were 250 mm long and 25 mm wide, and the test speed was 1 mm/min.Equation ( 7) was used to calculate the tensile strength.For the coated specimens, the designed thickness of the paint layer (120 µm) was subtracted from the measured value.
Polymers 2015, 7, 2494-2503 where, σ t is the tensile strength (MPa), P t is the maximum tensile load (N), b is the width of the coupon (mm), h is the thickness of the coupon (mm).Equations ( 8) and ( 9) were used to calculate the coefficient of variation (CV) based on the results of five coupons.

CV ts "
SD ts σ t (8) CV tm " SD tm E t (9) where CV ts is the coefficient of variation for the tensile strength, SD ts is the standard deviation for the tensile strength (MPa), σ t is the averaged tensile strength (MPa), CV tm is the coefficient of variation for the tensile modulus, SD tm is the standard deviation for the tensile modulus (GPa), E t is the averaged tensile modulus (GPa).
The in-plane shear test (tensile test at a 45 ˝direction) was carried out based on JIS 7059.The coupons were 250 mm long and 25 mm wide, and the test speed was 1 mm/min.Equation ( 10) was used to calculate the in-plane shear strength.For the coated specimens, the designed thickness of the paint layer (120 µm) was subtracted from the measured value.
where τ s is the in-plane shear strength (MPa), P s is the maximum load (N), b is the width of the coupon (mm), h is the thickness of the coupon (mm).Equations ( 11) and ( 12) were used to calculate the coefficient of variation (CV) based on the results of five coupons.
CV ss " SD ss τ s (11) CV sm " SD sm G s (12) where CV ss is the coefficient of variation for the in-plane shear stress, SD ss is the standard deviation for the in-plan shear stress (MPa), τ s is the averaged in-plane shear strength (MPa), CV sm is the coefficient of variation for the in-plane shear modulus, SD sm is the standard deviation for the in-plane modulus (GPa), G s is the averaged in-plane shear modulus (GPa).Retention ratios are used to discuss the change of the strength and elastic modulus with aging, and they are defined in Equations ( 13) and (14).
where Rr s is the retention ratio for the strength, f t exposed is the tensile strength or the in-plane shear strength after the exposure (MPa), f t 0 year is the tensile strength or in-plane shear strength before the exposure (MPa), E t exposed is the tensile modulus or in-plane shear modulus after the exposure (GPa), Rr m is the retention ratio for the elastic modulus, E t 0 year is the tensile modulus or in-plane shear modulus before the exposure (GPa).

Unpainted Specimens
Test results of unpainted specimens are discussed with the contribution ratios based on the rule of mixtures.Table 3 shows the tensile strength of unpainted specimens in comparison with the calculated values.The experimental coefficient, K, was set as 1.0 for the calculation.The ratio of the experimental values over the corresponding calculated values are from 0.532 to 0.832, as shown in Table 3.The average value of the experiment/calculation ratio among the values of the 0 ˝and 90 ˝directions was 0.636, which was smaller than the suggested value, 0.75, for unidirectional FRP in the previous study [12].Table 4 shows the tensile and in-plane shear strengths of unpainted specimens with the retention ratios.CV ts and CV ss of 0 years have a range from 2.6% to 4.3%, except for that of R43 of the 0 direction, which is 21.0%.In the case of one year, Rr s shows a range from 0.91 to 1.16, and CV ts and CV ss range from 1.0% to 6.2%.In the case of 10 years, Rr s shows a range from 0.67 to 1.07, and CV ts and CV ss range from 2.1% to 7.8%.Rr s of R43 at the 0 ˝direction is higher than those of the other cases probably because of its bigger CV ts .Overall, Rr s of the 90 ˝and 45 ˝(in-plane shear) directions show lower values than that of the 0 ˝direction.Table 5 shows the contribution ratios of each layer based on the rule of mixtures calculated by Equations ( 3)-( 6).In the case of the 0 ˝direction, R rov and R cloth show dominant values, and their summations give a range from 82.2% to 93.1%.On the other hand, in the case of the 90 ˝direction, R cloth gives a range from 75.1% to 80.0%.It is thought that the reduction of Rr s of the 90 ˝direction in 10 years can be caused by the deterioration of plain-woven cloth because the contribution ratios of the plain-woven cloth of the 90 ˝direction are dominant.By considering R cloth of the 90 ˝direction, Rr s of the 45 ˝direction (in-plane shear) in 10 years may be also reduced by the deterioration of plain-woven cloth.Table 6 shows the elastic modulus of unpainted specimens with the retention ratios.CV tm and CV sm in 0 years have a range from 1.7% to 4.3%, except for that of R43 of the 0 ˝direction, which is 12.3%.Rr m of one year shows a range from 1.06 to 1.20, and a CV tm range from 1.1% to 4.6%.In the case of 10 years, Rr m shows a range from 1.01 to 1.29, and a CV tm range from 2.1% to 5.8%.Rr m is higher than those of the tensile and in-plane shear strengths, probably due to the effect of post-curing the resin.Note that five coupons were used for each test.
Figure 5 summarizes results of unpainted specimens.It is shown that, overall, the strength of the unpainted specimens declines after 10 years of outdoor exposure as shown in Table 4, and that the plain-woven cloth layers were deteriorated during 10 years of outdoor exposure by considering the contribution ratios of each layer calculated based on the rule of mixtures.On the other hand, the elastic modulus of the unpainted specimens shows a slight increase as shown in Table 6.
Polymers 2015, 7, page-page 7 Table 6 shows the elastic modulus of unpainted specimens with the retention ratios.CVtm and CVsm in 0 years have a range from 1.7% to 4.3%, except for that of R43 of the 0° direction, which is 12.3%.Rrm of one year shows a range from 1.06 to 1.20, and a CVtm range from 1.1% to 4.6%.In the case of 10 years, Rrm shows a range from 1.01 to 1.29, and a CVtm range from 2.1% to 5.8%.Rrm is higher than those of the tensile and in-plane shear strengths, probably due to the effect of post-curing of the resin.Note that five coupons were used for each test.
Figure 5 summarizes results of unpainted specimens.It is shown that, overall, the strength of the unpainted specimens declines after 10 years of outdoor exposure as shown in Table 4, and that the plain-woven cloth layers were deteriorated during 10 years of outdoor exposure by considering the contribution ratios of each layer calculated based on the rule of mixtures.On the other hand, the elastic modulus of the unpainted specimens shows a slight increase as shown in Table 6.

Painted Specimens
Test results of painted specimens are discussed by comparing them with the results of the unpainted specimens.Table 7 shows the tensile and in-plane shear strengths of painted specimens with retentions ratios.In the case of one year, Rrs shows a range from 0.92 to 1.21, and CVts and CVss range from 2.0% to 10.7%.In the case of 10 years, Rrs shows a range from 0.86 to 1.24, which are higher values than those of unpainted specimens.Rrs of unpainted specimens of the 90° and 45° (in-plane

Painted Specimens
Test results of painted specimens are discussed by comparing them with the results of the unpainted specimens.Table 7 shows the tensile and in-plane shear strengths of painted specimens with retentions ratios.In the case of one year, Rr s shows a range from 0.92 to 1.21, and CV ts and CV ss range from 2.0% to 10.7%.In the case of 10 years, Rr s shows a range from 0.86 to 1.24, which are higher values than those of unpainted specimens.Rr s of unpainted specimens of the 90 ˝and 45 ˝(in-plane shear) directions show lower values than those of the 0 ˝direction, while Rr s of painted specimens Figure 6 summarizes the results of the painted specimens.In contrast to the case of the unpainted specimens, the strength of the painted specimens after 10 years of outdoor exposure indicates a close value to the one before outdoor exposure, as shown in Table 7.It is thought that a surface coating can prevent the plain-woven cloth layers from deteriorating due to outdoor exposure.The elastic modulus of the painted specimens shows a slight increase as shown in Table 8, which is also observed in the case of the unpainted specimens.
Polymers 2015, 7, page-page 9 Figure 6 summarizes the results of the painted specimens.In contrast to the case of the unpainted specimens, the strength of the painted specimens after 10 years of outdoor exposure indicates a close value to the one before outdoor exposure, as shown in Table 7.It is thought that a surface coating can prevent the plain-woven cloth layers from deteriorating due to outdoor exposure.The elastic modulus of the painted specimens shows a slight increase as shown in Table 8, which is also observed in the case of the unpainted specimens.

Conclusions
The main conclusions from the present study can be summarized as follows: In the case of the unpainted specimens, the tensile and in-plane shear strengths after 10 years of outdoor exposure were lower than those of the ones before the exposure test.By calculating the contribution ratios of each layer, plain-woven cloth, which had a relatively higher contribution ratio than the other layers, seemed to deteriorate during the 10-year exposure.For the elastic modulus of the unpainted specimens, a slight increase was observed, probably due to the effect of post-curing of the resin.
In the case of the painted specimens, the tensile and in-plane shear strengths after 10 years of outdoor exposure were close to the ones before the exposure test, which were caused by the effect of the surface paint.However, further studies are needed to understand how the surface paint protects the painted specimens.For the elastic modulus of the painted specimens, as they have the same tendency as the unpainted specimens, a slight increase was observed.

Conclusions
The main conclusions from the present study can be summarized as follows: In the case of the unpainted specimens, the tensile and in-plane shear strengths after 10 years of outdoor exposure were lower than those of the ones before the exposure test.By calculating the contribution ratios of each layer, plain-woven cloth, which had a relatively higher contribution ratio than the other layers, seemed to deteriorate during the 10-year exposure.For the elastic modulus of the unpainted specimens, a slight increase was observed, probably due to the effect of post-curing of the resin.
In the case of the painted specimens, the tensile and in-plane shear strengths after 10 years of outdoor exposure were close to the ones before the exposure test, which were caused by the effect of the surface paint.However, further studies are needed to understand how the surface paint protects the painted specimens.For the elastic modulus of the painted specimens, as they have the same tendency as the unpainted specimens, a slight increase was observed.

Figure 1 .
Figure 1.A schematic illustration of cross-section of the specimens.

Figure 1 .
Figure 1.A schematic illustration of cross-section of the specimens.

Figure 2 .
Figure 2. Maximum, average and minimum temperatures each month at Tsukuba exposure site.

Figure 3 .
Figure 3. Specimens placed on exposure racks facing south on a 5° slope.

Figure 2 .
Figure 2. Maximum, average and minimum temperatures each month at Tsukuba exposure site.

Figure 2 .
Figure 2. Maximum, average and minimum temperatures each month at Tsukuba exposure site.

Figure 3 .
Figure 3. Specimens placed on exposure racks facing south on a 5° slope.
Apr May Jun Jul Aug Sep Oct Nov Dec Temperature (degrees Celsius) max.avg.min.

Figure 3 .
Figure 3. Specimens placed on exposure racks facing south on a 5 ˝slope.

Figure 4 .
Figure 4. Test coupons cut out from the exposed specimens.

Figure 4 .
Figure 4. Test coupons cut out from the exposed specimens.

Figure 5 .
Figure 5. Summary of results of unpainted specimens.

Figure 5 .
Figure 5. Summary of results of unpainted specimens.

Figure 6 .
Figure 6.Summary of results of painted specimens.

Figure 6 .
Figure 6.Summary of results of painted specimens.

Table 1 .
Laminate system of the specimens.

Table 1 .
Laminate system of the specimens.

Table 2 .
Mechanical properties of each layer of the specimens.

Table 2 .
Mechanical properties of each layer of the specimens.

Table 3 .
Tensile strength of unpainted specimens in comparison with calculated values.
Note that five coupons were used for each test.

Table 4 .
Tensile and in-plane shear strengths of unpainted specimens with retention ratios.

Table 5 .
Contribution ratios of each layer based on the rule of mixtures.

Table 6 .
Elastic modulus of unpainted specimens with retention ratios.

Table 5 .
Contribution ratios of each layer based on the rule of mixtures.

Table 6 .
Elastic modulus of unpainted specimens with retention ratios.