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Vol. 8. Issue 4.
Pages 3653-3661 (July - August 2019)
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Vol. 8. Issue 4.
Pages 3653-3661 (July - August 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.003
Open Access
Mechanical testing and microstructure characterization of glass fiber reinforced isophthalic polyester composites
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Ritesh Bhat, Nanjangud Mohan
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ns.mohan@manipal.edu

Corresponding author.
, Sathyashankara Sharma, Ashu Pratap, Agastya Prasad Keni, Dev Sodani
Mechanical Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal 576104, Karnataka, India
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Figures (13)
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Tables (13)
Table 1. Thickness and the corresponding number of layers for the manufactured composite sheets.
Table 2. Details of specimens for the flexural test.
Table 3. Tensile test results concerning the ultimate tensile strength for 6, 8, and 10mm thick GFRP specimens.
Table 4. Tensile test results concerning Young's modulus for 6, 8, and 10mm thick GFRP specimens.
Table 5. ANOVA results for tensile strength.
Table 6. ANOVA results for Young's modulus.
Table 7. Mode of failures in 6mm thick specimen as per the ASTM D3039 standards.
Table 8. Mode of failures in 8mm thick specimen as per the ASTM D3039 standards.
Table 9. Mode of failures in 10mm thick specimen as per the ASTM D3039 standards.
Table 10. Flexural test results for the 6, 8, and 10mm thick laminate specimens.
Table 11. ANOVA results for flexural strength.
Table 12. Barcol hardness test results for the 6, 8, and 10mm thick laminate specimens.
Table 13. ANOVA results for Barcol hardness.
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Abstract

The present work investigates the effect of material geometry in terms of material thickness on the mechanical properties of glass fiber reinforced isophthalic polyesters. The tensile strength, flexural strength, and Barcol hardness quantify the mechanical properties in this article. The experimental tests as per the ASTM standards show that the tensile strength and hardness increase with the increase in the thickness of laminates. The flexural strength is determined to decrease with the increasing thickness of laminates. The statistical analysis through ANOVA test shows that material thickness is a significant factor affecting the mechanical properties with 83.88, 70.37, 97.63 and 62.19% contribution toward the variance in the tensile strength, Young's modulus, flexural strength, and hardness respectively within the selected range of values for glass fiber reinforced isophthalic polyesters. The tensile failure mode analysis indicates that the increase in the thickness of laminates increases delamination failure. It is also found that the anisotropic behavior of the composites leads to an angular failure of the specimen in the tensile test. The random orientation of fibers leading to anisotropic behavior and increasing voids/decreasing interfacial bonding with the increasing laminate thickness is visualized in the microstructural studies through SEM images.

Keywords:
Glass fiber
Isophthalic polyesters
Mechanical testing
Microstructure characterization
Tensile failure modes
ANOVA
Abbreviations:
FRP
GFRP
SEM
ANOVA
Full Text
1Introduction

Composite materials, particularly fiber reinforced polymer (FRP) are most often used in marine crafts such as canoes, fishing trawlers, patrol boats, and naval ships. It also finds the application in hull casing, sonar dome, and masts of the submarine. Other non-transportation application of FRP includes deck gates, low-pressure underwater pipes, storage tanks, and civil infrastructures [1].

The excellent functional properties compared to conventional metals, good resistance to fatigue, and corrosion emphasizes the use of FRP composites in the marine industry [2]. Though there exists a good number of choices of reinforcement, for its economic advantage, the emphasis for bulk use is firmly on the glass fiber compared to the carbon and aramid fiber for the marine applications. There do exist high-performance crafts which employ the combination of carbon and aramid fibers, but the glass fiber still accounts for over 95% of the utilization in the maritime applications [3].

Polyester resins are one amongst the most popular and widely used matrix material in the glass fiber reinforced polymer (GFRP) composites used for the maritime applications [4–6]. Polyester resins are categorized broadly into two types viz., orthophthalic polyester resins, also known as standard economic polyester resins, being widely used in several industrial applications and isophthalic polyester resins, which is trending to be the preferred matrix material in the marine industry for its superior water resistance properties [7]. The prior is prepared using phthalic anhydride, maleic anhydride or fumaric acid and glycols, whereas; the latter is prepared by replacing phthalic anhydride by isophthalic acid [8].

Evaluating the mechanical property of materials is of the utmost importance in any application. Various researchers have dedicated their time in investigating the mechanical property of fiber reinforced polymer matrix composites in aged as well as unaged conditions concerning the marine applications. Awaja et al. [9] in their comprehensive review, emphasized the prevention of crack propagation in the marine application-based glass fiber reinforced polymers. Jang et al. [10] in their research work investigated the effect of gel-coats used in the fiber reinforced polymer composites used in the marine industry on the tensile strength and hardness. Tran et al. [11] investigated the pyrolysis effect on marine-based GFRP composites and proposed a possible solution to improvise it. Jesthi and Nayak [12] proposed a way of hybridizing the glass and carbon fibers to improve the mechanical properties of marine application-based fiber reinforced composite materials.

Though isophthalic polyesters are the trending replacement to the general purpose orthophthalic polyester resins, only a few have focused on investigating the mechanical property of it concerning the marine application. Also, there has been no much focus of the researchers in investigating the effect of variation in thickness on the mechanical property. Considering the research gaps, the present work focuses on investigating the mechanical properties in terms of tensile, flexural strength, and Barcol hardness of glass fiber reinforced isophthalic polyesters used in marine applications. The work also discusses in detail regarding the modes of tensile failure as per the ASTM standards.

2Materials and methods2.1Material preparation

Hand lay-up method is one of the simplest and oldest techniques of manufacturing the fiber reinforced polymer composites [13–15]. It is also the most economical process compared to its counterparts like resin transfer molding and vacuum process. Considering the economic advantage and the popularity of the hand lay-up process, the same has been employed in the current work to make the required glass fiber reinforced polymer composites.

The chopped stranded mat of E-glass fiber of 450 GSM with randomly oriented fibers is used as the reinforcing material and the marine grade isophthalic polyester resin is used as the matrix or the binding material. The methyl-ethyl-ketone peroxide (MEKP) is used as the hardener and mixed in the ratio of 0.012:1 (hardener to resin). The resins, fiber mats, and the hardener are procured from Shri Mookambika poly products, Udupi, Karnataka, India. The standard 2:1 ratio (glass fiber t0 the polyester matrix) is followed for making the composite laminates. Composites of dimensions 300mm×300mm are made each for three different thickness values of 6, 8, and 10mm. The number of layers required for fabricating each of the three variants is calculated using Eqs. (1)–(3), where is the thickness of constituent, Tc1 and Tc2 are the thickness of reinforcement and matrix materials respectively, Ta is the total thickness of one layer of composite laminate and T is the total desired thickness. Table 1 details the number of layers calculated for each of the three variants of manufactured composite sheets. m and d in Eq. (1) represents the mass and density values.

Table 1.

Thickness and the corresponding number of layers for the manufactured composite sheets.

S. No.  Thickness (mm)  Number of layers (N) 
1. 
2.  11 
3.  10  14 
2.2Experimental method2.2.1Tensile test

The mechanical properties in terms of the ultimate tensile strength, tensile strain, and Young's modulus are determined by employing ASTM D3039 standard for determining the tensile properties of a polymer matrix composite [16]. Five flat strips specimens with rectangular cross-section are cut from each of three thick composites. The strips are mounted in the grips of Unitek 9450 computerized universal testing machine, as shown in Fig. 1, which is monotonically loaded in tension while recording the applied load. The ultimate strength of the specimen is determined for the maximum load carried before failure. The stress–strain responses are recorded to determine Young's modulus for each specimen. The mode of failure also is noted, concerning the ASTM D 3039 standards, as shown in Fig. 2.

Fig. 1.

Tensile strength test using Unitek 9450 computerized universal testing machine.

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Fig. 2.

Modes of failure during the tensile test in a polymer matrix composite specimen [16].

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2.2.2Flexural test

The mechanical property in terms of the flexural stiffness or strength is determined by employing ASTM D7264 standard for determining the flexural properties of a polymer matrix composite [17]. The specimens are supported as a beam and deflected at a constant rate. The point load is applied at the midpoint from the supporting end. The force thus applied to the specimen and the resulting specimen deflection at the mid-portion is measured and recorded until the failure occurs on either of the two surfaces. The Instron 3366 UTM is employed for accomplishing the discussed test and shown in Fig. 3. The details of specimens for flexural or three-point bending test are given in Table 2.

Fig. 3.

Flexural strength testing using Instron 3366 UTM.

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Table 2.

Details of specimens for the flexural test.

S. No.  Thickness (mm)  Length (mm)  Width (mm) 
1.  36  12 
2.  48  16 
3.  10  60  20 
2.2.3Hardness test

The mechanical property in terms of hardness is determined by employing ASTM D2583 standard for determining the Barcol hardness of a polymer matrix composite [18]. Five specimens with 30mm×30mm dimension are cut from each of three thick composites. The specimens are dry conditioned at room temperature for 48h before testing as per the requirement of ASTM 2583 standard. The hardness test is then conducted in the laboratory at ambient atmospheric conditions. The indentations created on each of the specimens are at least 3mm away from the edge of the specimen and each other. The indentation is done only on the smooth surface, which is coated by the polyester resin. On each specimen, five readings are recorded. The specimens prepared and tested for hardness is shown in Fig. 4.

Fig. 4.

Prepared and tested specimens for Barcol hardness test.

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3Results and discussion3.1Tensile test experimental and statistical results

The results obtained for the conducted tensile test as per the ASTM D3039 standard are given in Tables 3 and 4. The stress–strain graph representing the maximum recorded values concerning the three different thickness is shown in Fig. 5. The obtained stress–strain graph indicates the brittle nature resulting in the catastrophic failure of the prepared composite as there exist no yield points before the fracture mechanism. The bar chart of tensile strength versus thickness is shown in Fig. 6. The tensile strength increased by 11.86 and 11.06% when the material thickness is increased from 6 to 8mm and 8 to 10mm, respectively. Young's modulus decreased by 13.12 and 13.89% when for material thickness increased from 6 to 8mm and 8 to 10mm, respectively. The results indicate that the thickness has a significant effect on the tensile strength and Young's modulus in case of glass fiber reinforced isophthalic polyesters.

Table 3.

Tensile test results concerning the ultimate tensile strength for 6, 8, and 10mm thick GFRP specimens.

Sample No.  Ultimate tensile strength (MPa)
  6mm  8mm  10mm 
120.89  127.63  145.03 
121.65  138.45  150.68 
110.10  134.74  149.70 
115.89  128.44  144.80 
119.03  128.07  139.72 
Average  117.51  131.46  145.99 
Table 4.

Tensile test results concerning Young's modulus for 6, 8, and 10mm thick GFRP specimens.

Sample No.  Young's modulus (MPa)
  6mm  8mm  10mm 
3231.35  2587.14  2394.80 
3368.81  2858.18  2359.10 
2575.98  2844.73  2224.43 
3563.56  2587.43  2387.06 
3058.90  2835.81  2443.07 
Average  3159.72  2742.66  2361.69 
Fig. 5.

Comparison of stress–strain curves for the maximum tensile strength values.

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Fig. 6.

Analysis of tensile strength for the tested specimen.

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The one-way analysis of variance (ANOVA) test results for tensile strength and Young's modulus are given in Tables 5 and 6. The ANOVA result for tensile strength at 95% confidence interval validates the obtained experimental result and proves that there is a significant effect of thickness on the tensile strength, as the obtained ‘P-value’ is less than the α value of 0.05.

Table 5.

ANOVA results for tensile strength.

Source  DF  Adj SS  Adj MS  F-value  P-value 
Factor  2027.1  1013.54  46.63  0.000 
Error  12  260.8  21.74     
Total  14  2287.9       
Table 6.

ANOVA results for Young's modulus.

Source  DF  Adj SS  Adj MS  F-value  P-value 
Factor  1593207  796604  14.25  0.001 
Error  12  670786  55899     
Total  14  2263993       

The high range of ‘F-value’ of 46.63, being greater than the Fcrit=3.7, proves the fact that the variance within the group is lesser than the variance between the group. Thickness is proved to contribute 88.6% toward the variance in the tensile strength corresponding to a very high degree of correlation (94.12%).

The ANOVA result for Young's modulus at 95% confidence interval validates the obtained experimental result and proves that there is a significant effect of thickness on Young's modulus, as the obtained ‘P-value’ is less than the α value of 0.05. The high range of ‘F-value’ of 14.25, being greater than the Fcrit=3.7, proves the fact that the variance within the group is lesser than the variance between the group. Thickness is proved to contribute 70.37% toward the variance in Young's modulus, corresponding to a high degree of correlation (83.88%).

3.2Tensile failure mode analysis

Figs. 7–9 represent the pictures of the fractured specimens of 6, 8, and 10mm thickness respectively after the tensile test. From Fig. 7, it is evident that the mode of fracture varies in the 6mm thick laminates. The variety of fracture modes observed in comparison to the standard codes as per the ASTM standards is given in Table 7 for 6mm thick samples. Table 7 shows that all the specimen fail at the top location. The reason being the formation of the main chap causing the stress concentration at the top end portion. Though multiple chap formation is the actual phenomenon, the formed main chap at the top portion inhibits the further development of chaps at other portion of the specimen [19]. Table 7 also indicates that three out of five specimens had an angular type of failure instead of lateral failure. The reason for the angular failure is the anisotropic characteristic of the composites [20]. The orientation of the fibers has a significant influence on the mechanical property [21]. The anisotropic behavior is highly induced in the composites having randomly oriented fibers [22] as is the case of the composite specimens prepared for the current study. Specimens 3 and 4 faced delamination type of failure, wherein the damage primarily occurred due to the separation of layers from each other [23]. The delamination failure is developed generally due to excessive inter-laminar stresses generated at the interfaced between adjacent piles. The delamination failure in the specimen during mechanical testing can also be known as premature failure.

Fig. 7.

Fractured specimen of 6mm thickness after the tensile test.

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Fig. 8.

Fractured specimen of 8mm thickness after the tensile test.

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Fig. 9.

Fractured specimen of 10mm thickness after the tensile test.

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Table 7.

Mode of failures in 6mm thick specimen as per the ASTM D3039 standards.

Specimen number (left to right in Fig. 6Mode of failure as per the ASTM standards  First character (Failure type)  Second character (Failure area)  Third character (Failure location) 
AGT  Angular  Gage  Top 
AAT  Angular  At the grip  Top 
DGT  Delamination  Gage  Top 
DGM  Delamination  Gage  Middle 
AAT  Angular  At the grip  Top 

The same is evident from Table 3, wherein specimens 3 and 4 demonstrate a lower tensile strength compared to the other three for the 6mm thick laminate. Table 8 indicates that all the specimens bearing 8mm thickness demonstrate the angular type of failure at the top end. As discussed earlier, the angular failure indicates the anisotropic behavior due to randomly oriented fibers, and the top end failure represents the formation of the main chap at the top location [19–21]. Also, it is observed from Table 8 that the majority of specimens failed at the grip. The reason for this being the local damage caused resulting in excessive stress concentration due to a high clamping force and the absence of tabs, the presence of which provides a cushion effect to the applied grip force [24]. Table 9 indicates that four out of five specimens bearing 10mm thickness fails at the top, which is an indication of high-stress concentration resulting from the high clamping force and absence of the tab [24]. It also indicates that the majority of failures are of delamination type because of the combined effect of increased thickness of laminates and the high clamping force at the grips [25,26].

Table 8.

Mode of failures in 8mm thick specimen as per the ASTM D3039 standards.

Specimen number (left to right in Fig. 6Mode of failure as per the ASTM standards  First character (Failure type)  Second character (Failure area)  Third character (Failure location) 
AGT  Angular  Gage  Top 
AAT  Angular  At the grip  Top 
AAT  Angular  At the grip  Top 
AAT  Angular  At the grip  Top 
AAT  Angular  At the grip  Top 
Table 9.

Mode of failures in 10mm thick specimen as per the ASTM D3039 standards.

Specimen number (left to right in Fig. 6Mode of failure as per the ASTM standards  First character (Failure type)  Second character (Failure area)  Third character (Failure location) 
LGT  Lateral  Gage  Top 
DAT  Delamination  At the grip  Top 
DAT  Delamination  At the grip  Top 
DMV  Delamination  Multiple areas  Various 
DAT  Delamination  At the grip  Top 
3.3Flexural test experimental and statistical results

The results obtained for the conducted flexural strength test as per the ASTM D7264 standard is given in Table 10. The results indicate that the thickness has a significant effect on the flexural strength in case of glass fiber reinforced isophthalic polyesters and is found to reduce with the increase in the thickness of laminates. Fig. 10 represents the reduction observed in the flexural strength with the increase in the thickness of the composite laminates. The flexural strength decreased by 8.84 and 25.87% when the material thickness increased from 6 to 8mm and 8 to 10mm, respectively. The same has been validated statistically using the one-way ANOVA test given in Table 11. The ANOVA result for flexural strength at 95% confidence interval validates the obtained experimental result and proves that there is a significant effect of thickness on the flexural strength, as the obtained ‘P-value’ is less than the α value of 0.05. The high range of ‘F-value’ of 246.75, being greater than the Fcrit=3.7, proves the fact that the variance within the group is lesser than the variance between the group. Thickness is proved to contribute 97.63% toward the variance in the flexural strength corresponding to a very high degree of correlation (98.81%).

Table 10.

Flexural test results for the 6, 8, and 10mm thick laminate specimens.

Sample No.  Flexural strength (MPa)
  6mm  8mm  10mm 
329.36  310.43  213.60 
323.71  284.06  212.16 
317.91  295.61  225.26 
322.38  294.00  229.98 
325.28  291.38  212.79 
Average  323.73  295.10  218.76 
Fig. 10.

Analysis of flexural strength for the tested specimen.

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Table 11.

ANOVA results for flexural strength.

Source  DF  Adj SS  Adj MS  F-value  P-value 
Factor  29443.3  14721.7  246.75  0.000 
Error  12  716.0  59.7     
Total  14  30159.3       

The increase in the thickness of laminates increases the fiber volume or content in the composites. Though researchers state that the increase in the fiber content in the composites solely causes a reduction in the flexural strength of the laminates [27], the stated reason stands hypothetical as there is a constant ratio of glass fiber to matrix maintained inconsiderate to the thickness of the material. The actual reason for the tremendous decrease in the flexural strength is the decrease in the compression strength [28] of the upper surface of the fiber, which is less supported by the matrix [29].

3.4Hardness test experimental and statistical results

The results obtained for the conducted Barcol hardness test as per the ASTM D2583 standard is given in Table 12. The results indicate that the thickness has a significant effect on the hardness in case of glass fiber reinforced isophthalic polyesters and is found to increase with the increase in the thickness of laminates. The Barcol hardness increased by 7.86 and 6.51% when the material thickness increased from 6 to 8mm and 8 to 10mm, respectively. The same has been validated statistically using the one-way ANOVA test given in Table 13.

Table 12.

Barcol hardness test results for the 6, 8, and 10mm thick laminate specimens.

Sample No.  Barcol hardness number (HB) (average of 5 points on a specimen)
  6mm  8mm  10mm 
43.40  46.60  45.40 
45.80  49.20  52.40 
42.40  48.40  49.40 
42.20  45.40  48.80 
42.60  43.80  52.60 
Average  43.28  46.68  49.72 
Table 13.

ANOVA results for Barcol hardness.

Source  DF  Adj SS  Adj MS  F-value  P-value 
Factor  103.79  51.896  9.87  0.003 
Error  12  63.10  5.259     
Total  14  166.90       

The ANOVA result for flexural strength at 95% confidence interval validates the obtained experimental result and proves that there is a significant effect of thickness on the hardness, as the obtained ‘P-value’ is less than the α value of 0.05. The high range of ‘F-value’ of 9.87, being greater than the Fcrit=3.7, proves the fact that the variance within the group is lesser than the variance between the group. Thickness is proved to contribute 62.19% toward the variance in the hardness corresponding to a nearly high degree of correlation (78.86%). The hardness is defined as the resistance to permanent indentation or penetration. In simple words, it is the resistance to the plastic deformation [30]. The increase in thickness implies the increase in fiber layers. The increase in the fiber contents increases the hardness of the composites [31,32]. The same effect is seen in the experimental test results.

3.5Microstructural studies

The microstructure of the fabricated glass fiber reinforced isophthalic polyester composites is investigated using the scanning electron microscopic (SEM) analysis. The SEM images of 6, 8, and 10mm specimen are shown in Figs. 11–13, respectively. From the obtained SEM images, it is evident that the compact interfacial bonding between the glass fiber and the isophthalic polyester resins decreases with the increase in the thickness. The compact bonding between the reinforcing fiber and binding matrix is high in 6mm specimen and least in 10mm specimen.

Fig. 11.

SEM image for the 6mm thick glass fiber reinforced polyester composites.

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Fig. 12.

SEM image for the 8mm thick glass fiber reinforced polyester composites.

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Fig. 13.

SEM image for the 10mm thick glass fiber reinforced polyester composites.

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The voids increase with the increase in thickness, which validates the previously obtained results of decrease in the flexural strength and occurrence of the delamination during the tensile test with the increase in the thickness. The random orientation of glass fibers is also observed, which leads to anisotropic behavior, leading to angular failure of the specimen during the tensile testing.

4Conclusions

In the present work, the contingency of mechanical property over the material thickness is investigated on the glass fiber reinforced isophthalic polyester composite material. The specimens are made using simple hand lay-up method. The mechanical tests are conducted as per the relevant ASTM standards. ANOVA is carried out to determine the significant contribution of material thickness on the tensile strength, flexural strength, and Barcol hardness. The mode of failure in the tensile specimen is studied in comparison to the ASTM D3039 standards and reasons are validated using the scanning electron microscopic analysis. Based on the conducted work, the following conclusions are made.

  • The tensile strength increased by 11.86 and 11.06% when the material thickness is increased from 6 to 8mm and 8 to 10mm, respectively.

  • Young's modulus decreased by 13.12 and 13.89% when the material thickness is increased from 6 to 8mm and 8 to 10mm, respectively.

  • The flexural strength decreased by 8.84 and 25.87% when the material thickness is increased from 6 to 8mm and 8 to 10mm, respectively.

  • The Barcol hardness increased by 7.86 and 6.51% when the material thickness is increased from 6 to 8mm and 8 to 10mm, respectively.

  • The ANOVA results indicate that the material thickness significantly affects the mechanical properties and contributes 83.88, 70.37, 97.63 and 62.19% contribution toward the variance in the tensile strength, Young's modulus, flexural strength, and hardness respectively within the selected range of values for glass fiber reinforced isophthalic polyesters.

  • The failure mode analysis indicates that there exists an angular mode of failure in most of the specimen, which is due to the high anisotropic behavior of the prepared composites.

  • Due to the absence of tab and high clamping force, the specimens, during the tensile test, fails at top end location and at the grip area.

  • SEM images of the microstructure validate the experimental results and proves the fact that the compact interfacial bonding between the fibers and matrix reduces with the increase in the thickness, leading to a reduction in the flexural strength and delamination failure during the tensile test.

Competing interest

The authors declare that there is no competing interest.

Funding

The present work did not receive any funding from private, government, and non-government organization.

Availability of data

All the data are available with the authors and can be provided on request.

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