PSI - Issue 14

P R Krishna Mohan et al. / Procedia Structural Integrity 14 (2019) 176–183 P R Krishna Mohan/ StructuralIntegrity Procedia 00 (2018) 000–000

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UD prepregs. The laminates made of prepregs, B-staged at room temperature with vacuum and without vacuum are tested for 0° layups with an objective to improve the strength of the composite. The tests were conducted using the Instron 1195 Universal Testing Machine (UTM) with 50 kN load cell. The test samples are made according to ASTM D3039 and were tested at a speed of 0.5 mm/minute. The test data is reported in Table 1.

Table 1. Tensile test data for 0°, 90° and [±45] S samples Laminate Property

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Average SD 915.48 992.18 848.85 1059.63 1016.29 966.48 84.05

0 ° Non-vacuum Strength (MPa)

Modulus (GPa)

76.12

84.83

79.8

61.92

61.02

72.74 10.75

0 ° Vacuum Strength (MPa)

1158.44 1876.44 1611.11 1168.11 1763.42 1515.5 335.06

Modulus (GPa)

87.22

77.06

85.09

65.63

75.02

78

8.63

90° Laminate

Strength (MPa)

10.1

9.39

6.75

7.2

-

8.36

1.64

[±45] S Laminate Strength (MPa)

181.86 181.33 192.14 152.94 -

177.07 16.84

Axial Modulus (GPa) 9.56

9.1

9.73

9.65

-

9.51

0.28

3.4. Plasma treatment and its effect on carbon fiber We have performed oxygen plasma treatment on carbon fiber to improve the wettability between fiber and epoxy. By using plasma on fibers, it makes surface rough and induces oxygen functional groups. Surface roughness can help to increase wettability because the epoxy can adhere well and can penetrate through the pores created by plasma. But the fibers shouldn’t be exposed to the plasma for a long time and high powers as it destroys the fibers by excessive pitting which degrades its strength. We need to optimize the power and time. We have used 75W power for 30 seconds on each side of the tow. Fourier Transform Infrared Spectroscopy (FTIR) is performed on the plasma-treated and untreated fibers to check the effect of plasma on carbon fibers. 3.5. Fourier Transform Infrared Spectroscopy In FTIR, the light consisting of various frequencies is emitted on the sample giving the output in the form of intensity versus wavelength, which provides the amount of light with varying frequencies is absorbed by the sample. We have carried out FTIR on plasma treated and untreated carbon fiber to check the effect of plasma. We aimed to identify some of the essential functional groups which might have induced by plasma. These functional groups improve adhesion between carbon fiber and epoxy by chemical bonding and interaction. 4. Results and discussion 4.1. Tensile test for 0 ° laminate The uniaxial tensile tests on 0° laminate prepared with prepregs B-staged at room temperature with and without vacuum were conducted. Five samples were tested for each laminate to check the repeatability of the results. A typical load-displacement curve is shown inFig. 1. The elastic modulus of the material is calculated from the stress strain graph. The average strength obtained by earlier work done, without modifications in the prepreg machine, is 621.95 MPa. There is a significant improvement in the strength of 0° laminate after modifying the prepreg machine as shown inTable 1. The vacuum treated prepreg sheet laminate has more strength compared to the strength of the non-vacuum treated material. The consistency of the ultimate strength is observed in the non-vacuum 0° laminate. In the case of vacuum 0° laminate, there is a difference in the values for specimen 1 and specimen 4 with the other values of the specimens because of the tab failure. The other main reason for the early failure of the sample is the improper cutting of the testing sample. The maximum strength and modulus observed are1876.44 MPa and 87.22 GPa, respectively, and these samples are treated as successfully failed samples. From the images captured using scanning electron microscopy (SEM), as shown in Fig. 2, for cracked specimens made of laminates before and after modifications in the prepreg machine, it can be seen that the wetting of fibers has improved significantly. It is