RESULTS AND DISCUSIONS

6.4.1 RESULTS OF SEM INVESTIGATION

It has been observed in the following Figure that the untreated fiber surface appeared generally smooth and glossy due to being coated with non cellulosic materials, mainly hemi cellulose, lignin pectin, waxes, oils and other traces material standard for cellulosic fiber. But shock treated fibers surface were seen to have less glossy, rough and cracked , indicating removal of non cellulosic compounds due to etching mechanism caused by shockwave. From the SEM images of FRP it is also understood that the untreated jute fiber makes poor interfacial adhesion with the matrix. But SEM analysis result shows that the treated jute fiber surface became rougher which enhancing the mechanical interlocking with the resin. The removal of surface impurities on the jute fiber is advantageous for fiber -matrix adhesion as it facilities both mechanical interlocking or anchorage and bonding reaction due to the exposure of hydroxyl group fiber. Shock tread has two effects on the fiber properties :(ⅰ) increases the fiber surface roughness that results in a better mechanical inter locking (ⅱ) it increments the amount of cellulose exposed on the fiber surface , thus increasing the number of possible reaction sites. The fiber pre impregnation allows an improved fiber wetting which in a normal fiber –resin mixing procedure would not be possible because of high polymer viscosity. Thus, the impregnation increases the mechanical inter locking between the fiber and matrix.

６１

Treated Jute fiber,100 MPa Treated Jute fiber,150 MPa

Treated Jute fiber,184 MPa Treated Jute fiber,200 MPa Treated Jute fiber,250 MPa Untreated Jute fiber,100 MPa

Fig. 6.4 SEM micrographs of untreated and treated jute fiber reinforced composite.

Treated jute composite150 MPa Untreated jute composite Treated jute composite100MPa

Treated jute composite,184 MPa Treated jute composite,200 MPa Treated jute composite,250 MPa

Fig. 6.5 SEM micrographs of untreated and treated jute fiber reinforced composite.

６２

In the previous chapter it is found that treated jute fiber shows higher permeability and wicking due to better wettability. The pores in the cracked or rougher cell wall have been increased in size to an extent where the polymer chain can penetrate easily.

.

A B

Fig. 6.6 Treated and untreated jute fiber reinforced composite (A=treated fiber composite, B=untreated fiber composite)

6.4.2 WATER ABSORPTION

The water uptake of soaked samples were also measured as a function of shockwave pressure. Results are described in the following Figure 6.7 It can be observed that the un-shocked fiber-polyester resin composite shows substantially higher water absorption than the composite made of shock treated jute fiber. Indeed, it is well known that the interface between the matrix and fiber in composites is an ill-defined but extremely important part of the material, which can easily allow the absorption of water. Moisture diffusion in polymeric composites has shown to be governed by three different mechanisms [23-24]. The first involves of diffusion of water molecules inside the micro gaps between polymer chains. The second involves capillary transport into the gaps and flaws at the interfaces between fibre and the matrix. This is a result of poor wetting and impregnation during the initial manufacturing stage. The third involves transport of microcracks in the matrix arising from the swelling of fibres (particularly in the case of natural fibre composites).

The encapsulation of the shock treated fiber with resin decreases the water sensitivity of the composite. Due to having poor interfacial bonding, FRP made up of untreated jute

６３

fiber shows the capillary transport into the gaps and flaws interfacial between the fiber and the matrix. So that, it is less hydrophobic than that of treated jute fiber reinforced composite.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

0 10 20 30 40 50 60 70

Time , hr

Water Uptake, %

Controlled 100MPa 150MPa 184MPa 250MPa

Fig.6.7 Effect of shock treatment on water absorption of FRP.

6.4.3 RESULTS OF BENDING TEST

Figures 6.8 and 6.9 explain the mechanical properties of treated and untreated jute fiber reinforced polymer composite. Using underwater shock jute fiber we were able to make randomly oriented jute –polyester composite that showed increase in stiffness than that of composite made from untreated jute fiber.

We suggest that the increase in stiffness is the result of resin penetrating into the cracks of fiber surface and locking the structure. Mechanical strength was not increased and the general shapes of stress or strain curve shows that the mechanical properties is decreased which indicating that fiber failure initiates composite failure. Result found from the previous chapter that shock treated jute fiber losses strength during the shock wave treatment.

６４

0 50 100 150 200 250

0 100 150 184 200 250

FRP With Fiber Treatment Presure, MPa

Flexural Stess , MPa

Fig.6.8 Effect of shock treatment of jute fiber on flexural stress of FRP.

0 1 2 3 4 5 6 7 8 9

0 100 150 184 200 250

FRP With Fiber Treatment Pressure, MPa Flexural Strain , x 10 -3 mm/mm

Fig. 6.9 Effect of shock treatment of jute fiber on flexural strain of FRP.

６５

6.4.4 RESULT OF UNDERWATER SHOCK LOADING TEST

Table 5.1 Response of FRP to underwater shock wave loading.

✓✓

✓✓ 3

5 1

150

✓✓

✓✓ 3

5 1

135

✓✓

✓✓ 3

5 1

100

✓✓

✓✓ 3

5 1

75

✓

✓✓

✓ 3

5 1

50

✓

✓✓

✓ 3

5 1

30

Broken Micro

cracked Unchanged

No of shock loading No of

treated sample No of

untreated sample Shock

strength applied, MPa

The above table and following micrographs explains the results of response of fiber reinforced plastics (FRP) composite.

a b

Fig. 6.10 Optical micrographs of micro cracked FRP after shock loading.

The underwater shockwave explosion experiments revealed that both type of FRP can suffer microstructural damage when impulsively loaded by a high pressure shockwave.

The optical micrograph examination of the FRP tested at shock pressure at 75 MPa revealed that damaged was confirmed to some micro cracking of the polymer matrix.

６６

These shockwave generally generate impulse s of very high pressures but short duration, resulting in extremely high strain rates, which may cause severe structural damage [25].