Ananta Prasad Chakraverty - CORE

Bar graph) 79 4.12: Change in ILSS of hydrothermally treated sample with. time of exposure to the above thermal shock. Bar graph) 93 4.18: Change in ILSS of hydrothermally treated sample with. heat shock exposure time down.

Polymer Matrix Composite 7

The properties of PMCs are highly dependent on factors such as the matrix and fiber material and their volume fractions, the fiber orientation, the applied stress levels and strain rates, as well as the loading conditions and the nature of the fiber polymer interface. The local response of the fiber matrix interface within the composite plays an important role in determining the gross mechanical performance.

Applications of PMCs 8

35 Kootsookos, A; Burchill, P.J.; "The Effect of Cure Degree on the Corrosion Resistance of Vinylester/Glass Fiber Composites". The data shows the following:. i) The 2 days hydrothermally treated sample shows an increase in ILSS starting with shorter to longer duration of down-thermal shock with some fluctuations.

Glass Fiber Reinforced Plastics (GFRPs) 9

Major characteristics of GFRPs 9

Applications of GFRPs 10

Today, more than 95% of commercial fiberglass reinforcement is made from E-type [22]. In recent years, the Glass Fiber Reinforced Polymers (GRP), especially glass/epoxy composites, are widely used in various civil engineering applications ranging from seismic retrofit of columns and reinforcement of walls, slabs to new building frames and bridges[23]. Composite materials have been widely used in the shipbuilding industry for four decades, mainly because of the clear advantages they offer over more conventional materials such as aluminum and steel.

Glass fiber 12

Bulk fiber structures 13

The oxides in the glass fiber are classified into three groups according to their function in the network structure. All the remaining oxides in the table are called "modifiers" because they can form only one or two chemical bonds and thus they can disrupt the network structures.

Bulk fiber properties 13

1 to 11; where as E-glass fibers degrade significantly in strength either above or below a PH of 6. Second, the size layer covering commercial glass fibers has a significant influence on the kinetics and mechanism of weight loss in solution.

Sizing of glass fibers 15

Epoxy Resin 16

Curing agents 17
Curing of Epoxy resin 17
Properties of cured epoxy resin 18

Glass Transition Temperature 18
Mechanical Properties 18
Tensile stress-strain behavior 19
Moisture Transport 19
Physical ageing 20

Applications of Epoxy Resin 20

The Tg of crosslinked polymer can be related to the total conversion, the stiffness of the crosslinked chain and the free volume trapped in the network. It was believed that the initial curing of the resin may affect the extent of degradation of the mechanical properties during exposure.

Environments to which the GFRP composites

The environmental exposure effects of fiber-reinforced polymer composites (FRPC) and the long-term retention of properties are important concerns for such applications where service life may span decades and little or no maintenance is expected. Designing for such lifetime requires the ability to predict changes in material properties as a function of environmental exposure, including bulk properties and integrity of fiber-matrix interfaces.

Environmental degradations of GFRPs 22

In applications where corrosive environments are experienced by the material, further degradation of mechanical properties may occur. The main types of damage that occur with composites are interlaminar cracks, interlaminar delamination, fiber breakage, fiber-matrix interface failure, and fiber pull-out.

Moisture Sorption Process 24

Fick’s Diffusion 24
The temperature dependence of diffusion coefficient 25
Weight gain (Moisture uptake) 26
The different stages of Moisture diffusion process 27
Factors influencing the Moisture diffusion 29
Moisture Transport in Polymers 32
Effect of moisture diffusion on

Plasticization 35
Swelling 36
Hydrolysis 36

Reduction in ILSS due to moisture absorption 37

Parameters affecting ILSS 38

Reduction in Glass Transition Temperature (T g ) 38
Microstructural damage 40
Various moisture environments 41

Hygrothermal conditioning 41
Hydrothermal conditioning 42
Sea water conditioning 42

They proposed interface osmosis by leaching alkali metal oxides from E-glass in the presence of moisture. The amount of water absorbed at the interface also depends on the nature of the glass surface, i.e.

Fig: 2.3 (a) Control specimens, (b) Conditioned specimens with hydrogen bonds [48]

Thermal shock and its effect on E-glass/epoxy composite 43

Effect of up-thermal and down-thermal shock on

A Bird’s eye view on Literature Survey 46

Schneider, M; "The Combined Effects of Load, Moisture and Temperature on the Properties of E-Glass/Epoxy Composites" Composite Structures. The data reveals the following: i) The ILSS of hydrothermally treated sample decreased for two days exposed sample from 'as cured' condition. The data reveals the following. i) The ILSS of up-thermally shocked hydrothermally treated sample for 2 days initially increased for shorter duration.

The ILSS gradually decreased from shorter to longer duration of shock treatment. v) The ILSS of the entire shocked hydrothermally treated sample converged over a longer period of time between a given range of values. The data shows the following:. i) The 2 and 4 day hygrothermally treated sample showed a wide variation of ILSS during the shock, ranging from shorter to longer duration. The data shows the following:. i) The 2, 6 and 8 day hygrothermally treated samples show the response to the downward thermal shock in a similar manner.

The data shows the following:. i) The 1-week, 2-week, and 3-week seawater-treated samples show a similar trend in ILSS after down-thermal shock.

Fig: 3.8 (Breaking of laminated composite by 3-point bend test)

Conclusion 56

Introduction 57

The equipment/instruments used to carry out the experiments are listed in tabular form indicating their specific use in the project, along with their specifications and details. This chapter contains a clear description of the detailed step-by-step methods used for making test samples, taking fixed weight of samples by heating process, different moisture treatment, heat shock treatment with different time periods. Sample characterization including mechanical testing with 3-point bending test (ILSS determination), generation of micrographs by scanning electron microscopy for micrograph analysis and determination of glass transition temperature of samples in required situations.

Equipment / Instruments used in the present investigation 57

A detailed report is also provided on the raw materials used for the manufacture of FRP test samples.

Materials used 57

Experimental flow sheet 59

Fabrication of E-Glass/Epoxy composite 60

Sample Preparation 60

Weight after extraction of moisture 60

Moisture treatment 61

Hydrothermal treatment 61

Hygrothermal treatment 61

Sea Water treatment 62

Thermal shock treatment 62

Up-thermal shock treatment 62

Down-thermal shock treatment 62

This leads to a decrease in ILSS for a shorter to longer duration of the upward thermal shock. The ILSS remains unchanged for the 3-week seawater-treated sample for the longer duration of the upward heat shock. During the final stage of low heat shock (-40 °C), the remaining moisture freezes, which may be responsible for the improvement of interfacial adhesion with a mechanical key [39].

Characterization 63

Differential scanning calorimetry (DSC)

DSC measurements were performed on a Mettler-Toledo 821 (Figure: 3.9) with an internal cooler using STAR software with an Alternating DSC (ADSC) module. The temperature calibration and determination of the time constant of the instrument were performed with In and Zn standards, and the heat flux calibration was performed with In. To calibrate the heat flux signal, a blank procedure was performed before the sample measurements with an empty container on the reference side and an empty container and lid on the sample side.

Scanning electron microscopy 64

The net result is a lower initial rate of moisture absorption in the seawater immersion case compared to the other two cases as evidenced by the experimental findings. 56] propose that seawater immersion may result in fiber-related mechanisms for moisture transport in the composite body. The above would explain the anomalous nature of moisture ingress in the case of immersion in sea water.

Inter laminar shear strength without thermal shock 77

The reduction in ILSS of the hydrothermally treated sample for the initial moisture treatment may be related to the weakening effects of the higher swelling stresses induced by heat and moisture at the interface and/or in the matrix resin [30]. The continuous decrease in ILSS for the hygrothermally treated sample with oscillation can be attributed to the formation of a double hydrogen bond in the epoxy chain. The slight decrease in ILSS in the seawater treatment for the initial moisture exposure is due to the weakening of the matrix due to the formation of hydrogen bonds.

Inter laminar shear strength with up-thermal shock 82

For hydrothermally treated sample 82

At longer shock duration, the 6-day hydrothermally treated sample shows a decrease in ILSS. The decrease in ILSS for the 8-day hydrothermally treated sample from shorter to longer duration can be analyzed due to the absence of stress relaxation for this situation. Therefore, the residual stress resulting from differential thermal contraction and expansion is much greater than the post-hardening effect for shorter to longer impact durations.

For hygrothermally treated sample 87

The ILSS variation is very visible in the case of the 4-day hygrothermally treated sample. ii) ILSS decreased rapidly from a maximum value to a large extent for the sample hygrothermally treated 2 and 4 days for a longer exposure time of thermal shock upwards. For which the growth rate of ILSS is less pronounced compared to the hygrothermally treated sample 2 and 4 days. The ILSS value for the 8-day hygrothermally treated sample maintained an approximately constant value from the shortest to the longest duration of thermal shock upwards.

Fig: 4.14 (Variation of ILSS of hygrothermally treated sample with the time of exposure of up-thermal shock)

For sea water treated sample 92

The one week seawater treated sample shows the decrease in ILSS for the initial duration of op-thermal shock. The ILSS will be lowered for 1 week seawater treated sample for longer conditioning time of shock ie. The ILSS value is observed to increase for 4 weeks seawater treated sample for shorter duration of shock ie.

Inter laminar shear strength with down-thermal shock 97

For hydrothermally treated sample 97

The increase in ILSS over 2 days of hydrothermally treated sample for shorter to longer duration of down thermal shock can be attributed to the cryogenic hardening of the matrix phase at low temperatures [14]. As a result, the increase in ILSS is observed during down-thermal shocks, starting with a shorter to longer duration of the shock. The variation of ILSS for a 4-day hydrothermally treated sample may be due to a variation in the duration of the down-thermal shock in the first stage.

For hygrothermally treated sample 102

The increase in ILSS for 2, 6, and 8 days of the hygrothermally treated sample for shorter shock duration can be attributed to the improvement of interfacial adhesion by mechanical locking due to frozen moisture during the second phase of downward thermal shock, i.e. The ILSS for 2, 6 and 8 days hygrothermally treated sample for longer shock duration can be attributed to the weakening effect due to downward thermal shock. This occurred due to the absorption of more moisture in the case of the 8 days hygrothermally treated sample.

For sea water treated sample 107

The initial decrease of ILSS during 1 week, 2 weeks and 3 weeks with seawater treated sample can be attributed to the detrimental effect due to down-thermal shock. For further down-thermal shock conditioning time, the ILSS is increased for 1 week, 2 week and 3 week seawater samples. Hence, the depression of Tg for 4 weeks of seawater-treated sample is prominent in case of upward thermal shock treatment.

Fig: 4.25 (Variation of Glass transition temperature of different moisture treated sample w.r.t

Effect of up-thermal shock and down-thermal shock

Scanning electron micrographs 121

A fiber fracture is also visible in Figure: 4.37, indicating a brittle failure mode in this sample following a downward thermal shock. A graphic electron micrograph of a 4-week seawater treated sample without the use of thermal shock clearly shows the fiber/matrix delamination shown in Figure: 4.41. Due to the large stress mismatch between the glass fibers and the epoxy resin, a rapid drop in ILSS during the maximum duration of the downward thermal shock is observed in this sample, as shown in Figure: 4.22 earlier in the thesis.

ILSS with no thermal shock 125

The total moisture uptake under hydrothermal conditions is less than under hygrothermal conditions for a similar length of exposure time. In all cases of moisture penetration, the weight percent moisture absorption shows a continued absorption trend and not a plateau even after prolonged exposure to the respective atmosphere. Thus, it is concluded that in all the cases studied, longer exposures to moisture help further moisture absorption by delaying the saturation level of moisture in the composite body.

ILSS with up-thermal shock 126

Ray, B. C.; "Thermal shock on interfacial adhesion of thermally conditioned glass fiber/epoxy composites" Materials Letters 58(2004) p2175-2177. Ray, B. C.; Biswas, A; Sinha, P.K.; "Hygrothermal effects on the mechanical behavior of fiber reinforced polymeric composites" Metals Materials and Process: 3 (1991) p99-108. Bond, I; Hucker, M; Weaver, P; Bley, S; Haq, S; "Mechanical behavior of circular and triangular glass fibers and their composites" Composite Science and Technology.

ILSS with down-thermal shock 126

Both up-thermal shock and down-thermal shock do not affect Tg much compared to the value under no-shock conditions for samples with seawater as well as hydrothermal exposure. The lowest value of Tg is recorded for the 8-day hygrothermally treated sample under op-thermal shock. Variation of Tg during the same hygrothermal exposure is negligible for downthermic shock compared to the non-thermal shock condition.

Failure modes 127

Ray, B.C.; "Temperature effect during moist aging at interfaces of glass and carbon fiber reinforced epoxy composites Journal of Colloid and Interface Science 298(2006) p111-117. Kootsookos, A; Burchill, P.J.; "The effect of the degree of curing on the corrosion resistance on vinyl ester/glass fiber composites" Composites: Part A 35(2004) p501- 508. Abdel-Magid, B; Ziaee, S; Gass, K; Schneider, M; "The combined effects of load, moisture and temperature on the properties of E-Glass/epoxy composites" Composite Structures 71(2005) p320-326.

A Bird’s Eye View on Literature Survey. 46

ILSS of hydroscopic, hygroscopic and sea water treated sample

ILSS of hydrothermally treated sample after up-thermal shock treatment. 82

ILSS of hygrothermally treated sample after up-thermal shock treatment. 87

ILSS of sea water treated sample after up-thermal shock treatment. 92

ILSS of hydrothermally treated sample after down-thermal shock treatment. 97

ILSS of hygrothermally treated sample after down-thermal shock treatment. 102

ILSS of sea water treated sample after down-thermal shock treatment. 108

Network structure of epoxy resin. 17

Schematic curves representing four categories of Non-Fickian

Humidity Cabinet. 65

Schematic diagram of hygrothermal chamber. 66