Research Article EFFECT OF TEMPERATURE ON … I/IJAET VOL I ISSUE III... · International Journal of Advanced Engineering Technology E-ISSN 0976-3945 IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/225-231

International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/225-231

Research Article

EFFECT OF TEMPERATURE ON THE HYGROTHERMAL

AND MECHANICAL BEHAVIOR OF GLASS-EPOXY

LAMINATES Shivakumar S

a, Dr.Shivarudraiah

b

Address for Correspondence

*aResearch Scholar, Dept. of Mechanical Engg. UVCE, Bangalore-1

[Asst. Prof, Dept. of I&PE, GIT, Belgaum] bAsst. Professor, Dept. of Mechanical Engg. UVCE, Bangalore-1

[email protected], [email protected]

ABSTRACT

The aim of the work was to study the effects of temperature on hygrothermal and flexural behavior of glass/epoxy

composites exposed for 200 days at different temperature, 30, 50 and 70 °C. The failure mechanisms associated with

fractured damage were investigated under three-point bending loading. The flexural moduli, mass uptake, internal

laminar shear strength [ILSS] and flexural strength of glass-fiber reinforced epoxy matrix interface weakening and

debonding due to different hygrothermal ageing stages have been measured. These results showed that the combined

effect of the hygrothermal with the high temperature test, decrease the flexural and ILSS strength of the composite

materials but the thickness increased. Micro structural analysis was done for hygrothermally treated samples by

Scanning Electron Microscope.

KEYWORDS: Hygrothermal Degradation, Flexural Behavior, Epoxy-Glass Laminates

INTRODUCTION

The fiber reinforced plastics (FRPs), when

placed in service, are exposed to a variety of

ambient conditions in different types of

mechanical loading. The main atmospheric

agent causing ambient attacks are the

temperature, the relative humidity of air, the

effect of ultraviolet radiation, the chemical

exposition, the saline water and the contact with

hydraulic fuels, gases and fluids. Thermosetting

polymeric composites must perform under both

environmental conditions combined with

deformations and mechanical stresses [1].

Moisture penetration into the composite

materials is conducted by one major mechanism,

namely, diffusion. This mechanism involves the

direct diffusion of water into the matrix and in a

much less extent into the fibers. The capillarity

mechanism involves the flow of water molecules

along the fiber/matrix interface, followed by

diffusion from the interface into the bulk resin.

Transport of moisture by microcracks and voids

involves both flow and storage of water into the

microcracks and other forms of micro damages

[2]. The strength of the interface determines how

much of the applied stress can be transferred to

the load-bearing fibers. This strength is largely

determined by the contact area between fibers

and matrix and the level of adhesion at the

contact points. However, it is not clear how the

interface should be tailored to optimize

composite properties and whether optimum

adhesion is desirable or essential to achieve the

best balance of mechanical properties [3-5]. The

presence of moisture within a FRP can lead to

significant changes of the chemical and physical

characteristics of the polymeric matrix. Few

possible mechanisms have been suggested to



explain the effects of the moisture on most

composite systems matrix plasticization and

degradation of the fiber / matrix interface [6].

Moisture absorption by epoxy matrix composites

has plasticizer effect, as evidenced by Tg

reduction of the matrix [7].

EXPERIMENTAL STUDIES

Epoxy/glass composites were cured in ambient

condition according to the standard curing cycle

recommended by the material supplier. The

specimens were fabricated using wet hand-lay

process into flat panels measuring 250mmx

250mm with a thickness of 3 mm. The

specimens were cut to sizes as per ASTM

standards. The composites were cured at room

temperature without elevated temperature post

curing because most of the marine composite

structures are cured under ambient conditions. A

plane mould was treated with silicon based

releasing agent for easy removal of glass/epoxy

composites. A single strand of glass fiber was

placed on the mould and the layer of catalyzed

epoxy was poured on to it uniformly. Very light

rolling was then carried out to remove any gas

pockets if present and to uniformly distribute the

epoxy resin throughout the composite. The

micro-composites prepared were then cured at

room temperature for 24 hours. Then the

samples were treated in microprocessor

controlled climatic chamber for hygrothermal

conditioning. The samples were divided into six

batches, they were hygrothermally treated for 10

to 200 days in 95% RH and one batch remains

untreated [8]. The flexural strength of the

specimen (12.7mm width, 127mm length, and

3mm thickness) were determined for different

immersion times using the three-point bend test

as per ASTM-D790 using UTM. The flexural

strength of the composite was computed.

RESULTS & DISCUSSION

Fig. 1 shows the water uptake (% of mass

change), the rate of uptake and moisture content

were seen to increase with immersion time.

Overall results in terms of maximum percentage

weight gain, over the 200 day period of

investigation. The percentage of water uptake

with time is far greater in the case of higher

temperature exposure epoxy composites than the

room temperature exposure composites. All

temperature exposure composite specimens

showed saturation due to water uptake, since the

saturated levels of moisture uptake dictate the

property degradations in the materials employed

for underwater applications. As the sample

expands and shrinks, debonding between the

matrix and fiber creating voids which act as a

reservoir for moisture thereby increasing its

overall saturation level.

Fig. 2 shows change in ILSS of epoxy/glass

composites with respect to different temperature

exposure times. Though all the specimens

showed drop in flexural strength with respect to

immersion time because of moisture uptake,

vinyl ester-based specimens showed lower levels

of degradation, while epoxy-based specimens

showed a drop of 52% in ILS strength for an



exposure time of 200 days. The behavior of

epoxy-glass based composites was observed to

be very similar with respect to drop in flexural

strength shown in Fig. 3. Flexural strength is one

of the important properties in composites, which

determine the load sharing by the fibers, that is,

the interfacial strength. The moisture is seen to

attack the glass fiber surface with the free

hydroxides that form, further degrading the

silica structure at higher temperature. But this

work was conducted under room-temperature

conditions and hence higher degradation was not

observed in higher temperature. This indicates

that most of the damage mechanisms initiated by

seawater exposure are at the interface rather than

at the fiber level. A progression of change in

flexural modulus as a function of immersion

time and temperature is shown in Figure 4 for

the specimens immersed in water. It clearly

shows that the degradation increases

substantially with increase in immersion time. It

is of significant interest to note greater

degradation for 200 days followed by almost

saturation behavior. The amount of water uptake

by the epoxy-based composites is significantly

greater than that of the higher temperature

composites. This results in a mismatch in the

moisture-induced volumetric expansion at

interfaces. This leads to the evolution of

localized residual stress fields in the composites.

The water uptake most often leads to change in

the thermal, physical, mechanical, and chemical

properties of the composites. Integrity of the

composites in terms of matrix cracking and

fiber/matrix debonding/discontinuity by humid

aging may be reflected by studies on tensile

strength [10].

Mass change (%)

Fig.1. Effect of hygrothermal immersion duration on the water uptake of

epoxy/glass composites as a function of exposure duration and temperature.



Fig.2. Effect of hydrothermal immersion duration on inter-laminar shear strength of


Fig.3. Effect of hygrothermal immersion duration on the Flexural modulus of


Fle

xu

ral

mo

du

lus,

GP

a



Fig. 5 Hydrothermal effects on fracture failure of glass-epoxy composites

10µm

10µm

50µm



Due to the anisotropy of the construction of the

laminates, the process of failure in the

thermosetting composites submitted to the

loading in compression is very complex.

Different failure mechanisms can occur and they

are influenced by four main factors: the fiber

characteristics, the polymeric matrix behavior,

the angle lamination and the properties of the

fiber/matrix interface. Representative photos of

the samples tested in compression. All

specimens present kink band failure. It is

observed that the failure modes observed for the

laminates submitted to the room conditioning

are very similar to those ones observed for the

specimens submitted to the humidity

conditionings. It is also observed that the failure

is perpendicular to the applied load, revealing

interlaminar and translaminar failures. The

different environmental conditionings cause

changes on the resin physical and chemical

characteristics and on the fiber surface. The

decrease on the compressive property indicates

composite degradation. The fractography

analyses were carried out to elucidate what are

the probable causes that contribute to the

decrease of the compressive strength. The

effects of moisture on the fracture surfaces of

the specimens were examined by scanning

electron microscopy (SEM). Fig. 5 presents a

typical fracture surface of fractured compression

strength dry specimen of glass/epoxy laminate.

The smooth clean surface of fibers is caused by a

fracture micromechanism involving an

interfacial debonding in the glass/epoxy fabric

laminate. Cracks through a fiber tow, either in

the warp or fill direction, can also be identified

as a fracture micromechanism. The crack runs

along the matrix between fibers, this feature

suggests a brittle fracture micromechanism. In

this material the microcracks initiated at the

matrix at the interface growing along parallel

planes under the shear load.

CONCLUSION

Hygrothermic behavior of glass / epoxy FRP

was studied by exposing them to 30, 50 and 70

°C water for a maximum duration of 200 days.

The maximum water uptake and the mechanical

property degradations were studied. Moisture

absorption increased with increase in

temperature in all the cases. The property

degradation in the periods subsequent to

saturation was much less than that of pre-

saturation period.

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Documents

Research Article EFFECT OF TEMPERATURE ON … I/IJAET VOL I ISSUE III... · International Journal of Advanced Engineering Technology E-ISSN 0976-3945 IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/225-231