Influence of Fiber Orientation and Thickness on Tensile Properties of Laminated Polymer Composites

Headnote

Abstract: The objective of this research was to gain a better understanding of tensile properties of epoxy resin composites reinforced with glass fiber. The effect of fiber orientation & thickness of laminates has been investigated & experimentation was performed to determine property data for material specifications, the laminates were obtained by hand layup process. The laminates were cut to obtain ASTM standards. The test ready specimens were subjected to tensile loads on UTM machine. This research indicates that tensile strength is mainly dependent on the fiber orientation & thickness of laminated polymer composites.

Keywords: Fiber orientation, Epoxy resin composites, Glass fiber, laminated polymer composites.

Introduction:

The popularity/usage of bi-woven composites has increased recently in the aerospace, automobile and defense industries due to their lower production costs, light weight, higher fracture toughness, low thermal expansion, corrosion resistance and better control over the thermo-mechanical properties. Hingeless and bearingless helicopter rotor hubs that are designed using laminated composite materials experience centrifugal loads as well as bending in the flapping flexure region. The demand for improved performance of these structural materials makes it necessary to evaluate these materials under multi-axial loading. Fiber-reinforced composites show strong anisotropic mechanical behavior due to their fiber orientations. These orientations cause a variety of failure mechanisms, which are more complex under multi-axial loading conditions. Therefore, continuous effort has been made to make quasi-isotropic composite materials with controlling parameters, such as the orientation of adjacent plies, the stacking sequence and the properties of the constituents. Several researchers found that bending strength was greater than tensile strength in polymeric composite materials. Wisnom reported in his review that the ratios between 3-point flexural strength and tensile strength of different composite materials were in the range of 1.3-1.49. .The majority of engineering composites materials in demanding applications consist of continuous fibers of glass or carbon reinforcement in thermosetting epoxy polymer. There has been a tremendous advancement in recent days. Compared to metals, the polymeric composites have many advantages as higher fatigue strength, higher corrosion resistance and lower weight[1,2] polymeric composites are susceptible to mechanical damages when they are subjected to efforts of tension,flexural,compressionwhich can lead to material failure. Therefore it is necessary to use materials with higher damage tolerance & carryout an adequate mechanical evaluation. Damage tolerance of epoxy polymeric composites can be enhanced by improving the interlaminar properties by toughening matrix [3], reinforcement with bidirectional woven fabrics [4].

The basic concepts of composites material along with details of earlier works are explained by author at reference [5].The author investigated the influence of different sized glass fibers on the mechanical properties of glass fiber epoxy resin composites. The compressive experimental study to identify the effects of fiber cross sectional aspect ratio on tensile & flexural properties and failure modes of glass fiber/epoxy composites by using fibers of different cross sectional shapes was carried out by author[7].Author[8] investigated the influence of fiber orientation and fiber content of epoxy resin components on mechanical prosperities. The main aim of the present investigation was to study the influence of fiber orientation on tensile properties and also the influence of thickness of the specimen.

Experimental:

Materials:

Bi woven fabrics provide greater damage tolerance and increased popularity in different applications. The present investigation carried on epoxy resin bi woven glass fiber (238 gsm), the matrix materials are epoxy resin YD128 and hardener HY140 mixed in appropriate ratio with room temperature curing cycle of 48 hours duration.

Instrumental:

The composite laminates were subjected to various loads and computer controlled UTM. The specimens were clamped and tests were performed. The tests were closely monitored and conducted at room temperature. The load at which the completed fracture of the specimen occurred has been accepted as breakage load.

Sample Preparation:

Composites laminated were fabricated at room temperature in shape of rectangle plates by hand layup technique proper care was taken during fabrication of laminates to ensure uniform thickness minimum voids in the material and maintain homogeneity.

The laminates were fabricated by placing the glass fiber one over the other with a matrix in between the layers. Tools were used to distribute resin uniformly, compact plies and to remove entrapped air. The surfaces of the laminates were covered with 25 micron Mila film to prevent the layup form external disturbances. The laminates were cured in room temperature and at constant pressure for two days. The laminates were cut to suit ASTM dimension by a band saw cutter and edges were ground.

Test configuration:

All composites were processed such that fiber fraction was greater than the epoxy resin. For the purpose of investigation 3 different orientation and two different thicknesses were selected. Thickness selected were 2mm and 3mm. The fiber orientation selected for the experimentation was ±300, ±450, ±900. For the purpose of varying the volume fraction of the specimen the number of glass fabric layers was varied from 6 to 9. The tensile properties of the glass fiber reinforced polymer composites were determined according to ASTM 3039 [9] test standard specifications. The average tensile properties were determined from 3 specimen tests on each type of orientation and thickness.

Results and Discussion:

The experimental results clearly indicate that

a) Increase in thickness of laminates tends to decrease the tensile strength.

b) Load required to fracture the specimen completely depends on the thickness. If thickness increases by 1 mm then 20% more load is required for fracture

c) Extension of specimen completely depends on thickness of the specimen, if specimen is of 3 mm then extension varies from 3 to 22 % when compared with 2 mm

d) The load at the high yield point also increases upto 28 % when specimen is of 3 mm thickness

e) Young's modulus is decreased with increase in thickness of specimen, decrease seen to be about 12 to 30 % for each 1 mm increase in the thickness.

Effect of Fiber Orientation

When composite materials are designed, the reinforcements are always oriented in the load direction. However if the load direction is variable and not parallel to the fibers it becomes more important to investigate the laminate mechanical behavior. To investigate the effect of fiber orientation, ±300 , ±450and ±900 were selected under this study. Specimens with different fiber orientations were prepared under the same conditions as discussed earlier.

The experimental results show that the tensile strength is affected by the fiber orientation significantly as summarized below:

a) The tensile strength is superior in case of 300orientation.

b) More load is required for fracture of laminates in case of 900 orientation.

c) More elongation is observed in 300orientation. The elongation is minimal in case of 900 orientation

d) Maximum load at high yield point in case of 450 orientations.

e) Young's modulus is more in case of 900 orientations.

Conclusions

The experimental investigations used for the analysis of tensile behavior of glass fiber reinforced polymer laminates leads to the following conclusions.

i)The laminated specimens with lesser thickness leads to more ultimate tensile strength irrespective fiber orientations

ii)Specimen sustain greater load in 900orientation specimens than in other orientations.

iii)Young's modulus of specimens increases with decrease in thickness.

iv)Extension is minimum in case of 900 orientations and maximum in case 300 orientations.

References

References

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