Temperature Effect on Poisson’s Ratio of Woven Composites

ABSTRACT

Monotonic tensile tests were conducted following ASTM Standards D3039 (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials) and D3518 (Standard Test Method for In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a +45° Laminate), on non-hybrid plain weave composite materials. Strips (6.35mmx25mmx250mm) of non-hybrid IM-7 Graphite/SC-79 epoxy called GR for short, non-hybrid S-2 Glass/SC-79 epoxy called GL for short specimens were tensile tested. The tests were conducted at -60°C, -20°C, 75°C and 125°C. The Poisson’s ratios were measured using strain gages. It was observed that temperature had a small effect on the Poisson’s ratio.

INTRODUCTION

Due to the increasing use of polymer composites, a greater understanding these materials are necessary. Numerous researches have been conducted on composite in general to study their material properties [1-4]. Studies have also been done to study their responses under various types and rate of loading [5-8] and environmental conditions [9-10]. In this research woven composited will be studied. Experiments will be carried to study the effect of temperature on the Poisson’s ratio of these composites. Woven composites are known for their excellent dimensional stability and impact properties.

EXPERIMENTAL PROCEDURE

Materials:

The individual constituent materials combined to form the composite material used in this research are, IM-7 graphite (IM7-GP 6000) and S2-glass (S2-4533 6000) woven fabrics placed in SC-79 toughened epoxy resin matrix. The IM-7 graphite woven fabric and SC-79 epoxy matrix form the composite called GR. The S2-Glass woven fabric and SC-79 epoxy matrix form the laminate called GL. S2-glass fabrics and IM7-graphite fabrics were supplied by the Hexcel Corporation. The matrix, SC-79 toughened epoxy resin, which has Part A (Batch number: SC79A012307) and Part B (Batch number: SC79B012507), was supplied by Applied Poleramic Inc. The manufacturing of the composite was done by EDO Fiber Innovations. The vacuum assisted resin transfer molding (VARTM) technique was used to stack the plain woven fabrics together. The specimens were cured at 1770 C. Fiber volume fraction for all types were 55%.

Experimental setup:

Strips of GL and GR specimens, both of dimensions 6.35mmx25.4mmx254mm were tested under uniaxial tension at 125 °C, 75 °C, -20 °C and -60 °C following ASTM Standards D3039 (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials). The GL laminate consisted of 39 laminas stacked together, while those of GR consisted of 28. During the experiments, the desired temperatures were achieved using an environmental chamber, where each specimen was allowed to soak at the required temperature for thirty minutes before testing.

The chamber utilizes a heating coil for achieving high temperatures and liquid nitrogen for low temperatures. During this soaking period, one end of the specimen was clamped while the other end was free; this was done in order to allow the specimen to elongate freely. The tests were conducted using a universal testing machine also known as MTS while specimens were still in the environmental chamber. Figure 1 shows a schematic of the tensile specimen.

Figure 1 Tensile test specimen dimensions

In figure 2, the tensile testing machine, environmental chamber and liquid nitrogen used to achieve low temperatures are shown. All tests were displacement controlled. Specimens were clamped two inches each, thus the resulting gage length was six inches

Figure 2 MTS machine with environmental chamber and liquid nitrogen tank

In order to obtain the Poisson’s ratio of the composite, the strain along the longitudinal, transverse and thickness direction was needed. These strains were obtained by the use of strain gages (CEA-13-062UW-350). The output data obtained from the stain gages were then used to calculate the Poisson’s ration of each specimen.

EXPERIMENTAL RESULTS

Before plotting the data obtained from the experiment and finding the Poisson’s ratio, it was necessary to correct the lateral and thickness direction strain for transverse sensitivity. During tensile testing, the longitudinal strain in the specimen is several times larger than the lateral and thickness direction strains. The strain gages placed in the thickness and transverse directions will therefore be strained in a direction which is not their primary sensing direction. As a result of the finite width of the grid lines in the gage, the presence of end loops connecting the grid lines, strain gages are generally sensitive not only to strain parallel to the grid direction, but also to strain perpendicular to the grid direction. This property of strain gages is referred to as "transverse sensitivity".

Transverse and thickness direction strains were corrected for transverse sensitivity following guidelines from the manufacturer. Figure 3 below shows the placement of strain gages on the specimen.

Figure 3 Location of strain gages to measure the Poisson’s ratio of woven glass and woven graphite toughened epoxy composites

Figures 4 to 7 show therespectively, for GL specimen at various temperatures from whichare determined. Figures 8 to 11 shows the curves respectively, for GR specimens at various temperatures, from whichof GR are determined.

Figure 4 Poisson ratio V12 of woven GL

Figure 5 Poisson ratio v13 of woven GL

On each of the curves, an arrow bar is places to show the variation of the Poisson’s ratio from specimen to specimen. For each case, a minimum of three tests were done. The subscripts 1, 2, and 3 represent the longitudinal, transverse and thickness directions of the strips respectively. In this case, the fibers in the composite strips are parallel and perpendicular to the direction of loading. The subscripts x, y, and z represent the longitudinal, transverse and thickness direction of the strips respectively, however, in this case the fibers are at +45° angle with respect to the direction of loading. In figure 4 it can be  seen that v12 increase with a decrease in test temperature whereOn the other hand, v13 decrease with a decrease in test temperature.

Figure 6 Poisson ratio VXY of woven GL

Figure 7 Poisson ratio VXZ of woven GL

Figure 8 Poisson ratio v12 of woven GR

Figure 9 Poisson ratio v13 of woven GR

Figure 10 Poisson ratio VXJ of woven GR

Figure 11 Poisson ratio VXZ of woven GR

Table 1 below shows the Poisson’s ratios obtained from the above graphs for the GL and GR specimens respectively. The Poisson’s ratio for room temperature was taken from []

Table 1 Poisson’s ratio of woven glass and graphite fibers-reinforced toughened epoxy

Composite	Test Temperature	‘ 12	1 13= 1 23	>xy	lxz=’yz
Non- Hybrid Glass	-60 °C (-76 °F)	0.152	0.370	0.487	0.246
	-20 °C (-4 °F)	0.145	0.361	0.515	0.214
	R T (R T)	0.13	0.41	0.578	0.184
	75 °C (167 °F)	0.128	0.436	0.656	0.153
	125 °C (257 °F)	0.121	0.453	0.702	0.147
Non- Hybrid Graphite	-60 °C (-76 °F)	0.140	0.382	0.726	0.170
	-20 °C (-4 °F)	0.129	0.462	0.754	0.162
	R T (R T)	0.121	0.427	0.794	0.102
	75 °C (167 °F)	0.116	0.627	0.846	0.085
	125 °C (257 °F)	0.099	0.649	0.921	0.081

As seen in table 4.5, the Poisson’s ratio appears to be a function of temperature. As the temperature increases the Poisson’s ratios v12 and vxz= vyz decrease for both GR and GL specimens while the Poisson’s ratios n3=v23 andvxz = vyz increase. The Poisson’s ratio v13 tends to increase with an increase in temperature.

CONCLUSION

a) As the test temperature increases, the Poisson’s ratio 112, decreases where as 113= 123 increases.

b) As the test temperature increases, the Poisson’s ratio i xy, increases where as 113= 123 decreases.