Aspects concerning some polymeric composites behaviour at cryogenic temperatures.

Hancu, Liana ; Paunescu, Daniela ; Iancau, Horatiu 等

1. INTRODUCTION

Due to their properties and qualities, polymeric composites are widely applied in many fields, including aviation and aerospace industry. The parts that are manufactured to work in such equipments where temperatures are very low must be tested in order to know their behaviour at these temperatures. The manner in which they perform at these temperatures is influenced by the fabrication recipe that was used. Even the slightest modifications in the formula might lead to obtaining different values for the mechanical characteristics.

The structure of the composite material can be easily modeled, so that the products satisfy all requests. The proper selection of the nature, size, geometry and proportion of the reinforcing materials, as well as the nature of the auxiliary materials and the fabrication technology, enable the production of a wide and detailed range of new composites that can be used at cryogenic temperatures. In order to optimise the design of the structure of the composite material, it is necessary to determine the mechanical characteristics, both at ambient and cryogenic temperatures.

Generally, all plastic materials increase their strength and decrease their plasticity in cryogenic conditions. For composite materials, things are not always the same because of the filling materials used in the combination (Hancu et al., 2007). There is limited information about how different filling materials influence the mechanical characteristics of the composites. In order to use a certain combination for the parts used at very low temperatures, it is necessary to determine some of the mechanical characteristics of the composite, both at ambient temperature and under cryogenic conditions.

2. TESTING METHODOLOGY AND EQUIPMENT

In this paper the authors present the experimental research regarding the influence of auxiliary material "quartz" (chemical formula Si[O.sub.2]), upon the mechanical characteristics of a thermosetting material, namely epoxy resin. Traction and bending tests were performed at room temperature (considered to be 293K) and under cryogenic conditions (77K for the tensile test and 100K for the bending test), and the following characteristics were determined: tensile breaking stress, bending resistance and deflection.

The traction tests at room temperature and under cryogenic conditions were performed on a universal testing machine INSTRON 1196 that is equipped with an automatic system for recording the load variation diagram, as an electrical sensing device measures the load. The speed, kept at the same values during testing, is 5 mm/min for traction and 1mm/min for bending. The class of precision of 0.5 ensures an error of exactness of [+ or -] 0.5% from the applied load.

Samples for traction and bending were manufactured from epoxy resin, with different ratio of quartz as a filling ingredient and polymerisation was performed in oven at 60[degrees]C for two hours. The following values for quartz quantities (considering volume units) were used: 33%; 50% and 66%.

Tensile breaking tests in cryogenic conditions were performed by using a cryogenic container that works with nitrogen liquid at a 77K temperature. After measuring the section of each sample, traction was performed and the equipment measured the stress during the tests and recorded the load variation diagram. In the end the tensile breaking strength was calculated for each sample with the formula:

[R.sub.r] = [[F.sub.r]/[S.sub.0][MPa] (1)

Where: -[F.sub.r] is the maximum breaking load [N];

-[S.sub.0] is the cross section area of the test bar in the ganged section, measured before testing [[mm.sup.2]].

For bending tests, samples were immersed in cryogenic fluid (nitrogen) and quickly (no longer than 5 seconds) placed on the same device that was used for bending at room temperature on the INSTRON testing machine. In this way, the temperature of the sample is considered to be around 100K. The load was very slowly performed at the middle of the specimen and no chocks were allowed. The INSTRON machine automatically registers the load variation diagram.

With the measured values (width and thickness) and the obtained ones from the registered diagram, the bending breaking strength is calculated as follows:

[R.sub.i] = [3 x F x [L.sub.0]/2 x b x [h.sup.2]][MPa] (2)

Where: -F is the bending load, [N];

-[L.sub.0] is the distance between bearings, [mm];

-b is the specimen's width, [mm];

-h is the specimen's thickness, [mm].

The value of the deflection is determined by using the linear part of load-deflection diagram registered by the machine.

3. EXPERIMENTAL RESULTS

Figure 1 presents the results of the tensile test, as deducted from the experimental data. First, it can be observed that at cryogenic temperature, breaking strength is higher than at room temperature. It also appears from the diagram that the tensile strength decreases when an auxiliary material such as quartz is incorporated. The slope of the curve is approximately the same at room temperature and in cryogenic conditions.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Considering the bending experiments (figure 2), it also appears from the diagram that the bending stress is higher in cryogenic conditions than at room temperature. The difference is bigger when quartz is used in low quantities, with a minimum at 42,857%. Above this value, the strength goes higher and that has to be detailed in a more specific work.

The slope of the curve is higher in cryogenic conditions, so the influence of increasing the quantity of quartz is bigger. Bending stress values at cryogenic temperatures are bigger than those obtained at room temperature. The values obtained for higher quartz ratios do not vary proportionally with the quantity of the auxiliary material.

The values for deflection are lower under cryogenic conditions, as shown in figure 3. That means that the material is more fragile if it is cooled. Even a small amount of quartz reduces considerably the material's plasticity at room temperature but the difference is not so relevant in cryogenic conditions.

In order to validate the results of the bending tests, (considering the values obtained for 42,857% of quartz in composition), statistical approach was performed by using IRWIN test (Constantinescu et al., 1980). The test was applied for the value of 42,857% quartz in composition, which is an unexpected value for bending resistence and deflection. Considering the obtained values ([R.sub.i]: 94,8; 97,3; 104,9; 109,6; 112,1; 125,4; 131,7; 144,2; 152[MPa] ) for bending resistence at 42,857% quartz, the inquaring value [x.sub.n] was verified and a control parameter [lambda] was determined:

[lambda] = [[absolute value of [x.sub.n]--[x.sub.n-1]]/s] (3)

where "s" is the square mediam deviation for "n" dates.

[FIGURE 4 OMITTED]

Comparing [lambda] with a critical value [[lambda].sub.critical], the [x.sub.n] element is eliminated from the line if [lambda] > [[lambda].sub.critical].

Values for [lambda] are presented in figure 4. As [lambda] = 0,32 and [[lambda].sub.critical] = 1,20, it means that [lambda] < [[lambda].sub.critic], no matter the position of the [x.sub.n] element in the line so the values will be all considered.

4. CONCLUSION

Auxiliary materials determine a reduction of the mechanical characteristics of the composite, regardless of the the temperature used for tests.

Under cryogenic conditions, the tensile breaking strength and the bending resistance of the resin are higher than at room temperature, while the plasticity of the material is smaller. It is obvious that a composite material based on epoxy resin filled with quartz does not improve the bending stress values, regardless of the temperatures used for tests. When an auxiliary material, namely quartz, is added, both values vary, but with different percentages at different temperatures. The diagrams show that once the auxiliary material is incorporated, the influence upon the properties does not vary proportionally with the quantity. Even a small amount of the ingredient is enough to reduce the measured values. The research must be continued in order to establish the optimum recipe for the best mechanical behaviour.

5. REFERENCES

Constantinescu, I., Golumbovici, D. & Militaru, C., (1980). Prelucrarea datelor experimentale cu calculatoare numerice (Processing of experimental data with numerical computer), Editura Tehnica, Bucuresti

Hancu, L., Paunescu, D. & Borzan, M., (2007). Considerations about Filling Materials Influence upon Deflection of Epoxy Resin under Cryogenic Conditions, Revista Materiale Plastice, vol. 44, nr 2/2007, ISSN 0025-5289

Hancu, L., (2007). Researches Concerning the Influence of Clay upon Deflection of Composites under Cryogenic Conditions, Acta Technica Napocensis, ISSN 1224-9106

Hancu, L., Iancau, H. & Achimas, Gh., (2003). Criogenie si masini frigorific, Alma Mater, ISBN 973-8397-33-2

Iancau, H., Bere, P., Borzan, M., Hancu, L. & Crai, A., (2008). The Influence of Reinforced Materials on the Mechanical Characteristics of Polymeric Composite Materials, Revista Materiale Plastice, vol.45, nr 3/2008, ISSN 0025-5289