Effects of particle content and post curing thermal treatment on the effective modulus of multi phase composite materials.

Luca, Dana Motoc

1. INTRODUCTION

Considerable research in the field of material science has been directed towards the development of new light-weight, high-performance engineering materials, such as composites. Nonetheless, the huge number of papers in this subject area covers mainly all of the problems specific to material development and characterizing, leaving enough space for further possible combinations to satisfy any requirements.

Technical literature provides numerous references concerning of the elastic moduli evaluation for two-phase composite materials, no matter the reinforcement shape, type and materials, either at macroscopic or microscopic levels (Chen et. al., 2006; Torquato, 2002).

In such circumstances the multi-phase composite materials emerge not as new paradigm but challenge for people from various research fields aiming to push forward the line of material development for structural applications, particularly for aerospace and military industry, to computer and mechatronics, robotics ones. For this category, the theoretical models for effective elastic moduli prediction were natural consequences in the material development and characterization even there is the case of a relatively lack of information and references (Ji, 2004).

The present work aims to present the approach an analysis of variance (ANOVA), a method of search for factors having a considerable effect on experimental data, in order to assess the effects of particle volume fraction and a post-curing thermal treatment on the effective elastic coefficient of self-developed and manufactured multi-phase composite materials. The composites were considered reinforced both with random E- glass fibres and ceramic particles, the latter in different volume fraction, into a polymeric matrix (Motoc Luca & Teodorescu, 2008). The manufacturing technology will be not presented herein, and was not included as an influencing factor in the analysis, as was normally supposed to, due to the lack of an alternative. The effective elastic modulus was retrieved experimentally, from 3 point bending tests on representative composite samples, and compared with the theoretical values obtained using a two-step homogenization concept.

The theoretical approach and associated expressions are not included herein as the subject is beyond the concept (Curtu & Motoc Luca, 2008).

2. MATERIALS AND METHODS

2.1 Materials

In this study were used random, long E-glass fibre mats in a combination with a polymeric matrix from DSM Composite Resins (Switzerland), the latter being choose due to its availability and very good fibre wetting and impregnation properties. The fibres--MultiStrat[TM] Mat ES 33-0-25 (Johns Manville, USA) were made up from multidirectional continuous E glass fibres, leading to a 65% volume fraction in the composites' panels. The fibres can resist reasonably high temperatures and corrosion. It is a very versatile fibre and has been used for interior panels and structural aircraft parts and in combination with other fibbers and or arrangements in boat hulls and pressure vessels. The fillers considered were ceramic (CaC[O.sub.3]) particles embedded into different volume fraction into the composite's matrix material (0%, 5 %, and 10 %) not only to release the stresses during the manufacturing step but to obtain certain values of panels' rigidity. Neither visible surface defects nor edge delamination are present.

2.2 Determination of composite properties

The samples' effective elastic properties were evaluated at room temperature, for representative samples free or after subjecting to a post-curing thermal treatment (different temperature cycles). One post-curing thermal cycle applied, by aid of a controlled temperature oven, had the following variation: 1 hour of heating up (5 min. until the temperature was reached) and maintaining at 120[degrees] C followed by 1 hour of cooling down at room temperature. The samples were shaped using a standardized form (SR EN ISO 178) and subjected to a 3-points bending tests, all of them with a crosshead speed of 1 mm/min using a LR 5K Plus device from Lloyd Instruments Ltd. The results obtained applying a statistical analysis was considered as corresponding to the mean values and used for comparison with the theoretical values.

2.3 ANOVA analysis

A two-way ANOVA (Analysis of variance between groups) was applied to analyze the statistical significance of particle volume fraction and post-curing thermal treatment on the effective elastic modulus of the multi-phase composites considered in the present study and to determine which of the parameters has the most significant effect. The SPSS 17.0 software was used to carry the statistical analysis.

3. RESULTS AND DISCUSSION

Figure 1 shows the comparative values retrieved experimentally and those after applying a two-step homogenized scheme, firstly the self-consistent Mori-Tanaka for the matrix material and ceramic particles leading to a new effective matrix, next the Halpin-Tsai expressions that are particular for effective elastic modulus estimation of composites reinforced with long, random fibres, case in which the matrix role are taken by the effective ones from the previous step.

A general linear model (GLM) was used to perform the 2- way ANOVA analysis:

E([V.sub.p], T) = [E.sub.0] + [b.sub.1] [V.sub.p] + [b.sub.2]T + [b.sub.3] [V.sub.p] T + [epsilon] (1)

where [V.sub.p] and T are the effects on effective elastic modulus E [GPa] of the particle volume fraction, post-curing thermal treatment, respectively, [E.sub.0] the overall mean of the effective elastic modulus, bi are constants retrieved from the statistical table (output of the analysis), and e is the random error term. The analysis was carried out for a 95% probability value.

Figure 2 shows the residuals plots, from where can be seen the component of the linear relationship between the particle volume fraction and the influence factor due to the thermal treatment (standardized residuals vs. observed values), whereas the plot of standardized residuals vs. predicted values shows that the error variances are almost equal across the volume fractions (or thermal treatment).

From the statistical outputs can be concluded the fact that the thermal post-curing treatments has a significant influence on the effective elastic modulus (F(1,2)=25,140, p=0,038) in opposite with the influence of the volume fraction of the filler inclusions (F(2,2)=5,274, p=0,159) or combinations of both factors considered (F(2,12)=0,596, p=0,566). These effects can be easily sized even in the variation of the effective elastic moduli retrieved experimentally and relies on practical intuitive observations for which such small quantities of embedded particles do not show dramatically changes on the estimated property.

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Figure 3 shows the estimated marginal means of the effective elastic moduli vs. particle volume fraction for the two phases considered--without (1) and with post-curing thermal treatment.

4. CONCLUSIONS

The analysis of variance (ANOVA) carried out in this study in order to characterize the elastic properties of a particular type of multi-phase polymeric composite material (both fibers and particle reinforced) revealed the degree of influence of the most significant factors considered--one type of reinforcement (particle content) and a post-curing thermal treatment applied upon the samples.

The results are in accordance with the theoretical predictions, experimental observations and intuitive feelings, revealing the fact that small volume fraction of the fillers, at macroscopic level of the mechanical property investigated (effective elastic moduli from 3-point bending tests), do not have a significant influence comparatively with the results obtained after applying a certain thermal cycle. The latter seems to contribute to the improvement of the effective elastic moduli.

5. ACKNOWLEDGEMENT

The research was supported from grant ID_135, 108/1/10/2007, CNCSIS, Romania.

6. REFERENCES

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Curtu, I. & Motoc Luca, D. (2008). Theoretical-experimental comparisons of multi-phase composite materials elastic coefficients retrieved from tensile, compressive and bending tests. Influencing factors. Plastic Materials, Vol. 45, No. 4, 366-371, ISSN 0025/5289

Ji, S. (2004). Generalized means as an approach for predicting Young's moduli of multiphase materials, Materials Science and Engineering, Vol. A366, 195-201

Motoc Luca, D. & Teodorescu, H. (2008). Fillers' content influence on the mechanical properties of the glass mat reinforced polymeric composite, Proceedings of 19th International DAAAM Symposium "Intelligent Manufacturing & Automation: Focus on next generation of intelligent systems and solutions", Katalinic, B. (Ed.), pp. 00913-00914, ISSN 1726-9679, Trnava, October 2008

Torquato, S. (2002). Random heterogeneous materials, Springer, ISBN 0-387-95167-9, U.S.A