Some quality aspects regarding advanced ceramics mechanical parts for precision machinery.

Ivan, Ioana Carmen ; Enciu, George ; Ghinea, Mihai 等

1. INTRODUCTION

Advanced ceramics represent a wide range of very useful materials if they are very well processed with sophisticated technologies (HIP-Hot Isostatic Pressing) and if there are used high quality ceramic powders.

Before using these expensive technologies, it is important that it all researches and all comparative analysis (ceramics-traditional materials) to be done (Ghinea, 1999). Sometimes is better and cheaper to use Alumina ceramics (cheaper) instead Zirconia ceramics (expensive), or to use a ceramic materials obtained by extrusion or CIP (Cold Isostatic Pressing) instead HIP.

Of course, today there are a lot of specialists in mechanics that do not agree to use advanced ceramics for these parts that work under dynamic conditions, frictions, or if there is a contaminated working area. For them, classical materials (metals, steel, and composites) can still fix the problem. In certain cases a numerical method can make obviously the difference between advanced ceramics and other material. FEM analysis offers the possibility to obtain a complex and precise simulation especially for complicate geometry and shapes (Ispas et al., 1990). In this paper there were brought out few aspects from a research based on FEM analysis, which present the advanced ceramics advantages near to steel behavior (Dogariu, 1994; Ghinea & Vieru, 1997).

It was used a simple shape, a ring, that can be worked in a lot of mechanical applications. In table 1 there are the most important mechanical properties used in FEM analysis (for ceramic material and steel, also) and in figure 1 it can be observed the geometry of the ring and the finite element modeling. The chosen advanced ceramic material was Alumina ([Al.sub.2][O.sub.3]) and ordinary steel.

[FIGURE 1 OMITTED]

2. MODAL ANALYSIS

In FEM analysis equilibrium general equation is:

[M]{U}+[C]{W}+[K]{u} = {F(t)} (1)

where [M] is masses matrix, [C]--damping matrix, [K]--rigidity matrix, {[??]}--acceleration vector, {[??]}- speed vector, {u}--displacement vector, F(t)--external loads vector (Dogariu, 1994). Of course, for static phenomenon

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

so,

[K]{u} = {F} (3)

Obviously, [M] is known from ring geometry and materials properties and also, if there are concentrated masses, [M] can be determinate. [C] matrix can be obtained from experimental determinations and simulations (with Matlab), [K] matrix results also from materials properties (E and G) and ring geometry. The vectors {[??]}, {[??]} and {u} remain unknown. If [C] = 0 means that the system will work without dumping.

In the case of dynamical analysis by FEM a modal analysis it will considerate ({F}=0) and also, when a dynamical solicitation appears, ({F} [not equal to] 0), afterward it will be established the displacements {u(t)}, speeds {[??](t)} and accelerations {[??] (t)}, in nodes and stresses ([alpha](t)) which help to define the ring/system fatigue behavior.

Modal analysis supposes the vibration proper modes and frequencies, and these can be established based on solving of the general system that described the part behavior. In this system all values of the time dependent loads vector are null. In modal analysis proper modes are obtained (which, if they are the same with the work frequencies the resonance phenomenon can appear), the infinite displacements for undumped structures and huge displacements for dumped structures can also appear (Kunkel et al., 1990). A good result means that proper frequencies do not be similar with the work frequencies, and that they can be obtained by any parameters variation. So, if rigidity K increases, frequency f increases, or if mass m increases frequency f decreases (soft materials).

3. FINAL RESULTS AND CONCLUSIONS

In order to obtain a lot of numerical data linked by ring vibration modes, there were calculated lots of proper frequencies, but for our research only the first three vibration modes were chosen (fig.2, 3, 4, a results for metal ring, and b results for Alumina ring). The reason was that due to a large rigidity, these first three proper frequencies are already high (small part with a good fixation on X,Y,Z directions). The most important conclusion is the first vibration proper frequency for Alumina is with 35% higher then steel, and in this case a resonance phenomenon cannot appear.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Finally, it is obviously that advanced ceramics can successfully replace any classical material, in any mechanical application. Especially, top industrial activities like mechatronics, robotics, and IT branches, can fully use ceramic materials even in these applications where dynamic phenomenons often appear. The entire research, that contains also this FEM analysis, presents a lot of studies regarding advantages of ceramics properties, new obtaining technologies of ceramics part used in several mechanical assemblies. After that it will follow the ceramics parts utilization inside several precision machinery which are present in our research centers or laboratory.

4. REFERENCES

Dogariu, C. (1994). Theoretical and Experimental Researches Regarding the Concrete Utilization in Machine Tools Construction, PhD. thesis, University POLITEHNICA of Bucharest, Romania.

Ghinea, M, & Vieru, A. (1997) The Behavior of Ceramics Used in Mechanics, In: Interceram (International Ceramic Review), 1/97, Verlag Schmid GmbH, pp.16-22.ISSN 0020-5214.

Ghinea, M. (1999) Theoretical and Experimental Researches Regarding the Advanced Ceramics Utilization in the Machine Tools Construction, Ph.D. thesis, University POLITEHNICA of Bucharest, Romania.

Ispas, C.; Ghinea, M.; Dogariu, C. & Vieru, A.(1994) Technical Ceramics Used in Machine Tools Building, Proceedings of Materials Engineering conference, Haifa, Israel, pp.64-71,

Kunkel, H.A.; Locke, S.& Pikeroen, B. (1990). Finite-Element Analysis of Vibrational Modes in Piezoelectric Ceramic Disks, Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on Volume 37, Issue 4, pp.316-328, DOI 10.1109/58.56

IVAN, Ioana Carmen; ENCIU, George & GHINEA, Mihai *

Tab. 1. Mechanical properties for Alumina and steel

Property Steel Alumina

Elasticity modulus E, N/[m.sup.2] 2,1 x [10.sup.11] 4 x [10.sup.11]
Density, [rho] kg/[m.sup.3] 7850 3940
Poisson ratio, v 0,3 0,23
Coefficient of thermal 6,2 7,4
 expansion, [alpha],
 [10.sup.-6][K.sup.-1]