Dear Armin,

In the case of CMCs, I am sure that fiber is better than particle, especially, SiC or Carbon reinforcements in the ultrahigh temperature ceramics. however, you can find the attachment as an example for MMCs.

Hello,

Generally, the addition of long fibers (carbon or glass) to polymeric matrix improve significantly the mechanical properties of the composite. In the other hand, the addition of spherical charges can improve thermal properties of the composite (like isolation).

Agreed with Reda, just pointing out that other morphologies can also modify mechanical properties upon their compatibility with polymer matrix. This is done by surface modification mainly.

Problem with anisotropic fillers like carbon or glass fibers is their orientation dependent properties especially if the composites are made by extrusion. 

Rather than morphology, effective load transfer between the polymer and filler is the key to achieve good mechanical properties. 

Dear  All

thank  for your respond. Let me know about the metal matrix. 

Hi Armin, there are a lot of information regarding to the effect of particles (SiC, SiO2, Al2O3, Graphite, etc) in MMC´s, in general, a fine dispersion of particles (equiaxed) can improve the mechanical properties, but tit depends of the size, and distribution of them, also the interface matrix particles. In the case of fibers the material will be anisotropic. 

Dear Armin,

Generally, the higher aspect ratio, the better mechanical properties.

Hence, I think SiC whisker or fibre may be a good choice.

Dear Mehdi

you mean that fiber is better than particle. for this, you have papers that showed this phenomenon 

Dear Armin,

In the case of CMCs, I am sure that fiber is better than particle, especially, SiC or Carbon reinforcements in the ultrahigh temperature ceramics. however, you can find the attachment as an example for MMCs.

In MMC, usually spherical morphology of second phase particle are better to give rise to isotropy in physical and mechanical properties. Irregular shape of particles provides sharp corners which act as stress raisers leading to premature failure of composites.

Generally in MMCs, spherical morphology of second phase improves the mechanical properties specially elongation.

In general second-phase particles tend to be fillers and increase material stiffness (if they have a higher modulus than the matrix) but generally do not substantially increase strength. If the particles are not well-bonded to the matrix, and are brittle (e.g. ceramic powders) they often decrease tensile and bending strength of a specimen. Second-phase fibers (e.g. length/diameter ratio ca. 10) which have a higher tensile strength than the matrix tend to increase specimen tensile and bending strength--if the matrix/fiber bond is stronger than the strength of the matrix. 

The best I can suggest is to read the book by J.N.Reddy "Mechanics of laminated composite plates and shells"  (CRC Press), very  clesr and extensive on the topic you are interested to.

The general principles are that (a) particles which are stiffer than the matrix increase composite stiffness but do not increase tensile or bending strength, and (b) short fibers (e.g. L/D ratio range ca. 10 to ca. 60) that are stiffer than the matrix tend to increase both stiffness and strength, and (c) the bond between reinforcement (particles or fibers) and the matrix must be strong in order for the effects of reinforcement to be maximal.

Here is an abstract of an excellent paper on this topic, followed by a link to it. I do not have a copy of the paper.

Composites Science and Technology

Volume 56, Issue 10, Pages 1113-1222 (1996)

Effects of fiber length and fiber orientation distributions on the tensile strength of short-fiber-reinforced polymers

Authors: Shao-Yun Fu, Bernd Lauke

Abstract

This paper presents an analytical method considering the effects of fiber length and fiber orientation distributions for predicting the tensile strength (TS) of short-fiber-reinforced polymers (SFRP). Two probability density functions are used for modelling the distributions of fiber length and fiber orientation. The strength of SFRP is derived as a function of fiber length and fiber orientation distribution taking into account the dependences of the ultimate fiber strength and the critical fiber length on the inclination angle and the effect of inclination angle on the bridging stress of oblique fibers. Then the effects of the mean fiber length, the most probable length (mode length), the critical fiber length, the mean fiber orientation, the most probable fiber orientation and the fiber orientation coefficient on the tensile strength of SFRP have been studied in detail. This model provides the necessary information to determine what fiber length distribution, what fiber orientation distribution and what interfacial adhesion are required to achieve a desired composite strength. The present theory is then applied to existing experimental results and the agreement is found to be satisfactory.

http://www.sciencedirect.com/science/journal/02663538/56/10

Dear amin ,

I think it is depend on type of second phase (particle and fiber) and  kind of matrix . it is clear that spherical morphology and fine dispersion of reinforcement can influence on mechanical properties.

It should be mention that the way you produced composite is so important, I have experience in metal matrix composite which is produced by spark plasma sintering, microwave sintering and usual furnace. Method of fabrication can effect on interface between second phase and matrix.

Dear Armin,

In a recent paper (see below), we have thoroughly examined the influence of the inclusion (particle) shape on the effective mechanical properties of random two-phase media with arbitrarily shaped inclusions.

We have examined the cases of stiff and compliant inclusions, and shown that the effective properties can be significantly affected by the shape of the inclusions especially in the case of large volume fraction and (matrix-inclusion) stiffness ratio.

You can find more details in our paper:

D. Savvas, G. Stefanou, M. Papadrakakis, G. Deodatis. Homogenization of random heterogeneous media with inclusions of arbitrary shape modeled by XFEM, Computational Mechanics (2014), in press.

http://link.springer.com/article/10.1007/s00466-014-1053-x

I think that spherical particles will act as a filler, while acicular shape are more effective of increasing many properties like mechanical properties.  

Dear Armin Rajabi

The morphology of matrix and reinforcement material or second phase  is very important physical and mechanical properties of the composite materials because particle morphology effects densification or consolidation behavior of the composite powder mixture. However, it should be noted that the interface bonding and interface reactions determine the composite properties. Moreover, production process effect the interface reactions between matrix and second phase.

İf you want to learn about metal matrix composite fabricated powder metallurgy I can sad that you should be spherical matrix powder due to lower friction forces.

 As per as morphology is concerned reinforced phase should be uniformly distributed in the matrix i.e within the grain and along the grain boundary. Fibre reinforced composites are better to improve the fracture toughness of ceramic matrix composites as they have high aspect ratio which leads to good interaction with matrix. It also enhances toughening by crack bridging,pullout,debonding and crack deflection.

Dear Armin,

I think that for getting proper mechanical characteristics of composite should find the relationship between morphological and mechanical properties. For example, if you focus on hardness, fibers should be have small lumen diameter, opposite that fibers should be have big cell wall thickness. When cell wall thickness increases the stiffness of fiber increases. It means that the modulus of the fiber is increased, these mechanical properties are more important for composite. Also, fiber diameter effects to mechanical properties such as; tensile stress, tensile modulus. 

Sincerely,