Stress strain behavior of ceramics

Stress-strain behavior of ceramics:

Almost the entire ceramic compound consists of ionic bonds.

Ceramic materials:

  1. Glasses
  2. Glass ceramics
  3. Clay product
  4. Structural products
  5. White wares
  6. Abrasive
  7. Refractories
  8. Fire clay
  9. Cement
  10. Advanced ceramics

The stress-strain behavior of brittle ceramic is not usually obtained by tensile test.

In the testing of ceramic material 3 point loading scheme and four point loading scheme is used.

In 3 point loading scheme:-

  1. First the material is prepares in the required geometry (either rod having circular or rectangular cross section).
  1. Second, the ceramic materials are difficult to grip without fracturing. So, three point loading scheme is used. tn this scheme, load is applied in middle or the end of the ceramic rod placed over two knives support as shown in figure.
  1. The strain for brittle fracture in ceramics is too small only about 0.1 % strain. So, specimen is perfectly align to avoid the presence of bending stress.

fig: three load point
fig: three load point(www.chegg.com)

Stress is computed from the specimen thickness is bending moment, moment of inertia of cross-section (geometrical moment of inertia). The maximum tensile stress exist or the bottom specimen surface directly below the point of load application.

The stress of fracture using flexural strength of modulus of rapture or bend strength. It is denoted by \(\sigma_f\). The stress at fracture using this flexure test is known as the flexural strength, modulus of rupture, fracture strength, or bend strength, an important mechanical parameter for brittle ceramics i.e.

\(\sigma_{fs}\)=flexural strength

=fracture strength

=bend strength

=modulus of rupture

The stress at fracture using this flexure test is known as the flexural strength, modulus of rupture, fracture strength, or bend strength, an important mechanical parameter for brittle ceramics

$$\sigma_{fs}=\frac{3F_fL}{2bd^2}\dotsm(1)$$

For rectangular cross section

\(F_f\)=applied load at fracture

L=effective length of rod

B=breadth

D=thickness

For the rod having circular cross-section, the fracture strength is given by,

$$\sigma_{fs}=\frac{F_fL}{\pi R^3}\dotsm(2)$$where R=radius of specimen

The maximum bending moment for the specimen in this type of loading scheme is given by,

M=effective load\(\times\) load distance from centre

=\(\frac{F}{2}\times \frac{L}{2}\)

=\(\frac{FL}{4}\)

\(I_g\)=I=geometrical moment of intertia

=\(\frac{bd^3}{12}\) (for rectangular)

=\(\frac{\pi R^4}{4}\) (for circular)

The stress on specimen is given y,

$$\sigma=\frac{MC}{I}$$

Where,

C=distance from centre to outermost fiber

=R(radius)

=\(\frac{d}{2}\) (half thickness)

\(\sigma=\frac{FL}{4}.\frac{d}{2}\times \frac{1}{bd^3}\) (for rectangular)

\(\sigma=\frac{FL}{4}\times R\times \frac{4}{\pi R^4}\) (circular)

\(\therefore \sigma_{fs}=\frac{3F_fL}{2bd^2}\) (for rectangular)

\(\sigma_{fs}\)=\(\frac{FL}{\pi R^3}\) (for circular)

Characteristics flexural strength for different material is quite higher than that of modulus of elasticity. So magnitude of flexural strength is always measured flexural test except tensile strength.

Elastic behavior of ceramics:

The stress strain for ceramic using flexural test is similar to tensile as shown in figure.

fig:Elastic properties of ceramics
fig:Elastic properties of ceramics(www.slideshare.net)

Properties:

  1. The stress-strain is almost for ceramic material up to fracture strength.
  2. For given value of stress is too small in comparison to metals and polymers. This indicates the brittle nature of ceramic material.
  3. For crystalline material, plastic deformation occurs by the motion of dislocation.
  4. The rate of deformation in non-crystalline ceramics is due to applied stress and they deform by viscous force.
  5. In crystalline ceramic, the contribution to modulus of elasticity is due to ionic bond wheras for non-crystalline ceramics it is due to covalent bond.

The elastic stress–strain behavior for ceramic materials using these flexure tests is similar to the tensile test results for metals: a linear relationship exists between stress and strain.

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.

1. Process of 3 point loading system

  1. First the material is prepares in the required geometry (either rod having circular or rectangular cross section).
  1. Second, the ceramic materials are difficult to grip without fracturing. So, three point loading scheme is used. tn this scheme, load is applied in middle or the end of the ceramic rod placed over two knives support as shown in figure.
  1. The strain for brittle fracture in ceramics is too small only about 0.1 % strain. So, specimen is perfectly align to avoid the presence of bending stress.

2. Some formulas

$$\sigma=\frac{MC}{I}$$

\(\therefore \sigma_{fs}=\frac{3F_fL}{2bd^2}\) (for rectangular)

\(\sigma_{fs}\)=\(\frac{FL}{\pi R^3}\) (for circular)

 

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