Ceramics Have Greatest Resistance To Compression

Alumina for example has a tensile strength of 20 000 psi 1138 mpa while the compressive strength is 350 000 psi 2400 mpa.

Ceramics have greatest resistance to compression.

There is a need to know accurately the strength of brittle materials in compression both to distinguish between competing statistical micromechanical theories of strength and to provide data necessary for engineering designs for the optimum use of these materials as load bearing elements. Introduction structural ceramics have excellent mechanical properties such as high strength low specific weight and corrosion resistance even at high temperature. 4 2 generally resistance to compression is the measure of the greatest strength of a monolithic advanced ceramic. Ideally ceramics should be compressively stressed in use although engineering applications may frequently introduce tensile stresses in the component.

Ceramics are very sensitive to cracks impurities and porosity and thus generally have low toughness. In compression however the flaws in the material do not cause stress concentrations or crack propagation as they do in tension. Ceramics thus were considered an ideal material for heat engines because they would make it possible to raise the combustion temperature and would improve the thermal efficiency. The ionic and covalent bonds of ceramics are responsible for many unique properties of these materials such as high hardness high melting points low thermal expansion and good chemical resistance but also for some undesirable characteristics foremost being brittleness which leads to fractures unless the material is toughened by.

Aluminas have good creep resistance up to about 1 500 f above which other ceramics perform better. Another way at looking at resistance to failure for ceramic materials is to examine the energy required to drive cracks through the system. Ceramics tend to be weak in tension but strong in compression. In addition aluminas are susceptible to corrosion from strong acids steam and sodium.

Their strength depends on the number and type of defects that are present. Generally a ceramic with more defects is weaker. For a metal the compressive strength is near that of the tensile strength while for a ceramic the compressive strength may be 10 times the tensile strength.