How do mechanical characteristics (stress v strain) of metals and ceramics compare at room Temperature?
What are typical ceramic failure mechanisms?
Why are stress-strain characteristics of ceramic materials determined using transverse bending tests rather than tensile tests?
Be able to compute the flexural fracture strength of a ceramic from a flex test.
Be able to use fracture toughness to determine the max stress for a given ceramic with flaws of a known size and radius of curvature.
Why is there normally significant scatter in the fracture strength for specimens of the same ceramic material?
Why are crystalline ceramic materials so brittle?
Ceramics are generally extremely hard
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Stress vs. Strain
For many metals:
Elastic and significant plastic deformation
For most ceramics:
No appreciable plastic deformation
Brass Brass strains to 35% = Fracture Strength (stress at failure) Ceramics strain to 0.1%
Fracture strength Callister, Appendix B Note that the fracture strength for ceramics is about 10* better in compression than in tension. Body armor 69 Soda lime glass 13.8-69 Graphite 230-825 SiC (sintered) 1050 Diamond (natural) 800-1400 Diamond (synthetic) 37.3-41.3 Concrete 130 Si  cleaved 282-551 Al 2 O 3 (99.9% pure) Strength (MPa) Ceramic
Failure Mechanisms-single xtals
For single crystals, cleavage occurs
Very rapid crack propagation along specific crystallographic planes.
In rare cases (usually nanoscale polycrystalline ceramics), there can be limited ductility at room temperature.
At higher temperatures, plastic deformation may occur.
Typical mechanical property measurements
Standard tensile tests are problematic:
Failure usually occurs at low strains (<0.1%), where bending stresses can be significant unless the sample is perfectly aligned in the tensile stage.
Gripping brittle materials like ceramics often leads to fracture at the grips.
Test geometry is difficult to prepare.
Measuring ceramic mechanical properties
We can’t use the standard tensile test, but we still need elastic modulus and fracture strength.
Solution: bend test. Most appropriate for bars, rods, plates, and wafers. Where will cracks form? Which part is under tension and which is under compression?
MEASURING FRACTURE STRENGTH • 3-point bend test to measure room T strength. Adapted from Fig. 12.29, Callister 6e. • Flexural strength: Flexural fracture strength is higher than the tensile fracture strength. Why? Test specimens undergo compressive and tensile loads instead of pure tension. Si carbide Al oxide glass (soda) 550-860 275-550 69 Data from Table 12.5, Callister 6e. circ.
MEASURING ELASTIC MODULUS • Room T behavior is usually elastic, with brittle failure. • Determine elastic (Young’s) modulus according to slope: Adapted from Fig. 12.29, Callister 6e. Si carbide Al oxide glass (soda) 430 390 69 Data from Table 12.5, Callister 6e.
Measured Fracture Strengths
Practically, measured fracture strengths of ceramic materials are usually much lower than predicted.