Engineering Science 1021A/B Lecture Notes - Lecture 21: Fracture Toughness, Ductility, Burgers Vector
17 May 2018
School
Department
Professor
Ceramics: compounds of metallic and non-metallic elements bonded ionically or
covalently
Gives ceramics low ductility
Very sensitive to internal cracks and flaws
§
○
High melting temperatures
○
High young moduli
○
High hardness
○
Low electrical conductivity
○
•
Strength of Ceramics
Material
Fracture Toughness KIC
4340 Steel
99 MPa·m½
2024 Aluminum
26 MPa·m½
Ti-6Al-4V
55 MPa·m½
Al2O3
1.7 MPa·m½
Si3N4
5 MPa·m½
Very low fracture toughness of ceramics means the failure is almost
always due to flaws in the part
Design strengths of ceramic materials are described using statistics
§
○
Failure of ceramic parts is described by Weibull statistics
○
Probability of failure at a given stress:
○
𝑚: Weibull modulus
§
•
Defects
Point defects
Substitutional and interstitial defects are common•
Electrical neutrality must be maintained so defects often occur in pairs
Frenkel and Schottky defects
Schottky defect is the missing pair of + and – ions
§
○
Basis for semiconductors
○
•
•
Line Defects
Dislocations are present in ceramics but are very difficult to move
Due to the stiffness of atomic bonds (high bond strength) and a large
Burger's vector
○
•
Surface Defects
Grain boundaries are present in ceramics as they are metals•
Number of grain boundaries (grain size) becomes important during deformation
at high temperatures
•
Porosity (special type of surface defect)
Most ceramics are porous to a certain extent because of the way they are
made
○
•
Ceramics
Ceramics: compounds of metallic and non-metallic elements bonded ionically or
covalently
Gives ceramics low ductility
Very sensitive to internal cracks and flaws
§
○
High melting temperatures
○
High young moduli
○
High hardness
○
Low electrical conductivity
○
•
Strength of Ceramics
Material Fracture Toughness KIC
4340 Steel 99 MPa·m½
2024 Aluminum 26 MPa·m½
Ti-6Al-4V 55 MPa·m½
Al2O3 1.7 MPa·m½
Si3N4 5 MPa·m½
Very low fracture toughness of ceramics means the failure is almost
always due to flaws in the part
Design strengths of ceramic materials are described using statistics
§
○
Failure of ceramic parts is described by Weibull statistics
○
Probability of failure at a given stress:
○
𝑚: Weibull modulus
§
•
Defects
Point defects
Substitutional and interstitial defects are common•
Electrical neutrality must be maintained so defects often occur in pairs
Frenkel and Schottky defects
Schottky defect is the missing pair of + and – ions
§
○
Basis for semiconductors
○
•
•
Line Defects
Dislocations are present in ceramics but are very difficult to move
Due to the stiffness of atomic bonds (high bond strength) and a large
Burger's vector
○
•
Surface Defects
Grain boundaries are present in ceramics as they are metals•
Number of grain boundaries (grain size) becomes important during deformation
at high temperatures
•
Porosity (special type of surface defect)
Most ceramics are porous to a certain extent because of the way they are
made
○
•
Ceramics
