Cracks
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Where do cracks originate?
Cracks originate at stress concentrations
What is the equation for the stress intensity factor, K?
K = Ysigmasqrt(pi*a) where a is the crack length sigma is the applied stress Y is the geometrical factor
What are sources of stress concentration?
Surface defects and notches
How are stress concentrations prevented?
By polishing the surface of the material to reduce the number of surface defects such as notches and by radiussing all sharp corners which can act as stress concentrations
How are flaws found?
Using non-destructive testing such as dyes
The two mechanisms of crack propagation in composites
• Deflexion at the matrix-fibre interface 2. Crack bridging and fibre pullout
Describe deflextion at the matrix fibre interface
The strengthening fibre stops the growth of the crack/ Laminating at the surface of the interface because it is weaker.
Describe crack bridging and fibre pullout
The crack is held together by fibres. Eventually the fibre breaks and absorbs energy
Are deflextion and fibre pullout energy absorbing or energy releasing processes?
Energy absorbing processes
Is the propagation of cracks energy absorbing or energy releasing?
Energy releasing
What is the only energy absoroption mechanism in brittle materials?
The formation of new surfaces. It consumes energy to absorb new surfaces.
Other energy absorbing mechanisms in classes of materials
• Plastic Deformation - Fibre pullout in composites
What is Griffith Crack Theory?
Unstable crack growth occurs when the stress intensity is higher than the elastic strain energy. When the elastic energy released by crack growth is greater than the energy required in absorption to form new surfaces.
What class of materials does the Griffith Crack Theory apply to?
It applies to brittle materials. It doesn’t apply to ductile materials because plastic deformation in the ductile materials reduces the stress concentration at the tip of the crack.
What is fracture toughness?
The fracture toughness is a measure of the resistance of a material to crack propagation.
What is the equation for the critical stress intensity factor?
K ic = sqrt(2gammaE) where gamma is the surface energy
Define the critical stress intensity factor
It is the stress intensity required for unstable crack propagation. This is a material property and characterises the fracture toughness of a material.
What does the critical stress intensity factor depend on?
It depends on the released during crack growth. The crack growth releases strain energy in the material but consumes energy in creating new surfaces, in plastic deformation
When does unstable crack growth occur?
When the energy released iby the crack is greater than the energy required for the formation of new surfaces.
The three tests required to determine the fracture toughness of ceramics
• The tensile test • The bend test • The compressive test
Why is the fracture toughness measured in the tensile test of ceramics usually low?
Ceramics contain a large number of flaws. In tension, the largest flaw in the ceramic will propagate unstably.
Why is the fracture toughness measured by the compressive test for ceramics greater than that of the tensile test?
The flaws will propagate stably. Extension of small cracks distributed through the surfaces.
Why are there flaws in ceramics?
Production process but also in service and handling Flaws are locations of stress concentrations.
The ranking of the fracture toughness of materials
Polymer foams < Ceramics+Glasses < Polymers < Metals and alloys < Composites