Fracture of Brittle Materials Flashcards
How does stress and strain vary for metals and ceramics?
Metals can sustain a significant amount of strain for the stress applied linearly to its yield strength then it shall plastically deform to its failure.
Ceramics have brittle fracture at a much smaller strain than metals and will be catastrophic failure. Only linear elastic behaviour. Have a higher youngs modulus.
Why do ceramics vary as such against metlas?
Because of the strong atomic bonds in ceramics so they have a more rigid structure of covalent or ionic bonds. And so metals can have lower elastic regions as they have more metallic bonding and is directional.
What is super plasticity?
Where a ceramic plastically deforms slightly - not expected to happen and in rare cases.
what is an ideal brittle ceramic?
Elastic deformation followed immediately by fracture
Stress displacement curve
Atoms are pulled apart on the sine curve leading to a displacement from the strength that is applied.
What does high strength be dependent on in ceramic behaviour wise?
high modulus(stiffness), high surface energy and small lattice spacing
What is the griffith theory?
Where fracture begins at inherent flaws in the material due to stress concentrating effect of cracks. E.g. cut glass by scoring with diamond tipped implement then cracking along the score by applying stress and it should crack down that line.
What 2 flaws of the ceramic need to be met to have effective conditions for the griffith theory to propagate?
The local stress must exceed the stress applied.
Must have enough strain energy to form new surfaces .
What is the griffith equation ? REMEBR THIS
what are the two stresses that must be satisfied to satisfy the 2 flaws for griffiths?
Inglis and Griffith equation however the inglis value can be smaller than the griffith but only one condition would be met so griffith criterion gives the minimum stress for cracks to propagate.
What does the strength of individual samples rely on?
A closer look at this equation reveals that for a given series of
similar samples E and γ will be common. Thus the strength of
individual samples will depend on c, the size of the largest flaw
present in the stressed region of the material.
Dependent on largest flaw in the stressed region irrespective of any other flaws - provided the flaws don’t interact.
What is the effect of microstructure on flaw depth and flaw shape?
The shape of a pore whether spherical or near spherical will have lower stress concentration effect than an angular pore.
How doe the present of cracks adjacent to pore affect propagation?
intersection of the pore with grain
boundaries of the ceramic. If the pore is much larger than the grain size, the
extremities of the pore = critical flaw size. However, if the pore size approaches that of the grains, then the effect of cracks along the grain boundaries will probably predominate leading to an effective larger flaw size.
What is the distance between pores and between pores and the surface in propagation?
the physical location of a pore within the ceramic will affect strength. If a pore is close to the surface of the material, the bridge of material separating it from the surface
may break first, resulting in a critical flaw whose dimensions are now the size of the pore plus the length of the bridge.
What are pore clusters?
it is known that if a group of pores are close together the material
bridges between them can crack first, linking the pores together and producing a much larger flaw that results in much lower strength.
What are the effects of inclusions mechanically?
Contamination during ceramics processing
Effect is similar to that of pores
– Large angular inclusions are more detrimental
griffits theorh and rest of 2-4, 5-6
What are beneficial properties in ceramics?
high stiffness, high strenghtm high corrosion resistance, low density, high hardness, high refractoriness, high wear resistance.
what is fracture toughness?
metals are much higher than ceramics (as they have defectts) and ceramic are not good at resisting crack propagation.
What is the critical stress intensity factor, Kic?
The stress intensity factor at which a crack propagation will happen and lead to fracture.
If the load is perpendicular to the crack, as is typically the case in a tensile or
bend test, the displacement is referred to as mode I and is represented by KI. This
is also called the opening mode and is most frequently operational for ceramics. can be applies in 3 mode directions.
How is mode 1 stress intensity factor related to the applied stress and crack length?
[ref equation pg 62]
From the equation we see that the toughness determines the critical flaw size that will lead to catastrophic failure at a given stress, i.e. the higher the fracture
toughness the more difficult it is to initiate and propagate a crack.
A doubling of the material toughness will lead to a 4-fold increase in the tolerable flaw size at a given stress.
This will in turn result in an increase in the reliability of the component since the
larger the critical flaw size the greater the chance we have of ensuring no flaws
exceed it during processing.
What is the relationship between failure stress, flaw size and fracture toughness for brittle materials?
As the flaw size increases the stress will decrease or as the flaw size increases, the stress will decrease. [ref graph pg63].
Why is achieving reliability in ceramics hard to achieve?
The flaw sensitivity and low fracture toughness provides a challenge in struvtural applications
What are the 2 main approaches to making ceramics more reliable?
Reduce flaw size- ultra fine powders or sol gel processing (improves processing)
increase toughness- microstructural design(control of microstructure), zirconia toughening(secondary phase addition) or composite reinforcement(particles/whiskers) which improves inherent toughness.
