Lezione 8: Fracture and fracture toughness Flashcards
What is the difference between strength and toughness?
Strength: resistance of a material to plastic flow
Toughness: resistance to fracture, resistance of a material to the propagation of a crack
How are the tests for toughness done?
Either the dented sample is subjected to tension, or to impact with a mass.
A material is tough if the tip of a defect - a crack - will undergo strength hardening absorbing energy. A material is brittle if a defect is propagated quickly from the top of the crack and the material breaks.
Is stress concentration factor still valid for materials with cracks?
No, because is requires a radius, and the tip radius of a crack is near zero. We need to use instead the stress intensity factor K1. In particular, mode 1 is meant for tensile fracture normal to the crack. Cracks propagate when the stress intensity factor K1 exceeds a critical value, which is knows as the fracture toughness K1c.
What is parameter Gc?
Gc is the critical strain energy release rate, inappropriately called toughness. It describes the energy necessary to create extra surface or to grow a crack. Although polymers have low K1c, as ceramics, they have much higher Gc.
Talk about crack tip zone
For ductile materials, the crack tip will be surrounded by a plastic zone, limiting at yield strength the intensity of the local stress: if yield strength is low, they show a large plastic zone at the crack tip, absorbing energy of the propagation. Ductile materials have a large plastic zone, ceramics have a small one or none at all.
What does the critical crack length define?
The dimension of the critical crack length Ccrit for a given material drive the transition between material yield and fracture. For crack length < Ccrit, material yields. For crack length > Ccrit, materials fractures.
Tensile failure stress, σf, occurs when c>ccrit.
The transition crack length can be calculated when σf= σy
How do cracks influence different materials?
Tough metals are able to contain large cracks but still yield in a predictable, ductile manner.
Ceramics always contain small cracks, fail in brittle way at stresses far below their yield strength.
Polymer not cracked can be tough, but defects smaller than 1mm can cause brittle failure.
Talk about the fracture toughness-modulus chart
The fracture mechanics (linear elastic) provide valid values for brittle materials, while it’s only approximated for ductile materials. In engineering applications in which the materials are highly stressed it’s better to use materials with high K1c
Talk about the fracture toughness-strength chart
Transition crack length plotted on chart. Values can cange from near-atomic dimensions (for ceramics) to almost a meter (ductile materials)
Talk about brittle “cleavage” fracture
It’s a type of crack characteristic of ceramics and glasses. Local stress rises as 1/(r)^0.5 towards the crack tip. If it exceeds the one required to break inter-atomics bonds they separate, giving a cleavage fracture.
The progression of the crack increases K1, causing acceleration of the crack up to the speed of sound (makes noise).
Talk about ductile fracture
- Tough: materials contain inclusions which act as stress concentrations when loaded. The inclusions separate from the matrix, causing voids to nucleate and grow, causing fracture.
- Of a cracked sample: in a material is ductile, a plastic zone forms at the crack top- Within the plastic zone, voids nucleate, join and link to cause fracture. The plasticity blunts the crack tip, reducing the severity of the stress concentration. The plastic deformation adsorbs energy, increasing Gc.
What is the brittle-ductile transition
At low T, some metals and all the polymers become brittle and their fracture mode from ductile becomes brittle (cleavage fracture). Only a few FCC metals remain ductile.
Steels have a ductile-brittle transition T that can around room T if they’re not properly modified. For polymers it depends on the specific type.
Embrittlement can happen from chemical segregation: impurities in an alloy are normally found in grain boundaries: this leads to a network of low-toughness paths that can lead to brittle fracture.
How can polymers be modified to increase fracture toughness?
With fillers, impact modifiers, fiber reinforcements. In particular, fibers bridge the cracks.
