Resistência ao impacto
Testes Charpy: http://www.cimm.com.br/cimm/construtordepaginas/htm/3_24_8317.htm
1
1
Analysis of Charpy Impact Test
Ductile-to-Brittle Transition Temperature
2
2
shock test machines for component qualification (to various standards)
http://www.mpmtechnologies.com/index.htm
3
3
4
4
5
5
6
6
rupture (in engineering) describes a failure mode
mechanical failure; structural failure; electrical failure; systems failure
7
7
Mechanical Failure Causes:
overload = sobrecarga
ductile or brittle fracture = fractura dúctil ou frágil
impact = impacto
fatigue = fadiga
creep = fluência
corrosion = corrosão
stress corrosion cracking = corrosão sob tensão
corrosion fatigue = fadiga com corrosão
thermal shock failure = rotura por choque térmico
wear = desgaste
8
8
fracture of materials
fracture is the (local) separation of an object or material into two, or more, pieces
under the action of stress
associated concepts / conceitos:
- brittle fracture / fractura frágil
- ductile fracture (rupture) / fractura dúctil (rotura dúctil)
- fractography / fractografia
- fracture mechanics / mecânica da fractura
- fracture modes / modos de fractura
- fracture toughness / tenacidade à fractura
9
9
In brittle fracture, no apparent plastic deformation takes place
a fracture in glass
http://en.wikipedia.org/wiki/Fracture
10
10
In ductile fracture, extensive plastic deformation takes place before fracture
Ductile failure of a metal specimen strained axially
http://en.wikipedia.org/wiki/Fracture
11
11
basic steps of ductile fracture:
12
12
Fractography is the study of fracture surfaces of materials.
Fractographic methods are routinely used to determine the
cause of failure in engineering structures, especially in
product failure and the practice of forensic engineering or
failure analysis.
In material science research, fractography is used to develop
and evaluate theoretical models of crack growth behavior.
13
13
Fracture of an Aluminum Crank Arm. Bright: Brittle fracture. Dark: Fatigue fracture.
http://en.wikipedia.org/wiki/Fracture
14
14
Fatigued rubber brake seal
http://en.wikipedia.org/wiki/Fractography
15
15
The three fracture modes:
16
16
Fracture mechanics is the study of how cracks start, grow, and (hopefully)
stop.
Fracture mechanics also deals with methods for predicting failure of a
structure containing a crack (or several cracks).
It uses methods of analytical Solid mechanics to calculate the driving force on
a crack and those of experimental Solid mechanics to characterize the
material's resistance to fracture.
In modern materials science, fracture mechanics is an important tool in
improving the mechanical performance of materials and components. It
applies the physics of stress and strain, in particular the theories of elasticity
and plasticity, to the microscopic (crystallographic) defects found in real
materials in order to predict the macroscopic mechanical failure of bodies.
Fractography is widely used with fracture mechanics to understand the causes
of failures and also verify the theoretical failure predictions with real life
failures.
http://simscience.org/cracks/advanced/cracks2.html
17
17
The history of fracture mechanics
Part 1
Griffith's crack theory:
Fracture Mechanics was invented during World War I by English aeronautical
engineer, A.A.Griffith, to explain the failure of brittle materials.
Griffith was faced with the problem that theoretical calculations showed that the
stress at the tip of a sharp crack approaches infinity. Accordingly, any structure
containing a crack should fail, no matter how small the crack or how light the load.
18
18
Griffith's energy relation:
To solve this dilemma, Griffith
developed a thermodynamic
approach. He assumed that
growth of a crack requires
creation of surface energy,
which is supplied by the loss of
strain energy accompanying
the relaxation of local stresses
as the crack advances. Failure
occurs when the loss of strain
energy is sufficient to provide
the increase in surface energy.
strain energy release rate:
19
19
Watch the penny-shaped crack in the middle of
the box grow under tension
(the arrows are pulling on it).
http://simscience.org/cracks/image/crack_anim.gif
20
20
The history of fracture mechanics
Part 2
Irwin's modified Griffith crack theory: fracture toughness
A modification of Griffith’s solids theory emerged from the work of G.R. Irwin at the U.S. Naval Research
Laboratory (NRL) during World War II.
A term called stress intensity replaced strain energy release rate and a term called fracture toughness
replaced surface energy.
Both of these terms are simply related to the energy terms that Griffith used:
and
(for plane stress)
(for plane strain)
where KI is the stress intensity, Kc the fracture toughness, and ν is Poisson’s ratio.
Fracture occurs when KI ≥ Kc
For the special case of plane strain deformation, Kc becomes KIc and is considered a material property.
The subscript I arises because of the different ways of loading a material to enable a crack to propagate. It
refers to loading via Mode I - the most common form of loading.
21
21
The plastic zone size is obtained as:
22
22