Resistência ao impacto. Testes Charpy:

Transcription

1 Resistência ao impacto Testes Charpy: 1

2 Analysis of Charpy Impact Test Ductile-to-Brittle Transition Temperature 2

3 shock test machines for component qualification (to various standards) 3

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7 rupture (in engineering) describes a failure mode mechanical failure; structural failure; electrical failure; systems failure 7

8 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

9 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

10 In brittle fracture, no apparent plastic deformation takes place a fracture in glass 10

11 In ductile fracture, extensive plastic deformation takes place before fracture Ductile failure of a metal specimen strained axially 11

12 basic steps of ductile fracture: 12

13 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

14 Fracture of an Aluminum Crank Arm. Bright: Brittle fracture. Dark: Fatigue fracture. 14

15 Fatigued rubber brake seal 15

16 The three fracture modes: 16

17 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. 17

18 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

19 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

20 Watch the penny-shaped crack in the middle of the box grow under tension (the arrows are pulling on it). 20

21 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 K I is the stress intensity, K c the fracture toughness, and ν is Poisson s ratio. Fracture occurs when K I K c For the special case of plane strain deformation, K c becomes K Ic 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

22 The plastic zone size is obtained as: 22