Dislocations and Strengthening Mechanisms
Basic Concept of dislocation Dislocations can be edge dislocations, screw dislocations and exist in combination of the two. Their motion (slip) occurs by sequential bond breaking and bond reforming . The number of dislocations per unit volume is the dislocation density, in a plane they are measured per unit area. Characteristics of Dislocations There is strain around a dislocation which influences how they interact with other dislocations,…
Stress and Temperature Effects
Both temperature and the level of the applied stress influence the creep characteristics. The results of creep rupture tests are most commonly presented as the logarithm of stress versus the logarithm of rupture lifetime. Creep becomes more pronounced at higher temperatures. There is essentially no creep at temperatures below 40% of the melting point Creep increases at higher applied stresses. The behavior can be characterized by the following expression, where K, n and Qc are constants for a given material: dε/dt = K σn exp(-Qc/RT)
Creep
Creep is the time-varying plastic deformation of a material stressed at high temperatures. Examples: turbine blades, steam generators. Keys are the time dependence of the strain and the high temperature. The Creep Curve Creep in metals is defined as time dependent plastic…
Crack Initiation and Propagation
Stages is fatigue failure: I. crack initiation at high stress points (stress raisers) II. propagation (incremental in each cycle) III. final failure by fracture Stage I - propagation • slow • along crystallographic planes of high shear stress • flat and featureless fatigue surface…
Fatigue
Fatigue is the catastrophic failure due to dynamic (fluctuating) stresses. It can happen in bridges, airplanes, machine components, etc. The characteristics are: • long period of cyclic strain • the most usual (90%) of metallic failures (happens also in ceramics and polymers) • is brittle-like even in ductile metals, with little plastic deformation • it…
Ductile brittle transition
Ductile to brittle transition occurs in materials when the temperature is dropped below a transition temperature. Alloying usually increases the ductile-brittle transition temperature, for ceramics, this type of transition occurs at much higher temperatures than for metals. The notched-bar impact test can be used to determine whether or not a material experiences a ductile-to-brittle transition as the temperature is decreased. In such a transition, at higher temperatures the impact energy is relatively large since the fracture is ductile. As the temperature is lowered,the impact energy drops over a narrow temperature range as the fracture becomes more brittle. The transition can also be observed from the fracture surfaces, which appear fibrous or…
Impact Fracture
Impact fractures can best be described as a flute or strip of material that was cleanly sheared from a projectile point. The most common type of impact fracture starts at the tip of a point and runs down one blade edge possibly reaching the shoulder of a point. Some points were reworked into a useable point after having been damaged by an impact fracture. Normalized tests, like the Charpoy and Izod tests measure the impact energy required to fracture a notched specimen with a hammer mounted on a pendulum. The energy is measured by the change in potential energy (height) of the pendulum. This energy is called notch toughness.
Brittle Fracture
There is no appreciable deformation, and crack propagation is very fast. In most brittle materials, crack propagation (by bond breaking) is along specific crystallographic planes (cleavage planes). This type of fracture is transgranular (through grains) producing grainy texture (orfaceted texture) when cleavage direction changes from grain to grain. In some materials, fracture is intergranular. Fracture occurs due to stress concentration at flaws, like surface scratches, voids,
Ductile Fracture
Stages of ductile fracture 1. Initial necking 2. Small cavity formation (micro voids) 3. Void growth (ellipsoid) by coalescence into a crack 4. Fast crack propagation around neck. Shear strain at 45o…
Failure – Fundamentals of Fracture
Fracture is a form of failure where the material separates in pieces due to stress, at temperatures below the melting point. The fracture is termed ductile or brittle depending on whether the elongation is large or small. Steps in fracture (response to stress): • Crack formation • Crack propagation
