Material Science

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)

September 14, 2021 · Jacob

Material Science

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 deformation at constant stress (or load) and temperature. The form of a typical creep curve of strain versus time is in Figure. The slope of this curve is the creep rate dε/dt. The curve may show the instantaneous elastic and plastic strain that occurs as the load is applied, followed by the plastic strain which occurs over time. Three stages to the creep curve may be identified: Primary creep: in which the creep resistance increases with strain leading to a decreasing creep strain rate. Secondary (Steady State) creep: in which there is a balance between work hardening and recovery processes, leading to…

September 14, 2021 · Jacob

Material Science

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 Stage II – propagation Crack propagates by repetitive plastic blunting and sharpening of the crack tip.

September 14, 2021 · Jacob

Material Science

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 occurs in stages involving the initiation andpropagation of cracks. Cyclic Stresses These are characterized by maximum, minimum and mean stress, the stress amplitude, and the stress…

September 14, 2021 · Jacob

Material Science

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 dull for totally ductile fracture, and granular and shiny for totally brittle fracture. Over the ductile-to- brittle transition features of both types will exist. While for pure materials the transition may occur very suddenly at a particular temperature, for many materials the transition occurs over a range of temperatures. This causes difficulties when trying to…

September 14, 2021 · Jacob

Material Science

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.

September 14, 2021 · Jacob

Material Science

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,

September 14, 2021 · Jacob

Material Science

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 5. Final shear fracture (cup and cone) The interior surface is fibrous, irregular, which signify plastic deformation.

September 14, 2021 · Jacob

Material Science

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

September 14, 2021 · Jacob

Material Science

The Durometer

The Durometer is a popular instrument for measuring the indentation hardness of rubber and rubber-like materials. The most popular testers are the Model A used for measuring softer materials and the Model D for harder materials.The operation of the tester is quite simple. The material is subjected to a definite pressure applied by a calibrated spring to an indenter that is either a cone or sphere and an indicating device measures the depth of indentation.

September 14, 2021 · Jacob