Materials Chapter 8

fatigue life

(Nf) the number of cycles to cause failure at a specific stress level, as taken from the S-N plot.

fracture mechanics design

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fatigue

A form of failure that occurs in structures subjected to dynamic and fluctuating stresses. Fatigue failure is brittle like in nature even in normally ductile metals, in that there is very little, if any, gross plastic deformation associated with failure. The process occurs by the initiation and propagation of cracks, and ordinarily the fracture surface is perpendicular to the direction of an applied tensile stress. Know graphs and equations.

corrosion fatigue

Failure that occurs by the simultaneous action of a cyclic stress and chemical attack.

fatigue fundamentals

Fatigue is a common type of catastrophic failure in which the applied stress level fluctuates with time; it occurs when the maximum stress level may be considerably lower than the static tensile or yield strength.

fracture mechanics principles

Fundamental principles include; stress concentration, fracture toughness and design using fracture mechanics.

brittle fracture

Little to no plastic deformation with low energy absorption. The direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress and yields a relatively flat fracture surface. Unstable fracture.

fatigue life factors

Mean Stress - increasing the mean stress level leads to a decrease in fatigue life. Surface effects - Design effects such as stress concentrations decrease fatigue life. Surface treatments such as polishing, shot peening and case hardening can extend fatigue life. Environmental effects - thermal and corrosion fatigue decrease fatigue life. Know equation for thermal fatigue. - Reducing the mean stress level - Eliminating sharp surface discontinuities - Improving the surface finish by polishing - Imposing surface residual compressive stresses by shot peening - Case hardening by using a carburizing or nitriding process

cracks

Stable crack - one that takes place relatively slowly with extensive plastic deformation in the vicinity of an advancing crack such as in a ductile fracture. Unstable crack - one that may spread extremely rapidly with very little plastic deformation such as in a brittle fracture. Fatigue failure is characterized by three distinct steps: 1) crack initiation, wherein a small crack forms at some point of high stress concentration. 2) crack propagation, during which this crack advances incrementally with each stress cycle. 3) final failure, which occurs very rapidly once the advancing crack has reached a critical size.

steady state creep rate (Epsilon dot)

also known as secondary creep. The slope of the graph (know graph and equations) is linear in this section showing that the creep rate is constant. This is the stage of creep that is of the longest duration and the slope of this section of the creep curve is considered the most important parameter from the creep test. It is the engineering parameter that is considered for long-life applications.

rupture

also known as tertiary creep. results from microstructural and/or metallurgical changes such as: grain boundary separation, the formation of internal cracks, cavities or voids.

stress raiser

any place of stress concentration such as crack. Called this because the applied stress is amplified at the location of the stress concentration.

fracture variables

similar question to "fracture mechanics design". Know all the variables and equations or what is meant by this?

fracture mechanics

subject that allows quantification of the relationships between material properties, stress level, the presence of crack-producing flaws, and crack propagation mechanisms.

ductile fracture

substantial plastic deformation with high energy absorption before fracture. Material experiences "necking" until fracture, maximum shear stress is when shear deformation is at 45° with the tensile axis. This is sometimes called a cup-and-cone fracture.

fatigue limit (endurance limit)

the largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles.

fracture toughness (Kc)

the measure of a material's resistance to brittle fracture when a crack is present.

fatigue strength

the stress level at which failure will occur for some specified number of cycles.

creep

time-dependent permanent deformation of materials when subjected to a constant load or stress and at elevated temperatures. Know graphs and equations.

plane strain

when a load acts perpendicular to the crack (i.e. the load vector is perpendicular to the axis the crack exists around) there is no strain component perpendicular to the front and back faces (i.e. along the axis the crack exists around).

plane strain fracture toughness (Kic)

when specimen thickness is much greater than crack dimensions. Kic is low for brittle materials, but high for ductile materials. Kic is greatly influenced by temperature, strain rate and microstructure. Kic diminishes with increasing strain rate and decreasing temperature. It also decreases with increased yield strength. It increases with reduction in grain size. Know equation and variables.