Alternating Stress Levels
Cyclic stresses or stresses that change with time or revolution of a part.

Beach Marks
Beach marks are small ridges that develop on a fracture surface during cyclic load variations and are evidence of fatigue.

Bending Stress
Stress that is induced by a component having bending loads applied to it.

Charpy Impact Tests
A small test specimen (50mm x 10mm x 10mm) with a notch used to qualitatively measure toughness of a material by hitting the specimen with a calibrated anvil.  The ASTM standard is E23.

Compact Tension Tests
A test specimen used to measure fracture toughness or the ability of apart to resist crack growth.

Charpy V- Notch Specimen with a notch used in Charpy Impact tests.

Ductile is the ability of a material to be deformed plastically to some limit without fracture .

Ductile Overload
The appearance of a fracture surface or the mechanism at which a ductile materials exceeds it ability plastically deform and fractures. into two pieces.

Brittle Cleavage
The appearance of a fracture surface or material behavior when the material fractures without  appreciable or noticeable plastic deformation.

Corrosion Fatigue
Failure from the simultaneous application of cyclic stresses and chemical attack.  Corrosive environments can decrease the fatigue life.

Deformation of a part when exposed to elevated temperatures as well as static mechanical stresses.  Creep is time dependent and causes permanent deformation.

Dynamic Loading
Sudden application of a load on a component.

Dynamic Lower Bound
Description of the lower bound data on a Charpy Impact data curve.  The lower bound data would provide the most conservative estimate of impact toughness.

Energy Dispersive X-Ray Spectroscopy
A tool used with a scanning electron microscope (SEM) to identify the presence of elements via the energy levels of x-rays given off when a sample is hit by the electron beam.

A cracking mechanism caused by repeated application of cyclic/alternating loads to a component.  The resultant stresses are typically much lower than the yield strength of the material

The science of interpretation of  “topographic features in the examination of a fracture surface using the unaided eye along with various microscopes (e.g., scanning electron microscope)

Fine Fractographic Features
The microscopic features on a fracture surface that allow the mode of fracture to be identified or the origin area to be identified (e.g., fatigue striations, microvoids, etc.)

A description of the microstructure of steel.  Fine-grained means that the crystals composing the metallurgical microstructure are very small as compared to ASTM standards..

Finite Element Analysis (FEA)
A numerical method of modeling a complex component using discrete elements.  FEA can be used to model components to identify stress, strain, temperature, flow rates, etc.

Accumulation of debris on surfaces.  This can reduce efficiency and increase the probability of failure.

Fracture Toughness
A material property that measures the resistance of a crack/flaw to propagate (grow) under stress.

Gas Phase Embrittlement
(GPE)  Failure mechanism in turbine discs resulting from environmentally-assisted in-service cracking.  AKA hold-time cracking, stress assisted grain boundary oxidation (SAGBO).

Harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency.  Harmonics in turbines can cause unwanted vibrations in parts as the machine is ramped up and down.

High Cycle Fatigue
Describes fatigue cracking under low stress (far below the yield strength) with large number of cyclic stress events

High Temperature Oxidation
An oxide layer forms on a metal surface due to the exposure of the part to high temperatures and an oxidizing atmosphere.

Hot Corrosion
Chemical attack of the protective oxide layer on turbine blades from compounds in the fuel or possibly the atmosphere.  These compounds usually include alkaline metals, vanadium, and especially sulfur.

Hot Gas Path
(HGP)  Section of a gas turbine that is down-stream from the combustion section.  AKA Hot Section, Gas Generator, Turbine, HPT.

Laser Doppler Velocity Measurement System
A non-contact tool that uses a laser to measure the vibration of a moving component via the detection of the Doppler shift of coherent laser light that is scattered from a small area of a test object.

Low-Cycle Fatigue
Describes fatigue cracking under high stress (near the yield strength) with relatively low number of cyclic stress events

The description of the possible microstructures of low alloy steels such as the rotor steel used by MHI

Microvoid Coalescence
The description of the fracturing process of a ductile overload failure.  As the object is overloaded and it begins to plastically deform, small voids are created around particles in the microstructure and then the voids link up to cause a crack and ultimately a failure. Microvoid coalescence is a feature typical of ductile overload.

Modified Goodman Diagram
A widely accepted mechanical engineering tool used to illustrate the susceptibility of a component to fatigue. The diagram shows that as the mean stress (constant load) of a component increases the cyclic stress (alternating load) required for fatigue to occur decreases.

A term used to describe the different fractographic features (topography) of a fracture surface that can be used to identify the mode of cracking.

A part is exposed to temperatures well above the prescribed operating conditions.  Overheating can occur due to clogged cooling slots, exposure to fire, excess friction, among other causes.

Power Turbine
(PT)  Turbine section on the end of aero-derivative type machines to transfer the rotational energy into power rather than being emitted as thrust (as when they are in airplanes).

Quenched and Tempered Bainite with Carbide Precipitates
A description of a typical microstructure of a rotor steel.

Rockwell Hardness C Scale
The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.  The C scale describes the type of indenter (diamond) and load (150 kgf) applied while taking a hardness measurement.

Rockwell Hardness B Scale
The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.  The B scale describes the type of indenter (1/16” ball) and load (100 kgf) applied while taking a hardness measurement.

Rotor Dynamics
Rotor dynamics is applied mechanics concerned with the behavior and diagnosis of rotating structures. At its most basic level rotor dynamics is concerned with one or more mechanical structures (rotors) supported by bearings and influenced by internal phenomena that rotate around a single axis. As the speed of rotation increases the amplitude of vibration often passes through a maximum that is called a critical speed.

Normally referring to tip rubs of blades or vanes.  Generally the rubbing of a part on rotating or any other contact.

Scanning Electron Microscope
A laboratory tool that uses electrons to examine the surface of a component and allow for the taking of pictures and collecting surface chemistry data.

Semi-quantitative, Standardless Quantification Software
A widely accepted and used software algorithm that works in conjunction with a scanning electron microscope to semi-quantitatively measure the concentration of elements present on and near the surface of a component.

Static Loading
Very slow or essentially constant application of a load.

An optical microscope that uses multiple lenses to allow higher magnification viewing (e.g., 6X to 60X) of a component.  The multiple lenses a slight different angles provides a three-dimensional appearance with good depth of field.

Stress Corrosion Cracking
(SCC)  The sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature. SCC only occurs in certain alloys when exposed to a few, specific chemical environments. It is often undetected prior to a failure and can progress rapidly.

Tear Ridges
Tear ridges are steps created when two different crack planes link up tearing the ligament between them.  Tear ridges are parallel to crack propagation direction and perpendicular to the crack front.

Tensile Strength
A material property that measures the maximum stress a material can withstand when subjected to tension.

Technical Information Letters
(TIL)  Original equipment manufacturers will routinely issue TILs that address operation, maintenance, reliability, or performance. They are generally confidential or proprietary and often are serial number specific (do not address the entire fleet).

Thermal Mechanical Fatigue
(TMF) A type of fatigue where the part is exposed to fluctuating temperatures that induce cyclic thermal stresses as well as cyclic mechanical stresses.

Torque Stress
Another name for the torsional stress on a component.  The stress is created when a part/component is subjected to loading that causes it to twist.

Transgranular Cracking
Cracking that propagates through the grains or crystals of a microstructure as opposed to around the grains.

Transverse Crack
The description of a crack that indicates the crack propagated perpendicular to the axis of the component.  For example, a crack in a rotor that is perpendicular to the axial length of the rotor would be a transverse crack.

Vicker’s Microhardness Readings
The Vicker’s microhardness test determines the hardness by measuring the size of penetration that an indenter under a specific load for a specified time creates. A widely and universally accepted method for characterizing the hardness of metal.

API 691 defines wear abrasion  as the removal of material from a surface when hard particles slide or roll across the surface under pressure. The particles may be loose or may be part of another surface in contact with the surface being abraded.