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Definitions D

Decohesive Rupture: Decohesive rupture is a failure process in which the material that defines the edge of a metal grain/grain boundary [the area in between the individual metal grains] is weakened by high temperatures and/or the presence of a low strength phase such as a precipitate. 

Deep Drawing: A shaping process where a sheet or metal form is subjected to significant or major amounts of deformation, resulting in large changes in shape.

Defect: Flaws, which can grow in size or shape when subjected to normal operation stresses, are defined as defects. The term flaw and defect are sometimes inadvertently interchanged. All flaws are not defects but all defects are flaws. A flaw can exist and remain within a structure in a benign condition for years. If, however, the stress applied to that structure increases to a new and higher level and/or a previous static stress begins to alternate, the inactive benign flaw can begin to grow or extend. After a flaw begins to grow, it is classified as a defect.

[Flaws], which can grow in size or shape when subjected to normal operation stress [stresses which are expected during normal operation] are defined as defects. The term flaw and defect are sometimes inadvertently interchanged. All flaws are not defects but all defects are flaws. A flaw can exist and remain within a structure in a benign condition for years. If, however, the stress applied to that structure increases to a new and higher level and/or a previous static stress begins to alternate the previously benign flaw can begin to grow or extend. After a flaw begins to grow, it is classified as a defect.

Destructive Testing: Testing which destroys or physically changes the configuration of the object being tested in order to understand the structural performance. The purpose is to check for conformance in mechanical and metallurgical properties and is a process where load or stress is applied to the object of interest until the test specimen fails. These tests are generally easier to carry out, yield more information and are easier to interpret than the result of non-destructive testing. After destructive testing, the object may not be usable as originally designed or intended. Examples of destructive testing include, but are not limited to tensile testing, metallography, impact testing and chemical analysis.

[Nondestructive testing] of an object is often done in view of future use, which would make destructive testing pointless. However, destructive testing can be useful if the result gives information about similar specimens which are not tested.

Dezincification: A corrosion process where zinc is leached from brass [a mixture of copper and zinc]. The result is a porous zinc depleted structure which has reduced strength and increased ability for other chemical species to pass through the formerly solid brass structure.

Diffusion: A process where atoms or molecules move within a liquid or solid medium.

Digital Microscopy: An investigative tool that allows viewing fractures and components in stereo or 3D and records microscopic photographs of the images observed at magnifications up to 2500x to be sequentially recorded on different focal planes. The digital microscope computers then stitches the in-focus pixels from each focal plane into a three dimensional, in focus, view of the object. The range of magnification is far greater than a stereomicroscope. Digital microscope photographs are presented in color. The three-dimensional images obtained are similar to scanning electron micrographs, which are only visible in black, white and gray tones. The digital microscope is portable and is frequently used for field and scene examinations.

Diluent: A refined petroleum hydrocarbon which is a mixture of kerosene and diesel.

Dimple Rupture: A fracture mechanism that occurs in metals which fail under conditions of an overload or tearing stress. Tiny holes, or microvoids, are created in the interior of a piece of metal when the metal object is subjected to a load. As more load is applied, these microvoids internally gather together and form small void pockets in the metal. When the edges of these pockets meet and the metal separates, small cup-like features appear on the fracture surface. The appearance of such a fracture surface is referred to as dimple rupture and can be seen in the photograph below. Tiny holes, or microvoids, are created in the interior of a piece of metal when the object is subjected to a load. As more load is applied these microvoids internally gather together and form small void pockets in the metal. When the edges of the pockets meet, at the metal separates, small cup-like features appear [at high magnification] on the fracture surface. The fracture appearance of such a fracture surface is referred to as dimple rupture. Photograph A is a scanning electron micrograph showing dimple rupture at a magnification of 2500x.

SEM of Dimple Rupture
Photograph A


Types of Dimple Rupture:

1) Shear Dimples

Shear dimples are dimples which are formed on the metal fracture surface in the presence of large amounts of shear deformation. The tails of shear dimples point away from the source of the load causing the shear fracture.

2) Tearing Dimples

Tearing dimples are formed in tearing overload type failures. The dimple and dimple tail have an orientation which indicates the direction of tearing for application.

3) Tensile Dimples

Tensile dimples are formed in metals which deform appreciable amounts under the influence of tensile or pulling stress. Tensile dimples appear as small cups and are called equiaxed.

Discontinuity: An anomalous region in the metal structure usually discovered by x-radiography or other Non-Destructive Testing (NDT). A discontinuity indicates an abnormality which must be more fully investigated. Discontinuities may be considered a defect if they exceed the maximum flaw size specified in acceptance standards.

Dished or Dishing: Permanent deformation that results in a hollowed out or dish/saucered morphology.

Drill Bit: On the end of the drill string is a cutting tool called a drill bit, which cuts and breaks up the rock into small fragments as the oil well is drilled.

Drill String: A column of individual joints of pipe, each about 30 feet in length, joined together. The long pipe, drill string, is used to transmit drilling fluid and torque down to the drill bit at the bottom of the wellbore.

Ductility: The ability of a metal to stretch without fracturing. Most metals exhibit some degree of stretching when stressed. The photograph below shows a bolt from the propeller hub of a wind turbine that has bent or stretched without breaking, thus demonstrating ductility.

Ductile Fracture: Most metals exhibit some degree of ductility. Basically, when a metal component or part is stressed [or subjected to use] it will stretch. The ability to stretch without fracturing is called ductility. In Photograph A, this rather large bolt has bent in the bolt shank without breaking. Thus, the bolt would be described as exhibiting a considerable degree or amount of ductility.

Ductile Bolt
Photograph A the subject bolt bent at an angle of 30 degrees without fracturing.

 

Dutchman: A metallurgical term of art which refers to a small threaded stub which remains and is often stuck inside a female threaded fitting following a pipe, stud or screw failure.

Electromotive Series: A list of metals arranged in an order designating their tendency to oxidize. The most active, readily oxidized are at the anode or anodic end. Those least able to oxidize are at the cathodic end.

Elemental X-ray Mapping (XMAP): X-ray mapping (also referred to as a dot map) allows determination of the location in the sample of the chemical species which emits the characteristic x-ray and in turn yields the EDS spectra. Energy Dispersive Spectroscopy (EDS) allows determination of the chemical make-up of the sample. Assume that the sample is a chocolate chip cookie with pecans. EDS tells you that it is a chocolate chip cookie with pecans. The x-ray map allows the determination of the location each of the chocolate chips, each of the pecans and the rest of the cookie dough. The location of the various atomic species on the sample surface is mapped and displayed. The location of each atomic element is displayed in a different color.

Embedment: When the shape or form of one metal object transfers or is molded or embedded into the surface of another metal or metal object.

Embossed: Stamped imprint into the surface of metal for identification purposes.

Energy Dispersive Spectroscopy: An accessory attached to most scanning electron microscopes (SEM). When the SEM electron beam strikes the surface of the sample, the electron beam energizes the surface atoms and x-rays result from that interaction. The energy of those x-rays is unique and specific to the chemical species [atom] from which they initiated. These characteristic x-ray energies are collected and displayed as a spectra or chemical fingerprint of the sample area being examined. The spectra or x-ray energy spectra results can be used to determine a semi-quantitative analysis of what chemical elements are on the surface and approximately how much of each element is present.

Example EDS

Cases involving Energy Dispersive Spectroscopy (EDS):

Corrosion Failure Analysis
Clevis Pin Failure
Motor Cooling Fan Failure

 

ER Weld: Used to fabricate a pipe by welding. To fabricate a pipe, a thin flat plate of steel is rolled into a circle. The two plate edges are then welded to close the gap between the edges. This weld is often made by electric resistance (ER) welding. As the two edges are pushed together, the two surfaces touch, but the metal is not continuous where they touch. An electrical resistance is developed at this surface discontinuity. Passing a high electrical current through this resistance then heats and melts the plate edges and a solid weld is formed which closes the pipe.

Erosion Corrosion: An accelerated deterioration of a metal surface as a result of flowing liquid. Erosion corrosion occurs because of turbulence in the fluid, near the metal surface. This erosive attack is characterized by gullies, undulating surfaces and rounded holes in the metal surface. All types of metals, when subjected to fluid/liquid and/or gaseous flow are susceptible to erosion corrosion. An example of erosion corrosion is illustrated in the photograph below of a copper water pipe failure.

Example of erosion corrosion
Photograph A Interior of copper water pipe.
Size transition caused turbulence and erosion.

Explosion: Sudden release of energy in a violent manner. If the released energy was contained in a vessel numerous fragments will be created.

Extrados: The outer or exterior curve or surface of an arch or curved object.

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J.E.I. Metallurgical, Inc.

5514 Harbor Town
Dallas, Texas 75287

Phone: (972) 934-0493
Fax: (469) 737-3938
Email: r.c.jerner@metallurgist.com

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