Technical Jargon and Definitions
All professions have their own language and jargon. Engineering and metallurgy are no different. Below are some common terms mentioned in this website with simple explanations and definitions.
These are not intended to be comprehensive, but to merely assist the reader.
Scanning Electron Microscope
Often abbreviated as SEM. A microscope which utilizes a beam of electrons to generate an image of a surface. A sample is placed in a chamber from which the air is evacuated. A high energy beam of electrons is then focused on the surface using magnetic fields. This allows the sample to be viewed at wide range of magnifications, from ~15X to 200000X or more. From a practical standpoint, magnifications typically used in failure analysis are between 15X and 20,000X. In comparison, the maximum magnification of an optical microscope is limited to approximately 1500X with much less depth of field. Also optical microscopes require a polished surface where as rough fracture surfaces can be view directly in an SEM.
A common attachment to an SEM is EDS.
Energy Dispersive Spectroscopy
Often abbreviated as EDS. Energy Dispersive Spectroscopy is an analytical technique for determining chemical or elemental composition of a sample. The equipment consists of a detector often attached to a SEM specimen chamber and appropriate software to interpret the output of the detector. EDS can be use to provide a semi-quantitative analysis of the elemental composition of a selected area of an SEM image. The selected areas can be very small; for instance, allowing the determination of a small particle on a fracture surface. The technique utilizes characteristic x-rays generated by the interaction of the SEM electron beam with the atoms of the material being examined to determine the elemental composition. Maps of element can also be generated giving spatial location of elements within the specimen.
Optical Microscope
Sometimes referred to as a light microscope or metallograph. A microscope which utilizes visible light reflected off of the surface of highly polished specimens through a series of lenses to produce a magnified image. The optical microscope is used to examine cross-sections of fractures and the internal structure of materials. Magnifications typically range from 50X to 1000X in discrete increments depending on the specific objective lenses and eyepieces used in the microscope. The maximum magnification of an optical microscope is limited to approximately 1500X due to the wavelength of visible light and the associated physics of optical resolution. The depth of field of an optical microscope is very small, requiring careful sample preparation to produce a highly polished and flat specimen necessary for a good image.
Stereo Microscope
An optical microscope with two sets of lenses / optical paths slightly offset from each other producing a three dimensional image. Stereomicroscopes are used where the perception of depth and contrast assists in the interpretation of specimen structure. Fracture surfaces and other rough structures can be easily viewed. Magnifications range from 5X or less to 100X or more. From a practical standpoint, magnifications above 100X are typically not useful for rough surfaces due to the decrease in depth of field with increased magnification.
Stress
The intensity of internal forces in a material that resist distortion of the material when external forces are applied. Units of stress are force per unit area. Depending on the direction of loading, stresses can be categorized as tensile, compressive, or shear. Materials that are stretched have a tensile stress. Materials that are compressed have a compressive stress. Materials that are twisted have a shear stress.
Strain
The deformation or distortion of a material that occurs when an external load is applied.
Strain is measured in terms of elogation or compression of length per unit length and therefore strain is dimensionless. Strain is often expressed as a percentage.
All materials will deform to some extent when loaded regardless of strength.
For uniaxial tensile specimens, strain at failure may be between 5% and 50% for most metals.
In comparison, strains at failure for some plastics may be 500% or more.
Finite Element Analysis
Often abbreviate FEA. Finite Element Analysis is a computer-based numerical technique used by Advantec Engineering for determining the strength and behavior of engineering structures. This extremely powerful tool can be used to model how real world components react to forces, temperatures, and pressures. FEA can be used to determine deflections, stresses and strains in a component, fluid flow in a pipe, thermal expansion, elastic and plastic deformation, as well as many other phenomena. FEA can be especially useful in failure analysis of complex structures or components where direct numerical solutions do not exist.
In FEA, a computer model is created of the structure or component. The model is then divided into many small blocks or elements. Physical properties are assigned to each element with relatively simple equations. Forces, temperatures, pressures and other loads are assigned as appropriate. A powerful computer is then used to solve the system of equations and display the results.