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Metal testing is a process or procedure used to check composition and properties of an unknown metallic substance. There are destructive processes and nondestructive processes. Metal testing can also include determining the properties of newly forged metal alloys. With existing chemical-property databases available, identification of unmarked pure, common metals can be performed by comparing testing analyses to charts of known materials. Non-destructive testing involves leaving the original sample in complete, re-usable condition. When working with alloys (forged mixtures) of metals however, determining the exact composition could result in the original sample being separated into its starting materials, then measured and calculated. After the components are known they can be looked up and matched to known alloys. Breaking down or destroying the original sample during testing is destructive testing.
Contents
Destructive testing (DT)[edit]
In this kind of testing, the material undergoes mechanical testing and is discarded thereafter. Test results are compared with specifications. Subtypes include:
Tensile Test[edit]
Used to determine static mechanical properties of metals. This includes ductility, proportional limit, elastic limit, yield point and breaking strength.
Bend Test[edit]
Impact Test[edit]
Metals subjected to impact or shock loads may behave very differently than when that metal slowly has a force applied. Izod Test require clamping a notched material vertical in a vise, with the notch located flush with the top of the vise. A weighted hammer is swung from a pivot to strike the test material and a measure is made of the amount of force necessary to break the material.
Hardness Test[edit]
Hardness is the ability of the metal to resist permanent deformation when a force is applied. The Brinell hardness test is performed by pressing a hardened metal ball into a piece of metal under a known load and measuring the diameter of the resulting impression. The hardness is measured as the applied force divided by the area of the impression.[1] The Vickers test is identical to the Brinell test, but instead of using a round ball to make an impression a pyramid-shaped diamond is used. The Vickers test is favored over the Brinell method because the impression that it leaves in the material is more easily read. The Rockwell test is performed in a similar manner to the Brinell test, except that the depth of the impression is measured instead of the diameter.
Fatigue Test[edit]
Metals subjected to repeated shock loads might fail under a much smaller load than a single large shock load. Fatigue limit is the load force, typically expressed in psi which may be applied an infinite number of cycles without causing failure. In designing parts subjected to varying stresses, the fatigue limit of a material is often more important than its tensile strength or elastic limit.
Corrosion Test[edit]
The ability of metal to resist corrosion due to liquids or gases is very important. Corrosion can be measured by determining the loss of tensile strength, loss of weight of metal, or gain of weight of metal due to rust or oxidation formation.[1]
Nondestructive testing (NDT)[edit]
Test materials undergo testing according to specifications provided by organizations such as American Society of Mechanical Engineers (ASME) and American Society for Testing and Materials (ASTM). The tested material is not damaged by the test. Subtypes include:
Magnetic Particle Inspection[edit]
Magnetic inspection can only be performed on materials that have magnetic properties. There are two types of magnetic particle inspection: one to discover defects or flaws such as cracks or seams, and the other to determine variations in structure. In the Sperry test iron powder is spread on the test material and magnetic poles are applied at the ends of the material. The powder will stick to and defects which are present in the test material. Another test involves applying a magnetic field to a test material. The magnetic properties of the material are then compared to a database of known metals and alloys. Identical materials should have identical magnetic properties.[1]
Radiographic Testing[edit]
The ability of X-rays and gamma rays (symbol γ) to pass through metal makes them useful for spotting internal defects.[1] Radioactive material, typically radium or cobalt 60[2], is exposed to the test material. As the rays pass through the sample, they are recorded onto a photographic plate. When the rays pass through cracks, holes or areas of lower density, they will appear darker on the film, exposing any flaws in the material. Radiographic testing is used at high resolution to expose the crystalline structure of the metal itself.
Ultrasonic Inspection[edit]
There are several methods of ultrasonic testing, the most common of which uses a pulse-echo system. A transducer is attached to the material and induces electrical pulses into the test material. Those electrical pulses are picked up by a transducer that is also attached to the test sample, where they are read by an oscilloscope. The signal appears as a wave form on the oscilloscope, and any flaws will show as abnormalities in the wave.
Liquid Penetrant Inspection[edit]
Liquid penetrant testing can be used to detect surface cracks, porosity and other surface defects that are otherwise invisible upon visual inspection. The process involves thoroughly cleaning the surface to remove any contaminants. Once this is done a penetrating fluid is applied to the material, typically delivered by means of an aerosol can. It must be given an adequate amount of time so that it may soak into any holes, cracks or defects and any remaining penetrant must be cleaned from the surface. Then a developer is applied draw penetrant remaining in any of the cracks to the surface. Liquid penetrant testing can be used on a wide variety of materials, including metals, glass, plastics and ceramics. It is a quick and easy process as well as costing relatively little.[2]
Fluorescent Penetrant Inspection[edit]
In the fluorescent penetrant test, the developer used is designed to fluoresce under a black light.
Dye Penetrant[edit]
The dye penetrant test is a similar method to the fluorescent penetrant test, but without the need for a black light. It is much more portable and as such is advantageous for using in the field.
Eddy Current[edit]
Eddy current testing only works on electrically conductive materials. It can be used to detect flaws such as seams, surface cracks and variations in thickness. Differences in mass, density, dimension and shape can also be detected. One of its main advantages is that eddy current testing can be performed relatively quickly.
In eddy current testing, electrical current is passed through a coil, creating a magnetic field. Eddy currents are produced when the magnetic field is placed near the test material. These eddy currents cause a change in impedance in the magnetic field, which can be read on a meter or oscilloscope[2]. Due to eddy currents only being present near the surface of the test material, defects near the center of parts won’t be detected.
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- ^ a b c d Johnson, Carl G.; Weeks, William R. (1977). Anderson, John G. (ed.). Metallurgy. Alsip, Illinois: American Technical Publishers, Inc. pp. 105–130. ISBN 978-0826934826.
- ^ a b c Neely, John E.; Bertone, Thomas J. (2000). Practical Metallurgy and Materials of Industry. Upper Saddle River, NJ: Prentice Hall. pp. TBD. ISBN 0136245528.