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Experiment 2: Mechanical Testing- Impact & Hardness Testing

Henry Tan's picture

The mechanical properties of materials are ascertained by performing carefully designed laboratory experiments that replicate, as nearly as practical, the service conditions.

Factors to be considered include the nature of the applied load(s), its’ duration and the applicable environmental conditions. It is possible for the load to be tensile, compressive, or shear, and its magnitude may be constant with time, or it may fluctuate continuously. Application time may be for only a fraction of a second, or it may extend over a period of many years. Service temperature can also be an important factor.Mechanical properties are of concern to a variety of parties (e.g., producers and material consumers, research organizations and government agencies) that have differing interests.

Consequently, it is imperative that there be some consistency in the manner in which tests are conducted, and in the interpretation of their results. This consistency is accomplished by using standardized testing techniques. Establishment and publication of these standards are often coordinated by professional societies. In the United States the most active organization is the American Society for Testing and Materials (ASTM); its Annual Book of ASTM Standards comprises numerous volumes that are updated yearly. A large number of the ASTM Standards are relate directly to mechanical testing techniques including specifications for measuring the hardness and impact properties of materials. Directions for Charpy Impact TestDirections for Rockwell Hardness Test.

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Henry Tan's picture

Link: Indentation of polymer-matrix composites

For measuring hardness of nanograined materials, nanoindentation is used.

In nanoindentation small loads and tip sizes are used, so the indentation area may only be a few square micrometres or even nanometres. This presents problems in determining the hardness, as the contact area is not easily found. Atomic force microscopy or scanning electron microscopy techniques may be utilized to image the indentation, but can be quite cumbersome. Instead, an indenter with a geometry known to high precision (usually a Berkovich tip, which has a three-sided pyramid geometry) is employed. During the course of the instrumented indentation process, a record of the depth of penetration is made, and then the area of the indent is determined using the known geometry of the indentation tip. While indenting various parameters, such as load and depth of penetration, can be measured. A record of these values can be plotted on a graph to create a load-displacement curve. These curves can be used to extract mechanical properties of the material.

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