Foundries will sometimes cast a test coupon alongside the product casting. Quality test results from the test coupon are assumed to be true of the cast product as well. Some tests—such as tensile stress and impact—destroy the test piece in the process. However, non-destructive tests NDT do not destroy the metal sample to obtain a result. The advantage of NDT is that tests can be conducted on the cast metal product itself, as opposed to a test piece.
The term, hardness , typically implies a resistance to deformation. For metals, the property is a measure of their resistance to permanent, or plastic, deformation. Several different tests exist for measuring the hardness of metals and cast metals. It is a macro indentation test where a high load is used to obtain the measurement. Cast metals require macro hardness tests due to the course grain structure and the potential for an inhomogeneous material. To obtain a Brinell hardness number BHN , a carbide ball of fixed diameter presses into the metal at a fixed pressure for a set time.
On removal of the load, the operator measures the diameter of the indentation left behind and converts it into the BHN using the following formula:. In the United States, test loads for steel and iron are typically set at a maximum of kgf with a 10mm ball. Aluminium uses a lower test load of kgf, and sometimes a smaller indenter of 5mm. A typical BHN ranges from 50— for metals. Preparation of the metal surface for Brinell hardness testing is very important. A jagged surface or other imperfections will influence the result.
It is advisable to grind the metal surface in preparation for the test to minimise variability in the results. The origins of the Brinell hardness test stretch back to In the early years of the test, the results were strongly influenced by the perspective of the operator. Different operators would come up with different results leading to high variability in measurement. However, with the introduction of electronic measuring equipment, the level of consistency has significantly improved. The Rockwell test has two stages.
The equipment applies a preliminary test force to the sample using a diamond or ball indenter. The purpose of this stage is to break through the surface of the metal and reduce the effect of surface finish on the final result. The operator measures a baseline indentation depth at this point. After holding the preload for a set time, a major load is then applied.
Again, the force is held for a pre-set time, before reducing it to the preload force again. Once the time has elapsed, the operator takes a depth measurement of the indentation. The Rockwell hardness number is based on the difference between the baseline and final depth measurements. It is important for the accuracy of the Rockwell test for the test axis to be within two degrees of perpendicular.
A Rockwell hardness scale accompanies the test. The Leeb test is a measure of the rebound of an object from the test sample. The hardness of metals affects the rebound energy—harder materials produce a greater rebound, while softer materials dampen the rebound energy. The velocity of the object, before and after it strikes the sample, forms the basis for the rebound value. Leeb test equipment contains a coil, which measures the induced voltage of the magnetic ball used for the rebound test. This induced voltage is directly related to the velocity of the ball moving through the coil of the test equipment.
The Leeb hardness value is calculated using the following formula:.
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Advantages to the Leeb method include the fact that the indentation left on the test sample is much smaller than with other methods. It is also portable, easier to use, and quicker than the Brinell and Rockwell hardness testers. A disadvantage is that it may yield variable results where the sample surface is uneven. The thickness of the sample, and carbon content, may influence the result as well. ASTM Ebe1 gives standard hardness conversion tables for metals to convert from one hardness test method to another. It is important to note that these conversions are approximate and depend on factors such as material composition, microstructure, and heat treatment.
Although the tables are based on large numbers of tests across the different methods, a conversion result can only be considered as an estimate of comparable values.
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Cast metals are heat treated to manipulate their properties. Heat treatment involves raising the temperature of the material to a predetermined value. It is then cooled at a specific rate depending on the desired properties of the product. The final temperature of the heating cycle and the rate of cooling have a direct impact on the microstructure of the metal. Fine pearlite and ferrite microstructures caused by a faster cooling rate have a greater hardness value.
If the metal is quenched, the rapid cooling results in a martensite microstructure, which has the greatest hardness of all. Because of the direct relationship between microstructure and hardness, the hardness test is a quick indicator of whether the heat treatment has been successful or not.
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Tables show how tensile strength correlates to hardness for specific materials. With this method, we were able to grow disbond cracks of increasing lengths between the composite overlay and the concrete substrate. Lawrence T. Henjen Ho, in Comprehensive Composite Materials , Thermoset test coupons for the single-fiber fragmentation test can be fabricated by a casting method with the aid of a silicone RTV eight-cavity mold. Sprue slots are machined into the center of each dogbone to a depth of 0. The procedure for fabrication of the coupons is as follows:. Once selected, a filament is mounted in the mold and held in place with a small amount of rubber cement at the end of the sprue.
The rubber cement is not in contact with the cavity which contains the grip sections nor the gage length section in the mold. The rubber cement is allowed to dry, and the resin is added with the aid of a disposable pipet. The long, narrow tip should be removed so that the resin can readily enter and exit the pipet chamber.
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Air bubbles are avoided by first degassing the silicone mold and the resin in a vacuum chamber before filling the mold cavities. The assembly is transferred to an oven where the appropriate cure cycle for the fiber and matrix being investigated is completed. After cool-down to room temperature, the mold can be curled away from the specimens parallel to the fiber to prevent fiber damage. The test specimens can then be stored in a desiccator until ready for analysis.
Test Coupons and Casting Properties
Prior to testing, the coupons should be inspected for defects such as voids, wavy fibers, multiple fibers, etc. The specimens are tested in uniaxial tension, using a microstraining machine Her-rera-Franco et al. This allows the operator to assess the fiber fracture process along the entire gage length of the coupon.
A transmitted light polarizing microscope should be configured such that there is one polarizer below and one above the test coupon. The fiber diameter is measured using a calibrated eyepiece. Single fiber fragmentation technique apparatus. Fabrication of single fiber fragmentation specimens using a thermoplastic matrix and glass or carbon fiber reinforcement can also be completed with variations in the thermoset procedure as noted below:.
Lay fibers across the rubber gasket and the thermoplastic film. The fibers are held in place by static electricity forces between the fibers and the thermoplastic. Once all the fibers are in place, cut them with a scalpel into 2. This helps to keep the fibers straight during compression of the laminate. Place a thermocouple over one corner of the thermoplastic. The thermocouple can be held in place by taping it to the gasket material using high-temperature tape.