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productionmachining.com :: 21 Helping Precision Machine Shops Be More ProducƟ ve and Profi table Technical Regulatory Management Quality Technical Regulatory Management Quality Craftsman's Cribsheet Miles Free – Director of Technology and Industry Research NUMBER 50 Ductility as Measured by Tensile Testing The ability of a material to deform plastically without fracturing is called ductility. In the materials usually machined in our shops, ductility is measured by determining the percent of elongation and the percent reduction of area on a specimen during a tensile test. In this post, we will describe how we can measure ductility and use it to predict behavior based on values reported on certs and test reports. The percent elongation and percent reduction of area values shown on our test reports and material certifi cations from our material suppliers indicate the ductility of the material tested. In the tensile test, a cylindrical specimen is gripped securely and subjected to a uniaxial load and elongated until it breaks. At the end of the test, the pieces of the fractured specimen are fi tted back together again, and the change of length between the two gage marks put on the specimen before testing is determined. The change is then expressed as a percentage of the original gage length. The percent reduction of area is determined by measuring the minimum diameter of the broken test specimen after the two pieces are fi tted together, and the difference is expressed as a percentage of the original cross sectional area prior to the test. The differences in measurements after tensile test are used to calculate the percent elongation and percent reduction of area. A minimum of 12 percent elongation is recommended for consistent, trouble-free thread rolling applications. Rolled threads are stronger, so having the ductility to thread roll is important. However, too much ductility makes it diffi cult to get the chip to separate by cutting. Low ductility can be problematic for cold deformation manufacturing processes such as thread rolling, cold forming, swaging, staking and crimping. Original gage length Plastic deformation Gage length at failure (a) (b) Original diameter Reduction in diameter Diameter at failure All Craftsman's Cribsheets are available for viewing and download at short.productionmachining.com/cribsheets.