Tensile testing of an as-cast copper alloy

Steps in the process Marking the gauge length Loading the specimen Zeroing the crosshead Fitting the extensometer Proof stress measurement Plastic deformation and fracture The load/extension curve The fracture surface Measuring ductility

Marking the gauge length In order to be able to measure the DUCTILITY of the metal, we must first mark a measured gauge length on the sample. This is done by coating the test sample with lacquer, and then scratching two circumferential marks in the lacquer 70mm apart. [We must also measure the diameter of the test sample so we can calculate its area of cross section.]

Loading the specimen Once the sample has been measured, it is loaded into the tensile testing machine. Special split collars grip the head of the sample. The collars are held inside cylindrical grips which are free to pivot in the vertical plane. This allows the sample freedom to align itself once the grips begin to move apart.

Zeroing the cross-head At the moment, no load or force is being applied to the sample. The bottom crosshead is lowered until all the slackness is removed and the grips are about to begin to pull on the sample. The sample is almost ready for testing.

Fitting the extensometer An extensometer is attached to the gauge length of the sample. The extensometer grips the sample and measures how much the sample is being stretched as the tensile forces on the sample are increased. It therefore allows us to measure very accurately the extension on the sample as the load is increased.

Proof stress measurement As a tensile force is applied, the metal stretches elastically at first. The computer plots the load on the sample (in kN on the y axis) versus the extension of the metal (in mm on the x axis). Lines show the force required to extend the sample by 0.1% (Rp1) and 0.2% (Rp2). This allows us to calculate the 0.1% proof stress and 0.2% proof stress for the sample.

Proof stress calculation load to cause 0.1% extension Cross sectional area of the sample

Plastic deformation & fracture Once the sample has been stretched beyond the proof stress, force on the sample is so large that the strain in the sample becomes permanent. The sample can now break at any time, perhaps without warning. This would damage the extensometer, so the next step is to remove it from the sample. The load on the sample continues to increase until it is large enough to break the metal.

The load vs extension curve The final load versus extension curve shows an initial stage where the gradient of the line is constant. Here doubling the load doubles the extension, and the sample behaves elastically. In the second stage, the line begins to flatten off. Here the metal is being permanently stretched (or plastically deformed). [However, note that it requires an ever greater load to keep stretching the sample. The metal therefore gets harder and stronger as it is stretched. This phenomenon is called work hardening.]

The fracture surface It is always a good idea to look at the fracture surface of the sample to see if there was a defect inside the metal which may have affected the measured strength of the metal.

Measuring ductility We now wish to measure how far the sample stretched before it broke (its ‘ductility’). To do this we put the two broken pieces back together. We then re-measure the distance between the circumferential scratches in the lacquer on the gauge length of the sample.

The final extension The distance between the marks on the gauge length at the end of the test is 79mm. This gives an elongation to fracture of 13%.

Calculating % elongation Elongation to fracture = Final length – initial length x 100 Initial length