MY3200 Tensile Testing Fall 2002
Tensile tests can be used to determine the elastic modulii, yield and tensile strength, elongation, strain hardening rate, poissons ratio and many other mechanical properties of a material. In this laboratory you will learn how to measure each of these properties correctly, how to use the Instron testing machines and how to calibrate the extensometers used to measure strain. In addition to measuring the elastic modulus of a material by measuring load and deflection, both the shear and tensile modulus of the material will be measured using ultrasonics and poissons ratio determined.
The material to be tested is the same steel examined in the previous experiments on metallography and hardness. Each of the steels (1018, 1038, 1045 & 1065) is available in the annealed and quenched & tempered condition. Each group of students will conduct 3/4 tensile tests on a single steel in 1 of the two heat treated conditions.
Ultrasonic Determination of Elastic Properties
In the set up shown above, a piezo-electric crystal is stimulated with a high frequency voltage pulse, usually in the 1 to 10 GHz range. This cuases the crystal to expand and contract rapidly sending an elastic wave into the material studied. The wave travels through the material at a fixed velocity, is reflected from the far surface and travels back to the piezo-electric crystal where the elastic wave is converted back into a voltage pulse. The elastic wave is, of course, reflected back from this surface and makes the round trip through the material a second time. On reaching the piezo electric crystal the second time, the wave has been attenuated in power (amplitude) but the piezo-electric crystal can still feel the wave and as the latter deforms in response to the “impact” a second echo pulse of current is generated. The echo pulses are amplified and displayed on an oscilloscope. If the round trip transit time or the frequency of th epulses are measured, together with the thickness of the sample, the velocity of the wave can be calculated.
where f is the frequency of the echos and H is the thickness of the sample. For this method to work best, the two faces of the material to be used as reflectors must be parallel.
There are two type of elastic waves, shear and longitudinal. First measure the velocity of the shear wave using the 2.25GHz transducer. The shear modulus of the steel, G, is simply
where vs is the velocity of the shear wave and r is the density of the material.
Next measure the velocity of the longitudinal wave using the 5GHz transducer. At this point we can determine Poisson’s ratio, n .
where vl is the velocity of the longitudinal wave. Finally, the tensile or Young’s Modulus can be calculated
Having completed your determination of the elastic modulii, estimate the error in your answers based on the error in each of your measurements. You may assume that the density of the steel samples is 7.87g/cc ± 0.02g/cc.
Note: To ensure that the initial pulse in the piezoelectric crystal is transmitted to the sample and that the echos are transmitted back to the crystal, a thin layer of liquid or gel is introduced between the crystal and the sample - an acoustic couplant. The more viscous the better! Typical couplants are molasses or glycerin. The transuducer shoul dbe clamped to the sample to minimize the thickness of the liquid layer as its role is only to fill any residual small air spaces between the two contacting surfaces.
Measurement of Deflection/Strain.
In a tensile test carried out using a screw driven loading system, such as that employed by the Instron testing machine, a deflection is applied to the test piece and the force that opposes the imposed deflection is measured. The actual sample, or more accurately, the gauge length of the sample being tested, is not the only thing that moves when the screws are turned. The crosshead holding the grips is forced to move upwards/downwards, but as the test piece resists this motion and generates an opposing force, the crosshead itself deflects elastically, any slack in the “loading system” also needs to be removed, the grips holding the test piece deform elastically as does the head of the test piece itself and the gauge section which we are testing. In short, the actual deflection imposed on the gauge length is not the same as the motion of the crosshead which is actually measured by counting the number of light pulses transmitted by a rotating optical encoder attached to the screws themsleves. Thus we need both an accurate and very precise way of measuring the extension of the gauge length being tested. A high degree of precision is required since elastic deflections tend to be very small (<1% or 0.25mm for a 25mm gauge length). To make these measurements an extensometer is attached to the side of the sample.
The extensometer is essentially two thin cantilever beams separated by a fixed distance at each end. The measuring end separation is fixed, while the sample ends are free to move apart as the test piece to which they are attached stretches. Each arm of the cantilver has a strain gauge attached to its upper and lower surfaces so that as the arm bends, one gauge on each arm goes into compression, the other into tension. By arranging the resistive strain gauges into a 4 arm (full) wheatstone bridge circuit, the deflection of the sample can be measured independent of any changes in resistance of the gauges due to changes in ambient temperature during testing.
In using an extensometer, the first task is to select an extensometer with an appropriate deflection range to accommodate the expected strain. Stiff, or dynamic, extensometers measure small strains very precisely but in general cannot be used above about 5% extensions. Soft, or static, extensometers have a wider range of strain, up to 50% in many cases but are less precise and cannot be expected to measure the small elastic strains necessary to accurately determine the elastic modulus, yet are more useful for highly ductile materials giving good measures of elongation to failure.
In these experiments we will use a soft extensometer as we have already measured the elastic modulus by the most precise method available, namely ultrasonics.
Extensometer Calibration
The extensometers to be used in this experiment are 50% of either 25mm or 1” (25.4mm). With the extensometer connected to the testing machine - and hence the electronic circuit used to mesure the bridge voltage, use the following procedure to initialize the extensometer.
Clamp the extensometer onto the calibrator at zero deflection. ie. the arms of the extensometer should be separated by their nominal gauge length.
Select Strain Channel 1 (longitudinal strain)
STRAIN BALANCE followed by <ENTER>... readout should be zero
STRAIN CALIBRATE followed by <ENTER>.. readout should indicate full scale strain in %
Now using the calibrator, open the extensometer in 1mm or 0.040” increments. At each deflection note the strain reading until full scale strain is reached then repeat as the extensometer is closed to zero deflection. Plot actual strain vs. measured strain for both opening and closing. The graph should have a slope of 1 and pass through the origin. How accurate is the extensometer?
Tensile testing.
Before testing make sure that the load cell is plugged in to the test controller and that the load cell is both balanced and calibrated.
Measure the diameter of the gauge section of the tensile spaecimen. Clamp the specimen into the uppermost grips of the machine. Balance the load cell. Now fix the sample into the lower grips and clamp on the extensometer with the extensometer arms separated, as closely as possible, by the required gauge length. Wait at least a minute for equilibrium to be established. Re-balance the load and the strain to zero and reset the gauge length to zero deflection.
Use the Instron software to carry out a tensile test at an initial strain rate of 10-3/s. You may assume a gauge length of either 25 or 25.4 mm depending on the extensometer and must calculate the required testing velocity and enter this into the test set-up. Continue the test to failure and record the results obtained from the test software. Save the load deflection data to a disk. The data consists of columns of load in lbs and deflection in inches. Use a spreadsheet to import the data and produce three plots
(a) load in kiloNewtons vs. deflection in mm ,
(b) engineering stress (MPa) vs. engineering strain ,and
(c) true stress (MPa) vs. true strain assuming uniform plastic deformation.
From these graphs work out the following mechanical properties
(a) Youngs Modulus (GPa)
(b) Yield Stress or 0.1% Proof Stress (as appropriate) in MPa
(c) Ultimate Tensile Strength (MPa)
(d) Uniform plastic strain to the onset of necking (%)
(e) Tensile elongation at failure (%)
(f) The work hardening coefficient ,n, where
Is the work hardening exponent truely constant or is it a function of strain? How do the results you obtained compare with those calculated by the testing software.
From a collation of all results obtained by both sections, including errors, produce a plot of the yield strength, ultimate tensile strength and elongation to failure as a function of carbon content of the steel. Comment on the graph and relate the trends observed to the microstructure of the steel as observed in the previous experiment (The metallographic samples are available in the lab).
Scanning Electron Microscopy
The fractured samples will be examined in the sacanning electron microscope so that the mechanism of failure can be observed and any differences between the steels noted.
Lab Report
The report is to be written with the following title; “ Effect of carbon content on the tensile behaviour of steel”. It should consist of an experimental procedure, a results section with tables and graphs and a short discussion addressing some of the issues raised in the lab handout. The report will be graded on (a) organization and presentation of results, (b) the quality of the written experimental and (c) the metallurgical content of the discussion rather than its english syntax/grammar/style as in the experimental. Your discussion should focus on the difference in microstructure as affected by carbon content and by heat treatment, particulalrly the differences between the annealed and the quenched & temperaed microstructures. Where possible the structure should be related to the phase diagram and the effect of structure on the various mechanical properties explained.