R. J. FRIESEN, editor
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University of Waterloo Woterloo, Ontario, Canada
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Simple Tensile Testing
P D .McCormick Thorold Secondary School Ormond Street North Thorold, Ontario
deformation X
Fracture
Modem tensiometers costing several thousands of dollars are designed to measure the ratio of stress (defined as the force applied to a wire or rod divided by the cross-sectional area of the rod) to strain (the elongation of the rod divided by the original lengthof the rod). If the stress/strain ratio is measured for forces which are sufficiently small that reversible (elastic) deformation occurs, then the ratio is called Young's Modulus. If the forces are sufficiently large, non-reversible (plastic), deformation occurs and Young's Modulus has no meaning. A simple tensiometer giving good apprnximations to Young's Modulus and quite acceptable tensile test curves is descrihed below. The cost of special equipment should be less than one dollar.
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The Experiment
sile strength. Try this experiment with your students, and be prepared for some interesting questions concerning atomic reasons for elastic and plastic deformation.
From any solid support (such as a 6-ft step ladder or a hook in the ceiling) suspend a piece of wire of known diameter. A weiahed metal bucket is attached to the lower end and pe-weighed sand (or ~ e b b l e s or , water, or metal lumps) added to the bucket constitutes the stress. he strain is measured as the elongation observed hetween two pieces of masking tape placed on the zero load wire a t a separation of one meter (the gauge length). If the weights are added slowly, and if the load is supported by your hand when adding a weight, a stress/strain curve of the form shown in Figure 1 can be ohtained and Young's Modulus calculated. (Students, and perhaps yon, will be amazed a t the magnitude of the strain obtained using copper wire or brass "snare" wire.) The stress a t which permanent deformation occurs is called the elastic limit, and there is a small range of stresses just below the elastic limit for which strain is not proportional to stress. The minimum stress that will cause deviation from proportionality is called the proportional limit. The stress which causes 0.2% plastic deformation is called the yield strength, and the stress represented by a load that causes elongation even when the load is not increased ( a spectacular event to witness!) is called the ten-
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Elongation
Notes
1) One teacher uses a n archery target support for this experiment. 2 ) Almost any material in wire form can be investigated (even monofilament fishing line) and the results compared. 3) When attaching the wire to the support and the bucket, take several turns around each and wrap the wire upon itself about 20 turns. Further reinforce the wire by wrapping the point of attachment with plastic tape. 4) Use a cardboard box as a shock absorber if investigating the fracture point, and take precautions against injury when the wire whips hack upon fracturing. 5) Conversions from wire gauges to diameter in inches and centimeters can he ohtained from the "Handbook of Chemistry and Physics." 6) Can you discuss elastic and plastic deformation in terms of metallic bonding, closepacking and slip planes? 7 ) Comments?
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