Science of Engineering Materials

Equipment Required:

Creep Machine

Stop Watch

Materials Required:

Lead Specimens

Suggested reading:
Textbook - Pg. 220-224

Introduction:

When a materials is under a constant load or stress, it may undergo progressive plastic (permanent) deformation over a period of time. This time dependent deformation is called CREEP. The creep of materials is very important for some type of engineering designs, particularly those at elevated temperatures. For such designs at elevated temperatures the creep of engineering materials is the limiting factor with respect to the operating temperature. For some materials like lead and polymers creep occurs even at room temperature.

Let us consider a creep experiment in which a specimen is subject to a constant stress of sufficient magnitude and a high enough temperature to cause creep deformation. When the relative change in length or strain is plotted against time, creep curve is obtained.

The stress, , is defined as the load per unit area (newtons per square meter) and the strain, , is the relative increase in length, is the increase in length and and l0 is the initial length of the specimen.

In the creep curve there is first an instantaneous elongation of the specimen. This is followed by the primary creep stage in which the strain rate decreases with time. The slope of the curve is called the creep rate. After primary creep a second stage occurs in which the creep rate is essentially constant and is therefore referred to as steady state creep. In the third or tertiary stage the creep rate increases rapidly up to the strain at fracture. The shape of the creep curve depends strongly on the applied stress and the test temperature. Higher stress and temperature increase the creep rate.

During primary creep the metal strain hardens to support the applied load and the creep rate decreases as further strain hardening becomes more difficult. During secondary creep mobile dislocations counteract the strain hardening so that the metal continues to elongate at the steady-state rate. During this stage atoms diffuse from grain boundaries which are parallel to the applied stress to grain boundaries perpendicular to the applied stress for tensile loading. For compressive loading the atoms move in the opposite directions. In both cases this causes permanent deformation of metals.)

Since both the mobility of the dislocations and the diffusion of atoms are thermally activated processes, the steady-state creep rate, , can be described by

Where Co and n are constants, is the stress in , Q is the activation energy for the creep process in joules per mole, R is the gas constant (8.314 J/mol.K) and T is the temperature in degrees Kelvin.

If we take the natural logarithm we obtain

For constant temperature this expression is of the form y = a + bx + c , which represents a straight line with a slope b. Thus if we plot vs. we get a straight line with a slope n.

If we could do the experiment at constant stress but at different temperatures we could plot ln against 1/T and get the straight line with slope -Q/R. Since R is known, this should yield the activation energy, Q.

References:

J.F. Shackelford, Introduction to materials science for engineers, 2nd ed., Chapter 7, section 7f.

L.H. Van Vlack, Elements of material science and engineering,6th ed. ,chapter 14, section 5.

W.F. Smith, Principles of materials science and engineering, chapter 6, section 11 and chapter 7, section 11.

THE EXPERIMENT

The Sm 106 Mk II machine uses a simple lever, A to apply a steady load to the specimen. The specimen is attached at one end to the lever mechanism by a steel pin B, and fixed at the other end to the bearing block with an other steel pin C. Two U-shaped brackets prevent the deformation of the specimen fixing holes. Loads are applied to the lever arm by placing weights on the weight hanger,D. The weight hanger has two pinning positions. The upper position pins the hanger in the rest position and the lower position pins the hanger in the loaded position.

The lever arm has a mechanical advantage of 8. If a mass m kg. is added to the weight hanger, then the tensile load on the specimen is:

F = (2.96 + 8m) x g newtons

Where g is the acceleration due to gravity and 2.96 includes the mass of the hanger and the lever arm.

The specimen elongation is measured by a dial test indicator(DTI). A tube fixed to the bearing block is the housing for the DTI and a nylon pinch screw is used to restrain the DTI under steady load conditions. The top of the DTI is attached to the lever mechanism by means of a grooved plate E, which is bolted to the lever arm. The arrangement is such that the groove in the plate is twice the distance from the pivot than that of the center of the specimen. Therefore the extension given by the DTI is twice the extension of the specimen.

It should be noted when zeroing the DTI the nylon pinch screw should only be tightened finger tight i.e. just sufficient to prevent the DTI from sliding upwards when under steady load.

Over tightening could cause damage to the DTI when the specimen breaks.

Three lead specimens should be pulled to failure with different loads. Ask the TA which loads should be used. All specimens should be labelled and their cross sections should be measured using a micrometer enabling subsequent calculation of stress. ( Be careful not to damage the specimen with the micrometer). Also the gage length should be measured. This is the length of the straight middle portion of the specimen.

Now go through the following steps:

1. Gently raise the lever arm and pin it in the rest position.

2. Remove the thumb nut retaining the grooved plate E on the lever arm and slacken the nylon screw retaining the DTI in the tube.

3. Using both hands, gently lift the DTI and the grooved plate of the apparatus. Separate the plate from the DTI and store both in a safe place.

4. Remove the specimen retaining pins B and C.

5. Measure the thickness, width and the length of the middle portion of the specimen.

6. Fit the top of the specimen in to the lever arm and replace the specimen retaining pin B. Use the U-brackets for the polymers.

7. Fit the bottom of the specimen the bearing block and replace the pin C(It may be necessary to remove the rest pin to allow some movement of the lever arm; if this is done replace the rest pin when the specimen has been fitted).

8. Refit DTI and grooved plate but do not tighten the nylon screw.

9. Remove the rest pin and gently lower the lever arm to take up any free movement. Zero the DTI and turn the nylon screw FINGER TIGHT.

10. Refit the rest pin.

11. Record the ambient temperature and reset the stopwatch to zero, ready to start the test.

12. Load the weight hanger with required load, remove the rest pin and gently lower the lever arm to take up any slack.

13. Raise the hanger to the load position and refit the pin. Gently release the load and start the stop watch.

14. Record extension readings from DTI every 15 seconds for the primary stage of creep. When the extension rate slows down record readings every minute. One student sees the stopwatch and calls the time when DTI should be read, second takes the readings and the third one records them.

As the test approaches the tertiary stage record readings again every 15 seconds until fracture occurs or the weight hanger bottoms down.

The following graphs should be made:

a) Extension vs. time.

b) This graph has only three points for the three specimens.

Curves in graph (a) give the for the secondary stage of the creep process in mm/min. These values of are to be used for graphs (b). Since we take the logarithm this does not have to be reduced to the actual strain. According to equation (2) the slope of the line in graph (b) {The best straight line through the 3 points} will provide n. This is only correct if the temperature does not vary during the experiment.