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Lab 3: Tensile Test of Brittle and Ductile Metals (Clockhouse Machine)

Objective

The aim of tensile test experiment of brittle and ductile metals is to discuses the basic concept of stress and strain. Experimental methods will be used to show the relationship between stress and strain and determine the mechanical properties for specific materials which include:

1. Proportional Limit

2. Modulus of Elasticity

3. Ductility

4. Percent Elongation

5. Ultimate Strength

6. Yield Points (Upper and Lower)

Introduction

The strength of a material depends on its ability to sustain a load without undue deformation or failure. This property is inherent in the materials itself and must be determined by experiment. One of the most important tests to perform in this regard is the tension test. Although many important mechanical properties of a material can be determined from this test, it is used primarily to determine the relationship between the average normal stress and average normal strain in many engineering materials such as metals, ceramics, polymers, and composites.

To perform the tension test a specimen of the material is made into a standard shape and size. Then, measurements are taken of the specimen’s initial dimensions including cross section area Ao, the length Lo, and the thickness. A uniaxial tensile load is applied slowly to stretch the specimen at a very slow, constant rate until it reaches the breaking point. The machine is designed to read the load required to maintain this uniform stretching and display the final load at failure point. For low loads the elongation and slight lateral contraction take place as show in Figure 3.1.

Sample

Figure 3.1

As the load continues to increase on specimen, a brittle material tends to fail suddenly with very little plastic deformation. Whereas a ductile material undergoes a substantial reduction in cross section area, know as necking, before reaching a breaking point. The two mode of failure is show is Figure 3.2

Brittle Specimen Ductile Specimen

Figure 3.2

Ductile Materials

A ductile material is any material that can be subjected to large strains before it rapture. The ductile materials are often chosen for deign because these materials are capable of absorbing shock or energy, and if they become overloaded, they will usually exhibit large deformation before failing. The percent elongation (PE) or percent reduction in area at the time of rapture is the specimen’s fracture strain expressed as a percent as follows:

Lf  Lo

PE  (100) (3.1)

where Lf is the length at fracture, and Lo is the original length of the specimen. The percent reduction in area ( PRA ) is defined within the region of necking as follows:

Ao  Af

PRA  (100) (3.2)

where Ao is the specimen’s original cross section area and Af is the area at fracture. A stressstrain curve typical ductile material along with the 0.2% offset line is shown in Figure 3.3.

Ultimate Strength

1. Ultimate Strength

2. Yield strength

3. Proportional Limit Stress

4. Rupture

5. Offset Strain (typically 0.002).

Figure 3.3 Typical stress strain curve of a ductile material

Brittle Materials

A brittle material exhibits little or no yielding before failure. Brittle materials do not have well defined tensile fracture stress, since the appearance of initial cracks in a specimen is quite random and lead to complete sudden fracture. In a tension test, brittle material fracture when normal stress reaches the ultimate stress  ult .

The Stress-Strain Relations

The normal or engineering stress () can be determined by dividing the applied load P by the specimen’s original cross section area Ao as follows

 (3.3)

The nominal or engineering strain () is found directly from the strain gauge reading, or by dividing the change in the specimen’s gauge length, , by the specimen’s original gauge length Lo as follows

 LLo (3.4)  

Lo Lo

The Modulus of Elasticity (Young’s Modulus), E, is a measure of the stiffness of the materials. It is numerical equal to the slope of the stress-strain curve in the elastic range (linear), as represented by Hooke’s Law

E (3.5) Brittle materials such as concrete and carbon fiber do not have a yield point, and do not strainharden which means that the ultimate strength and breaking strength are the same. A stressstrain curve for a typical brittle material is shown in the Figure 3.4.

Ultimate Strength

Rupture

Figure 3.4 Typical stress strain curve of a brittle material

Some typical tensile strengths of some materials are listed in Tables 3.1 and 3.2:

Table 3.1

Table 3.2

Procedure for the Clockhouse Machine:

BEFORE YOU BEGIN THE EXERIMENT:

· Measure all specimens with micrometer or calipers before testing

· Turn panel switch on, on the back of the digital control unit. Allow 15 minutes for warmup.

· All letters in parenthesis ( ) are referring to controls shown in Figure 3.5 and Figure 3.6.

Figure 3.5 Control Panel of Clockhouse Machine

Top Jaw

Bottom Jaw

Rapid Up

Rapid Down

Top Jaw

Handle

)

(

Lower Jaw

Handle

(

)

(

)

(

)

(

)

(

)

(

)

Lower part of the machine

Figure 3.6 Clockhouse Tension Machine

Look at the Clockhouse machine shown in Figure 3.6. Make sure that both the top and the bottom jaws are closed and measure the distance from the bottom of jaw to the top of the bottom jaw. The distance should be 6”. If this is correct, go to step #3. If it is incorrect, reach down to the knob on the lower right (I) (all letters in parenthesis are referring to controlling parts and buttons are shown on Figure 3.5 and Figure 3.6) and make sure it is pull out. Turn the large black wheel (H) counterclockwise to lower it and clockwise to raise it to the desired setting of 6”. Now push the lever on the bottom jaw (J) down, and latch it open using your thumb on the small catch.

Push down on the lever (K) on the upper T-grip and place the test specimen in the upper grip making sure that the material is placed evenly with the top jaw and all the way back into the jaw, then release the lever (K). Leave the bottom grip open.

Now, on the control panel. Locate the range buttons shown in Figure 3.5 as (A) on the left front of the control panel. Push button “D” (forth button on the control panel). Now zero the reading, using the fine adjustment (B). Now, push button “A” (20K). Zero the peak load reading by pushing the top right button F2.

Go to Clockhouse machine. Now reach down to the knob (I), (lower right front of the machine) and pull out while moving the pushed in large black wheel (H) back and forth until it locks.

Now release the bottom grip (J) on the test material. NOTE: If you have a reading on the screen now, don’t worry about it. Do not try to read just the zero.

Placement of a dial gauge to measure specimen elongation:

1- To measure the specimen elongation place a dial gauge under the lower jaw of the tension test machine as shown in Figures 3.7 and 3.8.

2- Read the dial gauge and the control panel output at the same time and at equal intervals. For example, read at each one complete dial turn. The dial reading represents the elongation of the testing specimen and the control panel reading is the load applied to cause this elongation.

3- Record specimen elongations obtained and correspondent applied load in the table provided in the experiment data sheet.

Figure 3.7

Figure 3.8

Notes:

Make sure to mount

the gauge exactly

under the lower jaw

after placing the

specimen and

ithout exerting any

pressure on it.

Make sure that the

gauge is zero before

starting the test.

7. Before you begin the test, turn the feed control (D) on the control column to the desired feed rate. The dial is calibrated in MM/MIN. One full turn equals 1 mm travel per minute. For most material, .5 – 1 MM/MIN is sufficient.

8. Note the lower set of three buttons (Figure 3.6) on the control column that say stop (E), up (F) and down (G).

NOTE: All readings in the tension mode are negative.

9. Watch the reading on the control panel. When the peak reading has been reached the yield point of the material has also been reached, even though the specimen has not broken. In a short time, the reading will begin to reverse before breaking the material.

NOTE: The peak load reading will remain until cleared by pushing button (C)

10. Stop the machine by pushing button (F) on the control column. Graphs: Plot stressstrain diagrams for all specimens up to failure using one graph for each type of metal. Show all quantities that can be determined from the diagrams on the same graph.

POST-LAB QUESTIONS:

1- Calculate: PE, PRA, stress, strain, and modulus of elasticity for all specimens.

2- Determine the type of metal used for the test based on the comparison between results obtained experimentally and values given in METALS HANDBOOK or your texts on strength of materials/or use tables listed above.

3- What is the ultimate strength and how can be determined?

4- Determine the type of materials being tested – Is it brittle or ductile

Lab 3: Tensile Test of Brittle and Ductile Metals

Experiment Data Sheet

Name: --------------------- Date: ------------------------ Instructor Initial: ---------------------

1- Measure the specimens with micrometer or calipers before testing. Record measurement on the drawing below:

Lo =-3.77

Thickness= ------

Before Testing

Lf = 5.8

Thickness= ------

--------

After

Testing

Lab 3: Tensile Test of Brittle and Ductile Metals

Experiment Data Sheet

1- Tabulate your experimental results with the anticipated values.

Sample No.	Rupture Loading
1	3410

2- Tabulate your experimental results with the anticipated values.

Loads (kips)	Elongation x 10-3 (in.)
2840	0.1
3710	0.2
3860	0.3
3940	0.4
3940	0.5
3410	0.6