Fracture Toughness/Impact Strength Lab.
Aim
To perform impact test on materials
Objectives
• To determine the basic impact strength of materials.
• To learn how the Charpy method works to measure impact strength of polymer materials.
• To calculate fracture toughness of materials.
• To compare the impact strength of two materials.
Background
In any engineering design, the material/part will be subjected to either of the three types of loading, i.e., static load, repeating load and shock/impact load. When a book is placed on a table, the weight of the book will be considered as the static load on table. Dynamic loading is one of the examples for repeated load and is related to mechanical test called fatigue test in which the mechanical forces change in both magnitude and direction. In impact loading, force is applied suddenly to a particular part/region due to which the part might fail if it doesn’t have enough strength i.e., impact strength.
Impact tests are performed on metals to find the information related to mode of failure under sudden loads that might lead to a fracture in samples with a notch (as shown in above figure). The area under the stress-strain curve is related to the energy absorbed at fracture which can be termed as toughness of materials (according to some literature studies). For example, brittle materials have less area under stress-strain curves due to their reduced toughness values and hence only a small amount of energy is absorbed during impact test. Whereas for metals, the capability of plastic deformation (ductility) increases, so the area under the stress-strain curve also increases and the absorbed energy and hence toughness will increase. The fracture surfaces of the high energy fractures (ductile) have shear regions and the fracture surface will be inclined to tensile stress at about 450, will appear as highly deformed and rough. Whereas, for the low energy failure, the fracture surfaces will be relatively smooth, showing brittle nature.
In impact testing, two types of tests can be used, Izod and Charpy. The V-notch Charpy mehotd is the most commonly used testing method for impact. The major difference between the two tests is the way the material is placed i.e., the fixtures. During testing, the load will be applied as a blow using a weighed pendulum hammer which will be released from a particular height (h,. Energy absorbed at fracture (E, will be noted by calculating the potential energy difference before and after the test is performed. Impact energy can be directly noted down from the machine, from this the impact strength (KCU).can be directly calculated using the following equation.
E- Impact energy; A- fracture cross-section area
Equipment
1. HIT50P pendulum impact tester with Charpy fixture.
2. Vernier caliper
Sequence of Operations
1. Set the distance between the test piece supports to appropriate dimension, so that the midpoint between the supports coincides with the midline of the pendulum.
2. Set the pointer to energy of the pendulum hammer.
3. Make a blow with the pendulum, without the test piece in position, to check the zero-position accuracy, adjust the pointer if necessary.
4. Place the test piece on the support so that the midline of the notch coincides with the midline of the pendulum.
5. Raise the pendulum to its starting position.
6. Set the pointer to energy of the pendulum hammer.
7. Release the pendulum to break the test piece.
8. Now read the impact energy directly on the scale.
9. Calculate the KCU impact strength by dividing the impact energy/cross section area.
With reference to the results sheet on the next page, record the impact energy at which the materials fail and observe the type of failure in materials. Using the results obtained from the data, calculate the fracture toughness of the materials observing the mode of fracture.
Data Analysis
You will find it easiest to process these results using Excel or similar.
1. Calculate the Impact energy, Impact strength and fracture toughness quantities for both samples: Polyethylene and Nylon.
2. Plot the type of failure observed (this can be a hand drawing) showing which type of failure it is (ductile, brittle or mixed mode) and clearly show the calculation of fracture toughness using the crack length and mode of failure (for this you will need to do a literature search on fracture mechanics to understand type failure and fracture toughness formulae)
DATA SHEET
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Material
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Area of the fracture (mm2)
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Impact energy (J)
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KCU (J/mm2)
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Report
This is an individual report. Page limit: Main lab report maximum of 4 pages (appendices maximum of 2 pages)
In your report,
1. In the THEORY section, define quantities such as Impact energy, Impact strength, toughness, fracture toughness.
2. In LITERATURE REVIEW section, include relevant information about the types of failure that can be observed in the materials explored in the current lab, fracture toughness modes referring to either scientific publications or textbooks.
3. In the RESULTS section, present all the required results including the calculations using the recorded data, include detailed explanation/calculation of fracture toughness together with the references for formulae used.
4. In the DISCUSSION section comment on the different modes of failure that might occur under impact loading and reasons behind each failure. Compare the impact strength for both materials at the same working conditions and comment on why the impact strength of these two materials are different even though they are both are from the same family of polymers. Discuss which material has the highest strength and why? And what are the factors that might affect the impact strength of materials?
5. Under CONCLUSIONS state concisely and in general terms what useful information you have obtained and to what extent the experiment was successful in terms of meeting the initial aims.