Mechanical Properties of Metal - Measuring fracture toughness

The aim of this demonstration is to use measurements of the load required to propagate a crack (tear) in packaging tape to determine its fracture toughness.

We will also observe what happens when a load is applied to a material with a sub-critical sized crack.

Governing equations:

Cracks propagate when the stress intensity factor exceeds a critical value - the critical value is the fracture toughness K_1c

K_1c = Yσ∗√(Πc)

where σ* is the tensile stress at which the crack propagates and Y a geometrical factor.

1. Use scissors to put a crack (cut) in a strip of the packaging tape (perpendicular to the direction of loading). Carefully apply weights to the bottom of the tape strip until the crack fast-propagates through the rest of its width. Repeat this at least 7 times with different sized cracks (cuts). Generate a table of crack length measurements and fracture load.

 - Share your data with the rest of the class (each group has a different type of packaging tape).

2. Comment on the fracture behaviour between the 3 tapes, and give an explanation why they are different or the same

3. Convert load to stress and plot the data Stress at failure vs Crack size and comment on the trend. Then taking into account the governing equation above, calculate the fracture toughness for each crack size. What is the average fracture toughness (± Standard deviation) of the tape? Comment and explain the differences between the 3 various types of tape.

4. Plot the stress against 1/Y√nc and use the gradient of the fitted plot to derive the fracture toughness. Is it significantly different to the average value calculated above? Why/why not?

5. For one crack sizes (within the range of those in the table from question 1), place a vertical slit in the tape just in front of the crack tip and repeat the loading experiment.

Does the slit increase or decrease the stress required for fracture. Propose a reason why it increased/decreased.

6. The concept of yield-before-break is an important one used in many applications. In this concept, the critical crack size cannot be reached since the material will yield before a high enough stress can occur to propagate the crack.

Determine the material index for condition (this can be done by setting the critical stress required to propagate the crack equal to σy).

Use the CES software to select the best yield-before-break material for a pressure vessel that has an outer diameter of 0.5 m, must contain a pressure of 4 MPa and cannot have a thickness greater than 5 mm.