Physics and Chemsitry of Solids (PCS) Fracture Research Group

• physical phenomena (e.g. material strength, aging);
• chemical phenomena (e.g. combustion, explosions and pyrotechnics);
• how things break.

Fracture (how things break) is very important in the design of virtually every object produced by industry. Fracture might be desirable (e.g. opening a can of lemonade), or unwanted (e.g. dropping a cup on the floor). Some objects are moved, struck or even dropped on a daily basis. Some materials regarded as weak perform well under certain conditions. For example, paper can be deceptively strong if loaded correctly.

Over time objects may become damaged as a result of repetitive action such as erosion (e.g. the effect of wind-blown sand on buildings). In many cases it is easy to look at the state of the building or specimen at regular intervals. If a jet aircraft travels through a rain storm the result is thousands of high-speed water-drop impacts on the fuselage. The added friction caused by the damage to the fuselage can reduce fuel efficiency by up to 15%. In the food industry, cleaning can be performed by high-speed liquid jets. On the other hand, sometimes the violence of the impact is so great that the object is broken almost immediately (e.g. a car crash). Examination of the fragments may give some idea of how the object was damaged, but it can be very difficult to find out what damage occurred first and if one initial fracture allowed other fractures to develop.

Can you do tests slowly which show you how things behave when they are hit rapidly? If you pull a piece of plastic slowly it will stretch, become thin and eventually break. If you do the same test more rapidly you find the plastic stretches less, breaks earlier and the surface of the tear is more jagged. This shows that slow tests may not give you the same results as fast tests.

Does the total amount of energy put into a system allow you to predict what will happen? An experiment involving passing an electrical current through a wire will show energy is not an absolute guide. It is the rate at which the energy is delivered that controls many situations.

Chemistry is very important in high-speed events. Gun-powder burns at different rates depending on the pressure around it. The chemical energy released can be used to do work (e.g. push a cannon-ball) but sometimes the output is so great it can break the container and send out high-velocity fragments. Technically, burning is referred to as deflagration. Both gun-powder and fireworks deflagrate, the flames and reaction spread at a velocity of up to 800 ms^-1>. Gun-powder will be compared to a modern propellant. Detonation is a high-speed reaction, which moves through the explosive at a velocity above the speed of sound. In this case a velocity of 7000 ms^-1 would not be unusual. We will show the difference between deflagration and detonation in a bench-top demonstration.

After this talk it should be clearer what properties are important in breaking things and also how fast things can fracture.

Figure 1: This photograph shows a drop landing on the top of a pool of liquid. In the first frame the drop nears the surface. In the second frame the impact has occurred and liquid has been pushed aside producing a Rayleigh crown. In the succeeding frames the liquid rebounds and throws up a drop from the long central column. In the colour image of this sequence you can clearly see that this drop is the same material as the impacting drop - there has been no mixing. Spark photography, exposure time = 5 us.

Figure 2: This shows the growth of a fracture. A plate of glass has had a small crack put in it. The break is pulled from the top and bottom to open it up. Towards the bottom of the first column the crack starts to grow. However, because the energy is put into the system too quickly, a single crack cannot release the energy fast enough, so the single crack transforms into a double crack. Technically, this is called bifurcation. This image converter sequence shows that the crack grows at 1500 ms-1.