Introduction
Toughness is the ability of a material to absorb energy from impact or applied stresses and deform plastically without fracture. The tougher a material is, the more it can deform plastically, thereby dissipating energy before failure.
Ceramics will typically fail by brittle fracture. The fracture toughness of ceramics, and therefore the failure stress, is increased by mechanisms that slow crack growth by requiring more energy for crack propagation.
In the video below, samples of various commercial chocolate bars are impact tested. Without the samples in place, the pendulum seen in these experiments is free to swing after being released from a predetermined height, and no energy is lost. With the chocolate bars in place at the bottom of the pendulum swing, energy is absorbed, depending on the sample of particular chocolate bar and its temperature: room temperature (20 °C) or liquid nitrogen temperature (−196 °C) in these experiments.
The percentage of energy lost is determined by the height reached by the pendulum after it has interacted with the chocolate bar. The greater the energy lost by the pendulum, the tougher the chocolate bar.
Five commercial chocolate bars were tested in the video below: Sainsbury's 72% Cocoa, Sainsbury's 55% Cocoa, Cadbury's Dairy Milk, Cadbury's Fruit and Nut, and Snickers. A standard sample size was taken to be one row of sharing size chocolate bar.
Impact testing of chocolate
These results demonstrate the range of toughness exhibited by these chocolate products as a function of temperature. They agree with everyday experience: at 20 °C, chocolate bars high in cocoa are much easier to snap in two than a bar of Snickers, and the higher the cocoa content, the easier chocolate bars are to snap. Putting chocolate into the freezer compartment of a refrigerator makes it far more easy to snap cleanly in two than leaving it outside to warm up in the sun. Indeed, if chocolate becomes too warm in the sun, it is almost impossible to snap in two!
We can conclude that for chocolate, toughness depends on composition/structure of the chocolate product and also that some toughening processes are thermally activated.
For further information see Toughness of chocolate food products.