WJEC Physics for AS: Study and Rev Guide

Plastic = when the stress is removed the material is permanently deformed. Ductile = a material which can be drawn into a wire. Elastic limit = the stress at which the deformation ceases to be elastic. Yield point = the point on a stress–strain graph at which a large increase in strain occurs for little or no increase in stress. Yield stress = the stress at the yield point. Key Terms Grade boost If asked to draw a σ σ – ε graph for a ductile metal, draw one for steel – it is easier to label! Grade boost The stress–strain graph continues to rise in the plastic region because the edge dislocations become entangled. This process is called work hardening. Further dislocations are produced but only at increased stress. Pointer The breaking stress is also called the ultimate tensile strength (UTS). Edge dislocation = an additional part-plane of ions in a crystal. Key Term 1.5.5 The stress–strain properties of ductile metals (a) The stress–strain relationships Copper and steel are ductile materials. Features of their stress–strain curves are illustrated in Fig. 1.5.13 Fig. 1.5.13 σ σ−ε ε curves for copper and steel The materials both have a linear elastic region for low stresses. The Young modulus is constant here. At higher values of strain the graph curves to the right and material becomes plastic . The features on the graph for steel are more clearly defined: P – limit of proportionality E – the elastic limit Y – the yield point and s Y – the yield stress s X – the breaking stress X – the breaking point For low stresses, an increase in stress results in an increase in the separation of the lattice ions in the direction of the stress. When the stress is removed, the ions are pulled back by the metallic bonds. Thus the deformation is elastic . The explanation of plastic deformation is given in the next section. (b) Edge dislocations Because the crystals grow randomly they are not perfect and very often an edge dislocation is produced [millions per crystal]. This is an extra part plane of ions. In Fig. 1.5.14 the dislocation is at X. If large enough forces are applied (so the stress exceeds the yield stress) as shown the dislocation will move irreversibly to the right i.e. the crystal will suffer permanent deformation. There are many animations of this – type edge dislocation into a search engine and choose a suitable video or sequence of images. The movement of edge dislocations can cause large deformations in the following way: large stresses can cause a crystal plane to snap at a point of weakness (e.g. a missing ion) producing two edge dislocations, which migrate strain, ε stress, σ steel O copper strain stress, σ Y P E X σ X O 56 AS Physics: Study and Revision Guide