WJEC Physics for AS: Study and Rev Guide

QUI QUI a If a piece of copper is hammered, the sudden large (compressive) stresses cause the edge dislocations to move to the grain boundaries. If it is heated to red heat, the copper re-crystallises. What effect do these activities have on the tensile properties? Pointer The ‘stress’ on the σ – ε curve is the engineering stress , which is defined as the tension divided by the original csa. The true stress is the tension divided by the actual csa, which decreases at the neck. A graph of true stress (at the minimum csa) against strain continues to rise until the breaking point. Ductile fracture = breaking that occurs when a ductile material is stressed to breaking point. It involves plastic deformation and necking. Key Term Fig. 1.5.14 Plastic deformation owing to the motion of an edge dislocation in opposite directions. This makes the crystals elongate in the direction of the stress. Because there are large numbers of edge dislocations, quite large strains (20–30%) can be produced. The edge dislocation cannot move beyond the grain boundary: the larger the grains, the greater the plastic strains. Foreign (impurity) atoms, grain boundaries and other dislocations impede the movement of edge dislocations and so stiffen and strengthen the material. (c) Ductile fracture Fig. 1.5.15 represents ductile fracture with its typical necking and ‘cup and cone’ fracture. As the stress reaches s X , more and more edge dislocations are generated and migrate, causing the elongation. Because there is no increase in volume the cross-sectional area decreases, increasing the true stress at the neck, resulting in more dislocations in a runaway process. Flow marks are often seen in the necking region. Fig. 1.5.15 Ductile fracture 1.5.6 The stress–strain properties of brittle materials (a) The stress–strain relationship Brittle materials are elastic; Hooke’s law is obeyed for all stresses up to the breaking stress. This is illustrated in Fig. 1.5.16. Note that ‘elastic’ does not mean highly extensible. The strain at the breaking point is of the order of 0.001 (i.e. 0.1%) for most brittle materials. Example A rod of length 60 cm is made of glass with a Young modulus 70 GPa and a breaking stress of 100 MPa. Calculate the increase in length up to its breaking point. F F F F stress, σ strain, ε σ X X O Fig. 1.5.16σ σ – ε εgraph for a brittle material 57 1.5 Solids under stress