Question

Multiple Choice Section [Total = 10 Marks] Please circle the correct answer using a pen. There is only one correct answer per question. 1. (2 Marks) Which of the following statements is false: a) Two edge dislocations of the same sign and lying on the same slip plane exert a repulsive force on each other. b) Edge dislocations of opposite sign and lying on the same slip plane exert an attractive force on each other. c) When two edge dislocations of the same sign meet, they annihilate each other and leave a region of perfect crystal. d) When two edge dislocations of the opposite sign meet, they annihilate each other and leave a region of perfect crystal. 2. (2 Marks) Cold working of a material... a) Is conducted at room temperature. b) Is conducted inside a refrigerator. c) Is conducted below the recrystallisation temperature. d) Is conducted above the recrystallisation temperature. 3. (2 Marks) Which of the following statements is false: a) During recovery, the dislocation density is reduced, the mechanism of which proceeds via diffusion and/or dislocation motion. b) Heating (annealing) reduces dislocation density and increases the grain size. c) The driving force for grain growth is the reduction in grain boundary area. d) During recrystallisation, relatively large grains are formed that have a small dislocation density. 4. (2 Marks) The three modes of crack surface displacement are: a) Mode I, opening or tensile mode; Mode II, sliding mode; and Mode III, tearing mode. b) Mode I, tensile mode applied to an interior crack; Mode II, tensile mode applied to an edge crack; and Mode III, tensile mode applied to a surface crack. c) Mode I, twisting mode; Mode II, torsion mode; and Mode III, tension mode (also referred to as TTT - "The Triple T'). d) Mode I, sliding mode; Mode II, opening or tensile mode; and Mode III, closing or compressive mode. 5. (2 Marks) Which of the following statements is true: a) Sharp corners improve the fracture resistance of a particular design. b) Cracks will propagate more rapidly if the crack tip is blunt. c) The breaking of a ductile material under tensile testing follows the following path: necking > void coalescence → void nucleation → crack propagation → separation. d) Brittle materials tend to show a simple cleavage surface after failure.

Answer 1

Ans. (d) Basically dislocations are surrounded by Shrain fiel de like tensile cind compressive fields Case (i) suppose that two edge dis bcations of some sign and identical slip planet. In this case the *have compressive and tensile strain fields for both lie. on some side of slip plome so because of these shain fields they repluse each other Compressive 2 Compressive Replusico (a) , A: lecahn • R A,... (G) Tensile Tensile. casecilij If two dislocation of opposite sijn having some slip plane :- They will attractedeachrother and dislocation annihilation will occur when they meet compressive Tensile o It T = T perfect Tensile crystal compressive So option (d) is espect correct. printing on hilotion ie 70) Ans@ o sold cold working is done below the recrystallization temperature. The main aim of cold working is to decrease the mobility of dislocations and strengthens the material. If this will of done at temperature above recrystallization temperature the grain will start to recrystallize and the strength and hardness of material! chill be decreased.

Anoca (d) During recrystallization the formation of neus ad of Corin free and equiaxed grains are formed that have lour our location density i The new graine ferrys as young math muder and grow untill they completely consume the Parent maloval Aru E The three odes of crack surface displacement are 9 mode I opening or teneile mode (6 mode ili sliding mode (6 mode 1 ; Tearing mode.

Angol 2 Necking Cavity formation Carity caalescence to from a frack Crack propagation Final shear failure at a angle uso relative to nola icture Here corrept option is de Brittle material tend to show a simple cleavage surface after failure
I am going to attach some picture that would be helpfull to understand question 4 and 5
(a) The three modes of crack surface displacement: (a). Mode I: tensile mode; (b). Mode II: Sliding mode; (c). Mode III: Tearing mode.

(b) (©) failure for brittle material