SE104 Structural Materials Sample Final Exam. For each multiple choice problem (1.5 points), choose 1 and only 1 most relevant answer.

Transcription

1 1 lb = 4.45 N; 1 inch = 25.4 mm = 1/12 ft 1 nm = 10 Å = 10-3 µm = 10-9 m SE104 Structural Materials Sample Final Exam For each multiple choice problem (1.5 points), choose 1 and only 1 most relevant answer. 1. Which one of the following does NOT describe the elastic properties of material? (a) Young s modulus (b) Stiffness (c) Endurance limit (d) Shear modulus (e) Poisson s ratio 2. The fracture toughness of a material can be characterized by its (a) Poisson s ratio (b) Bulk modulus (c) Critical stress intensity factor (d) Ultimate tensile strength (e) Hardness 3. A material has a high critical energy release rate; that is (a) Its Young s modulus is large (b) It can rapidly react to an external loading (c) Its yield strength is high (d) It is tough (e) Its service life under cyclic loading is long 4. As shown in the figure on the right, a 0.1-m-large cubic block is subjected to a horizontal shear force (x) along its top surface. Assume that there is no relative motion between the block and the floor. The shear modulus of the material is 0.7 GPa. After deformation, the upper-left corner ( A ) moves by 5 mm. The shear force, x, must be kn. (a) 350 (b) 175 (c) 35 (d) 23 (e) 12 5 mm A x 0.1 m 5. The shear modulus and the Young s modulus of a material are 90 GPa and 180 GPa, respectively. Its Poisson s ratio is (a) 0.5 (b) 0.4

2 (c) 0.25 (d) 0.15 (e) 0 6. For an alloy, usually the yield strength is directly determined by (a) Atomic bond strength (b) Dimple formation (c) Precracks (d) Dislocations (e) Creep resistance 7. A stainless steel is formed by a large number of small grains, and its properties are the same along all the directions; that is, the steel is polycrystalline and isotropic. Its yield strength is 300 MPa. The critical resolved shear stress is around (a) 930 MPa (b) 97 MPa (c) 900 MPa (d) 100 MPa (e) 310 MPa 8. To increase the yield strength of a metallic material, we may: (a) Increase the grain size (b) Reduce the density of dislocations (c) Increase the amount of precipitates (d) Increase temperature (e) Reduce the precrack size 9. According to the materials handbook, the fracture toughness of a material is 50 MPa m 1/2. The material is subjected to a 100-MPa tensile stress. Assume that the geometric factor Y = 1. The maximum allowed precrack length is approximately a) 2 mm b) 0.5 mm c) 8 mm d) 80 mm e) 0.8 mm 10. Microvoiding is involved in fracture a) cleavage b) ductile c) both cleavage and ductile d) dynamic e) brittle 11. For the fractography shown on the right, which statement is NOT true : a) This is a cleavage fracture surface 10 µm

3 b) Each facet is a smooth crystallographic plane in a grain c) The material must be ductile. d) The fracture process is not heavily dependent on microvoids formation e) The fracture is triggered by local stress concentration 12. According to the materials handbook, the fracture toughness, the stiffness, and the yield strength of a material are 50 MPa m 1/2, 200 GPa, and 500 MPa, respectively. What is the fracture resistance (Gc)? (a) 12.5 kj/m 2 (b) 25 N/m (c) 12.5 N/m (d) 250 J/m 2 (e) 125 kj/m A steel was quite ductile in lab testing, but it was reported that the same steel abruptly cracked in a beam of a bridge; i.e. it suddenly broken apart without much plastic deformation. What can be a possible reason? (a) The working temperature is too high (b) The dislocation density is too low (c) The precrack size is too large (d) The endurance limit is too high (e) The stiffness is too high 14. describes the growth rate of long fatigue crack as a function of the change in stress intensity factor a) S-N curve b) Paris law c) Miner s rule d) Miller parameter e) Critical stress intensity factor 15. According to the testing data, a metallic material is quite tough. The most possible reason is a) It does not contain precracks b) It forms dimples (microvoids) when fracture occurs c) It cleaves when fracture occurs d) The atomic bonds are ductile e) The grain size is large 16. Brittle fracture is directly triggered by a) Rupture of the atomic bonds exactly at the precrack tip b) Microvoids in the crack-tip zone c) Grain boundary d) Dislocation motion e) None of the above

4 17. The S-N curve of a material is shown on the right. The material is subjected to a cyclic loading, with the minimum stress of 50 MPa and the maximum stress of 250 MPa. What is the expected fatigue life? a) 10 7 cycles b) 2000 cycles c) much more than 10 7 cycles d) 10 cycles e) 80 cycles 18. Consider the material in the above problem. At first a fatigue loading with the stress amplitude S1 = 200 MPa is applied for 20 cycles, and then the stress amplitude is changed to S2 = 100 MPa and the loading is applied for another 2.5 million cycles. Finally, the stress amplitude is changed to S3. If the material must continue to work for 100 more cycles, what is the maximum allowed value of S3? a) 200 MPa b) 170 MPa c) 100 MPa d) 80 MPa e) 250 MPa 19. A steel plate contains a 10-mm-long edge crack. A cyclic tensile loading is applied. The maximum stress is 40 MPa and the minimum stress is 10 MPa. The Paris law parameters are A = and m = 2.67, when the unit of stress intensity factor is set as MPa m 1/2 and the unit of length is set to m. Assume that the geometric factor Y = 2. The growth rate of the crack is µm per cycle a) 1.3 b) 2.6 c) 5.2 d) 32.4 e) In each fatigue loading cycle, although the overall material does not fail, incremental damage is accumulated at the crack-tip zone, leading to a small crack length increase. This is often associated with a) Plastic deformation in the crack-tip zone b) Dislocation motion in the crack-tip zone c) Stress concentration at the crack tip d) Cyclic crack-tip opening e) All of the above

5 21. According to the Larson-Miller chart on the right, for DS CM 247 LC alloy (longitudinal, 1232 o C/2h o C/20h/AC), the value of Larson-Miller parameter under 207 MPa is (in the imperial unit system) a) 49 b) 50.1 c) 50,100 d) 49,800 e) In the above problem, at 1540 o F, the material has a creep life of hrs under 207 MPa a) b) 79,000 c) 7,900 d) 790 e) As a rule of thumb, when temperature is higher than, we should take into consideration the creep behavior a) One half of the melting point ( o K) b) The evaporation point ( o F) c) Room temperature d) 1200 o C e) 350 o C 24. Creep is often caused by a) Dislocation motion along slip systems b) Microvoiding at precrack tip c) Diffusion of atoms and/or molecules d) Separation of atomic bonds along crystallographic planes e) Repeated opening and closure of crack tips 25. Usually, which one of the following is the most important stage in creep process? a) Primary creep b) Secondary creep c) Tertiary creep d) High-temperature creep e) Room-temperature creep 26. The phase diagram of steel is shown below. In general, the carbon content of steel is a) Around 3-4% b) Below 2%

6 c) From 0 to 6.7% d) Above 1% e) From 2-6% 27. If the carbon content in a steel is 0.9%, as it is slowly heated up from room temperature, at what temperature does it fully melt? a) 727 o C b) 800 o C c) 1394 o C d) 1495 o C e) 1370 o C 28. Carburization treatment of steel involves heating steel with a carbon source (e.g. fine carbon powders). It increases the carbon content in the steel. Consider a carburization process of a low-carbon steel at constant temperature 1200 o C. The initial carbon content is 0.1%. If we observed that during the carburization treatment, the steel partly melted, what is likely the reason? a) The carbon content has increased to more than 1.8%. b) The carbon content has increased to more than 4.1%. c) The carbon content has increased to more than 6.7%. d) Carbide has formed. e) Gamma phase transformed to alpha phase. 29. We slowly decrease the temperature of a steel (the carbon content is 0.5%). When temperature changes from slightly above 727 o C to slightly below 727 o C, a) gamma phase is quenched b) gamma phase entirely transforms to alpha phase c) carbide is formed d) alpha phase melts e) alpha phase partly transforms to gamma phase 30. Which one of the following does NOT characterize the creep resistance of a material? a) Endurance limit b) Creep strength c) Steady-state creep rate d) Time to rupture e) Larsen-Miller parameter 31. Aggregates are used in concrete mainly to a) Increase the early strength b) Increase the long-term strength c) Occupy space

7 d) Reduce weight e) Increase the workability 32. To build a bulky and massive concrete structure, such as a dam, usually people use type portland cement a) I b) II c) III d) IV e) V 33. We want to fast repair potholes in a concrete pavement. To avoid extensive interruption to traffic, we need to minimize the curing (hydration) time. We should use type cement. a) I b) II c) III d) IV e) V 34. In the above problem, we may also use to speed up the hydration process. a) fly ash b) chemical admixture c) water reducer d) natural pozzolan e) dicalcium silicate 35. For a structure working under seawater, to mitigate sulfate attack, we should consider portland cement. a) High-quality b) High-strength c) Type III d) Type IV e) Type V 36. There are altogether five stages in the hydration process of portland cement. Active hydration reactions of calcium silicates begin in stage a) I b) II c) III d) IV e) V 37. The water/cement (w/c) ratio of portland cement is usually 44~55%. Which one of the following is NOT true: a) The chemical reactions of hydration procedure need more than 50% water to complete b) Aggregates may affect the true water/cement ratio

8 c) Water/cement ratio affects many cement properties d) A certain amount of water is absorbed at cement and aggregate surfaces e) To optimize the w/c ratio, we need to take into consideration both strength and workability 38. Air entraining agents are used mainly to a) Increase strength b) Improve cement performance in cold-weather areas c) Reduce weight d) Increase cost efficiency e) Enhance air quality 39. In a portland cement, which component contributes most to the early strength? a) Calcium aluminate b) Calcium sulfate c) Calcium aluminoferrite d) Tricalcium silicate e) Dicalcium silicate 40. Using mineral admixtures such as fly ash and natural pozzolan may NOT a) improve the durability of cement b) help reduce the cost c) help reduce greenhouse gas emission d) increase the early strength of cement e) change the hydration process 41. A few woods have been treated under similar conditions. Which wood tends to be the strongest one? a) The most lightweight one b) The cheapest one c) The one having the largest mass density d) The one harvested from a young tree e) The one harvested in winter 42. We have two appropriately treated woods, A and B. Wood A has the Young s modulus of 15 GPa, the tensile strength of 90 MPa, the fracture toughness of 0.3 MPa m 1/2, and the weight density of 0.6 g/ml. The Young s modulus of wood B is 10 GPa. All the measurements are made along the grain (axial) direction. What is likely the density of wood B? a) 1 g/ml b) 0.9 g/ml c) 0.6 g/ml d) 0.5 g/ml e) 0.4 g/ml 43. Which one of the following is NOT a major load-carrying component in wood? a) Wood cells b) Cell walls

9 c) Wood microfibers d) Cellulose e) Lignin 44. When we increase the carbon content of a steel, in general, the steel would become a) stiffer b) stronger c) tougher d) less fatigue resistant e) more lightweight 45. Quenching can increase the strength of steel, because a) it changes the carbon content b) it reduces the dislocation density c) it increases the dislocation density d) it affects the phase structure at room temperature e) it closes precracks 46. Coke cans are made of 1000 series aluminum alloy, because this alloy a) has high stiffness b) has high strength c) has high ductility d) has high fatigue resistance e) has high creep resistance 47. Grain boundary is also phase boundary. This statement is a) True b) False

10 48. A cantilever beam has one end firmly fixed on solid wall, and the other end is free. A bending force is applied on the free end. The cross section must be rectangle. The height of the beam (h) must be 10 mm and the length of the beam (L) must be 1 m. The maximum bending force (Fmax) is 100 N and under this loading no plastic yielding should happen. The deflection of the beam should be as small as possible. For material selection, list (a) the constraints and (b) the objective(s). (c) Derive the performance index. (d) Which one of the following is the best candidate: concrete, wood, aluminum alloy, steel, polyurethane? (9.5 points)

11 49. In a fatigue test on a material containing an edge crack, it was observed that, after 200,000 cycles of fatigue loading, when the crack length grew to 40 mm the sample failed in catastrophic fracture. The maximum stress was 60 MPa, and the minimum stress was 20 MPa. The geometric factor Y = 1. (a) What is the fracture toughness of this material? (b) The Paris law parameters are A = and m = 3.2 in the MPa and meter unit system. What is the initial crack length before the fatigue loading is applied? (10 points)

12 50. The phase diagram of Pb-Sn alloy is given below. Consider a Pb-Sn alloy with the Sn content of 50%. Perform phase diagram analysis for (a) 200 o C, (b) 183 o C + T, and (c) 183 o C T. For each temperature, the phase diagram analysis should answer: (i) what are the phases, (ii) what is the Sn content of each phase, and (iii) what is the weight percentage of each phase. (10 points)

13 Bonus Question. For a coil spring made of metallic wire, denote n = the number of turns, d = the diameter of the wire, R = the radius of the spring, σy = the yield strength of the wire, Cv = the cost per unit volume of the wire material, U = the stored energy when the spring is deformed, and G = the shear modulus of the wire material. The radius of the spring (R) and the length of the spring (measured by n) are specified. The load carrying capacity of the spring, Fmax, is also given. We need to choose material to produce a spring that, without plastic yielding, can store elastic energy as much as possible. (a) List two relevant free variables. (b) Derive the performance index. (c) Which one of the following is the best candidate: aluminum alloy, Ni alloy, polypropylene, polyurethane, tungsten carbide (WC). (8 points)

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15 Figure 7.8