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[求助]
老师布置的题目，求高手解答啊啊啊
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Modified True/False: Full credit is given for identifying a completely true answer, half for identifying a false, and half for fixing the false statement to be true. Please “Track Changes” for the modifications (5 points each) 1. Viscoelasticity requires strain rates roughly equivalent to the frequencies involved in atomic vibrations. 2. Low stacking fault energy leads to wavy slip in FCC materials. 3. Crack bridging is an effective toughening mechanism in pure, cubic ceramics 4. Notched specimens with a given loading amplitude are an example of stress-controlled fatigue. 5. Steel samples often have infinite fatigue lives in strain-controlled fatigue. Multiple/Multiple Choice: Full credit is given for identifying all of the correct answers, and half for a subset, however zero credit results from the choice of a single incorrect selection. Please highlight the correct answers in yellow. (5 points each) 1. Fatigue failures: a. in the case of metals, are the result of crystallization after a critical number of cycles. b. often occur on the interiors of parts. c. cannot occur in perfectly polished materials due to no crack initiation sites available. d. are completely avoidable using S-N curves 2. Fatigue crack prevention in ductile metals is: a. possible by shot peening b. aided by alloying with atomic hydrogen c. unrelated to intrusions and extrusions from persistent slip bands d. exemplary in the case of flame hardening of aluminums. 3. Fatigue loading conditions for a material are completely defined by: a. R-value b. stress amplitude c. type of stress d. frequency e. very smart marsupials. 4. The spring and dashpot models of viscoelastic/viscoplastic behavior: a. have no analogous connection to actual material structures. b. represent the primary and secondary bonding occurring within polymers. c. cannot account for dampening within a material during fatigue d. are applicable only to polymers. 5. The plastic zone in front of a crack tip: a. achieves a maximum residual stress consistent with the ultimate strength of the material. b. is larger than the grain size of the material when in the Paris regime. c. does not change the effective stress intensity value. d. is a way of calculating the crack closure occurring by residual stresses during unloading. Problems: (75 points each) Describe the principle and derivation of the strain energy release rate. Further, describe/design and experiment by which one could determine the plane stress and/or plane strain fracture toughness of a semi-brittle material using the principles involved. After 10,000 cycles, a crack has extended from 2 cm to 4 cm. The panel has the typical Griffith geometry. What is the amplitude of the applied stresses (assuming constant frequency and amplitude of course)? At the crack length of 2cm, what is the striation spacing? E= 210 GPa Where da/dN and ΔK have units of m/cycle and Pa√m, respectively. |
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