《材料科学与工程基础》英文影印版习题及思考(15)

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thermosetting polymers at room temperature? Justify each choice.

(a) Epoxy having a network structure.

(b) Lightly crosslinked poly(styrene-butadiene) random copolymer that has a glass-transition temperature of _50_C.

(c) Lightly branched and semicrystalline polytetrafluoroethylene that has a

glasstransition temperature of _100_C.

(d) Heavily crosslinked poly(ethylene-propylene) random copolymer that has a glasstransition temperature of 0_C.

(e) Thermoplastic elastomer that has a glass-transition temperature of 75_C.

8.53 In terms of molecular structure, explain why phenol-formaldehyde (Bakelite)

will not be an elastomer.

9.4 Estimate the theoretical fracture strength of a brittle material if it is known that

fracture occurs by the propagation of an elliptically shaped surface crack of length 0.25 mm and having a tip radius of curvature of 1.2 _ 10_3 mm when a stress of 1200 Mpa is applied.

9.6 Briefly explain (a) why there may be significant scatter in the fracture strength for

some given ceramic material, and (b) why fracture strength increases with decreasing specimen size.

9.7 The tensile strength of brittle materials may be determined using a variation of

Equation 9.1b. Compute the critical crack tip radius for an Al2O3 specimen that experiences tensile fracture at an applied stress of 275 Mpa . Assume a critical surface crack length of 2 _ 10_3 mm and a theoretical fracture strength of E/10, where E is the modulus of elasticity.

9.8 If the specific surface energy for soda-lime glass is 0.30 J/m2, using data

contained in Table 7.1, compute the critical stress required for the propagation of a surface crack of length 0.05 mm.

9.9 A polystyrene component must not fail when a tensile stress of 1.25 MPa is

applied. Determine the maximum allowable surface crack length if the surface energy of polystyrene is 0.50 J/m2 . Assume a modulus of elasticity of 3.0 GPa

9.16 A specimen of a 4340 steel alloy having a plane strain fracture toughness of 45

Mpa m is exposed to a stress of 1000 MPa. Will this specimen experience fracture if it is known that the largest surface crack is 0.75 mm long? Why or why not? Assume that the parameter Y has a value of 1.0.

9.17 Some aircraft component is fabricated from an aluminum alloy that has a plane

strain fracture toughness of 35 MPa_m. It has been determined that fracture results at a stress of 250 MPa when the maximum (or critical) internal crack length is 2.0 mm. For this same component and alloy, will fracture occur at a stress level of 325 Mpa when the maximum internal crack length is 1.0 mm? Why or why not?

9.18 Suppose that a wing component on an aircraft is fabricated from an aluminum

alloy that has a plane strain fracture toughness of 40 Mpa m It has been determined that fracture results at a stress of 365 MPa when the maximum internal crack length is 2.5 mm. For this same component and alloy, compute the stress level at which fracture will occur for a critical internal crack length of 4.0 mm .

9.21. A structural component in the form of a wide plate is to be fabricated from a

steel alloy that has a plane strain fracture toughness of 77 MPa_m and a yield strength of 1400 Mpa . The flaw size resolution limit of the flaw detection

apparatus is 4.0 mm . If the design stress is one half of the yield strength and the value of Y is 1.0, determine whether or not a critical flaw for this plate is