2材料科学基础英文版课件_(9).ppt
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1、Impact Fracture Testing,Impact Fracture Testing (1),Impact testing is to measure the energy absorbed by a material during its impact fracture (the fracture with a high strain rate),When a material is subjected to a sudden, intense loading (e.g. impact), it often behaves in a more brittle manner than
2、 observed in the tensile test.,Tensile testing: the load is applied slowly Impact testing: the full load is applied very rapidly,Specimen for Charpy and Izod tests,Two types of test: Charpy test and Izod test,Impact Fracture Testing (2),Impact Testing Techniques,Size: 10mm10mm55mm,Impact Fracture Te
3、sting (3),Loading manners:,Charpy test,Izod test,In engineering practice, usually using Charpy testing to determine the impact energy,Impact Fracture Testing (4),Representation of the Charpy impact test,Impact energy = h0mg - hfmg,Impact Fracture Testing (5),Ductile-To-Brittle Transition,One of the
4、major functions of impact tests is to determine whether or not a material experiences ductile-to-brittle transition,Impact Fracture Testing (6),FCC materials (e.g. austenitic stainless steels, Cu, Ni, and Al alloys): there is no ductile-to-brittle transition BCC materials (e.g. ferritic steels, Cr a
5、nd Mo alloys) and HCP materials (Mg alloys): there is ductile-to-brittle transition,Ductile-to-brittle transition temperature (DBTT): the temperature at which a material changes from ductile to brittle state Definition 1: defined by the average energy between the ductile and brittle regions Definiti
6、on 2: defined by 50% ductile fracture (fracture appearance transition temperature, FATT),For engineering applications, the lower the DBTT, the better is the material,Impact Fracture Testing (7),Impact Fracture Testing (8),Ductile-to-brittle transition may cause disasters,e.g. During World War II, so
7、me ships suddenly split in half because the environmental temperature was approaching the DBTT of the constructing material (e.g. a structural steel) or below, but at that time, people did not know why.,Strengthening: solid-solution strengthening, precipitation strengthening, and strain hardening (y
8、ield strength increase) DBTT increase hardening embrittlement Grain boundary segregation of impurities: segregation of impurities such as P, S, Sn and Sb Grain boundary cohesion decrease DBTT increase non-hardening embrittlement,Factors affecting the DBTT,Impact Fracture Testing (9),-150,-100,-50,0,
9、50,0,20,40,60,冲击温度,韧性断裂百分数,80,100,韧-脆转变温度,Impact Fracture Testing (10),Effects of impurity segregation and strengthening on the DBTT,Temperature,T1 T3 due to hardening T1 T2 due to segregation T1 T4 due to both,Grain boundary concentrations of P, Mo and Cr as a function of ageing time at 540oC (erro
10、r bars represent the S.D.),D.-D. Shen, S.-H. Song, Z.-X. Yuan, L.-Q. Weng, “Effect of solute grain boundary segregation and hardness on the ductile-to-brittle transition for a CrMo low-alloy steel”, Mater. Sci. Eng. A 394 (2005) 5359.,The hardness of the sample as a function of ageing time at 540 C
11、(error bars represent the S.D.),Ductile-to-brittle transition temperature (DBTT) as a function of ageing time at 540 C (error bars represent the S.D.),Effects of impurity segregation and strengthening on the DBTT,T1 T3 due to hardening T1 T2 due to segregation T1 T4 due to both,S.-H. Song, J. Wu, L.
12、-Q. Weng, and Z.-X. Yuan, “Fractographic changes caused by phosphorus grain boundary segregation for a low alloy structural steel”, Materials Science and Engineering A 497 (2008) 524-527.,Also read the paper:,Typical SEM fractographs of the fracture surfaces for the tempered samples fractured at (a)
13、 -20oC, (b) -50oC, and (c) -150oC.,Typical SEM fractographs of the fracture surfaces for the aged samples fractured at (a) 10oC, (b) -50oC, (c) -100oC, and (d) -150oC.,应力,温度,O,沿晶断裂应力 解理断裂应力 屈服强度,Tc,Ta,Tb,Effect of impurity segregation on the fracture mode,Impact Fracture Testing (11),Effect of carbo
14、n content on the DBTT of C steel,Carbon content Strength DBTT and upper shelf energy ,Demonstration of hardening embrittlement,Fatigue,Fatigue (1),A form of failure occurring in structures subject to dynamic and fluctuating stresses Failure occurring at a stress level substantially lower than the te
15、nsile or yield strength for a static load. The term “fatigue” is used because this kind of failure occurs after a long period of repeated stress or strain cycling Fatigue failure occupies 90% of all metallic failures Fatigue failure is brittle-like in nature even in normally ductile metals because t
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