2材料科学基础英文版课件_(9)

Impact Fracture TestingImpact 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 observed in the tensile test. Tensile testing: the load is applied slowlyImpact testing: the full load is applied very rapidlySpecimen for Charpy and Izod testsTwo types of test: Charpy test and Izod testImpact Fracture Testing (2)Impact Testing TechniquesSize: 10mm10mm55mmImpact Fracture Testing (3)Loading manners: Charpy test Izod testIn engineering practice, usually using Charpy testing to determine the impact energyImpact Fracture Testing (4)Representation of the Charpy impact testStarting position Ending position h0mg hfmg(m mass; g - gravity acceleration)Impact energy = h0mg - hfmgImpact Fracture Testing (5)Ductile-To-Brittle Transition One of the major functions of impact tests is to determine whether or not a material experiences ductile-to-brittle transition High impact energy at all temperatures, no ductile-to-brittle transition Lower impact energy, ductile-to-brittle transitionImpact Fracture Testing (6)FCC materials (e.g. austenitic stainless steels, Cu, Ni, and Al alloys): there is no ductile-to-brittle transitionBCC materials (e.g. ferritic steels, Cr and Mo alloys) and HCP materials (Mg alloys): there is ductile-to-brittle transitionDuctile-to-brittle transition temperature (DBTT): the temperature at which a material changes from ductile to brittle stateDefinition 1: defined by the average energy between the ductile and brittle regionsDefinition 2: defined by 50% ductile fracture (fracture appearance transition temperature, FATT) For engineering applications, the lower the DBTT, the better is the materialImpact Fracture Testing (7)50oCImpact Fracture Testing (8)Ductile-to-brittle transition may cause disasters e.g. During World War II, some 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 (yield 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 embrittlementFactors affecting the DBTTImpact Fracture Testing (9)-150 -100 -50 0 500204060冲击温度韧性断裂百分数80100韧 -脆转变温度Impact Fracture Testing (10)Effects of impurity segregation and strengthening on the DBTTTemperatureIntergranular fracture stress (Large segregation)High yield strength (due to hardening)StressT1 T4T2Cleavage fracture stressLow yield strengthIntergranular fracture stress (Small segregation)TIC T3T1 T3 due to hardeningT1 T2 due to segregationT1 T4 due to bothGrain boundary concentrations of P, Mo and Cr as a function of ageing time at 540oC (error 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 (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 DBTTIntergranular fracture stress (Large segregation)High yield strength (due to hardening)StressT1 T4T2Cleavage fracture stressTemperatureLow yield strengthIntergranular fracture stress (Small segregation)TIC T3T1 T3 due to hardeningT1 T2 due to segregationT1 T4 due to bothS.-H. Song, J. Wu, L.-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:(a)50 m(b)50 m(c)50 mTypical SEM fractographs of the fracture surfaces for the tempered samples fractured at (a) -20oC, (b) -50oC, and (c) -150oC. (a)50 m(b)50 m(c)50 m(d)50 mTypical 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 TbEffect of impurity segregation on the fracture modeImpact Fracture Testing (11)Effect of carbon content on the DBTT of C steelCarbon content Strength DBTT and upper shelf energy Demonstration of hardening embrittlement FatigueFatigue (1) A form of failure occurring in structures subject to dynamic and fluctuating stresses Failure occurring at a stress level substantially lower than the tensile 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 there is very little plastic deformation before failure The failure process proceeds by initiation and propagation of cracks and the fracture surface is normally perpendicular to the applied tensile stressFatigue (2)Applied Cyclic StressesReversed stress cycleRepeated stress cycleRandom stress cycleStress range r:Mean stress m:Stress amplitude a:Stress ratio R:e.g., R = -1 for the reverse stress cycleFatigue (3)The S-N CurveThe fatigue properties of a material can be determined by fatigue testsAxial-beam fatigue testAxial loading specimen. Fatigue (4)Rotating cantilever-beam fatigue test Rotating cantilever beam specimen 30Fatigue (5)Two types of S-N curve, where S is normally the stress amplitude and N is the number of cycles to failureThere is a limiting stress level below which the failure will not occur, called the fatigue limitIt is a typical S-N curve for some ferrous (iron-base) and titanium-base alloysFatigue (6)It is a typical S-N curve for nonferrous alloys (e.g