2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: 10mm10mm55mm Impact Fracture Testing (3)Loading manners: Charpy testIzod testIn engineering practice, usually using Charpy testing to determine the impact energyImpact Fracture Testing (4)Representation of the Charpy impact testStarting position Ending position h0mghfmg(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 embrittlementGrain 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 -100-500500204060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)StressT1T4T2Cleavage fracture stressLow yield strengthIntergranular fracture stress (Small segregation)TICT3T1 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)StressT1T4T2Cleavage fracture stressTemperatureLow yield strengthIntergranular fracture stress (Small segregation)TICT3T1 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TcTaTb Effect 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:2minmaxmminmaxrStress amplitude a:22minmaxraStress ratio R:maxminRe.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. Al, Cu, and Mg alloys)Fatigue strength: the stress level at which failure occurs for some specified number of cyclesFatigue life: the number of cycles for which failure occurs at a specified stress levelFatigue (7)The fatigue behaviour may be classified into low-cycle fatigue and high-cycle fatigueLow-cycle fatigue: associated with relatively high loads which cause both elastic and plastic strains during each cycle relatively short fatigue lives (normally less than about 104 - 105 cycles)High-cycle fatigue: associated with relatively low loads which cause just elastic strain during each cycle relatively long fatigue lives (normally more than about 104 - 105 cycles) Fatigue (8)Crack Initiation and Propagation Three steps for the process of fatigue failure: (1) Crack initiation (a small crack forms at some point of high stress concentration)(2) Crack propagation (the crack advances with each stress cycle)(3) Final failure (occurring very quickly once the crack has reached the critical sizeFor fatigue failure, cracks are usually nucleated at some points of stress concentration on the surface of a component (surface scratches, sharp fillets, key-ways, threads, and dents) The crack propagation step is characterized by two types of markings on the fracture surfaces: beachmarks and striationsFatigue (9)Fatigue beachmark ridges Fatigue striations Fatigue (10)On fatigue fracture surfaces, beachmarks and striations do not appear on the rapid failure areasCrack initiation siteCrack slow propagationCrack fast propagationThe rapid failure may either ductile or brittle Fatigue (11)Factors Affecting Fatigue LifeThe fatigue behaviour is very sensitive to mean stress levels and surface conditions (1) Mean stress level Effect of mean stress m on the S-N fatigue behaviourFatigue (12)(2) Surface condition For many loading situations, the maximum stress within a component appears at its surface most cracks leading to fatigue failure result at surface positions The fatigue life of a component is very sensitive to its surface conditionFatigue (13)Issues to be considered for surface effects:A.Design factors Any notch or geometrical discontinuity can make stress concentration and act as crack initiation site. The design features include grooves, holes, keyways, threads and so on The fatigue life of a component can be improved by avoiding these irregularities or by making modifications to the sharp corners 21tomt)(2aKThe sharper the discontinuity, the smaller the curvature radius, and therefore the more severe the stress concentration Fatigue (14)B. Surface treatments During machining of a component, small scratches and grooves are inevitably produced, limiting the fatigue life. The fatigue life can be improved considerably by improving the surface finish with polishing One of the most effective methods of increasing fatigue life is to make the component surface have a residual compressive stress. This is because the fatigue failure is usually caused by external tensile stress on the surface. The residual compressive stress can offset some of the tensile stress to reduce the possibility of fatigue failureThe residual compressive stress can be made by localized plastic deformation within the outer surface layer. In industry, this is normally implemented by shot peening In the shot peening treatment, small and hard particles (e.g. steel balls) with diameters in the range of 0.1 to 1.0 mm are projected at a very high speed onto the surface, causing compressive stresses in the surface layerFatigue (15)The vehicle spring steel platesFor exampleFatigue (16)Schematic S-N fatigue curves for normal and shot-peened steelFatigue (17)C. Case hardeningA technique by which both surface hardness and fatigue life are enhanced for low-carbon steelsNormally, it is implemented by carburizing or nitriding. A component is exposed to a carbonaceous or nitrogenous atmosphere at a high temperature where carbon or nitrogen atoms diffuse into the surface layer of the component Consequences: high hardness and residual compressive stress on the surface layer improvement of the fatigue propertiesFatigue (18)Micrograph showing the carburized regions of a case-hardened steel BBBBCreep