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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
16、here 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 stress,Fatigue (2),Applied Cyclic Stresses,Reversed stress cycle,Repeated stress cycle,Random stress c
17、ycle,Stress range r:,Mean stress m:,Stress amplitude a:,Stress ratio R:,e.g., R = -1 for the reverse stress cycle,Fatigue (3),The S-N Curve,The fatigue properties of a material can be determined by fatigue tests,Axial-beam fatigue test,Fatigue (4),Fatigue (5),Two types of S-N curve, where S is norma
18、lly the stress amplitude and N is the number of cycles to failure,Fatigue (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 cycles,Fatigue life: the number of cycles for which failu
19、re occurs at a specified stress level,Fatigue (7),The fatigue behaviour may be classified into low-cycle fatigue and high-cycle fatigue,Low-cycle fatigue: associated with relatively high loads which cause both elastic and plastic strains during each cycle relatively short fatigue lives (normally les
20、s 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 fail
21、ure: Crack initiation (a small crack forms at some point of high stress concentration) Crack propagation (the crack advances with each stress cycle) Final failure (occurring very quickly once the crack has reached the critical size,For fatigue failure, cracks are usually nucleated at some points of
22、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 striations,Fatigue (9),Fatigue beachmark ridges,Fatigue striations,Fatigue
23、 (10),On fatigue fracture surfaces, beachmarks and striations do not appear on the rapid failure areas,The rapid failure may either ductile or brittle,Fatigue (11),Factors Affecting Fatigue Life,The fatigue behaviour is very sensitive to mean stress levels and surface conditions,Fatigue (12),(2) Sur
24、face 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 condition,Fatigue (13),Issues to be considered for surface effe
25、cts:,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 modification
26、s to the sharp corners,The 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 fatig
27、ue 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 th
28、e surface. The residual compressive stress can offset some of the tensile stress to reduce the possibility of fatigue failure The 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
29、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 layer,Fatigue (15),For example,Fatigue (16),Schematic S-N fatigue curves for normal and sh
30、ot-peened steel,Fatigue (17),C. Case hardening A technique by which both surface hardness and fatigue life are enhanced for low-carbon steels Normally, 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 properties,Fatigue (18),Micrograph showing the carburized regions of a case-hardened steel,BBBB,Creep,