Impact Testing. Ductile to Brittle Transition in Steel. Charpy Test. Outline. Impact Testing

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1 Impct Testing Outline Impct Testing Chrpy Test Ductile to Brittle Trnsition Temperture Metllurgicl Fctors Affecting T T Ftigue Ftigue Testing Types of Ftigue Chrpy nd Izod tests mesure impct energy or notch toughness Chrpy V- notch (CVN) most common Before frcture mechnics - impct testing ws used to mesure impct behviour nd likelihood of brittle frcture. Developed in response to onset of brittle filure in ductile mterils e.g. steel ships, bridges etc. - Still used in qulity control nd Stndrds (ship plte etc). Three min fctors were producing these frctures in service: Trixil stress stte (t notches, crcks etc) Low tempertures High strin or loding rtes Impct testing is used for: checking qulity tendency for brittle filure temperture dependence. Not tht useful for design clcultions Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/1 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/ Chrpy Test Ductile to Brittle Trnsition in Steel use stndrd sized br specimens with centrl notch weighted pendulum relesed from height, h impcts the specimen behind the notch (stress concentrtion) frcture of specimen occurs nd energy is bsorbed the pendulum trvels to point, h, where h < h obtin the mount of bsorbed energy from scle.... nd. test method Primry function of Chrpy test t high temperture, CVN for steel is reltively high but drops with decrese in temperture t low temperture steel cn be brittle the sudden drop in impct energy is the ductile-to-brittle trnsition (DBT) steels should lwys be used bove their DBT cermics nd polymers lso experience DBT Aluminum nd copper lloys show Impct Energy Trnsition Ductile frcture Brittle frcture Al nd Cu hve FCC structure Temperture Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/3 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/4

2 Ductile to Brittle Trnsition Behvior Ductile to Brittle Trnsition Behvior ctul DBT is. to define, insted minimum requirement of CVN = 0J (15ft.lb) is used DBT chnges with crbon content in steel Steels: %crbon Cn use frcture surfce ppernce to estimte the DBT Brittle, shiny, fceted, bright, flt overll, no or little deformtion Fibrous, grey, dull, possibly ridged. Sides my be pulled in. Hinged. Like tensile specimen, the frctured surfce of n impct specimen lso contins fibrous, rdil (crystlliztion zone) nd sher lip zones. Fibrous zones occur during the process of crck propgtion in mterils. The proportions of fibrous zone, rdil zone nd sher lip zone vry with temp. resulting in different test results. Notch Fibrous zone Rdil zone Sher lip zone A sketch of the ppernce of frctured surfce of impct specimen fter Chrpy test When temp. drops to certin vlue, the re of the fibrous zone.. nd the re of crystlliztion zone. shrply. The frcture behvior of the mteril trnsforms from ductile to brittle. Usully is determined (s the temp. t which the filure surfce is 50% shiny) s the re of crystlliztion zone ccounts for 50% of whole frctured surfce, nd it is denoted by FATT 50 (Frcture Appernce Trnsition Temp.) Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/5 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/6 Selecting Trnsition Temperture Chrpy Test T 1: Conservtive, bove T 1 frcture is 100% fibrous. Frcture Trnsition Plstic (FTP) very demnding. T : 50% clevge - 50% ductile Frcture Appernce Trns. Temp. (FATT). T 3: Averge of upper nd lower shelf vlues. (often pprox = T ) T 4: Arbitrry vlue of energy bsorbed, (CVN) e.g. 0 J (15 ft.lb) for low strength ship steel. Ductility Trnsition Temp. T 5: 100% clevge frcture. Nil Ductility Temperture (NDT) Figure Vrious criteri of trnsitiontemperture obtined from Chrpy tests The lower this temperture, the greter the frcture toughness An instrumented Chrpy test llows determintion of energy required to initite crck nd lso energy to propgte crck rther thn just totl energy for frcture. Figure 14-4 (Dieter) Lod-time history for n instrumented Chrpy test. Austenitic stinless steels, copper, most HCP metls e.g. low - medium crbon steels High strength steels, luminum, titnium Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/7 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/8

3 Metllurgicl Fctors Affecting T T Metllurgicl Fctors Affecting T T For steels: As %C σ y, σ TS, H, % El, CVN nd T T - This cn be countered by dding Mngnese - Mn : C should be 3:1 Phosphorous increses T T, Oxygen in steel increses T T : - semi-killed (dd Si) nd - fully-killed (dd Si +Al) to remove oxygen Rolled nd forged products my hve vrying impct behviour due to grin orienttion. Note tht the difference is not s lrge t lower tempertures. Remember lso: s grin size toughness.. nd T T. Niobium nd vndium dded to keep grin size smll. Figure 14-8 (Deiter) Effect of specimen orienttion on Chrpy trnsition-temperture curves. Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/9 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/10 Ftigue Cyclic Stresses Occurs under dynmic or fluctuting stresses exmples: bridges, utomobiles, ircrft nd mchine components filure cn occur t vlue thn under sttic loding ccumulted dmge (crcking) occurs over long period of time ctstrophic filure.. of metl filures occur in ftigue! Also occurs in cermics, polymers nd composites ppers s brittle-like filure even in ductile mterils Usully breks without wrning; no, or very little, observble plstic deformtion (some micro-deformtion). Ftigue consists of two stges: - Crck Initition - Crck Propgtion Applied stress: xil (compression-tension) flexurl (bending) torsionl (twisting) reversed stress cycle is where the sinusoidl stress is of equl mplitude bout men of zero σ is tensile nd σ min is compressive repeted stress occurs when σ nd σ min re symmetricl bout σ = 0 rndom stress often occurs in engineering nd is less esy to quntify Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/11 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/1

4 Men stress, σ m is defined s: rnge of stresses (σ r ) is the difference between σ nd σ min: σ r = σ stress mplitude, σ : σ σ + σ min σ = m = σ r σ = Cyclic Stresses σ σ rtio of. nd min. stress mplitudes is the stress rtio, R min min R = σ σ min Ftigue Testing nd S-N Curves Lbortory simultion using rotting bem test. cretes reverse cycle bending with rottion (compression/tension) pply stress of σ ~/3 σ TS mesure number of cycles (N) to filure repet using progressively lower σ (S) plot S vs. log N S-N curve Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/13 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/14 Ftigue Testing nd S-N Curves Two types of S-N curve 1. ftigue (..) limit for ferrous nd titnium lloys FL = σ TS (typiclly). no ftigue limit with non-ferrous lloys (Al, Mg nd Cu) define ftigue strength t specified number of cycles (10 7 ) ftigue life (N f ) is the number of cycles t specified stress level (S 1 ) the dt on S-N curves re scttered due to vribility nd test prmeters difficult to control ftigue strengths re usully verge vlues probbility of filure (P) defined, e.g. t 15MP 1% of smples fil t 10 6 cycles Sttisticl Nture of Ftigue Since, this type of curve shows probbilities of ftigue filure t certin stress level, it is more ccurte thn verge vlue s is normlly shown. Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/15 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/16

5 low cycle ftigue Types of Ftigue high lods short N f ( cycles) high stress environment with high design stress nd smll sfety fctor, DS σ ys scheduled inspection nd mintennce of prts (ircrft) most common cuse of ftigue crcking nd filure high cycle ftigue low lods long N f (>10 5 ) typiclly involves low design stresses, DS << σ ys less commoncuse of filure, results from poor design or environmentl effects Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/17.. Low-Cycle Ftigue This type of cycling is more likely in nucler pressure-vessels, stem turbines, nd similr components where repeted stresses re creted by therml fluctutions. e.g If mteril is constrined nd then heted, therml stresses re generted; If it is not constrined we hve cyclic therml strins (i.e. heting nd cooling of vessels). - Ftigue results from cyclic stress: this is induced by the restrint of the dimensionl expnsion nd/or contrction occurring due to vrying the temperture. Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/18 Low-Cycle Ftigue p Coffin-Mson reltionship: ( ) C where Δε p/ = plstic strin mplitude Δε = ε f N ε f = ftigue ductility coefficient true strin t frcture, ε f C = ftigue ductility exponent (-0.5 to -0.7 for most metls; lower C vlue = longer life) N = no. of cycles N = no. of strin reversls to filure (1 cycle is reversls) Next time: Continue Ftigue Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/19 Dr. M. Medrj Mech. Eng. Dept. - Concordi University MECH 31 lecture 10/0