Fatigue Corrosion

corrosion Fatigue

introduction: corrosion corrosion fatigue is a big and widely section. section. What this term term paper is going to represe represent nt , First irst the denition denition of  fatigue corrosion and what the most eective process on it , After that the process of corrosion fatigue in generally and how can the engineer know if there is a corrosion fatigue . Also, present the most important to understanding understanding the the fatigue fatigue life which which is S! diagram. diagram. "hen pre presentin nting g how how the corrosio sion fatig igue ue initia itiattes and pro propagat gates . what hat happen to fatig igue ue in corrosiv ive e envir environm onmen entt will will be dis discus cuss s and and sh show owin ing g some some grap graphs hs about about that. that. After fter that the evidence evidence of corro corrosion sion fatigue fatigue and the dierence between the corrosion fatigue and #ust fatigue . Also, presented some $haracteristics of corrosion fatigue with a graph and the %ariables %ariables aecting corrosion corrosion fatigue.

corrosion Fatigue: $orrosion $orrosion Fatigue Fatigue is process process in which the failure failure of a metal by cracking under repeated cyclic loading at lower stress stress level level and in a corro corrosiv sive e envir environm onment ent . A corro corrosive sive environment plays a signicant role in the fatigue of high strengt strength h struct structura urall materia materials ls like like steel, steel, aluminiu aluminium m and titanium alloys. Also "he corrosive environment can cause a faster faster crack crack gro growth wth.. fatigu fatigue e can be in ships, ships, air aircraf craft, t, heat e&changers tubes , pump shaft and steel e'uipment.

Fig()*+ $orrosion $orrosion Fatigue Fatigue $racks in rass

Process of corrosion fatigue + "his is a process process where where the fracture fractures s will happened under condition of corrosion. "he cracking begins at the surfa surface ce defec defect. t. -ole and and irr irregula egulari riti ties es and propa propaga gate te trans granularly . Failure occurs by crack propagation p ropagation . "he crack are unbranched and propagate perpendicular to the surf su rfac ace e of the the meta metal. l. u utt befo beforre all all that that We have have two two stages rst one s about the initiation of corrosion fatigue crack then the propagation of corrosion fatigue crack.

Fig(/*+ trans granular cracks in corrosion fatigue

diagram N-S: A fatigue curve is commonly known as S! diagram . is obtained by plotting the number of cyclic re'uired to $ouse failure against the ma&imum applied cyclic stress . Fatigue occurs when a material fails under a cyclical stress which is below the yield stress of the material and on each repeat epeated ed st strress cycle cycle,, the the crack crack grow grows. s. "he "he repeat epeated ed cyc cyclic will con contin inu ue repeating unt until the cracking is happened . in the g there is two lines the rst one is abou aboutt the fatig fatigue ue curv curve e in the the air air as you you can can see see the endurance limit . endurance limit is the ma&imum stress at which a material will never e&hibit fatigue. Some metals don0t have endurance limit. As long as the cyclic increase and the stress decrease the fatigue happened. the second one one is abou aboutt the the fati fatigu gue e cor corrosio osion n whic which h is abou aboutt the the aluminium aluminium alloys in tap water water. As you can see aluminium aluminium d o n 0t have endurance limit.

Fig(1*+ fatigue and corrosion fatigue curves for an aluminium alloy

n gure gure (1* there there are three three conditions conditions for the same metal metal . As shown in the the gure gure fatigue fatigue behavi behaviour our of the metal etal in air air st stop ops s at almos lmostt 23 233 3 !4mm !4mm5/ 5/ this this is the endurance limit for the metal in air. air. "he second and third third lines the metal is in corrosive corrosive environment For water and sodium chloride solution as the graph represented they are under the endurance limit for the metal .

Fig(1*+ fatigue and fatigue corrosion for some metal in air water and sodium chloride solution.

fig(5):low cyclic fatigue

Initiation of corrosion fatigue cracks:

"he a fatigue fatigue crack crack has two parts, parts, initia initiatio tion n and propagation . "here are several possible for starting the initiation of fatigue such as scratch on the metal ,twin boundaries, boundaries, and and slip band. band. "he cracks cracks are are initiated initiated start start often often at the surfa surface ce of the metal. metal. ut it could could also also start start from from under under surfac surface e in the prese presence nce of surfac surface e defect defect . during the loading part of the cycle , slip bands will occurs . the the su surf rfac ace e crea create ted d by slip slip may may o&idi &idi6e 6e durin during g the the unloading unloading period . Af After ter that the rst step cyclic make make the intrusion intrusion or e&trusion e&trusion . "hen by continued continued deforma deformation tion in the ne&t cyclic the intrusion may grow and form a crack.

Fig(7*+slip band intrusion and e&trusion

Propagation of corrosion fatigue: After fter the crack crack is happene happened d now the dissect dissection ion the phenomena of propagation of corrosion fatigue . "he crack start to propagate in the the direction of the stress a&is. "he stress stress concentration concentration cusses local deformation deformation in a 6one at the crack crack tip . becaus because e of the increas increasing ing in plastic plastic 6one 6one the crack will will growth. growth. "he 6one 6one continue grow growing ing until it reach the thickness of the the sample. Af After ter the 6one became

as the thickness of the sample there is no stress plain.  "he crack propagates in a direction perpendicular to the tensile stress. "he crack undergoes undergoes rotation. Af After ter that on each cyclic the crack start to growth.

Corrosion fatigue evidence:  "he dierence between the fatigue and the corrosion fatigue is that that in the fatigue fatigue rarely rarely there there is any evidence evidence that conform that in the fatigue there is more than one crack while in the the corros corrosion ion fatigue fatigue have more more than than one crack crack it0s it0s usually usually easy to determin determine e whether whether the failure failure was because of the corrosion fatigue or by fatigue . two things that special the corrosion fatigue the surface of the metal is pitted and the cracks are often are in the bottom of the pits.

Fig(8*+ corrosion fatigue failure fail ure from the origin

Corrosive Environment :

Fatig atigue ue crac crack k rate rate depen depends ds very very st strrongly ongly on the the environments as well as on the metal . n an Aggressive environment the rate of the metal cracking is higher than in the inert environment and the two environment have the same condition. "he lowest crack rate was observed in the inert environment . there is two type of environment li'uids and gaseous , generally generally the most most aggressive aggressive rate the cracking could increase in is in the li'uid.

gure(9*+ in:uence of corrosive environment on fatigue crack rate  "his gure is about corrosive and the inert environment environment . As the graph presented the in the ya&is there is the log dl4dn which is about the time and  the &a&is log ka which is thres hresho hold ld valu value. e. As you you can can see see the the the the in iner ertt environment are are less than corrosive environment in the kc (frac (fractu turre toughne toughness ss** , So based based on the graph graph you can know that rate of fatigue cracking in corrosive environmental more than the inert environment.

Characteristics of corrosion fatigue: ) "he main thing that that uni'ue the fatigue fatigue corrosion corrosion is the cracks near the fracture , in a normal fatigue usually there is one crack near the fracture. / "he corrosion corrosion fatigue fatigue cracks in certain certain steels are are trans granular . gure (/*.

2 "he crack generally generally initiated initiated at the surface surface where the stress is ma&imum. 1 "he striations in corrosion corrosion fatigue is less clearer than in fatigue failure. Figure Figure (;*.

Fi g(;*+ fatigue striations

Variable Variable aecting corrosion fatigue: )