Doprinos numeričkoj i eksperimentalnoj analizi lomnog ponošanja heterogenih struktura

Kozak, Dražen (2001) Doprinos numeričkoj i eksperimentalnoj analizi lomnog ponošanja heterogenih struktura. = Contribution to the numerical and experimental analysis of fracture behaviour of heterogeneous structures. Doctoral thesis , Sveučilište u Zagrebu, Fakultet strojarstva i brodogradnje, UNSPECIFIED. Mentor: Alfirević, Ivo.

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Abstract (Croatian)

U radu je analizirano lomno ponašanje zavarenih spojeva s pukotinama, različitih mehaničkih svojstava u odnosu na osnovni materijal ploče – visokočvrsti niskolegirani čelik NN70A. Pri tome je X-zavar izvođen kao homogen iii kao heterogen (2 ili 4 prolaza u korijenu su rađena s niskočvrstom Z3kor iii srednječvrstom elektrodom Z2k,,,, a popuna je uvijek bila s visokočvrstom elektrodom Zlpop). Iz tako izrađenih ploča, vađene su standardne jednostrano zarezane BxB i Bx2B epruvete za savijanje u tri točke. Kod BxB epruveta, inic jalni zarez je bio površinski, a kod Bx2B epruveta, zarez je išao kroz debljinu epruvete. Položaj vrška pukotine nakon zamaranja je bio u sredini ili u zoni utjecaja topline. Tijekom ispitivanja mjereni su: sila, pomak na liniji djelovanja sile LLD, neposredno otvaranje vrška pukotine CTOD (d5), otvaranje vrška pukotine na površini epruvete CMOD i prirast pukotine Aa. Dobiveni rezultati s jedne strane ovise o dubini pukotine (omjeru a/W) i pripadajućoj širini zavara na mjestu položaja vrška pukotine, a s druge strane o interakciji pojedinih materijala u spoju. Laboratorijske krivu je otpornosti u obliku CTOD (65) - Aa mogu se koristiti i za konstrukcije u eksploataciji, jer su prije nestabilnog loma zabilježeni mali prirasti pukotine. Eksperimentalno ispitivanje epruveta je simulirano i pomoću metode konačnih elemenata. Kod ravninskih modela je prikazan originalan način dodjeljivanja stanja naprezanja, koji kombinira stanje ravninske deformacije oko vrška pukotine i stanje ravninskog naprezanja da je od vrška. Pri tome je utvrđen udjel elemenata u stanju RD, kao ne. to manji od 1%. Provjerena je valjanost ovog mješovitog modela na primjeru BxB epruvete s površinskom pukotinom u sredini homogenog zavara. Uvidom u po je naprezanja oko vrška pukotine pri maksimalnoj sili, dokazano je da je do loma došlo zbog ograničenja tečenja uzrokovanog X-oblikom zone utjecaja topline. Na primjeru BxB epruvete s heterogenim zavarom i vrškom pukotine u ZUT-u, pojašnjeni su detalji koji se odnose na definiranje mehaničkih svojstava ZUT-a, strukture mreže oko vrška pukotine, rubnih uvjeta itd. Prikazane su zone tečenja i raspodjela 0,2% ekvivalentne plastične deformacije oko vrška pukotine za različite razine opterećenja. Najveće područje tečenja se nalazi u žilavom osnovnom materijalu, a najmanje u krhkom mater jalu ZUT-a. To je pokazatelj zašto pri napredovanju pukotine mole doći do njenog preusmjeravanja k području većeg tečenja. Određen je položaj maksimalnog glavnog naprezanja al i analizirana promjena faktora troosnosti naprezanja k u ravnini epruvete. Kod Bx2B epruvete s pukotinom kroz debljinu i sredinu heterogenog zavara uspoređeni su 2-D i 3-D rezultati za CTOD (65) s eksperimentalnim mjerenjima. Analizirana je raspodjela naprezanja i deformacija po debljini epruvete. Prikazana je promjena ograničenja tečenja kroz debljinu epruvete. Uz pomoć programa MATLAB prikazane su plohe naprezanja otvaranja pukotine ax i naprezanja a,, od djelovanja sile u smjeru osi y. Za modeliranje Bx2B epruvete s pukotinom kroz debljinu, čija je simetrala zamaknuta od simetrale homogenog zavara predložen je originalan pristup. Analiziran je utjecaj stvarnog oblika zamorne fronte kroz debljinu epruvete na 65. U to je svrhu napisan program POMP UK, koji pomiče čvorove mreže modela s ravnom frontom u nove položaje, koji opisuju stvarni oblik fronte. S ciljem nalaženja mjesta inicijac je pukotine, prostorni model je presijecan ravninama po deb jini. \Na osnovi prikaza po ja ekvivalentnih naprezanja i plasticnih deformac ja po pojedinim presjecima, utvrdeno je da je pukotina inicirana na mjestu prolaska njene fronte kroz ZUT. Prikazana je i promjena 0,2% ekvivalentne plasticne deformac je i 65 kroz debljinu epruvete sa stvarnim oblikom fronte pukotine. U zadnjem poglavlju je prikazana mogućnost numeričkog određivanja kritičnog prirasta pukotine. Za BxB i Bx2B epruvete s pukotinom u sredini zavara primijenjena je metoda otpuštanja čvorova. Određeno je područje inic /acne pukotine i sile razvoja pukotine za po tri karakteristične kombinacije materijala epruveta. Rezultati su pokazali da epruveta s homogenim zavarom ZIpop+kor i epruveta s heterogenim zavarom ZJpop+Z2kr,,, zadovoljavaju postavljene uvjete. Drugim riječima, prijedlog izvedbe srednječvrstog korijena u visokočvrstoj popuni sa svim svojim prednostima izloženim u radu, aplikativan je i sa stajališta mehanike loma.

Abstract

The fracture behaviour of weldments with cracks with different mechanical properties related to the base material of plate (HSLA NN70A steel) has been analysed in this work. The welded joint has been produced as homogenous or as heterogeneous (two or four passes in root were made by undermatched Z3kor wire or by Z2k„r with middle strength wire and the filler was always overmatched Z1p,p wire). From these plates, standard single edge notch bend specimens were extracted. BxB specimens were notched with surface cracks. The notch by Bx2B specimens has been placed through the thickness. The location of crack tip after fatigue was in the middle of weld or in the heat affected zone (HAZ). The force, load line displacement LLD, crack tip opening displacement CTOD (d5), crack mouth opening displacement CMOD and crack growth Aa were recorded during testing. The experimentally obtained results for mentioned specimens are depending on crack depth (a/W ratio) and corresponded weld width, and on the other hand on interaction of materials in the joint. Crack growth resistance curves in the form of CTOD (d5) - Aa determined on small laboratory specimens may be applied for the structures in exploitation because small crack growths are measured before unstable fracture. Experimental testing of specimens has been simulated by means offinite element method. The original approach to state-of-stress modelling by 2-D models is presented. This mixed-state-of-stress model combined plane strain state in the region close to the crack tip and plane stress state far from the tip. It was found out that percent offinite elements with prevailed plane strain is less than 1%. The validity of this mixed model on the BxB specimen with surface crack in the middle of homogeneous weld has been proved. Considering stress field around the crack tip for maximum load, one may conclude that constrained yielding in the weld metal caused fracture. The X-shape of HAZ was acting as a barrier toward plastic deformation in the base material. In the case of BxB specimen with the crack located in the HAZ of heterogeneous weld, some details with respect to HAZ mechanical properties determination, mesh refining around crack tip, setting of boundary conditions etc. are given. Plastic zones and 0,2% of equivalent plastic deformation fields near the crack tip for different levels of loading are presented. The region of the yielding is the largest in the ductile base material and the smallest in the brittle HAZ. This is the reason why the crack may change the direction of propagation to the softer material. The location of maximum principal stress al in the specimen and the variation of stress triaxility factor k in plane of specimen were determined. For the Bx2B specimen with crack in the middle and through the thickness of heterogeneous weld, 2-D and 3-D results for CTOD (d5) with experimental measuring were compared The distribution of stresses and strains and variation of yielding constraints through the thickness of specimen has been analysed. The surface of crack opening stress aX and stress caused by force acting a-y were presented by MATLAB application. For modelling of Bx2B specimen with crack through the thickness dislocated from the homogenous weld symmetry, the original approach has been proposed The influence of real crack front variation according to the thickness on the as value has been analysed. For this purpose POMP UK code has been developed. This code shifts mesh nodes from their original position to corresponding real crack front node position. Considering the crack front deviation it is possible to look at real plastic strains field and d5 variation through the thickness. Also, such analysis enables the finding of the plane section where the crack was initiated. Last chapter deals with numerical determination of critical crack growth. In that sense node releasing technique for the specimens with the crack in the middle of weld was applied. The region of crack initiation and crack driving forces for three characteristic material combinations were designated. The results showed that specimen with homogeneous weld Z/p(,p+kor and specimen with heterogeneous weld ZJp„ p+Z2kor, are satisfying all conditions. Because ZJp(,p+Z2k(,r weld has some other advantages described in this work, its application may be recommended

Item Type: Thesis (Doctoral thesis)
Uncontrolled Keywords: mehanika loma; zavareni spojevi; metoda konačnih elemenata; stanje naprezanja; visokočvrsti niskolegirani čelik; otvaranje vrška pukotine; epruveta za savijanje u tri točke; krivulja otpornosti; krivulja sile razvoja pukotine; odc
Divisions: 200 Department of Engineering Mechanics > 210 Chair of Mechanics and Strength of Materials
Date Deposited: 22 Sep 2014 18:00
Last Modified: 16 Oct 2015 13:19
URI: http://repozitorij.fsb.hr/id/eprint/162

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