UDK 539.55:620.17:669.14 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 41(1)35(2007) AN INTEGRITY ANALYSIS OF WASHING-MACHINE HOLDERS ANALIZA CELOVITOSTI NOSILCA KADI V PRALNEM STROJU Nenad Gubeljak1, Matej Mejac2, Jožef Predan1 1University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia 2Diploma student employed at Gorenje, gospodinjski aparati, d. d., Partizanska 12, 3320 Velenje, Slovenia nenad.gubeljakŽuni-mb.si Prejem rokopisa – received: 2006-05-17; sprejem za objavo - accepted for publication: 2006-11-13 This paper deals with a structure-integrity analysis of a holder designed to carry the cross of a washing machine. Premature fracture of the holder occurred during mechanical tests of the washing machine in the factory. In order to prevent fracture, the task was to determine the causes of the premature fracture of the holder and estimate the suitability of a new design of holder cross in the washing machine. The input data for the structure-integrity analysis were obtained from mechanical testing of the materials used. A stress-and-strain analysis of the holder’s limit load was performed using finite-element modelling of the holder. Dynamic tests of holders with two different thicknesses were made on a servo-hydraulic machine in order to find dynamically the strength and endurance of the holder. The fracture behaviour of the holders is defined by the initiation and propagation of a crack. The determined behaviour confirmed that a new design of holders (with thickness t = 2.5 mm instead of t = 1.5 mm) reduces the stress concentration in the critical region. Consequently, the new holder, subjected to the same dynamic load, can last for more cycles until it breaks. The total number of cycles exceeded the requirements set for industrial testing. Key words: structure-integrity assessment, fracture-toughness testing, high-cycle fatigue, washing-machine holder V članku je predstavljena analiza celovitosti križnega nosilca kadi pralnega stroja. Predčasna porušitev nosilca je nastopila med mehanskim preizkušanjem pralnega stroja v podjetju. Z namenom, da se prepreči predčasna porušitev nosilca, so bili raziskani vzroki za porušitev in ocenjena je bila primernost nove zasnove nosilca križa kadi pralnega stroja. Vhodni podatki za oceno celovitosti so bili dobljeni na osnovi mehanskih preizkusov materialov. Napetostna in deformacijska analiza nosilca pri mejnem stanju obremenitve je bila opravljena z numeričnim modeliranjem in izračunom po metodi končnih elementov. Dinamični preizkusi dveh nosilcev z različnima debelinama ob enakem vpetju, kot je to v pralnem stroju, so bili opravljeni na servohidravličnem preizkuševalnem stroju. Na osnovi opravljenih preizkusov je bila določena dinamična trdnost in vzdržljivost nosilcev. Lomno vedenje nosilcev je bilo ocenjeno glede na lomno žilavost materiala med utrujenostno rastjo razpoke kot tudi glede na iniciacijo končnega, nestabilnega loma nosilca. Dobljeni rezultati potrjujejo, da nova zasnova nosilca z debelino t = 2,5 mm namesto t =1,5 mm ob posledično spremenjenem polmeru zakrivljenosti zmanjša koncentracijo napetosti v kritičnem delu. Tako je pokazano, da novi nosilec pod enako obratovalno obremenitvijo prestane večje število ciklov do končne porušitve, kot je predpisano za preizkuse pri preverjanju kontrole kakovosti v podjetju. Ključne besede: ocena celovitosti konstrukcij, preizkušanje lomne žilavosti, visokociklično utrujanje, nosilec kadi pralnega stroja 1 INTRODUCTION Holders for carrying the cross of a washing machine’s drum are dynamically loaded components, see Figure 1. The premature fracture of the holder can cause severe damage to other mechanical and electrical parts in the housing of the washing machine. Therefore, the integrity of the holder is essential for the safe and reliable service of the whole washing machine. The mechanical testing of a washing machine with an eccentric load was performed in the factory. The results showed that the number of cycles without fracture or crack formation is insufficient for the quality-control requirements. A failure analysis and inspection of the fractured parts showed that the initial fracture occurred in the central holders of the cross, while the fracture of the outer holders occurred at the end, when the inner holder was already broken, see Figure 2. Therefore, the aim of this study was to carry out a stress-strain analysis and a structure-integrity analysis of the inner holders of a washing machine’s drum. Outer Holder Inner Holders čččč*Či « -, i A JSČ - A JČL Ď ¦ \ \lŮ -J t*l\ / M Č Č &Č Š"ČK. _______*> Figure 1: Holder cross welded on the drum of a washing machine Slika 1 : Križni nosilec kadi bobna pralnega stroja Materiali in tehnologije / Materials and technology 41 (2007) 1, 35-40 35 N. GUBELJAK ET AL.: AN INTEGRITY ANALYSIS OF WASHING-MACHINE HOLDERS 1.0E-04 Figure 2: Broken outer holder without fatigue-crack propagation Slika 2: Zlomljeni zunanji nosilci brez vidne utrujenostne rasti razpoke 2 MECHANICAL PROPERTIES The mechanical testing was performed on a steel sheet of the same material and the same thickness as used for the inner holders of the cross of the washing-machine drum. The nominal parent metal is DC03. The tensile mechanical properties were measured 200 0 0 0 0 0 0 0 0 A Figure 3: Middle-cracked tensile specimen (t = 2.5 mm) Slika 3: Plošča s sredinsko razpoko ob izvrtini za natezni preizkus (t = 2,5 mm) E E ? CO TJ —- al -e- ad y = 9E-09x28254 R2 = 0,9384 y=1E-08x2Jn6s R" = 0,9025 / 1.0E-05 10 20 Ig (AK/MPa m1/2) Figure 4: Results of fatigue-crack growth rate for left- and right-side measurements (specimens with t = 2.5mm) Slika 4: Rezultati hitrosti utrujenostne rasti razpoke za meritev na levi in desni strani izvrtine v preizkušani plošči z debelino t = 2,5 mm on flat tensile specimens with geometries according to the DIN10125 standard. The obtained mechanical properties are shown in Table 1. Fatigue-crack growth and fracture-mechanics testing were performed on a middle-cracked tensile specimen, M(T) 1, with the geometry shown in Figure 3. The initial notch of 0.5 mm in the hole was made with a razor blade. The growth of the fatigue crack was followed on both sides of the central hole. The fatigue loading of the sheets (t = 2.5mm) was performed in load control with a ratio R = Fmin/Fmax = 0.21 and frequency 20 Hz, Fmax = 25.4 kN. The Paris-Erdogan relationship 2 was used to describe the fatigue-crack growth law, as shown in Figure 4. The fracture-toughness measurement 3 was performed on cracked specimens with measurements of crack-mouth opening displacement (CMOD) in the specimen’s symmetry loading line, as shown in Figure Table 1: Obtained tensile mechanical properties for the parent material (DC03) Tabela 1: Dobljeni rezultati za mehanske lastnosti za osnovni material (po oznaki DC03) Thickness, t = 2.0 mm Thickness, t = 2.5 mm Standard prescription 01 02 01 02 (R0,005/Rp0,2)/MPa 152/203 184/217 135/188 123/188 Rp0.2 max = 240 Rm/MPa 300 306 284 286 270-370 .E/MPa 201012 202516 188284 159913 210000 Table 2: Obtained fracture-toughness values for parent material (DC03) Tabela 2: Dobljeni rezultati za lomno žilavost za osnovni material (DC03) t/mm W/mm a/mm <7p0.2/MPa (7y/MPa K I,i/MPa m1/2 Fi/kN CTODpl,m/mm CTODm/mm K I,mat/MPa m1/2 2.0 140 34.1 210 180 11.23 18.5 0.595 0.599 145.61 2.5 140 34.6 188 130 17.918 33.6 0.995 1.004 205.86 36 Materiali in tehnologije / Materials and technology 41 (2007) 1, 35–40 N. GUBELJAK ET AL.: AN INTEGRITY ANALYSIS OF WASHING-MACHINE HOLDERS Figure 5: Measurement of CMOD values for middle-cracked tensile specimen (t = 2.5 mm); a) start of test b) stable crack initiation c) end of test Slika 5: Meritev odpiranja ustja razpoke (ang. CMOD) med nateznim obremenjevanjem plošče s sredinsko razpoko (t = 2,5 mm); a) začetek preizkusa, b) začetek stabilne rasti, razpoke c) konec preizkusa * r" t * T ¦ ff ir ?» i ' d li r 0 2 4 6 8 CMOD /mm Figure 6: Measured data load vs. CMOD (t = 2.5 mm) Slika 6: Izmerjeni podatki v odvisnosti obremenitve in odpranja ustja razpoke (t = 2,5 mm) 5. During the test compliance the unloading method was used to observe stable crack-growth extension. The recorded data are plotted in Figure 6. The results of the fracture mechanics testing are listed in Table 2. 3 TENSILE AND FATIGUE TEST Tensile and fatigue tests were performed on the same holder (thickness and geometry) as was tested in the Figure 7: Holder welded by spots on pad for testing Slika 7: Nosilec, zavarjen s točkovnimi zvari na podlago za preiz-kušanje factory. The holder was welded with eight spot welds, as with the washing drum, but in the laboratory case this was on a pad for testing, as shown in Figure 7. The holder was tested statically with tensile pulling until fracture, as shown in Figure 8. A graph of load vs. stroke was recorded, as shown in Figure 9. The fatigue pull testing of both holders (with t = 1.5 mm and t = 2.5 mm) was performed with the same Figure 8: Static pulling test of holder Slika 8: Statični trgalni preizkus nosilca, ki je zavarjen na podlago za preizkušanje 10 Stroke, l/mm 15 20 Figure 9: Load vs. stroke during static pulling test of holder Slika 9: Obremenitev v odvisnosti od pomika, ki je posneta med statičnim trgalnim preizkusom nosilca Materiali in tehnologije / Materials and technology 41 (2007) 1, 35–40 37 N. GUBELJAK ET AL.: AN INTEGRITY ANALYSIS OF WASHING-MACHINE HOLDERS Figure 10: Fatigue crack at root region of holder (t = 1.5 mm) Slika 10: Med dinamičnim utrujenostnim obremenjevanjem nosilca se je pojavila razpoka v kritičnem upognjenem delu nosilca (t =1,5 mm) equipment. Since the fatigue-behaviour analysis was performed only to compare two holders (different in thickness and root radius), the same fatigue load was chosen (Fmax = 2 kN, R = -1). The fatigue crack appeared in the holder (t = 1.5 mm) in the expected region, like during the washing-machine test. The fatigue crack did not appear in the holder (t = 2.5 mm) after 1 million load cycles. As a result, a higher maximum fatigue load (Fmax = 3.5 kN) was used and the fatigue crack appeared in same region, as shown in Figure 10. The fatigue-crack growth sensitivity was estimated for both holders by using fatigue-crack growth rate testing results, e.g., from Figure 4 for t = 2.5 mm. The range of the fatigue stress-intensity factor was determined using A K = K max max K (1) since the loading ratio corresponds to the range of the fatigue stress-intensity factor is AKmax=2Kmax (2) A finite-element calculation shows that in the root region of the holder both tension stress and shear stress appear. The relevant maximum stress-intensity factor is K