Structure Integrity Assessment of a Windflow Power Plant s Axle

Transcription

1 Poc. of the 9th WSEAS Int. Conf. on Matheical and Computational Methods in Science and Engineeing, Tinidad and Tobago, Novembe 5-7, 7 74 Stuctue Integity Assessment of a Windflow Powe Plant s Axle N. GUBEJAK 1, M. CVETIC, J. PREDAN 1, A. POES 3, M. MADERCIC, Maks OBAK 1 -c -Konca-Powe Plant and Electic Taction Engineeing Inc., Falleovo setaliste, 1 Zageb, Coatia ( 3 -Konca-Electic Institute, Falleovo setaliste, 1 Zageb, Coatia ( Abstact: - A stuctue Integity assessment was pefomed accoding to the FITNET* stuctue integity pocedue on axle with multiple cacks in the citical section. In ode to detemined the stess-stain condition of axle, numeical modeling was pefomed by using finite element methods. The esults show that the ange of the stess integity facto fo dynamic loading can exceed the fatigue stess intensity facto at theshold egions. Theefoe, fatigue cack popagation by finding flaws is possible. Results also show, that the final collapse of an axle appeas at a significant fatigue cack size. On the basis of these esults, epaiing by the welding of the citical aea of an axle is ecommended. Key-Wods: - Stuctue integity assessment, windflow axle, cack s defects 1 Intoduction Nowadays a windflow powe has plant become a significant souce of electical enegy. It is an altenative to nuclea powe, steam powe o even hydo powe plants. Howeve, eliable poduction of electicity among othes paametes depends on the axle of the windflow powe plant. Mechanically, the axle is the most citical pat of the plant and is loaded by bending (not tosion). This pat consists of a mechanical beam, between the wind-otos blades and the geneato's housing. The axle is usually made of steel by a casting pocess. Changes in wall thicknesses and eduction of diametes can cause defects in eials as poes, flaws o emaining of slag. Theefoe, fo eliable use, it is necessay to pefom stuctual integity assessment of axle fo flaws such as cacks. The aim of this pape was to find the maximum load caying capacity fo a windflow powe plant s axle having a adial cack Assessment pocedue Duing the inspection pocedue, the pesence of a cack in the windflow Powe Plant s axle was detected in an aea of geometical change. The position and size of the detected cack in a citical section of the axle ae shown in Figue 1. Since the quality assessment equies non existing cacks in a component, a new axle had to be cast. Howeve, the main question was whethe the pesence of a cack was acceptable o not and if fatigue cack popagation is possible. In ode to find the answe, pieces of cast steel wee used fo manufactuing tensile and factue mechanic specimens. The obtained tensile popeties of cast steel ae listed in Tab. 1. Factue toughness values ae obtained by CTOD testing accoding to standad ASTM E-18 [1], and consevative values fo thee specimens ae also listed in Tab. 1 as well.

2 Poc. of the 9th WSEAS Int. Conf. on Matheical and Computational Methods in Science and Engineeing, Tinidad and Tobago, Novembe 5-7, 7 75 Table 1 : Measued mechanical popeties and factue toughness of cast steel Mateial Cast steel Yield stess R 549 MPa p, Ultie tensile stess Young's modulus E R m 653 MPa 185,6 GPa Poisson's ν,3 CTOD,B = CTOD,75 mm J,B =J 185 N/mm Accoding to the FITNET pocedue, it is allowed to assume wost cack geomety. This appoach leads to consevative esults. Theefoe, the idealization of cack geomety is shown in Fig. 1.b, whee a= mm is the cacks depth and 15 is adial suface cack s length. The wind-powe plant s axle is loaded by bending as scheically shown in Fig.. Figue.b) Idealized uniqe suface cack in the most citical egion. M u M u Figue Manne of loading axle Table : Geomety in citical section of the axle Dimension Outside diamete R mm Inside diamete R 57.5 mm i Figue 1.a) Citical section of axle with the position and sizes cack thickness T = R R 7.5 mm i Cack depth a mm Suface Cack length c 15

3 Poc. of the 9th WSEAS Int. Conf. on Matheical and Computational Methods in Science and Engineeing, Tinidad and Tobago, Novembe 5-7, 7 76 Stess intensity facto is calculted accoding to [] Eq. (1) a K = σ π a Y, β (1) T whee Y(a/T,β) is the stess intensity function, and σ is the pincipal opening load, obtained by finite element modelling. Distibution of cack opening stess along the axle is shown in Fig. 3, this loading coesponds to the highest loading ix accoding to the design of the wind-powe plant. Figue 3 shows that the highest stess appea in the aea of sicoveed cacks. In stuctue integity analysis the elavant stess is the cack's opening stess (mode I) σ x as shown in Fig. 4. Factue toughness of the eial is detemined using the factue mechanics appoach in tems of J integal o cack tip opening value CTOD=δ: K Axial stess, MPa. J E R δ 1 ν 1 ν E p, = = () distance along sufce of axle, mm Figue 3 Cack's opening stess distibution on the suface of the axle in the longitudinal diection (load case 7.1c5: M x =1581 knm, M y =4,5 knm, M z =38,3 knm, M yz =57,1 knm, F x =5,3 kn, F y =-39 knm, F z =-89,8 knm, F yz =9,4 knm) M y is limit bending moment at yielding the est of the non-cacked ligament and σ ef is the efeence stess, taking into account the cack's size and loading manne. Stess, MPa S-axial S-Mises though thickness distance, mm Figue 4 Distibution of opening stess σ x and equivalent von Mises though the axles wall Plastic collapse at the base of the FITNET pocedue is obtain + max 1 Rp, Rm = (4) Rp, In the FITNET pocedue the failue assessment cuve-fac is defined fo eials with continues hadening: f ( ) = 1 [,3+,7 e ] fo 1 µ (5) E,1 whee µ = min R p, (6),6 N 1 = f = 1 fo 1 max and ( ) ( ) N f (7) stain hadening exponent N is given by the empiical tem: The nomalized load duing the FITNET pocedue is defined as M σ b ef = = (3) M Y σ Y N Rp,,3 1 + (8) R = m whee R p, and R m ae yield stess and ultie tensile stess, espectively (see Tab. 1).

4 Poc. of the 9th WSEAS Int. Conf. on Matheical and Computational Methods in Science and Engineeing, Tinidad and Tobago, Novembe 5-7, 7 77 Table 3 Reduction of maximum bending stess at constant cack depth a= mm Suface length Suface angle Maximum bending stess bending moment is detemined fo each cack's length. Table 3 shows values fo maximum caying capactiy egading diffeent cack lengths. l (mm) β ( ) σ x, N/mm 5, a= 4 6, , , ,54 47 σx, MPa a=3 3 a=4 a= suface cack length, mm Each intesection point in the failue assessment diagam-fad between FAC and loading cuve epesents the potential dange of a factue, as shown in Fig. 3. Nomalized load contains one value at each intesection point. Theefoe, the coesponding value of the applied bending moment M b is known, if the limit bending moment M Y is calculted. K [ / ], f() 1, 1,8,6,4, potentialy unsafe safe citical intesection point,,4,6,8 1 1, [ / ] Figue 5 FAD plot fo axle with cack and suface cack length β=15 a= mm Figue 6 Reduction of maximum cay capacity in egad to cack depth and suface lengths 3 Conclusion The obtained esults show that in the case of oveloading, the bittle factuing of an axle is possible if cack popagates to citical cack length. FEM analysis shows that maximum load appeas on an axle accoding to a combination of bending and tensile loading. The loading code fo wind powe plants is used duing stuctual integity assessment. The esults of this analysis help to judgment about acceptance o efusal axle egading to the windflaw. Accoding to the FITNET stuctue integity pocedue, stuctual integity assessment has to be pefomed fo an axle with multiple cacks in citical sections. The esults show that the ange of the stess integity facto fo dynamic loading can exceed the fatigue stess intensity facto at theshold egions. Theefoe fatigue cack popagation afte finding flaws is possible. The esults also show, that the final collapse of an axle appeas at a significant fatigue cack size. On the basis of these esults epai by welding at a citical aea of an axle is ecommended. Figue 6 shows the vaiations in maximum loading capacity coesponding to diffeent suface adial cack length fo two-cack depths a = 3 mm and a = 4 mm. In ode to estie est loading capability it is possible to epeat the same pocedue fo a few cack sizes. imit

5 Poc. of the 9th WSEAS Int. Conf. on Matheical and Computational Methods in Science and Engineeing, Tinidad and Tobago, Novembe 5-7, 7 78 Refeences: [1] Nenad Gubeljak:»Celovitost konstukcij«, Univeza v Maibou, Fakulteta za stojništvo Maibo, 6 [] SINTAP: Stuctual integity assessment pocedue fo Euopean industy. Final Pocedue, Bitte-Euam Poject No., BE95-146, Bitish Steel, 1999 [3] ASTM E-18: 1 Standad Test Method o Measuement of Factue Toughness, Volume 3.1, ASTM Intenational, W. Conshohocken, PA, 4 [4] BS 7448: Factue mechanics toughness tests; Pat. Method fo detemination of K IC, Citical CTOD and citical J values of welds in metallic eials, [5] Euocode-3: Design Manual fo Steel Stuctues and Building, Bussels 1993 [6] BS 791: Guide on methods fo assessing the acceptability of flaws in fusion welded Stuctues, 4 th Daft, 8 Apil 1999 [7] Kuma V., Geman M. D. and Shih C. F. (1981), An Engineeing Appoach fo Elastic-Plastic Factue Analysis, EPRI, Final Repot to NP 1931 [8] R6 (1): Assessment of the integity of stuctues containing defects, Nuclea Electic Pocedue R/H/R6, Revision 4