Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 UDK - UDC 620.17:669.715 Pregledni znanstveni članek - Review scientific paper (1.02) Overitev trajnosti aluminijastih sestavnih delov Structural Durability Validation of Aluminium Components Vatroslav V. Grabišič (Reinheim, Germany) Zaradi povečane uporabe aluminijevih zlitin za sestavne dele vozil je treba povzeti najsodobnejša spoznanja o presoji trajnosti sestave v delovnih pogojih. V prispevku smo predstavili postopke za preizkusno in numerično vrednotenje delovne trdnosti vlitka, kovanih in varjenih sestavnih delov iz aluminijevih zlitin. Predstavili smo tudi rezultate raziskav vpliva korozije, prav tako pa tudi metode pospešenega odobravanja preizkusov Ti so nato potrjeni in priporočeni za uporabo v praksi. © 2007 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: delovni pogoji, trdnost materialov, trajnost, vplivi korozije, utrujenost materialov) The increasing trend to use aluminium alloys for vehicle components makes it necessary to summa- rize the state of the art related to the approval of their structural durability under operational conditions. In this paper the procedures for the experimental and the numerical service-strength evaluations of cast, forged and welded aluminium-alloy components are presented We also present the results of investigations of the influence of corrosion as well as the methods for accelerated test approval; these are then validated and a practical approach recommended © 2007 Journal of Mechanical Engineering. All rights reserved. (Keywords: service evaluation, strength of materials, structural durability, corrosion-fatigue-influence) 0 INTRODUCTION The demands of the automotive industry for lightweight design and weight saving can only be fulfilled by going to the limits of the materials’ prop- erties, the manufacturing process, the behaviour of the structural part and the behaviour of the system. An increase in the reliability is required, but at the same time there is a desire to reduce test costs, cou- pled with efforts to improve the methods of numeri- cal design. Structural durability validation covers, on the one hand, special event loading and loading during misuse, and, on the other hand, service fatigue load- ing, characterized by the criteria of the structural yield point, the fracture behaviour and the fatigue strength ([1] and [2]), Fig. 1. Whereas an experimen- tal proof of the strength safeguards the product safety, the numerical service strength evaluation serves as a pre-design procedure. An intensive cooperation between the Fraunhofer Institute for Structural Durability and System Reliability (LBF), Darmstadt, and the Tech- nical Faculty, Ljubljana, was started about 40 years ago by Jože Hlebanja and Ernst Gassner, and was continuously supported by co-workers and younger scientists. One of them, who contributed not only to this cooperation but also to the development of struc- tural durability validation, was Matija Fajdiga. This cooperative research, especially the investigations related to the structural durability of aluminium components, which were supported by the EU ([3], [4] and [6]), are of great value. This paper is a review of the results of all the available investigations about the structural dura- bility of aluminium components, the study of which was initiated and supported by experts from AUDI AG, BMW AG, Daimler Chrysler AG, Porsche AG and Volkswagen AG, and carried out by authors of Ref [1]. In it the procedures for the experimental and the 310 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 Structural durability of safety components Loading from special and misuse events ! Service loading(inclusive environment) Investigation of the structural yield point Investigation of the fracture behaviour under impact loading Investigation of fatiguestrength Gassner-curve (vaiab le amplitudes) Fp(---------- Local deformation eloc Global deformaion s N WL Cycles N Fig. 1. Partition of the structural durability numerical service-strength evaluation of cast, forged and welded aluminium-alloy components are pre- sented. Furthermore, the results of the investiga- tions of the influences of corrosion as well as the methods for accelerated test approval are presented, validated, and a practical approach is recommended. 1 THE EVALUATION OF SERVICE STRENGTH 1.1 Fracture strength The relevant design criterion for a special event, for example, pressing a wheel against the curbstone edge when parking or when hitting a pot- hole with front wheel at braking, is usually the com- ponent’s yield point. However, depending on the material and the component, a misuse criterion may become more relevant. The component’s yield point is defined as the local equivalent strain or the corre- sponding equivalent stress, causing a plastic defor- mation of an allowable size, Fig. 2. A prerequisite is that neither an unacceptable global deformation of the component remains nor the required fatigue strength of the component decreases. Experimental investigations to determine the component’s yield point are carried out with vehicle- and component-relevant quasi-static loading, simu- lating the relevant special event. A recording of the load-local strain behaviour is recommended, making it possible to evaluate the material’s fatigue behav- iour and make a comparison with calculation results. Fig. 3 shows the investigations on cast wheels (mate- rial G-AlSi7Mg T6), which were preloaded on the in- ner-rim side before the experimental structural dura- bility validation was carried out. During preloading a special, seldom-occurring event is simulated, when the user drives over a “speed bump” or curbstone. Under such a loading a plastic, simply non-detect- able, deformation can occur on the wheel, which could decrease the structural durability because of prema- ture fatigue cracks on the rim, as shown in Fig.3. Within the pre-design process the compo- nent yield point may be estimated by elastic/plastic finite-element analysis based on the monotonic stress-strain curve for the local stress state. If any indications exist that plastic deformations may cause damage, fatigue testing with a load spectrum de- rived from the component’s service-load history is recommended. The relevant design criterion for misuse events, for example, high-speed curbstone impact, is the fracture behaviour during impact loading, with which fractures without deformation, e.g., brittle frac- tures, must be excluded. 1.2 Fatigue strength The fatigue strength of aluminium alloys de- creases continuously for a large number of stress cycles. Based on experience, the knee point of the Woehler curve is assumed to be between 1106 and 2-106 cycles. However, it may diverge from this, de- Overitve trajnosti - Structural Durability Validations 311 1 N GL Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 a. S tres s-strain cu rv e (M at er ial beh av iou r , K t = 1.0) b . L o ad -strain cu rv e (Compon en t beh aviou r , K t > 1.0) RnD2 t E3 uctural yield point % - ODinponent‘s yield point 0.2% e ee eq,M Fy eq,0 .2 Local equivalent strain eeq Fig. 2. Structural yield point and plastic deformation 10.5 De5ign Weight [kg] Plastic Deformation AD [mm] Du rabili^ Test Life [km] A 10.9 -0.85 cracks at 4 973 =0.5 B 11 -0,55 cracks at 10 141 »1.0 C 11.35 -0,35 without cracks 14 920 >1.5 fatigue crack Fig. 3. Influence of the rim design on plastic deformation and durability pending on the component, the material and the load- ing mode. As separately manufactured specimens do not inherit the shape, the surface condition and the residual stresses of the component, a direct trans- mission of the specimen’s test results to the compo- nents is not possible. Fig. 4 shows the empirically derived shape of the Woehler curve recommended for the pre-design- ing of the components from wrought or cast alu- minium alloys according to the local stress concept when the specific component data are missing. The available, published quantitative material data are summarized in [1]. In Fig. 5 a generalized Woehler-curve for welded aluminium joints is presented. Compared to the aluminium base material the fatigue strength of the welded joint is less by a factor of between 1.3 and 3.0, assuming an equal stress distribution along the highly stressed section. The tensile strength, Rm, and the yield strength, Rp,0.2, of the base material have only a minor influence on the fatigue strength. Residual tensile stresses caused by the change of microstructure as well as the solidifica- tion may degrade the fatigue strength. Such a deg- radation prevails in the range of large numbers of cycles (N>1106). When the magnitude of the residual stresses is known they may be assessed like mean stresses. Otherwise, it is recommended to cover the effect of residual stresses by choosing the allow- able stresses resulting from the fatigue loading with R=0. In the case of a fatigue-life estimation under variable-amplitude loading the recommended slopes of the Woehler curve are k=5 in the cycle region of N<1107 and k’ = 2k-2 in the region of N >1107, as shown in Figs. 4 and 5. For the design of welded joints the automo- tive industry normally applies the structural stress concept ([5] and [6]). The structural stress incorpo- rates the influence of the weld geometry and the load- ing mode, but should not be mistaken for the maxi- mum notch stress or the hot-spot stress [5], Fig. 6. 312 Grubišič V. V. 0.2 % Si Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 ^^^^^^ a " = 0 30 ¦ f. ' R (accordng to the German Association ¦^ ' * '"of Mechanical Engneering FKM) f = f (surface, constrant, environment ...) ) 1 F ^' ~ 7i - -k' = 2k-2 = 8 ~ " ~ LBF-recommendation for j fatigue life stimatiion : ' 1 1Q' 1Q- 10° 10° 10' Cycls to failure Nf (log) Fig. 4. Schematical presentation of the Woehler-curve for components of wrought and cast aluminium alloys 1 "j ~ f (joint geometry, loadng mode) FAT-caegory (according to IIW or Eurocode) : ^^-^^.^ j,. LBF-recommendation for V' b j Z5 L ^ ^ ^ 2 - ii f atigue life estimation *) IIW or FKM: k = 3 Eurocode 9: k = 3.2 to 6 course accordi Eurocode 9 ng to 1 )' 10^ K 2 5 f 1 i K )* Cy cls to failure Nf (log) Fig. 5. Schematical presentation of Woehler-curves for aluminium welded joints The local geometry substantially influences the en- the analytical proof must take into account the influ- durable structural stresses of welded joints [6], Fig. 7. If no specific data are available a structural stress of CT *(R=-1, N=1106, P =90%) = ±40 MPa may be a s used for the pre-design. 2 THE INFLUENCE OF CORROSION If components are exposed to a corrosive en- ence of the corrosion [7]. For instance, for a cast steering rod and a welded rear-axle carrier the Woehler and Gassner curves re- sulting from constant and variable amplitude loading in air and in a corrosive medium (5% NaCl) are dis- played in Figs. 8 and 9. The applied stress variation and the spectra are presented in Figs. 10 and 11. Concerning the effect of corrosion on the fa- vironment during service the experimental as well as tigue strength special attention should be paid to Overitve trajnosti - Structural Durability Validations 313 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 s : structural stress s : max imu m n ot ch s t r es s hs : hot-spot-stress s 100 MPa 90 80 70 60 50 : f ( w eld geom et ry, loadingmode) structural stress dstribution (strain gauge or FE) linear extrapolation strain gauge d Notch str es s distri buti on (FE) Str ess-concentration factor: s s hs weld-toe radius weld angle 9 weldtoe' t = 1-5 mm: Gauge length lo = 1.5 mm, d = 1 mm t > 5 mm : lo = 3.0 mm, d = 2 mm Fig. 6. Definition of stresses in a weld A Base material 40 30 20 10 0 Material: AlMg 4,5Mn (filler metal AlMg5) Loading: Bending, R = -1 = 2 10 , P= 90% B Butt weld MIG C Butt weld, ground CT MIG D L ap joint MIG E L ap joint MIG F L ap j oi nt TIG ) " ! 1 HI 1 ^ y 1 8 - 9 , HI 1 1 1 V^^'^100-120° H I ^ 1 '^^¦-130-140° HI XT G Lap joint, milled J MIG \ I AB CDE F G *) in 2mm distance of the notch transition of the weld; length of the strain gage: l0= 3mm Fig. 7. Influence of geometry on supportable stresses of aluminium welded joints R the following: • To reveal the damaging influence of corrosion fa- tigue load levels we should allow at least 5-106 cycles. • During corrosion the slopes of the Woehler curves do not change. A constant slope of k = 4 is recom- mended. • Available results show a significant drop in the fatigue strength due to the corrosion being less during random loading. If the surface is not treated (coated, shot-penned) the fatigue strength after 5-106 load cycles reduces to 50% under constant- amplitude loading and to 20-25% under variable- amplitude loading. This applies to the intensified corrosive conditions imposed in the laboratory on aluminium alloys belonging to the 5000 and 6000 groups of the international alloy register. • Regarding components made of standard alloys, service experience has confirmed the following approach: the damaging influence of corrosion for a real component can be covered by proof testing in air with a 15% increase in the fatigue loads, provided the component’s yield point is not ex- ceeded. Otherwise, a design life increased by a factor of 2 must be proven [1]. • Although cast skin and shot-penning diminish the strength drop due to corrosion this effect should be neglected in the numerical design process. In the case of corrosion protection (surface coats) the design process may be conducted as if no 314 Grubišič V. V. s s 150° 8 J. 5 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 Component: Materi al: Surface Loadng: Frequency: Environment: Cast rearaxle steering rod A 361 T6 (GD-AlSi10Mg T6) vacum investment casting as cast axcial, R, R = -0.44 10 to 16 s1 RT Test in air A Constant amplitudes A Variable amplitudfes, L = 2.3 103 Tests under corrosion (5% NaCl) 0 Constant amplitudes O Variable amplitudfes, L = 2.3103 4.0 2.0 III."' "D 1.0 6 0.6 "8 M 0.4 0.1 2 4 6 8 4 6 2 4 5 6 8 7 2 4 10 8 104 105 106 Fig. 8. Fatigue strength of cast aluminium rear axle steering rods in air and under corrosion Component: Welded rea"axle ca"rier" Materi al: AA 5454 (AlMg3Mn) Weldng: MIG Loadng: axial, R, R = -1 Frenquency: 10 to 20 s-1 Environment: RT Tests in air A Constant amplitudes ^ Variable amplitudes, Ls = 5 104, Gauss, I = 0.99 Tests under corrosion Corrosion cycle spraying 5 min, dry 20 min # Constant amplitudes Q Variable amplitudes, L, = 5 104 , Gauss, I = 0.99 2.0 1 b 0.6 0.2 0.1 Gassner-curves Woehler-curvs TF = 1 : 1.30 Ps = 50% 10= 6 S Cycles to failure Nf (10% stiffness loss) Fig. 9. Fatigue strength of welded rear axle carriers in air and under corrosion corrosive environment were active. • So far, generally valid knowledge about the influ- ence of distinct corrosion cycles on fatigue be- haviour and knowledge about an appropriate se- lection of service-like environmental conditions are still missing. Thus, for future testing concepts, first of all a unification of simulated environmen- tal conditions is to be recommended, for example, a 5-min period of salt spraying with 5% NaCl solu- tion followed by a drying period of 20 to 25 min. Investigations on preconditioning with sub- sequent fatigue loading in air do not have a real influence on the fatigue strength. Therefore, corro- sive preconditioning should not be recommended to assess fatigue corrosion. 3 STRUCTURAL DURABILITY VALIDATION 3.1 Experimental validation The experimental validation of structural du- rability must be based, on the one hand, on repre- sentative operational stress spectra including the loading sequence, and on the other, be carried out in a time- and cost-saving way [8]. Therefore, it is nec- essary to accelerate the durability testing for which the different possibilities exist: • The increase of the maximum and of all other spec- trum loads with a simultaneous decrease of the spectrum size. This modification is suitable only when the structural yield point of the component Overitve trajnosti - Structural Durability Validations 315 0.2 -J- 1.0 0.8 0.4 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 a. Stress-time history b. Cu mu lat ive amplit u de dis t r ibu t i on Fig. 10. Test spectrum of cast aluminium rear axle steering rods a Stress-timehistor +1 b. Cumulative amplitude distibution +1 R= -1 I = 0.99 Spectrum size: Ls = 5 · 104 Fig. 11. Test spectrum of welded aluminium rear axle carriers is not exceeded by this measure. • Omission (cutting off the low loads); “real-time load-sequence testing” applies omission very of- ten. Omitting the small loads with their large number of occurrences may change the compo- nent’s behaviour, depending on the omission level, the material, the loading mode, the manufacturing and the environment, occasionally leading to a wrong evaluation. • A change of spectrum shape, keeping the maximum spectrum load, but with an increased load on the lower levels. Here, the omitted small spectrum loads and the corresponding decreased total frequency are compensated by the increased loads, maintain- ing the damage content of the design spectrum. The modified load spectra should obey the following rules: • The load spectrum must contain a sufficient number of cycles, 2-106 up to 5-106, to cover the eventually occurring effects of fretting and envi- ronmental corrosion as well as the degradation of the fatigue strength in the region of a large num- bers of cycles. • The equivalence of the damage caused by the test and the design spectrum, which should be veri- fied by calculation with a modified Palmgren-Miner rule using Woehler curves for wrought or cast aluminium-alloy components, Fig. 4, or for welded joints of aluminium alloys, Fig. 5. • The sequence length of the test spectrum must be fixed in such a way that a sufficient mix of loads with the appropriate repetition is achieved, as ap- plied during testing, Figs.10 and 11. 3.2 A theoretical estimation of the fatigue life To estimate the fatigue life two different con- cepts, namely the concept of local strain or stress for cast and forged components, and the concept of structural stress for welded joints, are used. Corre- spondingly, the local stress concept utilizes a local stress Woehler curve, as shown in Fig. 4, and the structural stress concept a structural stress Woehler curve, as shown in Fig. 5. A comparison of the ex- perimentally generated strength data with the data derived from a German design guideline (FKM) [9] or from the Uniform Material Law shows large differ- ences caused by the influences of component manu- facturing. It is therefore advisable to use data ob- tained with specimens removed from components or with components. The local stress concept covers the mean- stress effects with the material-related mean-stress sensitivity parameter M = [a (R=-1) / a (R=0)]-1. It 316 Grubišič V. V. 0 0 -1 -1 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 Damage sum of the spectrum: X ^ ='^sp8c. ¦D Cumulative frequency* distribution(spectrum) Woehler curve ¦N i = 1: steel , aluminium i = 2: cas t an d s in t ered mat er i als L s Cycles N, N Fig. 12. Modification of the Woehler-curve and calculation of fatigue life (schematically) has values between 0.2 and 0.3 for wrought alu- minium alloys (Mwrought=0.25) and between 0.4 and 0.5 for cast aluminium (Mcast=0.45), depending on the tensile strength. For welded joints, mean-stress sensitivity parameters between 0.2 and 0.7 are deter- mined, depending on the residual stress state; Mweld=0.45 is recommended. Furthermore, it is recommended to use Palmgren- Miners-Rule modified by Haibach ([1], [2] and [8]) within the local and structural stress concept to esti- mate the fatigue life, Fig. 11. The damage accumulation is based on the Woehler curves, Figs. 4 and 5, recom- mended above. If components are prone to corrosion effects, damage accumulation is recommended by the elementary Palmgren-Miner-Rule using a Woehler curve with a constant slope of kcorr=kair–1. Various investiga- tions revealed damage sums between 0.05 and 2.0, de- pending on the stress-time history, the stress distribu- tion and the failure criterion (the cracking or fracture of a test specimen). A real damage sum of Dreal = 0.5 is recommended for use with the fatigue-life estimation [10]. In the case of a large mean-stress variation, smaller damage values should be used. Assuming proportional loading and a constant direction for the principal stresses, multi-axial fatigue loading of wrought aluminium-alloy components can be assessed by equivalent stresses based on the dis- tortion energy (Mises) or the shear-stress criterion (Tresca); in the case of cast alloys the normal (princi- pal) stress criterion (Galilei) should be used. In the case of changing the principal stress directions the use of the distortion energy or the shear-stress criterion is not appropriate. They may lead to significantly over- estimated fatigue lives of components with ductile material (e>10%) behaviour; thus a ductility-depend- ent modification is needed. For less ductile cast alloys (e<2%) the normal stress criterion delivers correct equivalent stresses [11]. In welded joints a multi-axial stress state with the constant direction of the principal stresses due to proportional loading can also be repre- sented by equivalent stresses based on the conven- tional fracture criteria of Mises, Tresca or Galilei. In the case of non-proportional loading and a varying direc- tion of the principal stresses, the fatigue-life estimation applying these criteria results in an unrealistic increase in the life compared to the case of proportional loading. The local equivalent stress, based on a modified Mises criterion, is calculated for the combination of normal and shear stresses in different interference planes of a surface element; the maximum value of the combina- tion determines the critical plane and the equivalent stress [11]. The pre-designing of components, sub- jected to multi-axial loading with variable amplitudes, still contains large uncertainties requiring experimental verification. 4 CONCLUSIONS The state-of-the-art knowledge and the al- ready-existing experience make the reliable design of aluminium components feasible. However, in fu- ture experimental investigations of components’ structural durability the local stresses or strains, with their dependencies on manufacture, geometry, and loading, should be determined, because the result- ing data are more appropriate than the data derived from specimen testing or taken from guidelines. Such data may form a basis for verifying calculations and for the transfer of data to other components and for different loading modes. Overitve trajnosti - Structural Durability Validations 317 L N s cac. D spec. D Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 310-318 To profit by eventually extending the avail- special events on the fatigue life under subse- able lightweight design potentials, supplementary quent random loading. investigations to extend the database are desirable. • The influence of temperature combined with the pres- This mainly concerns: ence of a corrosive environment and random loading. • A determination of the maximum allowable plastic • The improvement of the accuracy of fatigue-life deformation with regard to the structural yield assessment methods for welded aluminium com- point of various components and the effects of ponents, also when they are multi-axially loaded. 5 REFERENCES [I] Sonsino C. M., Berg A., Grubisic V. 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(2000) Obratovalna trdnost mehanskih zvez delov iz jekla in aluminijevih zlitin (Fatigue life of mechanical connections made up from steel and aluminium alloys.). Ph.D.-Thesis on Technical Faculty of University of Ljubljana. [5] Radaj D., Sonsino C. M. (1998) Fatigue assessment of welded joints by local approaches. Abington Publishing, Cambridge. [6] Fischer G., Grubisic V (1999) Data for the design of welded aluminium sheet suspension components. SAE Paper 1999-01-0662, Detroit/USA. [7] Morgenstern C, Streicher M., Oppermann H. (2004) Leichtbau mit AluminiumschweiBverbindungen des Fahrzeugbaus unter korrosiven Umgebungsbedingungen und variablen Belastungsamplituden (Light- weight design of vehicle structures with welded aluminium joints under corrosive environment and variable amplitude loading). DVM- Report No. 131 (2004), pp. 75-88. [8] Grubisic V. (1994) Determination of load spectra for design and testing. International Journal of Vehicle Design 15 (1994) No. 1/2, pp. 8-26. [9] FKM-Richtlinie (German design recommendation): Rechnerischer Festigkeitsnachweis fiir Bauteile aus Aluminiumwerkstoffen (Numerical strength proof of aluminium alloy components). FKM-Forschungsheft No. 241 (1999), VDMA-Verlag, Frankfurt am Main [10] Grubisic V., Lowak H. (1986) Fatigue life prediction and test. Results of aluminium alloy components. „Fatigue Prevention and Design“, Ed. by J.T Barnby, EMAS, Warley, pp. 171-187. [II] Kiippers M., Sonsino C. M. (2003) Critical plane approach for the assessment of the fatigue behaviour, of welded aluminium under multiaxial loading. Fatigue & Fract. of Eng Mat. & Structures 26 (2003) No.6, pp.507-514. [12] Eurocode No. 9: Design of Aluminium Structures, Part 2: Structures susceptible to fatigue. European Committee for Standardization (CEN), Brussels, Ref No. ENV 1999-2: 1998 E. Author’ Address: Prof Dr. Vatroslav V. Grubisic Independent Consultant Zum Stetteritz 1 Reinheim, Germany vgrubisic@hotmail.com Prejeto: 10.72006 Sprejeto: 07 Odprto za diskusijo: 1 leto Received:. Accepted: 25.4.20 Open for discussion: 1 year 318 Grubisic V. V.