fatigue strength evaluation of the aluminum carbody of

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Fatigue Strength Evaluation of the Aluminum Carbody of Urban

Transit Unit by Large Scale Dynamic Load Test∗

Sung Il SEO∗∗, Choon Soo PARK∗∗∗, Ki Hwan KIM∗∗∗∗,Byung Chun SHIN† and Oak Key MIN††

Aluminum carbody for rolling stock is light and easily recycled, but includes severedefects which are very dangerous to fatigue strength. Strength evaluation of a carbody bystatic load test has been performed. However, true evaluation of fatigue strength could notbe performed because fatigue failure is caused by dynamic alternating loads. In this study, toevaluate fatigue strength of the aluminum carbody of urban transit unit, a large scale testingmethod to simulate actual dynamic loads is proposed and the fatigue failures of the carbodyare investigated. The test results are compared with the estimated results by static load testand structural analysis. Also, the differences between the results are discussed. The comparedresults show that flexural response of the carbody plays a significant role in occurrence offatigue failure. The estimation of fatigue failure based on static load test or static analysismay provide misleading results. It was also verified that the fatigue life characteristics ofthe aluminum carbody can be estimated from the published fatigue test data for aluminumcomponents with similar joint detail. Test and evaluation of fatigue strength based on theestablished static load test method needs to be modified to consider dynamic response of thecarbody.

Key Words: Aluminum Carbody, Crack, Dynamic Load Test, Fatigue, Rolling Stock, S – NCurve, Welded Joint

1. Introduction

As aluminum alloys are light and strong, they havebeen widely used as the main structural materials of trans-portation systems such as aircrafts, high speed vessels androlling stocks. Good extrudability as well as light weightare ideal property for rolling stocks which have uniform

∗ Received 7th November, 2003 (No. 03-5137)∗∗ High Speed Rail Division, Korea Railroad Research Insti-

tute, 360 Woulam-Dong, Uiwang-city, Kyonggi-Do 437–825, Republic of Korea. E-mail: [email protected]

∗∗∗ High Speed Rail Division, Korea Railroad Research Insti-tute, 360 Woulam-Dong, Uiwang-city, Kyonggi-Do 437–825, Republic of Korea. E-mail: [email protected]

∗∗∗∗ High Speed Rail Division, Korea Railroad Research Insti-tute, 360 Woulam-Dong, Uiwang-city, Kyonggi-Do 437–825, Republic of Korea. E-mail: [email protected]

† Structural System Division, Korea Institute of Machineryand Materials, 171 Jang-Dong, Yusong-Gu, Taejon 305–600, Republic of Korea. E-mail: [email protected]

†† Department of Mechanical Engineering, Yonsei Univer-sity, 134 Shinchon-Dong, Seodaemun-Gu, Seoul 120–749, Republic of Korea. E-mail: [email protected]

section profile. Aluminum alloys are more expensive thansteel or stainless steel which has been commonly used forrolling stocks, but increase of material cost can be com-pensated by decrease of labor cost, since aluminum extru-sion profiles minimize cutting and joining works to forma structure.

Aluminum carbody structures have several strongpoints, but also have various problems. Major defectsfound in aluminum structures are porosity and hot crack-ing in the weld joint. They lower structural reliability andmake railway companies reluctant to use the unproven car-body. To prove structural safety of the carbody of rail-way rolling stock, Japanese Industrial Standards regulateload test for the prototype carbody(1). However, the test-ing method is restricted to static loading. According to thetesting method, fatigue strength of a carbody is evaluatedby Goodman’s diagram based on the measured stressesby the static load test. The alternating stress limit ofGoodman’s diagram is given by the fatigue test results forthe components of structures. Simple extension of staticload test results to evaluation of fatigue strength of thewhole carbody may result in serious problems, because

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fatigue failure is caused by dynamic loads.Oomura et al. have performed fatigue test of an ac-

tual carbody structure(2), (3). They applied the concentratedload on the two points of the underfloor to realize dy-namic loading conditions in the laboratory. They showedvalidity of the testing method by comparing stress distri-bution around the supporting part of the carbody struc-ture. However, equivalent concentrated loading conditionscan’t substitute for distributed loading conditions aroundloading points. There are considerable differences in stressdistribution around the loading points. Also, the target car-body of Oomura’s research is not the aluminum structurebut the stainless steel structure fabricated by spot welding.

In this study, a dynamic load testing method for thewhole carbody structure is proposed and test results arepresented. Also, fatigue strength of the aluminum carbodyis evaluated.

2. Survey of Load Testing Methods for CarbodyStructure

2. 1 Static distributed load testJIS E 7105 regulates the static load test methods to

evaluate strength of carbody structure for railway rollingstocks(1). It presents seven kinds of load testing methodssuch as vertical load test, compressive load test, twist-ing load test, three point supporting test, flexural naturalfrequency measurement test, torsional natural frequencymeasurement test and pressure test. Among the testingmethods, the vertical load test is directly relevant to fa-tigue strength. The test set-up is shown in Fig. 1. Anequivalent dynamic load in addition to the basic load cor-responding to the weight of passengers and equipments isapplied on the floor of the carbody. Stresses and deforma-tions are measured. Static and dynamic components of themeasured stresses are separated and Goodman’s diagramis drawn based on the component fatigue test results. Thepossibility of fatigue failure at each weak point is evalu-ated by Goodman’s diagram.

Fatigue strength evaluation by static load testingmethod is simple and useful. But potential cracks maynot be revealed by the static load test.

2. 2 Dynamic concentrated loading testOomura’s dynamic load testing method is shown in

Fig. 2(2). An actuator generates the dynamic concentratedloads which are transferred to the underfloor of the car-body through the special jigs as shown in Fig. 2. The car-

Fig. 1 Static load test

body structure is subjected to four points bending moment.They proved the validity of the testing method by com-paring with analysis results and other test measurements.However, bending moments at center show some differ-ence between the concentrated loading case and the dis-tributed loading case. Also, discontinuity of shear force atloading points causes different stress distribution from thedistributed loading case.

3. Dynamic Load Testing Method

3. 1 Testing facilityIn order to evaluate actual fatigue strength of a car-

body structure, it is important to make the testing con-ditions very similar to the real dynamic load conditions.For this purpose, authors propose a dynamic load testingmethod as shown in Fig. 3. Four supporting beams at thepositions of air springs of the actual bogie system are fixedon the test bed. The carbody is sitting on the four support-ing beams through coil springs. Basic load correspondingto the weight of passengers and equipments is distributedon the underfloor. Taking into account the fact that dy-namic load is transferred from the bogie under the bodybolster of the carbody, two servo actuators to generate dy-namic motion of the whole carbody are placed under thebody bolster. Springs between the carbody and the sup-porting beams play the same role as the secondary sus-pensions of the actual bogie system. The actuators movethe carbody to vibrate at the constant acceleration.

3. 2 Testing equipment and control systemTo measure stresses and displacements of the car-

body, thirty four Rosette strain gages and fourteen singlestrain gages are attached on the stress concentration re-gions, which are identified by the detailed structural anal-ysis results. Typical locations of strain gages are shown inFig. 4. Seven LVDTs’ (linear variable displacement trans-ducer) are installed under the carbody to measure verticaldisplacement.

The measuring and control system is shown in Fig. 5.The system is composed of hydraulic actuator, hydraulicpower supply, digital controller and data processing com-puter. The data processing computer generates analog sig-nals for the target force or the target displacement andtransmits them to the digital controller. The digital con-troller controls the applying force or the displacement by

Fig. 2 Dynamic load test by Oomura(2)

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(a) Aluminum carbody on testing jig

(b) Detail of testing facility

Fig. 3 Set-up of dynamic load test

transmitting the input voltage signal to the servo valveof the hydraulic actuator. During the testing process, aclosed-loop control method is used. The feed back signalfrom the actuator is compared with the command signaland the difference is controlled to be zero.

3. 3 Testing procedureThe testing procedure is given by Fig. 6. The proto-

type aluminum carbody is put on the supporting beams.Actuators and the control system are installed on the car-body. Strain gages, displacement gages and accelerome-ters are also mounted on the carbody. For initial static load

test, basic load is uniformly applied by steel blocks on theunderfloor. The basic load corresponds to the weight ofpassengers and equipments amounting to 420 kN. Stressesand displacements at the full loaded condition are calcu-lated by the data acquisition system. To measure the natu-ral frequency of the carbody, spectrum analysis is carriedout after sudden excitation.

Before beginning dynamic fatigue load test, pre-testto determine the magnitude of exciting forces of the actu-ators is conducted. The actuators below the body bolstersexcite the carbody on the springs at the frequency of 5 Hz,


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Fig. 4 Typical locations of strain gages

Fig. 5 Configuration of load control system

Fig. 6 Procedure of dynamic load test

which is corresponding to the mean rotational speed ofwheel. Exciting force is determined so that the amplitudeof acceleration at the quarter of the carbody may be 0.2 g,which is required by the standard specifications for urbantransit units in Korea(4). Dynamic accelerating motion ofcarbody on the track is reproduced by forced vibration byactuators. The carbody undergoes the constant load due toweight plus the alternating load due to forced vibration.

Stresses and displacements at all the measuring pointsare calculated and recorded by the data processing com-puter. When crack initiation is detected, the test is stoppedand investigation is made. For the critical failures, gaug-ing and repair welding is done. At 6×105 cycles, overallexamination to detect invisible cracks is made. Dynamicfatigue load test is continued up to 2×106 cycles.

4. Test Results and Investigation

The locations of fatigue crack initiation and the cy-cles are presented in Fig. 7. The first crack occurred at thelower joint of the window post as shown in Fig. 8. Thedetail of the crack joint is shown in Fig. 9, where one sidefillet welding is needed to join the post with the under-floor. The root of fillet is thought to be crack initiationpoint. Another typical crack is shown in Fig. 10. The par-tial penetrated root of butt joint shown in Fig. 11 was thecause of the fatigue crack, where high stress was acting.

4. 1 Comparison of resultsBefore the load test is conducted, finite element anal-

ysis for the carbody subjected to vertical static load in-cluding equivalent dynamic load was carried out. Theanalysis results showed that maximum stress regions are


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Fig. 7 Fatigue cracks occurred in the carbody

Fig. 8 Cracks on the fillet joint of window post

Fig. 9 Detail of one sided fillet joint

Fig. 10 Cracks on butt joint of door corner

Fig. 11 Detail of partial penetrated butt joint

the lower corners of the first and the fourth doors near thebody bolster. The upper corners of entrance door as shownin Fig. 12 are also high stress regions. Since the support-ing springs of the bogies are located under the body bol-ster, maximum shear forces are acting on the side struc-


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Fig. 12 Finite element analysis results for aluminum carbody under vertical loading(equivalent stress in daN/mm2)

tures near the body bolsters. To make matters worse, thereare openings for doors in the side structures near the bodybolsters. As a result, maximum stresses are concentratedon the side structures near the body bolsters. Theses re-sults were supported by the static load test(5), (6). Since thejoint detail of the lower corner of door is the same withFig. 9 of which fatigue strength is low due to incompleteand unsymmetrical fillet, it was thought to be the first fa-tigue crack point. However, dynamic test results shown inFig. 7 revealed that the initial cracks are concentrated onthe lower joints in the central part of the carbody. It maybe assumed that all the crack joints in the central part in-cluded harmful welding defects. But, severe defects werenot found during initial inspection. The different locationsbetween two test results are thought to be due to the dy-namic response of the carbody during the test.

4. 2 Analysis on alternating stressesIn the static load test and analysis, uniformly dis-

tributed basic load plus 20 percent additional load dueto acceleration of carbody during running were appliedaccording to the specification(4). The additional load isalso applied uniformly on the floor based on the assump-tion that the carbody is rigid. When evaluating fatiguestrength, alternating stresses become ±20 percent of meanstresses due to basic load. So, the ratios between the alter-nating stresses and the mean stresses are constant alongthe length of the carbody. However, the stress rangesmeasured in the dynamic test show variation along thelength. The stress range is greatest at the center as shownin Fig. 13. It is thought that the variation of the stressranges is related with the concentration of crack points onthe center.

Fig. 13 Distribution of stress ranges along length

Fig. 14 Distribution of displacements along length

Because the carbody is not a rigid body but an elas-tic structure, the vibration acceleration at each location isdifferent. Particularly, in the central part of the carbodythe vibration amplitude is maximum as shown in Fig. 14and the stress change is also maximum. It is thought thatconstant additional load corresponding to dynamic mo-tion can’t adequately reflect actual dynamic loading dur-ing running on the track.


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Table 1 Mechanical properties of aluminum alloy

Fig. 15 S – N curve and fatigue life of crack point

4. 3 Analysis on fatigue lifeThe material properties of the carbody are presented

in Table 1(7). The recorded fatigue cracks are plotted onthe S – N curves as shown in Fig. 15, which indicates thatthe plotted principal stress ranges of fatigue points areclose to the design S – N curves for the component test re-sults. For the joint shown in Fig. 9, the S – N curve of one-sided fillet joint(8), (9) is suitable to predict the real fatiguelife. Also, for the butt joint in the corner of entrance door,the design S – N curve of partial penetration joint(8), (9) issuitable to predict the real fatigue life.

4. 4 Evaluation on testing methodsIn drawing Fig. 15, mean stresses were not consid-

ered. However, tensile mean stress can play a significantrole in fatigue initiation. Goodman’s diagram is a simpleand useful method to take into account the effect of meanstress. The standards of structural test and evaluation forrolling stocks(1) also suggest to use Goodman’s diagramto predict fatigue failure possibility. Based on the resultsof one-sided fillet joints shown in Fig. 15, Goodman’s di-agram is drawn to investigate the effect of mean stressesand to judge fatigue failure occurrence. Figure 16 presentsGoodman’s diagram and the failure points of the carbodybased on 5×105 cycles. The stress amplitudes of the statictest are the estimated results assuming that peak alternat-ing acceleration is 0.2 g and uniform along the carbodylength. However, the stress amplitudes of the dynamic testare the measured results presented above. Figure 16 showsthat fatigue failure prediction based on the static test canproduce underestimated results. This comes from ignoring

Fig. 16 Estimation for fatigue crack initiation

that the carbody is flexible as mentioned above. To con-struct a more reliable carbody structure, equivalent loadtesting procedure to consider dynamic effect is necessary.

5. Conclusion

In this study, a dynamic load testing method to evalu-ate the fatigue strength of the carbody of urban transit unitwas proposed and the results obtained by the test were an-alyzed. The proposed testing method is different from theprevious testing methods. It takes into account the actualloading condition. The test results can be summarized asfollows;

1 ) The initial fatigue cracks occurred in the weldjoints on the central part of the carbody. During the test,the measured stress range ratios showed maximum at thecenter and varied along the length. The large stress rangesat the central part are thought to be due to flexible vibra-tion of the carbody, of which acceleration is maximum atthe center.

2 ) The fatigue strength evaluation results based onthe static load test show some difference with the resultsbased on dynamic load test, because only constant stressrange is assumed in the static load test. The differencemay result in fault to predict crack initiation points at thecentral part of the carbody.

3 ) S – N curves based on component fatigue test re-sults are well fitted with the failure results of the actualcarbody under dynamic load, when the joint details aresimilar.

References

( 1 ) Test Methods for Static Load of Body Structures ofRailway Rolling Stock, Jpn. Ind. Stand., JIS E7105,(1989).

( 2 ) Oomura, K., Okuno, S., Kawai, S., Masai, K. andKasai, Y., Fatigue Test of an Actual Car Body Structure(1st Report, The Testing Method and Its Accuracy),Trans. Jpn. Soc. Mech. Eng., (in Japanese), Vol.58,No.545, A (1992), pp.20–25.

( 3 ) Oomura, K., Okuno, S., Kawai, S., Masai, K. and


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Kasai, Y., Fatigue Test of an Actual Car Body Struc-ture (2nd Report, Investigation of Strength Evalua-tion Methods for Spot-Welded Joints), Trans. Jpn. Soc.Mech. Eng., (in Japanese), Vol.59, No.562, A (1993),pp.131–137.

( 4 ) Standard Specifications for Urban Transit Unit, Bull. ofMin. of Con. and Transp. of Kor. Govern., (in Korean),No.1998-53 (1998), pp.173–182.

( 5 ) Seo, S.I., Kim, J.T., Park, I.C., Lee, D.H. and Shin,D.S., Design of Aluminum Carbody and Developmentof Production Technologies of Large Aluminum Extru-sion Profiles for Rolling Stocks, J. Kor. Rail. Soc., (inKorean), Vol.2, No.1 (1999), pp.1–13.

( 6 ) Seo, S.I., Kim, J.T., Park, I.C., Lee, D.H. and Shin,D.S., Development of Construction and Painting Tech-nology for the Aluminum Carbody of Rolling Stock, J.Kor. Rail. Soc., (in Korean), Vol.2, No.2 (1999), pp.1–5.

( 7 ) Aluminum and Wrought Alloys for Rail Vehicles,Deut. Ind. Norm, DIN 5513, (1983).

( 8 ) European Recommendations for Aluminum AlloyStructures Fatigue Design, ECCS-Technical Commit-tee 2 Aluminum Alloy Structures, (1992).

( 9 ) Dimitris Kosteas, Design Recommendations for Fa-tigue Loaded Structures, Training in Aluminum Appli-cation Technologies, Tech. Univ. Mun., (1994).


fatigue strength evaluation of the aluminum carbody of

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