Bir demiryolu yük vagonunun sayısal yöntemlerle seyir emniyeti ve yorulma dayanımının araştırılması
The numerical investigation of running safety and fatigue analysis of a freight wagon
Yazar: BERAT GÜRCAN ŞENTÜRK
Danışman: ÖĞR. GÖR. EMİN SÜNBÜLOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Katı Cisimlerin Mekaniği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Dinamik benzetim = Dynamic simulation ; Dinamik modelleme = Dynamic modelling ; Sonlu elemanlar analizi = Finite element analysis ; Sonlu elemanlar stres analizi = Finite elements stress analysis ; Yorulma dayanımı = Fatigue strength

Özet:
Yapmış olduğumuz bu çalışmada genel olarak, magnezyum karbonat (MgCO3) cevherini taşıyan yük vagonlarının dinamik testleri yanı sıra, sayısal yorulma analizleri yapılmıştır. Bu esnada analiz edilen modeller, vagonların gerçek ölçülerine uygun şekilde hazırlanan sonlu eleman modelleri ve çoklu cisim dinamiği (MBD) modelleridir.
Sayısal analizler yapılırken, vagonların gerçekte maruz kaldıkları dinamik zorlanmaların simule edilebilmesi amacıyla bir çoklu cisim dinamiği simülasyon programı olan Simpack yazılımı kullanılmıştır. Simpack, çoklu cisim sistemlerinin modellenmesini, sistemi oluşturan her bir cismin koordinat sistemlerinin ve cisimler arasında var olan kinematik kısıtların tanımlanabilmesini sağlayan bir, çoklu cisim dinamiği yazılımıdır. Özellikle demiryolu araçlarının analizlerinin kolaylıkla yapılabilmesi amacıyla, gerçeğe uygun değerlerde dever, eğim ve yatay kurp yarıçaplarına sahip hatların tanımlanabilmesine imkan vermektedir.  Daha sonra vagon bojileri ve vagon gövdesi bu hatlar üzerine yerleştirilerek kütle ve hız değerleri belirlenmekte, vagonlar üzerine yerleştirilen sanal sensörler vasıtasıyla da istenilen noktalardan kuvvet, ivme v.b. değerler okunabilmektedir. Böylece araçların maruz kaldıkları çalışma şartları simule edilerek zamana bağlı kuvvet, hız, ivme değişim değerleri elde edilebilmekte ve çeşitli özet grafikleri oluşturulabilmektedir. Elde edilen bu sonuçların,  uluslararası standart sürüş karakteristikleri v.b. konularda belirlenen limit değerleri arasında olmaları durumunda vagonların sürüş testlerinden başarıyla geçtiklerini söylemek mümkün olmaktadır. Bu yüzden gerek sonuçların değerlendirilmesinde, gerekse üzerinde test yapılan hatların tanımlanmasında ray standartları baz alınmıştır.
Vagonların statik yük analizlerinde, önceden tasarım aşamasında hazırlanan CAD modelleri kullanılarak yüzeyler üzerinde 2 boyutlu eleman ağları oluşturulmuş ve sonlu eleman modeli hazırlanmıştır. Bu aşamada  ağ oluşturmak amacıyla kullanılan program Hypermesh tir. 
Hazırlanan sonlu eleman modeli statik analizlerin yapılabilmesi amacıyla ABAQUS ortamına alınmaktadır. ABAQUS te yükleme durumlarına göre modele etkiyen statik yükler ve sınır koşulları tanımlanmış ve statik analizler sonucunda birim şekil değiştirme ve gerilme dağılımları gibi sonuçlar elde edilmiştir.
Simpack analizlerinden elde edilen zamanla değişen kuvvet verileri ile birlikte sonlu eleman modelinin statik analiz sonuçları, yorulma analizi programı olarak kullandığımız FeSafe için kullanılabilir girdiler oluşturmaktadırlar. Fe-Safe analizlerinde, statik analiz verileri ile birlikte Simpack sonuçları uyarı sinyalleri olarak kullanılarak yükleme senaryoları oluşturulmuş ve periyodik yükleme durumları sonucu yorulma meydana gelen kısımlar ve yorulma ömürleri hesaplanmıştır.

Abstract:
It is a well known fact that, freight wagons  have a large field of use in the transportation industry. For this reason,  new  types of railroad tracks and vehicles are being designed and produced for work.
 For a safe and efficienct use of this vehicles, several stationary tests are need to be done within the railway application standards, which are globally accepted by the responsible corporations.
Also fatigue life is another fact that should  be taken into account, because of the dynamic working conditions and unstable or moving loads which applied to the body of the vehicle. 
Nowadays, all the innovations and technical progress in the computer technologies has made it possible to make numerical analysis of models which have more complex geometric properties. Also with the help of software suites for finite element analysis and computer aided engineering,  quickly turning CAD models  into finite element models, helped a lot to prepare practical projects in the field of vehicle design. In addition to that, many of these projects are providing useful results for the maintanence and production processes.
Of course when compared to the experimental works which have done in the field, results of the computerized numerical methods can not give hundred percent correct but  in some means approximate values that, can be interpreted as an important fact according to the problem. 
This  project in the thesis, generates an obvious example for numerical method applications  in the field of railroad vehicle analysis. The main idea was to simulate the real working conditions of the vehicle and then, make a finite element analyse of the vehicle body under the forces which applied by the loads in the containers. By the help of the force values and accelerations that we obtained from the dynamic simulation and stress, strain values that we get from the FEM analysis, we have managed to make the fatigue analyse of the vehicle body part with the help of the obtained values.
The main model is a freight wagon, which carries magnesium carbonate (MgCO3) ore which usually called as magnesite. There are four bunkers on the vehicle body to carry the loads and the forces which applied to the body, are caused by the density of this material. It is important to make sure that the running safety criterias are ensured for all the loading types of the bunkers.  
The geometry of the model has been prepared with the CATIA software, according to the real dimensions of the vehicle body. After that, the CAD model has been imported into  SIMPACK, which is the multibody dynamics simulation program that we used to simulate the real conditions of the drive.
SIMPACK uses the physical relations between the bodies which forms our general  multibody system (in this case, the vehicle with the bogie structures). The joints and force elements like springs, vibration dampers and etc. provides the connections between multibody parts and general behaviour of these systems can be explained with the conventional mathematical equations which consists of the variables that symbolize the parts and connections between them. SIMPACK allows the user  to make any kind of adjustments in the coefficient values of the force elements and generally the mass and inertia values of the vehicle model. Also static equilibrium of the vehicle is an important case and can be tested with the changes load and preload values which applied on the force elements. 
So we had determined the center of gravity and inertia tensors of the vehicle and the loads inside the bunkers. Because of the general behaviour of the magnesite ore, we aassumed the loads as separate rigid bodies. We had calculated the volume and center of gravity of the bunkers with the CATIA software.    
When doing the simulations, we considered that the vehicle body can also  be assumed as a rigid object because, before the simulations, we had made a modal analysis test with the finite element model of the vehicle and seen that the natural frequency values are too high, so the model behaves nearly as a rigid body, in this case even if we consider the model as flexible during the dynamic simulations, that won't make any appreciable differences.  
To complete the simulation model preloads on the joints had been checked and also the coefficient values of the force elements been determined. Finally to measure the force and accelerations from the model we put some sensors on the vehicle body, which were on the side bearers, bogie wheelsets and pivot connection points.      
After forming the simulation model correctly, tests of running safety had been operated. The important parameters were the speed of the vehicle, radius of the track curves and the loading conditions. The vehicle had been tested for four different loading conditions, on eleven different tracks, all of them with excitation and no excitation conditions separately. Then the forces and acceleration values which determined by the sensors we set on the vehicle, running safety limits and assessment values that determined by the running behaviour and stationary test standards been checked.
The results had shown us that, even on the tracks which have distance excitations, the assessment values are between the safety limits so the vehicle doesn't lose its balance no matter what the circumstances are.
To have the fatigue analysis results of the vehicle body, we have made a static analyse with finite element model of the vehicle. To do that, the mesh model of the vehicle geometry had been prepared by using the HYPERMESH software. The mesh model indicates nearly one million two dimensional elements and the geometry of these elements are in the mixed type so there are both rectangular and triangle elements which are connected by the nodes and generates the mesh model. The both  static and modal analysis had been made by using the same mesh model, that we had import into the ABAQUS software. 
The main parameter that effects the loads which applied to the bunkers is the density of the material which carried by the vehicle. So we determined the stress values on each directions inside the inner surfaces of the bunker.
To do that, we had considered  the loads on each direction as a distributed load (like there is a concentrated force applied on the centre of the each element of the mesh  on the bunker surfaces). After that, determined the magnitudes of the loads like the way it has mentioned before, depends on the density of the carried material. Also we considered the loadings on each direction as a separate loading step.
Then the boundary conditions has been determined, in this case the boundaries were the connection points between the vehicle body and the bogies which we call pivots.
The most important thing is to specify the degrees of freedom for the boundaries correctly. This can change locations of the maximum stress points and others respectively.
For making the fatigue analyse, we have used the FE-SAFE software which puts together the finite element analysis results and the forces applied during the dynamic simulation process. 
For the fatigue analyse, we have made a long track simulation on the Krumovo Dimitrovgrad track which is about 70 kms long.
Then the static analysis results ( a file with .odb extension),  had imported inside the FE-SAFE software, also the force and acceleration values that we got from the Krumovo Dimitrovgrad track simulation, had been determined as a cyclic loading senario. Also the analyse had repeated with the distance excitations on the same track.
The fatigue analysis results showed us that, especially on the second test with excitations, on the critical points like; welded points, sharp corners on the bunkers, pivot points and etc. fatigue life declines obviously when compared to the other surfaces.