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İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Anabilim Dalı / Konstrüksiyon Bilim Dalı

Bir ticari taşıtın makas gözü bağlantı parçasının optimizasyonu ve yapısal analizler ile tasarımın doğrulanması

Leaf spring eye bracket optimization of a commercial vehicle and verification of design by structural analyses

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Özet:

Günümüzde bilgisayar programlarının gelişmesi ile birlikte parça veya sistem bazlı testlerde meydana gelebilecek problemlerin sanal ortamda yapılan analizler ile önceden tespit edilip tasarım değişimi yapılması mümkündür. Otomotiv sektöründe ise sanal ortamda yapılan analizler projelerin ilk safhasında yer almaktadır. Araçlarda meydana gelen yükler zamana bağlı oldukça değişken ve farklı yönlerde olabilmektedir. Bu yüzden ilgili araçlarda sanal analizler öncesi gelen yüklerin bilinmesi analizler sonrası çıkan sonuçların doğruluğu açısından oldukça önemlidir. Bu süreç, sanal ortamda yapılan CAE analizlerinin doğruluğunun arttırılabilmesi amacıyla prototip bir araç toplanıp, araç tekerlerine yerleştirilen kuvvet ölçerler ile araç tekerlerinden yük verileri toplanması ile gerçekleşmektedir. Bu veriler ile yine sanal ortamda tüm araç modeli oluşturularak araç şasisi ve gövdesi üzerinde ilgili noktalara gelecek yüklerin tayinini mümkün kılmaktadır. CAE analizleri ile aracın yapısal problemlerinin önceden tespit edilmesinin yanı sıra parçalar üzerinde optimizasyon analizi ile araç ağırlığının azaltılması da mümkündür. Bu çalışmada, Ford Transit modelinin mevcut makas gözü bağlantı parçasında optimizasyon çalışması yapılarak parçanın ağırlığının azaltılması ve bu arada ağırlık azaltılmış parçanın ilgili yapısal dayanım kriterlerin sağlaması amaçlanmıştır. Optimizasyon süreci için akış şeması oluşturularak daha hafif parça tasarımı oluşturuldu. Akış şemasının ilk adımında topoloji optimizasyonu ile parça hafifletilmesi gerçekleştirildi. Optimizasyon sonucuna istinaden üç boyutlu tasarım oluşturuldu. Bir sonraki adımda ise oluşan tasarımın dayanım kriterlerini sağlayıp sağlamadığı kontrol edildi. İlk olarak yer değiştirme analizi yapılarak optimizasyon sonucunda oluşan tasarımın mevcut tasarıma göre yer değiştirme analizi sonuçları kıyaslanıldı. Tasarımın ilgili yer değiştirme kriterlerini sağlayıp sağlayamadığı kontrol edildi. Eğer tasarım yer değiştirme kriterlerini sağlanmadığında optimizasyon modeli güncellenerek yeni tasarım oluşturuldu. Oluşan yeni tasarım ile birlikte yer değiştirme kriteri sağlanmış oldu. Tasarım doğrulama adımlarından ikincisi olan yorulma analizi ile tasarım kontrol edilerek yorulma kriteri de sağlanmış oldu. Yorulma analizi ile tasarımın araç ömrü süresince maruz kaldığı kuvvet verileri yardımı ile araç ömrü süresince tasarımda herhangi bir kırılma sorununun meydana gelip gelmeyeceği öngörülmüş oldu. Ancak, oluşturulan tasarımda dayanım kriterleri açısından istenilen seviyede olmasına rağmen mevcut tasarıma oranla az bir hafifleme elde edildi. Tasarımda istenilen ağırlık değerini sağlamak adına yorulma sonuçları ile birlikte tasarımda sağlam bölgelerinden yerel malzeme çıkarılarak tekrar üç boyutlu tasarım oluşturuldu. Sonrasında, Bu tasarımın yer değiştirme analizi ile tekrar sertlik değerlerine bakıldı. Parçanın ağırlığının azalmasına rağmen sertlik değerleri istenilen seviyede kaldı. Ağırlığı azalmış bu parça yorulma analizi kriterini de başarılı bir şekilde geçti. Böylelikle, optimizasyon ve doğrulama süreci tamamlanmış oldu.

Summary:

Nowadays, it is possible along with the development of computer programs to detect early the problems that may occur in component or system based tests by analyzing in virtual environment and possible to change design. In automotive sector, analyses done in virtual environment takes place in first project phase. Loads occurring in vehicles are time dependent, quite variable and can be different directions. For this reason, it is so important to know the loads in related vehicles before virtual analyses in terms of accuracy of results after analyzing. This process come true by building prototype vehicle and collecting load data on vehicle wheels by locating force transducers on the purpose of increase the accuracy of CAE analyses. With these data, it is possible to obtain loads in related nodes on vehicle chassis and body by creating full vehicle model in again virtual environment. Besides detecting vehicle structural problems early by CAE analyses, it is also possible to reduce weight of vehicle by the optimization analysis on the components. In this study, It is aimed to reduce the weight of part by optimization study on the current leaf spring bracket of Ford Transit model and in meantime, the structural criteria is aimed to achieve by weight reduced part. The flow chart is created for designing the lighter design of leaf spring optimization bracket. Optimization study carried out at the first stage of designing lighter design. Creation of three-dimensional geometry continues after the optimization study. The next stage goes on with the verification of the model. Firstly, displacement analysis check is finished. Secondly, fatigue analysis check is done. If all the structural design criteria satisfies, additional weight reduction opportunity is examined by changing the geometry by filling the weak spots on part with material and removing strong spots on part with material. Finally, optimization study is completed by the enough reduction in weight. Optimization definitions such as design responses, constraints and the design goal should be identified well. Volume fracture function was defined as the design constraint in order to control the reduction of weight. The restrictions comes from the manufacturing such as the cast type, minimum element dimension and symmetrical conditions was studied as well. Cast type is the one of the most important parameter that highly influence on the quality of optimization result. Three cast type is available in optimization software. One of them, no cast type definition. The second one is the single type and the last one is the split type cast. All cast type definition was created to see the effect of all. When no cast type parameter was applied to the optimization model, non-feasible geometry was found after post-processing the optimization analysis. The second one called single cast type was applied to optimization model. The output geometry was very complex. Therefore, the design is not applicable in reality. Finally, split cast type was created to the model. The related geometry was fine to manufacture, so split cast type parameter constraint was given to optimization model. The effect of minimum element dimension was investigated by changing minimum element dimension value which is starting 10 mm up to 50 mm with every 10 mm increment. The optimum minimum element dimension was chosen as 20 mm. The third manufacturing constraint is the symmetry. The influence of symmetry condition was examined in xy, yz and xz planes. Because of unsymmetrical connection of leaf spring eye bracket, the symmetry condition was satisfied in almos none of planes. Manufacturing restrictions affect the optimization results highly, so they should be taken into account. The design goal function was selected as minimum compliance to achieve more stiff design by the optimization analysis. For getting lighter design, it is important point to select maximum available design space of leaf spring eye bracket. The more design space, the more usage of material within the design space will be achieved. After optimization analysis was finished, three-dimensional design was drawn based on the geometrical shape of optimization result. At the end of stage of getting the new leaf spring eye bracket geometry by three-dimensional drawing, displacement and fatigue analyses were done as structural analyses. For displacement analysis, the displacement limits of leaf spring eye bracket corresponding to unit loads in vehicle three translational directions were defined as the same as the displacement limits of current leaf spring eye bracket corresponding to unit loads in vehicle three translational directions or a little less than the current design to get design, that is slightly more rigid. It is important to reach almost the same stiffness value of new leaf spring eye bracket design with the current one. Because the change of stiffness of the leaf spring eye bracket can affect the vehicle dynamic behavior. For fatigue evaluation, the nonlinear analysis was performed in two stages. The first stage is applying clamping forces to the bolts and the second one is the including unit force steps. After analyses were completed, the stress responds according to unit forces in vehicle translational directions were calculated. The stress results come from static analysis were then to be matched with proper load history channel so that fatigue solver can calculate each stress in time and finally the cumulative fatigue result can be viewed by using the parameters both with the fatigue property of material and the stress history. The fatigue criteria was described by the comparing the fatigue results of the current and the optimized leaf spring bracket design. It is desired to have higher life value than the current one and new design should have the life value higher than one in order to make sure that the failure will not happen in physical test. New leaf spring eye bracket geometry was checked by the displacement analysis at first if the displacement criterion is satisfied or not because it does not require much effort to generate displacement analysis model compared to the fatigue analysis model. If the model pass the displacement analysis criterion, fatigue criterion will be checked. After the new design was provided in accordance with structural analysis criteria, the reduction of weight was evaluated and additional weight opportunity was checked. There is not defined weight goal for the new design comes from the optimization result. The design of part was updated constantly. After repetitions, it was aimed to achieve the final design that meet the structural analyses criteria and the lightest. The total three design was evaluated and it was called as design 1, design 2 and design 3. Design 1 did not meet the displacement criterion and it was not feasible by the structural perspective. Design 2 satisfied both structural criteria but there is not significant weight reduction achievement. Design 3 is not only meet the structural design target but also have the lightest weight. Therefore, design 3 was selected as the optimum design among the three designs.