Üç tekerlekli elektrikli skuter motosiklet şasisinin mukavemet ve yorulma dayanımı değerlendirmesi
Strength and fatigue durability assessment of a three wheel electric scooter chassis
Yazar: LEVENT ÇELİK
Danışman: DOÇ. DR. MESUT KIRCA
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Katı Cisimlerin Mekaniği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Günümüzdeki araç tercihleri CO2 emisyonu, yakıt ekonomisi, kaynakların verimli kullanılması gibi faktörlerle birlikte elektrikli araçlara doğru bir kayma eğilimi göstermektedir. Bu eğilim kendini binek araçlar, skuterler, motosikletler, ticari araçlar gibi pek çok alanda göstermektedir. Diğer araç tiplerinde olduğu gibi, motosiklet ve skuter tarzı araçlar da bu eğilimden etkilenmektedir.
Elektrikli araç tasarımlarında dikkat edilen ön önemli unsurlardan bir tanesi araç ağırlığıdır. Günümüzdeki elektrikli araçların en büyük handikaplarından biri olan menzil problemini en aza indirmek için mümkün olan en hafif tasarımın yapılması elzemdir. Bu durumun da etkisi ile, elektrikli araçlardaki taşıyıcı sistem – şasi tasarımlarında mümkün olduğunca hafif bir tasarıma gidilmek istenir. Bu hafifletme eğilimi, taşıyıcı sistem dayanımı açısından da sınırların zorlanması anlamına gelmektedir. Tekrarlı yol yüklerine maruz kalacak olan taşıyıcı sistemin dayanım hesaplarının doğru yapılabilmesi, mümkün olduğunca hafif bir tasarım yapılmasına olanak sağlayacağı için son derece  kritik bir öneme sahiptir.
Bu yüksek lisans tez çalışmasında, bir 3 tekerlekli skuter motosikletin taşıyıcı sisteminin yol yükleri altındaki yorulma dayanımı incelenmiştir. Üretilen ilk deneme aracında önce sonlu elemanlar yöntemiyle statik analizler yapılmış ve yapının gerilme dağılımı incelenmiştir. Analiz sonuçları kullanılarak, yol testinde araçtan toplanacak birim uzama verileri için uzama ölçer lokasyonları belirlenmiştir. Gerekli enstrümantasyon yapıldıktan sonra yol testi gerçekleştirilmiştir. Yol testi sonrasında, araç üzerinde modal test için ivmeölçer enstrümantasyonu yapılmış ve bir modal test gerçekleştirilmiştir ve aracın modal parametreleri çıkartılmıştır. Bu testler tamamlandıktan sonra, araç tasarımını yönlendiren ilgili ekipler tarafından aracın ilk prototip seviyesindeki konsept tasarımı yapılmıştır. Daha önce elde edilen yol testi verileri kullanılarak, prototip araç SE modeli ile statik analizler yapılarak araç araç şasisinin tasarımı dayanım açısından iyileştirilmiştir. Sonrasında, iyileştirilmiş şasi tasarımı ile prototip araç üretilmiştir.
Üretilen prototip araçla yol testleri tekrarlanmış, bu araçtan güncel ivme ve birim uzama verileri toplanmıştır. Araç üzerinden toplanan ivme verileri kullanılarak, test pistindeki yolun gerçek müşteri kullanımındaki yolla yorulma ömrü açısından farkı değerlendirilmiştir. Bu değerlendirme sonucunda test pistindeki 12.000 kilometrelik kullanımın, araç ömrü hedefi olan 150.000 kilometrelik müşteri kullanımına karşılık geleceği tariflenmiştir. 
Aracın dinamik korelasyonundaki zorluklar düşünülerek, yorulma analizlerinde kullanılmak üzere statik bir SEM modeli kurulmuştur. Yol testinde 5 farklı lokasyondan toplanan birim uzama verileri, aynı lokasyonlarda benzer birim uzama çıktısı verecek bir statik SEM analiz modelinin kurulması için kullanılmıştır. Hem SEM analizi sonuçları hem de yoldan toplanan birim uzama verileri kullanılarak ayrı ayrı gerçekleştirilen yorulma hasar analizleriyle birlikte, SEM analizindeki bir yük çevriminden elde edilen hasarın, test yolundaki sürüş mesafesi karşılığı elde edilmiştir. Sonuç olarak, araç ömür hedefi 150.000 kilometrelik müşteri kullanımına karşılık gelecek, 883.000 çevrimlik bir statik yükleme koşulu tariflenmiştir. 883.000 çevrim olarak tariflenen yorulma ömür kriterine göre, FEMFAT yazılımı kullanılarak detaylı yorulma analizleri yapılmış, yorulma açısından en kritik bölgeler belirlenmiş ve bir tasarım önerisi yapılmıştır. Sonraki adım olarak, yorulma analizinin doğrulanabilmesi için bir yorulma test riginin detayları tariflenmiştir ve çalışma sonlandırılmıştır.

Abstract:
Today's vehicle preferences has a tendency towards electric vehicles with factors such as CO2 emissions, fuel economy and efficient use of natural resources. This trend shows itself in many areas such as passenger cars, scooters, motorcycles, commercial vehicles. Motorcycle and scooter style vehicles are also affected by this trend as much as other vehicle types.
One of the most important parameters in electric vehicle designs is the weight of the vehicle. In order to minimize the vehicle range problem, which is one of the biggest handicaps of today's electric vehicles, it is crucial to make the lightest possible design. With the influence of this range problem, it is desired to go as light as possible in the carrier system – chassis designs in electric vehicles.  Need of the lighter design means pushing the boundaries in terms of the strength of the carrier system. Making accurate strength and durability calculations of the carrier system under the influence of cyclic road loads is extremely critical to make the design as light as possible.
Recent durability studies at the literature about scooter and motorcycle carrier systems generally focused on fatigue durability under the effects of road loads. Field data acquision works for gaining the road load data is present at nearly all of the scooter and motorcycle carrier system durability studies. Also FEM and multi body dynamic analyses studies are widely used for lowering the amount of road load test data requirements. Apart from these studies, test rig setups and fatigue life test applications with cyclic loadings are present at the literature. The most critical part of these kind of studies are the correlation between the test data and analytical models to make the durability assessments.
This postgraduate thesis study contains the strength and fatigue durability studies for a three wheel electric scooter chassis. Since studying about this kind of vehicles is unfamilliar with the company that I am working for, a temporary three cycle electric scooter vehicle which is  called "trial vehicle" is built. The built quality of this vehicle is far from ideal. Some parts of the vehicle is spare parts of the vehicles on the market, rubber bushings are used instead of  ball bearings for lots of rotating connectors. Purpose of the trial vehicle is getting used to the concept of the three cycled scooter, electrification and making any possible test that could be beneficial. Using the trial vehicle design, static FEM analyses are made to understand the concept of the vehicle and determining the potential strain gauge positions for the road load test. Stress distribution at the chassis and tensoral stress resutlts were reviewed for strain gauge locations. Accelerometers were also used at the road test with the trial vehicle. Locations of the accelerometers are ends of all suspensions, swing arm brackets, and two location at the chassis for having an idea about the acceleration levels at the vehicle chassis. Road load test was made at the road test track inside the company's facilities. Both acceleration and strain results were checked. Maximum stress values at two strain gauge locations were exceeding chassis materials yield stress limit. For the final vehicle design, much more durable vehicle chassis is a must. Maximum acceleration values were checked at the chassis and with multiplying the safety factor of 1.25, maximum acceleration value of 5.6 g at Z direction was noted. This maximum acceleration value would be used at improving the prototype vehicles chassis design in terms of durability. After the road load test, a modal test was performed to check the modal dynamic parameters of the vehicle and compared it with the FEM modal analyses results.
After the studies on the trial vehicle, concept of the prototype vehicle was decided by the design teams. The prototype vehicle would have much more refined design in terms of durability, built quality, design metrics etc. Chassis of the prototype vehicle is made of steel profiles and sheet metals, just like the trial vehicle but more durable at first glance. Although the prototype vehicle's chassis was much better than the trial vehicle's, a durability study was needed for this chassi design. At this study, maximum chassis Z direction acceleration value, 5.6g is used at the statical FEM analyses to check the stress distribution and maximum stress levels on chassis. At these analyses, vehicle is fully loaded with 120 kg load mass at the rear of the chassis. The stress target of these analyses were chassis material's yield strength. After making many analyses and design proposals, some metal profiles were added to chassis and some thickness changes were made at existing profiles and sheet metals. At the final prototype chassis design, some locations at the chassis still had higher stress values than the material's yield point, but the prototype vehicle was built with that design to prevent possible overengineering issues.
After the built of the protype vehicle, the road tests were made with this vehicle to check the acceleration and strain data. These data were compared with the trial vehicle's road test data and the result was just as expected. The maximum acceleration value at Z direction is marginally lower, but the strain – stress values are much lower than the trial vehicle's. At first glance, is was clear that that design is a massive improvement in terms of durebility. After the acquisition of the road load data fot the new prototype vehicle, the fatigue durability studies were started. Using the acceleration data from the suspensions lower ends, near the wheels, fatigue life correlation between the road test track and roads that real customers use  was performed. For this study, a particular literature review was made because we did not have any customer road load data. Acceleration RMS data near vehicle wheels were used at this study. As a result, 12,000 km of road test track mileage correspons 150,000 km of customer mileage in terms of fatigue durability. Keep in mind that the vehicle's life was determined as 150,000 km at first by the design team.
Considering the possible issues about the dynamic correlation of the vehicle with analytical models, static FEM analysis based fatigue study is performed. There were 5 different strain gauge locations at the prototype vehicle's road load tests. These strain gauge's data were reviewed with rainflow cycle counting (RFCC) method. Cycle counts for particular mean strain and strain amplitude values were extracted using RFCC. A static fatigue analysis model was prepared to obtain same levels of strain at those exact 5 points at FEM analyses results. The loads at that model is applied at front wheel and rear wheels. After many iterations, two different load cases were prepared for obtaining strain amplitude and mean strain values. Loads for obtaining same strain amplitude are 1000 N at front wheel at X direction and 1200 N each at rear wheels at Z direction. For mean stress, 250 N at front wheel at X direction is applied. Boundary conditions at both analyses are same; Y and Z direction displacements are fixed, X direction displacement and rotational DOFs are fixed. After determining the loads of static FEM model, fatigue analysis with this FEM model with 1 load cycle was made using these mean and amplitude loads. Fatigue damage results at the strain gauge locations were noted. With road load strain data at these strain gauge locations, fatigue analyses were performed using the strain data directly. These fatigue analyses results gave us the amount of fatigue damage occurred during the road tests at particular strain gauge locations. After comparing the fatigue damage results from the road load test data and static FEM 1 load cycle, correlation was made. 530 meters of test track milage have same amount of fatigue damage with 39 load cycles with defined amplitude and mean loads. With combining these results and previous correlation study, fatigue life criteria is set. 883,000 cycles of loads with the defined mean and amplitude forces is the target fatigue life criteria for our chassis. 883,000 cycles of loads correspond 150,000 km mileage of a real customer in terms of fatigue durability.
Using determined life criteria, fatigue analysis was made with static FEM model. The results of the fatigue analysis shows that our chassis has less life than the target. A small design proposal is made and the fatigue analysis was also made with the proposal design and minimum fatigue life at vehicle chassis is increased from 24,000 km to 60,800 km. The results of the datigue analyses indicate that chassis design needs to be updated. But, as next step of this postgraduate thesis study, before the chassis design update, a fatigue test bench should be prepared with determined loads and boundary conditions to correlate the fatigue analyses results with the real life fatigue damage. In addition to that,  data acquisition  from a potential customer using the vehicle in real life would be beneficial to the study, since the correlation between real life and test track mileage correlation would be much more precise.