Ağır ticari araçların havalı süspansiyon sisteminde kullanılan boru denge çubuğu tasarımı
Design of hollow anti-roll bar for heavy duty vehicle air suspension systems
Yazar: NESLİHAN SAYILGAN
Danışman: PROF. DR. EKREM TÜFEKCİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Katı Cisimlerin Mekaniği Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Bu çalışmada, ağır ticari araçların havalı süspansiyon sistemlerinde kullanılan boru denge çubuğunun tasarımı, imalatı, yorulma ömür tespiti ve parça dayanım özelliklerinin belirlenmesi hedeflenmiştir. Boru denge çubuğunun burulma direnci mevcut tasarım olan dolu denge çubuğuna göre belirlenmiştir. Süspansiyon paket alanı ve denge çubuğunun diğer komponentlerle bağlantıları göz önünde bulundurularak boru denge çubuğunun tasarım kriterleri belirlenmiştir. Tasarım kriterleri belirlenen çubuğun ilk numuneleri imal edilerekz deney düzeneğinde test edilmiştir. Elde edilen test sonuçlarına göre yeni tasarımda iyileştirmelerin yapılmasına karar verilmiştir. Tüm bu hesaplar ve deneyler sonucunda yeni tasarım boru denge çubuğunun imalat kısıtları belirlenmiştir.
Taşıtın ömrüne karşılık gelen test parkuru seçilmiş, teker merkezinden kuvvet-moment ölçümleri ve denge çubuğu linki üzerinden gerinim ölçümü yapılmıştır. Süspansiyon kinematiği ve deplasman-yük değerleri araç üzerinden özel bir platformda ölçülmüş ve hızlandırılmış yol testinde toplanan datalar bu veriler ile birleştirilerek denge çubuğu linkine gelen yüklere dönüştürülmüştür. Denge çubuğunu şasiye bağlayan rigde kalibre edilmiş link üzerinden kuvvet datası toplamıştır. Parçaların ömür testlerini yapmak ve gerinim datası toplamak amacıyla laboratuvar ortamında parça için test düzeneği kuruluştur. Laboratuvar test düzeneğinde ise denge çubuğunda çeşitli lokasyonlara bağlanan strain gage ile gerinim datası toplanmıştır.Sonlu elemanlar analizi rig ile aynı koşulda yerdeğiştirme bazlı yapılmış olup, gerinim dataları analizin korelasyonu için kullanılmıştır. Test düzeneğinde denge çubukları araçta yapacağı en büyük yerdeğiştirme miktarına göre test edilmiştir. Test sonucunda elde edilen parça bazlı kırılma çevrim sayıları sonucunda malzemenin gerinim-ömür grafiğinden parçanın gerinim– ömür grafiği elde edilmiştir. Kurulan sonlu elemanlar modelinde toplanan ve araç süspansiyon kinematiğine göre dağıtılan yükler denge çubuğunun yük alma bölgesi olan göz bölgesine gelen yüklere dönüştürülmüş ve yükleme dataları uygun şekilde ayıklanmıştır. Denge çubuğunun gözüne gelen altı eksenli kuvvetlerin etkin olan yükleme yönleri tespit edilmiş ve sonlu elemanlar modelinde denge çubuğu gözünden birim yük olarak uyglanmıştır. Birim yüklü sonlu elemanlar analizi, yorulma analizi programında aynı yöne karşılık gelen yol data yükleri ile eşleştirilerek hasar bulunmuştur. Hasar analiz döngüsünde birim yüklü sonlu eleman modeli ve altı eksenli yol datası gerinim-çevrim eğrisine girdi olarak verilmiş ve karşılık olarak hasar sonuçlu sonlu eleman modeli çıktı olarak alınmıştır. Bu döngü sürecinde kullanılan yazılım öncelikle birim yüklü analiz sonuçlarını ayrı adımlarda almakta ve gelen yüklemeler ile eşleştirip vektörel çıktıları hesaplamaktadır. Hesaplama işlemi sonrasında rainflow çevrim sayma metodu ile sınıflandırıp tanımladığınız SN eğrisi üzerinden hesaplamalar yapmaktadır. Taşıtın ömrüne karşılık gelen hızlandırılmış yol parkurundan toplanan datalar ile parça dayanım testlerinde uygulanan datalar ömür olarak karşılaştırılmış ve parçaların komponent test çevrim sayıları hesaplanabilmiştir. Sonuç olarak, her iki tasarımın ömür karşılaştırılması yapılmış olup laboratuvar ortamında yapılan parça dayanım testi, araçtan toplanan datalar ve sonlu elemanlar metodu ile parça tasarımı tamamlanmıştır. Denge çubuklarının rig test ömrü belirlenmiş, yorulma ömür hesap metodolojisi tespit edilmiş ve parçanın dayanım kriterleri ortaya konulmuştur.

Abstract:
Fuel consumption is an important aspect for customer and vehicle manufacturers accordingly; therefore weight reduction plays a significant role in fuel economy to be competitive in heavy commercial vehicle market. Weight reduction can be achieved primarily by the introduction of better material, design optimization and better manufacturing processes. Anti-roll bar is a suspension part which transmits the loads from the outside to the inside wheel to minimize the body roll. ARB also reduces body roll which in result, enhances ride comfort and increases driving confidence. In this study, tubular type stabilizer bar designed for rear suspension on heavy-duty vehicles. Using hollow ARB has many advantages; the most important advantage is the weight reduction.
Surface defects such as roughness, penetration, decarburization, has negative impact on fatigue life of the components. Decarburization is a change in the structure and content of steel that some surface layers of steel and carbon are lost. Loss of carbon can make structural steel less stable. Decarburization normally takes place when steel goes through the heating process it was done in electrical resistance or natural gas heated furnaces where contain Oxygen resulting in loss of carbon and oxidation. ARB is generally manufactured with using hot bending or cold bending method.Decarburization occurs during hot bending process as a source of heating. This study investigates effects of decarburization on the heavy commercial vehicle hollow ARB fatigue life. Surface decarburization is reduces the durability fatigue of the anti-roll bars, so it is important that surface decarburization be at minimum level. Occurrence of decarburization primarily starts in the course of manufacturing raw material, rod or tube, surfaces produced with hot rolled method before heat treatment in the production of ARB.
Anti-roll bar is a crucial component in vehicle suspension; and it was chosen to perform the current study. Surface defects such as decarburization resulted in a reduction hardness in decarburization area. Experimental designs were performed to understand the relation between fatigue life and decarburization. In initial experiment, tubular black bars used on hot bending production oprations. Because of the high decarburization level on between shoulders and descaling on shoulder zones, observed breakage location of the fatigue test results of ARB was different from expected. Peeling operation was performed to decrease surface decarburization to improve fatigue behaviour of the part. According to statistical B10 life, 3,2 times life improvement has been obtained by peeling process. This study also proves that decarburization level and fatigue life has negative strong correlation. According to the test results, Hollow ARB was broken from shoulder, expected portion, and fatigue life increased. Hollow ARB was produced with peeling mehod and tested on rig test.
Hollow and solid ARB's fatigue life will be compared. In order to achieve this, at first a finite element model is constructed iteratively with using strain gauge measurements on rig test bench. This model and component S-N that will be found with performed tensile test, are used as an input for quasi-static fatigue analysis. Fatigue analysis will be correlated with rig test results. That correlated analysis is modified with multiaxial loading scenarios of proving ground. Stress combination methods are examined and the most suitable ones are selected for critical areas.
In the automotive industry, fatigue testing in a laboratory is defined as an accelerated test that is specifically designed to replicate fatigue damage and failure modes from proving grounds. The aim using proving ground is to increase the damage gathering by adding extreme events that would match the target usage mileage in short periods. To understand if the vehicle life cycle is equivalent or not, a customer clinic is performed that shows the customer usage and loading statistics. According to customer clinic results, the road load acquisition event is done on selected areas. For the same purpose, RLD collection is performed with the same instrumented vehicle in different proving ground events. Force and moment measurement, totally six different channels for every axle, are done from the wheel center from proving ground. After the comparison of damages, an equivalent event is produced for the vehicle. The total event has different loading paths and vehicle condition. During data acquisition phase, the wheels are equipped with wheel force transducers and critical parts are equipped with strain gauges. Strain gauge is implemented on anti-roll bar link and strain is collected from all different loads. Suspension geometry and displacement-load values are measured via a special platform that the vehicle mounted and the data acquired from durability test are converted to anti-roll bar link load after combining with the data gathered at the accelerated road test. Strain gages also implemented ARB that tested on test bench. Displacement is determined as maximum displacement that ARB exposes at the vehicle.
At first, in order to correlate the model, finite element model is constructed that simulates boundary condition of the vehicle and test rig for solid and hollow ARB. Boundary conditions of finite elements are simplified suitably and implemented to the model. ARB is modelled with hexahedral elements and the element size is decreased until get enough convergency. Finite element model is optimized with respect to the requested strains and measured strains. The other finite element model, which is constructed as an input for quasi-static fatigue analysis with proving ground data is "unit load analysis". This analysis consists of unit magnitude loads at each degree of freedom (DOF) and is used to determine the stress state of ARB for each DOF. At this type of analysis, unit loads are applied at all DOFs one by one and as a result, the stress state of the part is obtained at every DOF. Each step of analysis has to contain only one stress state as they will be matched with time dependent characteristics on post process. Empty analysis steps, where all the loads are deactivated, are constructed to be able to obtain a more stabilized analysis. For both designs, as hollow and solid ARB, these two finite element models are created.
Cyclic properties of materials are obtained from Ford material database. It should be noted that these material properties do not contain manufacturing effects as surface roughness, shot peening, cold or hot rolled. Therefore, at first these properties must be manipulated with respect to rig test results. This is achieved at rig test fatigue analysis and a component level S-N curve is obtained.
Proving ground data consist of different events with specific repeat numbers. By using these time series, a duty cycle schedule is built. During fatigue analyses, critical plane approach and absolute maximum principle methods are applied. At critical plane approach stress tensor is defined by the calculation of the most damaging plane simply by rotating the plane at each step and finding the most critical plane. Absolute maximum principle stress could be defined as the principle stress with the largest magnitude. Nonproportionality factor and biaxiality ratio is calculated and in the light these values the multiaxiality condition is checked. It is observed that non-proportionality factor, biaxiality ratio is between the thresholds, and therefore selected absolute maximum principle stress combination method is valid. While running whole model with relevant data, critical hot spot nodes are selected and compared. The results of most critical area presented as pseudo-damage.
As a result, two designs are compared with respect to pseudo-damage results and new design is assessed. Finite element correlation is performed with strain-gauge and vehicle durability test. FE models are optimized and verified for the future works. Correlated finite element model and damage analysis methodology is determined and used for test specifications of anti-roll bar.