Ağır ticari araç havalı süspansiyonunda kullanılan Z tipi makas tasarımı
Design of Z type leaf spring for heavy duty vehicle air suspension systems
Yazar: MAHMUT DURUŞ
Danışman: ÖĞR. GÖR. LEVENT KIRKAYAK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Bu çalışmada, havalı süspansiyon sistemlerine entegre edilerek kullanılan Z tipi makasların yorulma ömür tespiti ve parça dayanım özelliklerinin belirlenmesi hedeflenmiştir. Makasın tasarım yük kriterleri belirlenmiş ve parametreler üzerinden makas merkez - göz kalınlık ve parabolik eğri hesaplamaları yapılmıştır. Taşıtın ömrüne karşılık gelen test parkuru seçilmiş, teker merkezinden kuvvet-moment ölçümleri ve makas 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 makasa gelen yüklere dönüştürülmüştür. Toplanan yük dataları sonlu elemanlar analizinde yükleme olarak, gerinim dataları ise korelasyon için kullanılmıştır. Sonlu elemanlar sınır koşulları araçtaki duruma uyumlu şekilde basitleştirilmiş ve modele yansıtılmıştır. Sonlu elemanlar modeline uygun şekilde gerçek zamanlı bir parça test düzeneği oluşturulmuş ve seçilen farklı yükleme koşullarında parçalar test edilmiştir. Elde edilen bu parça bazlı kırılma bölgeleri ve çevrim sayıları sonucunda yorulma çarpanları bulunmuş ve malzemenin gerinim-ömür grafiğinden parçanın gerinim– ömür grafiği elde edilmiştir.Gerinim–ömür grafikleri parçanın hem sonsuz ömrünün bulunmasına hem de rig test hızlandırma çalışmalarında kullanılabilecek duruma gelmiştir. Toplanan ve araç süspansiyon kinematiğine göre dağıtılan yükler makasın yük alma bölgesi olan göz bölgesine gelen yüklere dönüştürülmüş ve yükleme dataları uygun şekilde ayıklanmıştır. Makas gözüne gelen altı eksenli kuvvetlerin etkin olan yükleme yönleri tespit edilmiş ve sonlu elemanlar modelinde makas gözünden birim yük olarak uyglanmıştır. Birim yüklü sonlu elemanlar analizi, aynı yöne karşılık gelen yol data yükleri ile eşleştirilerek hasar analizi yapılmıştır. 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 ve 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. Validasyon sürecinde araç testi de tamamlanmıştır. Araçtan çıkan parçalarda rigde yorulmaya devam edilmiş ve araç testi sonrası kalan ömrü tahmin edilebilmiştir.Sonuç olarak, tasarımı deplasman bazlı öngörüler ile hesaplanamayan havalı süspansiyon kullanımlı Z-tipi makasların rig test ömrü belirlenmiştir.

Abstract:
In this study, fatigue life calculation and system design specifications of Z_Type leaf spring that is integrated to air suspension system is aimed. Design and load criterion of leaf spring are designated. Design load of Z type leaf spring is determined by the geometric proportionality between air spring, axle and leaf spring eye center distances. Road load is directly distributed from axle center and then air spring and leaf spring will react from their loading points. Because of that type loading distribution, leaf spring loading condition could not be directly correlated by axle displacement so the durability criteria. Standard leaf spring test procedure applies maximum displacement which is the distance between metal-to-metal and rebound. Test cycle of the specified input is 100,000 cycle and it comes from the experience of the vehicle manufacturer. Even the standard test procedure was proven; test does not have a correlation directly to the every design. In addition to the vertical loading scenario, longitudinal loading scenario is also differs because of the air suspension system kinematic and compliance behaviours. Z type leaf springs have dominant loadings on both vertical and longitudinal direction so eye thickness calculation limits also differs. Leaf spring eye thickness and parabolic curve calculations are done without using current corrections and adapted to Z type leaf spring.
Proving ground test area corresponding to lifetime of the vehicle is selected. 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.  Force and moment measurement, totally six different channels for every axle, are done from the wheel center from proving ground and selected customer usage areas. 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 Z type leaf spring 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 leaf spring load after combining with the data gathered at the accelerated road test. 
Finite element model is constructed that simulates boundary condition of the vehicle and test rig. Boundary conditions of finite elements are simplified suitably and implemented to the model. Leaf spring is modelled with hexahedral elements and the element size is decreased until get enough convergency. After pre-process phase, two different correlations are done to understand the accuracy. First of all, leaf spring eye displacement was measured via test rig and same boundary conditions are applied to finite element model. The difference between the results can be corrected by changing elastic modulus at a certain level otherwise the model should be updated. 
Mathematical model correlation can be done with the leaf spring rate test results. Standard solutions for leaf springs should be updated and vertical rate should be optimized. The difference for the mathematical model is the leaf spring geometry. Z type leaf spring behave as half spring because air spring mounting section has a short length and do not have a parabolic section. Mathematical model was updated as a half spring and suitable vertical rate factor was found. 
The collected strain data are used for finite element analysis correlation. The spring eye load that has time series data is applied on finite element analysis by using SN glyph. Collected strain data are translated to stress values and then same data are applied on SN analysis. SN Analysis glyph shows finite element and collected data damage results of the whole proving ground cycle combinations. According to the results, the difference of the damages is %14.4 and the difference can be useful to understand for finite element correlation sensitivity. Both data have time series output, by comparing the results the correlation resolution for six different loading can be meaningful.  
In accordance with the finite elements model, a real time component test rig is generated and the parts are tested in selected different vertical load conditions. Six test are conducted for every specified vertical load. The load has minimum and maximum force, the minimum shows unladen condition and maximum shows laden condition of the vehicle. Obtained results of the rig tests are fracture area for FE correlation and cycle quantity for SN curve. The failure cycles of the test is changed to one cycle by statistical calculation B10 life analysis. Fatigue constants are gained so SN curve of the component is achieved. Stress-life curve enables to reach the endurance limit so 1,000,000 cycle is selected as endurance limit by the intersecting point of the slope. SN curve also provide the data that can be used for accelerated rig test studies.
Loads collected and distributed according to vehicle suspension geometry are converted to the loads reaching to eye area which is the loading area of the leaf spring. The active load directions of six-axes loads reaching to eye area of leaf spring are determined and converted to meaningful data by the use of signal processing methods. Load data selected at performed method are used directly to SN curve calculation and multiplied the durability vehicle test cycle. The result of the first step is time based stress history and the history is used for absolute maximum stress calculation. Rainflow cycle counting method is needed to simplify time series data for SN damage analysis. Same calculation is done again to take into account the multiaxiality assessment. Calculation results are checked and iteration number is defined. Damage result is under the limit of one so this means the part can complete one durability cycle. 
The durability test vehicle is equipped with the design phase leaf springs and durability test is completed without failure of leaf springs. The durability result shows the damage result should below the limit, one, however even the accuracy of the analysis cannot be calculated by the outputs. To understand the analysis accuracy, the parts that completed the durability tests are repeated the rig laboratory test. Four leaf spring are tested with the same input loads and results are used to understand the remaining life of the leaf springs.The remaining life is an aspect to understand the accuracy of SN and finite element model. The accuracy of durability model is % 14.7.
As a result, rig test life of air suspension is used both the material curve calculation and remaining life analysis after durability test. Finite element correlation is performed with strain-gauge and vehicle durability test. Correlated finite element model and damage analysis methodology is determined and used for test specifications of Z-type leaf spring whose design is not calculated by displacement basis estimations.