Yedek teker taşıyıcı sisteminin titreşim kaynaklı yorulma analizi
Vibration based fatigue analysis of spare wheel carrier system
Yazar: KADİR ORAY AKSOY
Danışman: PROF. ZAHİT MECİTOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Uçak Mühendisliği = Aircraft Engineering

Özet:
Günümüz otomotiv fimaları arasındaki rekabet günden güne artmaktadır. Artan rekabet firmaları daha güvenli daha ucuz ve daha ekonomik kulanımı olan araçlar üretmeye zorlamaktadır. Bu rekabet ortamında firmalar, istenilen araç özellik hedeflerini tutturabilmek için projelerin başlangıç safhalarında tasarlanan sistem ve komponentler dijital ortamda analiz edilip fiziksek olarak test edilirler. Bu testlerin en önemlilerinden birisi de uzun ömür yapısal dayanım araç testleridir
Bu çalışmada ağır ticari bir aracın uzun ömür yapısal dayanım araç yol testinde titreşim kaynaklı yorulma hasarına uğramış yedek teker ana taşıyıcı ana braketleri incelenmiştir.
Yedek teker ana taşıyıcı braketlerinin titreşim kaynaklı yorulma hasarını incelemek için aracın uzun ömür yapısal dayanım testinin yapıldığı yol güzergahlarının her birinden üç eksenli olarak ivme verileri toplanmıştır. Toplanan ivme verileri zaman ve frekans düzleminde incelenmiştir Yol yüklerinin 0-50 Hz frekans aralığında ve Z (düşey) ekseninde baskın olduğu gözlemlenmiştir.
Toplanan ivme verilerinin incelenmesinin ardından yedek teker taşıyıcı sisteminin titreşim kaynaklı yorulma analizi gerçekleştirilmiştir. Titreşim kaynaklı yorulma analizi için yedek teker taşıyıcı sisteminin ANSA yazılımı ile sonlu eleman modeli oluşturulmuştur. Model NASTRAN yazılımına alınarak serbest titreşim analizi yapılmıştır. Yedek teker taşıyıcı sisteminin doğal frekanslarının ve modları belirlenerek araç yol testinden toplanan ivme verilerinin frekans değerleri ile karşılaştırılmış ve sistemin ilk üç doğal frekansının yol yüklerinin etkinliğinin fazla oluğu 0-50 Hz frekans aralığında olduğu belirlenmiştir. Doğal frekans analizinden sonra NASTRAN yazılımı ile dinamik gerilme analizi gerçekleştirilmiştir. Dinamik gerilme analizi büyük kütle yaklaşımı kabulü ile mod süper-pozisyon yöntemi kullanılarak yapılmıştır. Dinamik gerilme analizinde lineer malzeme özellikleri kullanılmıştır. Yorulma analizi FORD Otomotiv A.Ş. tarafından geliştirilmiş FDYNAM yazılımı ile gerçekleştirilmiştir. Dinamik gerilme analizi sonucunda elde edilen asal gerilmeler Neuber gerilme düzeltme teorisi kullanılarak nonlineer gerilmelere dönüştürülmüştür. Elde edilen nonlineer gerilmeler Smith Watson Topper ortalama gerilme teorisi kullanılarak düzeltilmiş ve Miner birikimli hasar teorisi kullanılarak yedek teker ana taşıyıcı braketleri üzerindeki toplam hasar değerleri belirlenmiştir. Yedek teker ana taşıyıcı braketi üzerindeki hasar 1.59 değerinde ve araç testinde oluşan hasar ile aynı bölgede elde edilmiştir. Elde edilen sonuçların ardından tasarım iyileştirme çalışmaları yapılmış, farklı çözüm önerileri belirlenmiştir. Belirlenen çözüm önerilerinin arasından zaman, maliyet, ağırlık gibi kısıtları ihlal etmeden uygulanabilecek olan en uygun çözüm olarak yedek teker ana taşıyıcı braketlerinin kalınlığının arttırılması kararlaştırılmıştır. Yedek teker ana taşıyıcı braketinin kalınlığının arttırılması ile elde edilen yeni tasarımın titreşim
xx
kaynaklı yorulma analizi gerçekleştirilmiş. Yeni tasarım üzerinde çıkan hasar değerleri emniyet katsayısı olarak belirlenen 0.25 hasar değerinin altına indirilerek 0.20 hasar değeri elde edilmiştir. Elde edilen yeni tasarım yorulma hasarı oluşan araç ile aynı araç özellikleri taşıdığı kabul edilen farklı bir araç projesi için yapısal dayanım uzun ömür testi yapılacak olan araçta test edilerek tasarım doğrulanmıştır.

Abstract:
Competition in today's automotive industry forces the firms to produce cheaper, lighter and better quality vehicles. Automotive companies have been working to cover the most appropriate designs to compete with each other and to meet the customer expectations. The resulting designs are analyzed in digital format and tested after, to represent the usage conditions of customers. One of the most important expectancy for the vehicles produced to ensure the customer usage conditions is a successful complement of structural resistance testing procedures. In general, vehicles are designed to preserve their structural strength and to eliminate any potential functional problems throughout their useful lifetime.
Vehicles in automotive industry work in different circumstances throughout their useful lives. These conditions vary depending on the usage type, effective loads, local road type and driver profile. Structural parts on the vehicle are designed to fulfill their duties to be able to cover manufacturer's warranty period. Warranty periods are determined by the companies in accordance with their own usage strategies and these periods may differ according to type of the company and the vehicle. The warranty period for passenger and light commercial vehicles is around three hundred thousand kilometers, while it is nearly a million kilometers for heavy commercial vehicles.
Structural parts are damaged under a singular impact forces or cyclic loads throughout their lifetime. Structural damage which occurs due to the repeated loads on the parts is called fatigue damage. A structure becomes unable to fulfill its functionalities due to fatigue damage and potential dangerous consequences may occur as a result of life and property loss.
In this study, the vibration based fatigue damage occuring in a road test on the main supporting brackets of the built-hanging spare wheel carreir system for a heavy commercial vehicle is examined.
Importance of the fatigue in automotive structures are discussed in the first part of the study. Mainly, general information about the static and dynamic damage occuring in automotive structures is given and a literature survey is conducted regarding the type of damage.
Fatigue theory and analysis is explained in the second part of the study. General information about different fatigue life assumptions; stress-life, strain-life and crack propogation theories, is provided. Additionally, the required theoretical information for the planned vibration based fatigue analysis is given. This section explains basic concepts related to system dynamics, general stress calculations, a large mass approach and mode superposition method in detail.
xxii
In the third part of the study, information about the fatigue life in heavy commercial vehicles and fatigue damage types that heavy commercial vehicles experience are explained. Moreover, general information about the long life structural resistance road test procedure for a heavy commercial vehicle is given. In this respect, 6 different tracks which are used in long life structural resistance road test and road types in each track are explained in detail.
Vibration based fatigue analysis of the spare wheel carrier system is performed as a sample study in the fourth part of the current study. 3-axis acceleration data is collected via the road test which represents the real life usage conditions to evaluate the fatigue damage of the main supporting bracket in the spare wheel carrier system. It is found that the maximum effective loads are dominant in the Z-axis (vertical) direction and vary between the range of 0-50 Hz analyzing the collected acceleration data.
The acceleration data analyzed after the vehicle road test is used in the vibration based fatigue analysis. In the current study vibrational fatigue analysis is performed by using finite element method and ANSA package software is used for the modeling phase. For the finite element model, spare wheel carrier system and the chassis rail section where the system is connected are integrated. Supporting subsystem of the spare wheel carrier is meshed by using 'Quadratic' shell elements with an average size of 3 mm and the chassis rail is modelled by using 'Quadratic' shell elements with an average size of 5 mm. Furthermore, the spare wheel itself is also modelled with the finite element method because it is expected that the spare wheel has a direct effect on the results of finite element analysis due to its large mass, 135 kg, and high moment of inertia that results from the design's geometrical nature.
Vibration based fatigue analysis of the spare wheel carrier system modelled with finite element method is performed in 3 phases.
In the first phase, free vibration analysis is performed by using NASTRAN package software. It is accepted that due to the dynamic effects the system should be analyzed dynamically until 150 Hz, which is the 3 times of the road load bandwith. For this reason, all the natural frequencies of the spare wheel carrier system until 150 Hz are calculated. The natural frequencies of the spare wheel carrier system are determined as; 24.48 Hz, 26.71 Hz, 31.62 Hz, 111.39 Hz and 134.45 Hz. Additionally the data shows that the idea which predicts the possibility of resonance phenomenon and the necessity of analyzing the system dynamically is validated with the first 3 natural frequencies that overlaps the road data. By using the data, mode shapes of the spare wheel carrier system for each natural frequency is studied.
Dynamic stress analysis is performed in the second phase. The analysis is performed in NASTRAN package with a mode-superposition method by using the acceleration data obtained in vehicle road test and large mass theory. During the dynamic stress analysis, material properties are accepted as linear. Additionally, damping ratio which is a very important parameter for a dynamic stress analysis is set 3%. At the end of the dynamic stress analysis, linear stress values are obtained as an output.
Fatigue life of the structural parts on the spare wheel carrier system is calculated by using FDYNAM software, developed for fatigue analysis by Ford Company, in the third phase. The linear stress values obtained from dynamic stress analysis are converted to nonlinear values by using Neuber rule, corrected with Smith Watson
xxiii
Topper average stress correction method, and cumulative damage values of the output data is calculated by using Miner rule. The damage value on the spare wheel main supporting bracket is calculated as 1.59 as a result of the fatigue analysis. Moreover, the regions where the calculated peak damage value determined from the fatigue analysis and damaged in the vehicle road test are observed in the same locations. For this case, it is determined that there is a correlation between the outputs of the fatigue analysis and vehicle road test. Furthermore, for each track used in the vehicle road test, percentage effect on the calculated cumulative damage value for spare wheel main supporting bracket is determined.
After the results of the fatigue analysis are obtained and analyzed, failure reasons of the spare wheel main supporting bracket are invetsigated. Then, solution alternatives are discussed for the reasons within the existing constraints such as cost, time and weight. Without violating these constraints, best applicable solution through different alternatives is found as to increase the thickness of spare wheel main supporting bracket failed in the vehicle road test from 6 mm to 8 mm. However, it should be noted that 4.1 kg weight gain is introduced to the system with this modification.
In the next phase, vibration based fatigue analysis is performed on the new design updated with an increase of the spare wheel carrier bracket thickness from 6 mm to 8 mm to calculate the fatigue life. It is seen that the natural frequency values of the new design for the first 3 modes, which are working directly with the dominant road load between 0-50 Hz, increase when modal analysis that is the first phase of the vibration based fatigue analysis is examined. Additionally, principle stress range for the most effective road track on damage values is found to be decreased due to the increased mode values as a result of dynamic stress analysis. The damage value in the region where the failure occures in the road test decrease from 1.59 to 0.20 in the vibration based fatigue analysis. Furthermore, the result of the fatigue analysis for new design, 0.20, is found below the damage value, 0.25, which is determined as a safety factor.
It is quite necessary to validate the new designs obtained from the improvements as is done in all studies. Thus, for the current study, new design for spare wheel carrier system is accepted with a long life road test in a different project that represents the same structural properties with the vehicle used for old design. According to the results, the vehicle road test is completed without any failure on the spare wheel carrier system.
In the last section, the obtained results are evaluated and the potential future studies are explained.