Test rigi ve yol koşulları kullanılarak arka aks kovanı yorulma davranışının incelenmesi
Investigation of rear axle housing fatigue behavior using test rig and road conditions
Yazar: KEMAL ANIL ZEREN
Danışman: PROF. DR. İSMAİL MURAT EREKE
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Akma gerilmesi = Yield stress ; Kırılma dayanımı = Fracture strength ; Mekanik dayanım = Mechanical strength ; Yorulma dayanımı = Fatigue strength ; Çekme dayanım testi = Tensile strength test ; Çekme dayanımı = Tensile strength ; Çekme gerilme testi = Tensile stress test

Özet:
Yeni parça tasarımları yapılırken parçanın çalışma koşulları belirlenip üretilen parçalar test edilerek dayanımları ölçülür. İlgili test prosedürleri genel yükleme koşullarına bağlı olarak belirlenir. Günümüzde farklı çalışmalar göz önünde bulundurularak zorlayıcı koşullar incelenip test prosedürleri sürekli güncellenmelidir. Bu doğrultuda sahadaki araçlardan veriler alınarak, test prosedüründeki yüklerle karşılaştırılmalıdır. Yapılan karşılaştırma sonucunda büyük farklılıklar görülürse, zorlayıcı koşullar için test prosedürleri güncellenip yeni parça tasarımları buna göre yapılmalıdır. Parça tasarımı yapılırken parçanın hangi alanda kullanılacağı önceden belirlenerek, yükleme koşulları bu şartlar altında hesaplanmalıdır. Çalışma alanının ve yükleme test koşullarının belirlenmesi, yorulma kriterlerinin tespiti açısından oldukça önemlidir. Belirlenen yorulma koşulunu sağlayamamış olan parçalar için tasarım süreci en baştan başlatılır. Başa alınan tasarım süreçleri de ekonomik açıdan büyük kayıplara yol açmakla birlikte iş/zaman israfına neden olmaktadır. Verimli bir tasarım süreci için tasarım aşamasındaki test ve doğrulama işlemleri oldukça önemlidir.
Kamyon özelinde incelendiğinde çalışma koşullarından (yükleme ve yol şartları) en çok etkilenen parçalardan biri de aks kovanıdır. Yük aktarım parçalarının çoğu, aks kovanı üzerine bağlandığı için pek çok farklı dinamik yüklemelere maruz kalır. Farklılaşan çevre koşulları nedeniyle en çok hata gözlemlediğimiz parçalardan biri de aks kovanıdır.
Yaptığımız çalışmada temel amaç; genel test prosedürü uygulanan aks kovanının saha çalışmalarında neden kırıldığını belirleyerek yeni bir test yöntemi ortaya koymaktır. Kırılma nedenini belirlemek için test ortamından ve saha çalışmalarından toplanan veriler incelenip, iki senaryo arasındaki korelasyon hesaplanacaktır. Çalışmamızın ilk aşamasında, test riginden toplanan veriler baz alınarak aks kovanın üzerindeki birim yük analiziyle gerilme vektörlerinin dağılımı incelenmiştir. Değerlendirmeler sonucunda strain-gage lokasyonları belirlenmiştir. Test rigi pistonlarının üzerinde kuvvet sensörleri yer aldığından, kovan davranışını değerlendirmek için sadece kovanın üzerine strain gage yapıştırmak yeterli olacaktır. 
Bir sonraki adımda test rigindeki benzer çalışmalardan elde edilen veriler kullanılarak FEM analizinde korelasyon çalışmaları yapılacaktır. Strain gageden alınan veriler n-Code aracılığıyla işlenerek maksimum principal gerilme verisine dönüştürülmüştür. Kovan üzerine test rigi ile aynı kuvvetler uygulanarak stain-gage lokasyonundaki gerilme değerleri karşılaştırılmış ve doğrulanan model sonucunda yorulma analizi tamamlanmıştır. Korelasyon işlemi sonucunda, test riginden alınan kuvvet değerleri ve çevrim sayıları birim yük analizi ile süper pozisyon yöntemi uygulanarak yorulma analizi koşturulmuştur. Çıkan yorulma sonuçları, test rigindeki sonuçlar ile karşılaştırıldığında kırılma lokasyonlarının aynı bölgelerde olduğu gözlemlenmiştir.
Çalışmamızın ikinci aşamasında, yol testi için FEM ortamında sistemin tamamı modellenmiştir. Aks kovanı üzerinden alınan strain gage verileri n-Code programında işlenerek gerilme değerlerine çevrilmiştir. Veri işleme sonucunda elde edilen gerilme değerleri kuvvet verileri ile karşılaştırılıp, eş zamanlı kuvvet ve gerilme değerleri belirlenmiştir. Belirlenen kuvvetler, sistem modeline uygulanarak aks kovanındaki strain gage lokasyonlarının gerilme değerleriyle karşılaştırılmıştır. Karşılaştırma sonucunda, gerilme değerlerinin test ortamındakiyle benzer olduğu görülmüştür. Her eksen için birim yük uygulaması yapılıp, kuvvet zaman eğrileriyle süper pozisyon yöntemi kullanılarak yorulma analizleri tamamlanmıştır. Ek olarak her bir eksen için ayrı ayrı birim yük ve kuvvet-zaman eğrisi çerçevesinde yorulma analizleri gerçekleştirilmiştir. Yapılan yorulma analizleri sonucunda her eksendeki yükün yorulmaya ne kadar etki ettiği tespit edilmiştir.
Çalışmamızın sonucunda yol testi ile test rigi arasında karşılaştırma yapılmıştır. Yapılan karşılaştırmada yol testinin yorulma değerlerinin, test rigi koşullarındaki yorulma değerlerinden daha yüksek olduğu tespit edilmiştir. Eksenlerin yorulmaya etkisi değerlendirilerek bu farkın sebebi belirlenip, çıkan sonuçlar neticesinde yeni bir test rigi yöntemi geliştirilmiştir.
Yapılan çalışma sayesinde, yeni aks kovanlarının tasarım aşamasında kullanılmak üzere yol testi ve test rigi korelasyonları elde edilmiştir. Sonraki çalışmalarda yapılacak olan yeni tasarımların, teste sokulması gerekmeden FEM ortamında yorulma analizlerinin yapılabilmesi sağlanmıştır.

Abstract:
During the designing of the new component, the working conditions of the components are designated and the components are tested in terms of their durability. Predetermined test procedures are defined in relation with the general loading conditions. Nevertheless, the terms of use of the vehicles produced today show many differences and the predetermined tests are inadequate to give permission for durability. Trucks are one of the vehicles, which show a great variety in terms of their usage area. Even though trucks are used in many different areas, such as; road, tow and worksite, they are subject to different loading conditions in their use in similar areas. When we consider worksite trucks, their fields of work (mine, construction etc.) show difference in both the road condition and the loading conditions. That is why; making the trucks' durability tests according to a general procedure may cause a variety of problems. The most common problem faced related to the components that are designed is crossing boundaries of the loading conditions, which were defined in the vehicle's use in its field of work.
Considering the recent studies and investigation of the compelling conditions, test procedures should be constantly updated. In this direction, data taken from the vehicles in the field should be compared to the loads in the test procedure. If major differences are observed in the results of that comparison, test procedures must be updated for the compelling conditions and the designs of the new components should be done according to that. When the designs of the new components are being made, the field in which the component will be used should be determined and loading conditions should be calculated under these circumstances. Determining the field of study and loading test conditions are extremely important in terms of detection of fatigue criteria. For the components, which have not achieved the specified fatigue condition, the process of designing starts from the beginning. Processes started from the beginning not only lead to huge losses economically but also cause a waste of work/time. Test and verification operations at the design phase are crucial factors for an efficient designing process.         
One of the most affected components from working conditions (loading and road conditions), when examined in a truck-specific environment, is the axle housing. Many of the load-bearing components are subjected to many different dynamic loads as they are connected to the axle housing. One of the components, that we observe the most flaw due to differentiated environmental conditions, is the axle housing.
The main purpose in our study is to identify why the axle housing, on which the general test procedures was applied, gets broken in field studies. To identify the reason for that, the data collected from the test environment and field studies should be examined and the correlation of these two scenarios must be found out. With our study, we aim to put forth the optimum design and ensure that the new axle housings will not be damaged similarly in field study. 
When the studies are examined, it is observed that the axle housing is arranged according to the previously determined traditional test procedure. However, this test procedure is made with a loading type, independent from the working conditions in the field, and uniaxial . For this reason it is unknown whether it covers today's field conditions or not. In previous studies conducted on the test rig, only the refractive counts are determined and no data have been received from the housing. Failure to collect data in the test rig results in unpredictable behavior of the housing under test rig loading.    
It is not possible to make valid fatigue analysis since it is not possible to collect data from the field in current FEM analyses. For this reason, the loading conditions from the test rig environment are taken into account. In these analyses, which are done statistically, the maximum forces in test rig are chosen. It is expected to see that the stress results are improved when the new design's stress results are compared with the former design's results. 
With the differentiated worksite/field conditions, the variety and number of errors have been increased. Among the new worksites, the one with most errors is observed to be the 3rd airport's worksite. To build the test procedure as inclusive as possible, we have taken the 3rd airport's road conditions as a reference in our studies. While the new test procedure is constructed, data will be collected from the test rig firstly, later on, these data will be compared with the 3rd airport's road conditions in FEM environment while their correlations are found out. After the examination of the results, a new test procedure will be determined and a new design for the axle housing will be made. When the tests of the new design are completed in the test rig, according to the results, a final decision will be made about the design of the new axle housing.
We have dealt with our work in 3 different stages in order to determine the test procedures:
Since there is no data collected from the test rig, it is unknown what kind of a reaction would the housing give to the force defined in the test rig. At first, related data was collected from the similar studies done earlier. In the light of these data, strain-gage locations were determined by examining the distribution of stress vectors with unit load analysis on axle housing. Because the force sensors are located on the test rig pistons, it will only be possible to evaluate housing behavior by sticking/gluing  strain-gage on the housing.
Correlation studies were done in FEM analyses with the data collected from similar studies. The data taken from the strain gage were processed by the n-Code and were converted to maximum principal stress. The same forces were applied on the housing and the stresses at the strain-gage location were compared. As a result of the study, %6 tolerance level and stress results were validated. According to the validated model, fatigue analysis was done. The force values taken from the test rig (cycle counts) were carried out by unit analysis and the application of the superposition method. The results from fatigue analysis and test rig were compared. It was observed that the breaking locations were in the same area.
Previously, the axle housing did not have any road data. Therefore, the road tests were completed for the determination of the dynamic loads on the housing. Instrumentation was done on the axle housing and the related components. A force measure was hanged on to the pivot and the forces coming from the road were calculated. At the same time, at the verification stage strain-gages were glued to the suspension and chassis ports of the axle housing. With this method, the modelling of the axle housing's behavior under road loadings was done properly. At the next step, displacement meter was placed on the axle housing to identify suspension stiffness values. To calculate the amount of road the vehicle has taken, a gps was assembled on the vehicle.

For the road test, full system model was built in FEM environment. The strain gage data received from the axle housing were processed in n-code program and converted to stress values. Stress values taken from the results of data processing were compared to force data, depending on time and simultaneous force and stress values were identified. Identified forces were applied to the system model and compared to the stress values of the strain gage locations of the axle housing. It was observed that the stress values were similar with the values at the test environment as a result of the comparison. Unit load application was done for each axis and fatigue analysis was done with force-time curves and by using superposition method. In addition, fatigue analyses were performed by evaluating each axis within the framework of the unit load and force-time curve. Thus, how effective are the loads in each axis to fatigue was detected.  
As a result of the fatigue analyses, there was a comparison done between the road test and the rig test. In the comparison, it was detected that the fatigue values of the road test was higher than the ones under test rig conditions. The effect of the axes on fatigue was investigated and the reason for this diversity was discovered. A new test rig method was developed according to the result.
A new axle housing design was made for current working conditions, according to the correlation identified. Some improvements were made on the critical areas of the new axle housing and fatigue analyses were done. The newly designed housing was tested in both updated test rig and FEM environment. After that testing, the new axle housing was evaluated in road tests in the work field (3rd airport) where the most errors were observed previously. As a result of the tests, it was seen that the new axle housing fulfills the predetermined fatigue conditions.
With the help of the studies, it was possible to get correlations of road and test rig to use it during the designing of the new axle housing. It was made possible to not test the new designs, which will be done in the subsequent studies, in FEM environment and still make fatigue analyses. It became possible to check whether the new component produced will fulfill the durability conditions which were identified during the designing process. Not only this situation has provided us a significant gain in time, but also it helped us to make more reliable designs, in terms of production of the components. At the same time, it can also be eliminated with the decreasing number of physical tests and even the increasing number of correlations.