Kamyon kabin giriş basamağının gerilme analizi ve optimizasyonu
Stress analysis and optimizations on truck cab entrance steps
Yazar: ALPER AYKUT
Danışman: PROF. DR. HİKMET KOCABAŞ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Konstrüksiyon Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Yeryüzünde kara yolu ile yük taşımacılığının en önemli unsuru olan kamyonlar, değişen trendler ve güncellenen yönetmelikler ile birlikte günümüz tasarımına ulaşmışlardır. Kamyonlarda sürücü kabinleri, yaygın olarak yer seviyesinden merdiven ile çıkılabilecek yüksekliktedir. Bu sebeple ihtiyaç duyulan merdivenler için ülkemizde de kabul edilen 130/2012/EU direktifleri, kamyon basamaklarının tasarımında sınırlayıcı rol oynamaktadır. Tasarlanan basamak sistemlerinin sürücü ve yolcu kullanımına uygun, belirli yükleme koşullarına karşı dayanıklı ve mühendislik çözümleri açısından ucuz olması istenmektedir. 
Bu çalışmada, geliştirilen yeni basamak sistemi için deney standı oluşturulmuş ve basamaklar üzerinde bir noktaya kuvvet uygulanarak sistemin dayanımı incelenmiştir. Uygulanan kuvvetin basamak siteminin ana parçası olan SMC gövde üzerindeki belirli bir bölgede deformasyon yarattığı tespit edilmiştir. Deneyin ardından, basamak sisteminin FEM analizi için modellemesi yapılarak basamağın aynı yükleme şartı altındaki davranışı incelenmiştir. Çözüm ağı modellemesinde kullanılan ağ tipi 10 noldu tedrahedron olup analiz NX Nastran programı ile çözdürülmüştür. Elde edilen sonuçlarda en yüksek gerilmenin, deney sonucu deforme olan bölge ile aynı yerde ortaya çıktığı görülmüş ve malzemenin akma gerilme sınırının üzerinde seyrettiği tespit edilmiştir. Deney ve analiz sonuçlarından yola çıkılarak merdiven sistemleri için ucuz ve hafif bir malzeme olan SMC üzerinde bir güçlendirme yapılmasına karar verilmiştir. SMC üzerinde yapılan güçlendirme işlemi sonrası karoseri bağlantı braketlerinden L-Braket üzerindeki gerilmelerin arttığı görülmüş ve bu tez çalışmasının ikinci ana öğesi olarak bu parça üzerinde de güçlendirme yapılmasına karar verilmiştir. SMC gövde üzerinde feder güçlendirmesi, çelik braket üzerinde ise feder güçlendirmesi ile birlikte kabartma güçlendirmeleri, parça kalınlığı değişiklikleri ve şekilsel optimizasyon adımları aşamalı olarak uygulanmıştır. 
Yapılan iyileştirmeler sonucunda, SMC parça ile L-Braket için ayrı çıktılar elde edilmiştir. SMC parçalar için, daha düşük maliyetli olacak şekilde kalıp değişikliği yapılarak ekstra yeni bir parça oluşturmaksızın parça üzerinde bir feder tasarımı ile güçlendirme sağlanabildiği anlaşılmıştır. Feder kalınlığının gerilmeler üzerinde etkili olduğu anlaşılmış, şekilsel optimizasyon ile malzeme tasarrufu sağlanarak gerilmelerin istenen seviyede control edilebileceği görülmüştür. Çelik braketlerde ise feder ve kabartma tasarımlarının gerilmeler üzerine etkisi incelenmiş, malzeme değişikliğine gidilmeden L-Braket gibi küçük parçalarda büküm yeri düzenlemesi, parça kalınlığı değişikliği ve bağlantı noktalarında pul kullanılmasının gerilmeler üzerinde önemli etkileri olduğu tespit edilmiştir.
İleriki çalışmalarda, kamyon basamaklarında özellikle SMC parçalar ile çelikten mamül bağlantı braketlerinin bir arada montajlandığı sistemler için, bu çalışmada geliştirilen model ve optimizasyon tekniklerinin uygulanabilmesi amaçlanmıştır.

Abstract:
The trucks, which are the most important element of road haulage, have reached today's design with changing trends and updated regulations. In trucks, driver cabins are commonly located high enough to reach from the ground level by stairs. For this reason, the 130/2012 / EU directives, which are adopted in our country for the stairs needed, play a limiting role in the design of truck steps. Designed step systems should have proper position for driver and passenger use, resistant to certain loading conditions and desirable to be cheap in terms of engineering solutions. 
In this study, an experimental stand was built for the new step system. The strength of the system was examined by applying a force to a certain point on the steps. It has been found that the applied force causes deformation in a certain region on the SMC body, which is the main component of the step system. After the experiment, the behavior of the step under the same loading condition was investigated by modeling the step system for FEM analysis. "Tetrahedron with 10 nods" is used as mesh type and FEM model was solved with the analysis in NX Nastran software. The results showed that the highest stress occurred at the same place as the deformed region and it was found that these stresses were above the yield stress limit of the material. Based on the results of the experiment and analysis, it was decided to make a reinforcement on SMC which is a cheap and light material for truck entrance systems. 
In literature studies, studies on reinforcement methods in plastic-based materials and steel brackets have been examined. Firstly, for rib design on SMC component, studies on sink marks of ribs and the effect of rib parameters on tension were examined among the studies in literature. As a result of this, it was found that the fiber arrangement used in the production phase of SMC components also had effects on strength. In addition, reinforcement applications related to steel brackets have been investigated and the points related to how the dimple reinforcements affect the strength of parts in steel brackets have been considered.
It is aimed to guide the study by investigating the literature in order to determine the design limits and mechanical properties of SMC components and steel brackets, which are the main elements of the study. Production methods for SMC components, constituents of material, the use of SMC technique in the automotive field and researches on the superiority of SMC method over alternative production techniques have been emphasized. For the L-Bracket, which is the other main element of the work, material types and properties, molding and forming methods, mechanical properties and information of DC04 steel used as material for the bracket to be optimized were examined. 
On SMC component, It was decided to make a rib reinforcement behind the the region where deformation occurred and the stresses were high. In this study, linear rib design and curved rib design are examined under two main opitmization titles. For the initial optimization, it was preferred that the designed rib should have a thickness of 3 mm and a radius of 1.5 mm and that it would advance until the connection point of the component behind the deformed region. In the numerical analysis performed after the first optimization, a significant decrease was observed in the stresses and stresses were found to be 8% below the material yield stress limit. A second optimization was decided to bring the stresses closer to the yield stress limit and to save material. In this optimization, the thickness of the same rib design was reduced to 2 mm and the radius was reduced to 1.00 mm. In the numerical analysis, it was found that the stress in the region reached a value less than 3.8% below the material yield stress limit. It was decided to perform a third optimization in order to further increase the material savings and to examine the effect of formal optimization on stress. In this optimization, the length of the designed rib extending to the connection point was cut in half and decided to be angled to the ground. As a result of this formal optimization, numerical analysis showed that the stress reached a value of 2.5% below the yield stress limit of the material. Moreover, it was understood that the stresses could be kept under control with formal optimization. 
In the curved rib design, nine different thicknesses were tested by keeping the draft angle as 3°, radius for outer edge of the rib as 0.25 mm and ground radius of rib as 1.5 mm. Thickness of thinnest edge of the rib increased from 0.5 mm to 4.5 mm in each optimization step by 0.5 mm. As a result of examining the effect on the stresses only by changing the thickness, the ninth optimization design with 4.5 mm thickness was accepted as the most suitable design because of its advantage in weight compared to the optimization in linear rib design.
It was found that the stresses on the steel L-Bracket, which is the other main element of this study, increased after optimization of the SMC body component. In order to reduce the high stresses at the connection points, it has been decided to apply dimple reinforcement at all connection points and to add a rib reinforcement at bending axis. In the numerical analysis after the first optimization , it was observed that the stresses were displaced on the part. Stresses were decreased at the connection points and contrary to expectations, increased at the rib positioned at the bending axis. It was thought that the rib design applied to the bending axis had a negative effect due to its proximity to the connection point. Therefore, the rib was removed in the second optimization. In following numerical analysis, it was determined that the tension at the bending axis shifted to the connection point and increased greatly. It is thought that the rib reinforcements do not have a positive effect on the small parts. Contrary to expectations, they have a negative effect on the connection areas close to the bending axis. Therefore, in the third optimization it was decided to remove all rib reinforcements. In the third optimization, the thickness of the part was also increased by 0.5 mm and the part was extended 1.5 mm from all sides. Furthermore, the connection point of the part to the rod support member was moved to the other side so that the bending area was moved away from the connection points. In the numerical analysis performed after this optimization , it was found that all stresses decreased significantly. It was decided to use washers in order to further reduce the stresses at all connection points since the stress at one of the connection points remained at the yield stress limit of DC04 steel. Numerical analysis after the addition of washer showed that the stresses on the part decreased dramatically and the highest stress was found to be 33% below the yield stress limit.
In this thesis, a part produced by the SMC technique and the carrier system that connects this part to the body in white are examined and the cause of the deformation under certain loading conditions is investigated. The point of occurrence of the deformation was based on experimental data and validated by numerical analysis. Based on the results of the analysis, design improvements were made for strengthening in the problem area and the results obtained by numerical analysis were evaluated.
As a result of the improvements , separate outputs for SMC component and DC04 steel were obtained It was seen that SMC parts can be improved with rib design by making low cost modifications on the mold and without creating new additional parts costs. It was found that the thickness of the rib was effective on the stress besides, the material could be saved with the formal arrangement and the stresses could be controlled at the desired level. In steel brackets, the effect of rib and dimple designs on stresses was investigated. And It has been found that design change of the bending axis in small parts such as the L-Bracket, the thickness of the parts and using washers at the connection points have significant positive and negative effects on stresses.
In future studies, model and optimization techniques developed in this study can be applied in truck entrance steps, especially for systems where SMC components and steel brackets used for connection to cab in white are assembled together.