Yüksek basınçlı sistemlerde civata üzerindeki gerilmenin burç boyuna göre analizi ve optimum burç boyunun tespit edilmesi
Analyze stress on the bolt in high pressure systems according to bushing size and decide the right bushing size for the system
Yazar: SİNEM ÇEVİKALP
Danışman: YRD. DOÇ. DR. VEDAT TEMİZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Konstrüksiyon Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Civatalar, mekanik parçaların montajında oldukça yaygın kullanılan elemanlardır. Birbirinden farklı malzemeler için kullanılabilme özelliği, tamirat sırasında tekrar tekrar sökülüp takılma kolaylığı ve diğer bağlama methodlarına göre daha ucuz olması nedeniyle en çok tercih edilen bağlama elemanlarıdır. Karmaşık tasarımlar için kullanılabilme kolaylığı ve farklı malzemeleri montajlama gibi özellikleri sayesinde mühendislere kolaylıklar sağlayarak, tasarım olanaklarını artırır.
Birçok uygulamada, cıvataların ömrü işletme sisteminin ömrünü etkiler. Bu sebepten dolayı civatalar, sistemeler için çok önemli ve üzerlerinde birçok çalışma yapılan ve yapılacak olan elemanlardır.
Civatalar kendi içlerinde çok karmaşık tasarıma sahiptir. Geometrisinin yapısı, civata ve somun dişleri arasındaki temas yüzeyleri, civatanın üzerindeki ön yükleme, flanşlar ile civata başının ve somunun temas yüzeyleri, civata ve flanş deliklerinin temas yüzeyleri gibi birçok etken civatalı bağlantılarda önemli rol oynamaktadır. Günümüzün teknolojisi ve sayısal yetenekleri sayesesinde sonlu elemanlar yöntemi ile bu karmaşık fiziksel olayı en gerçekçi birşekilde yansıtılabilmesi, uygun malzemeler, eleman tipleri ve ağ (mesh) yapıları ile oluşturulan bir model ile mümkün kılınabilinmektedir. Eğer sistem hiç basitleştirilmeyip, bütün karmaşıklığı ile yansıtılmaya çalışılırsa, uygun sürede sonuçlar alınıp tasarım sürecinin gerektirdiği birçok değişik tasarım konfigürasyonları yapılarak, uygun tasarım seçilemez. Bu karmaşık sistem, işlem süresini kısaltmak için doğru sonuçlar elde edilecek şekilde basitleştirilmelidir.
Bu tezde güdülen amaç civata bağlantılı sistemlerde civatalardan kayanaklanan hataları en aza indirmek amacı ile civatalar üzerinde meydana gelen gerilmeleri, flanş tasarımını değiştirmemek kaydı ile çeşitli tasarımlar deneyerek, düşürmektir.
Bu tezde civata bağlantılı modeli oluşturmak için SIEMENS NX 9 yazılımı kullanılmıştır ve daha sonra model ANSYS WORKBENCH kullanılarak analiz edilmiştir.
3 boyutlu model oluşturulduktan sonra sonlu elamanlar yöntemi ile gerçek fiziksel koşulların oluşturulmasına çalışılmıştır.
Çalışma standart bir baz modeli geliştirmek amacı ile sürdürülmüş ve baz model DIN2628 ve flanş yapısına ait civatalar M20'dir.
Baz modelin sonlu elemanlar modeli oluşturulduktan sonra yüksek basınçlı işletme sistemi simule edilmiştir. Bu koşullar altında civata üzerinde yüksek gerilmeler görülmüş ve civata akmıştır.
Sisteme çeşitli boylarda burçlar eklenip civata üzerinde görülen gerilmenin düşürülmesi ve sistemin başarı ile çalışması hedeflenmiştir.
Bunun yanında sisteme eklenen burçlar hem sistemin ağırlığını hem de sistemin fiyatını artıracağı düşünülerek ikinci bir öneri olarak da civata başına kanal açarak civatanın verilen sistemde sağlıklı şekilde çalışması amaçlanmıştır.

Abstract:
Bolts are commonly used in assemblies of structural parts. Combining different materials, ease of disassembly for repair, relative low cost to other joining method properties are reasons for their preference. On the other hand, ability to use in complex structural parts and combining different materials enable greater design possibilities and because of these properties, the bolts provide great convenience for engineers.
Bolts can fail from different locations, but there are three critical locations, which are fillet under the bolt head, beginning of thread on the bolt shank and the first thread of contact threads between nut and bolt.  
Bolts life directly effects the life of the system, which the bolts are used in many applications. Because of these reasons, bolts are the most important part for the applications, and there are so many studies related to them and there will be. 
There are some ways, which can be followed to increase the bolt life. Joint stiffness factor can be improved. For this purpose, bolt stiffness can be minimized and clamped material stiffness can be maximized. Stress distribution can be improved. In addition, stress concentration factor in the bolt can be minimized.
Bolts design are complex when we investigate it in their own environment. Many factors play significant roles as design of the bolts geometry, the contact surfaces between threads of bolts and nuts, preload on the bolt, contact surface between flange and head of bolt, contact surface between flange and nut, contact surface between flange holes and bolt. With our recent technology finite element method allows us to simulate the bolts as it is working in a real application. Finite element method only works perfectly with convenient material, type of element and the construction of the mesh. In finite element methods, so many engineers try to simplify the system because of avoiding the time consumption. If they are modelled the system with whole complex features, the response computation time for only one simulation will be increased dramatically. That is why engineers renounce small details and so they can try different design options to find the best design.
This thesis presents the development of a finite element method for modeling bolted joints in structures, which have high pressure. This thesis aims to decrease the bolt failure because of high stress with trying different techniques on system at the same time without changing the flange design. 
In the history of engineering, maybe the hardest part is hand calculation of the products before produce the final design and after that compare the calculations and real outputs. Because product does not give the same output with the hand calculation every time. In that situation, engineer has to recalculate or double check everything on paper after that they have to produce the product again. It will continue till, output error can be acceptable. After the computer revolution, this type of paper work has transferred to the computer programs and the computer programs have started to calculate everything with acceptable errors. This case gave very flexible calculations to engineers, because after changing any part of the product, engineer does not have to calculate everything. Computer can do it easily with just changing some values on their design. Also with the 3D computer design opportunity, engineers have gained so many time to study different design options before produce the final products. With this time saving everything can be smaller, smarter and compact nowadays. 
3D model can be created by many different computer software and also analyzed. So many 3D design computer software compete each other and most of them working good enough to simulate real environment. In this thesis "SIEMENS NX 9" was used as a 3D model development tool and after modeling the 3D design "ANSYS WORKBENCH" software was used to analyze the model.
Bolt was modeled without concerning the geometry of the thread and so bolt shank is modeled as cylindrical sections. With this modeling method, plastic and elastic deformations can be read correctly and this bolt modelling method allows to avoid unnecessary details on the bolt and also is reduced degrees of the freedom of bolted joint system.
After the 3D model has created, finite element method was used to simulate real environment conditions. 
In this thesis study the standard bolted joint model was modeled as a base model. The base model consists of DIN2628 flange and M20 bolt and M20 nut. Besides bushings are used, to reach the aim of this thesis. 
DIN2628 flange, M20 nut, M20 bolt and bushing with various length were modelled in 3D computer software.
Axi-symmetric designs can be modeled as a sector to decrease the computation time. Base model was developed as 45° sector to decrease the computation time. When 3D model was modeled, 3D model split into many bodies to have good quality mesh conditions. Flange split two bodies, bolt split seven bodies which are more than flange body number.
After that they were assembled together in assembly tool. Later they have transferred to the analysis software and then model was simplified to avoid the time waste and meshed.
Mesh convergence is used to find good enough mesh size. Smaller mesh size always gives more realistic results. But engineers must optimize the mesh size, because if the mesh size is getting so smaller then computation time will be enormously increased. That is why, convenient mesh size is selected to decrease the computation time. Decreasing computation time will allow to try various design options. Mesh of the bolt was tried to make finer, because interested body was bold and stress will be read on the surface of the bolt. Flange mesh is rough relatively to bolt mesh. Having right results on the model is related to mesh quality. Mesh convergence method is used on the base model to find right mesh size. 7 different mesh size studied on the base model and one of them is selected as a mesh size.
Cyclic symmetry was performed to base model which is modeled as 90° sector. So with this command, 90° sector was completed to 360° whole model.
Contacts were defined for the model to reach realistic results. Right contact types and surfaces are also so important for taking right results from the model. Frictional contacts were used on between bolt head and flange, between two flanges, between nut and flange between bushing and flange surface and between bushing and nut.
Two steps were performed to simulate real environment conditions. At first step, preload was performed on the bolt and there was no pressure on the system, but at the second step, the bolt was locked and pressure was performed to flange. Fixed support was used to fix the model.
After waiting the computation time while computer was calculating, outputs were taken from the program. For the base model the maximum stress was seen on the fillet which is under the bolt head. It was about 3.54e8 Pascal, and that time our aim was to decrease the maximum stress on the fillet.
For the aim of the thesis, study has started with assembly an addition different length bushing to that system. The calculation for the bolt, nut, and flange and bushing system was performed with 10mm, 20mm, 30mm, 40mm and 50mm of the bushing sizes. The outputs of the calculations have shown us that the maximum stress on the bolt decreased with increasing the bushing size. After adding a 10 mm bushing to the system, maximum stress on the bolt was decreased to 3.45e8 Pascal and it means that the 2.5 percentage decreased when it is compared to the base model. Then, with added 20 mm bushing maximum stress is decreased to 3.35e8 Pascal, 30 mm bushing decreased to 3.30e8 Pascal, 40 mm bushing decreased to 3.20e8 Pascal and finally 50 mm bushing size decreased the maximum stress on the bolt to 2.99e8 Pascal and this final decreased almost 16% less when it is compared to the base model. This dramatic decrease on the stress showed us adding a bushing on system can affect the systems reliability. 
On the other hand, after adding the bushing on the system, in this thesis undercut technique was tried to decrease maximum stress under the bolt head with thinking that adding volume or mass to system is not convenient every time. With this second method for the stress decreasing multiple dimensions of undercut were studied. This trials started with 1mm diameter undercut and finalized with 6 mm diameter with 1mm interval. For the first try, 1 mm diameter undercut applied to the bolt head and maximum stress increased from 3.54e8 Pascal to 5.92e8 Pascal which means that 67 percentage increased when it is compared to base model. Then trials continued with 2 mm and the maximum stress increased as far as the base model, but at that time it decreased the stress comparing to the previous 1 mm trial with 4.05e8 Pascal. After that 3mm undercut applied to the bolt head and output was 3.68e8 Pascal. Then when the 4mm undercut was performed finally the output was less than base model with 3.51e8 Pascal, which means that approximately 1 percentage decreased. With 5 mm diameter undercut decreasing stress on bolt continued, the output of the new system decreased almost 10 percentage compared to the base model with 3.19e8 Pascal. After that, 6mm undercut has tried however output was not seeming good, maximum stress on the bolt increased enormously. The result of analysis of 6 mm was 6.33e8 Pascal and it meant that stress increased by 80 percent compared to the base model. 
To sum up, adding bushing to the system and undercut under the bolt head decrease the maximum stress on the bolt. Adding bushing to the system is more effective than undercut method in terms of decreasing maximum stress. Undercut method decreased the maximum stress by 10 percent with the best its modification, at the same time adding 50 mm length bushing decreased the maximum stress by 15 percent.