Palmgren-mıner Yöntemi İle Semı-swath Tipi Alüminyum Bir Teknenin Yorulma Analizi

Abstract

Yorulma analizleri, günümüz gemi inşaatı mühendisliği temel hesaplamaları arasında önemi hızla artan analizlerin başında yer almaktadır. Yorulma analizleri ile ilgili hesaplamaların gerçekleştirilmesinin en önemli nedeni, meydana gelen can ve mal kayıplarının kayda değer biçimde artıyor olmasıydı. Yorulmadan kaynaklanan hasarların yeni imal edilmiş olan gemilerde de sıklıkla görülüyor oluşu, özellikle armatörlerden gelen talepler üzerine gemi inşaatı mühendislerinin konu ile çok daha fazla ilgilenmelerini gerektirdi. Bu gerekliliklerin neticesinde yorulma analizleri, artık tüm klas kuruluşları tarafından talep edilen temel mühendislik hesaplamaları arasında yerini almıştır. Yorulma hesaplamaları gerçekleştirilirken, tekne bünyesinde meydana gelen gerilme yoğunluğu bölgelerinin (hot spot) analiz edilebilmesi ve çevrim sayılarının belirlenebilmesi en önemli iki basamağı oluşturmaktadır. Çevrim sayılarının belirlenebilmesi, tamamiyle dalga spektrumlarına bağlı olmakla birlikte, her deniz karekteristiğinde farklı ömür sürelerinin oluşmalarına neden olmaktadır. Teknenin global olarak analizleri gerçekleştirildiğinde yorulmadan kaynaklanan çatlakların ve gerilme yığılmalarının en fazla olduğu bölgelerin teknenin yapısında bulunan süreksiszliklerden kaynaklandığı anlaşılmaktadır. Genel olarak gerilme yığılmaları, kaynak geçiş cugullarının bulunduğu bölgelerde, braketlerin T profillere ya da hollanda profillerine bağlandığı noktalarda meydana gelmektedir. Bu çalışma dahilinde sonlu elemanlar analiz yönteminin seçilmesindeki en önemli amaç, gerilme yığılmalarının olduğu bölgeleri tespit edebilmekti. Sonlu elemanlar analiz program seçilirken, sadece gemi inşaatı mühendisliği hesaplamaları ile ilgili olduğu için Maestro program seçilmiştir. Maestro program kullanılarak dalga spektrumlarının tanımlanabilmesi kolaylıkla sağlanabilmiş ve aynı zamanda tekneye farklı açılarla gelen dalgaların yaratmış olduğu gerilmeler de ayrı ayrı analiz edilerek sonuçlar bu doğrultuda çok daha geniş bir spektrumda analiz edilebilmiştir. Analizi gerçekleştirilecek olan tekne semi-swath tipinde bir sahil güvenlik gemisi olduğundan, çok daha kötü hava koşullarında görevini sağlıklı bir biçimde sürdürebilmesi amacı ile çok daha dikkatli bir biçimde tanımlanmalıydı. Bu doğrultuda tekne yap inşaa malzemesi olarak AA5059 H321 alaşımı seçilerek hessaplamalar gerçekleştirilmiştir. Bölüm 4’te de oldukça detaylı bir biçimde aktarıldığı üzere AA5059 alaşımının yorulma dahil diğer tüm özellikleri, kendi rakibi olan 5xxx serisi diğer alaşımlardan çok daha üstündür. Özellikle yorulma dayanımı AA5083’e oranla çok dah ayüksek değerlerdedir. Sürtünme kaynağının uygulanabilmesi durumunda (FSW), yani ısıl işlem uygulanmadan kaynak edilebildiği durumda göstermiş olduğu mukavemet özellikleri, A grade gemi inşaa çeliğinden bile oldukça üstündür. Bu malzemenin kullanılması aynı zamanda teknenin taşıma kapasitesini artırarak çok daha fazla sayıda faydalı yükü taşıyabilmesine olanak sağlamıştır. Yorulma hesaplamaları gerçekleştirilirken hemen hementüm klaslama kuruluşları tarafından (Alman Loydu, Norveç Loydu, Amerina Loydu vb…) kabul gören analiz yöntemi olan Palmgren-Miner yöntemi seçilmiştir. Her ne kadar bu yöntemin eksik yönleri bulunsa da vermiş olduğu sonuçların gerçek değerlere yakınsaması ve uygulanabilirliğinin kolay olması bu yöntemi klas kuruluşları tarafından tercih edilen bir yöntem olmasını sağlamıştır. Palmgren-Miner yönteminin en önemli özelliği, geminin maruz kaldığı gerilmelerde olduğu gibi, sürekli değişen genlikli gerilmelerde başarılı sonuçlar vermesidir. Gerçekleştirilen hesaplamalar ve analizler neticesinde gerilme yığılmalırının olduğu bölgeler tespit edilerek bu bölgelerde gerilme yığılmalarını önleyici önlemlerin alınması gerekliliği vurgulanmıştır(braket takviyelendirmesi, sac kalınlığı artırılması, stifner aralığının küçültülmesi gibi). Ayni zamanda Ege Denizi, Ak Deniz ve Kara Deniz koşullarında çevrim sayıları hesaplanarak teknenin hangi deniz ortamlarında daha uzun süre yorulma hasarı olmadan kullanılabileceğinin analizi gerçekleştirilmiştir.

Fatigue analysis is one of the most important issue in ship building industry. Although, in most cases, fatigue cracks are observing for commercial ships, such as chemical and oil tankers, bulk carriers etc. by reason of loading and reloading operations, however loads caused by waves and the acceleration, high-speed military vessels are also in danger to the fatigue cracks. Especially coast guard ships, which are designed to fulfill the purpose of search and rescue operations in adverse weather conditions, should be designed taking into account fatigue cracks. Fatigue failure is an extremely complex physical process which is governed by a great number of parameters related to, for example, local geometry and material properties of the structural region surrounding the crack growth path. There are different approaches and methods which can be used in fatigue life predictions. The S-N approach is a suitable method for fatigue assessment of welded components or structures when fatigue life may be defined as a crack not exceeding a physically short crack (i.e. maximum 10 grains long). Since the past two decades probabilistic approaches for fatigue have become more common. Two different approaches are often considered to describe the fatigue limit state; S-N curve with Palmgren-Miner’s damage accumulation rule or fracture mechanics based approach. For both probabilistic S-N and fracture mechanics approaches, the appropriate modelling of the structural response due to fatigue loading is an important issue. In this study, wave spectral based fatigue analysis are studied for the Turkey’s adjacent coastal seasby using Maestro finite element analyzing software. Palmgren-Miner’s method is used for obtaining and analysing solutions. The reason for choosing Palmgren-Miner’s method for fatigue analysis is, well accepted fatigue analyzing method for almost all class societies, such as Germanischer Lloyd, American Bureau of Shipping, Det Norske Veritas. Maximum stress regions of the plates are obtained by using finete element method and the results are compared with the Wöhler diagram of AA5059 H321 aluminium alloy’s endurance limit. Forthe purpose of obtaining number of cycles for each of sea conditions, wave spectrum table, which is given below is used. By using wave spectrum table, wave lengths, wave speeds and cycles are obtained. This study is performed for understanding and calculating the life time estimation of semi-swath type coast guard boat, which is produced by using AA5059 H321 aluminium alloy, in different enviroment conditions. The results are also examined with respect to thewave angle of attack. Spectral based fatigue analysis methodology allows to determine the long-term stress range from the wave environment (assumed or actually) encountered by the ship. This approach assumes linear load effects and linear stress response and is performed in the frequency domain. Design wave approach is a simplification of the frequency domain analysis. In this approach, each load is defined by an equivalent wave corresponding to a certain probability of exceedance which gives the maximum load response. The structural stress approach, defines the stresses at the weld toe location taking all geometrical influences into consideration except for the local weld geometry. The research community encourages pursuing the development of stress concentration factor determination methods for structural components. Ship designers and builders have been constantly in search of alternative materials with the aim of reducing the weight of the boat. As a result of these concerns, aluminium alloys almost as the most suitable materials of ship building industry. While density of aluminium is approximately 2,7 tonnes/m3, density of steel is 7.8 tonnes/m3. However, while shipbuilding steel’s yield strength is 235 MPa, AA5059 H321 alloy’s yield strength is 160 MPa with welded condition and 270 MPa unwelded condition. As a result of these informations, aluminium produced boats provides a weight and powering advantage. At the same time, using of AA5059 ALUSTARTM aluminium alloy, brings a fairly large amounts of corrosion resistance compared with shipbuilding steel. Using of Alustarincrease strength, both before and after welding, by at least 20% so that the user can use a thinner gauge and thus build a lighter vessel or alternatively build a vessel with 20% higher overall strength, and keep the corrosion resistance at the same level or better than that of AA5083. By adding up to 6.0% Mg, andsubtle amounts of Zr and Mn it is guaranteed a minimum yield strength of 270 MPa (39 KSI) and UTS of 370 MPa (53 KSI), while elongation (which lends itself to formability) remains at 10%. By the application of a proprietary thermo-mechanical process and the addition of controlled amount of Zn we were able to keep all the major intermetallic particles inside the grain and prevent any excessive electro-chemical imbalance and thus avoid corrosion susceptibility. Extensive corrosion tests performed and documented under the supervision of classification societies such as DNV show that the corrosion resistance of Alustar is better or equal to that of AA 5083. Welding and heat affected zone (HAZ) strength is also improved by controlling the re-crystallization rate during welding and to avoid the formation of large grains. Alustar has also since found its way into the sailing and luxury yacht building industry. Because of the high final strength after welding and the good deformability, it is possible to design even more innovative and efficient hulls, which in turn have positive influence on the speed and form of a (sailing) yacht as well as fuel consumption. Alustar is also used in the defence industry for its strong ballistic properties. In the chaper 4.3.3 balistic properties of AA5059 is described in more detail. Fatigue calculations arecarried out taking into account the Turkey’s coastal seas. As a result of these analyzes, we will have calculated how long the fatigue event will not occur for each sea conditions and we will be calculated life time of safe. Maestro finite element software program was used with the aim of can be quickly and accurately represented in different sea states. Maestro is primarily a complete ship structural design system (though not limited to) for the design of marine structures. Maestro provides a highly interactive and intuitive graphical environment for structural design via FE modeling/analysis. By usingMaestro, variety of structures including monohull ships, multihull ships, offshore structures, submarines, foundations, etc. can be modelled easily. One of the most common and important problem in terms ofnaval engineering, calculation of stresses caused by waves from different angles. Maestro can easily analysis stresses due to waves coming from different angle of attack. As a result of this study, semi-swath type of coast guard boat’s,which is produced by using AA5059 H321 alloy, parametric fatigue analysis is performed taking into account theterritorial seas ofTurkey.For the purpose of calculate stress and hot spot areas, caused by the waves comingfrom different angle of attack, Maestro finite element analysis software was used. Calculations was performedfor three different seas and five different sea states. Stress values due to the waves from different angle of attack was calculated and analyzed.As a result of these analyzes, how much time the boat can be used in a safe manner withouttaking any fatigue damage is predicted. As a result of these calculations, if the lower limit value117 MPais not exceeded, there will not occur any fatigue crack for the Aegean Sea for 32.72 years, for Mediterranean Sea for 46.897 years and for the the Black Sea for 36.69 years. As can be seen here in the calculated values,while the Aegean sea offering the shortes period of use, whereas mediterranean sea offers longest opertating time. In summary of this study, the two most important parameters to be considered when performing fatigue calculations. One of them is to determine the number of cycleswhich varies depending on sea conditions. And the second one is the fatigue resistance value of aluminum alloy. The number of cycles, which are depening on with wave periods and sea characteristics, should be determined first. At the same time, the prediction of how many hours the boat will be in operation should be predicted. Related with this prediction, results of fatigue life calculation will also be changed. As can be seen from the calculations, yield stress is not the decisive factor for fatigue life calculations. While the AA5059 H321 alloy’s yield strength is 155 MPa, whereas the value of fatigue stress for 108 cycle is 118 MPa. When the performing fatigue calculations, this issue should be handled carefully. In that regions, where the stress values exceeds 118 MPa, structural measures should be taken into consideration initial stage of the design. Structural discontinuities should be strictly avoided as much as possible in the initial stage of design. The regions, which are expected to fatigue cracks occures, such as cut outs, menholes, soft bracket toe endings etc. should be carefuly taking into consideration for the purpose of analyzing and evaluting of stress concentration.