The experimental analysis of the fatigue strength of the spot welded different steel sheets

Nokta kaynaklı farklı çelik sacların yorulma dayanımlarının deneysel analizi

Abstract

Resistance spot welding is a weld process which iswidely used in the manufacturing areas. In the mostimportant levels of the steel sheet constructions andespecially automotive industry, the resistance spotwelding is used. This type welded joints are exposedto static and dynamic forces during the operations.The failure type which occurs during dynamic forcesis the most critical failure type, and it is called asfatigue failure. A fatigue failure starts from the ma terial surfaces or a defect in the material. This de fect becomes a crack during the operation, it propa gates and finally the material fractures. Because theresistance spot welded areas are a natural notch,they behave like a defect or crack initiation. Andthus those welded areas are most critical joints infatigue failure.Corrosion is one of the most important problems inthe steel structures. To improve the corrosion resis tance, the steel sheets are coated with Zinc, andthese Zinc coated steel sheets are called as galva nized steel sheets. To decrease the cost and height,in manufacturing, not only the same sheets but onlythe different sheets (for example galvanized steelsheets and austenitic stainless steel sheets) arewelded to each other by using resistance spot weld ing. Some problems can occur in spot welded differ ent sheets during the operation, and thus it is veryimportant to investigate those problems, especiallyfatigue strength of the joints. In this study, galvanized and austenitic stainlesssteel sheets were joined to each other by using resis tance spot welding. The thicknesses of the galva nized steel and austenitic stainless steel are 0.93 and1.03 mm, respectively. The experiment parametersare sheet combination and weld nugget diameter.Three weld nugget diameters were selected as 4, 5and 6 mm which are most widely used in the steelsheet joining industry. The pre-tests were performedto investigate the effect of the weld current on thenugget diameter. By using the pre-test results, resis tance spot welded steel sheets series were obtainedwith 4, 5 and 6 mm (± 0.1) nugget diameter, andgalvanized – galvanized steel, galvanized – austen itic stainless steel. The fatigue tests were performed to spot welded steelsheet specimens. The effects of the material composi tion on the fatigue strength of the spot welded steelsheets were investigated. The fatigue experiments inthis study were performed in a laboratory environ ment at room temperature (19 – 25o C) using a 60 kNservo-hydraulic test machine. Specimens were ex- posed to a constant load amplitude sinusoidal wave form until fracture occurred or to a maximum 1x107 cycles at R value 0.01. The cyclic displacement be tween specimen holders was measured. The variationof the specimen stiffness was evaluated after eachtest. Specimen stiffness was defined as ∆P /∆l. Where∆P is the cyclic load range and ∆l is the specimen’scyclic elongation range during fatigue testing. Thespecimen stiffness decreases as fatigue cracks propa gate. Specimen failure was defined at 25% stiffnessdrop. Some welded specimens which is exposed inhigh loads failure as rupture. The test frequency waskept constant during the test. By using the fatigue testresults, the load range versus number of cycles tofailure (S – N) curves were plotted. The results showthat galvanized steel sheet combination has the high est fatigue limit. The sheet combination which has theminimum fatigue limit is galvanized – austeniticstainless steel sheet combination.Due to the spot welding nugget formation, the nug get behaves like a surface crack. In this study, tomeasure the nugget diameter after the fatigue andthus crack length, four identical spot welded speci mens with 5 mm nugget diameter, galvanized steelaustenitic stainless steel combination were exposedto fatigue experiment 30000, 60000, 90000 and104000 number of cycles respectively. After the fa tigue tests, the remaining nugget of the weldedspecimens were measured, and crack lengths wereevaluated. According to results, while number ofcycles is increased, the crack length and crackgrowth rate increase.By using the results and measured crack length forspot welded galvanized steel – austenitic stainlesssteel sheet combination, C and m material constantin Paris – Erdogan crack growth rate equation wereobtained. The crack growth rate equation for thistype spot welded joint were obtained as4 . 2 3 . 11 10 K dN da ∆ = − .

Özet

Nokta direnç kaynağı, imalat sektöründe oldukça yaygın olarak kullanılan bir kaynak yöntemidir.Özellikle otomotiv ve beyaz eşya sektöründe imalatın en önemli aşamalarını bu kaynak yöntemioluşturmaktadır. Bu tip bağlantılar, işletme sırasında statik ve dinamik yükler altındazorlanmaktadır. Özellikle dinamik yükler altında zorlanan bölgelerde oluşan hasar tipi oldukçakritik olup, bunlar yorulma hasarı olarak adlandırılırlar. Yorulma hasarı, malzeme içerisindeki birhata veya süreksizlikten başlar, giderek ilerler ve sonunda hasarla sonuçlanır. Nokta kaynaklıbölgeler de, doğal bir çentik etkisi yapmaktadır ve dolayısıyla da yorulma hasarı açısından oldukçakritik bölgeler olarak karşımıza çıkmaktadır. İmalatta, sadece aynı özelliğe sahip saclar değil,maliyet ve konstrüktif açıdan farklı bileşime ve özelliğe sahip saclar da birbirleriyle nokta kaynağıyardımıyla kaynak edilmektedir. Bu tip nokta kaynaklı bölgelerde ortaya çıkabilecek problemlerinve bunların yorulma dayanımlarının belirlenmesi büyük önem taşımaktadır. Bu çalışmada,galvanizli ve ostenitik paslanmaz çelik saclar, hem kendi aralarında hem de birbirleriyle noktakaynağı ile birleştirilmişlerdir. Her malzeme kombinasyonu için 3 farklı nokta çapına sahipnumuneler elde edilmiştir. Kaynaklı numunelere uzun ömürlü yorulma deneyleri yapılmış ve herdeney numunesi için S-N (kuvvet – çevrim sayısı) eğrileri elde edilmiştir. Galvanizli çelik saclardanoluşan kaynaklı bağlantılar en yüksek yorulma dayanımına sahip bağlantılardır. En düşük yorulmaömrüne sahip bağlantı ise galvanizli çelik – ostenitik paslanmaz çelik sac bağlantılardır. Bubağlantı tipi üzerinde, yorulma sonrası çekirdek çapı ölçümleri yapılmıştır. Bu bağlantı tipi için,Paris–Erdoğan çatlak ilerleme hızı bağıntısını veren malzeme sabitleri belirlenmiştir.