Pre-Loaded high strength bolted connections of aluminium members with steel members

Alüminyum ve çelik elemanların öngerilmeli yüksek mukavemetli bulonlar ile birleşimi

Abstract

A certain amount of research and studies has been done on the connections of the same kind of material such as aluminum to aluminum or steel to steel con-nections. However, as far as the connections of two different materials are concerned, there are no sys-tematic works on the subject. In order to connect hot dip galvanized steel material with aluminum materi-al, it is necessary to use high strength pre-loaded bolts. The use of weld is not possible because of the different properties of materials. The presented work is the result of the studies con-ducted on two different materials- aluminium and steel and their friction grip pre-loaded high strength bolted connections. For this reason, two different types of experiments have been done. These tests are the short-term and the long-term tests, performed on four different dimensions of thickness: 10.0 mm, 9.7 mm, 8.0 mm and 5.0 mm, and on three different aluminum alloys: AlMgSi1 (6082), AlMgSi0.5 (6063) and AlZn4.5Mg1 (7020). In each specimen two 10.9 M16 bolts have been used. In the first five tests of the short-term tests, friction surfaces are on-ly treated oil-free and in the latter 9 test specimens are painted with alkali silicate zinc. In pre-loaded high strength bolted connections, the friction coefficient had been determined for the alu-minum and steel specimens with oil-free contact sur-faces and also for the alkali silicate zinc painted contact surfaces. For the above mentioned two dif-ferent cases of treatment of the contact surface, loading-displacement diagrams are studied for shear loads and after it then, up to the occurrence of the rupture. After having subjected the bolts to pre-loading, the loss of pre-load had been studied for 30 min. prior to the beginning of the test. During the test under load, the specimen was monitored to de-termine whether a sudden loss in pre-load occurred. In the short-term tests after applying the pre-loading to the bolts, no big loss on the pre-load was noticed, because of the creep behavior of aluminum in time. In the short-term tests, when no paint on the friction surfaces was used, the number of the bolts necessary for the proper connections would be too high to be considered economical. In case of using the friction-increasing alkali silicate zinc paint, the limit friction force of one bolt increases 3.5 times. A change of the thickness has no effect on the friction coefficient in short-term tests. When the aluminum alloy has lower yield stress than the steel, an indent under the wash-er will be observed. This observation is valid only for the AlMgSi0.5 (6063) alloy. That's why it is nec-essary to transfer the pre-load to a greater surface. To prevent the indent from happening, a thicker and wider washer could be used. After evaluating the short-term test results statisti-cally, it has been noticed that the standard deviation of the distribution of friction coefficient does not ex-ceed 10% limit. With the help of the results of four-teen short-term tests that were conducted, a long-term testing system was prepared and a total of fif-teen long-term connection specimens in three differ-ent frames with 90 kN, 78 kN and 60 kN of continu-ous loading was monitored. The loading on each frame was determined so that the friction surface of each specimen was loaded with 75%, 65% and 50% ratios. To prepare the friction surfaces of long-term test specimens, all surfaces were treated with alkali silicate zinc paint. The long-term test had been car-ried out within one - year time span, during which the loading of each frame, the pre-loading on bolts and the slip between aluminum and steel plates were observed. The effect of small thickness on friction in the long-term tests causes the slip of contact surfaces in time. By using thinner aluminum plates, contact pressure was distributed on a smaller area of contact surface. This explains the earlier shear observed in the long-term tests under constant tension force. The thinner the specimen, the earlier the slip occurs. For the study of the long-term test results using constant dis-tance separation - a method not found in the litera-ture, an attempt has been made to predict the behav-ior of specimens within 50 years onwards. Inspect-ing of the 10 mm thick aluminum plate specimens after the period of one year shows that the predicted displacement in 50 years' time will be within the 0.30 mm slip limit range. The conclusion can be drawn that the alkali silicate zinc paint increases the friction coefficient of connections made of two dif-ferent materials- aluminum and steel.

Özet

Bu araştırmada alüminyum ve çelik iki farklı malzemenin öngerilmeli yüksek mukavemetli bulonla sürtünme ile yük aktaran birleşimi incelenmiştir. Sürtünme ile yük aktaran birleşimlerin davranışını belirlemek amacı ile, iki farklı grup deney gerçekleştirilmiştir. Bunlar, kısa süreli ve uzun süreli kayma deneyleridir. Kısa süreli kayma deneyleri ile öngerilmeli yüksek mukavemetli bulonlu birle-şimlerde, hem alüminyum ve hem de galvanizli çelik temas yüzeylerine herhangi bir işlem yapılma-dan sadece yüzeyler yağdan arındırılmış durum için ve alkali silikat çinko boya ile boyanmış durum için, sürtünme katsayısı belirlenmiştir. Yukarıda belirtilen iki farklı temas yüzeyi haline ait yük-yer değiştirme diyagramları, kayma yükü ve sonrasında kopma haline kadar incelenmiştir. Deney sıra-sında yük altında bulonlardaki öngerilme kuvvetindeki değişim araştırılmıştır. Kısa süreli deneyler-de sürtünme yüzeylerine boya kullanılmaması durumunda birleşimler ekonomik olmaktan uzaktır. Sürtünme arttırıcı alkali silikat çinko boya kullanılması durumunda bir bulonun sürtünme ile taşı-yabileceği kuvvet yaklaşık olarak 3.5 kat daha fazladır. Yapılan ondört adet kısa süreli deneyden elde edilen verilerden yararlanılarak uzun süreli deney sistemi hazırlanmış ve toplam onbeş adet uzun süreli birleşim numunesi bir yıl boyunca üç farklı çerçevede 90 kN, 78 kN ve 60 kN’luk sürekli yük altında gözlenmiştir. Bu deney sonuçlarının irdelenmesinde daha önce literatürde rastlanma-yan sabit aralıklı ayrıklaştırma uygulanmak suretiyle elli yılsonundaki davranış tahmin edilmeye çalışılmıştır. Bu araştırma ile alüminyum ve çelikten oluşan farklı iki malzemenin birleşimi için al-kali silikat çinko boyanın kullanılabileceği ve sürtünme katsayısını arttırdığı gösterilmiştir.