Propertıes Of Adhesıves; Analytıcal And Numerıcal Study Of Bonded Joınt Subjected To Temsile Stress

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1996Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1996Bu yüksek lisans tez çalışmasında, havacılık ve uzay sanayiinde çok geniş kullanım alanına sahip yapıştırıcılar, yapışma ve yapışma birleşmeleri konulan genel olarak değerlendirilerek, yapıştırıcı çeşitleri, yüksek sıcaklıklara dayanıldı yapısal yapıştırıcıların imali ve mukavemeti, yapışma işlemi, yapışmaya yardım eden mekanizmaların başında gelen ıslanabilirlilik, yapışma işleminin en can alıcı ve nihai ürünün kalitesini etkileyen yapışma yüzeylerinin hazırlığı, yapıştırıcıların uygulanması, yapışmada kullanılan alet ve ekipmanlar, sertleştirme ve yapışmanın Test edilmesi hakkında geniş bilgi verilmiştir. Konu ile ilgili olarak daha sonra, birleşmede yapıştırıcı kullanmanın avantajları, yapıştırıcı ile birleşmenin kısıtlamaları ve değişik gerilmelere maruz yapışma birleşmeleri hakkında bilgi verilerek; yapışma birleşmelerinin hangi esaslara göre dizayn edildikleri, normal ve yüksek sıcaklıklara maruz yapışma birleşmelerinde birleşme mukavemetinin değişimi, yapışma uzunluğunu artırmanın yapışma birleşmeleri üzerindeki etkisi, yapışma ile birleşme dizaynında maximum kayma gerilmesinin önemi, yapışma birleşmelerinin yorulma mukavemeti, birleşen parçaların geometrisinin yapışma birleşme mukavemeti üzerindeki etkisi, yapışma birleşmelerinin modifikasyonu ve değişik yükler karşısında yapışma birleşmesinin davranışı konulan incelenmiştir. Yapışma ile birleşme dizaynında temel prensip, birleşenler arasında yükün emniyetli bir şekilde transferini gerçekleştirerek emniyetli, ekonomik ve yüksek performansa sahip yapışma birleşmesinin dizayn edilmesidir. Emniyetli, ekonomik ve yüksek performanslı bir birleşmeyi yapıştıncı ile dizayn edebilme; genel olarak yapışma birleşmesi için yapılan gerilme analizlerine ve yapıştıncının taşıyabileceği maximum yükü tespit etmek için yapılan güvenilir Testlere bağlıdır. Bu amaç ile çekme gerilmesine maruz, birim genişlikte, / yapışma uzunluğunda, ti ve fc kalınlığında iki levhanın birbirine yapıştincı ile birleşmesini temsil eden basit bir model yardımı ile yapışma birleşmesi analitik ve nümerik olarak incelenmiştir. Nümerik incelemede ANSYS programı kullanılmıştır. Analitik incelemede kullanılan model boyutiandınlarak, elde edilen denklemler yardımı ile yapışma hattı boyunca çekme ve kayma gerilmelerinin; yapışma uzunluğu, yapıştincı kalınlığı, elastik modül ve levha kalınlığı ile değişimlerini gösteren çeşitli grafikler bulunmuştur. Elde edilen grafikler, nümerik incelemede kullanılan ANSYS programı ile elde edilen grafiklerle karşılaştırılarak değerlendirilmiştir. Sonuç olarak yapışma ile birleşmenin mukavemeti; birleşen levhaların kalınlığına, levhaların elastik modüllerine, yapıştıncının kayma modülüne, yapışma uzunluğuna, yapışma yüzeylerinin ön işlemine, yapışma işlemine ve yapışma birleşmesinin maruz kalacağı şartlara bağlı olduğu ortaya çıkmışlar.in this work the bonding in general was studied and some general information about adhesives, production and strength of high temperature adhesives has been given. Bonding surface preparation, application of adhesives, equipment and tools for bonding, curing and inspection of bonding joints have been inspected by giving general information about bonding and wettability. Bonded joints by using simple lap joint model subjected to tensile stress have been studied analytically and numerically. Adhesives, which have existed for a thousands of years, have become very important in the last few decades. The growing availability of a variery of new materials and significant advances in bonding technology have allowed people to routinely trust theİr lives and fortunes to adhesive joint constructions. Adhesives are generally a mkture of materials that give the desired design properties in the final bonded joint. The adhesive materials may be in several forms prior to usage; such as films, pastes, liquids, powders, ete. Chemically, the adhesives may consist of öne ör several materials such as epoxy, phenol, polyamid, silicone, ete. The generic properties of a adhesive usually indicates the type of properties that the final bond will have such as flexibility, temperature resistance, fluid resistance, shear strength, ete. Modem adhesives are classified either by the way they are used ör by their chemical type. The strongest adhesives solidify by a chemical reaction. Less strong types harden by some physical changes. Adhesives solidified by a chemical reaction are as follovvs: a) Anaerobics b) Cyanoacrylates c) Toughened Acrylics d) Epoxies e) Polyurethanes f) Modified Phenols Adhesives harden by some physical change are as follows: a) Höt melts b) Rubber Adhesives c) PVA' s d) Pressure Sensitive Adhesives The adhesives that used in subsonic transport aircraft have considerable ductility in shear. it is good practice to restrict the adhesive to elastic-strain only for the regularly occurring fatigue loads, but extra strain capability is necessary for ultimate loads and for load redistribution around flaws. Dissimilar materials can frequently be bonded together to optimize a structure; however, long bonded interfaces between dissimilar materials require xvi special consideration as to the effects of differential thermal expansion. The special consideration is necessary because heat curing adhesives bond at elevated temperatures, locking the faying surfaces of the bonded area and thermal expansion. This can cause some buut in stresses and severely damage the assembly. A good bonded area should be uniform and bonding material thickness is in the range of 0.005 to 0.015 inches thickness. Bonding thickness control should be maintained by the good tooling. Each cured bond line contributes approximately 0.007 inch to the overall thickness of the bonded assembly in strength consideration. Bonding procedure is as follow; selection of adhesives and bonding primers, surface preparation, applying of the adhesives to the surfaces and curing. Önce the liquid adhesive has been applied to the adherents, start contact and wet the surfaces, and the bonding establish with the following three mechanism; mechanical interlocking, adhesion of öne material to another, and formation of direct chemical bonds across the interfaces. Mechanical interlocking can certainly be a significant factor in adhesion. Ice on glass is a common example of adhesion by mechanical interlocking. Another is the mechanical adhesion of dental fillings to teeth. in generally, mechanical interlocking is probably not a practical factor in most adhesive joining, although the roughness and porosity of a substrate and their effects on stress distribution in the interfacial region can be a significant factor. The formation of covalent chemical bonds across the interface is the another extreme. This type of bonding would be expected to be the strongest and most durable. it does require that there be mutually reactive chemical groups tightiy bound on the adherent surface and in the adhesive. Some coupling agents may be used to promote the formation of direct chemical bonds. The adhesion of solders to metals is probably increased by the formation of metallic bonds across the interface. Surfaces are prepared by öne of the following pretreatment procedures: a) Degrease only b) Degrease, abrade and remove loose particles. c) Degrease and chemically pretreat. Çare must be taken to avoid contaminating the surfaces during ör after pretreatment. Contamination may be caused by finger marking, cloths which are not perfectly clean, contaminated abrasives, standard degreasing ör chemical solutions. Contamination may also be caused by other work processes taking place in the bonding area. Particularly to be excluded are oil vapors from machüıery, spraying operation (paint, mold release agent, ete.) and processes involving powdered materials. Whatever the pretreatment procedure used, it is good practice to bond the surfaces as soon as possible after conıpletion of the pretreatment, i. e. when surface properties are at their best. The Water-Break Test is a simple metiıod to determine whether the surface to be bonded is clean. it is best suited to metals. If a few drops of distilled water applied to the surface wet and spread ör- if, on dravving the surface from distilled water, the water film does not break up into droplets - then the surface may be assumed to be free of Contamination. Uniform wetting of the surface by distilled water indicates that it will probably be likevvise wetted by adhesive. XVİİ Most bond lines are heated by placing the assembly in the bonding fkture, in an öven ör autoclave. Certain local bonds may be heated by electric blankets, höt air guns - heater strips ör lamps. Press heat is supplied by höt platens ör integrally heated tools. As the component is heated to the cure temperature, the adhesive will melt and flow. in order to produce a satisfactory bond the components must be held together without movement until after the adhesive has become solid and cooled. This is accomplished in a variety of ways depending on the type of adhesive, the type of component to be bonded and the production rate required. Bonding process using some special tools and equipment as follows can be performed: a) Hydraulic press with heated platens. b) Tooling fixtures c) Vacuum bağ: in the absence of a press, this is a way of applying holding pressure to flat ör curved components while öven curing. Bags may be purpose made to süit using a rubber diaphragm. Some reduction in strength may occur due to the sub-atmospheric pressure increasing the volume of the unavoidable air in the adhesive joint. d) Autoclave: For very large ör complicated, curved components requiring higher pressures, a large autoclave can be used and may cure several different components at the same time. The capital cost is high and the bonding time relatively slow. e) Riveting and weld-bonding Quality control can vary from simple strength Tests on bonded joints to destructive and non-destructive Testing (NDT) techniques. Non-destructive Testing techniques are performed by X-ray, ultrasonic and eddy current methods. For many applications Test specimens, pretreated and cured alongside the component, are sufficient for destructive Testing. These Test methods for lap shear strength, metal / metal peel strength and skin / core peel strength are used. Relating to the subject in this study, strength of the bonded lap joints, effect of overlap length on the strength of the bonded lap joints, strength of bonded lap joints at normal and elevated temperatures, maximum shear stress of bonded lap joints along overlap length, formability of bonded lap joints and fiıel tightness have been examined by giving information about bonded lap joints subjected to many kind of stresses. The terminology in the bonding joint has the following meaning: Âdherent: A body held to another body by an adhesive Primer: A coating applied to a surface, prior to the application of an adhesive, to improve the performance of the bond Adhesive: A substance capable of holding materials together by surface attachment Bonded joints derive their strength from the relatively large contact area (compared to mechanical fasteners) över which the transferred loads may be distributed. Bonded joints may be subjected to tensile, compressive, shear ör peel stresses, often in combination. Adhesives are strongest in shear, compression and tension. They perform relatively poorly under peel and cleavage loading. A bonded joint needs to be designed so that tiıe loading stresses will be directed along tiıe lines of the adhesive's greaTest strengths. Load concentrations due to deflection ör eccentricity should be avoided ör minimized. xviii Cleavage and peel loading are the most critical loading: They concentrate the applied force into a single line of high stress. in practice a bonded structure has to sustain a combination of forces. Fer maximum strength, cleavage and peel stresses should be, as far as possible, designed out of the joints. Generally bonding can't be advised in the joints which exist cleavage and peel forces The safe and effective design of bonded structure is fundamentally dependent on both a valid stress analysis of the adhesive in the bond and reliable Tests that determine allowable stresses in the adhesive. Safe design is achieved by comparing the calculated stresses with the allowable stresses, thereby predicting the performance of the bond. Effective design is the most lightvveight, economical configuration consistent with safety. This is achieved through the stress analysis itself because it contains parameters relating to the size and geometry of the adherents. This size and geometry can be selected for minimum weight and cost, consistent with safe adhesive stress levels as determined by the analysis. For stress analysis, a model is presented that embodies the fundamental stress distribution found in the adhesive in typical bonded joints. This analysis is shown to be valid when the adherents are in tension, compression, ör shear. it also shows that the indispensable property of the adhesive, for stress analysis, is stiffiıess in shear. A very important fundamental concept is to understand the behavior of an adhesive as a fastener. The usual material properties of tension, compression, flexure, and modules do not serve well. The indispensable property is stifmess in the shear mode. The stifmess of the glue line relative to the stiffiıess of the adherents determines the stress distribution İn the adhesive. in this study a simple lap joint between two sheets of thickness t] and ^ with / overlap length subjected to tensile stress has been examined in analytical and numerical using ANSYS, FEM sofbvare. Some assumptions have been made. a) There is a perfect bonding betvveen adherents. b) Thickness of adhesive along overlap is constant. c) Lap joint model is subjected to only tensile stress. The analysis methods that the engineer has available for the design of bonded joints can be divided into two groups: analytical and numerical (approximate). Each method has advantages and disadvantages, but the solution of problems with complex geometry and boundary conditions becomes possible only through numerical approximations. Analytical solutions also provide a means of checking numerical results for accuracy and convergence. in the analytical study, the shear stress along upper skin/ lower skin interface was related to the displacements of the upper and lower skins. According to this, following equation can be written, r = £(H,-w2) (1) where, k : Stifmess parameter u 2'. Displacement of upper skin u2: Displacement of lower skin xix Equation (1) indicates that relative sliding between upper and lower skins is necessary to generale shear along the interface. Performing analytical study, following equations have been obtained. T Ot x, COSH(ûJ(j)) ^-(*fe> E2t2SWH(fll(7»-(E2t2COSHW + EA) ^^ (2) er, l x SmH(ft(~)) ^=(¥77F7} ^oc^C^(7))+^-(^aoas»OflO+£iii)----£- (3) CTool .fll'l+-*i2'2 * SMH(ft) T A, [" x, COSH(ftfy) ^~ = (777T^) E^SINH(pl(^ - (EA + E2t2COSH(pl)} -__' (4) ff«2 ^l'l + A2'2 * SINH(Pl) a 1 SINH(ai(-y) , ^ ^"=(¥77¥7) (^+^c