Elastomerlerin zamana bağlı kayma davranışı
Başlık çevirisi mevcut değil.
- Tez No: 46165
- Danışmanlar: DOÇ.DR. HULUSİ ÖZKUL
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 115
Özet
ÖZET Bu çalışmada İki çelik tabaka arasındaki elastomerin zamana bağlı kayma davranışı incelenmiştir. Elastomer mesnetlerde takviye tabakalarının yerleştirilmesinin kaymaya etkisi olmaması nedeniyle bu davranış aynı zamanda, çok tabakalı mesnetlerin davranışı hakkında da bilgi verir. Elastomer olarak vulkanize olmuş doğal kauçuk kullanılmış ve ilk karışıma, kükürtün üç farklı oranı, karbon siyahı kullanılmaksızın; diğer karışıma ise kükürt oranı sabit tutularak, karbon siyahının üç farklı oranı katılmıştır. Farklı karışım oranlarında üretilen numuneler üzerinde gerilme gevşemesi deneyleri yapılmış; her farklı yükleme için numunelerin uzamaları okunmuş, sabit şekil değiştirme ve sıcaklık altında yükte meydana gelen azalmalar kaydedilmiştir. Chasset ve Thirion'un olgusal yaklaşımla geliştirdikleri ve zamana bağlı davranışı ifade eden a(t) = öe 1+ -L bağıntısı yardımıyla, denge gerilmeleri (ae) çeşitli aralıklar esas alınarak, her numune (karışım) için ayrı ayrı bulunmuştur. Yükle meydana gelen uzamalardan kayma şekil değiştirme değerleri hesaplanmıştır. Deney numuneleri ile aynı koşullarda üretilen numunelerin benzen içinde denge durumunda şişme oranları kullanılarak çapraz bağ yoğunlukları hesaplanmıştır. Chasset- Thirion denklemindeki, m ve log t değerlerinin çapraz bağ yoğunluğu ile arttığı gözlemlenmiştir. Gerilme şekil değiştirme grafikleri, şekil değiştirme 0.15 'den küçük olduğunda lineer olduğundan, bu eğrilerin eğiminden kayma modülü (G) hesaplanmış ve kayma modülünün çapraz bağ yoğunluğu ile arttığı gözlemlenmiştir. XV
Özet (Çeviri)
SUMMARY TIME DEPENDENT SHEAR BEHAVIOUR OF ELASTOMERS Polymer is a long molecule which contains a chain of atoms held together by covalent bonds. It is produced through a process known as polymerization whereby monomer molecules react together chemically to form either linear chains or a three-dimensional network of polymer chains. The main characteristic of the chain is that the chemical bonding is strong and directional along the chains but they are only bonded side ways by weak secondary Van der Waals bonding or occasionally with hydrogen-bonding. Polymers can be subdivided into three main categories; thermoplastics consist of individual long chain molecules, and in principle any product can be reprocessed by chopping it up and feeding it back into the appropriate machine; thermosets contain an infinite three dimensional network which is only created when the product is in its final form, and can not be broken down by reheating whilist. Rubbers contain looser three dimensional networks, formed by crosslinking of chains where the chains are free to change their shapes. The raw rubber must be compounded to form the elastomer, which must be vulcanized after the reinforcement has been bonded to the rubber. This assures a stable, durable bearing with a high quality bond between the elastomer and the reinforcement. During compounding, the rubber is mixed with fillers such as carbon black, and also oils and other additives which aid the manufacturing process, protective systems such as anti ozonants and antioxidants, and a vulcanizing agent such as sulfur. The mix is then vulcanized by the application of heat and pressure. The details of specific elastomer compounds vary and are frequently regarded as proprietary information by the bearing manufacturer. However, they affect the strength, stiffness, and other material properties of the rubber, and so the mechanical properties of the finished bearing will vary significantly from manufacturer to manufacturer. This is one of the major problems facing structural engineers in the design and selection of elastomeric bearings. Nearly all elastomeric bearings are made of either natural rubber or a synthetic rubber such as chloroprene. Other rubbers have been tried, but only these two have developed a history of satisfactory performance and are widely accepted by the profession. They have highly nonlinear, time and temperature dependent stress-strain behavior. Nonlinearity is introduced by creep, stress relaxation and large strains, since tensile strains (elongation at break, sB) of up to 600% are possible. Elastomers stiffen at low temperatures and under dynamic loading. Stiffening at low temperatures is a complex XVIphenomenon, but it is generally agreed that natural rubber is less susceptible to it than chloroprene. While elastomers are highly nonlinear materials the stress- strain relationships actually used in the design and analysis of bearings assume isotropic linear elastic behavior. This assumption is clearly not correct, but it is easy to use and provides adequate accuracy over the range of application. For design purposes, an elastic modulus, E, shear modulus, G, and Poisson's ratio, v, must normally be defined. Elastomers are virtually incompressible but are flexible under uniaxial stress. Thus v*0.5 (1) G« E/3 (2) and E is a small number [typically less than 1.000 psi (6.9 MPa)]. Variations from Eqs. 1-2 are sometimes used in practice, but these are empirical relations which account for factors other than true material properties. The structural designer is interested in the stiffness (i.e., E and G) of the elastomer, but he must also specify rubber properties such as the hardness and elongation at break for the manufacturer. It is well known that,[29-35] elastomers undergo extremely long term relaxation processes as seen in stress relaxation and creep experiments. Furthermore, the time required to achieve equilibrium decreases dramatically as the crosslink density incerases; Ferry [35] has suggested that the molecular mechanism responsible for this long term process is the diffusion of chain branches (dangling ends) in the presence of entanglements. Chasset and Thirion [29] recognized that an excellent phenomologic respresentation of typical stress relaxation behaviour of elastomers at long times is given by 0(t) = Oe 1+ -i- (3) for long times, t, where a(t) is the isothermal relaxation modulus, oe is the equilibrium modulus, and m ant tm are material parameters. This relationship holds for temperatures below which no chemical reaction or degradation occurs during the experiment. Dickie and Ferry [35], Curro and Pincus [31] also observed that this relationship respresents typical stress relaxation behavior of elastomers fairly good. They also investigated materials constants and found that they are being depended on temperature and the crosslinking density. In this work, the same approach has been used to examine the stress relaxation of elastomers. XVIIde Gennes suggested that, stress relaxation behavior of elastomers is a result of reptation of branched elastomers in the presence of topological constraints. Curro and Pincus [31] who made a suggestion based on de Gennes1 theory, showed that creep or relaxation phenomena which is a result of the movements of dangling end, fits the equation which is shown at Eq.(3). With the development of synthetic rubber during World War II, the elastomeric pad came into use as a bearing. Practically inert and unaffected by weather, it had no moving parts in the usual sense, yet it provided support and accommodated movements of several centimeters. Before this device could be perfected, however, many failures resulted from poor quality material. Improvements were made and are still being made. Today elastomeric bearing pad is the best expansion bearing for moderate movement that has been developed in the progression from sliding plates through the various types of roller devices. Elastomeric bearings are widely used in structures. In bridges they accomodate movements such as creep and thermal expansion and in precast concrete construction they act as seating pads which provide uniform bearing for members, absorbing small movement, and fabrication misalinement. Further, they are being used increasingly for seismic base isolation and machine vibration control. These bearings are economical and they have developed a history of satisfactory performance with few problems. They do not have any moving parts to corrode or freeze up. Although their use has increased dramatically in recent years, their behavior is complex and is not well understood by structural engineers, and thus bearings may not live up to the designer's expectations. There are several reasons for this lack of understanding. First, the material properties of elastomers are very different from those commonly encountered by structural engineers. Second, the mechanics of elastomeric bearings are also unusual, because of the different configurations employed and the large strains which may occur. The hardness of the elastomer must normally be specified by the structural engineer. Both the Shore A Durometer and International Rubber Hardness scales are used today, but they are nearly identical over the range 50-60 which is most commonly used in elastomeric bearings. For design purposes, the material moduli, E and G, are correlated to hardness. Generally, harder rubbers are suffer with a reduced elongation at break. The behavior of elastomeric bearings is dependent upon the material properties of the rubber. Since these materials are stiff against volumetric changes but flexible in uniaxial compression, large strains with very little change in volume are to be expected. Reinforced bearings have steel plates placed in the bearing in layers. The reinforcement has large in-plane stiffness compared to the elastomer, and, thus, the attached rubber can develop only out-of- plane shear strains. This results in bulging of the elastomer when the bearing is compressed. Lateral translation of the bearing produces a simple shear deformation with little or no bulging and rotation results in bulging. The magnitude of the bulging depends on xviiithe geometry of the bearing and the properties of the elastomer. Stiff rubbers bulge less than flexible ones, and thick layers bulge more than thin layers with the same plan dimensions. Since incerased bulging induces larger shear strains in the elastomer it must be controlled. Increasing the stiffness with a harder rubber reduces the shear strains, but it also makes the bearing less effective in accommodating structural movements. Thus, the ideal method for controlling the behavior of an elastomeric bearings is to adjust the geometry to the specific condition. The shape factor, S, is used to model this geometric effect, where for one layer of a reinforced bearing : (Fig.l) S= Loaded Area/ Area Free to Bulge (4) Figure 1 Introduction of many reinforcement layers will produce bearings with large shape factors, reduced bulging and shear strains, increased stiffness in compression and rotation, and no significant change in the resistance to translational movement. The shear stress in the elastomer produces tensile stress in the reinforcement, and both are proportional to the mean compressive stress. Steel is strong in tension and, thus, few problems have occurred due to tensile stress. Fiberglass fabric reinforcement has also been used. It is much weaker than steel in tension, and thus usually controls the strength of the bearing. In the experimental part of this work, a special testing specimen is used in order to represent the shear behaviour of elastomeric bearings (Fig 2). Vulcanized natural rubber is used as elastomer and it Is crosslinked by three different amounts of sulphur such as 0.5%, 1.5% and 3%. By keeping the sulphur content constant at the middle value in the previous mixture, three different amounts of carbon black, such as 20%, 40% and 60%, are introduced into the each batch as reinforcing particles. XIXRubber - steel ExWWil K\\\\\\\\N\^^^^ l^^^^^\\\\\\\^l ! fc\W3 Figure 2 After premixing raw natural rabber and ingredients through passing roller mills, the rubber is loaded into the preheated mould together with steel plates, which contain a special glue applied on the faces, and all were pressed and heated at a temperature of 150 °C for 15 minutes. The hardness tests were carried out on the sheet specimens prepared with the same batch of shear blocks and also the molecular weight of chain segment between the crosslinking points were determined from the equilibrium swelling test results in benzene by using the well known Flory-Rehner equation. The relaxation tests were performed by applying a predetermined displacement to the end plates of the specimen given in Fig 1 by the rapid movement of crosshead of Universal Testing Machine. The variation of load by time was recorded during the experiment and the displacement of plates on the ends with respect to each other was measured by using an optical catetometer. The relaxation tests results were evaluated by using the Chasset and Thirion approach given in Eq (3). After the rearrangement of this equation, the plotting of log((a(t)/oe)-l) with respect to logt, by using the experimental data, should give a linear relation if the equilibrium stress is chosen properly. By calculating the correlation coefficient of linear regression analyses, the constants m and m logtm in equation (3) were calculated together with the equilibrium stress By using the data obtained from the relaxation experiments the shear stress-shear strain curves were plotted for different relaxation times starting from 12 sec. to the equilibrium state and the shear moduli of rubbers tested here were calculated from these curves. xxThe following results can be derived from these experimental studies: 1-Chasset and Thirion approach can be used to predict the behaviour of elastomeric bearings under shear stresses. 2-Metarial constants m and tm in the Chasset and Thirion equation, both increase when the crosslinking density increases. 3-For the shear strains less than 0.15, the stress-strain relation in shear is linear, and the shear moduli calculated from the slope of these curves change with crosslinking density as expected. 4-During vulcanization, taking account of sulphur being the only variable effecting the crosslinking density was found incorrect; the heating agent (CZ-CBS) should be considered together with sulphur in order to get different crosslinking densities. First chapter of this study is about the general knowledge of elastomers and elastomeric bearing devices. In the second chapter, the specimen preparatation and experimental procedure are explained. The test results are evaluated in the third chapter, and the conclusions obtained from this study is given in the last chapter. xxi
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