Joints tension

Tension load Caused by the string weight during running in it will be highest at the top joints... 

Tension leg platforms (TLP) are used mainly in deep water where rigid platforms would be both vulnerable to bending stresses and very expensive to construct. A TLP is rather like a semi-submersible rig tethered to the sea bed by jointed legs kept in tension. Tension is maintained by pulling the floating platform down into the sea below its normal displacement level. The legs are secured to a template or anchor points installed on the seabed. 

Stress and Distortion. The forces acting on a stmcture are transmitted through the welded joints that is, the joint is subjected to simple tension (or compression), bending, shear, or torsional stresses, or to combinations of these stresses owing to combined loading situations. Weldments must be of a proper size, length, and location to withstand the loads imposed during service. 

Solders should flow promptly and smoothly over the surfaces of the parts to be joined. This property depends on the surface tension, viscosity, and adhesive properties of the molten solder. Finally, the color of a solder should match that of the metal employed, and its physical properties should be at least as good as those of the metal, in order for the joint not to be a source of weakness (150). 

There are four commonly occurring states of stress, shown in Fig. 3.2. The simplest is that of simple tension or compression (as in a tension member loaded by pin joints at its ends or in a pillar supporting a structure in compression). The stress is, of course, the force divided by the section area of the member or pillar. The second common state of stress is that of biaxial tension. If a spherical shell (like a balloon) contains an internal pressure, then the skin of the shell is loaded in two directions, not one, as shown in Fig. 3.2. This state of stress is called biaxial tension (unequal biaxial tension is obviously the state in which the two tensile stresses are unequal). The third common state of stress is that of hydrostatic pressure. This occurs deep in the earth s crust, or deep in the ocean, when a solid is subjected to equal compression on all sides. There is a convention that stresses are positive when they pull, as we have drawn them in earlier figures. Pressure,... 

Table 1 contains the metal-to-metal engineering property requirements for Boeing Material Specification (BMS) 5-101, a structural film adhesive for metal to metal and honeycomb sandwich use in areas with normal temperature exposure. The requirements are dominated by shear strength tests. Shear strength is the most critical engineering property for structural adhesives, at least for the simplistic joint analysis that is commonly used for metal-to-metal secondary structure on commercial aircraft. Adhesive Joints are purposefully loaded primarily in shear as opposed to tension or peel modes as adhesives are typically stronger in shear than in Mode I (load normal to the plane of the bond) loading. 

The principal failure modes of bolted joints are (1) bearing failure of the material as in the elongated bolt hole of Figure 7-44, (2) tension failure of the material in the reduced cross section through the bolt hole, (3) shear-out or cleavage failure of the material (actually transverse tension failure of the material), and (4) bolt failures (mainly shear failures). Of course, combinations of these failures do occur. 

Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts. Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole. Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes. A possible solution is to put a relatively ductile composite material such as S-glass-epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip concept, and was addressed in Section 6.4. 

Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out. The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode. The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both. 

Chain Tension. All chain drives should have some means of controlling the chain sag caused by normal joint wear. This is of utmost importance when the drive is subject to shock or pulsating loads or to reversals in direction of rotation. The most common methods taking up chain slack are (1) drive units mounted on adjustable base plat, slide rails, or similar units these are used extensively in motor-driven applications and (2) the use of adjustable idlers (Figure 3-40) and chain tensioners. 

Kelley spinners, when used as tension members An adapter between the swivel and the kelley that spins the kelley for rapid attachment and disattachment to joints of drill pipe. 

Run enough drill collars to keep all drillpipe in tension in order to reduce wear and stress on tool joints. 

Proper tension is essential when installing a new chain. A tight chain causes an additional load, which increases wear on chain joints, sprocket teeth, and shaft bearings. A slack chain produces vibration, which may result in excessive chain wear, noise, or shock loading. 

Brazements included in a piping system that is subjected to a temperature 1,000°F (538°C) and greater shall require tests in addition to those of ASME BPV Code Section IX. These tests shall be considered a part of the qualification procedure for such design temperatures. Two tension tests on production type joints are required, one at the design temperature and one at 1.05T (where T is the design temperature in degrees Fahrenheit). Neither of these production-type joints shall fail in the braze metal. 

Fig. 2 Top Freely jointed chain (FJC) model, where N bonds of length a are connected to form a flexihle chain with a certain end-to-end distance R. Bottom in the simplified model, appropriate for more advanced theoretical calculations, a continuous line is governed hy some bending rigidity or line tension. This continuous model can be used when the relevant length scales are much larger than the monomer size...

---