Joint creep

Stress distribution shear lag solution D A DILLARD Stresses in shear joints Creep A D CROCOMBE Occurrence protection against creep failure Durability creep rupture D A DILLARD Creep under sustained stress Durability fatigue D A DILLARD Effect of cyclic loads... 

Keywords Adhesive modulus Adhesys expert system Co-axial joints Compression Concealed joints Creep Elastic limit Epoxy Epoxy composite Einite element analysis Glue line thickness Goland and Reissner Hart-Smith Heat exchanger Hooke s Law Joint designs Joint thickness Lap shear strength Peel Plastic behaviour Polyurethane Pipe bonding Shear stresses Shear modulus Stress distribution Thick adherend shear test Tubular joints Volkersen equation Young s modulus... 

On the other hand, the reliability of the product improves, too, if each state of the plasticity deformation, the creep deformation, and the diffusion joint in the solid phase diffusion bonding as the bonding process, is accurately understood, and the bonding process is controlled properly. 

Snap-Fit and Press-FitJoints. Snap-fit joints offer the advantage that the strength of the joint does not diminish with time because of creep. Press-fit joints are simple and inexpensive, but lose hoi ding power. Creep and stress relaxation reduce the effective interference, as do temperature variations, particularly with materials with different thermal expansions. 

Relaxation is an important example of a creep phenomenon encountered in practice. Bolts, studs, flanges, and springs of all kinds are subject to relaxation when used at high temperatures. These members are loaded to a stress that must be maintained for proper functioning. If relaxation occurs, the stress decreases. Thus bolts can become loose so that bolted joints develop leaks after operation at elevated temperatures. 

PVAc is another important type of adhesive, especially in furniture manufacturing and for carpentry. They form the bond line in a physical process by losing their water content to the two wooden adherends. PVAc adhesives are ready to use, have short setting time and give flexible and invisible joints. They are easy to clean and show long storage life. Limitations are their thermoplasticity and the creep behavior. 

The weld joint strength reduction factor is the ratio of the nominal stress to cause failure of the weld joint to that of the base material for the same duration. In the absence of more applicable data (e.g., creep testing), the factor shall be taken as 1.0 at temperatures equal or colder than 510°C (950°F), and 0.5 at 815°C (1,500°F) for all materials. The strength reduction factor shall be linearly interpolated for intermediate temperatures. The designer is responsible for determining weld joint strength reduction factors for temperatures warmer than 815°C (1,500°F). 

Creep testing of weld joints to determine weld joint strength reduction factors should be full thickness crossweld specimens with test durations of at least 1 000 h. Full thickness tests shall be used unless the designer otherwise considers effects such as stress redistribution across the weld. 

Stress-relaxation is more difficult to measure than creep. It is necessary to include a stiff load cell in line with the grips to measure the residual load, while the joints connecting the specimen to the applied load should be free from any play. Feedback methods by which the load is regularly reduced to maintain a constant strain should be examined carefully to ensure that load is adjusted frequently and sensitively with no overrun. 

Polymers have found two specific applications in orthopedics. First, they are used for one of the articulating surface components in joint prostheses. Thus, they must have a low coefficient of friction and low wear rate when they are in contact with the opposing surface, which is usually made of metal or ceramic. Initially, poly(tetrafluoroethylene), PTFE, was used in this type of application, but its accelerated creep rate and poor stress corrosion caused it to fail in vivo. Ultimately, this material was replaced with... 

Equisetum A genus of herbaceous, pteridophytic plants of wide distribution which are perennial from creeping rhizomes, have hollow, grooved and jointed steins, and leaves reduced to more or less conspicuous nodal sheaths. 

Features Stem long and creeping, rooting at joints, as the strawberry. Leaf stalks one to two inches long with five obovate leaflets, serrate, scattered hairs, veins prominent below. Flowers (June-September) bright yellow, five petals, solitary, on long stalks from stem as the leaves. 

Features This ivy, as its common name and second synonym convey, creeps along the ground. The quadrangular, unbranched stem is six inches or so long. Two kidneyshaped leaves appear opposite each other at every joint. They are deeply crenate, the upper leaves purplish in colour and paler underneath. The roots issue at the corners of the jointed stalks, and the two-lipped, purplish flowers bloom three or four together in the axils of the upper leaves. The taste is bitter and acrid, the odour strong and aromatic. 

There are several possible solutions to the expansion mismatch problem. One is to use a resilient adhesive that deforms with the substrate during temperature change. The penalty in this case is possible creep of the adhesives, and highly deformable adhesives usually have low cohesive strength. Another approach is to adjust the thermal expansion coefficient of the adhesive to a value that is nearer to that of the substrate. This is generally accomplished by selection of a different adhesive or by formulating the adhesive with specific fillers to tailor the thermal expansion. A third possible solution is to coat one or both substrates with a primer. This substance can provide either resiliency at the interface or an intermediate thermal expansion coefficient that will help reduce the overall stress in the joint. 

Life prediction methodology embraces all aspects of the numerous processes that could affect the function of the element—in this case the bulk adhesive. The first step is to define the function of the adhesive clearly enough for a failure criterion to be derived. This failure criterion may be an unacceptable reduction in tensile strength, time to creep failure under a given stress, reduction in modulus due to moisture ingression, increase in modulus due to oxidation, unacceptable crack depth, or a variety of other possible criteria. It is also important that the criteria be related to practical adhesive joint performance. This is where it is difficult, and one must presume, at least for this limited analysis, that the adhesive will fail via a bulk (cohesive) property. 

With every statistical test we perform, we face the fact that if no real effect were present, there would still be a 5 per cent risk of a false positive. We accept this small risk along with every /-test or correlation analysis, etc. The real problem comes when we indulge in a plethora of tests and the generic term multiple testing covers this scenario. It is one thing learning to live with the standard 5 per cent risk, but multiple tests will jointly entail a much greater hazard that, somewhere, a false positive will creep in. 

Acetal Copolymer (CELCON ) Two types of non-solvent adhesives are used, structural and non-structural. Most structural adhesives are based on thermoset resins and require the use of a catalyst and/or heat to cure. This type of adhesive is normally used in applications which require maximum bond strength and minimum creep of the adhesive joint under sustained loading. Many structural adhesives can be used continuously at temperatures up to 350 F, which is higher than the... 

Although radiation may raise the softening temperature because of the crosslinks produced, the thermodynamic melting point is hardly changed in PE as demonstrated by Dole and Howard (20) by means of accurate specific heat measurements. Ultrahigh molecular weight polyethylene, DHMWPE, is used as Implantable prosthetic devices such as total joint replacements because of its mechanical properties, resistance to chemicals and to abrasion. Irradiation enhances its creep resistance (21). 

The lead-free solders are mostly based on Sn-containing binary and ternary alloys. Among them, the Sn-Ag system is one of the earliest commercially available lead-free solders and has been recommended for general-purpose use as a substitute for Sn-Pb eutectic solder. Addition of nano-particles of second phase helps in improving thermo-mechanical properties such as melting temperature, mechanical strength, mechanical fatigue resistance, creep resistance and solder-joint reliability. 

The DB-procedure was optimised in respect with the kinetic requirements and the high-temperature mechanical properties of the Ni-superalloy. From the kinetic point of view, the bonding temperature should be over 1000°C when alumina and transition metals are directly bonded [6]. The bonding procedure was always carried out in high vacuum, better than 2-10 mbar (0.2 mPa). The typical thermal and axial compression cycles are presented in Fig.la. It was experimentally found that the ambient bonding temperature is 1100"C or less due to the fast creep of the superalloy beyond this. The compression for the tests was selected as 10 MPa in ceramic-metal joints and 20 MPa in ceramic-ceramic joints [6]. 

R. L. Williamson, B. H. Rabin and G. E. Byerly, "FEM Study of the Effects of Interlayers and Creep in Redueing Residual Stresses and Strains in Ceramic-Metal Joints," Composites Eng., 5 [7] 851-863 (1995). 

What is the use of an optimum surface treatment if creep corrosion affects the adhesive layer from outside the glueline, which results in the destruction of the bonded joint This process is shown in Figure 7.8. 

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