Creep reduction

The meehanism for creep reduction is not understood although the relationship between dopant concentration and creep resistance has been confirmed to be a true solid-solution effect. One school of thought is that creep is controlled by grain boundary diffusion and that large segregated ions simply hinder diffusion in the core sites at the boundary. This theory is supported by kinetic measurements of self-difEiision in undoped and yttria-doped alumina and by studies of the oxidation of aluminum alloys (Le Gall et al., 1995). There is... 

Fillers. In practice most epoxy resin systems have fillers incorporated, often simply to reduce cost although they may also assist in gap filling, reduction of creep, reduction of exotherm, corrosion inhibition and fire retardation. Their incorporation will also alter the physical and mechanical properties of the adhesive. Construction resins in particular often include a large volume fraction of sand or silica. 

Creep Resistsince. Studies on creep resistance of particulate reinforced composites seem to indicate that such composites are less creep resistant than are monolithic matrices. Silicon nitride reinforced with 40 vol % TiN has been found to have a higher creep rate and a reduced creep strength compared to that of unreinforced silicon nitride. Further reduction in properties have been observed with an increase in the volume fraction of particles and a decrease in the particle size (20). Similar results have been found for SiC particulate reinforced silicon nitride (64). Poor creep behavior has been attributed to the presence of glassy phases in the composite, and removal of these from the microstmcture may improve the high temperature mechanical properties (64). 

Chlorinated rubber is also used to promote the adhesion of solvent-borne CR adhesives to metals and plasticized PVC. Addition of a low molecular weight chlorinated rubber (containing about 65 wt% chlorine) improves the shear strength and creep resistance of polychloroprene adhesives [75] but a reduction in open time is also produced. A heat reactivation (process in which the surface of the adhesive film is raised to 90-100°C to destroy the crystallinity of the film and allowing diffusion to produce polymer chain interlocking more rapidly) restores tack to the polychloroprene adhesives. 

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and superimposed on this is an alternating axial load of 100 N. If the fatigue limit for the material is 13 MN/m and the creep rupture strength at the equivalent time is 40 MN/m, estimate a suitable diameter for the rod. Thermal effects may be ignored and a fatigue strength reduction factor of 1.5 with a safety factor of 2.5 should be used. 

Under normal operating conditions speed reduction due to slippage and creep of the V-belt material is usually quite small. Neglecting these effects, the velocity V (in./s) of the V belt is... 

Creep behavior Creep is the deformation that occurs over a long period of time in a material subjected to a continuous load, and stress relaxation is the reduction in stress with time that occurs in a material when it is de-... 

The instant the load is removed there is a reduction in the elongation of the model equal to aJEx. The equation for subsequent creep recovery is... 

Figure 3 shows plots of p versus shear rate at three different temperatures for the same latex (20% w/w latex A) at full coverage with PVA. These curves are typical of a pseudoplastic system showing a reduction of n with increasing shear rate, 7 p reaches a limiting value at If > 50 s l. It is also clear from fig. 3 that at 7 < 10 s-- -, n increases rapidly with reduction in 7. Comparison with nQ values obtained from the creep curves would indicate the p should increase very steeply with reduction of 7, in the low shear rate region (p is the limit of p as Y+0). °... 

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. 

J.H. Greenwood, Creep data on HDPE and acrylic thermoplastics a reduction factor approach, ERA Report 99-0323R, ERA Technology Limited, Leatherhead, UK, 1999. 

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