Creep performance

While for many years, metal single crystals were used only as tools for fundamental research, at the beginning of the 1970s single-crystal gas-turbine blades began to be made in the hope of improving creep performance, and today all such blades are routinely manufactured in this form (Duhl 1989). 

Over the past century, many plastic products have been successfully designed for long-time creep performance based on the information and test data then available, but much more exists now and will in the future. 

A creep test is in essence very simple - a constant force is applied to the rubber and the change in deformation with time monitored - but detailed procedures were not standardised internationally until 1988 and there is still no general ASTM method. This reflects the relatively small amount of creep testing carried out on rubbers, which in turn is due to the relatively few rubber products where creep is a problem. This is in contrast to the situation with thermoplastics where creep performance is a prime engineering factor. However, for particular applications of rubber where creep is important, for example bridge bearings, a considerable amount of data has been generated. 

The present ISO standard for creep is ISO 80131 which specifies procedures for measurements in compression and shear. In earlier standards, creep and stress relaxation were covered in the same documents and creep in tension was included. One reason for the separation was that stress relaxation became more important for seal performance, whereas creep remained a more minority interest. Measurements in tension were dropped on the basis that engineering components are not generally stressed in this manner. However, it is worth noting that, if a general indication of creep performance is required, the strains in tension can be relatively large and only quite simple apparatus is necessary. Such a simple method is included in the ISO standard for tension set described in Section 3.2. The British equivalent, BS903 Part A152 is identical to ISO 8013. 

J. R. Porter and A. Chokshi, Creep Performance of Silicon Carbide Whisker-Reinforced Alumina in Ceramic Microstructures 86 The Role of Interfaces, eds. J. Pask and A. Evans, Plenum Press, New York, NY, 1987, p. 919. 

Creep performance may be inferred from the solution of simpler, desired product geometry, or a simple model of creep properties may be used. However, any such model must adequately represent the creep properties of the material, as with models for metals where creep contained in FEA software may not be adequate for plastics, particularly unreinforced TPs. Metals creep is usually approximated by its secondary (constant rate) component, whereas plastics creep is essentially primary creep (decreasing creep rate), one consequence of which is that creep strain for metals is usually plotted against time, but for plastics it is plotted against log time. 

Segregant-weakened interfaces are an attractive concept based on the possibility of eliminating the coating phase from the composite fabrication process. This concept has only been demonstrated, however, on model composite systems. A major technical issue is whether the matrix or fiber, or both, would have to be doped to control the fiber-matrix interfacial energy. If polycrystalline fibers require doping, then the effects of dopants on creep performance must be considered. Therefore, additional basic research of this interface concept, utilizing available polycrystalline fibers and candidate oxide matrices, is recommended. 

The Nextel 720 based composites show substantially improved creep performance in correlation with the superior fiber properties. A creep limit of 145 MPa, which is 80% of the ultimate strength, was identified by John et al. [158] as the level where the COI720/AS composite could survive 100 hours at 1100°C. The linear creep rate is 1 x 10 /s under these conditions, however the creep response at this high load is significantly greater during the initial 10 hours before decreasing to a steady rate [159]. 

Cahrera J.C. and A.F. Nikolaides. 1988. Creep performance of cold dense hituminous mixtmts. Journal of the Institution of Highways and Transport, Vol. 35, No. 10, p. 7. 

Mays, G.C. Fatigue and Creep Performance of Epoxy Resin Adhesive Joints, To be published in TRRL Contractors Report Series, 1990. 

A Mukhopadhyay and S Ghosh, Creep performance of short stretch bandages , Indian J of Fibre and Textile Research, 2005 30 331-334. 

Figure 13.2 Comparison ofthe creep performance in different grades of silicon nitride ceramics at 150 MPa [21],...

For example, single-lap shear joints can have a known static load applied, then inserted into a specified environment, and the time-to-failnie of joints noted as a function of variables such as adhesive type, pre-treatment and stress (see Shear tests). Static loads can also be applied to other joint configurations. Many studies have shown that such tests exhibit good discrimination between different surface pre-treatments. The creep performance of adhesives can also be measured by means of static load testing (see Durability creep rupture and Durability sub-critical debonding). 

Polypropylene Special rubber reinforced mineral-filled grade Good impact behaviour Good stiffness Cheaper than ABS Poor creep performance... 

Zhang G, Karger-Kocsis J and Zou J (2010) Synergistic effect of carbon nanofibers and short carbon fibers on the mechanical and fracture properties of epoxy resin, Carbon 48 4289-4300. Varela-Rizo H, Weisenberger M, Bortz D R and Martin-Gullon I (2010) Fracture toughness and creep performance of PMMA composites containing micro- and nanosized carbon filaments. Compos Sci Technol 70 1189-1195. 

At present only special programs are available which take into account the creep performance of statically loaded plastics. The usual method is to use a creep-adjusted apparent modulus of elasticity in a second cut structural analysis to determine whether there will be serious creep effects on the product which must be accounted for in the design. With increased sophistication of the computer programs available it will soon be possible to do a direct computer analysis of the creep effects on a plastics component. In fact, there is current work on programs that produce a time model of the plastics component under load that will show how the stresses affect shape and load capabilities with time. 

Anchors are in many cases pretensioned in order to limit the deformation to activate the anchor. The anchor system is therefore very stiff. Failure of structure resulted form anchor rupture often occurs very quickly and catastrophically. Pretension may also be lost over time because of creep in some types of rock and soil. Anchors should be tested carefully for their design capacity and creep performance. 

FEP contains CFj side groups due to copolymerisation with 10-12% hexafluoropropylene. The side groups tend to lock together and improve mechanical properties at processable molecular weights. FEP can be processed by extrusion, and its creep performance is better than that of PTFE. However steric stress due to the bulky side group reduces the CUT from 260 to 200 °C. PFA introduces a perfluoroalkoxy side chain - typically OC3F7. PFA can be injection moulded. Both PFA and FEP have PTFE-like chemical resistance and... 

The use of a commercial Cloisite 20A organoclay to prepare SBS-based nanocomposites by melt processing was recently reported [63]. In this case, the nanocomposite morphology was characterized by a combination of intercalated and partly exfoliated clay platelets, with occasional clay aggregates present at higher clay content. For this particular thermoplastic elastomer nanocomposite system, well-dispersed nanoclays lead to enhanced stiffness and ductility, suggesting promising improvements in nanocomposite creep performance. The use of stearic acid as a surface modifier of montmorillonite clay to effectively improve the clay dispersion in the SBS matrix and the mechanical properties of the SBS-clay nanocomposites was reported [64]. 

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