Mechanical properties creep

Lead—copper alloys are specified because of superior mechanical properties, creep resistance, corrosion resistance, and high temperature stabiUty compared to pure lead. The mechanical properties of lead—copper alloys are compared to pure lead, and to lead—antimony and lead—calcium alloys in Tables 4 and 5. 

The mechanical properties at low strain rates, dynamic mechanical properties, creep-recovery behaviour, thermal expansion and thermal conductivity of foams manufactured from blends of LDPE with an EVA and with an isoprene-styrene block copolymer were studied as a function of the LDPE content in the blends. The experimental results demonstrated important aspects related to the modification of the foam properties by blending. 16 refs. 

Polysulfone Plastics. These plastics which were commercialized by Union Carbide are actually aromatic polyethers containing periodic sulfone groups which provide additional resonance stabilization. They have good mechanical properties, creep resistance, and dimensional stability but their outstanding quality is their high heat distortion temperature (345°F.) and resistance to thermal oxidative degradation. Limitations are difficult thermoplastic processability, amber color, and sensitivity to organic solvents. 

Sample CPVC PVC Content, wt % Vicat, °C Mechanical Properties Creep Izod... 

As the grain boundaries move through the casting, they are trapped on defects and impurities. This can result in boundaries that are more prone to corrosion. It has been suggested [25] that heat treatment of the alloy can redissolve the PbsCa particles near the boundary and, thereby, reduce the rate of corrosion of the alloy. Certainly, the mechanical properties, creep rate, and corrosion rates of lead-calcium alloys are... 

Another application of carbon fibres has been realised by an attempt to overcome creep effects in polyethylene by random incorporation of chopped carbon fibres into the polymer. However, when the proportion of carbon fibre is optimised with respect to the mechanical properties (creep) the rate of wear against a stainless steel counterface in a pin on disc machine is increased. 

Wrought lead—calcium—tin alloys contain more tin, have higher mechanical strength, exhibit greater stabiUty, and are more creep resistant than the cast alloys. RoUed lead—calcium—tin alloy strip is used to produce automotive battery grids in a continuous process (13). Table 5 Hsts the mechanical properties of roUed lead—calcium—tin alloys, compared with lead—copper and roUed lead—antimony (6 wt %) alloys. 

Lead—tin (1.8—2.5 wt %) is used both as a cable sheathing ahoy (BS 801 ahoy A and DIN 17640) and as a battery connector ahoy ia sealed lead—calcium—tin batteries (15). Tia is generahy added to lead—arsenic cable ahoys ia smah amounts. The arsenic ahoys have excehent creep resistance and mechanical properties, but are unstable and lose arsenic readily by oxidation. The addition of smah amounts of tin (0.10—0.20 wt %) eliminates arsenic loss. Lead ahoys having 0.4 wt % tin and 0.15 % cadmium, which are used for cable sheathing, do not age harden, show excehent corrosion and creep resistance, and are very ductile. 

Mechanical properties of plastics can be determined by short, single-point quaUty control tests and longer, generally multipoint or multiple condition procedures that relate to fundamental polymer properties. Single-point tests iaclude tensile, compressive, flexural, shear, and impact properties of plastics creep, heat aging, creep mpture, and environmental stress-crackiag tests usually result ia multipoint curves or tables for comparison of the original response to post-exposure response. 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

A fully automated microscale indentor known as the Nano Indentor is available from Nano Instmments (257—259). Used with the Berkovich diamond indentor, this system has load and displacement resolutions of 0.3 N and 0.16 nm, respectively. Multiple indentations can be made on one specimen with spatial accuracy of better than 200 nm using a computer controlled sample manipulation table. This allows spatial mapping of mechanical properties. Hardness and elastic modulus are typically measured (259,260) but time-dependent phenomena such as creep and adhesive strength can also be monitored. 

Solders. In spite of the wide use and development of solders for millennia, as of the mid-1990s most principal solders are lead- or tin-based alloys to which a small amount of silver, zinc, antimony, bismuth, and indium or a combination thereof are added. The principal criterion for choosing a certain solder is its melting characteristics, ie, soHdus and Hquidus temperatures and the temperature spread or pasty range between them. Other criteria are mechanical properties such as strength and creep resistance, physical properties such as electrical and thermal conductivity, and corrosion resistance. 

Mechanical history, heat, and impurities gready affect the mechanical properties. Pure zinc is ductile at room temperature and does not have a definite yield point as do most stmctural metals. Rather, it creeps under sufficient constant load. The impurities of commercial zinc and alloying metals are carefully controlled to achieve the desired mechanical properties. 

Sintered sihcon carbide retains its strength at elevated temperatures and shows excellent time-dependent properties such as creep and slow crack growth resistance. Reaction-bonded SiC, because of the presence of free sihcon in its microstmcture, exhibits slightly inferior elevated temperature properties as compared to sintered sihcon carbide. Table 2 (11,43) and Table 3 (44) show selected mechanical properties of sihcon carbide at room and elevated temperatures. 

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). 

Mechanical Properties Mechanical properties of wide interest include creep, rupture, short-time strengths, and various forms of ductihty, as well as resistance to impact and fatigue stresses. Creep strength and stress rupture are usually of greatest interest to designers of stationary equipment such as vessels and furnaces. 

Alloys of antimony, tin, and arsenic offer hmited improvement in mechanical properties, but the usefulness of lead is limited primarily because of its poor structural qualities. It has a low melting point and a high coefficient of expansion, and it is a veiy ductile material that will creep under a tensile stress as low as 1 MPa (145 IbFin"). 

In the last chapter we saw how a basic knowledge of the mechanisms of creep was an important aid to the development of materials with good creep properties. An impressive example is in the development of materials for the high-pressure stage of a modern aircraft gas turbine. Here we examine the properties such materials must have, the way in which the present generation of materials has evolved, and the likely direction of their future development. 

As with other plastics materials, temperature has a considerable effect on mechanical properties. This is clearly illustrated in Figure 13.5 in the case of stress to break and elongation at break. Even at 20°C unfilled PTFE has a measurable creep with compression loads as low as 3001bf/in (2.1 MPa). 

Copolymers of chlorotrifluoroethylene and ethylene were introduced by Allied Chemicals under the trade name Halar in the early 1970s. This is essentially a 1 1 alternating copolymer compounded with stabilising additives. The polymer has mechanical properties more like those of nylon than of typical fluoroplastic, with low creep and very good impact strength. Furthermore the polymers have very good chemical resistance and electrical insulation properties and are resistant to burning. They may be injection moulded or formed into fibres. 

It must, however, be stressed that for design purposes such data have little value. Like the nylons, which are also widely used for load-bearing light engineering applications, the polyacetals exhibit a small but finite creep under load. It is thus necessary to consider mechanical properties under those main headings. 

Mechanical properties. Properties commonly of importance here are rigidity, creep resistance, strength and toughness. For some applications resistance to repeated flexing or abrasion may be critical. 

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