Thermal Expansion and Contraction

There are plastics that have equal or less than those of other materials of construction (metals, glass, or wood). In fact with certain additives such as graphite powder contraction can occur rather than the expected expansion with the application of heat. However many plastics typically have coefficients that are considerably higher than those of other materials of construction such as metals. This difference may amount to a factor of 10 to 30. Also available are plastics, particularly TS-RPs, with practically no change. 

Obviously, the designer must take thermal expansion and contraction into account if critical dimensions and clearances are to be maintained during use where material is in a restricted design. Less obvious is the fact that products may develop high stresses when they are constrained from freely expanding or contracting in response to temperature changes. These temperature-induced stresses can cause material failure. 

Plastic products are often constrained from freely expanding or contracting by rigidly attaching them to another structure made of a material (plastic, metal, etc.) with a lower coefficient of linear thermal expansion. When such composite structures are heated, the plastic component is placed in a state of compression and may buckle, etc. When such composite structures are cooled, the plastic component is placed in a state of tension, which may cause the material to yield or crack. The precise level of stress in the plastic depends on the relative compliance of the component to which it is attached, and on assembly stress. 

To minimize the stresses induced by differential thermal expansion/contraction one must (1) employ fastening techniques that allow relative movement between the component parts of the composite structure, (2) minimize the difference in coefficient of linear thermal expansion between the materials 

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Temperature and Humidity. Temperature is probably the easiest environmental factor to control. The main concern is that the temperature remains constant to prevent the thermal expansions and contractions that are particularly dangerous to composite objects. Another factor regarding temperature is the inverse relation to relative humidity under conditions of constant absolute humidity, such as exist in closed areas. High extremes in temperature are especially undesirable, as they increase reaction rates. Areas in which objects are exhibited and stored must be accessible thus a reasonable temperature setting is generally recommended to be about 21°C. 

Fretting corrosion (36,37) can lead to high contact resistance of base metal contacts, such as tin plate in electronic connectors. Small cycHcal displacements of the connector halves occur because of external vibration or differential thermal expansion and contraction of the mating contacts. The wear debris that is formed remains in the contact zone. The accumulation of oxide debris in the contact region leads to increased contact resistance. Solutions to this problem are stmctures that do not permit movement of contact surfaces with respect to one another, the use of gold as a contact finish, and the appHcation of thick coatings of contact lubricants and greases, which reduce the rate of wear and restrict access of air to the contact surfaces. 

Thermal-expansion and -contraction loads occur when a piping system is prevented from free thermal expansion or contraction as a result of anchors and restraints or undergoes large, rapid temperature changes or unequal temperature distribution because of an injection of cold liquid striking the wall of a pipe cariying hot gas. 

Stresses may be applied and/or residual. Examples of applied stresses are those arising from thermal expansion and contraction, pressure, and service loads. Applied stresses may be continuous or intermittent. Examples of residual stresses are those arising from welding, fabrication, and installation. The importance of residual stresses in SCC should not be underestimated. Residual stresses may... 

The cyclic stresses responsible for this failure were apparently bending stresses associated with cyclic thermal expansion and contraction. 

The orientation of the cracks indicates that cyclic bending stresses or cyclic axial stresses generated by thermal expansion and contraction provided the responsible stresses. The large number of crack initiation sites and the tightness of the cracks indicate high-level stresses. 

Stresses from welding result principally from the effects of differential thermal expansion and contraction arising from the large temperature difference between the weld bead and the relatively cold adjacent base metal. Shrinkage of the weld metal during solidification can also induce high residual stresses. Unless these residual stresses are removed, they remain an intrinsic condition of the weldment apart from any applied stresses imposed as a result of equipment operation. 

The thermal expansion and contraction of solids can also have safety implications. For a given material the amount of its linear expansion, or contraction, in one direction is directly related to temperature and its original size (i.e. length, diameter, circumference). Thus ... 

In some hydraulic systems, it is necessary to maintain the system pressure within a specific pressure range for long periods. It is very difficult to maintain a closed system without some leakage, either external or internal. Even a small leak can cause a decrease in pressure. By using an accumulator, leakage can be compensated for and the system pressure can be maintained within acceptable range for extended periods. Accumulators also compensate for thermal expansion and contraction of the liquid due to variations in temperature or generated heat. 

Fines should normally be kept as short and free of bends as possible. However, tubing should not be assembled in a straight line, because a bend tends to eliminate strain by absorbing vibration and compensates for thermal expansion and contraction. Bends are preferred to elbows, because bends cause less loss of power. Some of the correct and incorrect methods of installing tubing are illustrated in Figure 40.26. 

They are applied by wet paint spray techniques and have the advantage, over other paint systems, of long-term flexibility. Conventional alkyd systems may have an initial degree of flexibility, but within 12 months outside become rigid and then crack due to thermal expansion and contraction of the substrate. This phenomenon is less likely to occur with a well formulated vinyl solution. 

An influence on dimensions and tolerances involves the coefficient of linear thermal expansion or contraction. This CLTE value has to be determined at the product s operating temperature (Chapter 2, THERMAL EXPANSION AND CONTRACTION) Plas tics can provide all extremes in CLTEs. As an... 

When materials with different coefficients of linear thermal expansion (CLTE) are bolted, riveted, bonded, crimped, pressed, welded, or fastened together by any method that prevents relative movement between the products, there is the potential for thermal stress. Most plastics, such as the unfilled commodity TPs, may have ten times the expansion rates of many nonplastic materials. However there are plastics with practically no expansion. Details are reviewed in Chapter 2, THERMAL EXPANSION AND CONTRACTION. 

Metal inserts are usually not feasible, because thin walls are not sufficiently strong to hold inserts, particularly if thermal expansion and contraction of the product takes place in service. Figure 3-40 shows a method of holding metal fittings. It may be desirable to in-... 

Products are affected dimensionally by the difference between their forming temperature and their product-use temperature. Thus, a plastic s coefficient of thermal expansion and contraction has a significant effect on service conditions. The thermoforming pressure, time, and temperature variations that can exist will affect the final dimensions. Of these factors, evenness in heating throughout the sheet thickness before forming is usually the most important control. Type of heater has a direct effect on obtaining uniform heat... 

Thermal Loads Due to Restraints. These loads consist of thrusts and moments, which arise when free thermal expansion and contraction of the piping are prevented by restraints or anchors. 

Pressure design of straight-threaded joints shall be based on calculations consistent with design requirements of this Code. These calculations shall be substantiated by testing in accordance with to-be-determined procedures and protocols. The testing shall consider such factors as assembly and disassembly, cyclic loading, vibration, shock, hydrogen embrittlement, thermal expansion and contraction, and other factors to be determined. 

In addition to the design requirements for pressure, weight, and other loadings, hydrogen piping systems subject to thermal expansion and contraction or to similar movements imposed by other sources shall be designed in accordance with the requirements for the evaluation and analysis of flexibility and stresses specified herein. 

Supporting elements shall be designed to permit the free movement of piping caused by thermal expansion and contraction. 

Thermal expansion and contraction are reversible effects of temperature which may be very important in some applications. Usually expansion is measured using thermomechanical analysis (TMA) (see ISO 11359-2 [4]). 

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