Creep, construction materials

Our first non-Newtonian liquids are solutions or melts of a polymer (Figure C4-4). In equilibrium the polymer strands tend to form more or less spherical coils. However, when the liquid is sheared, the coils are stretched and tend to become aligned. This stretching increases their energy, and when the shear is removed, they rebound . So the fluid has elastic properties in addition to viscous ones. An important property of the polymer in solution is its relaxation time. This is a measure for the time that it takes to rebound. It is of the order of nanoseconds for small molecules, of seconds for polymers in processing equipment, and of centuries in construction materials. Yes, these last ones also flow or creep . 

One of the most valuable properties of polysulfones is well creep resistance, especially at high temperatures. The polysulfone s creep deformation is 1.5% at 100 °C and after 3.6 x 10 sec loading of 21 MPa, which is better than that of others. Their long-term strength at high temperature is also better. Thus, the polysulfone can be used as constructional material instead of metals. 

Materials. To validate the design and ensure satisfactory performance over the entire lifetime of the plant there must be an adequate data base for the construction materials. This includes long-term data on creep, fiitigue, and d adation of mechanical properties, reflecting typical plant design lifetimes of up to 40 years. As a result well-established and documented materials have to be used. It is a lengthy and expensive process to develop a new material to design standards. 

The tube materials are limited by design temperature and creep rupture strength. Recent progress in the formulation of construction materials (Jones, 90/91) and (Thuillier et al., 1978) has allowed safe designs with exit gas temperatures exceeding 950°C corresponding to tube wall temperatures up to 1050°C. The new materials which have superior creep rupture characteristics, allow a design with a considerable reduction in tube wall thickness. The use of a thin tube wall... 

There are materials-of-construction problems associated with high-temperature operation. The main problem is creep. 

Examination of reinforced structures has so far shown the creep strain to be less than predicted, generally because the soil carries more load than is allowed for in design. Tests on materials buried and then exhumed after 10-20 years have so far shown little significant evidence of degradation, although little would be expected in the relatively benign soil conditions experienced. In common with industrial products, early failures have been shown to be due to errors in design or construction any failures due to poor durability will occur later, hopefully very much later. 

CNF is an industrially produced derivative of carbon formed by the decomposition and graphitization of rich organic carbon polymers (Fig. 14.3). The most common precursor is polyacrylonitrile (PAN), as it yields high tensile and compressive strength fibers that have high resistance to corrosion, creep and fatigue. For these reasons, the fibers are widely used in the automotive and aerospace industries [1], Carbon fiber is an important ingredient of carbon composite materials, which are used in fuel cell construction, particularly in gas-diffusion layers where the fibers are woven to form a type of carbon cloth. 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

Master curves can be used to predict creep resistance, embrittlement, and other property changes over time at a given temperature, or the time it takes for the modulus or some other parameter to reach a critical value. For example, a rubber hose may burst or crack if its modulus exceeds a certain level, or an elastomeric mount may fail if creep is excessive. The time it takes to reach the critical value at a given temperature can be deduced from the master curve. Frequency-based master curves can be used to predict impact behavior or the damping ability of materials being considered for sound or vibration deadening. The theory, construction, and use of master curves have been discussed (145,242,271,277,278,299,300). 

Materials selection process can be depicted in terms of Figure 1.40. Materials selection involves many factors that have to be optimized for a particular application. The foremost consideration is the cost of the material and its applicability in the environmental conditions so that integrity can be maintained during the lifetime of the equipment. When the material of construction is metallic in nature, the chemical composition and the mechanical properties of the metal are significant. Some of the important mechanical properties are hardness, creep, fatigue, stiffness, compression, shear, impact, tensile strength and wear. 

In selecting metals and alloys as materials of construction, one must have knowledge of how materials fail, for example is, how they corrode, become brittle with low-temperature operation, or degrade as a result of operating at high temperatures. Corrosion, embrittlement, and other degradation mechanisms such as creep will be described in terms of their threshold values. Transient or upset operating conditions are common causes of failure. Examples include start-ups and shutdowns, loss of coolant, the formation of dew point water, and hot spots due to the formation of scale deposits on heat transfer surfaces. Identification and documentation of all anticipated upset and transient conditions are required. 

In selecting a material for the construction of high pressure reactors a number of factors must lie considered. The more important of these are as follows 1, Ultimate tensile strength 2, proportional limit 3, corrosion resistance 4, workability 5, recrystallization if operating temperatures are to be as high as 500° C. or higher 6, creep stress if the service period is to be long. 

Aluminum is a low melting point metal, therefore microstructural stability (and consequently creep) is an important issue even for near ambient temperature. Many common structural aluminum alloys are precipitation strengthened at heat treatment temperatures on the order of 423 K, implying that the service temperature must be considerably lower. There are a few aluminum alloys designed for high-temperature applications however, creep makes them unacceptable as materials of construction for contaimnent of pressurized gas at elevated temperature. On the other hand, aluminum alloys are commonly employed at cryogenic temperatures. 

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