Design creep

Products are used at different temperatures, and each has a unique stress distribution. Consequently, design requires creep data under a wide variety of conditions. However the test programme to generate such data is excessively long. For most plastics there will be tensile creep data, for times up to 

A cross-plot can be made of the tensile creep strains for a specific time versus the creep stress. When a smooth curve is fitted to the discrete 

There is linear viscoelastic behaviour in the stress region where the isochronous stress-strain curve is linear (to within 5%). The creep compliance /( ), defined by Eq. (7.4), is independent of stress. However, above this stress region (stresses 1 MPa for the data in Fig. 7.7 for a time of 1 year) there is non-linear viscoelastic behaviour and the creep compliance becomes stress dependent 

A simple product that undergoes creep is shown in Fig. 7.8. A cantilever arm, with a float at one end, operates a water valve in a domestic cold-water tank. The load at the free end, due to the buoyancy of the float, is 5 N. The deflection of the free end of the arm should not exceed 30 mm after 1 year. The arm is to be injection moulded from polyethylene, and the cross section of the arm must be determined. 

We start with analysis for the deflection of an elastic arm. For a point force F on the end of cantilever beam of length L, Appendix C shows that the deflection 8 at the free end is 

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Engineering codes such as ASME B31.3 [4] for piping and ASME Section VIII, Div. 1 [1] for vessels contain provisions for creep design. If creep is a concern, coarse grained materials are favored. Carbon steels killed with silicon are usually recommended when creep is a concern, e.g., ASTM A106 for pipe and ASTM A515 for plate. 

Findley W N (1971) Combined stress creep of non-linear viscoelastic material, in Advances in Creep Design (Eds. Smith A I and Nicolsou A M) Applied Science Publication, Loudon, Ch. 14. 

Load-ceU performance is limited by properties such as nonlinearity, hysteresis, and creep, which are inherent to the particular design or spring element material. 

Static friction decreases with an increase in load, and the static coefficient of friction is lower than the dynamic coefficient. The tendency to creep must be considered carefliUy in FEP products designed for service under continuous stresses. Creep can be minimized by suitable fillers. Fillets are also used to improve wear resistance and stiffness. Compositions such as 30% bronze-fiUed FEP, 20% graphite-filled FEP, and 10% glass-fiber-filled FEP offer high PV values ( 400(kPa-m)/s) and are suitable for beatings. 

Creep of Thick-walled Cylinders. The design of relatively thick-walled pressure vessels for operation at elevated temperatures where creep caimot be ignored is of interest to the oil, chemical, and power industries. In steam power plants, pressures of 35 MPa (5000 psi) and 650°C are used. Quart2 crystals are grown hydrothermaHy, using a batch process, in vessels operating at a temperature of 340—400°C and a pressure of 170 MPa (25,000 psi). In general, in the chemical industry creep is not a problem provided the wall temperature of vessels made of Ni—Cr—Mo steel is below 350°C. 

Division 1. Below the creep range, design stresses are based on one-fourth of the tensile strength or two-thkds of the yield, or 0.2% proof stress. Design procedures are given for typical vessel components under both internal pressure and external pressure. No specific requkements are given for the assessment of fatigue and thermal stresses. 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

At high temperature, the behavior is different. A stmcture designed according to the principles employed for room temperature service continues to deform with time after load apphcation, even though the design data may have been based on tension tests at the temperature of interest. This deformation with time is called creep because the design stresses at which it was first recognized occurred at a relatively low rate. 

The role, design, and maintenance of creepproof barriers in traps, especially those in oil DPs, remain to be fully explored. In general, uncracked oil from a DP is completely inhibited from creeping by a surface temperature of <223 K. On the other hand, a cold trap, to perform effectively in an ordinary vacuum system, must be <173 K because of the vapor pressure of water, and <78 K because of the vapor pressure of CO2. For ultracontroUed vacuum environments, LN temperature or lower is required. CO2 accumulation on the trap surface must be less than one monolayer. The effectiveness of a LN trap can be observed by the absence of pressure pips on an ionization gauge when LN is replenished in the reservoir. 

The maximum operating temperature is arbitrarily set at 260 C (500 F) because harder temper adversely affects design stress in the creep-rnptnre-temperatnre ranges. 

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. 

Fig. 17.3. Creep is important in (our classes of design (a) displacement-limited, (b) failure-limited, (c) relaxation-limited and (d) buckling-limited.

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