Creep guidelines

Creep guideline. Here is a summation of the factors to consider when reviewing creep properties ... 

Extensive amount of these type data has been plotted but unfortunately most of it is privately owned. Creep data available from material suppliers, college and government projects, etc. can provide guidelines. However where the product has to meet critical requirements that usually include safety of people and data from previous work does not exist, creep test have to be conducted and properly applied by the designer. 

Since the net drag on an arbitrary particle is generally not parallel to the direction of motion, a particle falls vertically without rotation only if it possesses a certain symmetry or a specific orientation. The following guidelines for solid particles with uniform density are derived from general results for creeping flow (H3) ... 

P(6) If data are not available then the guideline data presented in Figure 4.13 may be used. In order to accommodate composites with varying volume fraction and fibre orientations the figure shows normalised creep modulus against time. It is assumed that initial modulus and therefore creep modulus are linearly proportional to fibre volume fraction. This is a reasonable approximation in most circumstances. 

Creep begins at different temperatures for different materials and the following may be used as a very general guideline ... 

Bailey power law is used to determine creep strain and creep strain rate. The MFC Omega method can he used to calculate accumulated and future strain, total damage and damage rate, creep rate, and remaining life. In addition there are several industry standards that provide guidelines, criteria, and design data for materials in the creep range. These are AFI-530, AFI-579, and WRC-443. 

Several design standards may be used for the design of reactor tubes, for example, the guidelines AP530 of the American Petroleum Institute (51), which gives guidelines for creep rupture stress calculations. 

To account for the material evolutions, guidelines dedicated to the external or internal reinforcements of concrete structures using FRP composites usually introduce substantial durability parameters (reduction factors on FRP tensile properties and on shear characteristics of FRP/concrete bonded interfaces, creep stress levels and fatigue limits) see, for instance, ACI 440.2R-08 (2008) and ACI 440.1R-06 (2006). However, further research is still needed to refine the design codes and better calibrate the durability reduction factors taking into account synergistic effects between various environmental factors. 

The dwell time is determined by the creep behavior of the solder. At and above 100 °C (212 °F), 15 min. of dwell should be allowed, and between 80 to 100 °C (176 to 212 °F)> 20 min. is advisable. Unfortunately, it is rare that enough time can be given to allow for creep at low temperatures. Tables 5 and 6 can be used as guidelines once the specimen has reached the dwell temperature. Even if the numbers are only valid for the 2220 chip capacitors used for this estimate, the decrease in strain compared to the maximum strain possible can serve as a general guideline. 

In extending the allowable stresses to 500,000 h, the most important point is to use a reliable database of creep mpture data. Fig. 18.1 shows an example of creep data on Mod.9Cr-lMo steel [10—12]. A number of data whose rupture time is over 100,000 h have been obtained at 550°C. This leads to a time-wise extrapolation of a factor of 4—5 to determine allowable stresses up to 500,000 h. There is no unique scientific way to determine how much extrapolation would be allowed. ASME Boiler and Pressure Vessel Code Section IB Division 5 Subsection HB Subpart B Appendix HBB-Y Guidelines for Design Data Needs for New Materials (ASME Appendix HBB-Y) [13], which was published as a nonmandatory appendix in 2015, provides some guidance on the extrapolation of creep rupture time as part of the requirements for adoption of new materials into the Code. It assumes use of time—temperature... 

The appendix states that extrapolations by more than a factor of 3 will require metallurgical justification for metastable alloys such as the creep strength-enhanced ferritic-martensitic steels. When the maximum rupture time experimentally obtained is less than 166,667 h, for example, in order to achieve a 60-year design, an extrapolation of factor of 3 would be necessary. If we follow the guidelines, metallurgical justification would be required. 

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