Influence of Thermal Ratcheting on the Creep and Mechanical Properties of High-Density Polyethylene

The objective of this research is to describe the consequence of thermal ratcheting on the long-term creep property of the high-density polyethylene (HDPE) material. The thermal ratcheting phenomenon increases significantly the creep strain of HDPE. The magnitude of the creep strain of HDPE increases by 8% after just 20 thermal cycles between 28 and 50 °C. The creep modulus, which is inversely proportional to the creep strain, depletes further under thermal ratcheting. Both the properties change significantly with the number of thermal cycles. The coefficient of thermal expansion (CTE) of HDPE varies with the applied compressive load, with successive thermal cycles, and with the thermal ratcheting temperature. The impact of thermal ratcheting diminishes with an increase in initial steady creep exposure time period, but still the magnitude cumulative deformation induced is noteworthy. The magnitude of growth in creep strain drops from 8% to 2.4% when thermal ratcheting is performed after 1 and 45 days of steady creep, respectively. There is a notable change in the thickness of the material with each heating and cooling cycle even after 45 days of creep; however, the thermal ratcheting strain value drops by 80% in comparison with the thermal ratcheting strain after 1 day of creep and under similar test conditions.

Ratcheting due to Thermal Stratification

Thermal ratcheting is required to be checked by most of the piping design codes, specifically the ASME B&PV Code. For cases where the variation of temperature distribution is not uniform, the existing ratchet check methodology for piping is inadequate and therefore the finite element analysis (FEA) is often used to perform ratchet checks. Thermal stratification, in which cold and hot fluid flows are layered in a relatively steady state condition, is a good example of non-linear/non-uniform temperature distribution across the pipe. This paper develops straightforward equations to address thermal stratification in piping. Finite element analysis is used to benchmark the results.

Influence of Thermal Ratcheting on the Creep and Mechanical Properties of High Density Polyethylene (HDPE)

The objective of this research is to describe the consequence of thermal ratcheting on the long-term creep property of HDPE material. The thermal ratcheting phenomenon amplifies significantly the creep strain of HDPE in comparison to the steady creep strain under constant temperature. The magnitude of creep strain of HDPE increases by 8% after just 20 thermal cycles between 28 and 50°C. The creep modulus which is inversely proportional to the creep strain depletes further under thermal ratcheting. Both properties change significantly with the number of thermal cycles. The coefficient of thermal expansion (CTE) of HDPE varies with the applied compressive load, with successive thermal cycles and with the thermal ratcheting temperature. The impact of thermal ratcheting diminishes with increase in initial steady creep exposure time-period but still the magnitude cumulative damage induced is noteworthy. The magnitude of growth in creep strain drops from 8 to 2.4% when thermal ratcheting is performed after 1 and 45 days of steady creep, respectively. There is a notable change in thickness of the material with each heating and cooling cycle even after 45 days of creep however, the thermal ratcheting strain value drops by 80% in comparison to thermal ratcheting strain after 1 day of creep and under similar test conditions.

Proposal of Failure Mode Map Under Dynamic Loading—Ratcheting and Collapse

Ratcheting, collapse, and fatigue are the probable failure modes which can occur under alternate dynamic loading like seismic loading. The objective of this study is to propose a failure mode map for rectangular beams by determining the conditions of occurrence of the ratcheting and collapse failure modes. The paper considers the analogy between thermal ratcheting and dynamic ratcheting. The nonlinear dynamic finite element method was used to analyze a rectangular beam model for different loading conditions. The results were plotted on a nondimensional primary and secondary stress parameter graph similar to the Bree diagram for thermal ratcheting. The similarity between thermal load and dynamic load was observed. The main difference between thermal and dynamic loading is the effect of the frequency of dynamic loading on the occurrence of ratcheting and collapse. Experimental observations of ratcheting have been obtained and are used for comparison to validate the analytical predictions. From the above results, a failure mode map has been proposed which can evaluate the occurrence conditions of ratcheting and collapse under dynamic loadings.