Rudolph J. Scavuzzo

Oscillating Stress on Viscoelastic Behavior of Thermoplastic Polymers

Polymers are used in many applications where they are subjected to cyclic stresses. PVC and HDPE piping are often used in systems that include rotating machinery that cause mechanical vibration. Recent testing of thermoplastics indicates that there may he a large effect on the viscoelastic strains of thermoplastics from oscillating stresses. Cyclic loading on the permanent set of cross-linked elastomers has been studied. Perhaps, as expected, the effect of the oscillating behavior is measurable. Two types of tests have been conducted. First, tensile tests on HDPE standard specimens were conducted where oscillating stresses were superimposed onto an initial static or mean stress. These measurements showed a rapid decrease in the oscillating stresses when compared to measurements when steady nonoscillating stresses are applied to the same type of specimen. In the second test series, pressurized HDPE piping was subject to oscillating bending stresses. Ratcheting of the hoop strains in the pipe occurs. Results show that these strains follow the constitutive relationships of linear viscoelasticity and experimental results imply that viscoelastic changes are accelerated by stress oscillations. These preliminary results seem to indicate that the effects of oscillating stresses on the viscoelastic behavior of thermoplastics may be significant. A systematic study is required to further understand this behavior.

Tensile Testing and Material Property Development of High Density Polyethylene Pipe Materials

Degradation of service water systems is a major issue facing nuclear power plant owners, and many plants will require repair or replacement of existing carbon steel piping components. High Density Polyethylene pipe has been used in non-safety service water systems for over nine years and found to perform well, but it is not currently permitted in the ASME Section III Boiler and Pressure Vessel Code, Division 1 for use in nuclear safety-related systems. To assist in the implementation of High Density Polyethylene pipe in the ASME Boiler and Pressure Vessel Code, Section III, Division 1 for Safety Class 3 applications, EPRI initiated a High Density Polyethylene pipe and pipe material testing program. This test program includes tensile testing and fatigue testing of High Density Polyethylene piping and piping components and the development of slow crack growth data. To determine the material and engineering properties needed, extensive tensile testing of specimens cut from High Density Polyethylene pipe was conducted. The initial tensile test program was conducted on PE 3408 with cell classification 345464C and a second, not yet finalized, phase was added to test PE 4710 with cell classification 445474C. The data developed during the testing were used to establish ultimate strain, elastic moduli, yield stress and yield strain values for both new and aged materials. Because extruded HDPE properties vary in the hoop and axial directions and the properties are highly affected by temperature, specimens were cut in both the hoop and axial directions and were tested at temperatures ranging from 50° F to 180° F. This paper provides a description and overview of the PE 3408 cell class 345464C test program. In addition, an overview and summary of the test results for the PE 3408 cell class 345464C are provided.Copyright

Bending Fatigue Testing of Pipe and Piping Components Fabricated From High Density Polyethylene Materials

Degradation of service water systems is a major issue facing nuclear power plant owners, and many plants will require repair or replacement of existing carbon steel piping components. High Density Polyethylene pipe has been used in non-safety service water systems for over nine years and found to perform well, but it is not currently permitted in the ASME Section III Boiler and Pressure Vessel Code, Division 1 for use in nuclear safety-related systems. To assist in the implementation of High Density Polyethylene pipe in the ASME Boiler and Pressure Vessel Code, Section III, Division 1 for Safety Class 3 applications, EPRI initiated a testing program that includes tensile and fatigue testing of HDPE piping and components and the development of slow crack growth data. Straight cantilever bending fatigue tests on PE 4710 pipe with a minimum cell classification of 445474C were conducted. The tests were designed to comply with the requirements for fatigue testing given in Appendix II of the ASME Boiler and Pressure Vessel Code, Section III, Division 1. They were also designed to achieve failure at the fusion butt welds near the cantilever support. S-N curves developed from both sets of data were found to fit well to power formulas of the type S = C/Nb required by mandatory Appendix II. The tests were conducted at various temperatures from 50° F to 160° F and in addition the effects of cyclic rate and aging were evaluated. Based on the straight pipe tests, stress intensification factors were calculated for 5-segment miter bends in both the in-plane and out-of-plane directions. The test elbows were fabricated from PE 4710 material with cell classification 445474C. Two sizes of 5-segment miter bends were tested, 4” and 12” diameter. The fatigue testing results showed one of the unique characteristics of High Density Polyethylene pipe: a significant decrease in material stiffness from the first few test cycles to a lower value that remains almost constant until failure. Thus, S-N curves and SIFs were determined twice: first based on the initial cycle results and again at the midlife of the fatigue tests. This paper provides a description and overview of the test program, testing methods and materials tested. In addition, an overview and summary of the test program results are provided.© 2008 ASME

Seismic and Concurrent Load Design Criteria for Buried High Density Polyethylene Pipe in ASME BPVC Section III, Division 1, Applications: Part II — Piping Soil Interaction Design Basis Criteria

The commercial Light Water Reactors operating within the United States have been in service from about 20 to 35 years. These plants include buried Service Water piping systems primarily made from low carbon steel. This piping at several plants has been subject to aging over the years, resulting in degradation and corrosion that may require replacement of the piping. Due to the advantageous cost and durability of High Density Polyethylene (HDPE) piping (as demonstrated in other commercial industries), the nuclear power industry has expressed interest in replacing steel buried Service Water Piping in Nuclear Power Stations with HDPE Pipe. To assist in this effort EPRI has funded and supported the work summarized in this paper to develop design criteria for HPDE Pipe. The paper provides design criteria for High Density Polyethylene (HDPE) pipe made from PE 3408 resin. It also provides the technical basis for the proposed criteria. This paper deals primarily with the design of the piping in relation to its interface with the soil in which it is buried. The criteria primarily is derived from current analysis methodology for steel and concrete buried pipe while incorporating changes required to account for the properties and behavior of HDPE pipe. The proposed analysis methodology described herein has evolved into a proposed ASME Boiler and Pressure Vessel Code, Section III, Division I, Design Code Case for consideration by the Section III, Subcommittee on Nuclear Power.Copyright

The “Spectrum Dip”: Dynamic Interaction of System Components

In the early 1960s, many full sized surface combatants, submarines and structural models were tested with underwater explosive in order to evaluate the shock load to the ship and internal equipment and structures. Initially, shock spectra were calculated from base motion measurements of internal equipment and components. Attempts were made to envelop these spectra to develop shock design spectra inputs. At that time, earthquake engineers were using this enveloping method to develop design procedures from ground motion measurements to protect structures from earthquakes. However, for the measurements on ships, this procedure resulted in calculated loads that would have caused catastrophic failure of the equipment; yet the equipment had not failed on the ship tests. As a result, the data was reanalyzed over a period of over a year. It was concluded that the dynamic interaction of each component or structure reduced the measured spectral motion at the fixed-base frequencies of the structure by about an order of magnitude. In many cases, there was a dip in the shock spectra at the fixed-base frequencies: the “spectrum dip” phenomenon. This reanalysis led to shock spectra design curves for navy ships. This paper will present a review of an experimental study and an analytical model to explain the effect of dynamic interaction on the shock or response spectrum. In addition, a practical example of interaction of four single mass dynamic systems mounted on a realistic deck and subjected to a high impact shock input was studied by the authors.© 2007 ASME