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- Aging (8)
- Ageing (6)
- Elastomer (6)
- Seal (6)
- Rubber (5)
- Compression (4)
- Leakage (4)
- Simulation (4)
- Compression set (3)
- Cask (2)
Organisationseinheit der BAM
To determine a polymer’s spectral response of photo degradation, two main methods have been established. The so-called “filter technique” uses polychromatic radiation, which is filtered by a set of various sharp cut-on filters. This procedure provides separated reading points of an activation spectrum of the investigated material. With the "spectrographic technique” a specimen is irradiated with pectrally dispersed radiation, locally quasi monochromatically, and the local property changes.are related to the radiant exposure at the respective wavelengths. A third method, the irradiation of specimens with quasi monochromatic radiation behind interference filters, is rarely used.
These methods have advantages and disadvantages and therefore, there are limits for conclusions from these spectral data. Concerning the quasi monochromatic Irradiation of specimens the development of new UV radiation sources looks promising.
With the rapid development of inexpensive UV Light-emitting diodes (LEDs), new : sources of nearly monochromatic radiation in the UV ränge are available, even down to about 250 nm. To determine the spectral response, the use of such UV-LEDs in TO-18 ) and TO-39 metal cases combines the advantages of low cost and energy efficiency, stable spectral irradiance, temperature.and humidity control, larger specimen areas, and an easy and stable analysis.
Combinations of different UV LEDs are also discussed as replacement for xenon arc ' radiation sources, see patent application EP01528388A1.
The excellent mechanical properties of elastomer seals at a wide range of temperatures as well as their high versatility and recovery potential under several load conditions make these materials well suitable for the application in containers designed for transport and disposal of negligible heat generating radioactive waste. While a seal exchange at defined intervals is typical in many conventional applications, it is impossible or at least hard to perform when principles of minimization of radiation exposure have to be considered which prohibit an avoidable cask handling. An extensive knowledge of the change of the elastomer properties during aging and the availability of reliable end-of-lifetime criteria to guarantee the safe enclosure of the radioactive material for the required time are mandatory. As BAM is involved in most of the national cask licensing procedures and in the evaluation of cask-related long-term safety issues, great efforts have already been made and are still ongoing to scientifically support this task.
Among other representative types of elastomers, specimen made from ethylene propylene diene rubber (EPDM) were tested before, during and after aging to capture the with respect to application most important of their complex mechanical properties.
Exemplary results of these investigations were used to calibrate material models implemented in the commercial finite element software ABAQUS/Standard®. The finite element model already presented in previous works uses a sequential temperature displacement coupling. The calculated compression stress relaxation (CSR) and compression set (CS) values do satisfactorily match the experimental results.
In many investigations performed at BAM both values (CSR and CS) were identified as key indicators of elastomer’s long-term performance.
However, the possibility to correlate these equivalent indicators with performance values such as tightness and leakage rate, measurable in the mounted state, is an important goal of our future work. In the presented study the ABAQUS® feature of “pressure penetration” is introduced in the suggested finite element model for this purpose. It provides the possibility to simulate the penetration of a gas into a possible gap between flange and O-ring causing an opening of a leakage path. Three dimensional and axis-symmetric finite element models were generated to represent flat and grooved flanges of different dimensions.
The sensitivity of the feature to several input parameters is investigated and the observed behavior of the O-ring is correlated with the results of performed leakage tests.
Elastomer seals are mounted as barrier seals in lid systems of containers designed for transport and disposal of negligible heat generating radioactive waste and as auxiliary seals in spent fuel storage and transportation casks (dual purpose casks (DPC)). When the behavior of mounted seals under normal and hypothetical accident conditions of disposal and transport is to be simulated, a comprehensive knowledge of their complex mechanical properties at every state of aging is necessary. In previous works, BAM’s efforts in experimental investigations on specimen artificially aged at different temperatures and times and the implementation of the found results in finite element material models were presented. Additionally, our approaches to reproduce the aging process itself and to extrapolate the results of artificially accelerated aging to longer times were presented. Numerical simulations have shown that the behavior of the seal during mounting and one-sided pressurizing and the resulting performance values such as leakage rate strongly depend on the coefficient of friction (COF) between flange and seal. The friction coefficient, in turn, depends on the aging state of the elastomer material as several publications suggest (see below).
Dynamic COF between an exemplary ethylene propylene diene rubber (EPDM) material and a stainless steel ball were determined by using a self-designed linear oscillation tribometer. Unaged and artificially aged EPDM specimen stored for 30 days and 100 days at a temperature of 150 °C were tested. A stainless steel ball (d=10 mm) is brought in contact with the specimen’s surface und loaded by normal forces of 2.5 N, 5 N, 10 N and 20 N. During a reciprocating movement of the EPDM sheet, the horizontal force/friction force is continuously measured, and the COF can be derived.
It is well known that friction is a complex phenomenon especially in soft materials. It cannot be excluded that the measured friction force is influenced by additional force components, resulting from the ball’s grooving through the elastomer’s surface. This force depends on the penetration depth of the ball and on the resistance of the elastomer in its different states of aging. The latter results from microstructural changes i.e., chain scission and additional crosslinking that occur during aging which in turn influence the softening or hardening of the material.
A finite element (FE) ABAQUS® model was developed to reproduce the measurement process. It should help to better understand the physical mechanisms and to quantify the percentage of measured forces resulting from real friction on the one hand and forces resulting from unintended side effects that could falsify the result on the other hand. The behavior of the elastomer in its different states of aging is reproduced by a FE material model already presented in previous works of BAM.
In this study, bulked adhesives were examined regarding changes in properties for their use in the restoration of stained-glass windows. Two epoxy adhesives commonly used in the restoration of flat glass, Araldite® 2020 and Hxtal NYL-1™, were investigated. The adhesives were mixed with various silica-based micro- and nano-particles as fillers. Different tests were carried out on the adhesives with and without fillers in order to characterise their chemical and mechanical properties. The tests included colorimetry, dynamic-mechanical methods of analysis and tests on bending strength. No property changes could be ascertained in the bulked adhesives that would be negative for the restoration of delicate flat glass.
Testing and numerical simulation of elastomeric seals under consideration of time dependent effects
(2016)
Due to delays in the siting procedure to establish a deep geological repository for spent nuclear fuel and high level waste and in construction of the already licensed Konrad repository for low and intermediate level waste, extended periods of interim storage will become more relevant in Germany. BAM is involved in most of the cask licensing procedures and is responsible for the evaluation of cask-related long-term safety issues. Elastomeric seals are widely used as barrier seals for containers for low and intermediate level radioactive waste. In addition they are also used as auxiliary seals in spent fuel storage and transportation casks (dual purpose casks (DPC)).
To address the complex requirements resulting from the described applications, BAM has initiated several test programs for investigating the behavior of elastomeric seals. These include experiments concerning the hyperelastic and viscoelastic behavior at different temperatures and strain rates, the low temperature performance down to -40°C, the influence of gamma irradiation and the aging behavior.
The first part of the paper gives an overview of these tests, their relevant results and their possible impact on BAM’s work as a consultant in the framework of approval and licensing procedures.
The second part presents an approach of the development of a finite element model using the finite element code ABAQUS®. The long-term goal is to simulate the complex elastomeric behavior in a complete lid closure system under specific operation and accident conditions.
At BAM, which is the federal institute for materials research and testing in Germany, it is one of our tasks to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes assessment of elastomeric seals applied in the containers. Besides examining the low-temperature behaviour and irradiation effects of elastomeric seals, it is our goal to evaluate the service lifetime of the seals with regard to the requirements for long-term safety (40 years and more) of the containers. Therefore, we started an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) which are aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) up to 1 year. In order to assess sealability, O-rings are aged in compression by 25 % (corresponding to the compression during service) between plates as well as in flanges that allow leakage rate measurements. For comparison, uncompressed O-rings are aged as well. Further methods characterising seal performance are compression stress relaxation (CSR) reflecting the loss of sealing force of a compressed seal over time, and compression set (CS) which represents the recovery behaviour of a seal after release from compression. Additionally, hardness is measured for information about the change of mechanical properties. The experimental results indicate that while hardness, CSR and CS show considerable degradation effects, the leakage rate stays constant or even decreases until shrinkage and the loss of resilience of the aged seal leads to the formation of a leakage path. This indicates that static leakage rate, which is the only available direct seal performance criterion, has only limited sensitivity for the degradation of the seal material.
Elastomeric seals are used in many containers, including casks for radioactive waste. However, like all polymers, elastomers are prone to aging, which leads to a loss of sealing force and the ability for recovery which can ultimately result in leakage. Therefore it is important to be able to define an end-of-lifetime criterion and to judge the lifetime of elastomeric seals. For this reason, we started an aging program on three kinds of rubbers (HNBR, EPDM, FKM), monitoring the change of properties at four different aging temperatures over extended periods up to five years. The measured data is used for lifetime prediction by applying a suitable model.
High and ultra-high molecular weight polyethylenes are high performance materials, which are predestined for a wide range of applications due to characteristics like high density, low weight, good dimensional stability, high chemical resistance, and high hydrogen content. These polymers are used in two demanding areas: as implant material in medical technology (only ultra-high molecular weight polyethylenes) and as a component for neutron shielding purposes in casks for storage and transport of radioactive waste. In the medical field (joint replacements) as well as during neutron radiation shielding application, high and ultra-high molecular weight polyethylenes are exposed to gamma irradiation: in the first case requested as sterilization process and for surface-crosslinking and in the second case existing as a side effect of inserting the radioactive material in the cask.
Given that polyethylene as shielding material has to withstand any type of degradation affecting safety relevant aspects to be applicable for long-term radiation shielding purposes for instance over a period of 40 years, the durability of the material is of special interest. High molecular weight polyethylene (HMW-PE; LUPOLEN 5261Z; Lyondell Basell) and ultra-high molecular weight polyethylene (UHMW-PE; GUR 4120; Ticona) were subjected to gamma radiation and afterwards thermally treated. The gamma doses used are in the range of 50 to 600 kGy and irradiation takes place at RT using a Co-60 source. The planned thermal treatment will take place at a temperature of 125 °C for periods of minimum 30 days and maximum of five years. With the applied conventional analytical techniques it is possible to detect structural changes of both types of polyethylene induced by gamma irradiation and certainly of thermal treatment. Through gamma irradiation melting temperature, crystallinity, and density, respectively increased. Furthermore both polyethylenes get oxidized and cross-linked. With regard to the special application as neutron radiation shielding material in casks for storage and transport of radioactive materials, the impact of irradiation lead to changes of material properties. A consolidated view indicates that the detected changes of the irradiated (U)HMW-PE are not safety relevant for long-term neutron radiation shielding purposes over a period of 40 years in Germany.
Due to delays in the site-selection procedure to establish a deep geological repository for spent nuclear fuel and high level waste and in construction of the already licensed Konrad-repository for low and intermediate level waste without heat generation, extended periods of interim storage become more relevant in Germany. BAM is involved in most of the cask licensing procedures and especially responsible for the evaluation of cask-related long-term safety issues.
The long-term performance of elastomer seals for lid Systems of transport and storage casks whether used as auxiliary seals in spent fuel caslcs or as primary seals for low and intermediate level waste packages is an important issue in this context (Jaunich, 2013; Jaunich, 2014; Kömmling, 2015). The polymeric structure of these seals causes a complex mechanical behaviour with time-dependent elasticity reduction and loss of elastic recovery.
The paper presents first results of a comprehensive test Programme consisting of several static and dynamic mechanical short- and long-term tests which have been carried out at BAM on specimens made of representative types of elastomers, fluorocarbon rubber (FKM) and ethylene propylene diene rubber (EPDM). The investigation of the test results used to identify material models and their Parameters as well as the development of two finite element models for the numerical Simulation of tension and compression tests using the finite element code ABAQUS® are described. The calculation results are presented in comparison to the test results. The influence of important material and test parameters was investigated and discussed in sensitivity analyses.
BAM is the federal institute for materials research and testing in Germany. One of our tasks is to evaluate the safety of casks designed for transport and/or storage of radioactive waste. As elastomeric seals are used in the containers as safety-relevant parts, it is our goal to be able to evaluate the service lifetime of the seals with regard to the requirements for long-term safety (40 years and more) of the containers. For this reason, an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) was started. Ageing was performed at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) for up to 2 years. For assessing properties related to the sealability, O-rings were aged in compression by 25 % (corresponding to the compression during service) between plates. For comparison, uncompressed O-rings were aged as well. The aged materials were characterized with conventional polymer analysis methods such as hardness and tensile tests, but also with more seal-specific methods such as compression stress relaxation (CSR, reflecting the loss of sealing force of a compressed seal over time), and compression set (CS, representing the recovery behaviour of a seal after release from compression). CS is chosen as the property for lifetime prediction as it is both sensitive to degradation and related to the seal performance. CS data is extrapolated to 60 °C, which yields lifetimes of approximately 5 years for HNBR and 64 years for EPDM for a criterion of 85 % CS respectively, and approx. 40 years for FKM for a criterion of 65 % CS (the highest value measured so far).