Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (23)
- Beitrag zu einem Tagungsband (12)
- Monografie (1)
- Buchkapitel (1)
Sprache
- Englisch (37)
Referierte Publikation
- ja (37) (entfernen)
Schlagworte
- Compression set (12)
- Ageing (7)
- Elastomer (6)
- Rubber (6)
- Low temperature behaviour (5)
- Sealing material (5)
- Leakage rate (4)
- Seal (4)
- Aging (3)
- Compression (3)
- DLO (3)
- Degradation (3)
- Dynamic mechanical analysis (3)
- Elastomeric seals (3)
- Leakage (3)
- Low temperature (3)
- O-ring (3)
- Simulation (3)
- Compression stress relaxation (2)
- Crosslinking (2)
- DMA (2)
- Dichtungen (2)
- Druckverformungsrest (2)
- EPDM (2)
- Gamma irradiation (2)
- HNBR (2)
- Low temperature behavior (2)
- Metal seal (2)
- Metal seals (2)
- O-Ringe (2)
- Polyethylene (2)
- Seal performance (2)
- Testing (2)
- Tiefe Temperatur (2)
- Time dependent effects (2)
- Absorption (1)
- Aging management (1)
- Arrhenius (1)
- B1uTHz-region (1)
- Bauteilversuche (1)
- Cask (1)
- Chemical processes (1)
- Coefficient of thermal expansion (1)
- Colour center (1)
- Component test (1)
- Compression Set (1)
- Creep (1)
- Cross-linking (1)
- Crystallinity amorphization (1)
- Degradative processes (1)
- Dielectric properties (1)
- Drop testing (1)
- Dual purpose casks (1)
- Dynamic loading (1)
- Dynamics in confining space (1)
- Elastomers (1)
- Extended storage (1)
- Extrapolation (1)
- FKM (1)
- Fourier transform spectroscopy (1)
- GUR (1)
- Gamma-Bestrahlung (1)
- Gamma-Co-60 radiation (1)
- HMW (1)
- HMW-PE (1)
- Hardness (1)
- High molecular PE (1)
- High temperature (1)
- IRHD microhardness (1)
- Indenter modulus (1)
- Infrared spectroscopy (1)
- Integral absorption (1)
- Interim storage (1)
- Irradiation (1)
- Kinetic study (1)
- Leak-tightness (1)
- Lifetime (1)
- Lifetime prediction (1)
- Light absorption (1)
- Liquid-crystalline polymers (1)
- Long-Term Behaviour (1)
- Long-term behavior (1)
- Long-term investigation (1)
- Lupolen (1)
- Metal Gasket (1)
- Metal Seal (1)
- MidIR Fourier transform spectroscopy (1)
- Modeling (1)
- Modelling (1)
- Molecular dynamics (1)
- Molecular electronic states (1)
- Molecular structure modifications (1)
- Neutron shielding material (1)
- Nnetwork (1)
- Oxidation (1)
- Oxygen permability (1)
- Package (1)
- Partial release (1)
- Physical relaxation (1)
- Polymers (1)
- Radiation effects (1)
- Recovery (1)
- Replacement (1)
- Rubber seals (1)
- Safety (1)
- Scission (1)
- Seal failure (1)
- Sequential analysis (1)
- Spectroscopy (1)
- Spent fuel (1)
- Stress relaxation (1)
- THz spectroscopy (1)
- TTS (1)
- Temperature measurement (1)
- Terahertz spectroscopy (1)
- Thz and Mid IR Fourier transform spectroscopy (1)
- Tightness (1)
- Transport safety (1)
- Transvinylene unsaturation formation (1)
- UHMW-PE (1)
- UHMWPE (1)
- Ultra high molecular weight (1)
- Ultra high molecular weight polyethylene (1)
- Ultrahigh molecular PE (1)
- Vinyl decay (1)
- Yellowness index (1)
- stress relaxation (1)
- γ-Co60radiation (1)
Organisationseinheit der BAM
Paper des Monats
- ja (2)
Seals used in containers for dangerous goods need to maintain leak tightness for several decades. For investigating the change in seal material properties during aging, a comprehensive aging program on both uncompressed and compressed EPDM and HNBR seals was started. In order to obtain results closely related to practical conditions, O-rings with a full-scale cord diameter of 10 mm were aged. However, this set-up can lead to heterogeneous aging caused by diffusion-limited oxidation (DLO) effects. These effects depend on material, dimensions, time and temperature. Heterogeneous aging results in distorted bulk properties such as compression stress relaxation and compression set (CS) suggesting that HNBR has better performance than EPDM at 150 °C but which is not the case at 100 °C. The presence of heterogeneous material properties was shown by hardness measurements across the seal cross-section. If DLO-affected data is excluded, extrapolations of CS data are possible using time-temperature shifts and Arrhenius graphs. Exemplary CS values of 50 % and 80 % would be reached after approx. 10 years and 29 years, respectively for HNBR and after approx. 400 years and 1100 years, respectively for EPDM.
To improve the predictive capability of long-term stress relaxation of elastomers during thermo-oxidative ageing, a method to separate reversible and irreversible processes was adopted.
The separation is performed through the analysis of compression set after tempering. On the Basis of this separation, a numerical model for long-term stress relaxation during homogeneous ageing is proposed. The model consists of an additive contribution of physical and chemical relaxation.
Computer simulations of compression stress relaxation were performed for long ageing times and the results were validated with the Arrhenius treatment, the kinetic study and the time-temperature superposition technique based on experimental data. For chemical relaxation, two decay functions are introduced each with an activation energy and a degradative process. The first process with the lower activation energy dominates at lower ageing times, while the second one with the higher activation energy at longer ageing times. A degradation-rate based model for the evolution of each process and ist contribution to the total system during homogeneous ageing is proposed. The main advantage of the model is the possibility to quickly validate the interpolation at lower temperatures within the range of slower chemical processes without forcing a straight-line extrapolation.
Experimental and numerical studies of shock absorbing materials for containers for radioactive waste
(2012)
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 the assessment of the service lifetime of elastomeric seals that are part of the container lid system 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 1.5 years. 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 relatively constant or even decreases until shrinkage combined with the loss of resilience of the aged seal leads to leakage. This demonstrates that static leakage rate, which is the only available direct seal performance criterion, has only limited sensitivity towards the degradation of the seal material. CS data is extrapolated using time-temperature shifts and Arrhenius graphs. An exemplary CS of 50 % would be reached after approx. 1.2, 17 and 29 years at 60 °C for HNBR, EPDM and FKM respectively.
In many fields the function of seal materials is required at low temperatures. Therefore the understanding of failure mechanisms at low temperatures is of high importance. In some applications elastomeric seals are used in environments with increased irradiation backgrounds, as e.g. in containers for radioactive waste. This paper reports on the influence of gamma irradiation on the low temperature properties of different fluorocarbon rubber (FKM) materials.
The samples were irradiated with gamma doses of up to 600 kGy and the induced changes of material properties were tested by methods sensitive to the rubber-glass transition, like DSC and DMA. Additionally, compression set measurements were performed to give information about recovery behaviour at low temperatures.
O-rings made of HNBR, EPDM and FKM were aged in the compressed and uncompressed state at 150 °C, 125 °C, 100 °C, 75 °C, 60 °C and 23 °C for aging times of up to five years. Hardness was measured and increased with aging time and temperature for HNBR and EPDM, but it remained practically constant for FKM. Indenter modulus measurements were performed on the lateral O-ring surface (that was free of DLO effects) to assess an influence of the compression during aging, but none was detected. The equilibrium compression set (CS) exhibited faster and stronger degradation than hardness and was used for lifetime predictions using the time-temperature superposition (TTS) principle. With an end-of-lifetime criterion of 70 % CS, lifetimes of 4.5 years, 50 years and 526 years at 75 °C were estimated for HNBR, EPDM and FKM, respectively. The activation energies derived from an Arrhenius plot of the shift factors from the TTS were 85 kJ/mol, 99 kJ/mol and 78 kJ/mol for HNBR, EPDM and FKM, respectively, revealing that a higher activation energy does not necessarily mean that the material has a higher lifetime at lower temperatures. Furthermore, the measured lifetime of EPDM O-rings at 100 °C (5 years) was compared to that predicted on the basis of the lifetime at 150 °C as well as 125 °C using the corresponding shift factors. The error of the prediction was only ± 4 %. However, this precise prediction could only be achieved using the five-year long-term aging data. When using only data from aging times up to 0.5 years and 2 years, the lifetime of EPDM O-rings at 100 °C was underestimated by 31 % and 22 %, respectively.
Rubbers are widely used as main sealing materials for Containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste containers. The save enclosure of the radioactive container inventory has to be guaranteed according to legislation and appropriate guidelines for long term storage periods as well as down to temperatures of -40 °C for transportation. Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is necessary to ensure a proper material choice which is certainly also influenced by additional factors as e.g. the aging behavior.
The Bundesanstalt für Materialforschung und –Prüfung (BAM) runs an investigation program on the long-term behavior of multi-component metal seals. Such seals are used in a wide area of applications including transport and storage casks for spent nuclear fuel and high level radioactive waste.
The seal function is mainly based on the compression of the inner helical spring, which generates the necessary seal force to keep the sealing surfaces in close contact. This in turn leads to a plastic deformation of the outer jacket of the seal, comprised of highly ductile aluminum or silver that adapts to the sealing surfaces of cask body and bolted lid, thus providing high Level leak tightness. In Germany, those casks are licensed for Interim storage periods of up to 40 years or more if extended Interim storage would become necessary before a final repository is available. Thus, the sealing performance has to be evaluated, including factors like elevated temperature due to decay heat or mechanical loads due to transport under normal as well as accident conditions. Long-term investigations at BAM have been running over the last nine years to identify and evaluate the seal performance by measuring the remaining seal force, the useable resilience and the leakage rate after various time intervals at temperatures ranging from room temperature up
to 150 °C. It was found that the seal force and useable resilience decrease with time and temperature, caused by creep deformation of the outer jacket. In order to obtain an analytical description for the seal behavior and to achieve more information on the material behavior under application conditions a comprehensive investigation program with Focus on aluminum as outer jacket material was launched. The program includes material investigations such as compression and tension creep tests with representative basic materials. An additional test setup allows for the continuous measurement of the remaining seal force at temperatures of up to 150 °C. Furthermore, seal segments are compressed and stored in heating chambers, thus producing segments at different stages of the aging process. The segments are investigated regarding the development of the contact area width, jacket thickness and microstructural changes. This data will be used to develop material models and an analytical description of the time and temperature dependent long-term sealing behavior. This paper explains the current status of gained test results and modelling approaches and closes with an outlook to the future Project plans.
Due to their simple and reliable application, elastomers are used for many tasks. Many fields of application like transport, aviation and process technology, entail low temperatures where proper functionality of the employed seals has to be ensured. Therefore, understanding the low temperature behavior of elastomeric seals is important for safe seal operation to restrict the temperature application range.
For all areas of application, the question of seal lifetime is also important for replacement or maintenance intervals. The lifetime is especially important in applications that allow no seal replacement. Since the seal can determine the lifetime of storage in such cases, reliable lifetime estimation is necessary for these applications, and requires an extensive knowledge of the specific aging behavior.
In technical applications, static seals are sometimes also subjected to dynamic loadings. Therefore, the leakage behaviour under dynamic conditions has to be evaluated as well. For this purpose, FKM elastomer seals have been tested by using newly designed equipment that allows for rapid partial release of the seal and simultaneous leakage rate measurement at a wide range of test temperatures. Furthermore, material characterisation was done by using Dynamic Mechanical Analysis, Differential Scanning Calorimetry and Compression Set. It was shown that, under static conditions, the leakage rate increased significantly during cooling at temperatures around 18 K lower than the glass transition range. On reheating, the seal’s functionality was restored in the high temperature region of the glass rubber transition. In the subsequent dynamic release tests, that comprised a reduction of the seal compression within 1 s from 25 % to 23 %, increased leakage rates were observed in the high temperature region of the glass transition range. It was shown that the temperature that is critical for increased leakage is significantly lower under static conditions compared to dynamic conditions. The obtained leakage rates for static tests and dynamic release tests at different temperatures were analysed with reference to results of the material characterisation.