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Exposure of structural epoxy adhesive to combination of tensile stress and γ-radiation
Abstract
Mass is money! This sentence applies even more in space transportation than in aviation. With the development of high temperature adhesives and the rise of composite materials like CFRP, adhesive bonding has experienced enormous growth in the space sector. Unlike in terrestrial application, the adhesive joint has to withstand unique environmental cinditions. From the chemical point of view, adhesive systems belong to the group of polymers. Thus the effect of radiation, especially ionizing radiation, has an enormous influence on the structural integrity of adhesive systems. On an exploration space mission eg. on the lunar surface, the adhesive has additionally to withstand mechanical loads in combination with radiation environment. This paper investigates such a combined effect of tensile stress and γ-radiation on the mechanical properties of a 2-component epoxy adhesive. It is of high importance to investigate the possible degradation of the adhesive under combination of irradiation and tensile loading due to increased chain scission of stressed molecules. For structural adhesive, such phenomena are not covered well in the literature. In the present work, bulk specimens of the investigated adhesive are milled out of a plate and exposed to a 60 Co source. During the irradiation they are loaded in tensile direction by a uniquely designed fixture. Afterwards, the specimens are inspected by FTIR spectroscopy and then tested in order to determine the potentially degradated mechanical properties. The results show that the mechanical properties of the adhesive stay constant for non-irradiated, irradiated non-loaded and irradiated loaded specimen. This could be explained with the effect of crosslinking and chain scission induced by irradiation that occur simultaneously and level each other out.
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The modular satellite concept iBOSS (intelligent Building Blocks for On-Orbit Satellite Servicing and Assembly) enables on-orbit servicing and reconfiguration of satellite systems and has the potential to be a game changer in the space industry. Such building blocks have to withstand all environmental loads in space, e.g.: radiation, vacuum and thermal cycling. The present paper investigates the mechanical properties of the two component epoxy adhesive 3M SW9323 under the environmental effect of radiation. This adhesive is part of the building block's primary structure. Furthermore adhesive bonding is the sole joining technique used in the whole structure. It is therefore critical to know the influence of ionizing radiation on its load-carrying capacity. For this purpose bulk specimens were manufactured and exposed to γ-radiation, generated by a 60 Co source. Four different doses were achieved by varying the distance to the source and irradiation time. Afterwards the specimen were tested under tensile loading. Using the digital image correlation technique properties like elastic modulus, shear modulus, tensile strength and elongation at break were determined. 2 The results show that the mechanical properties of the bulk specimen of 3M SW9323 are not influenced by γ-radiation up to a dose of 17.6 kGy. This is explained by the phenomena of crosslinking and chain scission which occur simultaneously and cancel each other out. In addition, Fourier-transform infrared spectroscopy (FTIR) was carried out to investigate if one mechanism is predominant. A slight shift in the spectra indicates the supremacy of chain scission.
Ultra–high molecular weight polyethylene (UHMWPE) sheet was cross-linked by γ irradiation in air with a dose of up to 300 kGy at a dose rate of 5 kGy/h and further treated by post-annealing at 120 °C for 4 h in vacuum. Variations in chemical structure, thermostability, crystallinity, creep resistance, and tensile properties were investigated and compared mainly by gel content, TGA, DSC, and creep and tensile measurements. Gel content measurements indicated that cross-linking was predominant over chain scission during irradiation and post-annealing. Radiation cross-linking resulted in an obvious improvement in the creep resistance and tensile properties of UHMWPE. Through cross-linking, the operational temperature and yield strength of the irradiated and subsequently annealed UHMWPE sheet were improved by more than 100 °C and 14%, respectively, at a dose of 300 kGy. Simultaneously, Young's modulus was increased to 1413 MPa, compared with 398 MPa of pristine UHMWPE. Annealing after irradiation further improved the creep resistance and Young's modulus. Highly cross-linked UHMWPE can even be maintained at 250 °C for a long time without any obvious deformation.
This chapter focuses on the chemistry of polymer resins and the changes that occur when resins are exposed to environmental factors that can cause degradation. The exposures that are considered are elevated temperatures (with or without oxygen); contact with water and other fluids; radiation; and mechanical loads.
Some general observations about the types of effects to be expected for each exposure condition are outlined first. Then, the chemistries of the various classes of resins that are in current use as matrices for high-performance composites are described. Separate sections treat epoxies, bismaleimides, PMR-type thermosets, phenyethynyl-terminated imides, and high-temperature thermoplastics. For each resin type, the formulation and curing are briefly discussed. This discussion is followed by a review of the available literature on mechanisms of long-term degradation as determined by spectroscopy, chromatography, and other analytical techniques. Consequences to constituent properties are also described: increases or decreases in glass transition temperatures; shrinkage and cracking; and changes in mechanical stiffnesses, strengths and toughnesses.
This introductory chapter gives a brief description of adhesive bonding and adhesion-related phenomena. The major definitions of the terms associated to this technology such as adhesion, cohesion, adhesives, sealants, and adherends are given so that there is uniformity of language throughout the handbook. The reasons that drove the editors to prepare this book are explained. Finally, the organization of the book is described.
NASA is studying the effects of long-term space radiation on potential
multifunctional composite materials for habitats to better determine
their characteristics in harsh space environments. Two epoxy-matrix
composite materials were selected for the study and were mounted in a
test stand that simulated the biaxial stresses of a pressure vessel
wall. The samples in the test stand were exposed to radiation at fast
(0.1478 krad/s) and slow (0.0139 krad/s) dose rates, and the strain and
temperature were recorded during the exposure. During a fast dose rate
exposure, negative strain was recorded, decreasing with time, an
indication of matrix shrinkage. Given previous radiation studies of
polymers, this is expected to be a result of radiation-induced
crosslinking in the epoxy matrix. However, with a slow dose rate, the
materials exhibited a positive strain that increased with time,
corresponding to stretching of the materials. This result is consistent
with scission or degradation of the matrix occurring, possibly due to
oxidative degradation.
This work outlines an elasto-plastic investigation of two common peel tests which use high and low yield strength aluminium adherends. An elastic, large-displacement, finite element program has been extended to include elasto-plastic material behaviour. This has been used to analyse both peel tests. The adhesive stresses near the crack tip have been shown to be finite while the corresponding strains remain singular. A failure criterion based on a maximum adhesive strain has been used to predict the relative strengths of the peel test. The amount of energy dissipated in the plastic deformation of the peeling adherends has been assessed by a series of tests and has been shown to be a considerable amount of the total energy supplied to the peeling system. Further, although the two aluminium alloys considered have grossly different yield strengths the energies dissipated in plastic deformation are similar. Material data for the finite element analysis and the plastic work calculations have been obtained from uniaxial tensile tests of both the adherends and the adhesive and actual peel strengths have been measured in a series of peel tests.
The influence of electron bombardment on the time-to-rupture and creep of standard capron fibers in nitrogen and air was studied. An effect of the reversible increase in the creep rate was observed. It was shown that the time-to-rupture and creep rate of capron were substantially dependent on the radiation intensity, load, temperature, and ambient atmosphere. A formula describing the time-to-rupture and creep rate of polymers in radiation fields was derived, and it was shown that this formula correlates with a formula for the electrical conductivity of irradiated dielectrics.
Creep tests of an epoxy resin during bending and irradiation have been carried out to investigate the synergistic effects of radiation and stress on mechanical properties of the resin. Simultaneous application of stress and irradiation on the epoxy resin enhanced creep rates in comparison with the application of stress on an irradiated sample. In order to clarify the mechanism of the radiation-induced creep, measurements of solvent swelling of specimens have been performed. The swelling increased with the dose and the increase of the swelling corresponds to the increase of the chain scission. The mechanism of increased deformation of the resin during irradiation is proposed to be caused by increased chain scission following the release of the local strain energy.
Bisphenol-A polysulfone, poly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1, 4-phenyleneisopropylidene-1,4-phenylene), PSF (I) showed greatly reduced resistance to electron beam irradiation when subjected simultaneously to an applied tensile stress. The creep rate increased, and the time (dose) to failure of the sample decreased with increasing stress. The failure strain was constant for different applied stresses. Air, oxygen, and moisture caused decreases in radiation resistance compared with a dry nitrogen atmosphere. Increasing the irradiation temperature from 0 to 90°C resulted in substantially decreased radiation resistance.
Sterilization of medical devices by gamma (gamma)-irradiation is common. The effect of irradiation on a bone replacement material, HAPEX (hydroxyapatite-reinforced polyethylene composite), was investigated. Unfilled and hydroxyapatite-filled polyethylene at 0.20 and 0.40 filler volume fractions were gamma-irradiated at 2.5 Mrad, and the modified properties were studied by differential scanning calorimetery, isochronous experiments, and creep tests. The effect of thermal annealing of the samples from 140 degrees C also was examined. The results suggest that both irradiation and annealing increase creep resistance of the materials. These are associated with the formation of crosslinks and an increase in crystallinity, respectively.
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ISO 15856 Space systems -Space environment -Simulation guidelines for radiation exposure of non-metallic materials
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