Osamu Fujita’s research while affiliated with Hokkaido University and other places

We irradiated 5 eV Atomic Oxygen (AO) to PEEK sheets with 0.4 mm thickness under tensile load, and estimated strength of the specimen. Three phenomena caused by exposure were observed : (1) appearance of numerous conical pits a few urn deep on the specimen surface ; (2) reduction of thickness at the irradiated area ; (3) change of chemical structure on the surface, which is probably caused by UV generated slightly during the AO irradiation test. As a result of tensile tests, tensile properties after AO irradiation were almost same as those of pristine samples regardless of applied stress. The depth of conical pits was so small and the thickness of layer changed after AO irradiation test was so thin that they did not affect mechanical properties of PEEK sheets. Consequently, residual strength of this material after AO exposure can be estimated by consideration of thickness decrease.

To find out the degradation behavior of polymer in the real space, space exposure experiments utilizing the International Space Station (ISS) were scheduled. PEEK sheets under tensile stresses were exposed to the environment around the ISS orbit, and were irradiated by atomic oxygen (AO), ultraviolet ray, and electron beam (EB) in the ground test facility., This study introduces the outline of these experiments, and shows the results of AO and EB pilot irradiation tests as follows: (1) Test piece surfaces after AO exposure exhibited significant morphological damages characterized by micron-sized conical pits. (2) Thickness reductions of the test pieces by AO exposure increased with increasing tensile stress. (3) Residual strength after AO exposure could be estimated by taking account of thickness reduction. (4) No significant change was observed on surface morph, mass, chemical structure, and tensile properties of the test pieces after EB exposure regardless of tensile stress.

Polymeric flexible films are candidate materials for the fabrication of inflatable structures, which is a key technology in constructing space facilities. These materials are also central to the design of future artificial satellites and solar panels for space stations. These space-based structural members have to hold a certain amount of tensile load in severe environments irradiated by atomic oxygen and radiation. The effects of radiation on polymeric film have been studied, but the damage resulting from processes under tensile force have not been fully elucidated. To investigate this phenomenon, we conduct space experiments that expose PEEK sheets under tensile stress to a real space environment utilizing the International Space Station's Russian Service Module, and carry out ground control experiments that irradiate single and combined atomic oxygen, ultraviolet ray, and electron beam to the same materials. This report introduces an outline of these experiments and presents results of pilot irradiation tests carried out before the regular ground control experiments.

The relationship between the polymeric material degradation by atomic oxygen (AO) and tensile load was investigated. AO irradiation experiments were carried out at the facility of National Space Development Agency (NASDA). AO was irradiated to PEEK sheets under tensile stress. After AO exposure, surface observation, reaction efficiency (Re) measurements, and tensile tests were made on the sample. As a result, the following were obtained : (1) The circular irradiated region can be observed on all exposed samples. (2) Re was increased by AO exposure under tensile stress. (3) Conical pits were observed allover on all exposed specimens. (4) Elastic modulus was decreased by AO irradiation and this tendency was enhanced by applied stress. It was clear that stress affected polymer degradation by AO.

A polymer, PEEK, was investigated how tensile load would effect on its characteristics when irradiated by electron beam (EB). PEEK sheets under different tensile stresses were irradiated to EB by using the Combined Space Effects Test Facility of NASDA. After EB radiati on, mass measurements, surface observation and tensile tests were made on the samples. As a result, the following were obtained : (1) No major changes were not observed in mass measurements. (2) Irradiated surface showed no difference from non-irradiated surface. (3) Decreases in tensile strength and elongation were detected for the sheets irradiated under tensile stress.