Abstract
Thin polyimide (PI) films have been widely used for spacecraft polymeric materials. In the use in low earth orbit environment; however, small defects often initiate at the surface of PI films by the erosion of atomic oxygen. Mechanical strength of PI films are significantly reduced by the crack propagation from these defects; therefore, to evaluate fracture toughness in the direction of film thickness is important. This paper conducted tensile tests of 125-μm-thick PI films with two types of surface pre-cracks having the depth of 20μm and 40μm, and estimated fracture toughness J_<in> and CTOD_<in>. As a result, J_<in> of the specimen with the pre-crack of 40μm depth (40μm pre-crack specimen) was larger than that of 20μm depth (20μm pre-crack specimen). In contrast, CTOD_<in> showed almost same values regardless of the depth of pre-cracks. Thus, CTOD_<Ic> can be considered as elastic-plastic fracture toughness CTOD_<Ic>. J_<Ic> was estimated from the CTOA_<Ic> based on the commonly-used relationship between J and CTOD. Then, J_<in> of 20μm pre-crack specimen was same as J_<Ic> with an error of 12%. Therefore, J_<in> of 20μm pre-crack specimen was approximately considered as J_<Ic>.
