抄録
In order to ascertain the workability of the coating films adhering to a metal-plate, the impact wedge bending test and the cup punching test were carried out with the same specimen at room temperature. During the course of those tests, a great deal of force acting upon the test specimen is produced in the metal-plate, whereby deformation results, and hence it seems most reasonable to conclude that the rupture of the coating films in the case of those tests may be due to their strains produced. The strains of the thin coating films, produced by those tests, are equal to those of the surface of the metal-plate. The results of the two tests were treated by means of dynamic analysis for the strains produced. Thus those two methods of test can be used for the coating films which have the tensile-strain of rupture under 50% limit.
While the impact wedge bending test produces simple tensile strain for the coating film, the cup punching test, on the other hand, simultaneously produces tensile strain in the radius direction and compressive strain in the circumferential direction of the cup, and the magnitudes of those strains at any point of the cup are equal but of opposite signs with each other.
As the results of this analysis, we have found that the ratio of εr/εt is 1.7∼2.1 for the coating films of phenol resin-epoxy resin-polyvinyl butyral system, where εr and εt are the limit of the tensile-strains of rupture at the impact wedge bending test and at the cup punching test, respectively. To explain this result, the stress-strain relation of the coating films is treated as an analogous one in the case of an isotropic elastic-body, because at those tests all the cross sections of the broken coating films are perpendicular in respect to the direction of the tensile strain produced, and show a typical pattern of brittle rupture of polymeric materials. Denoting the ratio of the limit of tensile-strains of rupture by y, we can get the following equation:
Y=εr/εt=2(μ/λ+1)/2(μ/λ+1)+a'
where λ and μ are the constants corresponding to Lamé constant of coating films, a is the ratio of the strains of circumferential direction to the strain of radius direction at two-dimensional deformation, thus a in the case of the cup punching test becomes -1. Assuming Poisson's ratio of the coating films to be ranging from 0.3 to 0.48, an example of applicability of the above equation indicates that y-value becomes ranging from 1.43 to 2.0. From the check described above, a better approximation for y-value is obtained by the use of this equation, although there is no more detail to support that the dynamic equations of an isotropic elastic-body may be used in this case too. Therefore, it appears to be of interest to investigate further the dynamic behavior of the thin coating films adhering to the metal-plate.
