Strength Of Such Materials Research Articles

Plastic Flow, Creep, and Stress Relaxation: Part II. Creep

The general equation covering the deformation of plastic substances given in Part I is applied to creep. Creep is defined as a mechanism of deformation for systems which have a curvilinear relationship between stress and strain rate and a curvilinear relationship between strain and time at constant stress. Creep is connected with changes in the internal structure of a plastic substance and results in an increase in strength of such materials through work-hardening. Equations are derived which give the stress as a function of strain rate and time (time-hardening), as a function of strain rate and strain (strain-hardening) and as a function of strain rate, strain, and time. The difference between time-hardening and strain-hardening is discussed. Expressions are given for the coefficients of viscosity of such systems which are independent of the stress applied. The relaxation of stress at constant strain is discussed, and it is shown that the stress relaxation depends upon the history of the substance under test. The concept of creep is also applied to thixotropic systems which are considered as cases of work-softening.

The Effect of Scratches and Cuts on the Strength of Motion Picture Film

In An earlier paper on this subject <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> the effect of scratches on the strength of film support, not coated with emulsion or processed, was discussed. It was concluded from the measurements that the “principal effect of a scratch is due to its depth” reducing the effective thickness, and “the idea sometimes expressed that the mechanical strength of such materials as support and similar plastics is greatly dependent upon superficial scratches, and a surface skin does not hold for motion picture film support.” In the discussion the point was raised whether the age of the film and its brittleness would not change the matter, so that scratches would have relatively more effect. A further study has now been made on processed film brought to equilibrium with an atmosphere of definite temperature and humidity.