Measurement Of Thickness Changes Research Articles

Plastic deformation at a stably growing crack tip

The deformations around the tip of a straight, through-the-thickness crack in sheets of cellulose nitrate, celluloid, in tension have been studied, as well before as after the onset of stable crack growth. A tensile testing apparatus and a device for engraving a fine grid directly on the specimen sheet have been designed. The development of the plastic enclave also after the start of stable crack extension has been studied in connexion with investigations of the deformations in gross fields of size up to 30 × 40 mm2 performed on a digital computer (line element length 1 or 0.5 mm, virgin crack length 80 mm). Unexpected deformations very close to the crack tip were found in the near field where line elements of length 50 µm were analyzed. A modified Dugdale model where the cohesive stresses vary linearly is examined and is applied on polyvinylehloride, PVC, specimen the thickness of which is strongly reduced near the crack tip. A method for measurement of thickness changes of the order of a few µm by tracing isohypses is described. Finally, to enable comparison with the experimental results, the elastic deformation pattern for a square mesh in the plane strain and plane stress case is obtained by a digital computer.

An interpretation of inelastic birefringence

The relationship between plastic thickness change and plastic isochromatics occurring beyond the plostic tensile instability point of a uniaxial tensile specimen is investigated. Mechanical thickness-change measurements and a hologram of thickness change are shown to be in close qualitative agreement with isochromatics in a region of gross plastic yielding. Analytical observations are discussed to aid the interpretation of the experimental results obtained. Finally, possible extensions and applications are discussed.

On the determination of the sum of the principal stresses in two-dimensional problems

The well-known principle of the determination of the sum of principal stresses in plates by measuring thickness changes is applied in this paper to “frozen” plates and to models made with low-modulus materials, such as rubbers, deformed inside portable frames. It is shown that a sufficiently precise measurement of thickness changes is possible with a machine-shop comparator, rather than with the more delicate laboratory-type instruments. The necessary corrections to be introduced to the comparator readings when rubber models are used are described in the Appendix.

A method for measuring thickness changes in plane transparent models

A method for measuring thickness changes in plane transparent models

Action of Solvents on Dried Linseed Oil Films

THE action of a range of organic solvents on aged linseed oil films, both pigmented and unpigmented, has been investigated1. A specially designed apparatus based on the parallel plate viscometer was used2 to make precise measurements of changes in thickness of films coated on a rigid substrate, while free films (these proved to be isotropic in behaviour) were observed under a measuring microscope. The results thus obtained, together with related gravimetric measurements, made it possible to determine rates of swelling and deswelling, and to examine the characteristics of the leaching process in which constituents of low molecular weight are removed when the felt-like structure is swollen.