The Journal of Strain Analysis for Engineering Design

Digital fringe multiplication of three-dimensional photoelastic images using the image-division technique and multiple-angle relations of the cosine function is described and demonstrated. Fringe multiplication of two slices from a stress-freezing model to a factor of 9 is shown. The fractional fringe orders counted by this method agree well with that determined by the Tardy method and a null-balance compensator. The multiplication process is very simple and effective for practical use.

Three-point-bend (3PB) specimens with real cracks and very sharp notches have been manufactured from Araldite CT200 with HT907 hardener. The plane strain fracture toughness, K_IC, has been obtained from cracked specimen tests and the apparent plane strain fracture toughness, K^App_IC, has been obtained for various very sharp notched widths. A systematic variation of apparent fracture toughness with notch width has been observed.

Burst tests have also been performed on cylinders with similar, very sharp, axially aligned, external, semi-circular notches. It has been shown that the burst pressure for the cylinders, with different depths of crack, can be predicted from the corresponding 3PB apparent fracture toughness data. The notches in the cylinders and 3PB specimens must be very sharp, and manufactured in a similar way, for the predictions to be accurate. Therefore, by loading model components containing very sharp crack-like flaws to obtain the failure loads, the failure loads of similar actual components having a real crack and behaving in a brittle manner can be predicted.

The residual stresses in the three layers of a composite averaged over the layer thickness were determined by means of neutron diffraction experiments using the Fourier stress spectrometer (FSS) at GKSS Forschungszentrum Geesthacht (Germany). For stress analysis, the lattice strains of many different Bragg reflections have to be measured in the directions along and normal to the rolling direction employed during fabrication of the composite. These results have been compared with residual stress measurements using the ring-core method (RCM) performed at the IFAM (Fraunhofer Institut für Angewandte Materialforschung), Bremen. In addition to the stress states in the rolling and transverse directions, averaged over the whole thickness, the normal components of the stress tensor in the different layers of the composite—which are not detectable by RCM—have been investigated by neutron diffraction.

The solution of crack problems by employing dislocation loops (or dipoles) as strain nuclei is described. The relationship between this technique and dislocation density methods is outlined, the advantages of the former being illustrated by practical examples. The procedure also has the merit of being capable of extension to three-dimensional problems.

This paper deals with the influence of optical retarders on the isochromatic fringes obtained by automated white light photoelasticity using methods such as those based on spectral content analysis (SCA) and on primary colour (red, green and blue) analysis. In the following the light intensity equations of dark- and light-field polariscopes with both crossed and parallel optical retarders are reviewed. In particular, it is shown that the retardance error of the quarter-wave plates produces an attenuation of the maximum intensity in dark field and an increase of minimum intensity in light field. Experimental evidence of the influence of optical retarders is also shown.

Neutron strain scanning has been used to determine the residual stress distribution over the area of the mid-thickness plane of a small austenitic steel piate across which a weld had been laid. The longitudinal and transverse stresses in the centre of the plate and weld are strongly tensile. Stresses normal to the plane of the plate are low everywhere except near the ends of the weld where tensile ‘hot-spots’ are observed. Near the edges of the plate the effects of boundary and balancing conditions are evident. Longitudinal and transverse stresses tend to zero along the edges of the plate to which they are perpendicular but become strongly compressive near the middle regions of the edges to which they are parallel. The results are compared with published data from other welds that were made and constrained differently. The accuracy of the technique is discussed and the results obtained by measuring with different reflections are compared. It is shown that neutron strain scanning is an acceptable, and unique, method for determining non-destructively the residual stress distribution throughout small austenitic steel components with sufficient accuracy to provide data for design purposes and to validate model calculations.

The frictional behaviour of heavily loaded wire ropes bent over relatively small sheaves has been investigated with 6 × 36, 28 mm and 35 mm diameter, IWRC (independent wire rope core) and FC (fibre core), bright and galvanized ropes being tested, over sheaves with D/d ratios between 12 and 20. The groove profiles used were plain U and undercut U, with groove diameters 2.5 per cent smaller than the rope nominal diameter. Some pilot studies were also run on high-strength synthetic fibre ropes. The measurements showed that the effective friction coefficient increased with decreasing line tension and increasing sheave diameter for unused surfaces but became almost constant for used surfaces. It was shown that the undercut U groove had a higher frictional hold than the plain U groove but gave rise to early erratic slip. Galvanized rope had a lower friction coefficient while increased speed of sliding decreased friction.

Governing equations for predicting the steady state flexural behaviour of rotating sandwich cantilever beams with two orthotropic facings and an isotropic inner core are formulated by using the variational principle. Those for describing the steady state deformation and displacements of an orthotropic beam are also formulated by using the same method. The effect of the curing temperature of the orthotropic material upon steady state stress distributions is also included in this analysis. Exact solutions of these differential equations are obtained to calculate displacements and stress distributions in both symmetric and non-symmetric sandwich beams subjected to uniformly distributed loads. Steady state stress values for an orthotropic beam loaded with a flatwise tip force are calculated so that they can be compared to dynamic measurements for the type of beam reported in the literature. The present analysis also predicts both the lateral and the transverse inertio-elastic instabilities due to the rotational action of the beam.

The subsequent creep behaviour of polymers is influenced by prior plastic deformation. The constitutive creep equation is deduced by introducing the concept of a back stress. The creep relation obtained with back stress is compared with the experimental results for the subsequent creep after prior plastic tension. This creep relation gives good agreement with the experimental results for the small creep strain.