G O Rading

Stress Characterization of Autofrettaged Thick-Walled Cylinders

The inaccessibility of commercial software has necessitated the development of low-cost, general-purpose finite element method (FEM) computer programs for structural analysis. Using the FEM program, the elastic, elastic-plastic, residual and service stresses and displacements in a closed ended, thick-walled cylinder under internal pressure were established. The displacement formulation was implemented and eight-noded brick isoparametric elements chosen. The frontal solution technique and the incremental theory of plasticity were used, as only limited computing facilities were available. The results were found to be in very good agreement with the through-thickness analytical values. The benefits of autofrettage were demonstrated and an optimum overstrain of 16% established for a cylinder with a thickness ratio of 2. The material economy achieved through autofrettage and the limitations imposed are illustrated. The FEM program could therefore be reliably used for other complex geometries and load conditions.

Geometric Constants in Plain Cross-Bored Cylinders

A three-dimensional FEM computer program was developed to establish the stress distributions and SCFs in thick walled cylinders with flush and nonprotruding plain cross bores under internal pressure. The displacement formulation and eight-noded brick isoparametric elements were used. The Frontal solution technique was used due to the limited computing facilities. In the far field, the FEM stresses and displacements were in good agreement with the through thickness analytical values. The variation of SCF with d was established for various thickness ratios. For k> 1.25, the maximum hoop stress occurred away from the crotch corner when d≤0.2. For k=1.25, the maximum hoop stress occurred away from the crotch corner when d≤0.15. For k≥1.75, d was found to be a geometrical constant equal to 0.2 where the k values have a SCF of 2.734. For k <I.75, d was found to be a geometrical constant equal to 0.11 where the k values have a SCF of 2.67. A new categorization of cylinders has been proposed: thin shells, thick shells, and thick cylinders.

Effect of runner design on mechanical properties of permanent mould aluminium castings

Abstract A comparison of the influence of different runner designs on the mechanical properties of aluminium castings has shown that filtered rectangular runners (FRRs) yield aluminium castings with higher and more reliable mechanical properties than the conventional unfiltered rectangular runners. Unfiltered vortex flow runners have also been shown to improve the reliability of the modulus of rupture of cast aluminium alloys over unfiltered rectangular runners. In the present paper, experimental results of a comparative study on the effect of the unfiltered vortex flow and FRR designs on the tensile strength of permanent mould aluminium castings are reported. The results show that an FRR yields castings with higher and more reliable tensile strengths than the unfiltered vortex flow runner. Castings poured into a mould with an FRR had strengths between 269 and 291 MPa and a Weibull modulus of 50·2 while castings poured into the vortex flow runner had strengths between 255 and 280 MPa and a Weibull modulus of 40·3.

Overstraining of flush plain cross-bored cylinders

Abstract A three-dimensional finite element method computer program was developed to establish the elastic-plastic, residual and service stress distributions in thick-walled cylinders with flush and non-protruding plain cross bores under internal pressure. The displacement formulation and eight-noded brick isoparametric elements were used. The incremental theory of plasticity with a 5 per cent yield condition (an element is assumed to have yielded when the effective stress is within 5 per cent of the material yield stress) and von Mises yield criterion were assumed. The frontal solution technique was used. The incipient yield pressure and the pressure resulting in a 0.3 per cent overstrain ratio were established for various cylinder thickness ratios and cross bore-main bore radius ratios. For a thickness ratio of 2.25 and a cross bore-main bore radius ratio of 0.1, the stresses were determined for varying overstrain and an optimum overstrain ratio of 37 per cent was established. To find the accuracy of the results, the more stringent yield condition of 0.5 per cent was also considered. The benefits of autofrettage were presented and alternative autofrettage and yield condition procedures proposed.

Electrolytic Etching of Platinum- Aluminium Based Alloys

Department of Mechanical and Manufacturing Engineering, University of Nairobi, PO Box 30197-00100, Nairobi, Kenya, and African Materials Science and Engineering Network (AMSEN) The microstructures of as-cast and annealed platinum alloys of different compositions were revealed after electrolytic etching in hydrochloric acid/sodium chloride solution using direct current. It was shown that the etching process enhances good microstructural images of platinum-aluminium based alloys.

An evaluation of residual stress distribution in welded compact tension specimens using neutron diffraction

The neutron diffraction technique was used to determine the residual stress field in welded compact tension specimens of the aluminium-lithium alloy AA 2095. The deep penetrating characteristic of neutrons was exploited to evaluate the through-thickness variation in residual stress. Moreover, insight into the redistribution of these stresses was gained by extending a fatigue crack through the residual stress field and re-examining the stress distribution. The specimen without a crack was found to have a high compressive stress (of the order of - 135MPa) ahead of the notch. This rose to a maximum tensile stress of about 50MPa, 22 mm from the notch, followed by a drop to negative values further ahead of the notch. It was observed that the magnitude of the stresses changed on moving into the thickness of the specimen. However, the form of the graph showing stress versus distance ahead of the notch remained unchanged. When fatigue cracks of different lengths were introduced, the magnitude of the stress close to the tip first increased with crack length, before decreasing and then rising again. Nevertheless, the form of the graph remained unchanged and the stress at the crack tip remained compressive. The paper concludes that any study of the response of a component to mechanical loading involving a residual stress field must take these factors (i.e. through-thickness stress variation and stress redistribution) into consideration.

Overstrain in flush optimal-chamfered cross-bored cylinders

Abstract A three-dimensional finite-element method computer program was developed to establish the elastic—plastic, residual, and service stress distributions in cylinders with flush and non-protruding optimal-chamfered cross-bores under internal pressure. Eight-noded brick and four-noded tetrahedral isoparametric elements and the displacement formulation were used. The incremental theory of plasticity with a 5 per cent yield condition and von Mises yield criterion were assumed. The incipient and 5 per cent overstrain (ov) pressures were established for various thickness ratios and cross-bore to main bore radius ratios. For the optimum chamfer angle geometrical configuration, the stresses were determined for varying ov. The maximum and minimum effective stresses were located 7.5° from the meridional and transverse planes, respectively. Meridional plane through thickness yielding occurred at an ov of 41 per cent. The service stress gradients at the cross-bore chamfer end increased with ov for ovs >30 per cent. Stress reversals were eliminated for overstrain >27 per cent. Alternative autofrettage and yield condition procedures were proposed.