Effect of mean stress on hydrogen assisted fatique crack propagation in duplex stainless steel

Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, Kmax. A superposition model is developed to predict simultaneously the effects of stress intensity range (ΔK) and K ratio (Kmin/Kmax). The effect of Kmax is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip. © 1991.

An investigation of the mechanics of wire drawing with the superposition of an oscillatory drawing stress

A number of investigators have studied the application of oscillatory energy to a metal undergoing plastic deformation. Their results have shown that oscillatory stresses reduce both the stress required to initiate plastic deformation and the friction forces between the tool and workpiece. The first two sections in this thesis discuss historically and technically the devolopment of the use of oscillatory energy techniques to aid metal forming with particular reference to wire drawing. The remainder of the thesis discusses the research undertaken to study the effect of applying longitudinal oscillations to wire drawing. Oscillations were supplied from an electric hydraulic vibrator at frequencies in the range 25 to 500 c/s., and drawing tests were performed at drawing speeds up to 50 ft/m. on a 2000 lbf. bull-block. Equipment was designed to measure the drawing force, drawing torque, amplitude of die and drum oscillation and drawing speed. Reasons are given for selecting mild steel, pure and hard aluminium, stainless steel and hard copper as the materials to be drawn, and the experimental procedure and calibration of measuring equipment arc described. Results show that when oscillatory stresses are applied at frequencies within the range investigated : (a) There is no reduction in the maximum drawing load. (b) Using sodium stearate lubricant there is a negligible reduction in the coefficient of friction between the die and wire. (c) Pure aluminium does not absorb sufficient oscillatory energy to ease the movement of dislocations. (d) Hard aluminium is not softened by oscillatory energy accelerating the diffusion process. (e) Hard copper is not cyclically softened. A vibration analysis of the bull-block and wire showed that oscillatory drawiing in this frequency range, is a mechanical process of straining; and unstraining the drawn wire, and is dependent upon the stiffness of the material being drawn and the drawing machine. Directions which further work should take are suggested.

Mechanical properties of sheet metal under forming conditions

In the bulge test, a sheet metal specimen is clamped over a circular hole in a die and formed into a bulge by the hydraulic pressure on one side of the specirnen. As the unsupported part of the specimen is deformed in this way, its area is increased, in other words, the material is generally stretched and its thickness generally decreased. The stresses causing this stretching action are the membrane stresses in the shell generated by the hydraulic pressure, in the same way as the rubber in a toy balloon is stretched by the membrane stresses caused by the air inside it. The bulge test is a widely used sheet metal test, to determine the "formability" of sheet materials. Research on this forming process (2)-(15)* has hitherto been almost exclusively confined to predicting the behaviour of the bulged specimen through the constitutive equations (stresses and strains in relation to displacements and shapes) and empirical work hardening characteristics of the material as determined in the tension test. In the present study the approach is reversed; the stresses and strains in the specimen are measured and determined from the geometry of the deformed shell. Thus, the bulge test can be used for determining the stress-strain relationship in the material under actual conditions in sheet metal forming processes. When sheet materials are formed by fluid pressure, the work-piece assumes an approximately spherical shape, The exact nature and magnitude of the deviation from the perfect sphere can be defined and measured by an index called prolateness. The distribution of prolateness throughout the workpiece at any particular stage of the forming process is of fundamental significance, because it determines the variation of the stress ratio on which the mode of deformation depends. It is found. that, before the process becomes unstable in sheet metal, the workpiece is exactly spherical only at the pole and at an annular ring. Between the pole and this annular ring the workpiece is more pointed than a sphere, and outside this ring, it is flatter than a sphere. In the forming of sheet materials, the stresses and hence the incremental strains, are closely related to the curvatures of the workpiece. This relationship between geometry and state of stress can be formulated quantitatively through prolateness. The determination of the magnitudes of prolateness, however, requires special techniques. The success of the experimental work is due to the technique of measuring the profile inclination of the meridional section very accurately. A travelling microscope, workshop protractor and surface plate are used for measurements of circumferential and meridional tangential strains. The curvatures can be calculated from geometry. If, however, the shape of the workpiece is expressed in terms of the current radial (r) and axial ( L) coordinates, it is very difficult to calculate the curvatures within an adequate degree of accuracy, owing to the double differentiation involved. In this project, a first differentiation is, in effect, by-passed by measuring the profile inclination directly and the second differentiation is performed in a round-about way, as explained in later chapters. The variations of the stresses in the workpiece thus observed have not, to the knowledge of the author, been reported experimentally. The static strength of shells to withstand fluid pressure and their buckling strength under concentrated loads, both depend on the distribution of the thickness. Thickness distribution can be controlled to a limited extent by changing the work hardening characteristics of the work material and by imposing constraints. A technique is provided in this thesis for determining accurately the stress distribution, on which the strains associated with thinning depend. Whether a problem of controlled thickness distribution is tackled by theory, or by experiments, or by both combined, the analysis in this thesis supplies the theoretical framework and some useful experimental techniques for the research applied to particular problems. The improvement of formability by allowing draw-in can also be analysed with the same theoretical and experimental techniques. Results on stress-strain relationships are usually represented by single stress-strain curves plotted either between one stress and one strain (as in the tension or compression tests) or between the effective stress and effective strain, as in tests on tubular specimens under combined tension, torsion and internal pressure. In this study, the triaxial stresses and strains are plotted simultaneously in triangular coordinates. Thus, both stress and strain are represented by vectors and the relationship between them by the relationship between two vector functions. From the results so obtained, conclusions are drawn on both the behaviour and the properties of the material in the bulge test. The stress ratios are generally equal to the strain-rate ratios (stress vectors collinear with incremental strain vectors) and the work-hardening characteristics, which apply only to the particular strain paths are deduced. Plastic instability of the material is generally considered to have been reached when the oil pressure has attained its maximum value so that further deformation occurs under a constant or lower pressure. It is found that the instability regime of deformation has already occurred long before the maximum pressure is attained. Thus, a new concept of instability is proposed, and for this criterion, instability can occur for any type of pressure growth curves.

A failure criterion for fillet welds

The work constitutes a study of the strength of mild steel fillet welds subject to static loading, and the behaviour of flange welded beam-column connections under combined bending and shear. Tests are conducted on short welds in the as-welded and stress relieved conditions, and also on full-size beam-column connections. It is shown that welds under compression have a lower strength than when under tension. Failure of the fillet weld is initiated at the weld root, the important factor controlling the initiation being weld ductility. The greater the residual stress, the lower the weld ductility and ultimate strength. Thermal stress relieving increases strength by as much as 30%. Weld failure plane is rarely at the throat and varies from 0° to 45° depending upon loading condition. Failure plane average stresses are related by a circular function which is expressed in terms of externally applied forces at limit state. The tension weld of a flange-welded beam-column connection always fails before the compression weld. The shear load sharing between the welds is a complex function of elastic compression of the web, elastic/plastic deformation of the flanges, load/deformation characteristics, and the type of load application. Bearing forces between the compression flange and column face produce low level bearing stresses and frictional forces which make a negligible contribution to shear load resistance. Three modes of connection failure are possible; 'end mode', 'bending mode' and 'shear mode', with a sudden change taking place between the two latter.

Influence of macroscopic residual stress fields on fatigue crack growth measurement in SiC particulate reinforced 8090 aluminium alloy

The effect of residual stresses, induced by cold water quenching, on the morphology of fatigue crack fronts has been investigated in a powder metallurgy 8090 aluminium alloy, with and without reinforcement in the form of 20 wt-%SiC particles. Residual stress measurements reveal that the surface compressive stresses developed in these materials are significantly greater than in conventional metallurgy ingot 8090, because surface yielding occurs on quenching. The yield stresses of the powder route materials are greater than those of ingot produced 8090 and hence greater surface stresses can be maintained. In fatigue, severe crack front bowing is observed in the powder formed materials as a result of the reduction of the R ratio (minimum load/maximum load) by the compressive residual stresses at the sides of the specimen, causing premature crack closure and hence reducing the local driving force for fatigue crack growth ΔKeff. This distortion of the crack fronts introduces large errors into measurements of crack growth rate and threshold values of ΔK.

MRI pressure and stress measurement in novel homogeneous soft solids

This paper presents MRI measurements of a novel semi solid MR contrast agent to pressure. The agent is comprised of potassium chloride cross linked carageenan gum at a concentration of 2% w/v, with micron size lipid coated bubbles of air at a concentration of 3% v/v. The choice for an optimum suspending medium, the methods of production and the preliminary MRI results are presented herein. The carageenan gum is shown to be ideally elastic for compressions relating to volume changes less than 15%, in contrast to the inelastic gellan gum also tested. Although slightly lower than that of gellan gum, carageenan has a water diffusion coefficient of 1.72×10-9 m2.s-1 indicating its suitability to this purpose. RARE imaging is performed whilst simultaneously compressing test and control samples and a maximum sensitivity of 1.6% MR signal change per % volume change is found which is shown to be independent of proton density variations due to the presence of microbubbles and compression. This contrast agent could prove useful for numerous applications, and particularly in chemical engineering. More generally the method allows the user to non-invasively image with MRI any process that causes, within the solid, local changes either in bubble size or bubble shape. © 2008 American Institute of Physics.