R.J. Klassen

Evolution of plastic strain during a flow forming process

The distribution of equivalent plastic strain through the thickness of several AISI 1020 steel plates formed under different conditions over a smooth cylindrical mandrel using a single-roller forward flow forming operation was studied by measuring the local micro-indentation hardness of the deformed material. The equivalent plastic strain was higher at the inner and outer surfaces and lowest at the center of the workpiece. Empirical expressions are presented which describe the contribution of the roller and mandrel to the total local equivalent plastic strain within the flow formed part. The dependence of these expressions upon the thickness reduction during flow forming is discussed.

Control of surface topography in biomimetic calcium phosphate coatings.

The behavior of cells responsible for bone formation, osseointegration, and bone bonding in vivo are governed by both the surface chemistry and topography of scaffold matrices. Bone-like apatite coatings represent a promising method to improve the osteoconductivity and bonding of synthetic scaffold materials to mineralized tissues for regenerative procedures in orthopedics and dentistry. Polycaprolactone (PCL) films were coated with calcium phosphates (CaP) by incubation in simulated body fluid (SBF). We investigated the effect of SBF ion concentration and soaking time on the surface properties of the resulting apatite coatings. CaP coatings were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and energy dispersive X-ray spectrometry (EDX). Youngs modulus (E(s)) was determined by nanoindentation, and surface roughness was assessed by atomic force microscopy (AFM) and mechanical stylus profilometry. CaP such as carbonate-substituted apatite were deposited onto PCL films. SEM and AFM images of the apatite coatings revealed an increase in topographical complexity and surface roughness with increasing ion concentration of SBF solutions. Youngs moduli (E(s)) of various CaP coatings were not significantly different, regardless of the CaP phase or surface roughness. Thus, SBF with high ion concentrations may be used to coat synthetic polymers with CaP layers of different surface topography and roughness to improve the osteoconductivity and bone-bonding ability of the scaffold.

Variation in dislocation cell size with local strain in a low alloy steel

Abstract The role of dislocation cell structure is well established in the work hardening of metallic materials. The cell diameter in transmission electron microscopy specimens taken at various places along the gauge length of broken specimens of a low alloy steel containing niobium was correlated with the local strain. Also, microhardness measurements were made and the variation in hardness with strain was obtained. It is shown that the cell size decreases with increasing strain until a critical size is reached. After this critical size, the cell size remains constant, even while work hardening continues but apparently with decreasing rate as in stage III. These results are consistent with the principle of “similitude” in stage II and support the hypothesis of the mesh length theory of work hardening that stage III arises when dislocation cells cease to shrink and similitude breaks down.

A finite element model on effects of impact load and cavitation on fatigue crack propagation in mechanical bileaflet aortic heart valve

Pyrolytic carbon mechanical heart valves (MHVs) are widely used to replace dysfunctional and failed heart valves. As the human heart beats around 40 million times per year, fatigue is the prime mechanism of mechanical failure. In this study, a finite element approach is implemented to develop a model for fatigue analysis of MHVs due to the impact force between the leaflet and the stent and cavitation in the aortic position. A two-step method to predict crack propagation in the leaflets of MHVs has been developed. Stress intensity factors (SIFs) are computed at a small initiated crack located on the leaflet edge (the worst case) using the boundary element method (BEM). Static analysis of the crack is performed to analyse the stress distribution around the front crack zone when the crack is opened; this is followed by a dynamic crack analysis to consider crack propagation using the finite element approach. Two factors are taken into account in the calculation of the SIFs: first, the effect of microjet formation due to cavitation in the vicinity of leaflets, resulting in water hammer pressure; second, the effect of the impact force between the leaflet and the stent of the MHVs, both in the closing phase. The critical initial crack length, the SIFs, the water hammer pressure, and the maximum jet velocity due to cavitation have been calculated. With an initial crack length of 35 μm, the fatigue life of the heart valve is greater than 60 years (i.e. about 2.2×109 cycles) and, with an initial crack length of 170 μm, the fatigue life of the heart valve would be around 2.5 years (i.e. about 9.1×107 cycles). For an initial crack length greater than 170 μm, there is catastrophic failure and fatigue cracking no longer occurs. A finite element model of fatigue analysis using Patran command language (PCL custom code) in MSC software can be used to evaluate the useful lifespan of MHVs. Similar methodologies can be extended to other medical devices under cyclic loads.

Characterization of the effect of alloying elements on the fracture toughness of high strength, low alloy steels

Abstract The effect of small alloy additions on the ductile fracture toughness of ferritic high strength, low alloy steels was characterized by determining the fracture toughness parameter J Ic and by microstructural analysis on six compositions of these steels. Small additions of niobium (0.10 wt.%) or titanium (0.08 wt.%) resulted in material with significantly improved J Ic values. Additions of vanadium (0.085 wt.%), however, resulted in little improvement in J Ic values compared with the base steel and the other two alloying elements. The improvements in fracture toughness due to the additions of niobium and titanium are explained in terms of the dependence of ductile fracture toughness on the effects of grain size, inclusion diameter and inclusion volume fraction.

Thermal relaxation of internal strain in two-phase Cu-Nb wire

Abstract Thermal relaxation of internal strain in high-strength two-phase Cu–18wt.%Nb wire was studied using neutron diffraction at temperatures from 100 to 450 °C. The average axial tensile strain in the Nb phase reduced significantly at temperatures above 300 °C. This was shown to correspond to the onset of spheroidization of the Nb phase.

Void nucleation in constrained silver interlayers

The process of void nucleation during fracture in thin brazed Ag interlayers has been investigated. Tensile tests were performed on interlayers of five thicknesses. The tensile strength increased rapidly with the ratio of interlayer diameter to thickness(D/T) for the thick interlayers(D/T < approximately 40) while the rate decreased significantly in the thin interlayers. All specimens fractured in the Ag interlayer along a plane near, and parallel to, the steel interface. The fracture surfaces showed silicon oxide inclusions at the bottom of many of the dimples. A series of finite element models were constructed to determine the general stress state in the interlayers and to determine the development of local stresses around inclusions in the interlayer. The finite ele- ment calculations indicated that the distribution of triaxial tension radially across the interlayer varied withD/T. Triaxial stresses, at failure, up to 10 times the uniaxial yield stress of the Ag were predicted from the model. The local stresses around a rigid inclusion in the interlayer developed more quickly, with applied stress, in the thicker interlayers as a result of the increased plastic deformation. The development of local stresses also increased with the proximity of the inclusion to the steel interface. By assuming a critical stress criterion for void nucleation at an inclusion interface, the finite element model was able to predict the experimentally observed nonlinear relationship between the interlayer failure stress andD/T.

Fracture topography of HSLA steels

Abstract The topography of the fracture surfaces of several high strength, low alloy (HSLA) steels was determined using scanning electron microscopy. The steels contained various additions of titanium, vanadium and niobium. Two types of specimens were investigated. They were either fractured tensile specimens which had undergone uniform strain followed by necking to fracture or fractured three-point bending specimens broken during determination of the J-integral fracture parameter. The distribution of the dimple size from both types of specimens was determined. It was found that the average dimple size for the tensile specimens is about 1.5 μm for all steels. For the three-point bending specimens, however, the average dimple size is about 15 μm. Also, the relative depth of the dimples is higher for the tensile specimens than it is for the three-point bending specimens. These results are interpreted in terms of the plastic strain in the specimens where, in the tensile specimens, the large uniform strain contributes to the formation of elongated dimples and, in the three-point bending specimens, the state of triaxity at the crack tip contributes to large shallow dimple formation.

Analytical solution of the tooling/workpiece contact interface shape during a flow forming operation

Flow forming involves complicated tooling/workpiece interactions. Purely analytical models of the tool contact area are difficult to formulate, resulting in numerical approaches that are case-specific. Provided are the details of an analytical model that describes the steady-state tooling/workpiece contact area allowing for easy modification of the dominant geometric variables. The assumptions made in formulating this analytical model are validated with experimental results attained from physical modelling. The analysis procedure can be extended to other rotary forming operations such as metal spinning, shear forming, thread rolling and crankshaft fillet rolling.

Mechanisms of plastic deformation prior to ductile fracture in a low alloy steel

Abstract The fracture toughness of metallic materials is related to their ductility. Thus the fracture process is characterized by the mechanisms of plastic deformation which take place prior to fracture. In this study, such a characterization was obtained on tensile specimens of a low alloy steel containing niobium. The dislocation cell structures formed in the necked sections of tensile specimens were examined as a function of local strain, expressed as reduction in area. It was found that these cell structures decrease in size as the strain is increased up to a certain diameter where the diameter stays constant with further increase in strain. This behavior is related to observations on the variation in the fracture criterion J Ic with grain size to provide qualitative understanding of the dependence of fracture toughness on microstructural and dislocation arrangements in metallic materials.