Matt Cullin

A generalized analytical compliance model for transversely symmetric three-segment flexure hinges.

This paper presents a generalized compliance model for a three-segment notch flexure hinge with transverse symmetry. This flexure hinge configuration is most frequently employed in planar-motion, small-displacement compliant mechanisms. The axial and bending compliances are derived for this flexure hinge based on the compliances of two flexure components. The derivation is generalized such that it can be applied to various segment geometries. Using this open-ended model, a three-segment right elliptical corner-filleted flexure hinge design was analyzed. This geometric configuration introduces additional geometric parameters, which can be used to optimize the compliance of the flexure hinge without modifying its gross dimensions. The results of the analysis were validated in part by modifying the geometric parameters of the center segment and elliptical corner fillets to form limiting cases corresponding to several previously investigated configurations, namely right elliptical, three-segment right circular corner-filleted, and right circular geometries. Finite element analysis simulation and experimental testing were used to further validate the three-segment right elliptical corner-filleted analytical model. Additional simulations based on the analytical model were performed to highlight the influence of geometric parameters on compliances and to investigate shear effects for short flexure hinges.

Planar compliances of thin circular-axis notch flexure hinges with midpoint radial symmetry

The new class of flexure hinges with circular longitudinal axis and midpoint radial symmetry is introduced. Using rotation and mirroring, the symmetric flexure hinge is obtained from one half flexure. The six planar-bending compliances of the full hinge are determined analytically for small deformations by combining only three compliances of the half flexure. To illustrate the general flexure hinge category, the novel circular-axis, right circularly corner-filleted design is introduced. Experimental and finite element results correlate well with the analytical model predictions. The new flexure hinge design is compared to the circular-axis, constant-thickness flexure and the straight-axis, right circularly corner-filleted hinge.

Planar Compliances of Symmetric Notch Flexure Hinges: The Right Circularly Corner-Filleted Parabolic Design

This study proposes a general analytical compliance model for symmetric notch flexure hinges composed of segments with constant width and analytically defined variable thicknesses. Applying serial combination and longitudinal/transverse mirroring of base segments, the in-plane compliances of the full flexure are obtained as functions of one quarter-flexure compliances. The new right circularly corner-filleted parabolic flexure hinge is introduced as an illustration of the general analytical modeling algorithm. Experimental testing and finite element simulation confirm the analytical model predictions for an aluminum flexure prototype. The planar compliances sensitivity to the relevant geometric parameters is also studied.

Modeling and simulation of constituent particle clustering and distribution in rolled aluminum alloys

It is recognized that corrosion damage in aluminum alloys is the direct result of local galvanic coupling between constituent particles and the metal matrix. Heavily clustered groups of constituent particles have been shown to form the most severe corrosion pits, and the morphology of these pits has been found to exhibit a strong correlation with that of the instigating particle cluster. To predict damage evolution accurately in three dimensions, the spatial distribution of the constituent particles in a given material must be quantified. The tomographic reconstruction methods currently available are time consuming and possess characteristic length scales that are insufficient for large-scale corrosion studies. To accommodate this larger length scale, a model for three-dimensional constituent particle microstructure is proposed. This model employs a fusion of classic stereological techniques and qualitative observations to describe the constituent particle microstructure of rolled aluminum alloys in three dimensions. A method is described for generating statistically representative three-dimensional constituent particle microstructure from two-dimensional orthogonal cross sections. This novel simulation algorithm allows for detailed three-dimensional particle microstructure simulations to be carried out using standard optical microscopy and image analysis techniques.

Compliances of symmetric flexure hinges for planar compliant mechanisms

A general analytical compliance model is presented for symmetric flexure hinges formed of serial segments of known compliances and with thicknesses defined analytically. Serial combination, mirroring, and translation of individual segments yield the flexure compliances. As an illustration, the compliances of a right elliptical corner-filleted flexure hinge are studied. For an aluminum sample with dimensions of l = 0.0254 m, a = 0.006 m, b = 0.008 m, t = 0.0015 m, and w = 0.00635 m, experimental testing, finite element simulation, and analytical results are very close. Results of this study include plots of the rotary compliance in terms of geometric parameters.

Fatigue Behavior of Stainless Steel, Titanium, and Cobalt Chromium Molybdenum Spinal Rods

The objective of this study was to measure and compare the fatigue behavior of 316L stainless steel, titanium (Ti-6Al-4V), and cobalt chromium molybdenum (CoCrMo) spinal rods in vitro. Spinal rods are used to immobilize the spine while fusion of the vertebrae occurs (spinal arthrodesis). Implanted spinal rods are subjected to cyclic loading and are therefore susceptible to fatigue failure if fusion does not occur sufficiently quickly. A significant number of spinal rod fatigue failures have been observed between six months to one year following surgical implantation. On average, the spine will experience about 3 million cycles per year. Stress overloads can result in permanent deformation or immediate failure of the rod, however these overloads are seldom the root cause of failure—rods typically fail by fatigue [1].Copyright