FATIGUE CRACK GROWTH - THE COMPLICATIONS.

A vast body of experimental data has been accumulated on the constant amplitude crack growth response of structural metals in moist laboratory air. Usually the data is presented as plots of crack growth rate, da/dN, against stress intensity range, DELTA K. In order to extrapolate this data to fatigue crack growth in more active or more inert environments, to crack growth under variable amplitude loading, or to crack growth under multi-axial or mixed mode loading, the mechanisms of crack advance and crack closure should be considered. This paper briefly reviews the crack closure phenomenon and discusses the dominant causes of accelerated and retarded growth under changes in environment or type of loading. It is argued that simple constant amplitude data is often surprisingly accurate when used to predict crack growth in more complex situations. However, there are some cases where constant amplitude data lead to dangerously non-conservative predictions of fatigue life.

Integrated high pressure microhydraulic actuation and control for surgical instruments

To reduce the surgical trauma to the patient, minimally invasive surgery is gaining considerable importance since the eighties. More recently, robot assisted minimally invasive surgery was introduced to enhance the surgeon's performance in these procedures. This resulted in an intensive research on the design, fabrication and control of surgical robots over the last decades. A new development in the field of surgical tool manipulators is presented in this article: a flexible manipulator with distributed degrees of freedom powered by microhydraulic actuators. The tool consists of successive flexible segments, each with two bending degrees of freedom. To actuate these compliant segments, dedicated fluidic actuators are incorporated, together with compact hydraulic valves which control the actuator motion. Especially the development of microvalves for this application was challenging, and are the main focus of this paper. The valves distribute the hydraulic power from one common high pressure supply to a series of artificial muscle actuators. Tests show that the angular stroke of the each segment of this medical instrument is 90°. © 2012 Springer Science+Business Media, LLC.