Quality control of the induction hardened layer and the grinding process in aerospace ball screws via magnetic Barkhausen noise analysis
Reducing the weight of aircraft and helicopters is a common way to reduce fuel consumption and achieve greener operations. Nowadays, hydraulic actuators are used to move the control surfaces that are responsible for commanding the flight. Hydraulic systems have a considerable weight and therefore one of the major challenges in the aeronautic sector is to substitute these actuators with lighter electrical actuators. However, electrical actuators must be highly reliable and able to work under severe conditions. In this respect, electro-mechanical actuators (EMA) represent the next generation of actuation systems for commercial aircraft and aero engines. EMAs are equipped with a motor, a ball/roller screw and the control electronics. Actuators are safety-critical components of an aerospace system and an undetected failure can lead to serious consequences. Therefore, the quality of these ball screws has to be controlled and certified with the most advanced non-destructive techniques. In this context, the present work presents magnetic Barkhausen noise analysis (BNA) as a non-destructive technique for controlling the quality of ball screws, thereby ensuring the induction hardened layer has formed properly and detecting the local overheating regions, known as grinding burns, which may have occurred during the grinding process. With the aim of analysing the sensitivity of the BNA, several induction hardening treatments were applied to produce samples with different profiles and hardened layer depths. Moreover, grinding conditions were varied to obtain grinding burns of different intensities. Magnetic Barkhausen noise measurements were taken with a system designed and implemented by the authors, and the derived parameters were compared with microhardness measurements made at various depths after the different induction hardening and grinding processes. The results show that various parameters derived from the magnetic Barkhausen noise envelope can be used to identify both the different layer depths after induction hardening and the decrease in hardness produced by the grinding burns in such a way that a set of thresholds can be used to determine and assure the quality of the hardened layer and the grinding process of the ball screws. More info about this article: http://ndt.net/?id=19690
A model for the Barkhausen frequency spectrum as a function of applied stress
We derive a two parameter multi-exponential model to describe the frequency spectrum of Barkhausen noise in bulk steel under high excitation rates and applied tensile stress. We show how the amplitude and shape of the frequency spectrum depend on two directly measurable quantities, Barkhausen voltage and effective magnetic permeability, respectively, and how these change with stress. By incorporating frequency and depth dependence components into our model, we provide a framework for identifying stress variations along depth, which can be used for the purposes of non-destructive characterization.
Aristocratic Century: The sinews of power: political
In his excellent book on the Secretaries of State, Mark Thomson offered ‘the paradox of continuing domination of the aristocracy and the simultaneous insignificance of the House of Lords, per se ’. It is true that, by the beginning of the eighteenth century, the supremacy of the House of Commons in financial matters was a wellestablished convention. The claims of the Commons to an undisputed control over money matters, which had been urged before the Civil War, were resumed immediately after the Restoration and a protracted dispute with the House of Lords began. In 1661 the Lords resolved that there were no precedents in history for this new claim by the House of Commons. In 1671, the Commons retorted that taxation ought not to be questioned by the Lords and in 1678 they reiterated that supply was ‘the undoubted and sole right of the Commons’ and ‘ought not to be changed or altered by the House of Lords’. The Lords' attempt in 1692 to assess their own taxes was also abandoned, though they retired under cover of a rather insubstantial smoke-screen that they maintained their power of amendment to be a ‘fundamental, inherent and undoubted right from which their lordships can never depart’. That established, they departed. Geoffrey Holmes quotes a nice example from 1702 when the bishop of Carlisle found himself the only peer answering to a money bill, ‘no other lord in the House regarded what was doing, this being only (pro forma) to preserve a seeming right to dissent from, or amend, any part of a money bill’.