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
Surface Integrity Inspection on Gears Using Barkhausen Noise Analysis
Abstract Hard finishing of gears is the last step in the process chain and it is mostly responsible for the properties and performance of the workpiece. A grinding process basically offers the possibility of high temperatures in the cutting zone, possibly resulting in tensile residual stresses, reduced hardness and in metal structure. Thus the control of physical properties of the workpiece is of utmost importance for quality assurance. This paper shows the testing of finished gears in industrial production with a nondestructive, ecological and micro-magnetic method called Barkhausen noise analysis. The strength of Barkhausen signal is influenced by the elastic stresses and the micro structure. So, changes in residual stresses and material structure are measurable. Moreover they are indicators for improvement of the used grinding process. This paper focuses on the possibility to substitute other more hazardous methods by Barkhausen noise analysis. In addition process advantages and drawbacks factors influencing measurements industrial applications and options for measuring automation are also discussed.
Surface and subsurface stress evaluation in case-carburised steel using high and low frequency magnetic barkhausen emission measurements
Abstract The effect of near-surface and subsurface stresses on the magnetic Barkhausen emission (MBE) profile has been studied in case-carburised and tempered En36 steel. The high- and low-frequency MBE measurements were made, on both tensile and compressive sides of the rectangular bar specimens with case-depth of 0.95 mm loaded in cantilever beam, under different stress levels as well as after unloading from different higher stress levels. The high-frequency MBE profile showed a single peak while the low-frequency MBE profile showed two peaks. Under applied elastic stresses, both types of measurement showed increase in MBE under tension and decrease in MBE under compression. But, the MBE profiles measured after unloading from higher stresses showed different behaviour. On the tensile side, the high-frequency MBE profile did not change significantly due to pre-stress. But, in the low-frequency MBE profile, the first peak increases and the second peak decreases with increase in pre-stress. On the compressive side, the peak height of high-frequency MBE profile decreased gradually with increase in pre-stress. The first peak of the low-frequency MBE profile also decreased gradually with increase in pres-stress level. But, the second peak of the low-frequency MBE profile decreased by about 10% at a pre-stress level of −1094 MPa and remained more or less the same even after unloading from −1783 MPa. The MBE behaviour has been correlated to the residual stress (RS)-depth profile measured using X-ray diffraction method. This correlation clearly indicates that the high-frequency MBE reflects only the changes in surface RS level. It does not indicate RS changes occurring at depths >20 μm below the surface. The low-frequency MBE profile reflects the changes in the RS distribution occurring in the near-surface as well as deeper subsurface layers.