Key Engineering Materials Vols. 569-570

Abstract: This paper deals with experimental investigations of single hat longitudinal rail under quasi-static crush loading. A set of identical specimen which was simplified from actual automotive longitudinal rail was fabricated, and quasi-static testing (i.e., compression and bending test) were conducted. The main objective of this research was to study the failure mode, bending and tensile behaviour of the structure under bending and axial compression. In order to get the compressive load-displacement response and its corresponding deformation pattern, the compression test was carried out using a Universal Testing Machine. Stress-strain curve and energy absorption of the specimen were derived from the compressive load-displacement response. It was found that the quasi-static response of the specimen in compression case was affected by the number and position of spot welds. For the bending test, the bending load was found to be fluctuated with the displacement in a way analogous to the folding behaviour and deformation of the cross-section of the specimen. Bending moment-rotation characteristic was derived from bending load-displacement curve to investigate the energy absorption of the specimen in bending case. Lastly, the von Mises stress at the top and bottom surface of the specimen is increased with increment of the punch head travel distance.

Abstract: This paper discusses the benefits to using direct strain measurements when undertaking a fatigue assessment of a railway bridge. The paper gives a brief overview of the assessment methodology for assessing fatigue in a bridge carrying a single track before outlining a method suitable for assessing multi-track bridges. The accuracy of a fatigue assessment depends on the level of knowledge of many different parameters including: an accurate loading history comprising axle loads and train frequencies over its service life; details of changes to the track form, including changes in the depth of ballast; and changes to the structure including damage or repairs. Many of these parameters are not known with certainty. However, using monitoring systems to measure the strain cycles in a bridge helps to address some of these uncertainties and hence improves the accuracy of the assessment.

Abstract: In this work the crack initiation stress of notched specimens of filter paper is studied. The paper in the microstructure has a random array in their fibers while macroscopically it behaves anisotropically. The self-affine crack mechanics is used to study the size effect in the tensile behavior of this kind of paper under the presence of several conditions of geometrical notches. While in the traditional fracture mechanics the crack initiation stress is a material parameter when is reached a critical level at the crack tip, in the self-affine crack mechanics, depends moreover of the resulting tortuosity of the crack. Four geometrical arrangements in two sizes we considered: centered circular notch, centered lineal notch, sided circular notches and without notch at 10 and 300 mm width with a relation 2a/w = 0.25 under the same loading conditions. In this, the without notch specimens present the higher stress, all other notched specimens presented a similar crack initiation stress about 1 % of difference among them, and the crack growth is not affected by the geometry of notch. In spite of this difference, no one of the specimens reach the theoretical stress concentration of 3 such as predicted the classical stress theory.

Abstract: Ball grid arrays (BGAs) embedded in aerospace devices should satisfy strict standards in the purpose to ensure their mechanical safety, particularly in fatigue. In fact, critical phases of BGAs service life such as launch lead to high cycle fatigue (HCF) failure due to severe random accelerations.To face this problem, designers are still using experimental qualifications based on deterministic time-domain fatigue methods. This work is motivated principally to study the applicability of the principal spectral fatigue models as cost effective alternative to assess BGA HCF. The study includes an assembly made up of a BGA chip and a support board. Finite element spectrum analysis brings out that the fatigue failure is expected to occur at different interconnect locations like for instance a critical solder joint made of a ductile tin-lead alloy. Among all the studied spectral models, it emerges that the Dirlik’s fatigue prediction is the most relevant in the typical range of the solder fatigue coefficients.

Abstract: Experimental works carried out in recent years have demonstrated the feasibility of detecting a bearing fault through the spectral analysis of the Instantaneous Angular Speed (IAS) in the angular domain. Since these works have been carried out on complex mechanical systems (automotive gearboxes, vehicle wheels, wind turbines), neither the influence of operating parameters, nor the influence of structural parameters over the observed angular speed variations have been clearly identified. However, the implementation of effective tools for condition monitoring prospects requires a deep understanding of these interactions. In this regard, a test bench has been designed to allow defective bearing monitoring through IAS observation of a simple shaft running under varying loads and speeds, the system being simple enough to be easily described in various kind of mechanical or phenomenological models. The aim of this paper is to present a better understanding of the relationships between the speed variation induced by the monitored fault, the structural response and the observed phenomena. In the first part results obtained for a healthy bearing will be analyzed. These initial results serve as a reference for analysis of the results obtained with bearing defects. Coupled with dynamic modeling, they will also highlight the existence of a low frequency torsion mode. The results of this first part also highlight the wider interest of the IAS analysis for the study of rotating systems. In a second part, the measurements are performed with bearings having spalling type defects on their outer ring. The aim of this section is to estimate the influence of operating conditions on IAS monitored indicators. All these results will provide further phenomenological explanations of coupling between bearing fault and rotating speed.

Abstract: Raw vibration signals measured on the machine housing in industrial conditions are complex and can be modeled as an additive mixture of several processes (with different statistical properties) related to normal operation of machine, damage related to one (or more) of its part, some noise, etc. In the case of local damage in rotating machines, contribution of informative process related to damage is hidden in the raw signal so its detection is difficult. In this paper we propose to use the statistical modeling of vibration time series to identify these components. Building the model of raw signal may be ineffective. It is proposed to decompose signal into set of narrowband sub-signals using time-frequency representation. Next, it is proposed to model each sub-signal in the given frequency range and classify all signals using their statistical properties. We have used several parameters (called selectors because they will be used for selection of sub-signals from time-frequency map for further processing) for analysis of sub-signals. They have base in statistics theory and can be useful for example in testing of normality of data set (vibration time series from machine in good condition is close to Gaussian, damaged not). Results of such modeling will be used in the sub-signals classification procedure but also in defects detection. We illustrate effectiveness of novel technique using real data from heavy machinery system.

Abstract: Synchro-squeezing transform has recently emerged as a powerful signal processing tool in non-stationary signal processing. Premised upon the concept of time-frequency (TF) reassignment, its basic objective is to provide a sharper representation of signals in the TF plane and extract the individual components of a non-stationary multi-component signal, akin to empirical mode decomposition (EMD). The rich mathematical structure based on continuous wavelet transform (CWT) makes synchro-squeezing powerful for gear fault diagnosis, as faulty gear signal is frequently constituted out of multiple amplitude-modulated and frequency-modulated signals embedded in noise. This work utilizes the decomposing power of synchro-squeezing transform to extract the IMFs from a gear signal followed by the application of standard gearbox condition indicators which promises greater prognostic power than that can be achieved by applying condition indictors directly to the inherently complex gear signals. The efficacy and the robustness of the algorithm are demonstrated with the aid of practical experimental data obtained from a helicopter gear box.

Abstract: The use of composites in the aircraft industry has generated a great need for structural health monitoring and damage detection systems, to allow for safer use of complex materials. Such is the case with helicopter blades - these components nowadays are mostly composed of carbon fiber or glass fiber reinforced plastics laminates, epoxy and honeycomb filled core structures. The use of composite materials on the main rotor blade also allows for more complex and efficient shapes to be designed, but at the same time, their use requires an additional effort when it comes to structural monitoring, since damage can occur and go unnoticed. This work presents experimental results for structural health monitoring method based on strain energy. The test subject is a full-scale composite helicopter main rotor blade, which is a highly flexible, slender beam that can display unusual dynamic behavior with orthotropic behavior. This damage detection method is based on the modal strain properties, and a damage detection index is used to identify and quantify damage. A test setup was built to carry out an experimental modal analysis on the main rotor blade. For that purpose, a total of 55 uniaxial accelerometers were used on the helicopter blade to measure the displacement modes of the structure. To compute the strain modes from the displacement modes, central differences approximation is used. Damage is introduced on the blade by attaching a small mass to two different locations. Experimental results show the possibility of locating damage in this case.

Abstract: Conventional condition monitoring techniques such as vibration, acoustic, ultrasonic and thermal techniques require additional equipment such as sensors, data acquisition and data processing systems which are expensive and complicated. In the meantime modern sensorless flux vector controlled drives can provide many different data accessible for machine control which has not been explored fully for the purpose of condition monitoring. In this paper polynomial models are employed to describe nonlinear relationships of variables available from such drives and to generate residuals for real time fault detection and performance comparisons. Both transient and steady state system behaviours have been investigated for optimal detection performance. Amongst 27 variables available from the drive, the torque related variables including motor current, I_d, I_q currents and torque signals show changes due to mechanical misalignments. So only these variables are explored for developing and optimising detection schemes. Preliminary results obtained based on a motor gearbox system show that the torque feedback signal, in both the steady and transient operation, has the highest detection capability whereas the field current signal shows the least sensitivity to such faults.

Abstract: The paper deals with the local damage detection in rolling element bearings in presence of a high level non-Gaussian noise. In many theoretical signal processing papers and engineering application related to damage detection, a simple model of the vibration is assumed. Basically it consists of signal of interest (SOI) and some unwanted (deterministic and/or random) components masking SOI in acquired observation. So, damage detection problem has to concern filtering, decomposition or extraction issue. Unfortunately, in the most of the industrial systems mentioned unwanted sources are in fact not Gaussian, so many of de-noising techniques cannot be applied directly or at least might give unexpected results. In this paper an industrial example will be discussed and novel approach based on advanced cyclostationary-based technique will be proposed. In the paper disturbances include periodic impacts in reciprocating compressor on an oil rig. Existing classical detection techniques (statistics in time domain, analysis of envelope spectrum, time-frequency decompositions) are insufficient to perform the task due to high power of disturbance contribution in comparison to damage signature. In the proposed technique, the Spectral Coherence Density Map (SCDM) is estimated first. Next step is related to analysis of SCDM contents and selection of informative part. If informative and non-informative components lay in separate frequency regions, such selection should fix the problem immediately