Fadi Al-Badour

Non-stationary vibration signal analysis of rotating machinery via time-frequency and wavelet techniques

This paper introduces an efficient and powerful approach to fault detection in rotating machinery using time-frequency analysis based on both Fourier and wavelet transforms of the monitored vibration signal. Time-frequency techniques are powerful tools for analyzing transient information in vibration signature for both condition monitoring and fault detection purposes. Our work on fault detection reported in this paper is two-fold: (1) application of the short-time Fourier transforms (STFT) and the exploitation of the spectrogram-based time-frequency distribution to detect various mechanical faults during the start-up & coast down phases in rotating machinery and (2) application of a novel wavelet-based technique combining both the continuous wavelet and the wavelet packet transforms. This novel technique exploits the use of the modulus of the local maxima lines in the wavelet domain, to detect impulsive mechanical faults such as impact blade-to-stator rubbing in turbo machinery. Both the analysis and the extensive simulation work carried out here show in particular the superiority of our proposed combined wavelet-based approach over the traditional Fourier Transform (FFT) method, in reliably diagnosing impulsive mechanical faults and start-up and cost down signals.

Vibration monitoring and faults detection using wavelet techniques

This paper introduces an efficient approach for fault detection in rotating machinery by analyzing its vibration signals using wavelet techniques. Specifically our approach uses the wavelet packet transform (WPT) to decompose the vibration signals in the wavelet packet space, in order to reveal the transient information in these signals. Faults are efficiently detected by exploiting the mean values of the energy in the detail signals. The wavelet-based approach is also compared with the traditional Fourier-based one. Both analysis and an extensive simulation of the two approaches clearly show the superiority of the WPT-based approach over the Fourier-based one, in efficiently diagnosing faults from vibration signals.

Experimental and Finite Element Modeling of Friction Stir Seal Welding of Tube-Tubesheet Joint

Tube-tubesheet joints are critical in some applications, where contact between shell and tube side fluids is not tolerable. To ensure joint tightness, standards (ASME and TEMA) recommend performing a combination of rolling-or expansion of tube-tubesheet and seal welding. Available techniques for seal welding are based on fusion welding that sometimes results in a number of defects such as cracking and porosity formation, and such defects may take a newly fabricated heat exchanger out of service. In this work, friction stir welding (FSW) was used for tube-tubesheet seal joint and simulated using a 3D thermo-mechanical finite element model (FEM). The model was analyzed using a commercial finite element (FE) package. The model included the thermal effect of the tool workpiece interaction along with axial load, ignoring the metal flow around the tool. The material model took into account temperature dependency of thermal and mechanical properties. The model objectives were to evaluate the temperature distribution and residual stress in the workpiece resulting from the thermal cycle and axial load during welding for various process parameters, and to study how residual stresses in adjacent roller expanded tubes are affected during welding. The FE results show that the maximum temperature at the welding zone does not exceed the solidus temperature (except at high tool rotational speeds); the process can thus be classified as cold working. Moreover, adjacent tubes temperature does not exceed the annealing temperature. An experimental setup was designed and manufactured to show the feasibility of the process in this constrained size joints and to validate the numerical results. A test cell and a special FSW tool were designed and manufactured for this purpose. Many tests were performed with welding quality depending on process parameters.

Friction Stir Seal Welding (FSSW) Tube-Tubesheet Joints Made of Steel

This paper demonstrates the feasibility of successful seal welding process of tube-tubesheet joints using the relatively new friction stir welding (FSW) process. The purpose of the reported study is to develop the process parameters and to test the feasibility of friction stir welding ASTM 179 seamless cold-drawn carbon steel tube into an ASTM A516 Grade 70 tubesheet. All welds were performed in position control on a fully instrumented experimental friction stir welder using a water-cooled tool holder and a shroud for argon shielding gas. A proprietary tungsten-rhenium pin tool with a 1.54 mm pin length and shoulder diameter of 4.9 mm was used to perform the seal weld between the roller expanded tube and tubesheet. A steel plug was employed during each weld in order to maintain support for the weld throughout the circumference of the weld.Sound welds were achieved in the FSW of ASTM 179 seamless cold-drawn carbon steel tube into an ASTM A516 Grade 70 tubesheet. Welding was improved by avoiding tool contact with the tube plug or the flash of the previous weld. Though a root void has been observed, which is typical of partial penetration butt welds, it is believed that the weld would still achieve a hermetic seal. Tool wear features of the W-Re pin has also been documented.Copyright

Effect of pin tool profile on mechanical and metallurgical properties in friction stir spot welding of pure copper

In this paper, the influence of thread feature on mechanical and metallurgical properties of friction stir spot weldments is studied. Two pin tools, plain cylindrical and threaded cylindrical, having the same size are used to produce lap joints by friction stir spot welding (FSSW) of commercially pure copper. All experiments have been performed under the same welding parameters. The results revealed that joints produced with threaded pin tool have more than double the tensile shear strength of those produced by plain cylindrical pin. Moreover, the samples that were FSS welded with threaded pin tool failed in a plug failure mode, which is a desirable ductile failure mode. On the other hand, the weldments that were produced using plain cylindrical pin tool failed in interfacial failure mode or shear fracture mode. Effective upper sheet thickness analysis showed upward hook formation and less effective upper sheet thickness, which indicate better material mixing in the case of threaded pin tool. Furthermore, the transition bonding region was found to be very narrow in the case of plain tool while that of the threaded pin tool showed a transition region that is almost equal to complete bonding region, indicating gradual change from unbonded to complete bond; resulting in the above observed higher strength. Scanning electron microscopy analysis of the hook formation region revealed that, contrary to the weldments performed by plain pin tool, which showed the presence of large voids around the hook region, weldments performed with threaded pins were free of defects.

Microstructure and Hardness of Friction Stir Weld Bead on Steel Plate Using W-25%Re Pin Tool

One of the challenges that impede the use of the relatively new friction stir welding (FSW) process in joining steels and high temperature alloys, as well as dissimilar materials, is the development of the right pin tool material that can stand the severe welding conditions of these alloys. Recent developments in FSW tool materials include tungsten rhenium (W-Re) alloys. The ductile to brittle transition temperature of pure tungsten is reduced by the addition of rhenium (Re).. The addition of Re also improve fracture toughness of the alloy. The major focus of this paper is studying the process of making a friction stir welding bead on mild steel using a proprietary W-25%Re alloy pin tool and investigating the effects of process parameters (i.e. tool rotational and welding speeds) on microstructure, microhardness as well as tool reaction loads. Grain refining of the steel microstructure was observed in all beads. Certain process conditions produced a bead with needle like microstructure with the highest values of hardness. Reaction forces were found to increase with the increase in the tool welding speed and to decrease with the increase of the tool rotational speed. Although the spectroscopic analysis of the beads confirmed the diffusion wear of the tool, the overall tool has shown excellent resistance to mechanical wear.Copyright

Experimental Investigation of Friction Stir Seal Welding of Tube-Tubesheet Joints

To ensure heat exchanger tube-tubesheet joints tightness, industrial standards may recommend performing a combination of roller expansion and seal welding, using conventional fusion welding processes. Solid state friction stir welding (FSW) has several advantages over the conventional fusion welding but has not yet proven its usefulness in seal and strength welding of heat exchanger tube-tubesheet joints where the available space is very limited and weld pass is of a relatively complex contour.In this work, a newly designed tool and procedure are applied to friction stir seal weld (FSSW) tube-tubesheet joints. The effects of process conditions such as welding speed and tool offset on dependent process parameters including welding loads and joint quality are investigated on a 600-series aluminum three-tube test cell.The results revealed that the quality of the seal weld of tube-tubesheet joints is affected by the preceding parameters at different levels.Copyright

Experimental and Numerical Investigations of Friction Stir Welding of Aluminum to Copper

In this paper, results of performing successful butt-welding of aluminum grade Al6061-T6 to commercial pure copper using the relatively new friction stir welding (FSW) process are presented and discussed in conjunction with finite element results. A sound weld joint between these dissimilar materials has been achieved only when the pin tool was offset a certain distance from the center of the weld line and the harder material (copper) was placed at the advancing side of the FSW tool. On the other hand, the tool offset was not required when joining the similar materials of aluminum to aluminum and of copper to copper. A combined use of temperature distribution of the aluminum to copper weld nugget determined by a finite element model, the elemental concentration of copper and aluminum in the weld nugget from EDS analysis, and aluminum-copper phase diagram have been used to identify the phases present in the different weldment zones. The aluminum-copper joint was found to consist of several single and two phase intermetallic compounds such as Al2Cu, Al4Cu9, and (Al + Al2Cu). The results revealed higher hardness values for the weld nugget compared with the two base metals.

New Approach for the Indirect Detection of Blade-to-Stator Rubbing in Turbo-Machinery Using Wavelet Techniques

This paper introduces an efficient and powerful approach to fault detection in rotating machinery using time-frequency analysis based on the wavelet transform of the monitored shaft vibration signal. Wavelet techniques are one of the latest powerful tools in analyzing the transient information for condition monitoring and fault detection using vibration signature. The proposed technique combines both the Continuous Wavelet and the Wavelet Packet Transforms. In particular, it exploits the use of the modulus of the local maxima lines in the wavelet domain, to detect impulsive mechanical faults through shaft vibration such as impact blade-to-stator rubbing in turbo machinery.The proposed new wavelet-based signal processing method was able to detect the singularity in the measured shaft vibration, which was generated by blade rubbing. The singularity detection achieved by the new method was very well supported by its counterpart based on the direct blade vibration measurements.Our proposed technique was favorably compared with both the time wave and the traditional Fourier Transform techniques. In fact, both the analysis and the extensive simulation work show the superiority of the combined approach (Wavelet Packet Transform and Maxima Lines) over the traditional Fourier-based method, in reliably diagnosing impulsive mechanical faults.Copyright

Residual Stresses in Friction Stir Welded Tube-Tubesheet Joint

In this paper, a 3-D thermo-mechanical finite element model (FEM) is developed to simulate the process of friction stir seal welding (FSSealW) of tube-tubesheet joint, using a commercial finite element (FE) package considering temperature dependent material properties. The model is used for the prediction of temperature and stress distributions, as well as the prediction of the residual stresses in the seal welded joint, including the expanded tube and surrounding ligaments. Validation of the model is achieved using experimental temperature measurements. The FEM results are found to be in good agreement with experimental ones. Temperatures of the joint material away from the processed zone are below the annealing temperature. The calculated residual stresses are found to be compressive and help to enhance the contact stress in the tube-tubesheet joint.Copyright