M. Zaoui and N. Menasri
The welds are the site of high residual stresses, localized in the vicinity of the weld cracks. Their presence caused many type of damage mode as stress corrosion, fatigue, sudden…
Abstract
The welds are the site of high residual stresses, localized in the vicinity of the weld cracks. Their presence caused many type of damage mode as stress corrosion, fatigue, sudden failure and increasing the temperature of ductile/brittle transition. Residual stresses in restrained welds and weld repairs are very complex. The heat treatment affects the value and distribution of residual stress in the specimen. This peak stress in all three samples occurred not at the toe, but in the middle of the weld bead, where the yield stress is higher. The use of the neutron diffraction (ND) technique for residual stress measurements is described. In addition, studies of macrostructure and hardness were conducted. The results of different tests conclude the influence of heat treatment on Residual stresses in welds.
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N. Menasri, M. Zaoui and A. Bouchoucha
Rotating machines play a strategic role in a manufacturing process, and it is the case of a cement mill. These machines are composed of fragile bodies (bearings and gears, etc.…
Abstract
Rotating machines play a strategic role in a manufacturing process, and it is the case of a cement mill. These machines are composed of fragile bodies (bearings and gears, etc.) subjected to mechanical stress and harsh industrial environment.
Sources of failure are many; the wear and the spalling of bearing, the wear and fracture of a tooth of a gear, the misalignment of the axes, etc. To avoid unexpected downtime and costly, those bodies must be monitored continuously. There are several ways to do that; vibration analysis, lubricant analysis, infrared thermography and acoustic analysis. Vibration analysis (frequency) is most often used by operating a signal from an accelerometer. Whenever a fault participated in a contact, it changed the vibration characteristics of the system. By analyzing the acceleration measurements from an accelerometer, we can identify and quantify these changes in order to establish a relationship between the measures and the type of defect, which will be used as signatures for fault diagnosis system. In a system of power transmission (gear unit of a cement mill) the frequency of occurrences of faults can be known from the geometry and kinematics of the system.
This article deals with the detection of bearing defects isolated in a gear unit of a cement mill by spectral analysis of vibration; which based on a systematic analysis spectral and envelope spectrum of vibration signal for the presence of images of all vibrational defects may affect the body in question.
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Menasri Noureddine and Bouchoucha Ali
Rotating machines play a strategic role in the process; it is the case of a gear unit of a cement mill. These machines are composed of tribo-pairs (bearings and gears, etc.…
Abstract
Purpose
Rotating machines play a strategic role in the process; it is the case of a gear unit of a cement mill. These machines are composed of tribo-pairs (bearings and gears, etc.) subjected to mechanical stress and harsh industrial environments. This study aims to make an experimental investigation of bearing wear of a gear unit DMGH 25.4 of a horizontal cement mill.
Design/methodology/approach
Several tests were carried out in collaboration with industrial Algerian companies, for example, spectroscopy, micro hardness, X-Ray diffraction analysis and microscopic examination of wear-characterized QJ1244 N2MA bearing.
Findings
Experimental results reveal that there are two types of bearing wear: the first is a continuous mode of damage. The second mode is the wear by a third body (wear debris in suspension in oil). The spalling results from the propagation of cracks through the thickness of the material generally give rise to cracks in the sub-layer depth hertz. An unusual hardening observed suggests that the damage was caused by the dissipation of a large surge of frictional energy lasting several milliseconds. It is found also that heating due to insufficient lubrication of the bearing led to the formation of high-density fine carbides in the microstructure of 100Cr6, which tends to decelerate the movement of dislocations during plastic deformation.
Practical implications
This study allowed the characterization of bearings wear in industrial mechanical systems.
Originality/value
Characterization of the bearings showed that the unusual hardening observed suggests that the damage was caused by the dissipation of a large surge of frictional energy lasting several milliseconds. The microhardness of the worn bearing is very high compared to the beginning wear bearing, which is due to the quantity of carbides formed. The thermo-mechanical stresses do not affect the main phases such as martensite, but they accelerate the fracture propagation at the surface.
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Leyla Bouzid, Mohand Hamizi, Naceur-Eddine Hannachi, Aghiles Nekmouche and Karim Akkouche
The purpose of this study is to establish a relationship between causes and effects, the respect of materials characteristics values [concrete compressive strength (fc) and steel…
Abstract
Purpose
The purpose of this study is to establish a relationship between causes and effects, the respect of materials characteristics values [concrete compressive strength (fc) and steel yield stress (fy)] and the norms of the construction dispositions value (covers). This study is motivated by the post-seismic damages related to the plastification of the reinforced concrete (RC)/beams sections, named plastic hinges. The results are given by fragility curves representing the failure probability (Pf) of the plastic hinges versus covers value.
Design/methodology/approach
A mechanical-reliability coupling methodology is proposed and performed on three frames (three, six and nine storey). For each frame, seven covers the value of reinforcement steel bars has been taken into account in the beams. After definition of the limit state function G(x), a process of idea to twin-track; deterministic and probabilistic, is considered. Thus, numerical simulations are carried out under ETABS© software, to extract a soliciting moments Ms(x). Then, ultimate moments Mu(x), the result of reliability approach are calculated using Monte Carlo Simulations. In this step, two random variables; concrete compressive strength in 28 days of age (fc) and steel yield stress (fy), have been studied.
Findings
In the mechanical study, the results show that, the first plastic hinge appears at the beams for all frames. In the reliability study, the (fy) variation shows that all plastic hinges are in failure domain, nevertheless, the (fc) variation leads to have all sections in the safety domain, except A7 and B7 models. The failure probability (Pf) calculation according to (fc) and (fy) shows that an absolute error of 0.5 cm in the steel bars covers can switch the frame from the safety domain to the failure domain.
Originality/value
The plastic hinges reliability of the RC/ frame structures is independent on the high of the structure. The (fc) random variable according to the used distribution law does not affect the reliability (safety or failure). However, the impact of the steel yield stress variation (fy) is not negligible. The errors in covers affect considerably the strength of the elements.