Meftah Hrairi

Research and Progress in Incremental Sheet Forming Processes

Incremental sheet forming (ISF) is an emerging metal-forming technology in which the tool motion is controlled numerically. The process is economical to form complex parts in small to medium batches and provides a short and inexpensive way of forming products having a relatively simple but interesting shape. In this article, a review of the present state-of-the-art technologies and the potential applications of incremental sheet metal forming are presented in brief. This article seeks to address the approaches and methods that are prevalently applied. Furthermore, the article also seeks to guide researchers for future work, by identifying inadequacies of the current approaches and potential for valuable contributions in the field of incremental forming.

Significant Parameters for the Surface Roughness in Incremental Forming Process

The features of the incremental sheet forming (ISF) process allow it to meet a wide array of customer preferences. In this paper, the variation of surface roughness (SR) in a negative ISF process was systematically studied. The variation was investigated by means of four different process parameters, namely, the vertical step size, forming tool diameter, spindle speed, and feed rate. By using Taguchi analysis with the help of design of experiment and analysis of variance (ANOVA), the effects of the above four process parameters have been studied to optimize parameter levels to realize minimum SR. The results illuminated which parameters have the greatest effect on SR variation, namely, tool size and vertical step size. The confirmation test also showed that the response tables and graphs from Taguchi analysis and ANOVA constitute effective and efficient methods for determining each design parameters optimal level to produce the minimum value of the SR.

Modelling and simulation of impedance-based damage monitoring of structures

Electromechanical impedance (EMI) based monitoring techniques are successfully in use in current engineering structures. With the help of piezoelectric sensors, the EMI technique is used for monitoring the health of such structures. Generally, potential damage to the host structure is detected by examining the EMI signature and identifying changes in that unique signature. Since this technique has the potential to offer greater safety and reliability while lowering maintenance costs, it is becoming increasingly popular. This paper investigates the use of finite element method (FEM) to simulate the electro-mechanical impedance technique. A numerical analysis of simple models, such as free piezoelectric patches of various shapes and thicknesses is used to comprehend the fundamentals of this technique. Then, studies on different parts of the structure are conducted to find the effect on the output of system when both damage and loading co-exist, and investigate the effect of temperature for structural health monitoring based on EMI. The simulation results are then compared to experimental data and a very good agreement is observed.

Effects of Adhesive Bond on Active Repair of Aluminium Plate Using Piezoelectric Patch

Research activities on active repairs and stress control of structures using piezoelectric actuators and adhesive bond have received much attention in recent years. The function of the adhesive bond on active repair is to transmit the induced stresses by the piezoelectric actuator to the host structure in order to reduce the stress intensity on the crack front. Assessment of repair performance of adhesive bonds is done based on the transfer of the shear and peel stress concentration in the adhesive layer. In the present work, three dimensional finite element analyses have been carried to understand the effects of adhesive properties on active repair performance of a cracked aluminium plate under mode I. Adhesive efficiency is evaluated by the stress intensity factor (SIF) as a fracture criterion. The results show that SIF varies inversely with the adhesive layer’s shear modulus.

Modeling approach to evaluating reduction in stress intensity factor in center-cracked plate with piezoelectric actuator patches

Active repairs using piezoelectric actuators can play a significant role in reducing the crack damage propagation in thin plate structures. Mode-I crack opening displacement is the most predominant one in tension, and it is responsible for the failure which in turn affects the load carrying capability of the cracked structure. In addition, there are limited studies that investigated the effect of the piezoelectric actuator over mode-I active repair. In this study, the mode-I stress intensity factor for a plate with a center crack, and a bonded piezoelectric actuator was modeled using the linear elastic fracture mechanics. For this, an analytical closed-form solution is developed using the virtual crack closure technique taking into account mode-I as the only effective mode, coupling effects of the piezoelectric patch, and the singular stress at the crack tip. In addition, the total stress intensity factor was obtained by the superposition of the stress intensity factor obtained from the stresses produced by the piezoelectric actuators on the crack surfaces as the only external loads on the cracked plate and the stress intensity factor due to the far-field tension load. The proposed analytical model for mode-I stress intensity factor was verified by a finite element–based approach using ANSYS finite element software. The results demonstrated a good agreement between the analytical and finite element models with a relative error of less than 4% in all the cases studied. The results illustrated that the piezoelectric patch is efficient in reducing stress intensity factor when an extension mode of the actuator is applied. However, applying a contraction mode of the piezoelectric actuators produced negative strain which increased the stress intensity factor and thus the severity of the cracked structure and could lead to damage propagation.

Impedance based detection of delamination in composite structures

Nowadays commercial and military aircrafts are increasingly using composite materials to take advantage of their excellent specific strength and stiffness properties but impacts on composites due to bird-strike, hail-storm cause barely visible impact damage (BVID) that underscores the need for robust structural health monitoring methods. Hence, damage identification in composite materials is a widely researched area that has to deal with problems coming from the anisotropic nature of composites and the fact that much of the damage occurs beneath the top surface of the laminate. This paper focuses on understanding self-sensing piezoelectric wafer active sensors (PWAS) to conduct electromechanical impedance (EMI) in glass fibre reinforced polymer composite to perform structural health monitoring. With the aid of a 3D ANSYS finite element model, an analysis of different techniques for the detection of position and size of a delamination in a composite structure using piezoelectric patches had been performed. The real part of the impedance is used because it is known to be more reactive to damage or changes in the structure’s integrity and less sensitive to ambient temperature changes compared to the imaginary part. Comparison with experimental results is presented to validate the FE results. The experimental setup utilizes as its main apparatus an impedance analyser HP4194 that reads the in-situ EMI of PWAS bonded to the monitored composite structure. A good match between experimental and numerical results has been observed for low and high frequencies. The analysis in this paper provides necessary basis for delamination detection in composite structures using EMI technique.

Kinetics of mullite formation from kaolinite and boehmite

ABSTRACT The excellent electrical, thermal and mechanical properties, and good stability of mullite made it a candidate material to produce ceramics for advanced applications such as catalyst supports, filters, optical devices, heat exchangers and electronic packaging. In this work, monolithic mullite was synthesized from kaolinite and boehmite and the kinetics of its formation was investigated using dilamometry. The activation energies for the transformation of kaolinite to metakaolinite, transformation of metakaolinite to spinel, and transformation of the kaolinite-boehmite powder to mullite were evaluated through non-isothermal treatment following the Kissinger and Ozawa equations. The growth morphology parameters were evaluated for the transformation of metakaolinite to spinel, and for the transformation of the spinel to mullite.

Comparison of diesel emission experiments in view of the environment: A case study at Dubai

Abstract Because of the nature of the diesel engine combustion process, such engines produce more toxic emissions, more visible smoke and odour than gasoline engines. This has led to an increasing concern about the possible effects of diesel emissions on the environment and human health. This study examines emissions form two experimental diesel powered vehicles pilot schemes at a Dubai municipality; an inspection and maintenance experiment and a diesel particulate filter experiment. Comparing the two experiments with a baseline vehicle, the results indicate that after the inspection and maintenance experiment, a significant reduction in emissions was achieved in carbon monoxide (300%), nitrogen oxides (500%) and hydrocarbons (88%). The implementation of a diesel particulate filter lead to even better emission reductions but at a higher cost, with hydrocarbon emissions decreasing by 150%, carbon monoxide by 500% and nitrogen oxides by 700%. The results are promising for both experiments for future investigation.

Mixed-Convection Heat Transfer from Simulated Air-Cooled Electronic Devices: Experimental and Numerical Study

Air-cooling characteristics of an electronic-device heat sink have been experimentally and numerically investigated under various operating conditions for air. Flowing air velocities of 0-7.1 m/s were circulated through a wind tunnel with a rectangular section. The lower surface of the wind tunnel was equipped with 5 by 3 heat sources subjected to uniform heat flux. From the experimental measurements, surface temperature distributions of the discrete heat sources were obtained and effects of Reynolds numbers on these temperatures were investigated. A computational fluid dynamics analysis of the cooling process was conducted to simulate the system and to calculate the required cooling rate. A relation was identified between the thermal-wake function and capability of the software to give better estimations of the circuit-board temperatures. Overall, the obtained results showed good agreement between the simulation and the experimental results. In particular, it was found that surface temperatures of heated modules decrease with increasing Reynolds number.

Validation of ESP oil wells measured parameters using simulation olga software

The significant challenge in the oil and gas industry is the concurrent measurement of commingled gas, oil and water production, either using three phase test separator or multiphase flow meter (MPFM). A major issue in these applications is the uncertainty of the measurements, due to different measurement operations conditions. A new computational approach has been generated to estimate oil well flow rate of 48 oil wells using Electrical Submersible pump (ESP) from D, G, and W oil fields located in North Africa. The idea is to close the wellhead wing valve and the ESP is kept running normally and the wellhead flowing pressure before shut-in the well and the build-up of wellhead flowing pressure after shut-in the well are measured. OLGA software has been used to make comparison with multiphase flow model available in the OLGA software against each nominated ESP oil well parameters obtained from measured field data. The objective was to verify the obtained shut-in wellhead pressure after closing the choke wing valve (WHPa) from the measured field data with the obtained shut-in wellhead pressure valve from the simulation model. In this paper the simulation results showed that the estimated WHPa are in agreement with the measured WHPa. The relative errors for individual oil field are within accuracy standard specification (typically +/- 10%). The overall relative errors are low and within acceptable uncertainty range, where the aggregate relative error for all wells was less than +/-4% which is considered acceptable. Therefore, the results have demonstrated that the new computational method can work under ESP oil wells conditions and has the ability to perform accurate results even when closing the wellhead wing valve for short time span.