R. El Abdi

New procedure to determine steel mechanical parameters from the spherical indentation technique

Abstract A new numerical and experimental approach for determining mechanical properties of steels is presented. This method uses the instrumented spherical-indentation technique. A relationship between applied load, indenter displacement, flow stress and strain hardening exponent of steels, is given. This method is based on the minimisation of error between the experimental curve (applied load–indenter displacement curve) and the theoretical curve which is a function of the mechanical properties of studied materials. Comparison between the results obtained by the proposed method and experimental tensile tests, confirms the interest of the proposed method.

New method to determine the mechanical properties of heat treated steels

Abstract A new numerical approach to indentation problems is developed for hardened materials. A relationship between load, displacement, flow stress and strain hardening exponent of heat treated materials with a hard film, is given. This method is based on the minimisation of the error between the experimental curve (load–displacement of the indenter) and the theoretical curve, function of the mechanical and geometrical properties of the studied materials. Comparison of the numerical results with those experimentally obtained from known materials confirms the interest of the method proposed.

Cyclic plastic and creep behaviour of pressure vessels under thermomechanical loading

Abstract Structures such as pressure vessels are required to operate under thermal and mechanical loading and low-cycle fatigue conditions. Design of these structures is related to their response under these conditions. This work considers the cyclic plastic and creep behaviour of a thick-walled sphere and a cylindrical pressure vessel subjected to cyclic pressure and/or temperature. We investigate the steady state behaviour of the vessels using linear kinematic hardening in the plastic condition and Norton power law in the creep condition. Each loading cycle consists of four steps: loading, creep, unloading and creep. A numerical program has been developed to calculate final stress–strain state. We observe that because of the plastic strain and the creep strain in unloading, for both pressure and thermal loading, ratchetting is obtained. Thus, with these conditions secondary stresses lead to the ratchetting in the same way as the primary stresses.

Influence of material properties on the drilling thrust to hardness ratio

A drilling method developed by the authors for the measurement of the hardness profile for surface heat treated steels is based on the proportionality between drilling thrust and material hardness. However, parameters other than hardness related to the material can modify the cutting thrust level. In order to study the influence of the microstructure, the ductility, the toughness and the work hardening of material on the drilling thrust, a few tests were carried out in this work. Tensile and impact tests were carried out on different materials. Moreover, micrographic observations and Vickers micro-hardness were performed on the chips. The study allowed two groups of materials to be distinguished according to the drilling thrust to hardness ratio (R/HV). The first includes not very ductile materials presenting an about constant and low value of R/HV, short, thin and not very work hardened chips. The second includes ductile materials presenting a non constant and high value of R/HV, long, thick and very work hardened chips. As a consequence, the hardness profile determination by drilling method is reliable only if the R/HV ratio is about the same for the treated layer and for the substrate, i.e. if the difference in ductility between the treated layer and the substrate is not very high.

Hardness profile analysis of elasto-plastic heat-treated steels with a gradient in yield strength

An elasto-plastic indentation study on materials with a yield strength gradient like steels which have undergone thermal hardening such as nitriding, is carried out using experimental and finite element methods. The analysis of the normalized mean contact pressure as a function of a dimensionless strain parameter for graded materials shows that the mean contact pressure presents the same tendency as equivalent homogeneous materials. A simple model for the average plastic zone radius of graded materials is given. A relation is developed to predict the hardness variation as a function of the indentation depth. Lastly, a theory based on the effective evolution of hardness is proposed to determine the yield stress and hardness profile for materials with a decreasing yield stress with depth. Results for carbo-nitriding steels obtained by the standard Vickers micro hardness techniques are compared with those obtained by the proposed method.

Experimental and theoretical investigations on connector insertion phase

Inside a connector, an interface with low insertion force and contact resistance is required for low cost materials and coating such as copper alloys with tin coating. In addition, related to the application, the operating parameters have a wide range of values of currents, forces and materials. In this paper, we present a new experimental method based on nonintrusive probing of the deflection of the spring terminal, using a laser technique. The main feature is that the reflection of the laser beam on the spring allows the determination of the contact force of the lamella-spring inside the female part. The technique acquires the following insertion parameters during the insertion stroke: contact deflection /spl delta/ which allows contact force Fc, insertion force Fi and contact resistance Rc. It was found that the insertion force takes a maximum value and decreases to the stable value, which depends on the size and the material of the pin. However contact resistance decreases sharply on first insertion and tends towards stable values on completing the insertion process, which are less sensitive to the pin diameter. Finally, discrepancies were observed between the experimental and calculated data with simple numerical models. More complex models are in progress, which should improve the convergence of the theoretical approach to experimental results and proceed to the optimization of the connector parameters.

Determination of the Hardness Profile on Heat Treated Steels by Hole Drilling and Indentation Methods

A new approach to determine the hardness profile of superficially heat treated steels is proposed. This method uses the experimental hole drilling technique based on the measurement of the penetration load of the drill for the treated steels with a non negligible hard film thickness and the indentation technique for the case of thin hard films. The thrust measured during a drilling test is reliable to the hardness of the material being tested. The approach using the indentation technique gives a relationship between load, displacement, flow stress and strain hardening exponent. This method is based on minimization of the error between the experimental curve (load-displacement of the indenter) and the theoretical curve, as a function of the mechanical and geometrical properties of the materials studied. Results for carbo-nitriding steels obtained by the standard Vickers micro hardness technique are very close to those obtained by both methods.

A non-linear kinematic hardening model for a steel under complex loadings

Abstract A viscoplastic three-dimensional model is proposed in order to predict the response of the 28CrMoV5-8 steel used in brake discs of a High Speed Train, which are subjected simultaneously to mechanical and thermal loadings. This viscoplastic law is based on the internal thermodynamic variables and takes into account the non-linear kinematic and isotropic hardenings with plastic strain memory effect.

Experimental and numerical analyses of instrumented and continuous indentation of treated steels

Abstract The present paper uses the indentation of materials with a yield strength gradient, by spherical indenters in order to obtain Vickers hardness profile. Using finite element modelling, a simulation of elasto-plastic spherical indentation of materials with a yield stress was carried out. A theoretical model for effective mean pressure evolution of the spherical indentation on the surface of heat treated steels was obtained. Using the preceding model and steepest descent optimisation algorithms, a method for determining Vickers hardness profile (surface hardness and layer thicknesses) of these steels, is given. Results for carbo-nitriding steels obtained by the standard Vickers micro-hardness techniques (on the section taken through the surface) are compared with those obtained by the proposed method.

Fretting corrosion in power contacts: Electrical and thermal analysis

Fretting corrosion phenomenon is known as the main cause of contact resistance increasing in signal contact. But for power connectors this undesirable phenomenon in embedded systems is more complex because high current induces high voltages and subsequent Joule overheating can be expected during vibration. Our study deals with contact similar power connectors, ones submitted to vibration amplitudes up to 25 micrometres at frequencies of a few 10Hz under a current ramp from 0.1 to 40A under 14VDC. The main measured parameters are the contact voltage during vibration up to a hundred thousand cycles. The average voltage by micro-voltmeter and its fluctuations during one cycle are acquired permanently by oscilloscope. Thermal aspects are also investigated by the use of miniature thermocouples placed as close as possible to the contact zone. It was found that a limiting voltage is reached, similar to the fritting voltage in a tarnished metal film reported by Holm. So contact resistances and the power brought by Joule effect are reduced and the temperature rise is limited. The main factors which playing a role are current, contact materials and its susceptibility to wear and debris oxidation at the interface. This behaviour is confirmed by the SEM-EDX analysis of fretting zone at different steps of fretting.