Augustin Gakwaya

Modeling of the metal powder compaction process using the cap model. Part I. Experimental material characterization and validation

Abstract In order to produce crack free metal powder compacts that respect both the dimensional tolerances and the mechanical strength requirements, both tooling design and compaction sequence have to be adequately determined. The finite element method, through the use of an appropriate constitutive model of the powder medium, has recently been used as an efficient design tool. The accuracy of this method highly depends on the faithfulness of the constitutive model and the quality of the material parameter set. Furthermore, in order for the simulation results to be reliable, they should be experimentally validated on real parts featuring density variations. Hence, the main concerns of this paper are the development of a standard calibration procedure for the cap material model as well as the development of a reliable technique for the experimental validation of the powder compaction simulation results. The developed calibration procedure, applied for the case of 316L stainless steel powders, is based on a series of isostatic, triaxial and uniaxial compaction tests as well as resonant frequency tests. In addition, a sensitivity study was performed in order to determine the relative importance of each factor and basic simulations served to validate the parameter set extraction procedure. On the other hand, a local density measurement technique was developed for the experimental validation of the model results. This technique is based on correlation with Vickers macro-hardness. Finally, an application featuring the compaction of a 316L stainless steel cylindrical component is presented to illustrate the predictive capabilities of the cap material model as well as the accuracy of the acquired material parameter set.

Modeling of the metal powder compaction process using the cap model. Part II: Numerical implementation and practical applications

Abstract The finite element (FE) simulation method has recently been used as an alternative design tool in powder metallurgy (PM) industry. It allows for the prediction of density and stress distributions in the pressed compact prior to the actual tooling design and manufacturing activity. It thus makes possible the validation of the PM part and associated tooling design. However, the accuracy of FE prediction highly depends on the choice of an appropriate and well calibrated powder material model, as well as on the effectiveness of the computational environment. While the first point was presented in a previous work, the present paper addresses some computational aspects of compaction process modeling approach in the context of industrial production environment. Hence, this paper presents a discussion of the choice of stress and strain measures used in this large deformation context. It also presents the implementation of the cap constitutive model into abaqus FE software using the closest point projection algorithm. Furthermore, an integrated simulation module has been developed and is described herein. This module, designed in order to render the modeling approach practical and industrially attractive to PM engineers, permits an easy definition of the tooling and the powder geometry, as well as the prescription of compaction sequence and all other boundary conditions. Finally, the simulation of the compaction of an industrial PM part, intended to illustrate the usefulness of the simulation approach in the task of improving the design of PM part and process, is presented.

A formulation of the non linear discrete Kirchhoff quadrilateral shell element with finite rotations and enhanced strains

This paper presents a new formulation of the non-linear discrete Kirchhoff quadrilateral shell element applicable for the analysis of geometrically nonlinear structures undergoing finite rotations. The shell director is directly interpolated and the exact linearization of the discreet form of the equilibrium equations is derived in closed form. The consistent tangent stiffness matrix is symmetric and is given explicitly in this paper. Two or three rotational variables are used at each node. To improve the in-plane deformation enhanced incompatible modes are introduced. The formulation is then illustrated by a comprehensive set of numerical experiments selected from the literature.

Study of True Stress-Strain Curve after Necking for Application in Ductile Fracture Criteria in Tube Hydroforming of Aerospace Material

Tube hydroforming (THF) is an advanced metal forming process that is used widely in automotive industry, but the application of the THF process in aerospace field is comparatively new with many challenges due to high strength and limited formability of aerospace materials. The success of THF process largely depends on many factors, such as mechanical properties of the material, loading path during the process, tool geometry and friction condition. Due to complexity of this process, finite element modeling (FEM) can largely reduce the production cost. One of the important input in FEM is the material behavior during hydroforming process. The true stress-strain curve before necking can be easily determined, using either tensile testing or bulge testing, but for an accurate failure prediction in a large deformation, such as hydroforming, the study of true stress-strain curve after necking is important because it improves the quality of the analysis due to utilizing a real extended stress-strain curve. Hence, the objective of this research was to establish a methodology to determine the true stress-strain curve after necking in order to predict burst pressure in the THF of aerospace materials. Uniaxial tensile tests were performed on standard tensile samples (ASME E8M-04) to determine the true stress-strain before and after necking, using an analytical method presented in this study. To validate the approach, burst pressure in the THF process was predicted using the extended stress-strain curve in conjunction with Brozzos decoupled fracture model. The approach was evaluated using data obtained from the free expansion (tube bulging) tests performed on stainless steel 321 tubes with 2 inches diameter and two different thicknesses, 0.9 mm and 1.2 mm. The comparison of the predicted and measured burst pressures was promising, indicating that the approach has the potential to be extended to predict formability limits in THF of complex shapes.

Mechanical performance of repaired sandwich panels: Experimental characterization and finite-element modelling

This paper describes the static performance of adhesively bonded repairs on sandwich panels made with carbon-epoxy composite skins and a Nomex core. First, the mechanical behaviour of pristine and repaired panels under tensile loading was studied. All tests were conducted under room temperature conditions. Then, finite-element analyses were performed to predict the behaviour of repaired panels. Two material models were developed for the adhesive joint: one was linear elastic and the second was elastic-plastic with a shear failure criterion. For the composite skins, an orthotropic linear elastic model was used. Numerical model predictions are in good agreement with the experimental results. It was found also that the strength recovery of the repaired structure increases with the decrease of the scarf angle.

Effect of Material Model on Finite Element Modeling of Aerospace Alloys

Increasing acceptance and use of hydroforming technology within the aerospace industry requires a comprehensive understanding of critical issues such as the material characteristics, friction condition and hydroformability of the material. Moreover, the cost of experiments that can be reduced by accurate finite element modeling (FEM), which entails the application of adapted constitutive laws for reproducing with confidence the material behavior. In this paper, the effect of different constitutive laws on FEM of tubular shapes is presented. The free expansion process was considered for developing the FEM. Bulge height, thickness reduction and strains were determined at the maximum bulge height using different constitutive models, including Hollomon, Ludwik, Swift, Voce, Ludwigson. In order to minimize the effect of friction, the free expansion experiments were performed with no end feeding. The simulation results were compared with the experimental data to find the appropriate constitutive law for the free expansion process.

Numerical and experimental modeling for bird and hail impacts on aircraft structure

Aircraft bird-strike events are very common and dangerous. Hailstone impacts represent another threat for aircraft structures. As part of the certification process, an aircraft must demonstrate the ability to land safely after impact with a foreign object at normal flight operating speeds. Since experimental studies can be cost prohibitive, validated numerical impact simulation seems to be a viable alternative. Modelling of these soft body impacts still represents a challenge, involving modelling of both the target and the projectile. Here the smooth particle hydrodynamics method (SPH), which has been used successfully in ballistic applications involving bird strike scenarios, is extended to hail impact. The paper thus presents the meshless SPH numerical method as a novel modeling approach. The method is applied to model bird and hail impacts which are problems that traditional FEM based modeling methods typically struggle to solve because of involved mesh distortion problems. The numerical results are then evaluated by comparing with the data collected during recent experimental tests. The data acquisition methods are also described and evaluated for applications where the short duration of the impact presents a challenge. The accuracy of the numerical results allows us to conclude that the models developed can be used in the certification and/or design process of moving (aircraft) and stationary (wind turbines) composite structures subject to bird and hail impact.

FEA-Based Comparative Investigation on High Speed Machining of Aluminum Alloys AA6061-T6 and AA7075-T651

Independent research studies have shown notable dissimilarity in the machining behaviour of aluminum alloys AA6061−T6 and AA7075−T651 commonly used in automotive and aeronautical applications. The present work attempts to investigate this dissimilarity based on experimental and numerical data with a focus on chip formation and generated residual stresses under similar high−speed machining (HSM) conditions. The numerical data were calculated by a finite element modeling (FEM) developed using DeformTM 2D software. The results showed that both studied alloys exhibit different chip formation mechanisms and residual stress states at the machined surfaces. On one hand, the AA6061−T6 alloy generates continuous chips and tensile residual stresses whereas the AA7075−T651 alloy produces segmented chips and compressive residual stresses. FEM results showed that the AA6061−T6 alloy generates lower cutting temperature at the tool−chip interface along with higher equivalent total strains at the machined surface as compared to the AA7075−T651 alloy. Based on the experimental and numerical results, it was pointed out that the differences in terms of thermal conductivity and initial yield stress are the main reasons explaining the dissimilarity observed.

Prediction of Burst Pressure in Multistage Tube Hydroforming of Aerospace Alloys

Bursting, an irreversible failure in tube hydroforming (THF), results mainly from the local plastic instabilities that occur when the biaxial stresses imparted during the process exceed the forming limit strains of the material. To predict the burst pressure, Oyans and Brozzos decoupled ductile fracture criteria (DFC) were implemented as user material models in a dynamic nonlinear commercial 3D finite-element (FE) software, ls-dyna. THF of a round to V-shape was selected as a generic representative of an aerospace component for the FE simulations and experimental trials. To validate the simulation results, THF experiments up to bursting were carried out using Inconel 718 (IN 718) tubes with a thickness of 0.9 mm to measure the internal pressures during the process. When comparing the experimental and simulation results, the burst pressure predicated based on Oyanes decoupled damage criterion was found to agree better with the measured data for IN 718 than Brozzos fracture criterion.

Prediction of Burst Pressure in Multi Stage Tube Hydroforming of Aerospace Alloys

Bursting, an irreversible failure in tube hydroforming (THF), results mainly from the local plastic instabilities that occur when the biaxial stresses imparted during the process exceed the forming limit strains of the material. To predict the burst pressure, Oyane’s and Brozzo’s decoupled ductile fracture criteria were implemented as user material models in a dynamic nonlinear commercial 3D finite element (FE) software, Ls-Dyna. THF of a round to V-shape was selected as a generic representative of an aerospace component for the FE simulations and experimental trials. To validate the simulation results, THF experiments up to bursting were carried out using Inconel 718 (IN 718) tubes with a thickness of 0.9 mm to measure the internal pressures during the process. When comparing the experimental and simulation results, the burst pressure predicated based on Oyane’s decoupled damage criterion was found to agree better with the measured data for IN 718 than Brozzo’s fracture criterion.Copyright