Barbara Gouveia

Fracture predicting in bulk metal forming

Abstract An important concern in forming is whether the desired deformation can be accomplished without failure of the work material. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element method for predicting failures in cold bulk metal forming. Four previously published ductile fracture criteria are selected, and their relative accuracy for predicting and quantifying fracture initiation sites is investigated. Experiments with ring, cylindrical, tapered and flanged upset samples are performed to investigate the validity of the workability criteria under conditions of stress and strain similar to those usually found in bulk metal forming processes. The implementation of ductile fracture criteria into a rigid—plastic finite element computer program is presented. Local stress and strain distributions throughout the deformation are computed and compared with experimental measurements. A general good agreement is found. However, only two of these workability criteria have successfully predicted the location at which fracture initiates for all the upset tests performed in this work. The paper concludes with a discussion of the importance of the critical damage at fracture to remain independent from the technological processes.

Ductile fracture in metalworking: experimental and theoretical research

Abstract An important concern in metalworking is whether the desired deformation can be accomplished without failure of the material. This paper describes the utilisation of ductile fracture criteria in conjunction with the finite element method for predicting surface and internal failures in cold metalworking processes. Four previously published ductile fracture criteria are selected, and their relative accuracy for predicting and quantifying fracture initiation sites is investigated. Ring, cylindrical, tapered and flanged upset test samples are utilised for providing the experimental values of the critical damage at fracture under several different loading conditions. Two of the ductile fracture criteria are then utilised to predict the initiation site and the level of deformation at which surface or internal cracking will occur during finite element simulation of three types of metalworking processes, namely, radial extrusion, open-die forging and blanking. The analysis is made in conjunction with metal experiments, good agreement being found to occur.

Fabrication of computationally designed scaffolds by low temperature 3D printing

The development of artificial bone substitutes that mimic the properties of bone and simultaneously promote the desired tissue regeneration is a current issue in bone tissue engineering research. An approach to create scaffolds with such characteristics is based on the combination of novel design and additive manufacturing processes. The objective of this work is to characterize the microstructural and the mechanical properties of scaffolds developed by coupling both topology optimization and a low temperature 3D printing process. The scaffold design was obtained using a topology optimization approach to maximize the permeability with constraints on the mechanical properties. This procedure was studied to be suitable for the fabrication of a cage prototype for tibial tuberosity advancement application, which is one of the most recent and promising techniques to treat cruciate ligament rupture in dogs. The microstructural and mechanical properties of the scaffolds manufactured by reacting α/β-tricalcium phosphate with diluted phosphoric acid were then assessed experimentally and the scaffolds strength reliability was determined. The results demonstrate that the low temperature 3D printing process is a reliable option to create synthetic scaffolds with tailored properties, and when coupled with topology optimization design it can be a powerful tool for the fabrication of patient-specific bone implants.

Application of a 3D printed customized implant for canine cruciate ligament treatment by tibial tuberosity advancement.

Fabrication of customized implants based on patient bone defect characteristics is required for successful clinical application of bone tissue engineering. Recently a new surgical procedure, tibial tuberosity advancement (TTA), has been used to treat cranial cruciate ligament (CrCL) deficient stifle joints in dogs, which involves an osteotomy and the use of substitutes to restore the bone. However, limitations in the use of non-biodegradable implants have been reported. To overcome these limitations, this study presents the development of a bioceramic customized cage to treat a large domestic dog assigned for TTA treatment. A cage was designed using a suitable topology optimization methodology in order to maximize its permeability whilst maintaining the structural integrity, and was manufactured using low temperature 3D printing and implanted in a dog. The cage material and structure was adequately characterized prior to implantation and the in vivo response was carefully monitored regarding the biological response and patient limb function. The manufacturing process resulted in a cage composed of brushite, monetite and tricalcium phosphate, and a highly permeable porous morphology. An overall porosity of 59.2% was achieved by the combination of a microporosity of approximately 40% and a designed interconnected macropore network with pore sizes of 845 μm. The mechanical properties were in the range of the trabecular bone although limitations in the cages reliability and capacity to absorb energy were identified. The dogs limb function was completely restored without patient lameness or any adverse complications and also the local biocompatibility and osteoconductivity were improved. Based on these observations it was possible to conclude that the successful design, fabrication and application of a customized cage for a dog CrCL treatment using a modified TTA technique is a promising method for the future fabrication of patient-specific bone implants, although clinical trials are required.

Finite-element modelling of cold forward extrusion

Abstract Simulating metal forming processes using an updated Lagrangian finite-element formulation is not ideal when steady-state material flow conditions prevail. Firstly, repeated calculations of large non-linear finite element systems are needed for continuously updating the mesh, and secondly, remeshing operations must be undertaken to avoid excessive mesh distortion and to introduce localised refinements in regions where large gradients are likely to occur. The combined Eulerian–Lagrangian formulation overcomes these difficulties by using a temporary incremental mesh to calculate the strain and stress fields, coupled with a mathematical scheme to interpolate the updated mechanical state into a spatially fixed mesh. In this paper the cold forward extrusion of rods is analysed using both the updated Lagrangian and the combined Eulerian–Lagrangian finite-element formulations. The theoretical background for both formulations is reviewed, and the numerical results obtained with the two formulations are compared with experimental extrusion data. Excellent agreement is found for the flow pattern and for the distribution of strain within the plastically deformed region. In what concerns the extrusion load curve, the results demonstrate that the latter can be predicted more accurately using a combined Eulerian–Lagrangian finite-element formulation.

Finite element modelling of cold forward extrusion using updated Lagrangian and combined Eulerian–Lagrangian formulations

Abstract Finite element analysis of metal forming processes is predominantly treated by the updated Lagrangian formulation in which the mesh moves and deforms in space, in accordance with the deformation history of the material. However, the steady-state deformation characteristics of some metal forming processes such as extrusion, drawing and rolling, can be advantageously analysed using a combined Eulerian–Lagrangian formulation. In this paper cold forward extrusion of rods is analysed using both the updated Lagrangian and the combined Eulerian–Lagrangian finite element formulations. The numerical results obtained with the two formulations are discussed and compared with experimental data obtained from metal experiments carried out with steel rods. Excellent agreement is found for the flow pattern and for the distribution of strain within the plastically deformed region. In what concerns the extrusion load curve results demonstrate that it can be predicted more accurately using a combined Eulerian–Lagrangian finite element formulation.

Physical modelling and numerical simulation of the round-to-square forward extrusion

Abstract In this paper, three-dimensional forward extrusion of a square section from a round billet through a straight converging die is analysed using both physical modelling and numerical simulation (finite element and upper bound analysis). Theoretical fundamentals for each method are reviewed, and comparisons are made between the numerical predictions and experimental data obtained through the utilisation of physical modelling. Assessment is made in terms of flow pattern and strain distribution for two different cross-sections corresponding to the axial symmetry planes of the three-dimensional extrusion part. The experimental distribution of strain is determined from the shape change of quadrilateral grids previously printed on the surface of the axial cross-sections of the undeformed billets by means of large deformation square-grid analysis. Good agreement is obtained between physical and numerical modelling of the three-dimensional extrusion process.

The role of shell/core saturation level on the accuracy and mechanical characteristics of porous calcium phosphate models produced by 3Dprinting

Purpose – This paper aims to study the influence of the binder saturation level on the accuracy and on the mechanical properties of three-dimensional (3D)-printed scaffolds for bone tissue engineering. Design/methodology/approach – To study the influence of the liquid binder volume on the models accuracy, two quality test plates with different macropore sizes were designed and produced. For the mechanical and physical characterisation, cylindrical specimens were used. The models were printed using a calcium phosphate powder, which was characterised in terms of composition, particle size and morphology, by X-ray diffraction (XRD), laser diffraction and Scanning electron microscopy (SEM) analysis. The sample’s physical characterisation was made using the Archimedes method (porosity), SEM, micro-computer tomography (CT) and digital scan techniques, while the mechanical characterisation was performed by means of uniaxial compressive tests. Strength distribution was analysed using a statistical Weibull approac...

Deformation analysis of the round-to-square extrusion: a numerical and experimental investigation

Abstract In this paper the three-dimensional forward extrusion of a square section from a round billet through a straight converging die is analysed using both the updated Lagrangian and the combined Eulerian–Lagrangian finite element formulations. The theoretical background for both formulations is reviewed, and the numerical results obtained with the two formulations are compared with experimental data obtained through the utilisation of physical modelling. Assessment is made in terms of flow pattern and strain distribution for two different cross-sections corresponding to the axial symmetry planes of the three-dimensional extrusion profile. The experimental distribution of strain is determined from the shape change of quadrilateral grids previously printed on the surface of the axial cross-sections of the undeformed billets by means of large deformation square-grid analysis. Good agreement is obtained between physical and numerical modelling.

A metal‐forming approach to automatic generation of graded initial quadrilateral finite element meshes

This paper presents an algorithm for automatic generation of graded initial quadrilateral meshes targeted for the finite element analysis of metal‐forming processes. Meshing the domain geometry deals with a universe of shapes, and the procedure therefore takes into account the initial geometry of the billet. A grid‐based approach is utilised for generating an initial coarse mesh with well‐shaped (internal) elements, and in cases where non‐rectangular shapes are to be discretized, linking with the boundary is performed on the basis of constrained Delaunay triangulation. By analysing the contact situation between dies and mesh, an attempt is made to identify regions where plastic deformation is likely to be concentrated during the early stages of processing, and accordingly refinement of the mesh is performed locally by elemental subdivision. Simulation examples for closed‐die forging, forward rod and backward can extrusion substantiate the feasibility of this approach in terms of lowering the overall calculation error and limiting the interference between mesh and die.