R.J. Alves de Sousa

A new volumetric and shear locking‐free 3D enhanced strain element

This paper focuses on the development of a new class of eight‐node solid finite elements, suitable for the treatment of volumetric and transverse shear locking problems. Doing so, the proposed elements can be used efficiently for 3D and thin shell applications. The starting point of the work relies on the analysis of the subspace of incompressible deformations associated with the standard (displacement‐based) fully integrated and reduced integrated hexahedral elements. Prediction capabilities for both formulations are defined related to nearly‐incompressible problems and an enhanced strain approach is developed to improve the performance of the earlier formulation in this case. With the insight into volumetric locking gained and benefiting from a recently proposed enhanced transverse shear strain procedure for shell applications, a new element conjugating both the capabilities of efficient solid and shell formulations is obtained. Numerical results attest the robustness and efficiency of the proposed approach, when compared to solid and shell elements well‐established in the literature.

Motorcycle helmets—A state of the art review

This paper tries to make an overview of the work carried out by scientific community in the area of road helmets safety. In an area that is constantly being pushed forward by market competition, self-awareness of danger and tighter standards, several research groups around the world have contributed to safety gear improvement. In this work concepts related to head impact protection and energy absorption are explained. It also makes reference to the theories related to the development of helmets, as well as to the different existing types nowadays. The materials that are typically used in impact situations and new design concepts are also approached. In addition, it is presented a literature review of current--and most commonly used--helmet test standards, along with new tests and helmet concepts to assess the effects of rotational motion. In a non-restrictive, and never up-to-date report, a state-of-art review on road helmets safety is done, with a special insight into brain injury, helmet design and standards.

On the use of EAS solid‐shell formulations in the numerical simulation of incremental forming processes

Purpose – Incremental sheet forming represents a promising process in the manufacturing of metallic components, particularly its variant known as single point incremental forming (SPIF). The purpose of this paper is to test and validate the results coming from numerical simulation of SPIF processes using the reduced enhanced solid‐shell formulation, when compared to the solid finite elements available in ABAQUS software. The use of SPIF techniques in the production of small batch components has a potential wide application in fields such as rapid prototyping and biomechanical devices.Design/methodology/approach – Incremental forming processes differ from conventional stamping by not using a press and by requiring a lower number of tools, since no dedicated punches and dies are necessary, which lowers the overall production costs. In addition, it shows relative simplicity and flexible setup for complex parts, when compared with conventional technologies. However, the low speed of production and low‐dimensi...

Finding the Best Machine for SPIF Operations - a Brief Discussion

Single Point Incremental Forming (SPIF) given its easy implementation and absence of dedicated tooling is a promising manufacturing technology concerning the production of customized products, low batches or prototyping of ready-to-use parts. The range of application is wide, covering many materials and virtually unlimited geometries. Indeed, current process boundaries are more related to machine limitations than to the procedure itself. In this paper, research is carried out on the state-of-the-art of existing SPIF machine technology, in order to determine an appropriate configuration for an incremental forming equipment that overcomes such limitations. A comparative analysis is carried out to evaluate the different types of currently used equipment: adapted milling machines, serial robots and purpose built machines. Comparison parameters include among many others the maximum payload, tool path flexibility, stiffness and overall cost of the machine, based on information gathered on publications mainly from the last decade. Alternatively, other solutions used for different technological processes and assembly operations, such as precision positioning, are also taken into account. Based on the comparison of all solutions, and on the objectives of the current project carried out at the University of Aveiro, it is concluded that an equipment with parallel kinematics, driven by hydraulic servo-cylinders, could be the best choice to achieve the established goals.

Enhanced Assumed Strain Shell and Solid‐Shell Elements: Application in Sheet Metal Forming Processes

Reliable numerical analysis of sheet metal forming processes using commercial finite element programs involves a variety of fields within computational mechanics area. Material models, contact algorithms, robust and fast incremental and iterative solution techniques are among the key factors that a finite element package must rely upon. Nevertheless, the finite element formulation itself still represents a milestone of crucial importance in the overall quality of the final solution. As sheet metal forming processes present very strong test cases to the performance of finite elements, robust formulations are then desired. In this work, classes of finite elements involving only the Enhanced Assumed Strain (EAS) method are analyzed. Starting from an innovative approach to eliminate transverse shear locking in shell elements (S4E6P5 shell element) and going through a new approach for a volumetric and transverse shear locking‐free solid‐shell element with a low number of internal variables (HCiS12 solid‐shell ...

Finite element analysis of incrementally formed parts

Single point incremental forming (SPIF) processes represent a breakpoint from traditional forming processes, and possibly a new era in small batches production, customised parts or rapid prototyping. Given its easy implementation, SPIF is being currently a subject of intensive experimental and numerical research. Consequently, dimensional inaccuracy in formed parts is still a real problem to tackle, demanding a correct prediction of forming forces, springback defects and strain path effects. This work concerns a first step of a detailed numerical study, validated with available experimental results in the literature raised about simulation of SPIF processes. To this end, it is proposed a general simulation framework of SPIF processes using the finite element method with implicit solution schemes, along with innovative solid-shell elements for 3D analysis. Finally, the use of solid-shell elements provides several advantages compared to shell elements, as considering automatically full 3D stress states and constitutive laws, thickness variations, double sided contact and normal stresses.

On the Development and Computational Implementation of Complex Constitutive Models and Parameters’ Identification Procedures

Nowadays, the automotive industry has focused its attention to weight reduction of the vehicles to overcome environmental restrictions. For this purpose, new materials, namely, advanced high strength steels and aluminum alloys have emerged. These materials combine good formability and ductility, with a high tensile strength due to a multi-phase structure (for the steel alloys) and reduced weight (for the aluminum alloys). As a consequence of their advanced performances, complex constitutive models are required in order to describe the various mechanical features involved. In this work, the anisotropic plastic behavior of dual-phase steels and high strength aluminum alloys is described by the non-quadratic Yld2004-18p yield criterion, combined with a mixed isotropic-nonlinear kinematic hardening law. This phenomenological model allows for an accurate description of complex anisotropy and Bauschinger effects of the materials, which are essential for a reliable prediction of deep drawing and springback results using numerical simulations. To this end, an efficient computational implementation is needed, altogether with an inverse methodology to properly identify the constitutive parameters to be used as numerical simulation input. The constitutive model is implemented in the commercial finite element code ABAQUS as a user-defined material subroutine (UMAT). A multi-stage return mapping procedure, which utilizes the control of the potential residual, is implemented to integrate the constitutive equations at any instant of time (pseudo-time), during a deformation process. Additionally, an inverse methodology is developed to identify the constitutive model parameters of the studied alloys. The identification framework is based on an interface program that links an optimization software and the commercial finite element code. This methodology compares experimental data with the respective results numerically obtained. The implemented optimization process aims to minimize an objective function, which defines the difference between experimental and numerical results using the Levenberg-Marquardt gradient-based optimization method. The proposed integrated approach is validated in a number of benchmarks in sheet metal forming, including monotonic and cyclic loading, with the goal to infer about the modelling of anisotropic effects.

CAD/CAM Strategies for a Parallel Kinematics SPIF Machine

Single point incremental forming has attracted the interest of researchers in the last decade for the production of prototypes and small batch production of sheet-based parts [1, 2]. This technique allows the manufacture of parts without using expensive die sets. The SPIF (Single point incremental forming) process can be performed on different equipments such as adapted CNC milling machines, serial robots and built proposed machines [3]. Every solution has advantages and disadvantages. This work presents the CAD/CAM strategies for a parallel kinematics SPIF machine, designed and built at the University of Aveiro [3]. This machine brings a new approach to the SPIF industry. The machinery used to perform SPIF operations has limitations in their work volume with limited movements and in the magnitude of applicable forces. With that in mind, this machine was projected to overcome that obstacle, and was provided with a system with 6 degrees of freedom, while maintaining the ability to apply high loads. The disadvantage is the increase in volume occupied by the kinematic system. The manufacture of new parts could be reached out with more flexibility on the chosen tool path. The first step is the product design in the commercial CAD system. Next step is generating the tool path of the forming tool. This step is very important to achieve the desired part shape. It is used a commercial CAM system (EdgeCAM 2012®), which has resources from three up to five axis strategies. The last step is to send the information to the machine’s control system, based on real-time software. This paper will describe each step with more details.

On the Study of Distinct Algorithmic Strategies in the Implementation of Advanced Anisotropic Models with Mixed Hardening

In this study, suitable distinct stress integration algorithms for advanced anisotropic models with mixed hardening, and their implementation in finite element codes, are discussed. The constitutive model studied in the present work accounts for advanced (non-quadratic) anisotropic yield criteria, namely, the Barlat et al. 2004 model with 18 coefficients (Yld2004-18p), combined with a mixed isotropic-nonlinear kinematic hardening law. This phenomenological model allows for an accurate description of complex plastic yielding anisotropy and Bauschinger effects, which are essential for a reliable prediction of deep drawing and springback results using numerical simulations.In the present work distinct algorithm classes are analysed: (i) a semi-explicit algorithm that accounts for the sub-incrementation technique; (ii) the cutting-plane approach (semi-implicit integration); and (iii) the fully-implicit multi-stage return mapping procedure, based on the control of the potential residual. The numerical performance of the developed algorithms is inferred by benchmarks in sheet metal forming processes. The quality of the solution is assessed and compared to reference results. In the end, an algorithmic and programming framework is provided, suitable for a direct implementation in commercial Finite Element codes, such as Abaqus (Simulia) and Marc (MSC-Software) packages.

Numerical Simulation and Prediction of Wrinkling Defects in Sheet Metal Forming

The goal of the present work is to analyse distinct numerical simulation strategies, based on the Finite Element Method (FEM), aiming at the description of wrinkling initiation and propagation during sheet metal forming. From the FEM standpoint, the study focuses on two particular aspects: a) the influence of a given finite element formulation as well as the numerical integration choice on the correct prediction of wrinkling in walls and flange zones of cup drawing formed parts; and b) the influence of the chosen anisotropic constitutive model and corresponding parameters on the correct prediction and propagation of wrinkling deformation modes during forming operations. In this sense, this work infers about the influence of accounting for distinct planar anisotropy behaviours within numerical simulation procedures. Free and flange-forming examples will be taken into consideration, with isotropic and anisotropic material models. Additionally, the influence on wrinkling onset and propagation as coming from different numerical formulations will be accounted for shell and tridimensional continuum finite elements, along with implicit numerical solution procedures. Doing so, the present work intends to provide some insights into how numerical simulation parameters and modelling decisions can influence FEM results regarding wrinkling defects in sheet metal formed parts.