C. Vallellano

A Study to Predict Failure in Biaxially Stretched Sheets of Aluminum Alloy 2024-T3

The 2024-T3 aluminum alloy is extensively used for making aircraft parts. This alloy has a relatively low ductility at room temperature, and is generally heat-treated before forming. The present study experimentally and numerically analyzes the formability of AA2024-T3 sheets subjected to biaxial stretching. The aim of this study is to provide an appropriate criterion to accurately predict sheet failure, which will allow for the optimization of the stretching operations, and provide the means to evaluate when heat treatment can be avoided. The fracture process of AA2024-T3 stretched sheets is experimentally analyzed, obtaining the FLD at fracture. A selection of classical ductile failure criteria (integral and local criteria) is implemented in ABAQUS, and their capability to predict sheet failure is studied in detail. It is found that the local criteria, in particular Trecas criterion, provide a better description of the failure process than the integral criteria. Finally, the influence of bending strains in the initiation of failure is also discussed.

Compact formulation for modelling cracks in infinite solids using distributed dislocations

Abstract This work presents compact formulae for the representation of a rectilinear crack in an infinite medium whose plastic zones extend over an arbitrary number of regions with different frictional stresses, and with bounded and unbounded end conditions. The solutions given in a previous paper by Navarro and de los Rios (1988, Phil. Mag. A, 57, 43) are improved in three important aspects. Firstly, it has been found that the procedure proposed then as a general method for calculating an arbitrary constant that appears in the unbounded solution is erroneous. This has been corrected now and it is proved that the expressions derived by Navarro and de los Rios, which have been extensively used since, are indeed valid for the important case where symmetry conditions exist. Secondly, the need for the constancy of the friction stresses over each region is removed. It is shown that, as long as the variation in the applied and frictional stresses is well behaved (piecewise continuous and bounded functions), the solutions are still valid. Thirdly, formulae are presented which relate the relative displacement across the crack plane and the stresses ahead of the plastic zone directly to the effective stresses acting upon the dislocations. Thus this eliminates the necessity of obtaining first the dislocation density explicitly.

A functional methodology on the manufacturing of customized polymeric cranial prostheses from CAT using SPIF

Purpose This paper aims to propose a functional methodology to produce cranial prostheses in polymeric sheet. Within the scope of rapid prototyping technologies, the single-point incremental forming (SPIF) process is used to demonstrate its capabilities to perform customized medical parts. Design/methodology/approach The methodology starts processing a patient’s computerized axial tomography (CAT) and follows with a computer-aided design and manufacture (CAD/CAM) procedure, which finally permits the successful manufacturing of a customized prosthesis for a specific cranial area. Findings The formability of a series of polymeric sheets is determined and the most restrictive material among them is selected for the fabrication of a specific partial cranial prosthesis following the required geometry. The final strain state at the outer surface of the prosthesis is analysed, showing the high potential of SPIF in manufacturing individualized cranial prostheses from polymeric sheet. Originality/value This paper proposes a complete methodology to design and manufacture polymer customized cranial prostheses from patients’ CATs using the novel SPIF technology. This is an application of a new class of materials to the manufacturing of medical prostheses by SPIF, which to this purpose has been mainly making use of metallic materials so far. Despite the use of polymers to this application is still to be validated from a medical point of view, transparent prostheses can already be of great interest in medical or engineering schools for teaching and research purposes.

On the estimation of microstructural effects in the near-threshold fatigue of small cracks

The growth of small fatigue cracks is strongly influenced by the microstructure of the material concerned. Particularly in the early stages, the crystallographic orientation of the grains through which the crack must propagate plays a fundamental role. Cracks are generally initiated in well-oriented grains, but afterwards they are forced to grow through less favourably oriented grains. This work examines the influence of this crystallographic resistance to small crack growth in micromechanical terms. It is argued that the crystallographic orientation alone cannot explain the difference between small- and long-crack growth thresholds found in metals. Other phenomena must be called upon to account for this difference. For instance, crack closure exerts a resistance to crack growth whose evolution with crack length is similar to that due to the crystallographic orientation. Finally, it is shown that, when microstructural and mechanical thresholds are interpreted within the context of micromechanical models, a number of classical parameters and expressions usually employed in engineering practice can be naturally obtained and understood.

Prediction of Ductile Failure in the Stretch‐Forming of AA2024 Sheets

A number of ductile failure criteria are nowadays being used to predict the formability of aluminium alloy sheets. Generally speaking, integral criteria (e.g. those proposed by Cockcroft and Latham, Brozzo et al., Oyane et al Chaouadi et al., etc.) have been probed to work well when the principal strains are of opposite sign, i.e. in the left side of the Forming Limit Diagram (FLD). However, when tensile biaxial strains are present, as occurs in stretch‐forming practice, their predictions are usually very poor and even non‐conservatives. As an alternative, local criteria, such as the classical Tresca’s and Bressan & Williams’ criteria, have demonstrated a good capability to predict the failure in some automotive aluminum alloys under stretching. The present work analyses experimentally and numerically the failure in AA2024‐T3 sheets subjected to biaxial stretching. A series of out‐of‐plane stretching tests have been simulated using ABAQUS. The experimental and the numerical FLD for different failure crite...

Evaluation of total alloplastic temporo-mandibular joint replacement with two different types of prostheses: A three-year prospective study.

Background Temporo-Mandibular Joint (TMJ) replacement has been used clinically for years. The objective of this study was to evaluate outcomes achieved in patients with two different categories of TMJ prostheses. Material and Methods All patients who had a TMJ replacement (TMJR) implanted during the study period from 2006 through 2012 were included in this 3-year prospective study. All procedures were performed using the Biomet Microfixation TMJ Replacement System, and all involved replacing both the skull base component (glenoid fossa) and the mandibular condyle. Results Fifty-seven patients (38 females and 19 males), involving 75 TMJs with severe disease requiring reconstruction (39 unilateral, 18 bilateral) were operated on consecutively, and 68 stock prostheses and 7 custom-made prostheses were implanted. The mean age at surgery was 52.6±11.5 years in the stock group and 51.8±11.7 years in the custom-made group. In the stock group, after three years of TMJR, results showed a reduction in pain intensity from 6.4±1.4 to 1.6±1.2 (p<0.001), and an improvement in jaw opening from 2.7±0.9 cm to 4.2±0.7 cm (p<0.001). In the custom-made group, after three years of TMJR, results showed a reduction in pain intensity from 6.0±1.6 to 2.2±0.4 (p<0.001), and an improvement in jaw opening from 1.5±0.5 cm to 4.3±0.6 cm (p<0.001). No statistically significant differences between two groups were detected. Conclusions The results of this three-year prospective study support the surgical placement of TMJ prostheses (stock prosthetic, and custom-made systems), and show that the approach is efficacious and safe, reduces pain, and improves maximum mouth opening movement, with few complications. As such, TMJR represents a viable technique and a stable long-term solution for cranio-mandibular reconstruction in patients with irreversible end-stage TMJ disease. Comparing stock and custom-made groups, no statistically significant differences were detected with respect to pain intensity reduction and maximum mouth opening improvement. Key words:Temporo-mandibular joint, temporo-mandibular joint replacement, prosthesis, biomaterials, biomedical engineering, computer-aided design and manufacturing.

On the Experimental Detection of Necking in Stretch‐Bending Tests

The standard ISO 12004‐2 has recently established a methodology for estimating limit strains in Marciniak and Nakajima tests, that is, situations in which the strain gradient through the sheet thickness does not play an important role. This method, known as a “position‐dependent method,” is based on the analysis of the major strain profile just before fracture occurrence. The present paper proposes an alternative method for detecting the onset of necking and determining the forming limits, based on temporal analysis of the major strain. This time‐dependent methodology is applicable to more general situations where strain gradients in the sheet cannot be neglected, such as severe stretch‐bending tests, small radius punches, etc. A series of stretching and stretch‐bending experiments, close to plane strain conditions, have been carried out to check the ability of a number of methods to predict the onset of necking. Finally, a comparison of these methodologies is presented and their advantages and limitation...

Failure Prediction in Stretched Sheets of Aluminium 2024-T3

The present work analyses experimentally and numerically the failure limit of a 2024-T3 aluminium alloy sheet subjected to stretching. The capability of a number of ductile fracture criteria to predict sheet failure is examined and compared with experimental results. The influence of the hydrostatic pressure in the Freudenthal’s, the Cockcroft and Latham’s and the Bressan and Williams’ criterion is analyzed. The effect of the normal stress on the fracture plane in the Bressan and Williams’ criterion is also discussed.

Teaching Experience for the Virtualization of Machine Tools and Simulation of Manufacturing Operations

This work aims to generate the digital documentation related to a number of manufacturing processes on different machine tools. The project is developed with the contribution of engineering students doing their final thesis within this field. Different machine tools and machining and incremental forming processes have been virtualized by using the CAD/CAM software CATIA V5. Some of the modeled parts were finally manufactured after checking and post-processing the NC code. Digital documentation is developed on different formats (e.g. photographs, videos, images and simulations) in order to be used as a teaching complement.

Multidisciplinary Learning of Manufacturing Engineering through Bachelor and Master Theses in Incremental Sheet Forming

This work presents a novel teaching experience in the framework of final master and bachelor theses within the research line on incremental sheet forming processes. This forming process involves a series of competences dealing with manufacturing technology at a wide multidisciplinary level. This paper analyses these different manufacturing topics and the knowledge acquired by students doing their final theses within this field. This knowledge will be very useful during their future professional career.