Modelling the mechanical behaviour of metal powder during Die compaction process

In this work, powder compaction process was investigated by using a numerical material model, which involves Mohr-Coulomb theory and an elliptical surface plasticity model. An effective algorithm was developed and implemented in the ANSYS finite element (FEM) code by using the subroutine USERMAT. Some simulations were performed to validate the proposed metal powder material model. The interaction between metal powder and die walls was considered by means of contact elements. In addition to the analysis of metal powder behaviour during compaction, the actions transmitted to die were also investigated, by considering different friction coefficients. This information is particularly useful for a correct die design

The interaction of frictional slip and adhesion for a stiff sphere on a compliant substrate

How friction affects adhesion is addressed. The problem is considered in the context of a very stiff sphere adhering to a compliant, isotropic, linear elastic substrate, and experiencing adhesion and frictional slip relative to each other. The adhesion is considered to be driven by very large attractive tractions between the sphere and the substrate that can act only at very small distances between them. As a consequence, the adhesion behavior can be represented by the Johnson-Kendall-Roberts model, and this is assumed to prevail also when frictional slip is occurring. Frictional slip is considered to be resisted by a uniform, constant shear traction at the slipping interface, a model that is considered to be valid for small asperities and for compliant elastomers in contact with stiff material. A model for the interaction of friction and adhesion, known to agree with some experimental data, is utilized. This model is due to Johnson, and its adhesion-friction interaction is assumed to stem, upon shrinkage of the contact area, from a postulated reversible energy release associated with frictional slip. This behavior is considered to arise from surface microstructures generated or eliminated by frictional slip, where these microstructures store some elastic strain energy in a reversible manner. The associated reversible energy release rate is derived from the energy exchanges that occur in the system. The Johnson model, and an asymptotic analysis of it for small amounts of frictional slip, is shown to be consistent with the reversible energy release rate that we identify.Comment: 11 page

Matrici di pressatura di ingranaggi cilindrici a denti diritti: analisi numerica FEM sull’influenza del modulo e del numero di denti sugli stati tensionali

Dopo una premessa sulla competitività della metallurgia delle polveri per la realizzazione di componentimeccanici di geometria complicata, si espongono i dati impostati per uno studio sistematico sull’influenza deiparametri di progetto, della ruota dentata e dello stampo, sul livello tensionale che si può raggiungere a finepressatura, ipotizzando una pressione radiale di 400 MPa, orientativamente corrispondente ad una pressioneassiale di almeno 700 MPa. Nell’indagine sono stati considerati 8 valori del modulo, 3 valori del rapporto fraraggio di raccordo di testa e modulo, 4 altezze di matrice, 3 spessori di parete dei nuclei, 3 materiali per inuclei (acciaio rapido tradizionale, Vanadis 60 della Böhler-Uddeholm e metallo duro con 10% Co). L’analisiFEM ha evidenziato come un aumento del raggio di raccordo di testa consente una diminuzione dei livellimassimi di tensione nel nucleo atta a garantire il rispetto delle condizioni di sicurezza. L’interferenza relativafra nucleo e anello è la variabile di progetto cui spetta la necessità di attenta valutazione, connessa – secondoi risultati di un’analisi “Taguchi” - alla scelta del materiale del nucleo, specialmente per le matrici più alte. Èstata considerata anche l’influenza dello spessore relativo del nucleo, rapportato cioè al diametro esterno delladentatura. Le condizioni di sollecitazione degli anelli di cinturazione, per i quali è stato imposto un valore 4 delrapporto fra diametri, non implicano mai condizioni di insufficiente sicurezza delle matrici cinturate. Non siriportano i risultati dell’analisi FEM per moduli m = 4 e m = 5 poiché la verifica delle condizioni d’ingranamentoha fatto rilevare la necessità di modificare i profili dei denti, in varia misura, rispetto alle classiche evolventi.I risultati ottenuti possono portare a progetti più affidabili sia delle ruote dentate – da produrre mediantemetallurgia delle polveri – sia degli stampi di formatura

Coupled FEM-DBEM Simulation of 3D Crack Growth under Fatigue Load Spectrum

Abstract Numerical predictions of fatigue crack growth under load spectrum are obtained by coupled FEM-DBEM approach. An initial part-through corner crack, in a pre-notched specimen undergoing a traction fatigue load, propagates becoming through the thickness. A two parameter crack growth law ("Unified Approach") is calibrated by in house made constant amplitude experimental tests and the crack growth retardation after an overload application is reproduced. The residual stresses responsible for such retardation are calculated by a sequence of elastic-plastic static FEM analysis; such stresses are then applied to the crack faces for the propagation simulation in a DBEM environment. A satisfactory agreement between numerical and experimental crack growth rates are displayed, for both part-through crack and through the thickness crack. This approach provide general modeling capabilities, with allowance for general crack front shape and fully automatic propagation

Colorectal Cancer Stage at Diagnosis Before vs During the COVID-19 Pandemic in Italy

IMPORTANCE Delays in screening programs and the reluctance of patients to seek medical attention because of the outbreak of SARS-CoV-2 could be associated with the risk of more advanced colorectal cancers at diagnosis. OBJECTIVE To evaluate whether the SARS-CoV-2 pandemic was associated with more advanced oncologic stage and change in clinical presentation for patients with colorectal cancer. DESIGN, SETTING, AND PARTICIPANTS This retrospective, multicenter cohort study included all 17 938 adult patients who underwent surgery for colorectal cancer from March 1, 2020, to December 31, 2021 (pandemic period), and from January 1, 2018, to February 29, 2020 (prepandemic period), in 81 participating centers in Italy, including tertiary centers and community hospitals. Follow-up was 30 days from surgery. EXPOSURES Any type of surgical procedure for colorectal cancer, including explorative surgery, palliative procedures, and atypical or segmental resections. MAIN OUTCOMES AND MEASURES The primary outcome was advanced stage of colorectal cancer at diagnosis. Secondary outcomes were distant metastasis, T4 stage, aggressive biology (defined as cancer with at least 1 of the following characteristics: signet ring cells, mucinous tumor, budding, lymphovascular invasion, perineural invasion, and lymphangitis), stenotic lesion, emergency surgery, and palliative surgery. The independent association between the pandemic period and the outcomes was assessed using multivariate random-effects logistic regression, with hospital as the cluster variable. RESULTS A total of 17 938 patients (10 007 men [55.8%]; mean [SD] age, 70.6 [12.2] years) underwent surgery for colorectal cancer: 7796 (43.5%) during the pandemic period and 10 142 (56.5%) during the prepandemic period. Logistic regression indicated that the pandemic period was significantly associated with an increased rate of advanced-stage colorectal cancer (odds ratio [OR], 1.07; 95%CI, 1.01-1.13; P = .03), aggressive biology (OR, 1.32; 95%CI, 1.15-1.53; P < .001), and stenotic lesions (OR, 1.15; 95%CI, 1.01-1.31; P = .03). CONCLUSIONS AND RELEVANCE This cohort study suggests a significant association between the SARS-CoV-2 pandemic and the risk of a more advanced oncologic stage at diagnosis among patients undergoing surgery for colorectal cancer and might indicate a potential reduction of survival for these patients

Investigation of mode III fracture behaviour in bonded pultruded GFRP composite joints

In this work, a numerical-experimental characterization of pure mode III fracture toughness of bonded pultruded fibreglass composite joints is presented. Three types of specimen were considered, which differ for the fibres orientation of the bonded surfaces. The analysed angular orientations are: parallel (0 °–0 °), inclined (0 °–45 °), and perpendicular (0 °–90 °). The experimental investigation was realized by using the recently proposed torque shells test (TST). Numerical analyses, based on Finite Element Method (FEM), were performed in order to model a cohesive law of the adhesive interface. A minimization algorithm was also used for identification of cohesive zone model (CZM) parameters by comparing numerical and experimental outcomes. The results show how the TST provides with an almost pure mode III fracture loading condition when the adherends are non-isotropic materials. The comparison of experimental and numerical torque-angle curves, for every test case, shows a satisfactory agreement. As a result of the analyses, the influence of fibres direction on the mode III fracture behaviour of pultruded GFRP joints can be considered negligible for practical applications

DBEM Simulation of a FML Full Scale Aeronautic Panel Undergoing Biaxial Fatigue Load,

This paper concerns the numerical characterization of the fatigue strength of a flat stiffened panel, designed as a fibre metal laminates (FML) and made of Aluminium alloy and Fibre Glass FRP. The panel is full scale and was tested under fatigue biaxial loads, applied by means of a multi-axial fatigue machine: an initial through the thickness notch is created in the panel and the aforementioned biaxial fatigue load is applied, causing a crack initiation and propagation. The fatigue test is simulated by the Dual Boundary Element Method (DBEM) in a two-dimensional approach, followed by a 3D BEM sub-modelling analysis, needed to justify the assumptions on delamination area and fibre rupture made in the 2d crack propagation. Due to the lack of experimental data on the size of the increasing delamination area (varying as the crack propagates), the latter is assessed by the aforementioned DBEM sub-model simulation, considering the inter-laminar stresses and a delamination criteria. This approach aims at providing a general purpose evaluation tool for a better understanding of the fatigue resistance of FML panels, providing a deeper insight into the role of fibre stiffness and of delamination extension on the stress intensity factors. The experimental test was realized in the context of a European research project (DIALFAST)

A two-parameter model for crack growth simulation by combined FEM-DBEM approach

This paper describes the application of a two-parameters crack growth model, based on the usage of two threshold material parameters (DKth and Kmax,th) and on the allowance for residual stresses, introduced at the crack tip by a fatigue load spectrum or by material plastic deformations. The coupled usage of finite element method (FEM) and dual boundary element method (DBEM) is proposed in order to take advantage of the main capabilities of the two methods. The procedure is validated by comparison with available experimental results, in order to assess its capability to predict the retardation phenomena, introduced by a variable load spectrum or by a plastic deformation introduced with a tool on the panel (indentation). In particular two different tests are made: the first test involve a CT specimen undergoing a load spectrum and the second one involve a dented panel undergoing a constant amplitude fatigue load. In both cases a satisfactory numerical–experimental correlation will be proved. The main advantages of the aforementioned procedure are: the simplicity of the crack growth law calibration (few constant amplitude tests are sufficient without the need for any non-physical calibration parameters), and the possibility to simulate residual stress effects on crack propagation with a simplified approach, based on linear elastic fracture mechanics