Numerical and experimental assessment of the static behavior of 3D printed reticular Al structures produced by Selective Laser Melting: progressive damage and failure

Abstract In recent decades, the interest of the manufacturing industry towards additive manufacturing techniques has increased considerably. Speed and ease of implementation are just some of the factors that helped making this type of production one of the most developed in the world, considering also the possibility of creating complex geometries. The present research uses of a series of Al A357 specimens produced by SLM method. The experimental measurements on a first geometry have been used to calibrate the ductile damage model implemented in the FE code. The material model is based on both classical incremental model of plastic response with isotropic hardening and phenomenological concept of damage in continuum mechanic. The result of the calibration process was verified through the comparison of FE simulation of reticular specimens with the measured experimental response. Comparison between experimental data and numerical results will be discussed

Numerical simulations of normal and oblique impact on single and double-layered aluminium Al6061-T6 plates

Studies of ballistic penetration into metal plates and their numerical simulation currently present an important topic in ballistics, however, no congruent results have been presented so far, especially when it comes to impacts on multi-layered plates. Presently, as far as ballistic limits are concerned, the choice between layered and monolithic structures is not completely straightforward and unproblematic. The effect of introducing air gaps between metallic layers is not fully understood and explained either. Furthermore, these issues are more investigated for normal impacts than for oblique impacts for which only limited results are available. Therefore, the aim of this paper is to conduct a numerical analysis in order to evaluate the effect on the ballistic limit on layered targets for both normal and oblique impacts. A validated numerical methodology will be used, though validated with a limited number of experiments. The target material is an Al6061-T6 aluminium alloy the mechanical behaviour of which (hardening, strain rate, failure, etc.) is already known and described. Several configurations will be numerically tested and the results critically evaluated

Thermally activated magnetization reversal in bulk BiFe0.5Mn0.5O3

We report on the synthesis and characterization of BiFe0.5Mn0.5O3, a potential type-I multiferroic compound displaying temperature induced magnetization reversal. Bulk samples were obtained by means of solid state reaction carried out under the application of hydrostatic pressure at 6 GPa and 1100{\deg}C. The crystal structure is an highly distorted perovskite with no cation order on the B site, where, besides a complex scheme of tilt and rotations of the TM-O6 octahedra, large off-centering of the bismuth ions is detected. Below T1 = 420 K the compound undergoes a first weak ferromagnetic transition related to the ordering of iron rich clusters. At lower temperatures (just below RT) two distinct thermally activated mechanisms are superimposed, inducing at first an enhancement of the magnetization at T2 = 288 K, then a spontaneous reversal process centered at T3 = 250 K, finally giving rise to a negative response. The application of fields higher than 1500 Oe suppresses the process, yielding a ferromagnetic like behaviour. The complementary use of SQuID magnetometry and M\"ossbauer spectroscopy allowed the interpretation of the overall magnetic behaviour in terms of an uncompensated weak competitive coupling between non-equivalent clusters of interactions characterized by different critical temperatures and resultant magnetizations. PACS numbers: 75.85.+t, 75.60.Jk, 76.80.+y, 75.30.Et, 75.30.KzComment: 30 pages, 13 figure

Strain wave acquisition by a fiber optic coherent sensor for impact monitoring

A novel fiber optic sensing technology for high frequency dynamics detection is proposed in this paper, specifically tailored for structural health monitoring applications based on strain wave analysis, for both passive impact identification and active Lamb wave monitoring. The sensing solution relies on a fiber optic-based interferometric architecture associated to an innovative coherent detection scheme, which retrieves in a completely passive way the high-frequency phase information of the received optical signal. The sensing fiber can be arranged into different layouts, depending on the requirement of the specific application, in order to enhance the sensor sensitivity while still ensuring a limited gauge length if punctual measures are required. For active Lamb wave monitoring, this results in a sensing fiber arranged in multiple loops glued on an aluminum thin panel in order to increase the phase signal only in correspondence to the sensing points of interest. Instead, for passive impact identification, the required sensitivity is guaranteed by simply exploiting a longer gauge length glued to the structure. The fiber optic coherent (FOC) sensor is exploited to detect the strain waves emitted by a piezoelectric transducer placed on the aluminum panel or generated by an impulse hammer, respectively. The FOC sensor measurements have been compared with both a numerical model based on Finite Elements and traditional piezoelectric sensors, confirming a good agreement between experimental and simulated results for both active and passive impact monitoring scenarios

Predicting the spatio-temporal dynamics of Popillia japonica populations

We developed a reaction–diffusion model to describe the spatio-temporal dynamics of the Japanese beetle (Popillia japonica Newman), based on adult abundance data collected by the Regional Phytosanitary Service during the monitoring activities in the infested area in Lombardy (northern Italy), from 2015 to 2021. The model simulated the abundance of the pest, with a discrete time step of one year along linear trajectories departing from an initial point of establishment. The model allowed the determination of the rate of expansion (i.e. the speed at which the leading edge of a population wave moves over time) of the pest along 13 different trajectories, ranging from 4.5 to 13.8 km/y, with a mean value of 8.2 km/y. Finally, we developed a land suitability index that summarises the effect of land use on the trajectory-specific rate of expansion of P. japonica. Specifically, the model revealed an increase in the rate of expansion of 260 m per year for each additional percentage point in the land suitability index. The model presented and the knowledge acquired in this work represent an important step forward in the comprehension of P. japonica population dynamics, and they represent important elements for the development of a decision support tool for pest risk managers to design and implement scientifically driven management actions

Analytical Model to Describe Damage in CFRP Specimen When Subjected to Low Velocity Impacts

Abstract In the light of an increment of the safety of CFRP components subjected to low-velocity impacts, the identification of a damage onset threshold is desired. Hence the suitability of an analytical model for the estimation of the critical load of delamination onset and the resulting delaminated area, as well as for the approximation of the load-displacement curve, has been investigated. The Olsson's analytical model, available in the literature, is considered and applied in this study for the prediction of the mechanical behaviour of composite specimens subject to low-velocity impacts. Comparisons with experimental results have been carried out to demonstrate the accuracy of the presented model. Impact tests were performed in accordance with ASTM D7136 standard and damage was assessed by means of ultrasonic testing and computed tomography

Simulazione della diapausa e della fenologia del coleottero Giapponese, Popillia japonica

Il coleottero giapponese (Popillia japonica Newman) è una specie polifaga e invasiva originaria del nord-est asiatico. Fin dalla sua prima individuazione negli Stati Uniti nel 1916, la specie è stata in grado di invadere vaste aree del nord America ed alcune aree del Canada meridionale. Negli anni '70, la P. japonica è stata introdotta nelle Azzorre e dal 2014 la specie si è insediata in Europa continentale (Italia). Dal 2017, in Svizzera sono stati segnalati alcuni avvistamenti della specie. In Italia, la P. japonica è distribuita lungo la Valle del Ticino al confine tra Lombardia e Piemonte e, dalla sua prima individuazione, l’area di infestazione è incrementata nel tempo. La specie è considerata un organismo da quarantena e quindi il Servizio Fitosanitario Nazionale Italiano e i due Servizi Fitosanitari Regionali stanno attuando misure volte al monitoraggio e al contenimento di P. japonica. Il controllo delle popolazioni di P. japonica può essere supportato dall’uso di modelli in grado di predire le tempistiche di emergenza degli stadi vitali suscettibili (ad esempio, larve e adulti). In questo lavoro, presentiamo un modello meccanicistico per simulare l’influenza della temperatura sull’inizio dei voli e sulla curva fenologica degli adulti di P. japonica. Il modello è stato calibrato e validato utilizzando serie temporali relative alla cattura di individui adulti tramite trappole a feromoni. I dati sono stati raccolti dal Servizio Fitosanitario della Regione Lombardia (Italia). Il modello è in grado di simulare realisticamente l’influenza della temperatura del suolo sui pattern di sviluppo e sulle strategie dei cicli di vita della specie. L’applicazione del modello a scala locale (ad esempio, simulazioni a livello puntuale) possono supportare i decisori (agricoltori, cooperative agricole ecc.) nella pianificazione e implementazione di azioni volte al monitoraggio e al controllo delle popolazioni basate sulle tempistiche di emergenza stimate dal modello. Il modello può essere inoltre applicato su scala regionale per ottenere mappe fenologiche che permettano di identificare le aree dove è prevista una emergenza anticipata della specie. Questa informazione può supportare i decisori che agiscono a livello regionale (ad esempio Servizi Fitosanitari Regionali) nella prioritizzazione delle aree di intervento, nell’implementazione di sistemi di allerta e nell’implementazione di piani di gestione della specie su scala regionale. Finanziamento: Il Progetto GESPO è Finanziato da “Direzione Generale Agricoltura - Regione Lombardia - D.d.s. 28 marzo 2018 - n. 4403D.g.r. n. X/7353 14 novembre 2017”