VALUTAZIONE DEL DANNO DA IMPATTO IN PANNELLI SANDWICH CON LA TECNICA TERMOGRAFICA

Oggetto di questo studio sono dei pannelli sandwich per applicazioni automobilistiche, costituiti da pelli esterne in acciaio separate da poliolefina. Lo scopo del lavoro è lo studio del loro comportamento quando sono soggetti a impatti a bassa velocità. In particolare, si vogliono valutare le modalità e l’entità del danneggiamento subito, al variare della geometria e delle caratteristiche meccaniche dei componenti dei sandwich. Il lavoro è distinto in una parte sperimentale e in una numerica. Quella sperimentale consiste nella valutazione del danneggiamento, tramite la tecnica termografica, dei pannelli sottoposti a impatto. La seconda parte, numerica, sviluppa dei modelli a elementi finiti che simulano i pannelli durante la fase di impatto e le successive prove di trazione. Questi modelli numerici permettono di determinare parametri direttamente confrontabili con le grandezze termografiche

Sheep in Aesop’s and Phaedrus’s fables

Sheep feature in various animal fables. Marino & Merskin suggest that “we” view sheep as “docile, passive, unintelligent, and timid,” but animal fables do not support this view. In Aesop’s and Phaedrus’s fables, sheep are a primary target of injustice; but they are not passive targets. Sheep endure injustice actively and honestly. They are intelligent, aware and outspoken about their own condition

thermographic applications for the rapid estimation of fatigue limit

Abstract The AIAS group studying Energetic Methods for Experimental Analysis, MEAS, is performing round robin experimental tests for the rapid determination of fatigue limit on steels by different thermographic techniques. This work is part of the project and describes the experimental activity performed at Politecnico di Milano, based on stepwise cyclic tests. Thermograms are processed in terms of: 1) amplitude of the first order harmonic, in-phase with respect to the loading signal; 2) amplitude of the second harmonic, out-of-phase; 3) slope of the thermal signal with respect to the number of cycles. These values typically show bilinear trends, allowing to define a breakup point and a corresponding stress which is the thermographic estimation of the fatigue limit. The paper presents and discusses the results of tests with different stress ratios, i.e. fully reversed cycling with R=-1 and tensile-tensile cycling with R=0.1

On the influence of T-Stress on photoelastic analysis under pure mode II loading

According to the classical definition for in-plane modes of crack deformation, the constant stress term T exists only in the presence of mode I. However, recent studies show that this term can exist in mode II conditions as well, and significantly affect the elastic stress field around the crack tip. These effects can be visualized using the experimental method of photoelasticity. Based on the analytical studies, presence of the T-stress in mode II cracks transforms the isochromatic fringe patterns from symmetric closed loops to asymmetric and discontinuous shapes. In this paper, presence of the T-stress in mode II cracks and its effects on the fringe patterns is experimentally investigated. The test specimens are Brazilian disks containing very sharp central cracks: experimental results indicate that these specimens contain negative values of T-stress. Experimental values are then compared to numerical results. To better understand the differences between experimental and numerical values, a thee dimensional analysis is performed with the finite element method: results show the influence of the real geometry of the crack front on the stress intensity factors

REVISIONE DEI MODELLI NUMERICI ALLA MACROSCALA PER LO STUDIO DEL FENOMENO DI INFRAGILIMENTO DA IDROGENO

Il fenomeno di infragilimento da idrogeno, noto da diversi anni, rimane un argomento di grande interesse scientifico. Di recente, in letteratura l’attenzione si è spostata prevalentemente sullo sviluppo di modelli numerici in grado di riprodurre il fenomeno o di chiarire e/o interpretare alcuni aspetti che lo caratterizzano. Diverse sono le scale dimensionali considerate: scala atomistica, nano, meso, macroscala. I modelli sviluppati alla macroscala sono prevalentemente modelli ad elementi finiti di tipo coesivo il cui scopo è la valutazione della resistenza meccanica di un materiale sotto effetto combinato di sollecitazione meccanica applicata e ambiente aggressivo, in particolare ricco di idrogeno atomico. Il presente lavoro propone una revisione dei modelli numerici alla macroscala presenti in letteratura al fine di riassumere lo stato dell’arte, di valutare pro e contro dei modelli presenti e individuare quali possano essere gli sviluppi futuri per l’ottimizzazione di un approccio numerico a questa scala

A review of thermographic techniques for damage investigation in composites

The aim of this work is a review of scientific results in the literature, related to the application of thermographic techniques to composite materials. Thermography is the analysis of the surface temperature of a body by infrared rays detection via a thermal-camera. The use of this technique is mainly based on the modification of the surface temperature of a material, when it is stimulated by means of a thermal or mechanical external source. The presence of defects, in fact, induces a localized variation in its temperature distribution and, then, the measured values of the surface temperature can be used to localize and evaluate the dimensions and the evolution of defects. In the past, many applications of thermography were proposed on homogeneous materials, but only recently this technique has also been extended to composites. In this work several applications of thermography to fibres reinforced plastics are presented. Thermographic measurements are performed on the surface of the specimens, while undergoing static and dynamic tensile loading. The joint analysis of thermal and mechanical data allows one to assess the damage evolution and to study the damage phenomenon from both mechanical and energetic viewpoints. In particular, one of the main issues is to obtain information about the fatigue behaviour of composite materials, by following an approach successfully applied to homogenous materials. This approach is based on the application of infrared thermography on specimens subjected to static or stepwise dynamic loadings and on the definition of a damage stress, D, that is correlated to the fatigue strength of the material. A wide series of experimental fatigue tests has been carried out to verify if the value of the damage stress, D, is correlated with the fatigue strength of the material. The agreement between the different values is good, showing the reliability of the presented thermographic techniques, to the study of composite damage and their fatigue behaviour

Conformorality. A Study on Group Conditioning of Normative Judgment.

How does other people’s opinion affect judgments of norm transgressions? In our study, we used a modification of the famous Asch paradigm (1951, 1955) to examine conformity in the moral domain. The question we addressed was how peer group opinion alters normative judgments of scenarios involving violations of moral, social, and decency norms. The results indicate that even moral norms are subject to conformity, especially in situations with a high degree of social presence. Interestingly, the degree of conformity can distinguish between different types of norms