numerical and experimental validation of a non standard specimen for uniaxial tensile test

Abstract Material testing is a fundamental activity for the characterization of materials mechanical properties and for the certification of product quality. With concepts such as Smart Factories and Industry 4.0 coming to the fore, testing and measurement is moving away from laboratories and closer to the production floor: MaCh 3D is a miniaturized tensile testing machine developed for products and materials certification directly on the production site. The heart of the technology is the specimen with non-conventional geometry for tensile tests developed so as to be easily installed on the machine. The objective of this work is to illustrate the process of determining the geometry of specimen and fixtures by numerical analysis and their experimental validation, comparing the results with those obtained from specimens according to ASTM D638 standard, ASTM International, (1999)

fracture toughness of structural adhesives for the automotive industry

Abstract Adhesive bonding is currently employed by automotive manufacturers to complement (or replace) welding in joining dissimilar materials. In order to reduce the impact on the existing manufacturing infrastructures, structural adhesives are deployed in the body shop but hardening is accomplished in the paint cure oven. Various adhesive formulations have been specifically developed for the implementation in the automotive manufacturing chain. However, it is very important to assess the mechanical behaviour of the joints which results from the peculiar curing strategy. In the present work, automotive grade single component epoxy and two component epoxy modified acrylic adhesives were evaluated. T-joints were fabricated using a cold rolled galvanized steel (FeP04) employed in the production of car body parts. The fracture toughness of the joints was determined using the test protocol proposed by the European Structural Integrity Society (ESIS). Optical microscopy was employed to ascertain the mechanisms of failure. The results indicated that both adhesives were able to provide a fairly good mechanical response with minimum preparation of the mating substrates. Moreover, the obtained values of fracture toughness were shown to be essentially independent of the adhesive layer thickness

Studio sull'influenza delle configurazioni di processo per l'ablazione laser sulla morfologia superficiale e il comportamento meccanico di giunti incollati in alluminio

Scopo della tesi è lo studio dell’effetto dell’ablazione laser utilizzando un laser a impulsi a fibra di itterbio per pre-trattare superfici in lega di alluminio AA6082-T6 destinate ad essere incollate tra loro. Varie configurazioni di processo del laser sono esaminate, variando la potenza e la velocità di scansione, la cui combinazione è espressa dall’indice rappresentativo della densità di energia, nonché modificando la distanza nominale tra i solchi e la direzione di scansione del laser. Le modifiche indotte dal laser sul materiale sottoposto ad ablazione sono valutate sia in termini di stato della superficie che in quelli di risposta meccanica di giunti incollati. In particolare, la morfologia superficiale è valutata misurando la rugosità superficiale e il primo coefficiente di “skewness” di Pearson. Le modifiche chimiche sono valutate con misure EDS, da cui appare evidente un aumento di uno spesso strato di ossido all’aumentare della densità di energia. Infine, si effettuano misure di angolo di contatto da cui si rivela che la bagnabilità della superficie è totale quando la densità di energia è al di sopra di un certo valore. Il comportamento meccanico è valutato tramite il valore critico del rateo di rilascio di energia di deformazione di Modo I calcolato sulla base di una campagna di prove quasi-statiche su giunti “Double Cantilever Beam” (DCB) i cui substrati sono preventivamente trattati con il laser. La tenacità a frattura cresce con la densità di energia fino a raggiungere un massimo, dopo il quale un ulteriore aumento della densità di energia ha l’effetto di abbassare la tenacità del giunto. È possibile diversificare i risultati a seconda del valore di distanza tra i solchi quando tale grandezza è maggiore del diametro nominale dell’impronta del laser, mentre, relativamente all’effetto della direzione di scansione del laser, i risultati di tenacità a frattura sono simili sia per i giunti trattati unidirezionalmente sia per i giunti con texture incrociata. Ciò è essenzialmente dovuto alla presenza di aria rimasta intrappolata dentro i solchi quando la densità di energia è sufficientemente alta, il che consente di trovare una correlazione diretta, validata statisticamente, tra la tenacità a frattura e il coefficiente di “skewness”, che sembra essere un buon indicatore del fenomeno di intrappolamento dell’aria. Si testa anche la propagazione di cricca a fatica, che consente di classificare le configurazioni di processo del laser in base all’andamento della velocità di crescita. L’esito di tale classificazione appare coerente con i risultati dei test quasi-statici. Infine, si applica un ciclo di invecchiamento accelerato in controllo di temperatura e umidità relativa ad alcuni provini DCB che vengono poi sottoposti a test quasi-statici. Anche se si osserva un generale abbassamento di tenacità nei giunti invecchiati rispetto a quelli non condizionati, tale peggioramento è più marcato nei provini sabbiati, suggerendo l’esistenza di un beneficio insito nell’utilizzare alcune configurazioni di processo per ablazione laser al posto dei metodi tradizionali di trattamento meccanico, in presenza di condizioni ambientali critiche.In this thesis, the effect of a pulsed-mode Yb-fiber laser ablation as a pre-treatment for AA6082-T6 aluminium alloy surfaces for adhesively bonding purposes is studied. Several laser configurations are tested, by varying the power and the scan speed, whose combination is summarized in an energy density representative index, as well as the hatch distance and the laser scanning direction. The laser induced changing over the ablated material is evaluated both in terms of the surface status and of the mechanical response of bonded joints. In particular, the surface morphology is assessed by measuring the surface roughness and the Pearson's first coefficient of skewness. The chemical modifications are evaluated through EDS measurements, by which an increase of the thick oxide layer with the energy density appears evident. Finally, contact angle measurements reveal that the wettability of the surface is complete when the energy density is risen beyond a specific value. The mechanical behavior is assessed by computing the critical value of the Mode I strain energy release rate with a campaign of quasi-static tests over laser pre-treated Double Cantilever Beam (DCB) joints. The fracture toughness increases with the energy density until a maximum, after which the tendency reverses. A diversification according to the hatch distance value is possible when it is higher than the nominal spot diameter, while, with respect to the laser scanning direction, the fracture toughness results similar both in the unidirectional and in the crossed textured joints. This is essentially due to the presence of air remained entrapped within the grooves when the energy density is quite high, which allows to find a direct correlation, statistically validated, between the fracture toughness and the coefficient of skewness, which appears to be a good indicator for the air entrapment phenomenon. Also the fatigue crack growth is tested on the same geometry, allowing a ranking of the laser configurations according to the crack growth rate which appears consistent with the results of the quasi-static tests. Finally, an accelerated aging cycle in control of temperature and relative humidity is applied to some specimens which then undergo quasi-static DCB tests. Even though a general lowering of the toughness is recorded on the aged joints with respect to the unconditioned ones, it is more marked in the grit blasted specimens, suggesting a benefit in using some specific laser ablation configurations instead of the traditional mechanical pre-treatment methods in presence of some critical environmental conditions

Experimental Study of the Influence of the Surface Preparation on the Fatigue Behavior of Polyamide Single Lap Joints

The low quality of adhesion performance on polymeric surfaces has forced the development of specific pretreatments able to toughen the interface between substrate and adhesive. Among these methods, atmospheric pressure plasma treatment (APPT) appears particularly suitable for its environmental compatibility and its effectiveness in altering the chemical state of the surface. In this work, an experimental study on adhesively bonded joints realized using polyamide as substrates and polyurethane as the structural adhesive was carried out with the intent to characterize their fatigue behavior, which represents a key issue of such joints during their working life. The single lap joint (SLJ) geometry was chosen and several surface pretreatments were compared with each other: degreasing, abrasion (alone and followed by APPT) and finally APPT. The results show that the abrasion combined with APPT presents the most promising behavior, which appears consistent with the higher percentage of life spent for crack propagation found by means of DIC on this class of joints with respect to the others. APPT alone confers a good fatigue resistance with respect to the simple abrasion, especially at a low number of cycles to failure

Effects of Mediterranean Diet Combined with CrossFit Training on Trained Adults' Performance and Body Composition

CrossFit is a high-intensity training discipline increasingly practiced in recent years. Specific nutritional approaches are usually recommended to maximize performance and improve body composition in high-intensity training regimens; notwithstanding, to date there are no targeted nutritional recommendations for CrossFit athletes. The Mediterranean Diet (MD) is a diet approach with a well-designed proportion of macronutrients, using only available/seasonal food of the Mediterranean area, whose health benefits are well demonstrated. No studies have evaluated this dietary strategy among CrossFit athletes and practitioners; for this reason, we tested the effects of 8 weeks of MD on CrossFit athletes' performance and body composition. Participants were assigned to two groups: a diet group (DG) in which participants performed CrossFit training plus MD, and a control group (CG) in which participants partook in the CrossFit training, continuing their habitual diet. Participants were tested before and after the 8 weeks of intervention. At the end of the study, no significant difference was noted in participants' body composition, whereas improvements in anaerobic power, explosive strength of the lower limbs, and CrossFit-specific performance were observed only in the DG. Our results suggest that adopting a MD in CrossFit athletes/practitioners could be a useful strategy to improve specific strength, endurance, and anaerobic capacity while maintaining overall body composition