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The use of components obtained through the additive manufacturing (AM) technique has become increasingly widespread in recent years, playing a central role in industrial production, and in particular in some fields such as automotive, biomedical, aerospace and electronics. Among all AM techniques, FDM (Fused Deposition Modelling) represents the most used printing technique to produce polymeric and composite components, thanks to the flexible printing process, the low cost and the diversity of the materials adopted. The aim of the present work concerns the comparison between the mechanical properties of three plastic materials printed with the FDM technique (polylactic acid PLA, polyethylene terephthalate glycol-modified PETG and Acrylonitrile-butadiene-styrene ABS) using an Original Prusa i3 MK3S, by varying the raster angle between 0°, 45° and 90° degrees. Infrared Thermography has been adopted to monitor the temperature evolution during static tensile tests and to assess stress level that can initiate damage within the material. Failure analysis was performed to correlate the mechanical behaviour with the microstructural characteristics of the materials.

The aim of this study is the evaluation of fatigue strength of glass-fibre-reinforced polypropylene composite (PPGF35) by applying both the Risitano Thermographic Method (RTM) and the new static Thermographic Method (STM). Fifteen tensile tests and twenty cyclic tests were carried out to evaluate the fatigue strength of PPGF35 by applying traditional and innovative energy methods. The results show how the energy methods used are very useful for rapid identification of the material’s fatigue strength, saving companies time and money.

In this paper thermal analysis was applied to determine the “Critical Stress” of concrete, different from its ultimate strength, able to produce the first damage in the structures under compressive loads. The Critical Stress can be thought as the stress able to produce the beginning of fatigue rupture within the material. Several specimens of high strength concrete were tested in order to define the incipient crack phenomena, also in internal part of the specimen not accessible by direct inspections, with the aid of infrared thermography. A finite element analysis completes the study and compares, for the same static loading conditions, the stress state with the experimental thermographic images. The final results show as the coupling of normal compressive test and the acquisition of the thermal images can be a useful aid to estimate a security stress value, indeed the Critical Stress, before the Ultimate Serviceability Limit (SLU) of the structure, defined as the maximum load condition before its failure.

Economical composite materials such as PA66GF35 (glass-fibre-reinforced polyamide matrix) are being increasingly used in the automotive industry. Their good mechanical characteristics combined with low density, very high workability, low production cost and high availability are attractive prerogatives that induce engineers to adopt it in complex technological challenges. Injection moulding is the most common production technology used to realize composite components. While in industrial design this type of material is considered as isotropic, it is well known that injection moulding process gives orientation to the reinforcing fibres, leading to anisotropic mechanical behaviour. Starting from these considerations, attention has been turned to the vibrating properties of such materials and to the comparison between vibration modes and mechanical properties. In fact, composite materials are also used to produce components in the automotive field significantly affected by noise problems. Since the noise derives from a fluid-structure interaction, the own frequencies and the vibrating modes cover an important role on the Noise, Vibration and Harshness performance of the components. A comparison of the vibration modes of a plate in PA66GF35, numerically modeled both as isotropic and anisotropic material according to Folgar and Tucker theory, was carried out and compared with experimental measurements. The anisotropy of the composite material is demonstrated by the variation of the mechanical characteristics obtained from the static tensile tests. Results show that injection moulding confers different mechanical properties to real components due to the intrinsic fluid-dynamic phenomena of the production process.

This work evaluates the limit stress of the thermo-elastic phase of deformation by thermo-analysing the surface of a specimen during a static traction test. By adding a temperature curve measured over a small area of the surface to the classical stress–strain curve, it is possible to evaluate a limit temperature T0 that is coincident with the beginning of the curve's nonlinear trend. The corresponding stress value is as estimation of the fatigue limit of the component under analysis. The authors derive an expression to evaluate temperature during a mono-axial static traction test. As an example, temperature curves recorded during traction tests performed on two notched steel specimens are reported and compared with this proposed expression. The change to the linearity in the temperature curve during the static traction test is evident in these examples and the corresponding stress value is an estimation of the fatigue limit of the component under analysis.

The traditional methods of fatigue assessment of metallic and composite materials are extremely time consuming. The thermographic measurements during static tests can be used to predict the fatigue limit. This procedure has been already applied to composite materials. The aim of this study is the application of this procedure for the fatigue assessment of steel welded joints. Static tensile and fatigue tests were carried out on butt welded specimens, made of S355 steel. Two full-field techniques were applied during the tests: digital image correlation and infrared thermography techniques. Fatigue tests at increasing loads were performed by a stepwise succession in order to apply the Thermographic Method. The value of the fatigue limit, obtained by the traditional procedure, was compared with the values predicted by means of the Thermographic Method during stepwise succession fatigue tests and by means of the infrared thermography technique during static tensile tests. The predicted values are in good agreement with the experimental values of fatigue life.

Estimation of the fatigue limit for steel ductile materials using non-destructive methods is a topic of great interest to researchers today. In recent years, the method adopted has implemented infrared sensors to detect the surface temperature and correlate it with the fatigue limit. In previous paper, a new energy approach was proposed to investigate the fatigue limit during tensile test. The numerical procedure proposed by Chrysochoos is adopted to clean infrared images and applied to analyse the surface heat sources during tensile test. AISI 304 specimens with rectangular cross-sections are tested. Moreover fatigue tests at increasing loads were carried out on steel by a stepwise succession, applied to the same specimen, for applying the thermographic method. The predictions of the fatigue limit, obtained by the analysis of the energy evolution during the static tests, were compared with the predictions obtained applying the thermographic method during fatigue tests.

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Following the first experimental results concerning the possibility to estimate the fatigue limit by semi-static tensile traction test, the authors are continuing the experimental thermo-analysis of the curve zone between the origin and the yield stress; that is rich of indication about the fatigue failure process. The tests carried out on the AISI 304 steel by thermal analysis, confirmed the good approximation between the values of the fatigue limit estimated by fatigue test and by the quasi-static test. The results shoed the importance of the thermo-analysis of the part of the stress-strain curve (distant from the yield point) where only in macroscopic terms the behavior is perfectly elastic.

In this work is indicated how it could be possible to evaluate the limit stress of the thermo-elastic phase of deformation by thermo-analysing the surface of the specimen during a static traction test. Adding the temperature curve measured on a small area of the surface (the hottest) to the classic stress-strain curve, it is possible to evaluate a limit temperature T0 coincident with the beginning of the non linear trend of the curve. The corresponding stress value is coincident with the fatigue limit of the analyzed component. As an example, the results of traction tests performed on two notched specimens, where the change of linearity in the temperature curve during static traction test was evident, are reported.

Following previous results showing that under static loads it is possible to detect the first plasticization of the specimens at the end of the thermoelastic phase, the authors have conducted experimental trials to verify that this effect can be pointed out in notched and unnotched polyvinyl chloride (PVC) specimens. The goal is to define the real elastic phase also for materials for which the elastic limit and the yield stress are not easily defined, different from the case of steel. The results show a variable thermal behaviour depending on the distance from the notch. The thermal behaviour, proportional to the stress in the totally elastic phase, accordingly with the thermoelastic effect, deviating from the linearity, points out the beginning of the local plasticization. The thermoelastic limit, moving from the notch edge to the specimen boundary, allows to follow the paths of plasticization. The results are also compared with those found by cyclic loading using the thermographic methodology already verified by the authors and other researchers.

Le esperienze maturate nel campo dell’analisi termica di materiali utilizzati nelle costruzioni meccaniche [1,2,3] hanno permesso di evidenziare come il rilievo della temperatura (terzo parametro) in prove statiche e dinamiche costituisca un indicatore molto importante ai fini della caratterizzazione dinamica del materiale. Essendo il rilascio termico funzione dell’energia applicata per portare a rottura il materiale, il rilievo di parametri ad esso legati, induce a nuovi ipotesi e definizioni di limiti di fatica e resistenza a tempo. Mediante l’analisi termica è possibile valutare anche parametri correlabili con il valore limite di energia a rottura Eldel materiale. In [4] era stato già messo in evidenza da A. Risitano e Altri che, in prove statiche di trazione, l’inizio della zona di prima plasticizzazione del materiale, in termine di tensione, era osservabile dalla curva di variazione di temperatura ΔT con il procedere della prova. Nello stesso lavoro si evidenziava come la velocità di prova avesse poca influenza sui valori della variazione di temperatura specialmente durante la fase elastica. Operando con sensori sempre più precisi e per obbiettivi rivolti alla ricerca dell’energia limite a rottura è stato osservato dagli autori che il seguire la variazione della temperatura sulla superficie del provino, in prove statiche di trazione, permette di legare i classici valori di resistenza all’oscillazione σ0 con una “temperatura limite” T0 corrispondente all’inizio di andamenti non lineari della stessa. In questa sede si evidenzia un modello di comportamento fisico del materiale durante le prova di trazione che giustifica, in modo semplice, la capacità di risalire, attraverso la conoscenza sperimentale del limite di comportamento termo-elastico, ai classici parametri di resistenza a fatica. Viene riportato, a titolo di esempio, il risultato relativo a provini piatti forati in acciaio facenti parte di una serie utilizzati per altri scopi (formeranno oggetto di altra pubblicazione) con i quali anche mediante prova statica si è determinata la loro resistenza all’oscillazione.

The mechanical properties of injection moulded short glass fibre-reinforced polyamide composites was investigated using experimental techniques. The Digital Image Correlation technique was applied during the tensile tests in order to obtain the stress–strain curves and to identify the fracture location at the early stages of the tests. Moreover, the thermographic technique was used during the static tests in order to identify the fracture zone and also during the fatigue tests, carried out at different frequencies, to study the temperature evolution of the specimen. A theoretical model was developed to analyze the temperature evolution during the fatigue test and the effect of the test frequency. The aim of this study is the application of the Thermographic Method for the fatigue assessment of short glass fibre-reinforced polyamide composites.

Nowadays, many components and structures are subjected to fatigue loading with a number of cycles higher than 107. In this scientific work, the behaviour of two kinds of tool steel was investigated in very high cycle fatigue regime. The fatigue tests were carried out at the frequency of 20 kHz and in fully reversed tensioncompression mode (R = -1) by means of an ultrasonic fatigue testing equipment. The radiometric surface temperature was detected during all the test by means of an IR camera in order to extend the Thermographic Method and the Energetic Approach in very high cycle fatigue regime. The failure mechanism of the investigated steels was evaluated by means of several experimental techniques (scanning electron microscopy, Energy Dispersive X-ray spectroscopy and Optical Microscopy).

The quickly identify of fatigue limit of a mechanical component with good approximation is currently a significant practical problem not yet resolved in a satisfactory way. Generally, for a mechanical component, the fatigue strength reduction factor ( ? i) is difficult to evaluate especially when it is in service. In this paper, the procedures for crack paths individuation and consequently damage evaluation (adopted in laboratory for stressed specimens with planned load histories) are applied to mechanical components, already failed during service. The energy parameters, proposed by the authors for the evaluation of the fatigue behavior of the materials [1-5], are defined on specimens derived from a flange bolts. The flange connecting pipes at high temperature and pressure. Due to the loss of the seal, the bolts have been subjected to a hot flow steam addition to the normal stress. The numerical analysis coupled experimental analysis (measurement of surface temperature during static and dynamic tests of specimens taken from damaged tie rods), has helped to determine the causes of failure of the tie rods. The determination of an energy parameter for the evaluation of the damage showed that factors related to the heat release of the material (loaded) may also help to understand the causes of failure of mechanical components.

Partendo dalla osservazione che provini precedentemente danneggiati hanno un limite di fatica differente (vedi Miner) viene esaminato il comportamento di un acciaio soggetto a serie di carichi affaticanti al fine di verificare la conseguente risposta termica. L’esame, mediante analisi termica, delle prove di fatica su provini di acciaio C40 danneggiati a diverso grado, da interessanti indicazioni sul ruolo dei carichi applicati in relazione alla possibilita di produrre danno nel materiale. Viene messo in evidenza come la temperatura, conseguenza dell’energia consumata dal provino, possa essere elemento indicativo dello stato del materiale. Vengono riportate le curve di fatica e le curve di temperatura di provini lisci sottoposti a storie di carico diverse. Da esse si puo evincere che mentre e valida la legge di linearita del danno (?ini/Ni= cost), la curva definita secondo la regola del Miner-Manson (?ini/Ni=1), che tiene solo conto del numero di volte in cui la tensione supera il limite di fatica del materiale (provini lisci), invece, non sempre rispecchia il reale stato di danno. Questo specialmente quando si e in presenza di carichi prossimi al limite di fatica che, per la sequenza dell’applicazione, possono diventare carichi affaticanti.

This work reports fatigue stress test results for AISI 304 steel ( R = −1) that has been subjected to a high cycle number. It shows that the energetic effect (employing different temperature increments for equal loading of uniaxial stress) can be used to evaluate the cumulative damage caused by any prior loading. Various load histories were applied to AISI 304 specimens, and the resulting damage was evaluated in reference to the energy factor, Φ , which is dependent on the maximum temperature at the sample surface when irreversible plastic deformation begins. The time curves (Wohler curves) and the damage ratio based on the consumed energy were compared to those obtained according to Palmgren–Miner rules; this comparison served to show how under-evaluations can occur with high damage ratios.