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.

This paper regards the rapid determination of fatigue limit by using thermal data analysis. Different approaches available in the literature to estimate the fatigue limit of cold-drawn AISI 304L bars are analyzed and compared, namely, temperature- and energy-based methods. Among the temperature-based approaches, the Risitano Method (RM) and the method based on material temperature evolution recorded during a static tensile test were analyzed. Regarding the energy-based approaches, the input mechanical energy density stored in the material per cycle (i.e., the area of the hysteresis loop), the heat energy dissipated by the material to the surroundings per cycle, and the 2nd-harmonic-based methods were considered. It was found that for the material analyzed, all the considered energy-based approaches provided a very good engineering estimation of the material fatigue limit compared to a staircase test.

In this work, more specimens were tested at equal conditions (static compression test) and, during the tests, the released heat for irreversible phenomena was monitored by means of the analysis of the temperature surface of the specimen. In this way, it was possible to estimate the average value of the macroscopic stress ("critical stress") for which local micro cracks begin. The compression static tests (load - machine time) and the related thermal analysis (temperature-machine time) of spots located on the specimen face of the specimens, highlighted the possibility to estimate the value of the compressive load to which there was loss of linearity in the temperature - machine time diagram (Delta t-t). This effect is due to internal heat generated for irreversible phenomena (internal micro fractures). The results show that the "critical stress" has values practically coincident for the points (spots) located in different zones (center or corners of the specimens' surface). Copyright (C) 2017 The Authors. Published by Elsevier B.V.

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.

The energetic methods to determine the fatigue limit, proposed by various researchers in the last decades, are based on the evidence that fatigue is an energy dissipation process, and most of the dissipated energy is converted into heat, which manifests itself in the form of temperature change. The study of fatigue behaviour through the determination of the heat quantity released during the fatigue tests, and the analysis of the temperature variation of material under fatigue loading, by mean of thermography-based methods, constitutes a well-established approach.Reaching very high temperature in the specimen is a critical aspect of this phenomenon of conversion of the dissipated energy into heat, particularly with regards to materials characterized by high thermal releases, that could be subject to microstructural changing and behaviour decay.This effect can be exalted by the use of ultrasonic fatigue testing systems for the characterization of very high cycle fatigue behaviour of material.To highlight the influence of the fatigue test frequency on the specimen temperature that already occurs at low frequencies, in this paper a study of the thermal behaviour of AISI 304 stressed by fatigue loads in high cycle fatigue testing at different frequency values, based on energetic method to determine the fatigue curve, is presented. (C) 2016 Elsevier Ltd. All rights reserved.

In this paper are specifically derived parameters useful to estimate the fatigue behaviour of concrete subject to uniaxial compression. For this, methodologies and experience already adopted in the study of fatigue steel and composite materials are used. These parameters are obtained by detecting the surface temperature of the specimen in the traditional static compression tests. In this way, the beginning of the crisis of the concrete for fatigue stress is linked to the loss of linearity of the temperature test time curve (Delta T-t) and correlated to stress-test time curve (sigma r-t) of the tested cubic concrete specimens. In fact, the thermal analysis performed on the cubic specimen surface extended to the whole test time, shows interesting data on the crack beginning and on the subsequent evolution that after a certain number of loading cycles could determine the complete material failure. The slope variation in the interpolating curve temperature-test time allows to identify the critical points of the start fracture. This suggests a methodology to apply to civil infrastructures to evaluate in-situ, during the approval phase or during the working, critical situations.In this paper we propose a method to estimate the value of the "stress limit" (fatigue limit) of concrete material by means of an easy static uniaxial compression test according to an energetic method already proposed by Risitano. Copyright (C) 2016 The Authors. Published by Elsevier B.V.

A historical review was carried out, in order to point out the phases leading to investigate the fatigue resistance of materials and mechanical components, using quick and direct methodologies. Starting from researches performed in the early 1980s, the progresses in the research conducted by the Catania group and other investigators, significantly contributing to the increasing interest in the thermographic analysis of the fatigue phenomenon and, more specifically, to the experimental evaluation of the energy release under fatigue loading, were described.

The evaluation of the temperature rising during a fatigue test is extremely important to define the energy method to be used. In order to define the best load program and the more appropriate test frequency, it is necessary to estimate the surface temperature of the material, in particular when it's needed to arrange some tests on mechanical components. With regards to the energetic approach to the study of fatigue phenomena, in the past the internal damping factor was considered an important parameter to define the fatigue quality of the material used for the mechanical application. In the present paperthe role of another important work parameter ishighlighted, when linked to the plastic strain caused by the high number of fatigue tests. An approach to the detection of first microplasticization and crack beginning is also proposed, with a model for the estimation of the crack propagation during the last cycles of fatigue test.

Static traction tests on material samples for mechanical constructions have pointed out the loss of linearity of the specimen surface temperature with the applied load. This phenomenon is due to the heat generation caused by the local microplasticizations which carry the material to deviate from its behavior, perfectly thermoelastic. The identification of the static load which determines the loss of linearity under the temperature stress becomes extremely important to define an initial dynamic characterization of the material. The temperature variations that can be read during the static loads applications are often very limited (a few tenths of degree for every 100 MPa in steels) and they require the use of special temperature sensors able to measure the temperature variations. The experience acquired in such analysis highlighted that, dealing with highly accurate sensors or with particular materials, the identification of the first loss of linearity can be influenced by the investigator himself mainly for the above mentioned limited temperature variations which can lead to incorrect estimations, sometimes really significant. Checking the validity and the above mentioned observations on the different steels, this work proposes the application of the autocorrelation function to the data collected during the application of a static load in order to make the results of the thermal analysis free from the sensitivity of the operator and also to make the result as objective as possible in order to detect the time of the loss of linearity of the temperature-time function.

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 (Wöhler 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.

ABSTRACT 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 T 0 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 paper presents a study performed on steel specimens under static loading using thermal infrared techniques to determine whether thermal analysis can also provide information on the fatigue behavior. In particular, the traditional static elastic zone was investigated. The results highlight that, even in the macroscopically elastic phase, a variation from the theoretical behavior is also detectable using thermographic techniques. In some previous works the authors demonstrated the possibility to estimate the fatigue limit of the material by mean of static tensile tests. In this paper, the procedure is adopted to analyze the effect of the cross-head speed on the thermoelastic limit, confirming the possibility to define the fatigue limit as the end of the totally linear thermoelastic behavior.

According to Miner’s rule, the fatigue life of a material (or a mechanical component) is not a function of the order of the application of load. Many authors have already observed that the model proposed by Palmgren/Miner leads to underestimation of the damage and one of the reasons could be the disregard of the sequence of loads. Referring to the energy loss related to irreversible damage of the material, it was observed that the Miner’s rule gives exact results only if the damage caused by the fatiguing load is low compared to the limit energy of the material. Whereas, if the damage caused by the fatiguing load is high, Miner’s rule is no longer valid. In this work, according to previous observations reported in other authors’ works, tensile fatigue tests on AISI304 steel specimens were performed. The load stories consisted of loading ramps in subsequent blocks. For each group of specimens the same blocks were applied but with inverted sequence. The surface temperature, recorded during application of loads, has been chosen as a benchmark of the damage.

Following the first experimental results concerning the possibility to estimate the fatigue limit by quasi-static traction test, the authors give a physical explanation about the process of the failure of a specimen. It was put in evidence the link between the complete thermo-elastic phase under traction stress and the beginning of the punctual plastic deformation in the zone of the classic stress strain curve yet distant from the yield limit of the material. 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. KEYWORDS. fatigue, plastic energy, infrared thermography, mechanical characterization of steel.

In this paper a proposal for a control system that provides liquid cargo movement from one side to another is made. The system acts to minimise the effects of lateral load transfer and fluid oscillation. This consists of a main tank, two lateral tanks, regulators and a set of electropumps effecting the movement of the fluid. According to literature, in order to assess the risk of rollover, a normalised lateral load transfer has been used. Simulations of the lateral load transfer response have also been carried out to assess the improvements in lateral stability of the vehicle.

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.

In base all’esperienza maturata in anni di sperimentazione sull’analisi delle caratteristiche meccaniche dei materiali mediante indagine termografica, nel presente lavoro è proposta una procedura per il controllo di qualità di componenti meccanici in linea di produzione, che è già stata argomento di brevetto. Lo sviluppo di questo lavoro si colloca nell’ambito del progetto FIRB “Sistemi di produzione intelligenti, flessibili e riconfigurabili”. L’attività svolta dal DIIM riguarda il controllo avanzato dell’affidabilità di componenti meccanici per l’industria automobilistica. Viene proposta, quindi, la realizzazione di una cella di controllo in linea di produzione capace di valutare la presenza di eventuali componenti difettosi attraverso l’analisi termica degli stessi, sollecitati secondo un modello predefinito. L’attività, svolta in questa prima fase in laboratorio, è facilmente trasferibile in linea di produzione, considerando la possibilità di realizzare celle di prova in ambiente controllato, con condizioni praticamente identiche a quelle di laboratorio, eliminando gli effetti di disturbo che possono influenzare la risposta dell’indagine termografica in ambiente non strutturato.

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.