The specific heat loss combined with the thermoelastic effect for an experimental analysis of the mean stress influence on axial fatigue of stainless steel plain specimens (original) (raw)
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Engineering Fracture Mechanics, 2012
Development of experimental equipments able to provide the whole temperature field have given impulse to energy related fatigue studies, since temperature is an indicator of the heat loss. In the present paper the specific heat dissipation is assumed to be a fatigue damage parameter. A simple equation and the associated experimental technique previously published by the authors to estimate the specific energy loss is first recalled. Then it is shown that the fatigue behaviour of a stainless steel material can be summarised by a log-log plot of the specific heat loss versus number of cycles characterised by a constant slope from few hundreds to two million cycles. Further it has been found that plain and notched fatigue data collapse into the same energy-based scatter band. Finally a comparison of the proposed approach with the classical local strain method is performed.
International Journal of Fatigue, 2015
Thermography represents an important tool to study fatigue behaviour of materials. In this work, the fatigue limit of martensitic and precipitation hardening stainless steels has been determined with thermographic methods. Despite their use in corrosive and cryogenic environments, there is a data lack in literature concerning the study of fatigue behaviour. The peculiarity of these materials is the brittle behaviour: therefore, during fatigue tests the characteristic small deformations determine small changes of temperature. Thus, to properly determine the fatigue limit of aforementioned stainless steels, a more accurate setup is necessary in order to correctly detect surface temperature of specimens due to dissipation heat sources. In literature, different procedures have already been proposed to evaluate the fatigue limit from thermal data but very few works lead to an early detection of dissipation process which can obtain a further reduction of overall testing time. The aim of the paper is to propose a new robust thermal data analysis procedure for estimating fatigue limit of stainless steels in automatable way.
Fatigue Behaviour of Stainless Steels: A Multi-parametric Approach
Conference proceedings of the Society for Experimental Mechanics, 2016
In recent years different experimental methods have been experienced to enhance the fatigue characterisation of materials with the aim to overcome the Standard long-lasting tests, i.e. Wohler curve determination. Standard fatigue treatment requires at least 15 specimens being tested to get an estimation of material fatigue limit and it is worth noting that this kind of tests do not provide any information on damage phenomena occurring in the material. Thus, topic to be addressed in this research have to do with development of lock-in infrared measurement based thermal method for rapid evaluation of fatigue limit. By performing a single test , the adopted method leads to match different parameter information. The Assessed parameters are in number more than the ones provided by TSA, as well. Moreover, the adopted technique points to study damage by analysing the different phenomena involved in fatigue and in this regard, the aim of this paper is to show how a thermal technique can attain an early assessment of the failure processes during a cyclically loading test. The author is, also, focused on to illustrate the strong points of a method based on infrared measurements for assessing endurance limit for both austenitic and martensitic stainless steels while considering, as reference, the Standard Test methods.
Comparison of Experimental Thermal Methods for the Fatigue Limit Evaluation of a Stainless Steel
Metals
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.
The specific heat loss combined with the thermoelastic effect for an experiment.PDF
The energy dissipated to the surroundings as heat in a unit volume of material per cycle, Q, was recently proposed by the authors as fatigue damage index and it was successfully applied to correlate fatigue data obtained by carrying out fully reversed stress-and strain-controlled fatigue tests on AISI 304L stainless steel plain and notched specimens. The use of the Q parameter to analyse the experimental results led to the definition of a scatter band having constant slope from the low-to the high-cycle fatigue regime. In this paper the energy approach is extended to analyse the influence of mean stress on the axial fatigue behaviour of unnotched cold drawn AISI 304L stainless steel bars. In view of this, stress controlled fatigue tests on plain specimens at different load ratios R (R=-1; R=0.1; R=0.5) were carried out. A new energy parameter is defined to account for the mean stress effect, which combines the specific heat loss Q and the relative temperature variation due to the thermoelastic effect corresponding to the achievement of the maximum stress level of the stress cycle. The new two-parameter approach was able to rationalise the mean stress effect observed experimentally. It is worth noting that the results found in the present contribution are meant to be specific for the material and testing condition investigated here.
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Energy analysis of fatigue damage by thermographic technique
2002
The main aims of this paper are to describe the thermographic methodologies currently used in Italy for the rapid evaluation of the fatigue limit and to describe the local energy approach actually under development by the authors. Thermographic methodologies currently used in Italy for the rapid evaluation of the fatigue limit were applied to two stainless steels (AISI 304 and AISI 409). All the experimental results here obtained are in good agreement with the respective values reported in literature. An experimental program for the local energy approach is under development: its main characteristic consists in doing, besides the usual thermal measurements made by thermography, mechanical measurements in order to evaluate the mechanical energy locally dissipated inside the material. This experimental research is part of an interuniversity research program and it is made on stainless steel (AISI 304) notched specimens.
Energy analysis of fatigue damage by thermographic technique
Thermosense XXIV, 2002
The main aims of this paper are to describe the thermographic methodologies currently used in Italy for the rapid evaluation of the fatigue limit and to describe the local energy approach actually under development by the authors. Thermographic methodologies currently used in Italy for the rapid evaluation of the fatigue limit were applied to two stainless steels (AISI 304 and AISI 409). All the experimental results here obtained are in good agreement with the respective values reported in literature. An experimental programme for the local energy approach is under development: its main characteristic consists in doing, besides the usual thermal measurements made by thermography, mechanical measurements in order to evaluate the mechanical energy locally dissipated inside the material. This experimental research is part of an interuniversity research programme and it is made on stainless steel (AISI 304) notched specimens.
International Journal of Fatigue, 2004
The fatigue of dual phase steel was examined in terms of calorimetric effects in order to match the energy manifestations of fatigue and constitutive equations drawn up in a thermomechanical framework. A simplified method, assuming a homogeneous fatigue test, is proposed to determine heat source development from a temperature field provided by an infrared camera. Thermoelastic and dissipative sources were then separately identified. Experimental results concerning thermoelastic effects are in close agreement with theoretical estimates. Dissipation depends on the loading frequency and stress amplitude applied to the fatigue specimen. However, as the marked decrease in dissipation observed when testing a block at high stress was not easily interpretable in terms of material effects, we questioned the homogeneous fatigue test assumption.
Analysis of the fatigue strength under two load levels of a stainless _Issue17Paper02.PDF
In questo lavoro è stato analizzato il comportamento a fatica di un acciaio inossidabile AISI 304L. Nella prima parte del lavoro sono presentati i risultati ottenuti da prove ad ampiezza di sollecitazione costante sintetizzati sia in ampiezza di tensione sia in termini di densità di energia dissipata dal materiale per ciclo, Q. Successivamente alcuni provini sono stati sollecitati ad un livello di carico superiore al limite di fatica per circa il 70% della presunta vita e poi ad un livello di tensione inferiore al limite di fatica ad ampiezza costante precedentemente determinato ed è stata confrontata l'energia dissipata nella seconda parte della prova con quella trovata in una prova ad ampiezza costante, allo stesso livello di tensione. Il confronto ha mostrato come per il materiale analizzato il parametro Q sia sensibile al danneggiamento precedentemente subito.