investigation of the crack tip stress field in a stainless steel sent specimen by means of thermoelastic stress analysis

Abstract In this work a Thermoelastic Stress Analysis (TSA) setup is implemented to investigates the Thermoelastic and Second Harmonic signals on a fatigue loaded Single Edge Notched Tension (SENT) specimen made of stainless steel AISI 304L. Three load ratios are in particular applied, R=-1, 0, 0.1. The thermoelastic signal is used to evaluate the Stress Intensity Factor via two approaches, the Stanley-Chan linear interpolation method and the over-deterministic least-square fitting (LSF) method using the Williams' series expansion. Regarding least-square fitting, an iterative procedure is proposed to identify the optimal crack tip position in the thermoelastic maps. The SIF and T-Stress are then evaluated considering the influence of the number of terms (up to 20) in the Williams' series function, and the extent and position of the area used for data input. The study also investigates the Second Harmonic signal observed on the wake of the crack with varying load ratio R. An interpretation is proposed that considers the rise of the Second Harmonic as the result of the modulation of the compression loads between the crack flanks, rather than dissipation phenomena. This interpretation enables the possibility to use this parameter to reveal the presence and extent of crack-closure

Evaluating the specific heat loss in severely notched stainless steel specimens for fatigue strength analyses

Abstract In the last years, a large amount of fatigue test results from plain and bluntly notched specimens made of AISI 304L stainless steel were synthetized in a single scatter band adopting the specific heat loss per cycle (Q) as a damage parameter. During a fatigue test, the Q parameter can be evaluated measuring the cooling gradient at a point of the specimens after having suddenly stopped the fatigue test. This measurement can be done by using thermocouples in the case of plain or notched material; however, due to the high stress concentration at the tip of severely notched components analysed in the present paper, an infrared camera achieving a much improved spatial resolution was adopted. A data processing technique is presented to investigate the heat energy distribution close to the notch tip of hot-rolled AISI 304L stainless steel specimens, having notch tip radii equal to 3, 1 and 0.5 mm and subjected to constant amplitude cyclic loads

correlation among energy based fatigue curves and fatigue design approaches

Abstract In this paper, with reference to the strain controlled fatigue characterization of AISI 304L stainless steel, the correlations between plain material fatigue curves based on different definitions of the strain energy densities, namely the elastic, plastic and elastoplastic strain energy densities evaluated under the cyclic stress-strain curve and the plastic strain hysteresis energy density (per cycle and total at fracture) are investigated. On this basis, a diagram showing the link among the different energy-based fatigue curves is proposed and is applied to find the correlation between plain material strain energy density fatigue curves and some fatigue strength assessment methods for notched structural components, namely the one based on the experimental evaluation of the heat energy dissipated by the material per cycle and the one based on the evaluation of the linear elastic strain energy density, averaged in a properly defined structural volume

The Heat Energy Dissipated in a Control Volume to Correlate the Fatigue Strength of Bluntly and Severely Notched Stainless Steel Specimens

Abstract In previously published papers by the authors, the specific heat energy loss per cycle (the Q parameter) was used to rationalize about 120 experimental results generated from constant amplitude, push-pull, stress- or strain-controlled fatigue tests on plain and notched hot rolled AISI 304 L stainless steel specimens as well as from cold drawn un-notched bars of the same steel, tested under fully-reversed axial or torsional fatigue loadings. It has been shown that Q can be estimated starting from the cooling gradient measured at the critical point immediately after the fatigue test has been stopped. Concerning notched specimens, it was noted that a 3 mm notch tip radius was close to the limitation of applicability of the adopted temperature sensor, consisting in 0.127-mm-diameter thermocouples, because of the 1.5-to-2 mm diameter spot of the glue which prevented to measure the maximum temperature level. In this paper, the fatigue-damage-index effectiveness of Q parameter was investigated, carrying out fully reversed axial fatigue tests on 4-mm-thick AISI 304L specimens, having 3, 1 and 0.5 mm notch tip radii. The cooling gradients were measured by using an infrared camera, characterized by a 20 μm/pixels spatial resolution. As a result, all new fatigue data could be rationalized using the same scatter band published previously by the authors

analysis and comparison of some lefm parameters

Abstract This paper presents and analyses a possible extension of the well-known mean Strain Energy Density approach, proposed and developed by Paolo Lazzarin for the strength characterization and for the structural analysis of sharp notches. The new parameter, that here will be defined and discussed only for the case of a crack subjected to mode I loading conditions, will be shown to be able to characterize the superficial energy per unity of area due to the presence of a crack in a plate. Then it can be considered to be an Intensity Factor, in analogy to the Stress Intensity Factor KI. For this reason it will be called the Strain Energy Density Intensity Factor (SEDIF). Aim of the introduction of this new approach is to simplify both the characterization of the material and the structural analysis of the components, since the proposed parameter does not depend on the strength of an un-notched specimen taken as reference and does not need the evaluation of the radius R0 of the area to be considered for the evaluation of SED. Two in some way similar parameters (the J integral and the S factor proposed by Sih) will be discussed and compared to the proposed Strain Energy Density Intensity Factor

Uniform scatter bands to analyse the fatigue strength of welded joints

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the peak stress method applied to bi material corners

Abstract Bi-material interfaces are unavoidably present in many engineering applications, such as microelectronics, adhesive joints, fiber-reinforced composites and thermal barrier coatings. Under the hypothesis of linear elastic material behaviour, the local stress field at the point located at the free-edge of the bi-material interface has a singular behaviour, of which the intensity can be quantified by a generalized stress intensity factor, H. However, the numerical evaluation of H usually requires very accurate meshes and large computational efforts, hampering the use of H-based criteria in the engineering practice. The main aim of the present work is to overcome this limitation by extending to isotropic bi-material corners the Peak Stress Method (PSM), first proposed by Meneghetti and co-workers to estimate the stress intensity factor at the tip of a geometrical singular point with relatively coarse mesh patterns

Analysis of dissipated energy and temperature fields at severe notches of AISI 304L stainless steel specimens

In the last years, a large amount of fatigue test results from plain and bluntly notched specimens made of AISI 304L stainless steel were synthetized in a single scatter band by adopting the specific heat loss per cycle (Q) as a damage parameter. During a fatigue test, the Q parameter can be evaluated measuring the cooling gradient at a point of the specimens after having suddenly stopped the fatigue test. This measurement can be done by using thermocouples; however, due to the high stress concentration at the tip of severely notched components analysed in the present paper, an infrared camera achieving a much improved spatial resolution was adopted. A data processing technique is presented to investigate the heat energy distribution close to the notch tip of hot-rolled AISI 304L stainless steel specimens, having notch tip radii equal to 3, 1 and 0.5 mm and subjected to constant amplitude cyclic loads. A thermal finite element analysis was also performed by assigning heat generation in the appropriate region close to the notch tip. Then the numerical temperature values were compared with the experimental measurement

analysis of dissipated energy and temperature fields at severe notches of aisi 304l stainless steel specimens

In the last years, a large amount of fatigue test results from plain and bluntly notched specimens made of AISI 304L stainless steel were synthetized in a single scatter band by adopting the specific heat loss per cycle (Q) as a damage parameter. During a fatigue test, the Q parameter can be evaluated measuring the cooling gradient at a point of the specimens after having suddenly stopped the fatigue test. This measurement can be done by using thermocouples; however, due to the high stress concentration at the tip of severely notched components analysed in the present paper, an infrared camera achieving a much improved spatial resolution was adopted. A data processing technique is presented to investigate the heat energy distribution close to the notch tip of hot-rolled AISI 304L stainless steel specimens, having notch tip radii equal to 3, 1 and 0.5 mm and subjected to constant amplitude cyclic loads. A thermal finite element analysis was also performed by assigning heat generation in the appropriate region close to the notch tip. Then the numerical temperature values were compared with the experimental measurement

On relation between J-integral and heat energy dissipation at the crack tip in stainless steel specimens

In this paper, an experimental procedure to evaluate the elastic-plastic J-integral at the tip of a fatigue crack is presented. According to this new approach, the elastic component of the J-integral is derived from Thermoelastic Stress Analysis, while the plastic component of the J-integral is derived from the heat energy loss. An analytical link is proposed to apply this new experimental technique. Therefore, the elastic-plastic J-integral range was evaluated starting from infrared temperature maps measured in situ during crack propagation tests of AISI 304L stainless steel specimens. It was found that the range of the infrared thermography-based J-integral correlated well the crack growth data generated in small as well as large scale yielding conditions. Finally, the experimental values of the J-integral were successfully compared with the corresponding numerical values obtained from elastic-plastic finite element analyses