Modelling size effects for static strength of brittle materials

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VHCF Response of H13 Steels Produced with Different Manufacturing Processes

Experimental results have recently shown that failures at stress amplitude below the conventional fatigue limit and at Very High Cycle Fatigue (VHCF) are possible. In case of high-strength steels, VHCF failures generally originate from defects/inclusions present within the material and, consequently, the defect population properties (size and quantity) significantly affect the VHCF response of high-strength steels. Different refinement processes are commonly adopted in order to improve steel cleanliness: among the other, the ElectroSlag Remelting (ESR) process allows to obtain very clean high-strength steels, thus possibly inducing a significant enhancement of the VHCF response. The present paper aims at investigating the actual effect of the ESR process on the VHCF behavior of an AISI H13 steel. Fully reversed ultrasonic tension-compression tests are carried out on hourglass specimens manufactured with and without the ESR process. The estimated P-S-N curves highlight the positive effects of the ESR process on the VHCF response of the investigated H13 steel

VHCF response of AISI H13 steel: assessment of size effects through Gaussian specimens

The paper aims at assessing the influence of the Size-Effect (SE) parameter on the VHCF response of the AISI H13 steel. Ultrasonic tests were performed on Gaussian and hourglass specimens characterized by different risk-volumes (volume of material subjected to an almost uniform stress amplitude). Experimental results showed that a significant difference exists between the VHCF strengths of the investigated material obtained by using specimens with different risk-volumes

VHCF response of heat-treated SLM Ti6Al4V Gaussian specimens with large loaded volume

Abstract Among the materials used for the production of components through Additive Manufacturing (AM) processes, the Selective-Laser-Melting (SLM) Ti6Al4V alloy is widely employed in aerospace applications for its high specific strength and in biomedical applications for its good biocompatibility. Actual structural applications are generally limited to static loading conditions where the large defects originating during the SLM process do not play a significant role for the static failure. On the contrary, the same defects strongly affect the fatigue response of the parts since they act as crack initiation sites that rapidly lead to fatigue failure. In the literature, a lot of research has been carried out to investigate the quasi-static and the High-Cycle Fatigue properties of the SLM Ti6Al4V alloy but there are still few studies on its Very-High-Cycle Fatigue (VHCF) response. In the paper, the VHCF response of Ti6Al4V specimens, which are vertically orientated during the SLM building and then subjected to a conventional heat treatment (2 hours heating in vacuum at 850°C), is experimentally assessed. Ultrasonic VHCF tests are carried out on Gaussian specimens with a large risk-volume (2300 mm3). Fracture surfaces are investigated for revealing the defect originating the fatigue failure. The Stress Intensity Factor Threshold associated to the experimental failures is finally estimated

Comparison between dog-bone and Gaussian specimens for size effect evaluation in gigacycle fatigue

Gigacycle fatigue properties of materials are strongly affected by the specimen risk volume (volume of material subjected to a stress amplitude larger than the 90% of the maximum stress). Gigacycle fatigue tests, performed with ultrasonic fatigue testing machines, are commonly carried out by using hourglass shaped specimens with a small risk volume. The adoption of traditional dog-bone specimens allows for increasing the risk volume, even if the increment is quite limited. In order to obtain larger risk volumes, a new specimen shape is proposed (Gaussian specimen). The dog-bone and the Gaussian specimens are compared through Finite Element Analyses and the numerical results are validated experimentally by means of strain gages measurements. The range of applicability of the two different specimens in terms of available risk volume and stress concentration effects due to the cross section variation is determined

Duplex S-N fatigue curves: statistical distribution of the transition fatigue life

In recent years, very-high-cycle fatigue (VHCF) behavior of metallic materials has become a major point of interest for researchers and industries. The needs of specific industrial fields (aerospace, mechanical and energy industry) for structural components with increasingly large fatigue lives, up to 10 10 cycles (gigacycle fatigue), requested for a more detailed investigation on the experimental properties of materials in the VHCF regime. Gigacycle fatigue tests are commonly performed using resonance fatigue testing machines with a loading frequency of 20 kHz (ultrasonic tests). Experimental results showed that failure is due to cracks which nucleate at the specimen surface if the stress amplitude is above the conventional fatigue limit (surface nucleation) and that failure is generally due to cracks which nucleate from inclusions or internal defects (internal nucleation) when specimens are subjected to stress amplitudes below the conventional fatigue limit. Following the experimental evidence, the Authors recently proposed a new statistical model for the complete description of S- N curves both in the high-cycle-fatigue (HCF) and in the VHCF fatigue regions (Duplex S-N curves). The model differentiates between the two failure modes (surface and internal nucleation), according to the estimated distribution of the random transition stress (corresponding to the conventional fatigue limit). No assumption is made about the statistical distribution of the number of cycles at which the transition between surface and internal nucleation occurs (i.e., the transition fatigue life). In the present paper, the statistical distribution of the transition fatigue life is obtained, according to the statistical model proposed. The resulting distribution depends on the distance between the HCF and the VHCF regions and on the distribution of the random transition stress. The estimated distribution can be effectively used to predict, with a specified confidence level, the number of cycles for which an internal nucleation may probabilistically occur in a VHCF test and it is also informative for properly choosing the end of HCF tests in terms of number of cycles. A numerical example, based on experimental datasets taken from the literature, is provided

Adhesive stresses in axially-loaded tubular bonded joints - Part II: development of an explicit closed-form solution for the Lubkin and Reissner model

The literature presents several analytical models and solutions for single- and double-lap bonded joints, whilst the joint between circular tubes is less common. For this geometry the pioneering model is that of Lubkin and Reissner (Trans. ASME 78, 1956), in which the tubes are treated as cylindrical thin shells subjected to membrane and bending loading, whilst the adhesive transmits shear and peel stresses which are a function of the axial coordinate only. Such assumptions are consistent with those usually adopted for the flat joints. A former investigation has shown that the L-R model agrees with FE results for many geometries and gives far better results than other models appeared later in the literature. The aim of the present work is to obtain and present an explicit closed-form solution, not reported by Lubkin and Reissner, which is achieved by solving the governing equations by means of the Laplace transform. The correctness of the findings, assessed by the comparison with the tabular results of Lubkin and Reissner, and the features of this solution are commente

Conservative likelihood inference for type I censored samples from the log-location-scale distributions

A new method, theoretically justified, is proposed to overcome difficulties in analysing reliability data coming from Type I censored samples. The article shows that, by means of Monte Carlo simulations, it is possible to obtain quasi-exact likelihood estimator properties and conservative confidence intervals for log-location-scale distributions. In the case of the exponential distribution, comparisons with the exact estimator properties show that the Monte Carlo approach allows to calculate the properties with very good accuracy. Finally, for the exponential distribution it is demonstrated that, if the number of failures can only be different from zero, confidence intervals based on the asymptotic properties of the likelihood estimators may give statistically meaningless results in the case of small sample size (3-10) and low probability of failure (.05-.20)

GIUNTO TUBOLARE INCOLLATO: RICERCA DI UNA SOLUZIONE ESPLICITA

La letteratura tecnica fornisce numerosi modelli analitici e soluzioni per giunti piani incollati a singola o doppia sovrapposizione, in minore quantità per il giunto tra tubi cilindrici; per quest'ultima geometria il caso capostipite è il modello di Lubkin e Reissner (Trans. ASME 78, 1956). In esso i due tubi sono descritti come gusci sottili cilindrici in regime assialsimmetrico membranale e flessionale, mentre nell'adesivo agiscono le tensioni di taglio e pelatura, funzioni della sola coordinata assiale; tale impostazione è coerente con quella normalmente adottata per i giunti piani. Altri modelli successivamente presentati in letteratura non forniscono risposte migliori, al confronto con risultati FEM. L'obiettivo del presente lavoro è di ottenere una forma esplicita per la soluzione, non riportata da Lubkin e Reissner; questa è raggiunta risolvendo le equazioni che governano il problema mediante la trasformata di Laplac