On specimen design for size effect evaluation in ultrasonic gigacycle fatigue testing

Literature datasets showed that gigacycle fatigue properties of materials may be affected by the specimen risk-volume, i.e., the part of the specimen subjected to applied stress amplitudes above a prescribed percentage of the maximum applied stress amplitude. The paper proposes a Gaussian specimen shape able to attain large risk-volumes for gigacycle fatigue tests, together with a general procedure for its design: wave propagation equations are analytically solved in order to obtain a specimen shape characterised by a uniform stress distribution on an extended length and, as a consequence, by a larger risk-volume. The uniformity of the stress distribution in the Gaussian specimen is numerically verified through a finite element analysis and experimentally validated by means of strain gauge measurement

VHCF response of AM materials: A literature review

The present paper is an overview of the recent experimental results on the Very High Cycle Fatigue (VHCF) response of materials produced through additive manufacturing (AM) processes. At present, VHCF tests have been carried out on AlSi12, AlSi10Mg, and Ti6Al4V specimens produced through AM processes (SLM and EBM). The VHCF strength and the main findings provided in the literature are analyzed in the paper, focusing on the main factors that affect the fatigue response

Cidofovir for BK Virus-Associated Hemorrhagic Cystitis: A Retrospective Study

Background.BK virus-associated hemorrhagic cystitis (BKV-HC) is a severe complication after allogeneic hematopoietic stem cell transplantation (HSCT), but antiviral treatment for this condition has not been evaluated. Methods.We conducted a retrospective survey on the safety and outcome of cidofovir treatment for patients with BKV-HC in centers affiliated with the European Group for Blood and Marrow Transplantation. Results.From 1 April 2004 to 31 December 2007, 62 patients received a diagnosis of BKV-HC after a median interval of 35 days after HSCT (range, 3-577 days). Fifty-seven patients (92%) received intravenous cidofovir, whereas 5 patients received cidofovir intravesically. Complete response (CR) was recorded in 38 (67%) of 57 patients with HC treated with intravenous cidofovir, whereas partial response (PR) was documented in 7 patients (12%). CR was documented in 3 patients and PR in 1 patient with HC treated with intravesical cidofovir. A reduction of 1-3 logs in BKV load was documented in 8 of the 10 patients achieving CR. Mild-to-moderate toxic effects were recorded in 18 of 57 patients who received intravenous cidofovir administration. In a multivariate analysis, the factors significantly associated with response to cidofovir were the stem cell source (P=.01) and the use of total body irradiation (P=.03). After a median follow-up of 287 days, overall survival and total treatment-related mortality rates were 63% and 40% for patients achieving CR, compared with 14% and 72% for patients with PR or no response to cidofovir, respectively (P<.001 and P=.001, respectively). Conclusions.Cidofovir may be a potentially effective therapy for BKV-HC, but evidence supporting its use requires randomized controlled trial

Experimental and Numerical Investigation of a Lattice Structure for Energy Absorption: Application to the Design of an Automotive Crash Absorber

In this work, an experimental and numerical analysis of a lattice structure for energy absorption was carried out. The goal was to identify the most influencing parameters of the unit cell on the crushing performances of the structure, thus guiding the design of energy absorbers. Two full factorial plans of compression tests on cubic specimens of carbon nylon produced by fused deposition modeling (FDM) were performed. The factors were the beam diameter and the number of unit cells. In the first factorial plan, the specimen volume is constant and the dimensions of the unit cell are varied, while the second factorial plan assumes a constant size of the unit cell and the volume changes in accordance with their number. The results showed that the specific energy absorption increases with the diameter of the beam and decreases with the size of the unit cell. Based on these results, a crash absorber for the segment C vehicle was designed and compared with the standard component of the vehicle made of steel. In addition to a mass reduction of 25%, the improved crushing performances of the lattice structure are shown by the very smooth force-displacement curve with limited peaks and valleys

Modelling size effects for static strength of brittle materials

The paper proposes a new model for the assessment of size effects affecting the fracture strength of brittle materials. The proposed model permits to accurately estimate the relation between the specimen strength, the initial defect size and to take into account the strength variation with respect to the tested volume. The proposed methodology is analytically defined and thereafter validated with the literature data obtained through tests on different types of brittle materials, and on specimens with increasing volume. A simple procedure for parameter estimation is also defined in the paper. The literature validation proves the effectiveness of the proposed methodology, with the resulting fitting models in well agreement with the experimental dataset and characterized by high values of coefficients of correlation, similar or larger than those obtained in the literature with different approaches

An innovative nondestructive technique for the local assessment of residual elastic properties in laminated composites

In this work, an innovative experimental methodology is presented for the assessment of damage severity in composites. The technique aims at determining the local variation of the elastic properties in the damaged region of a composite component. Based on the Impulse Excitation Technique (IET), the vibrational response of the inspected region is isolated by clamping its extremities through vacuum, thus allowing the assessment of local variations. Complementarily, a new analytical approach is derived for the assessment of the residual elastic properties of the damaged area from the measurement of the first resonant frequency. Validation of the proposed methodology is performed on two glass-fibre woven fabric composites, damaged by impact. The material properties of the damaged zone determined through the proposed technique are compared to the results of tensile tests performed on specimens cut from the impacted plates. In particular, the specimens are equipped with optic fibre in order to punctually measure the elastic parameters. Results show that the residual elastic properties assessed with the proposed technique are in very good agreement with those measured through the optic fibre, thus proving the effectiveness of the methodology

Machine learning methods to predict the fatigue life of selectively laser melted Ti6Al4V components

The aim of the present paper is to predict the fatigue life of Selectively Laser Melted (SLMed) Ti6Al4V components via the process parameters, the thermal treatments, the surface treatments and the stress amplitude, adopting machine learning techniques to reduce the cost of further fatigue testing, and to deliver better predictive fatigue designs. The studies resulted in reliable algorithms capable of predicting trustful fatigue curves. The methods have been trained with experimental data available in the literature and validated on testing sets to assess the extrapolation limits and to compare the different methods. The behavior of the networks has also been mapped by varying one SLM process parameter at the time, highlighting how each one affects the life

Blunt notch effect on the fatigue response up to 10^9 cycles of selective laser melting Ti6Al4V specimens

In this paper, the influence of a blunt notch on the VHCF response of SLM Ti6Al4V specimens is investigated. Ultrasonic fully reversed tension-compression tests up to 10(9) cycles were carried out on unnotched specimens and specimens with a blunt notch. Unnotched specimens show a slightly larger fatigue response, with limited differences. All fatigue failures originated from defects, which are bigger in unnotched specimens, mainly due to the different risk volume of the tested specimens and the related size effect. Interactions between notch, stress gradient, and defect size distribution must be considered to properly assess the influence of notch on the VHCF response

A new methodology for thermostructural topology optimization: Analytical definition and validation

In the last few years, the rapid diffusion of components produced through additive manufacturing processes has boosted the research on design methodologies based on topology optimization algorithms. Structural topology optimization is largely employed since it permits to minimize the component weight and maximize its stiffness and, accordingly, optimize its resistance under structural loads. On the other hand, thermal topology optimization has been less investigated, even if in many applications, such as turbine blades, engines, heat exchangers, thermal loads have a crucial impact. Currently, structural and thermal optimizations are mainly considered separately, despite the fact that they are both present and coupled in components in service condition. In the present paper, a novel methodology capable of defining the optimized structure under simultaneous thermomechanical constraints is proposed. The mathematical formulation behind the optimization algorithm is reported. The proposed methodology is finally validated on literature benchmarks and on a real component, confirming that it permits to define the topology, which presents the maximized thermal and mechanical performance