225 research outputs found
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
Gaussian Specimens for Gigacycle Fatigue Tests: Damping Effects
Experimental tests investigating the gigacycle fatigue properties of materials are commonly performed with ultrasonic testing procedures, which allow for a significant reduction of testing time but induce a significant temperature increment in specimens. In order to evaluate the significance of size effects on the fatigue strength of materials, the Authors recently proposed to adopt Gaussian specimens for gigacycle tests. Fatigue specimens were designed without taking into account the hysteretic damping and its effects both on the stress distribution and on the heat power dissipation. However, in order to evaluate the temperature increment and the feasibility of ultrasonic fatigue tests with Gaussian specimens, the total dissipated heat power as well as the distribution of the dissipated heat power density along the specimen must be taken into account. The present paper proposes an analytical model validated through a finite element analysis, which permits to evaluate the effects of the hysteretic damping on the stress distribution, on the dissipated heat power density distribution and on the total dissipated heat power in Gaussian specimens
VHCF strength decrement in large H13 steel specimens subjected to ESR process
Failures at very high number of cycles (Very-High-Cycle Fatigue, VHCF) generally originate from inclusions or defects present within the material. VHCF response of materials is therefore strongly affected by the defect population and, in particular, by the characteristic defect size, which statistically increases with the material volume. According to this well-known dependency, Size Effects were found to significantly affect the VHCF strength of high-strength steels.
The paper aims at assessing the influence of Size Effects on the VHCF response of a high performance AISI H13 steel subjected to Electro Slag Remelting (ESR) refinement process. Ultrasonic VHCF tests were carried on specimens characterized by different loaded volumes (hourglass and Gaussian specimens). Experimental results showed that Size Effects strongly influences the VHCF response of the investigated high performance steel, even if it is characterized by a high degree of purity and by a population of inclusions with limited size
A general model for crack growth from initial defect in Very-High-Cycle Fatigue
It is well-known in the literature that internal defects play a major role in the Very-High-Cycle Fatigue (VHCF) response of metallic materials. Generally, VHCF failures nucleate from internal defects characterized by a limited size. Unexpectedly, it has been found that cracks can grow from the initial defect even if the Stress Intensity Factor (SIF) is quite below the characteristic threshold for crack growth. Even though researchers unanimously accept this singular experimental evidence, they still dispute about its physical justification. Different micromechanical explanations have been proposed in the literature: local grain refinement, carbide decohesion, matrix fragmentation, hydrogen embrittlement, numerous cyclic pressure and formation of persistent slip bands are the most famous proposals. Regardless of the specific micromechanical explanation, it is generally acknowledged that a weakening mechanism occurs around the initial defect, thus permitting crack growth below the SIF threshold.
The present paper proposes an innovative approach for the quantitative modeling of the weakening process around the initial defect. The proposed model considers an additional SIF that reduces the SIF threshold of the material. Starting from a very general formulation for the additional SIF, possible scenarios for crack growth from the initial defect are also identified and described. It is theoretically demonstrated that, depending on the scenario, a VHCF limit may also be present and its final formulation recalls the well-known expression previously proposed by Murakami
Effect of defect size on P-S-N curves in Very-High-Cycle Fatigue
It is well-known that internal defects play a key role in the Very-High-Cycle Fatigue (VHCF) response of metallic materials. VHCF failures generally nucleate from internal defects, whose size strongly affects the material strength and life. Therefore, S-N curves in the VHCF regime are defect size dependent and the scatter of fatigue data is significantly influenced by the statistical distribution of the defect size within the material.
The present paper proposes an innovative approach for the statistical modeling of Probabilistic-S-N (P-S-N) curves in the VHCF regime. The proposed model considers conditional P-S-N curves that depend on a specific value of the initial defect size. From the statistical distribution of the initial defect size, marginal P-S-N curves are estimated and the effect of the risk-volume on the VHCF response is also modeled. Finally, the paper reports a numerical example that quantitatively illustrates the concepts of conditional and marginal P-S-N curves and that shows the effect of the risk-volume on the VHCF response
Crack growth from internal defects and related size-effect in VHCF
It is well-known in the literature that fatigue cracks in VHCF originate from small internal defects. More than 95% of the total VHCF life is consumed in originating the so-called Fine Granular Area (FGA) around the small initial defect. Within the FGA, crack growth takes place even if the Stress Intensity Factor (SIF) is smaller than the threshold value for crack growth. Researchers proposed different explanations for this unexpected phenomenon but they unanimously accept that a weakening mechanism occurs around the initial defect, which permits crack growth below the SIF threshold.
In the present paper, crack growth in the VHCF regime is innovatively modeled and a general expression for the fatigue limit is then obtained. The statistical distribution of the fatigue limit is also defined and a model for the fatigue limit as a function of the risk-volume is proposed. Finally, the proposed model is successfully applied to an experimental dataset
Effect of friction on a crashworthiness test of flat composite plates
The diffusion of fiber reinforced plastics in crashworthiness applications is continuously growing thanks to the
excellent balance between high mechanical performances and low weight, resulting in most cases in a Specific
Energy Absorption (SEA) of composite structures higher than that of the corresponding metallic structures. In
this paper, a new fixture to test composite plates applying an in-plane load has been used to investigate the effect
of the impact velocity and of the friction caused by the fixture on the SEA of carbon fiber reinforced epoxy plates.
The tests have been carried out using a drop tower testing machine and the effect of the friction has been studied
varying the clamping force given by the fixture. Splaying is the main failure mechanism found in the specimens
during the tests; SEA values (43.6 kJ/kg in average) increase with the clamping force due to the higher friction
level induced by higher clamping force; impact velocity does not significantly influence the results. To avoid an
overestimation of the SEA due to the excessive friction force (+5.6% when the clamping force increases from 0.8
kN to 8 kN), a Polytetrafluoroethylene (PTFE) coating has been applied to the anti-buckling supports to reduce
the friction. The effect of this modification has been studied by carrying out a new test in which the specimen
slides between the anti-buckling supports with a given clamping force. A significant reduction (-48% with same
clamping force) of the friction force is obtained when the lubricant is applied
Taxonomy, biology and natural history of the Milos viper Macrovipera schweizeri (Werner, 1935): literature review and observations on autumnal activity and importance of residual pools
An innovative fixture for testing the crashworthiness of composite materials
Despite the growing diffusion of composite materials in
automotive and aerospace sectors, a standard procedure for testing their
crashworthiness has not been developed yet. At present, the international
standards for testing composite materials under impact conditions are not
adequate to test their crush behavior.
In this paper, a procedure for measuring the energy absorption due to the
compressive crushing of a composite flat specimen along its mid plane is
proposed. The experimental setup requires a fixture to hold the specimen and
to avoid its buckling and an instrumented drop weight tower to obtain the
force-displacement curves with the aim of calculating the Specific Energy
Absorption.
The paper describes the adopted test procedure and some of the features of
the newly developed experimental setup. The effectiveness of the procedure
is demonstrated by testing several glass fiber-epoxy specimens under different
impact energies
Cross-Linked Hyaluronan Derivatives in the Delivery of Phycocyanin
An easy and viable crosslinking technology, based on the “click-chemistry” reaction
copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (click-crosslinking), was applied to graft
copolymers of medium molecular weight (i.e., 270 kDa) hyaluronic acid (HA) grafted with ferulic acid
(FA) residues bearing clickable propargyl groups, as well as caffeic acid derivatives bearing azidoterminated
oligo(ethylene glycol) side chains. The obtained crosslinked materials were characterized
from the point of view of their structure and aggregation liability to form hydrogels in a water
environment. The most promising materials showed interesting loading capability regarding the
antioxidant agent phycocyanin (PC). Two novel materials complexes (namely HA(270)-FA-TEGECCL-
20/PC and HA(270)-FA-HEGEC-CL-20/PC) were obtained with a drug-to-material ratio of 1:2
(w/w). Zeta potential measurements of the new complexes (−1.23 mV for HA(270)-FA-TEGECCL-
20/PC and −1.73 mV for HA(270)-FA-HEGEC-CL-20/PC) showed alterations compared to the
zeta potential values of the materials on their own, suggesting the achievement of drug–material
interactions. According to the in vitro dissolution studies carried out in different conditions, novel
drug delivery systems (DDSs) were obtained with a variety of characteristics depending on the
desired route of administration and, consequently, on the pH of the surrounding environment, thanks
to the complexation of phycocyanin with these two new crosslinked materials. Both complexes
showed excellent potential for providing a controlled/prolonged release of the active pharmaceutical
ingredient (API). They also increased the amount of drug that reach the target location, enabling
pH-dependent release. Importantly, as demonstrated by the DPPH free radical scavenging assay,
the complexation process, involving freezing and freeze-drying, showed no adverse effects on the
antioxidant activity of phycocyanin. This activity was preserved in the two novel materials and
followed a concentration-dependent pattern similar to pure PC
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