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

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

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

INTERACTION BETWEEN NANOFILLED COMPOSITES AND POLYWAVE MULTILED CURING LAMPS: AN IN VITRO STUDY

8nonenoneBattaglia V; Bergantin E; Paolino D; Coero Borga FA; Cadenaro M; Breschi L; Berutti E; Scotti N.Battaglia, V; Bergantin, E; Paolino, D; Coero Borga, Fa; Cadenaro, Milena; Breschi, Lorenzo; Berutti, E; Scotti, Nicol