376 research outputs found
Analysis of hydrogen permeation tests considering two different modelling approaches for grain boundary trapping in iron
The electrochemical permeation test is one of the most used methods for
characterising hydrogen diffusion in metals. The flux of hydrogen atoms
registered in the oxidation cell might be fitted to obtain apparent
diffusivities. The magnitude of this coefficient has a decisive influence on
the kinetics of fracture or fatigue phenomena assisted by hydrogen and depends
largely on hydrogen retention in microstructural traps. In order to improve the
numerical fitting of diffusion coefficients, a permeation test has been
reproduced using FEM simulations considering two approaches: a continuum 1D
model in which the trap density, binding energy and the input lattice
concentrations are critical variables and a polycrystalline model where
trapping at grain boundaries is simulated explicitly including a segregation
factor and a diffusion coefficient different from that of the interior of the
grain. Results show that the continuum model captures trapping delay, but it
should be modified to model the trapping influence on the steady state flux.
Permeation behaviour might be classified according to different regimes
depending on deviation from Fickian diffusion. Polycrystalline synthetic
permeation shows a strong influence of segregation on output flux magnitude.
This approach is able to simulate also the short-circuit diffusion phenomenon.
The comparison between different grain sizes and grain boundary thicknesses by
means of the fitted apparent diffusivity shows the relationships between the
registered flux and the characteristic parameters of traps
Influence of charging conditions on simulated temperature-programmed desorption for hydrogen in metals
Failures attributed to hydrogen embrittlement are a major concern for metals
so a better understanding of damage micro-mechanisms and hydrogen diffusion
within the metal is needed. Local concentrations depend on transport phenomena
including trapping effects, which are usually characterised by a
temperature-programmed desorption method often referred to as Thermal
Desorption Analysis (TDA). When the hydrogen is released from the specimen
during the programmed heating, some desorption peaks are observed that are
commonly related to detrapping energies by means of an analytical procedure.
The limitations of this approach are revisited here and gaseous hydrogen
charging at high temperatures is simulated. This popular procedure enables
attaining high concentrations due to the higher solubility of hydrogen at high
temperatures. However, the segregation behaviour of hydrogen into traps depends
on charging time and temperature. This process and the subsequent cooling alter
hydrogen distribution are numerically modelled; it is found that TDA spectra
are strongly affected by the charging temperature and the charging time, both
for weak and strong traps. However, the influence of ageing time at room
temperature after cooling and before desorption is only appreciable for weak
traps
Cold Isostatic Pressing to Improve the Mechanical Performance of Additively Manufactured Metallic Components.
Additive manufacturing is becoming a technique with great prospects for the production of components with new designs or shapes that are difficult to obtain by conventional manufacturing methods. One of the most promising techniques for printing metallic components is binder jetting, due to its time efficiency and its ability to generate complex parts. In this process, a liquid binding agent is selectively deposited to adhere the powder particles of the printing material. Once the metallic piece is generated, it undergoes a subsequent process of curing and sintering to increase its density (hot isostatic pressing). In this work, we propose subjecting the manufactured component to an additional post-processing treatment involving the application of a high hydrostatic pressure (5000 bar) at room temperature. This post-processing technique, so-called cold isostatic pressing (CIP), is shown to increase the yield load and the maximum carrying capacity of an additively manufactured AISI 316L stainless steel. The mechanical properties, with and without CIP processing, are estimated by means of the small punch test, a suitable experimental technique to assess the mechanical response of small samples. In addition, we investigate the porosity and microstructure of the material according to the orientations of layer deposition during the manufacturing process. Our observations reveal a homogeneous distribution independent of these orientations, evidencing thus an isotropic behaviour of the material
Pre-notched dog bone small punch specimens for the estimation of fracture properties
In recent years, the pre-notched or pre-cracked small punch test (P-SPT) has
been successfully used to estimate the fracture properties of metallic
materials for cases in which there is not sufficient material to identify these
properties from standard tests, such as CT or SENB specimens. The P-SPT
basically consists of deforming a pre-notched miniature specimen, whose edges
are firmly gripped by a die, using a high strength punch. The novelty of this
paper lies in the estimation of fracture properties using dog-bone-shaped
specimens with different confinement levels. With these specimens, three
confinement variations have been studied. The results obtained enable the
establishment of a variation of fracture properties depending on the level of
confinement of each miniature specimen and selection of the most appropriate
confinement for this goal
Simulation of hydrogen permeation through pure iron for trapping and surface phenomena characterisation
There is a need for numerical models capable of predicting local accumulation
of hydrogen near stress concentrators and crack tips to prevent and mitigate
hydrogen assisted fracture in steels. The experimental characterisation of
trapping parameters in metals, which is required for an accurate simulation of
hydrogen transport, is usually performed through the electropermeation test. In
order to study grain size influence and grain boundary trapping during
permeation, two modelling approaches are explored; a 1D Finite Element model
including trap density and binding energy as input parameters and a
polycrystalline model based on the assignment of a lower diffusivity and
solubility to the grain boundaries. Samples of pure iron after two different
heat treatments - 950C for 40 minutes and 1100C for 5 minutes - are tested
applying three consecutive rising permeation steps and three decaying steps.
Experimental results show that the finer grain microstructure promotes a
diffusion delay due to grain boundary trapping. The usual methodology for the
determination of trap densities and binding energies is revisited in which the
limiting diluted and saturated cases are considered. To this purpose, apparent
diffusivities are fitted including also the influence of boundary conditions
and comparing results provided by the constant concentration with the constant
flux assumption. Grain boundaries are characterised for pure iron with a
binding energy between 37.8 and 39.9 kJ/mol and a low trap density but it is
numerically demonstrated that saturated or diluted assumptions are not always
verified, and a univocal determination of trapping parameters requires a
broader range of charging conditions for permeation. The relationship between
surface parameters, i.e. charging current, recombination current and surface
concentrations, is also studied
The Essential Work of Fracture parameters for 3D printed polymer sheets
Additive manufacturing is becoming increasingly popular in academia and
industry. Accordingly, there has been a growing interest in characterizing 3D
printed samples to determine their structural integrity behaviour. We employ
the Essential Work of Fracture (EWF) to investigate the mechanical response of
polymer sheets obtained through additive manufacturing. Our goal is twofold;
first, we aim at gaining insight into the role of fibre reinforcement on the
fracture resistance of additively manufactured polymer sheets. Deeply
double-edge notched tensile (DDEN-T) tests are conducted on four different
polymers: Onyx, a crystalline, nylon-reinforced polymer, and three standard
polymers used in additive manufacturing - PLA, PP and ABS. Results show that
fibre-reinforcement translates into a notable increase in fracture resistance,
with the fracture energy of Onyx being an order of magnitude higher than that
reported for non-reinforced polymers. On the other hand, we propose the use of
a miniature test specimen, the deeply double-edge notched small punch specimens
(DDEN-SP), to characterize the mechanical response using a limited amount of
material. The results obtained exhibit good alignment with the DDEN-T data,
suggesting the suitability of the DDEN-SP test for measuring fracture
properties of additively manufactured polymers in a cost-effective manner
Notch fracture predictions using the Phase Field method for Ti-6Al-4V produced by Selective Laser Melting after different post-processing conditions
Ti-6Al-4V is a titanium alloy with excellent properties for lightweight applications and its production through Additive Manufacturing processes is attractive for different industrial sectors. In this work, the influence of mechanical properties on the notch fracture resistance of Ti-6Al-4V produced by Selective Laser Melting is numerically investigated. Literature data is used to inform material behaviour. The as-built brittle behaviour is compared to the enhanced ductile response after heat treatment (HT) and hot isostatic pressing (HIP) post-processes. A Phase Field framework is adopted to capture damage nucleation and propagation from two different notch geometries and a discussion on the influence of fracture energy and the characteristic length is carried out. In addition, the influence of oxygen uptake is analysed by reproducing non-inert atmospheres during HT and HIP, showing that oxygen shifts fracture to brittle failures due to the formation of an alpha case layer, especially for the V-notch geometry. Results show that a pure elastic behaviour can be assumed for the as-built SLM condition, whereas elastic-plastic phenomena must be modelled for specimens subjected to heat treatment or hot isostatic pressing. The present brittle Phase Field framework coupled with an elastic-plastic constitutive analysis is demonstrated to be a robust prediction tool for notch fracture after different post-processing routes
Damage modeling in Small Punch Test specimens
Ductile damage modeling within the Small Punch Test (SPT) is extensively investigated. The capabilities of the SPT to reliably estimate fracture and damage properties are thoroughly discussed and emphasis is placed on the use of notched specimens. First, different notch profiles are analyzed and constraint conditions quantified. The role of the notch shape is comprehensively examined from both triaxiality and notch fabrication perspectives. Afterwards, a methodology is presented to extract the micromechanical-based ductile damage parameters from the load-displacement curve of notched SPT samples. Furthermore, Gurson-Tvergaard-Needleman model predictions from a top-down approach are employed to gain insight into the mechanisms governing crack initiation and subsequent propagation in small punch experiments. An accurate assessment of micromechanical toughness parameters from the SPT is of tremendous relevance when little material is available.The authors gratefully acknowledge financial support from the Ministry of Economy and Competitiveness of Spain through grant MAT2014-58738-C3
Pre-notched dog bone small punch specimens for the estimation of fracture properties
In recent years, the pre-notched or pre-cracked small punch test (P-SPT) has been successfully used to estimate the fracture properties of metallic materials for cases in which there is not sufficient material to identify these properties from standard tests, such as CT or SENB specimens. The P-SPT basically consists of deforming a pre-notched miniature specimen, whose edges are firmly gripped by a die, using a high strength punch. The novelty of this paper lies in the estimation of fracture properties using dog-bone-shaped specimens with different confinement levels. With these specimens, three confinement variations have been studied. The results obtained enable the establishment of a variation of fracture properties depending on the level of confinement of each miniature specimen and selection of the most appropriate confinement for this goal
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