Search

Understanding the evolution of dislocations and twinning at the crack front is critical for designing micro-mechanical systems with improved performance. In this work, the dislocation evolution at ...

The present work aims to investigate the role of hydrogen induced blisters cracking on degradation of tensile and fatigue properties of X65 pipeline steel. Both tensile and fatigue specimens were electrochemically charged with hydrogen at 20 mA/cm2 for a period of 4 h. Hydrogen charging resulted in hydrogen induced cracking (HIC) and blister formation throughout the specimen surface. Nearly all the blisters formed during hydrogen charging showed blister wall cracking (BWC). Inclusions mixed in Al-Si-O were found to be the potential sites for HIC and BWC. Slow strain rate tensile (SSRT) test followed by fractographic analysis confirmed significant hydrogen embrittlement (HE) susceptibility of X65 steel. Short fatigue crack growth framework, on the other hand, specifically highlighted the role of BWC on accelerated crack growth in the investigated material. Coalescence of propagating short fatigue crack with BWC resulted in rapid increase in the crack length and reduced the number of cycles for crack propagation to the equivalent crack length.

Correlating hydrogen embrittlement phenomenon with the metallic microstructural features holds the key for developing metals resistant to hydrogen-based failures. In case of fatigue failure of hydrogen charged metals, in addition to the hydrogen-based failure mechanisms associated with monotonic loading such as HELP, HEDE etc., microstructural features such as grain size, type of grain boundary (special/random), fraction of special grain boundaries; their network and triple junctions can play a complex role. The probable sites for fatigue crack initiation in such metals can be identified as the sites of highest hydrogen concentration or accumulated plastic strain. To this end, we have developed an experimental framework based on in-situ fatigue crack initiation and propagation studies under scanning electron microscope (SEM) to identify the weakest link in the metallic microstructure leading to failure. In-situ fatigue experiments are performed on carefully designed polycrystalline nickel (99.95% pure) specimens (miniaturised, shallow-notched & electro-polished) using a 10 kN fatigue stage inside the SEM. Electron Back Scattering Diffraction (EBSD) map of the notched region surface helps identify the distribution of special/random grain boundaries, triple junctions and grain orientation. The specimen surface in the shallow notched region for both the hydrogen charged and un-charged specimens are then carefully studied to correlate the microstructural feature associated with fatigue crack initiation sites. Such correlation of the fatigue crack initiation site and microstructural feature is further corroborated with the knowledge of hydrogen trapping and grain’s elastic anisotropicity to be either the site of high hydrogen concentration, accumulated plastic slip or both.

Hydrogen Embrittlement (HE) leads to deterioration of the fracto-mechanical properties of metals. In spite of vast literature, it is still not clearly understood and demands significant research on this topic. For better understanding of the hydrogen effect on fatigue behaviour of metals, present work focuses on developing a computational framework for fatigue crack initiation studies in metals in the presence of hydrogen. The developed framework consists of a nonlocal crystal plasticity model coupled with hydrogen transport model to study the fatigue behaviour of hydrogen charged metals. The nonlocal crystal plasticity model accounts for the statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) in polycrytalline metal. Hydrogen transport model, on the other hand, accounts for diffusion and trapping behavior of hydrogen due to concentration gradient, pressure gradient, plastic strain-rate with dislocations as the only trapping sites along the slip systems. A polycrystalline representative volume element (RVE) with periodic boundary conditions is used in this study. Fatigue crack initiation criterion is proposed for the simulated RVE with controlled microstructure by considering a critical value of the fatigue indicator parameter (FIP). FIP is formulated based on the experimental observations of several crack initiation sites along the grain boundaries, their normal direction with respect to loading direction and the accumulated plastic strain in nickel polycrystalline samples. Developed simulation framework correctly accounts cyclic stress-strain behavior and multiple fatigue crack initiation sites observed experimentally in the presence of hydrogen.

Histidine is an essential α-amino acid that plays a crucial role in tissue development and helps in the transmission of metallic ions during biological events. However, an abnormal level of histidine in the body is associated with various physiological conditions such as arthritis, liver cirrhosis, kidney diseases, and asthma. Herein, a unique ratiometric fluorescence sensing system has been developed for the recognition of histidine. The sensing system was developed using carbon quantum dots (CQDs) as an energy donor and a rhodamine 6G derivative (HS30) as an energy acceptor unit. Interestingly, upon the addition of Fe(III) into the mixture of CQDs and HS30, the phenomenon of fluorescence resonance energy transfer (FRET) was observed when excited at 350 nm. The emergence of a strong emission peak at 551 nm on the addition of Fe(III) suggested the formation of a ratiometric fluorescent complex “CQDs–Fe–HS30”. The ratiometric behavior of “CQDs–Fe–HS30” was studied by monitoring fluorescence emissions at 425 nm and 551 nm with an excitation wavelength of 350 nm. Furthermore, “CQDs–Fe–HS30” was employed for the recognition of histidine in an aqueous system. Due to the high affinity of histidine to Fe(III), the addition of histidine to an aqueous solution of “CQDs–Fe–HS30” resulted in the displacement of the Fe(III) cation from the complex, and the simultaneous quenching and enhancement of the emission peaks at 551 nm and 425 nm, respectively, was observed. The developed sensing system was successfully employed for a histidine recovery experiment in human urine samples with satisfactory results. Furthermore, the mixture of CQDs and HS30 was successfully utilized to implement an inhibit logic gate with Fe(III) and histidine as inputs and emission at 551 nm as output.

Understanding hydrogen embrittlement phenomenon that leads to deterioration of mechanical properties of metallic components is vital for applications involving hydrogen environment. Among these, understanding the influence of hydrogen on the fatigue behaviour of metals is of great interest. Total fatigue life of a material can be divided into fatigue crack initiation and fatigue crack growth phase. While fatigue crack initiation can be linked with the propagation of short fatigue cracks, the size of which is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack growth behaviour using conventional CT specimens. Extensive literature is available on hydrogen effect on long fatigue crack growth behaviour of metals that leads to the change in crack growth rate and the threshold stress intensity factor range (ΔKth). However, it is the short fatigue crack growth behaviour that provides the fundamental understanding and correlation of the metallic microstructure with hydrogen embrittlement phenomenon. Short fatigue crack growth behaviour is characteristically different from long crack growth behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as a strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. To this end, a novel experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged materials involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs a curves. Various metallic materials ranging from austenitic stainless steel (AISI 316L) to reactor pressure vessel steel (SA508 Grade 3 Class I low alloy steel) and line pipe steels (API 5L X65 & X80) are studied in this work.

WC-NiCr and WC-Hastelloy C coatings were deposited on Monel K-500 substrate by HVOF-spray with an aim to enhance cavitation erosion resistance of the alloy. The cavitation tests were performed for 10 h following ASTM G32-10 standard. Both WC-NiCr as well as WC-Hastelloy C coatings successfully reduced the erosion volume loss of the alloy by 59% and 9% respectively. The relatively superior performance of WC-NiCr coating could be attributed to better combination of its microhardness and fracture toughness. Formation of craters, cavities, and debonding of splats were found to be the signatures of cavitation erosion in the coatings. Whereas, microplastic tearing and microcracks were observed as the primary erosion mechanism in Monel K-500.

Highly selective azophenol-based chromogenic probe was synthesized that gave sharp color change in presence of cyanide. Based on colorimetric response of probe, a simple and economic colorimetric device has also been developed. Recently smartphones were employed for colorimetric analysis however there are number of limitations associated with it. Therefore, colorimeter was built using color sensor (TCS3200) and Arduino microcontroller for quantification of analytes using sensor coated paper chip. Initial colorimetric experiments revealed that sensor coated paper chip gave most linear response for change in the intensity of green component with change in the concentration of the cyanide. Thus, the device was calibrated using sensor coated paper chip and known concentrations of cyanide. It produced a best linear response over the range of 0−20 μM concentration of cyanide with R2 value of 0.9858 and limit of detection was calculated to be 0.86 μM which is lesser than WHO’s permissible limit of 1.9 μM. Finally, the applicability of device was successfully evaluated for quantification of cyanide concentration in spiked river water and food samples. Thus, the device can be successfully calibrated and used for quantitative analysis of other hazardous analytes such as cyanide through colorimetric sensing chips.

Hydrogen deteriorates the fatigue behaviour of metals by attacking their microstructure in different ways. For better clarity about hydrogen-based degradation of metals, it is vital to identify the microstructural configurations that promote fatigue crack initiation (FCI) in the presence of hydrogen. Intergranular regions in polycrystalline metals are more prone to hydrogen attack due to the prevailing atomic structure, elastic anisotropy, and plastic inhomogeneities causing increased hydrogen accumulation in this region. In this work, FCI is studied in a model material nickel in the uncharged and hydrogen charged state during in-situ strain controlled low cycle fatigue (LCF) testing under a scanning electron microscope (SEM). Crack initiation sites are characterized by investigating the elastic modulus in the loading direction as well as the maximum Schmid factor of the crack neighbouring grains extracted through the electron backscattered diffraction (EBSD) data. The crack frequency for the uncharged and hydrogen charged specimens is then analyzed using the difference in the elastic modulus ( Δ E ), the difference in the maximum Schmid factor ( Δ m ), and Δ E / Δ m ratio between the crack neighbouring grains. The comparison shows that for the hydrogen charged specimens, intergranular FCI sites show high values of Δ E / Δ m compared to the uncharged specimens. These findings provide a predictive model for hydrogen linked FCI in metals. In addition, the synergistic role of the Hydrogen Enhanced Local Plasticity (HELP) mediated Hydrogen Enhanced Decohesion (HEDE) mechanism responsible for FCI is also demonstrated.

To understand hydrogen embrittlement and predict probable damage sites based on high hydrogen segregation, it is important to identify hydrogen distribution mechanisms in metallic microstructure with deformation. Hydrogen segregation in metals is affected by various factors such as grain-size, the character of grain boundaries (GBs), loading direction and strain-rate. To this end, a computational framework consisting of non-local dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is presented to study the role of (i) grain-size, (ii) loading direction, (iii) strain-rate and (iv) GB character on hydrogen distribution and segregation in the pre-charged metallic microstructure. The computational framework is capable of accounting for the change in local hydrogen concentration due to prevailing hydrostatic pressure, trapping by dislocations, GB energetics and local slip-rates in the metallic microstructure. The difficulty in dislocation motion at the inter-granular regions of polycrystal due to high cross-hardening offered by geometrically necessary dislocations (GNDs) is incorporated in the model as additional isotropic hardening, whereas the back stress due to the GND pileups is included as a kinematic term in the flow rule. In addition, GNDs act as hydrogen trap sites leading to increased hydrogen concentration in the inter-granular regions. The strain-rate factor initially provided by Krom & Bakker (1999) is modified in the present work to make it compatible with the dislocation density-based coupled framework, which is able to calculate trapped hydrogen in dislocations along the slip systems. Plastically deformed polycrystals of various grain-sizes, along different directions, with varying strain rates and containing Σ 3 [ 1 1 ¯ 0 ] ( 111 ) and Σ5[001](210) GBs show that the gradients of hydrostatic pressure and GB character are major factors controlling hydrogen segregation in the microstructure.

Present work aims to unveil the mechanism of hydrogen embrittlement (HE) in SA508 Grade 3 Class I low alloy reactor pressure vessel (RPV) steel. In-situ tensile and low cycle fatigue (LCF) tests are performed on specially designed specimens using tensile/fatigue testing stage under scanning electron microscope (SEM). Electrochemical hydrogen charging resulted in localized void formation at prior austenite grain boundaries (PAGBs) during tensile loading. Alongside the hydrogen induced weakening of PAGBs due to synergetic HELP (hydrogen enhanced localized plasticity) and HEDE (hydrogen enhanced decohesion) mechanisms of HE, fish-eyes formation around Al2O3–SiO2 type inclusions are the primary factors for hydrogen enhanced tensile properties degradation in subject RPV steel. During LCF loading, crack initiation and propagation is facilitated by long rod inter-lath cementite particles distributed along the bainitic ferrite lath boundaries in the un-charged specimen. In case of hydrogen charged specimen, the edge crack formed during LCF loading propagated through the specimen by cleavage. Predominantly plasticity (slip) driven transgranular crack propagation occurred in un-charged specimen. In contrary, hydrogen charging resulted in LCF crack to propagate in mixed intergranular and transgranular manner during early stages of propagation, whereas once the crack length exceeded 5 to 6 grains, cleavage type transgranular crack propagation was observed.

From thick to very thin specimens, fracture toughness of metallic materials peaks out before hitting much lower value compared to the constant plane strain fracture toughness value obtained using standard fracture mechanics testing of thick specimens. Understanding this behavior is essential for improving reliability of small-scale metallic devices currently being used for various critical applications. To understand this behavior, it is essential to study the process of dislocations emission and interaction at the crack front and its variation with specimen thickness. To this end, atomistic fracture simulations of pre-cracked single crystal FCC metal (Nickel) specimens representing both thick and thin specimens are performed. At first, stress-state dependent single crystal yield function based on the generalized Schmid-law is associated with the dislocation emission process at the crack front for both thin and thick specimens. Fracture simulations are then performed on single crystal specimens representing different thickness cases. Due to low stress triaxiality prevailing throughout the thickness of thin specimens, dislocation interaction with each other inside the specimen and then with the specimen surface leading to wedge-shaped groove formation on opposite surfaces at crack front is found to be the responsible mechanism of crack propagation in thin specimens. This mechanism provides enhanced fracture toughness to thin specimen compared to thick specimen in which crack propagation is based on high stress triaxiality at the core of the crack front making formation of microvoids and their coalescence as a dominant mechanism of crack propagation. The dislocation configurations generated at the crack front for thick and thin specimens are also studied and the mechanisms for dislocation multiplication in thin specimens compared to thick specimens are highlighted.

The ductile versus brittle fracture in crystalline materials depends on the relative values of K Ic and K Ie as defined by well-known Rice theory, where K Ic and K Ie are the critical values of stress intensity factor corresponding to cleavage and dislocation emission, respectively. For K Ic < K Ie , the brittle fracture (or cleavage) takes place in atomically sharp pre-cracked crystal subjected to Mode I loading. For K Ie < K IC , the dislocations are emitted from the crack front resulting in ductile fracture. To this end, molecular static simulations are used to explain the crystal orientation dependent fracture behaviour of FCC single crystal and its contradiction with respect to Rice theory based on stress triaxiality at the crack front. The stress triaxiality at crack front changes with crystal orientation due to transformation of stiffness tensor C ijkl . It is shown that high stress triaxiality suppressed the dislocation initiation leading to cleavage failure even for the case when K Ie < K Ic .

cited By 8; International audience; Physical ageing of amorphous polymers well below their glass transition temperature leads to changes in almost all physical properties. Of particular interest is the increase in yield stress and post-yield strain softening that accompanies ageing of these materials. Moreover, at larger strain polymers seem to rejuvenate, i.e. aged and non-aged samples have identical stressstrain responses. Also, plastically deforming an aged sample seems to rejuvenate the polymer. In this work we use molecular dynamic simulations with a detailed force field suitable for macromolecular ensembles to simulate and understand the effects of ageing on the mechanical response of these materials. We show that within the timescales of these simulations it is possible to simulate both ageing and rejuvenation. The short range potentials play an important role in ageing and rejuvenation. A typical yield drop exhibited by glassy polymers is a manifestation of a sudden relaxation in the short range structure of an aged polymer. Moreover, the aged polymers are known to be brittle. We show that this is intimately related to its typical stressstrain response which allows it to carry arbitrarily large mean stresses ahead of a notch. © 2010 Elsevier Ltd. All rights reserved.