Fracture Limit Research Articles

The Determination of the Limit Load Solutions for the New Pipe-Ring Specimen Using Finite Element Modeling

To estimate the acceptable size of cracks and predict the loading limit of the pipeline or its resistance to the initiation and crack growth by following the structural integrity, the fracture toughness and limit load solutions are required. Standard fracture toughness testing of thin-walled pipelines is often difficult to perform in order to complete standard requirements. To find an alternative technique for the measurement of the fracture toughness of the already delivered pipeline segment, the new pipe-ring specimen has been proposed; however, the limit load solutions have not been investigated yet. The limit load depends on the geometry of the specimen and loading mode. The ligament yielding of pipe-ring specimens containing axial cracks through the thickness under combined loads was calculated by the finite element method. This paper provides limit load solutions of several different pipe-ring geometries containing two diametric symmetrical cracks with the same depth ratio in a range of 0.45 ≤ a/W ≤ 0.55. The limit load (LL) solutions calculated by numerical analysis are shown as a function of the full ring section’s size and the corresponding crack aspect ratio for determining the normalized load. These can potentially construct the failure assessment diagram to estimate the crack acceptance in a part of the pipe.

Formability analysis of electrically assisted double-side multi-point incremental sheet forming

Double-side multi-point incremental forming (DMISF) process combines local and progressive forming features with the advantages such as high forming precision, high production flexibility, and high fracture limit, so it is suitable to form complex aerospace parts and difficult-to-shape materials. High-strength aluminum alloy is widely used in aerospace fields, but its poor formability at room temperature limits its further development. Pulse current was introduced into DMISF process to improve the formability of high-strength aluminum alloy. So equipment for electrically assisted double-side multi-point incremental sheet forming (E-DMISF) process was set up. However, the E-DMISF process is a complicated electro-thermal-structure multi-field coupling process; the flow of the material in the forming process is very complicated. In this research, the hyperbolic truncated cone and hyperbolic truncated pyramid were chosen as study objects and the stress triaxiality was selected as the characteristic function of ductile fracture criterion. Based on all above, the fracture limit experiments and simulation in the E-DMISF process were performed and the fracture criterion was established correspondingly. The result indicates that the electric field could help enhance the fracture limit effectively.

Gauge length and frame rate dependence of the onset of instability and the fracture limit of DP 980 sheets

Despite the importance of the triaxiality failure diagram (TFD) for damage prediction, the quantitative influences of the gauge length Lo and the frame rate f of the DIC (digital image correlation) system’s camera on the necking strain εu and the fracture strain εf from tensile testing have hardly been dealt with. In this study, DP 980 sheets were subjected to uniaxial tensile, shear, and near plane strain conditions, and deformations were recorded by DIC. The investigation of the influence of varying Lo and f on the ability to capture εu and εf accurately reveals the following: the influences on εu is negligible; to avoid overly conservative values for εf, it is essential to limit Lo to the smallest value possible, i.e. the necking band width; the influence of f becomes relevant for low Lo; at a test velocity of 5 mm min−1, f should not be lower than 10 frames per second (fps) (i.e. 120 frames mm−1) when using the minimum Lo. Nonetheless, even with the minimum Lo, εf values are below those obtained on a local level by DIC. The TFDs for different f make clear that the influence of f holds also for other loading modes and is most pronounced for shear. Therefore, application in conjunction with a high-speed camera (≥10 fps) is necessary for the measurement of realistic parameters, which is critical to sheet forming analysis.

Forming and fracture limits of IN718 alloy at elevated temperatures: Experimental and theoretical investigation

Forming and fracture forming limit diagrams are significant performance indexes for evaluating the formability of a material. In the present study, experimental and theoretical investigations of the forming and fracture behavior for precipitate-hardenable Inconel 718 superalloy, have been performed at different temperatures (Room temperature (RT) to 700 °C). Firstly, uniaxial tests have been conducted over a temperatures range and quasi-static strain rates (10−4-10−1s−1). Flow stress (tensile) has been found to be significantly affected by variation in test temperatures and strain rates. Further, yielding behavior of IN718 alloy has been predicted based on Hill'48 (r and σ based) and Barlat'89 criteria. Barlat'89 criterion has better predictability of yielding behavior for IN718 alloy at all test temperatures. Subsequently, experimental forming and fracture limit curves have been plotted at different temperatures using Nakazima test. Limiting true strains have been found to be increasing with test temperature for all deformation regions in forming limit diagrams. Marciniak Kuczynski (M-K) and Bao-Wierzbicki (B-W) model coupled with anisotropic yield criteria have been used for theoretical prediction of limiting and failure strains for IN718. The M-K and B-W models coupled with only Barlat'89 criteria have shown a good prediction ability of limiting strains with least root mean square error (RMSE) and average absolute error (AAE).

Omnidirectional Stretchable Inorganic‐Material‐Based Electronics with Enhanced Performance

AbstractInorganic material‐based devices are well known for their high performance, excellent stability, and hence suitability for fast computation and communication. But their nonflexibility and nonstretchability often hinder their application in several emerging areas where conformability with irregular 3D surfaces is required in addition to the high performance. Herein, with honeycomb like patterns, the omnidirectional stretchability and conformability of inorganic material‐based device are demonstrated without sacrificing the performance. The simple method presented here facilitates the transfer of patterned inorganic material‐based devices from rigid poly(methyl methacrylate) (PMMA)/glass substrate onto flexible/stretchable substrate such as polydimethylsiloxane simply by placing a water droplet at the PMMA/glass interface. As a proof of concept, the intrinsically brittle indium–gallium–zinc oxide (IGZO)‐based stretchable photodetector devices are fabricated. These devices can be stretched up to 10% without performance degradation, which is a significant improvement considering the less than ≈1% fracture limit of IGZO. With Au decoration, these devices show 127‐fold higher responsivity (295.3 mA W−1) than planar IGZO devices. The higher fracture strain together with the omnidirectional stretchability underpinned by the honeycomb pattern could allow presented devices to conform to complex hemispherical surfaces such as the human eyes, thus showing significant potential for future high‐performance stretchable electronics.

Fracture criteria applied to numerical simulation of blowout preventer ram shearing

The interest in predicting the force required for a subsea blowout preventer (BOP) to cut and isolate the drilling pipe has been growing since the Macondo accident. In previous research, numerical simulation using the nonlinear finite element method combined with the blowout preventer (BOP) shear test has been a common approach. As various fracture criteria exist, the selection and application of a suitable fracture criterion are of great interest to achieve a successful ram shearing simulation. In this paper, the CrashFEM criterion, previously employed in crashworthiness simulation, is first adopted in BOP ram shearing simulation to capture the onset of damage to the pipe. The Johnson-Cook (J-C) and Modified Mohr-Coulomb (MMC) criteria are utilized to carry out a comparative study. The TRIP 690 experimental data are used to derive the fracture parameters. These parameters enable the building of numerical models based on boundary and loading conditions for the BOP shear test. Based on the numerical model, a mesh sensitivity study is carried out. The simulation results using refined meshes show that the shearing forces predicted by the CrashFEM and MMC criteria are generally consistent with the experimental data. The results from the three criteria indicate differences originating from the shape of the fracture limit curve, the damage initiation concerning the equivalent plastic strain accumulation, and the damage distribution in the shearing area. These findings are explained in detail in the paper. A comprehensive application of the CrashFEM criteria for BOP shearing simulation is presented and recommendations are provided about the selection of suitable fracture criteria.

Evaluation of the VDA 238–100 Tight Radius Bend Test for Plane Strain Fracture Characterization of Automotive Sheet Metals

Accurate characterization of the fracture limit in plane strain tension of automotive sheet metals is critical for the design and crash performance of structural components. Plane strain bending using the VDA 238–100 V-bend test has potential for proportional fracture characterization by avoiding a tensile instability. The VDA 238–100 V-bend test was evaluated using DIC strain measurement to characterize the plane strain fracture limit under proportional plane stress loading and to evaluate the effect of the VDA pre-straining methodology for ductile alloys upon the material response. The load-based failure criterion of the V-bend test was evaluated with DIC to monitor the development of surface cracking. The influence of the non-linear strain path imposed by the pre-straining procedure for ductile materials was then evaluated for three automotive alloys: an advanced high strength dual phase steel, DP1180, a rare-earth magnesium, ZEK100, and an AA5182 aluminum. A fracture criterion based on the load threshold was reasonable for the three alloys considered. Pre-straining in uniaxial tension prior to plane strain bending affected each alloy differently. The DP1180 was not affected by the non-linear strain path whereas the cumulative equivalent strain for the AA5182 and ZEK100 increased by strains of 0.07 and 0.05 strain, respectively. The non-linear strain path within the VDA pre-straining methodology creates ambiguity in comparing the fracture limits of different materials. The plane strain fracture limit for proportional loading can be readily obtained in the V-bend test with DIC strain measurement.

Theoretical Analysis of Strain‐Optoelectronic Properties in Externally Deformed Ge/GeSi Quantum Well Nanomembranes via Neutral Plane Modulation

For indirect‐bandgap germanium (Ge), strain‐engineering provides a controllable means to transform Ge into direct‐bandgap material with strongly enhanced light emission efficiency. Flexible electronics offer a powerful platform for introducing external strain into semiconductor nanomembranes (NMs) through deformation with flexible substrates. So far, the Ge/GexSi1−x alloy quantum well (QW) NMs have not been sufficiently investigated to comprehend how the external deformation tunes the wavelength and light‐emitting properties. Herein, a theoretical model based on the elasticity theory and k·p method is established to analyze the strain‐optoelectronic properties of the externally deformed Ge/GexSi1−x QW NMs via neutral plane (NP) modulation in both embedded and sticked form. Calculated results reveal that the elastic strain is linearly distributed, inducing tilt of the band edge similar to the electric field. Considering the fracture limit of the material, the NP layout determines the critical bending curvature that the NM can achieve. Large tunable range of wavelength Δλ of 1615 nm and enhanced transverse electric (TE)/transverse magnetic (TM) peak gain of 949/3778 cm−1 are predicted for r = 1000 nm (the distance between NP and QW) under the bending limit. Further calculations conclude that larger r allow for smaller bending curvature to achieve wider range of wavelength and more enhanced optical gain.

The Effect of Lamellar and Globular α-Phase on Mechanical Behavior of Strongly Textured Ti–6Al–4V alloy

In this study, the effect of lamellar and globular α-phase on mechanical properties of strongly textured Ti–6Al–4V alloy was investigated. The strong transverse texture was developed in Ti–6Al–4V alloy via α + β rolling at 950 °C. In the following, lamellar and equiaxed microstructures have been separately promoted in the Ti–6Al–4V alloy during annealing at β- and α + β regions, respectively. According to EBSD analysis, it was revealed that deformation texture was not considerably changed during different annealing processes. Then, room temperature mechanical properties of the studied alloy including uniaxial tensile, compression and bending tests were evaluated. According to the tensile test, work hardening capacity was obtained as 1.10 and 0.64 for equiaxed and lamellar microstructures, respectively. Fracture limit curve revealed that equiaxed microstructure had more extent of homogeneous deformation in comparison with lamellar one. In addition, bending properties of the Ti–6Al–4V alloy were significantly improved as a result of equiaxed microstructure.

Chemical-Physical Properties Characterization of White Snapper Fish Skin Rambak Crackers Based on Boiling and Drying Duration

HighlightsThe utilisation process of snapper skin from fish fillet industry by products for cracklings was not optimalThe optimization of snapper skin crackling production by boiling and drying methodThe optimal method of snapper skin crackling production was 30 minutes of boiling and sun-drying for 30 hoursAbstractFish skin is a byproduct of fish filet production, its use is still limited, but some of the terrestrial animal skin is partially processed into rambak crackers, which is quite well known in Indonesia. The purpose of this study was to determine the duration of boiling and drying time on the chemical-physical properties of rambak crackers from white snapper skin. The research method used was an experiment with a boiling time treatment of 0, 10, 15, and 20 minutes and drying time with 10, 20, and 30 hours in sunlight. The results showed that the boiling time and drying duration had an effect on the chemical-physical properties of the rambak crackers of white snapper fish skin. The resulting rambak crackers have physical characteristics of 15.13-25.39 N / m2, 70.44-100.07% enlarging capacity, and chemical characteristics of the water content was 1.09-4.95%, protein content of 65.26-70.43%, and fat content of 13.43-15.83%. The best crackers are those which resulted from 30 minutes of boiling and 30 hours of frying process with a fracture limit of 15.13 N/m2, an enlarging capacity of 100.07%, moisture content of 1.09%, protein content of 66.25%, and fat content of 13. 43%.

On the Determination of Forming Limits in Polycarbonate Sheets.

By proposing an adaptation of the methodology usually used in metal forming, this paper aims to provide a general procedure for determining the forming limits, by necking and fracture, of polymeric sheet. The experimental work was performed by means of Nakajima specimens with different geometries to allow to obtain strains in the tensile, plane, biaxial and equibiaxial states for Polycarbonate sheet with 1 mm of thickness. The application of the time-dependent and flat-valley approaches used in metals has been revealed appropriate to characterize the onset of necking and obtain the forming limits of polycarbonate, despite the stable necking propagation typical of polymeric sheets. An analysis of the evolution of the strain paths along a section perpendicular to the crack allowed for a deeper understanding of the steady necking propagation behaviour and the adoption of the methodology of metals to polymers. The determination of the fracture strains was enhanced with the consideration of the principal strains of the DIC system in the last stage, just before fracture, due to the significant elastic recovery typical of polymeric sheets. As a result of this analysis, accurate formability limits by necking and fracture are obtained for polycarbonate sheet, together with the principal strain space, providing a general framework for analysing incremental sheet forming processes where the knowledge of the fracture limits is relevant.

Biomechanical impact testing of synthetic versus human cadaveric tibias for predicting injury risk during pedestrian-vehicle collisions

Objective: The tibia is the most commonly fractured long bone in a pedestrian-vehicle collision. The standard injury assessment tool is the “legform,” a device that mimics the human lower limb under impact loads. These devices are designed to identify the impact load that will cause the onset of injury, rather than replicate the type and severity of fracture. Thus, this study is the first to determine if composite tibias made by Sawbones (Pacific Research Labs, Vashon, WA, USA) designed for orthopedic biomechanics research, could also potentially be used for traffic safety research by simulating both the damage tolerance of human cadaveric tibias for peak force and bending moment and the fracture patterns themselves, thereby more accurately predicting injury type during real-world pedestrian-vehicle collisions.Methods: Synthetic tibias (n = 6) and human cadaveric tibias (n = 6) were impacted at midshaft at 8.3 m/s (i.e., 30 km/h) under 3-point bending using a pneumatic impacting apparatus. Fracture force, bending moment, and fracture patterns were compared between the two groups, and Weibull survivability curves generated for force and moment results, to identify injury risk thresholds.Results: There was no difference for synthetic vs. cadaveric tibias regarding impact force (4271+/-938 N vs. 4889+/-993 N, p = 0.44) or bending moment at fracture (275+/-64 Nm vs. 302+/-107 Nm, p = 0.69). Force-time curves for all tibias were similar in shape based on the first three Principal Components (p > 0.14). Weibull survivability curves had differences in shape and in the 10% risk of fracture limits, with force thresholds of 2873 N for the synthetic vs. 3386 N for the cadaveric, and bending moment limits of 180 Nm for the synthetic compared to 157 Nm for the cadaveric. All fracture patterns were clinically realistic, but not consistent between groups. The coefficient of variation for synthetic tibias was >0.2 for both peak force and bending moment, which precludes their use as a reproducible test surrogate for injury prediction.Conclusions: Synthetic composite tibias offer the potential for developing a frangible test surrogate, and matched cadaveric response in several respects. However, the repeatability was not high enough for them to be used in their present form for injury prediction.

Investigations on strain distribution, stress-based fracture limit and corrosion behaviour of titanium Grade 2 sheets during single point incremental forming

Purpose The purpose of this paper is to investigate the strain distribution, stress-based fracture limit and corrosion behaviour of titanium Grade 2 sheets during single point incremental forming (SPIF) process, with various computerized numerical control (CNC) spindle rotational speeds and step depths. The development of corrosion pits in 3.5 (%) NaCl solution has also been studied during the SPIF process. Design/methodology/approach A potentiodynamic polarization (PDP) study was performed to investigate the corrosion behaviour of titanium Grade 2 deformed samples, with various spindle rotational speeds in 3.5 (%) NaCl solution. The scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis was carried out to study the fracture behaviour, dislocation densities and corrosion morphology of deformed samples. Findings The titanium Grade 2 sheets exhibited better strain distribution, fracture limit and corrosion resistance by increasing the CNC spindle rotational speeds, tool diameters and vertical step depths (VSD). It was recorded that varying the spindle speed affected plastic deformation which in turn affected corrosion rate. Research limitations/implications In this study, poor corrosion rate was observed for the as-received condition, and better corrosion rate was achieved at maximum speed of 600 rpm and 0.6 mm of VSD in the deformed sheet. This indicates that corrosion rate improved with increase in the plastic deformation. The EDS analysis report of corroded surface revealed the composition to be mainly of titanium and oxides. Practical implications This study discusses the strain distribution, stress-based fracture limit and corrosion behaviour by using titanium Grade 2 sheets during SPIF process. Social implications This study is useful in the field of automobile and industrial applications. Originality/value With an increase in the spindle rotational speeds and VSD, the titanium Grade 2 sheets showed better strain distribution, fracture limit and corrosion behaviour; the same is evidenced in fracture limit curve and PDP curves.

Status of Investigation to Ensure Applicability of ECCS Acceptance Criteria to High-Burnup Fuel

Light-water reactors (LWRs) are equipped with an emergency core cooling system (ECCS) that is designed to maintain the coolable geometry of the reactor core and finally minimize the release of radioactive fission products to the public and environment even in a loss-of-coolant accident (LOCA). Acceptance criteria for the ECCS of LWRs were determined to evaluate the safety function and performance in the design and to ensure a sufficient safety margin in the results of the evaluation. The latest revision of the criteria was made in 1981 in Japan, referring to the additional knowledge obtained after the previous revision. Fuel burnup has been extended by changing cladding materials, fuel design, etc., since the latest revision. Correspondingly, knowledge has been accumulated through studies on high-burnup fuel behavior under LOCA conditions to confirm the safety during the LOCA. This paper is a summary of the investigation and remaining issues on the applicability of the current Japanese ECCS acceptance criteria to high-burnup fuel, considering the history and basis of the current acceptance criteria. Results of the investigation conducted up to now reveal that the influence of burnup extension is small in terms of the cladding behavior of high-temperature oxidation and the fracture limit in quenching during the LOCA condition, and the current criteria are applicable even in the case of high-burnup fuel.

Predictions of Forming Limit Curves of AA6014 Aluminium Alloy at Room Temperature

The aim of present work is to examine D-Bressan mathematical model to accurately predict the surface limit strains by the critical fast shear stress criterion of conventional AA6014 aluminium alloy sheets, after loading with non-linear strain paths at room temperature. Two kinds of limit strain curves can be plotted in the Map of Principal Surface Limit Strains (MPLS): the surface local necking limit, FLC-N, and the fast shear stress fracture limit curve FLC-S. D-Bressan´s critical shear stress criterion model combined with Barlat´s Yld 2000-2D yield criterion is proposed for theoretical prediction of FLC-S curve in sheet metal forming operations and were compared with experimental outcomes of AA6014. In order to calibrate the Bressan-Barlat macroscopic model, tensile tests at two different strain rates on specimens cut at 0o, 45o and 90o to the rolling direction and bulge test were carried out at room temperature to obtain the material coefficients of plastic anisotropy, strain and strain rate hardening law. Experimental bi-linear strain paths were carried out in specimens with 1.04 mm thickness by pre-stretching in uniaxial stress direction up to two strain levels before performing Nakajima testing experiments for obtaining the forming limit strain curves. D-Bressan critical fast shear stress modelling, combined with Barlat´s Yld 2000-2d yield function, predicted quite well both formability of AA6014 sheet as received and the reduced FLC-S after 10% and 20% uniaxial pre-stretching and bi-linear strain path. Comparisons of experimental FLC-S and predictions with Barlat´s Yld 2000-2D and Hill 79 yield criteria were analyzed and discussed. D-Bressan approach with Barlat´s Yld 2000-2d yield function, for exponent m=6, provided better fitting to the experimental FLC-S curves than D-Bressan model combined with Hill 79 yield function.

Mixed mode I/II fracture criterion to anticipate behavior of the orthotropic materials

The new energy-based criterion, named Reinforcement Strain Energy Density (ReiSED), is proposed to investigate the fracture behavior of the cracked orthotropic materials in which the crack is embedded in the matrix along the fibers. ReiSED is an extension of the well-known minimum strain energy density criterion. The concept of the reinforced isotropic solid as an advantageous model is the basis of the proposed mixed-mode I/II criterion. This model introduces fibers as reinforcements of the isotropic matrix in orthotropic materials. The effects of fibers are qualified by defining reinforcement coefficients at tension and shear modes. These coefficients, called Reduced Stress (ReSt), provide the possibility of encompassing the fiber fraction in a fracture criterion for the first time. Comparing ReiSED fracture limit curve with experimental data proves the high efficiency of this criterion to predict the fracture behavior of orthotropic materials.

Fracture Investigation in Draw Bending of AZ31B Sheets using Fuzzy Logic Prediction

Automotive and aircraft industries are facing an increased demand of light-weighted materials for reducing fuel consumption and CO2 emissions. Magnesium alloy sheets are considered in manufacturing components of complex geometries and with flexible processes. Fracture investigation of AZ31B alloy sheets is of interest in continuous bending processes to achieve better formability. Certain forming processes of AZ31B sheets are restricted due to its limited ductility at room temperature. Draw bending of AZ31B sheets provides us the necessary forming state to investigate fracture limits. Our investigation relies on data extracted from draw bending tests organised according to a design of experiments plan. The experimental studies were conducted for different punch radii, traverse speeds and rolling directions. The predicted fracture behavior via Fuzzy Logic provides essential data to the setup procedures needed for our subsequent research, which is focusing on reducing the limitations of AZ31B sheets formability.

Characterization of plasticity and fracture of an QP1180 steel sheet

Experiments are designed to experimentally characterize the plastic behaviour and fracture limits of an QP1180 steel sheet under complex loading conditions. The plastic behaviour is modelled by the Drucker yield function, while the fracture behaviour is illustrated the DF2016 [1] fracture criterion. The pressure-coupled Drucker yield function is calibrated by inverse engineering approach. The predicted load-stroke curves are compared with experimental results, which shows that the prediction matches with the experimental results with good agreement. Then the fracture data are then extracted from experiments and numerical simulations to calibrated two fracture criteria. The predicted fracture loci are compared with experimental results. The comparison demonstrates that both fracture criterion matches with the experimental results very well. Therefore, the Drucker function together with the Swift-Voce hardening law is recommended to model plastic deformation up to large plastic strain for various loading conditions. The fracture behaviour from shear to plane strain tension is recommended to be modelled by the DF2016 criterion and the pressure-coupled Drucker functions of sheet metal forming processes.

Comparing the fracture limits of the proximal femur under impact and quasi-static conditions in simulation of a sideways fall

Sideways falls onto the hip are responsible for a great number of fractures in older adults. One of the possible ways to prevent these fractures is through early identification of people at greatest risk so that preventive measures can be properly implemented. Many numerical techniques that are designed to predict the femur fracture risk are validated through performing quasi-static (QS) mechanical tests on isolated cadaveric femurs, whereas the real hip fracture is a result of an impact (IM) incident. The goal of this study was to compare the fracture limits of the proximal femur under IM and QS conditions in the simulation of a sideways fall to identify any possible relationship between them.Eight pairs of fresh frozen cadaveric femurs were divided into two groups of QS and IM (left and right randomized). All femurs were scanned with a Hologic DXA scanner and then cut and potted in a cylindrical tube. To measure the stiffness in two conditions of the single-leg stance (SLS) and sideways fall (SWF), non-destructive tests at a QS displacement rate were performed on the two groups. For the destructive tests, the QS group was tested in SWF configuration with the rate of 0.017 mm/s using a material testing machine, and the IM group was tested in the same configuration inside a pneumatic IM device with the projectile target displacement rate of 3 m/s.One of the IM specimens was excluded due to multiple strikes. The result of this study showed that there were no significant differences in the SLS and SWF stiffnesses between the two groups (P = 0.15 and P = 0.64, respectively). The destructive test results indicated that there was a significant difference in the fracture loads of the two groups (P < 0.00001) with the impact ones being higher; however, they were moderately correlated (R2 = 0.45). Also, the comparison of the fracture location showed a qualitatively good agreement between the two groups.Using the relationship developed herein, results from another study were extrapolated with errors of less than 12%, showing that meaningful predictions for the impact scenario can be made based on the quasi-static tests. The result of this study suggests that there is a potential to replace IM tests with QS displacement rate tests, and this will provide important information that can be used for future studies evaluating clinical factors related to fracture risk.

H induced decohesion of an Al grain boundary investigated with first principles: General conditions for instant breakage and local delayed fracture

The uniaxial tensile test response of a H decorated Σ5 [1 0 0] twist grain boundary (GB) in face-centred-cubic Al has been examined with first principles. The impurity shows a strong tendency to relocate during loading. To capture these H movements, the standard model framework was extended to probe loading-unloading hysteresis. Due to the strong monotonic decrease in the H formation energy with rising GB elongation, the maximum tensile stress accepted by the H decorated GB in the slow fracture limit generally is reached before breakage becomes thermodynamically favourable. For any intact GB configuration visited upon exceeding this stress, the assumption of global chemical equilibrium is argued to be violated. Moreover, while breakage remains ensured from the slow fracture limit considerations, it may in practice require the influx of H to the GB vicinity. A quantitative analysis of GB destabilisation in this scenario requires multi-scale modelling even in the slow fracture limit.