Muscles and some tough hydrogels can maintain perfect mechanical properties after millions of loading cycles owing to the anisotropic microstructures inside them. However, applications of intrinsic ...anisotropic microstructures in biological tissues and tough hydrogels are limited by the poor mechanical performance in the perpendicular direction relative to the alignment direction. Here, a universal strategy is proposed for developing hydrogels with unprecedented isotropic crack propagation resistance only depending on the interpenetrating entanglements of polymer chains (polyacrylamide (PAAM) or poly‐(1‐acrylanmido‐2‐methylpropanesulfonic acid) (PAMPS)) in deformable polymeric microspheres (PAMPS or PAAM). The deformable interpenetrating network in microspheres can transform the hydrogel from isotropic to anisotropic instantaneously in any load direction, and effectively alleviate the stress concentration at the crack tip, dissipate energy, and eliminate notch sensitivity. The best isotropic hydrogel displays an ultimate strain of 5300%, toughness of 18.9 MJ m–3, fracture energy of 157 kJ m–2, and fatigue threshold of 4.2 kJ m–2. Furthermore, the mechanical strength of hydrogels can be simply tuned by solvent replacement. The strategy presented here can be expanded to prepare other isotropic hydrogels with super tear‐resistant and anti‐fatigue properties, based on a wide variety of deformable microspheres and matrix polymers.
A universal strategy for the preparation of hydrogels with isotropic and unprecedented crack propagation resistance is presented. The hydrogels integrate the seemingly contradictory mechanical properties such as ultra‐stretchability, fatigue resistance, tear resistance, and high toughness by the interpenetrating entanglements of polymer chains in the deformable polymeric microspheres. The prepared hydrogel displays a strain of 5300%, toughness of 18.9 MJ m–3, fracture energy of 157 kJ m–2, and fatigue threshold of 4.2 kJ m–2.
Featured Cover Liu, Zhenhai; Ye, Hongfei; Qian, Dong ...
International journal for numerical methods in engineering,
04/2021, Letnik:
122, Številka:
7
Journal Article
Recenzirano
The cover image is based on the Research Article A time‐discontinuous peridynamic method for transient problems involving crack propagation by Zhenhai Liu et al., https://doi.org/10.1002/nme.6602.
•Mechanical properties of the caprock are significantly changed under subsurface reservoir conditions and cyclic loading of hydrogen.•Hydrogen-brine multi-phase flow in the pore spaces of the caprock ...may increase capillary stress and fracture apertures in the rock resulting shrinkage cracking.•Stress-induced critical cracks and subcritical crack under the influence of geochemical reactions may cause significant impacts on carpock integrity during UHS.•Hydrogen diffusion and capillary trapping are amplified with the mechanical weakening and crack formation in the caprock.
This paper comprehensively reviews mechanical weakening and crack development in the caprock during underground hydrogen storage in depleted gas reservoirs. Hydrogen loss due to the geochemical interactions of hydrogen and caprock minerals critically impacts caprock integrity. As shown in the review, it is conspicuous that the mechanical properties of the caprock also change with the hydrogen injection, affecting its brittle-ductile behaviour. Furthermore, the stress–strain behaviour of the caprock is changed, and it undergoes irreversible deformations under the influence of confining pressure, cyclic loading, and changes in the mineral composition.
The fracturing of the caprock is another critical impact on the storage integrity, which may create new routes for the hydrogen permeation through the caprock. Cracks may form in the caprock in multiple ways, mainly 03 ways; 1) Highly pressurized hydrogen injection creates critical cracks in the caprock when the pore pressure exceeds the fracture toughness, called critical cracks, 2) The injected hydrogen accumulates under the caprock due to gravity segregation and, eventually, diffuses into the caprock, displacing its pore fluid (brine). Consequently, capillary stress on the caprock minerals and the pores may increase with developing cracks, called shrinkage cracks, and 3) Geomechanical interaction between hydron-pore fluid-rock minerals under the biotic environment (micro-organisms) available at underground storage sites can cause mechanical properties degradation in caprock, forming new cracks under low injection pressure conditions, called sub-critical cracks.
Although the critical or tensile crack formation process has been widely studied in the existing studies, minor attention has been given to other possible crack formation processes in the caprock, including the hydrogen-induced shrinkage cracking and the geomechanical reactions causing sub-critical cracking. In addition, the impact of this mechanical weakening of the caprock on its overall structure and flow characteristics hasn't been properly understood, adding extra uncertainty to the caprock's integrity during the underground hydrogen storage process.
•The fracture toughness and crack propagation velocity of sandstone decreased with the increase of water content.•The cumulative AE counts of wet specimens in the NSCB tests were much less than those ...of dry ones.•The DIC technique was used to measure and analyze the progress of fracture on specimen surface.
This study investigated the effect of water content on quasi-static fracture behavior of sandstone. Notched semi-circular bending (NSCB) tests were conducted on a total number of 20 sandstone specimens with different water contents (0, 1.0, 2.0 and 3.5%) to determine their fracture toughness. During the NSCB tests, the cracking process and acoustic emission (AE) signals were recorded continuously with the aid of a charged couple discharge (CCD) camera and an AE system, and the crack propagation velocity was also measured accurately via a crack propagation gauge (CPG). Test results demonstrated that both the fracture toughness and crack propagation velocity observably decreased with the increase of water content, the variation trend of which could be described by exponential equations. The cumulative AE counts of wet specimens in the NSCB tests were much less than those of dry ones, which indicated that the sandstone specimens underwent more ductile failure and released less elastic energy due to water-softening effects.
Fatigue crack propagation tests of Ti‐6Al‐4V fabricated by the Wire+Arc Additive Manufacturing (WAAM) process are analysed. Crack growth rate and trajectory are examined before and after the crack ...tip crossing an interface between the WAAM and wrought alloys. The study has focused on the microstructure and residual stress effect. First, the differences in crack growth rate and path between WAAM and wrought alloys are attributed to their different microstructure; the equiaxed wrought alloy has straight crack path, whereas the WAAM lamellar structure causes tortuous crack path resulting in lower crack growth rate. Second, based on measured residual stress profile in the as-built WAAM piece, retained residual stress in the much smaller compact tension specimens and its effect on crack growth rate are calculated by the finite element method. Numerical simulation shows considerable residual stress in the test specimen and the stress magnitude depends on the initial crack location and propagation direction in relation to the WAAM-wrought interface. Residual stress is released immediately if the initial crack is in the wrought substrate; hence it has little effect. In contrast, when crack grows from WAAM to wrought, residual stress is retained resulting in higher stress intensity factor; hence greater crack growth rate.
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•Crack growth pattern is examined near the interface between wrought and WAAM deposited Ti-6Al-4V.•Crack grows straight in wrought but tortuously in WAAM Ti-6Al-4V, which could be attributed to the difference in microstructure.•There are considerable residual stresses retained in the WAAM built wall and C(T) specimens.•The effect of residual stress depends on the crack location and growth direction.
The microstructure evolution and the fatigue damage mechanism of hydrogen-charged high-strain pipeline steel during the cyclic strain loading is studied. Hydrogen promotes the formation of ...dislocation walls and cells, and causes the lattice distortion iron atoms to move to the grain boundary, resulting in grain refinement. The fatigue life is reduced to more than 40 % when the strain amplitude reaches 0.5 % after hydrogen-charged. The main reasons for the reduce are the decrease of the number and the change of propagation mode of secondary cracks, and the increase of cleavage fracture and the decrease of deflection angle in the main cracks.
•Hydrogen accelerates the grain refinement in cyclic strain loading.•Hydrogen promotes the rapid formation of dislocation walls and cells.•Hydrogen causes the change of fatigue main crack and secondary crack growth modes.
Selective-laser-melting (SLM) is a powder-bed fusion additive-manufacturing process that has the potential to deliver three-dimensional complex parts with mechanical properties comparable or superior ...to parts produced via traditional manufacturing using cast and wrought alloys. Concerns for metallic parts built via SLM are the process-induced residual stresses, and anisotropic mechanical properties. This paper investigates the effect of residual stresses on the fatigue crack growth rate of SLM Ti6Al4V in as-built and stress-relieved conditions. Neutron diffraction and the contour method are employed to measure residual stresses in compact-tension samples. Neutron diffraction results are in good agreement with the contour method. It was found that tensile stresses are present at the notch root and the free edge areas, and compressive stress is seen in the middle of the sample. The tensile stresses in the as-built condition resulted in a higher fatigue crack growth rate. After stress relieving by heat treatment, the tensile residual stress diminished by around 90%, resulting in decreased crack growth rate. The build direction was seen to affect the crack growth rate, although the trend was different between the as-built and stress-relieved conditions.
In this recommendation, standard testing methods for determination of the double-K criterion for Mode I crack propagation in concrete using wedge-splitting tests and three-point bending beam tests ...are specified for the fracture parameters of the initial cracking toughness
K
Ic
ini
and the unstable fracture toughness
K
Ic
un
. Along with the recommendation of the standard testing methods, the theoretical background of the double-K criterion, the calculation methods and the results of round-robin testing for determining the double-K fracture parameters are presented in technical reports
1
,
2
. The recommendation of the standard testing methods includes geometry for specimens, fabrication of specimens, testing machine, load transmission system and supports, measuring instruments, determination of initial cracking load
P
ini
,
determination of measured maximum load
P
max
and initial compliance
c
i
,
calculation methods for wedge-splitting test and three-point bending beam test, as well as the testing results and testing report forms. According to these methods, the measured fracture parameters of double-K criterion can be used to describe the onset of cracking, and the onset of unstable cracking or failure for predicting crack initiation, structure failure and crack stability in concrete structures.
Fatigue crack propagation behavior in ultra-high-performance concrete (UHPC) containing different volume fractions of steel fibers is investigated under cyclic flexural loading at various stress ...levels. Evolutions of the crack mouth opening displacement, crack tip strain, and crack propagation length are characterized during continuous loading cycles via digital image correlation. Test data are fit to smooth, continuous logarithmic functions to discern relationships between evolving crack length and the number of loading cycles. The crack propagation rates of different UHPC specimens are then determined from the first derivative of these logarithmic functions. A critical crack length of approximately 20 mm is found in UHPC containing a fiber volume fraction (Vf) of 0.5%, while that of other specimens (Vf = 1.0%, 1.5%, and 2.0%) is found to approach 60 mm under the stress levels of 0.80, 0.75, 0.70 and 0.65. Strain diagrams obtained via DIC reveal no obvious changes for the crack growth along the y-direction in UHPC specimens containing fiber volume fractions of 0.5% and 1.0%, while a tendency for strain concentrations oriented at 45° characterizes crack development in specimens containing 1.5% and 2.0% volume fractions of fiber. Crack propagation rates during the stable stage of crack propagation decrease upon reducing applied stress level and with increased content of steel fiber reinforcement, providing predominant means to enhance fatigue life.
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