Crack Active Reports 7
Traditionally the development of engineering materials has been predicated on the design of new materials with greater damage tolerance, alongside the development of non-destructive evaluation methods for their inspection such that the part can be withdrawn from service prior to failure. By contrast, inspired by nature, self-healing materials are a relatively new and underexplored class of materials that exhibit the ability to repair themselves and to recover their functionality1. Certain self-healing materials have the ability to partially, or completely, repair cracks that may arise in service, such that the original properties are fully or partially recovered. Self-healing of damage can therefore extend the lifetime and reliability of the material/component. This can be particularly useful in cases where repair or replacement might be difficult or hazardous, or where frequent inspections are difficult. In this respect applications range from oil and gas structures, to aerospace polymer composites, through space applications, to corrosion resistant coatings and even to medical implants.
crack active reports 7
While various strategies (capsules, vascular and intrinsic healing) have been developed to deliver self-healing for different material classes (metals, ceramics, polymers), all rely on a mobile phase or reactive healing agent which can repair the crack. This is triggered either by the occurrence of damage itself (the ideal case, defined as autonomic self-healing), or by external stimuli (non-autonomic self-healing) such as heat or light or targeted triggers such as a laser beam, or inductive or resistive heating1,2. After healing, the repair should have properties approaching that of the undamaged material, retaining structural integrity and extending the lifetime of the material. For structural polymeric materials one of the most successful approaches has been to incorporate microcapsules that are filled with a liquid healing agent (see Fig. 1)3. When a crack propagates through the material the healing agent is released and becomes the mobile phase. Polymerization of the healing agent is triggered by contact with an embedded catalyst, resulting in a bonding of the crack faces.
Schematic of the self-healing process using embedded microcapsules. (a) A crack forms in the matrix due to damage; (b) the growing crack ruptures microcapsules in its path, thereby releasing the healing agent into the crack plane; (c) polymerization occurs and the crack faces are bonded closed3.
The occurrence of self-healing in microcapsule based polymer composite systems has been demonstrated using fracture experiments3,4,5,6, showing as much as 75% recovery of the original fracture toughness and a high healing efficiency reaching values of up to 90%. The average critical fracture load required to propagate a crack in an undamaged self-healing polymer was found to be 20% higher than the average value for neat epoxy samples containing no microcapsules or catalyst3. The inherent toughness of the epoxy is therefore shown to increase with the addition of microcapsules/catalyst. This microcapsule induced toughening has been observed to have a strong dependence on the size and density of the embedded microcapsules4. Furthermore, the presence of the microcapsules has been shown to significantly decrease the fatigue crack growth rate and increase the fatigue lifetime (more than 100 times increase at low K/K1c ratios)5. Given that the microcapsules impact on the stiffness and strength of the polymer it is essential to optimize their size, spacing and activation to effect healing at microcapsule loadings that are as low as possible.
Here we use synchrotron X-ray micro-CT to follow in situ the growth of a crack in a self-healing material for the first time. The material comprises an epoxy matrix containing a microencapsulated reactive epoxy resin and solvent. Epoxies are versatile engineering polymers with excellent mechanical and chemical properties often used as matrices for polymer composites. They can undergo a low-temperature cross-linking reaction when discrete healing agents are introduced2. Upon rupture of the microcapsules and subsequent release of the solvent, the epoxy matrix swells and enables the resin to react with residual amines in the matrix.
A limitation of using conventional absorption contrast X-ray tomography is that the epoxy and the microcapsules have very similar X-ray absorption coefficients making the capsules difficult to differentiate from the matrix. Here we exploit phase contrast and an associated phase filter during reconstruction to significantly improve their detectability. The sample was mounted in a tension-compression mechanical testing rig which could be accommodated on the synchrotron X-ray beamline such that the filling of the crack could be monitored by X-ray CT during loading. This has allowed us to establish a quantitative picture as to the activation sequence over time; in particular how near the microcapsules must be to the crack, how they progressively release their contents, and the extent to which this fills and repairs the crack.
Quantifying the release of solvent into the crack. Percentage solvent released from capsules as a function of position after (a) step 1 and (b) step 2, integrated through thickness (y direction), and (c) the amount of solvent released as a function of distance from the crack plane by the capsules ruptured by step 1 and step 2.
While the increase of the Ni ratio in NCM contributes to an enhanced specific discharge capacity, it also results in severe capacity degradation caused by cation mixing, surface side reactions, and crack propagation with structural instability1. To better understand these challenges, Jung et al. investigated the degradation mechanism of the phase transformation induced by cation mixing from the surface to bulk using ex situ structural analysis7. Similarly, Lin et al. described the surface reconstruction and chemical evolution of the rhombohedral layered structure to a cubic spinel structure using high-throughput X-ray absorption spectroscopy8. As theoretical approaches, electronic correlations for the redox reactions between the multivalent transition metals in the Ni-rich NCM9 and stability analysis with respect to the various ratios of the Ni, Co, and Mn components in the NCM10 have been performed through first-principles calculations. On the bases of these fundamental data, many researchers have suggested solutions to resolve the cyclic degradation problem. Along with diverse approaches such as morphology control11, elemental doping12,13,14,15 and surface coating16,17,18,19,20, Sun et al. have suggested various effective ways to reduce cyclic degradation and improve electrochemical performance through the design of core-shell21, gradient core-shell3,22, and full concentration gradient structures2,23,24 for Ni-rich NCM cathodes.
Recently, Meng et al. reported that the severe crack generation in NCM811 particles induces a significant performance degradation29. According to the scanning electron microscopy (SEM) observation, particle fractures and fragmentation of NCM811 particles were evident after cycles. Due to this crack generation, the discharge capacity of NCM811 was remarkably decreased with increasing overpotentials during cycles. From our fundamental understanding, we suggest that the origin of crack generation is the contraction of primary particles with a mechanical instability caused by heterogeneous phase transformation and anisotropic strain changes. In addition, the lower Gc at delithiated states contributes to a severe crack propagation. Finally, it is expected that these could be resolved by reducing the inhomogeneity and anisotropy of structural changes and increasing Gc.
Flavor innovation in the beverage market continues to soar. From collaborations, themed or limited-edition launches to capitalizing on emerging consumer trends, flavorists are tasked with staying ahead of the curve when it comes to creation. Here are some of the latest beverage market trend reports and product launches.
A release from Celsius reads, "Lemon and lime continue to be one of the most popular flavor combinations in the sparkling beverage category, and Celsius plans to leverage the popularity of the flavor combination to fuel those seeking an active, fit, and healthy lifestyle."
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As many as 34,000 homes constructed in northeastern Connecticut between 1983 and 2000 may have concrete foundations containing pyrrhotite and are at risk of cracking or crumbling. Pyrrhotite is an iron sulfide that can be found naturally in aggregates, or rocky materials such as gravel, sand, or stone that are added to cement to make concrete. When iron sulfides are exposed to oxygen and water, a series of chemical reactions convert the iron sulfides into other compounds.
Reports of crumbling foundations first began in 2015. By May 2017, the Connecticut Department of Consumer Protection (CDCP) had received reports of more than 550 homes with faulty foundations, and in December 2017 began processing 522 verified reports to determine compensation eligibility. 350c69d7ab