Room temperature crack-healing in an atomically layered ternary carbide

Ceramic materials provide outstanding chemical and structural stability at high temperatures and in hostile environments but are susceptible to catastrophic fracture that severely limits their applicability. Traditional approaches to partially overcome this limitation rely on activating toughening mechanisms during crack growth to postpone fracture. Here, we demonstrate a more potent toughening mechanism that involves an intriguing possibility of healing the cracks as they form, even at room temperature, in an atomically layered ternary carbide. Crystals of this class of ceramic materials readily fracture along weakly bonded crystallographic planes. However, the onset of an abstruse mode of deformation, referred to as kinking in these materials, induces large crystallographic rotations and plastic deformation that physically heal the cracks. This implies that the toughness of numerous other layered ceramic materials, whose broader applications have been limited by their susceptibility to catastrophic fracture, can also be enhanced by microstructural engineering to promote kinking and crack-healing.

Mechanical Properties of WC-Si3N4 Composites With Ultrafine Porous Boron Nitride Nanofiber Additive

WC-10 wt.% Si3N4 composites toughened with ultrafine porous boron nitride nanofiber (0, 0.01, 0.05, 0.1, and 0.15 wt.%) were prepared for the first time by spark plasma sintering. Compared with the WC-Si3N4 composite sintered in the same condition, the obtained WC-10 wt.% Si3N4 composites with ultrafine porous boron nitride were found to possess better hardness and fracture toughness. In addition, the Si3N4 phase in the UPBNNF toughened composites did not exhibit traditional catastrophic fracture as indicated in most investigations. In this study, the phenomena are discussed, and a probable mechanism is elucidated. It is deduced that the approach could be extended to materials with a feature of internal liquid phase during the sintering process and could improve hardness and fracture toughness.

Catastrophic Carpal Fractures in Racehorses during Traditional Competitions: Assessing Mechanobiology using Multibody Modelling and Comparative Analysis

Background: City circuit competitions (Palio) in Italy are traditional horse race held in the heart of some cities centre once a year. The prevalence of accidents during these competitions is only anecdotally reported; however there is a diffuse perception that these events adversely affect racehorse industry and impact on equine welfare. The aim of the study is to understand the basic physiopathology of catastrophic fracture of the carpus in a 7-y old thoroughbred euthanized following catastrophic fracture of the left carpus during a traditional horse race, comparing the localization and the magnitude of the contact forces at the level of the affected joint, obtained in simulated conditions with the macro- and microscopic structural damages.Methods: A retrospective analysis of a thoroughbred racehorse galloping at high speeds on an urban racetrack conditions was set up. Computational modelling of the carpal joint was generated using a multibody code for dynamics simulation, considering the circuit design, the speed of the animal, and the surface characteristic. The results were compared to the findings observed during computed tomography, micro-computed tomography and histological evaluation following hematoxylin and eosin and safranin-O fast green staining.Results: The articular surfaces of the radial, intermediate and ulnar facet of the radius together with the proximal articular surface of small carpal bones exhibited diffuse wear lines, erosions of the articular cartilage and subchondral bone exposure. The fracture line along the radial carpal bone initiating the injury does not travel in a region with bone sclerosis. In the computational model, the peak force for the contact between the radius and the radial carpal bone has a value of 6880 N. Conclusions: This study highlight how during traditional racing circuits it develops elevate impact forces at the level of contact surfaces of the carpal joint, due to effect of speed and curve, causing catastrophic bone fracture and hyaline cartilage breakdown in the absence of pre-existing pathology.

Non-Extensive Statistical Analysis of Acoustic Emissions: The Variability of Entropic Index q during Loading of Brittle Materials until Fracture

Non-extensive statistical mechanics (NESM), introduced by Tsallis based on the principle of non-additive entropy, is a generalisation of the Boltzmann–Gibbs statistics. NESM has been shown to provide the necessary theoretical and analytical implementation for studying complex systems such as the fracture mechanisms and crack evolution processes that occur in mechanically loaded specimens of brittle materials. In the current work, acoustic emission (AE) data recorded when marble and cement mortar specimens were subjected to three distinct loading protocols until fracture, are discussed in the context of NESM. The NESM analysis showed that the cumulative distribution functions of the AE interevent times (i.e., the time interval between successive AE hits) follow a q-exponential function. For each examined specimen, the corresponding Tsallis entropic q-indices and the parameters βq and τq were calculated. The entropic index q shows a systematic behaviour strongly related to the various stages of the implemented loading protocols for all the examined specimens. Results seem to support the idea of using the entropic index q as a potential pre-failure indicator for the impending catastrophic fracture of the mechanically loaded specimens.

Seismic events associated with catastrophic fracture propagation in rock under compression

<p>Fracture growth produced by compressive stress is typically restricted to the sizes of the pre-existing defect seeding the fracture. However the biaxial compression acting near a free surface (e.g. a wall of an opening or at a large scale, the Earth’s surface) can change the fracture growth mechanism. Our experiments demonstrate that in the presence of the second, intermediate principal stress (the minor principal stress is nearly zero in the vicinity of free surface) leads to extensive fracture propagation. Furthermore, the interaction of the propagating fracture with the free surface makes the growth unstable (catastrophic). This produces a seismic event and can lead to such a dangerous and hazardous dynamic rock failure such as skin rockburst.</p><p> </p><p>Our previous experiments on brittle transparent samples with an internal initial crack under biaxial compression showed that a small magnitude of the intermediate principal stress, around 5% of the major principal stress, is sufficient to ensure extensive fracture propagation [1- 3]. The catastrophic fracture propagation is then induced by the interaction between the fracture and the free surface as the presence of the free surface imposes additional tensile stresses on the growing fracture. The type of the associated seismic event is the Compensated Linear Vector Dipole (CLVD) source. We present a simple model that allows the determination of the conditions of unstable fracture propagation and the energy of the associated seismic event. The results of this research contribute to the understanding of the nature of seismic events and the mechanics of skin rockburst.</p><p> </p><ol><li>Wang, H., Dyskin, A.V. Pasternak, E Dight, P Sarmadivaleh, M. 2018. Effect of the intermediate principal stress on 3-D crack growth. Engineering Fracture Mechanics, 204, 404-420.</li> <li>Wang, H., Dyskin, A.V. and Pasternak, E. (2019) Comparative analysis of mechanisms of 3-D brittle crack growth in compression, Engineering Fracture Mechanics220, 106656.</li> <li>Wang, H., Dyskin, A. Pasternak, E., Dight, P. and Sarmadivaleh, M. (2019) Experimental and numerical study into 3D crack growth from a spherical pore in biaxial compression, Rock Mechanics and Rock Engineering, doi.org/10.1007/s00603-019-01899-1.</li> </ol><p> </p><p><strong>Acknowledgements</strong>. AVD and EP acknowledge support from the Australian Research Council through project DP190103260. The first author acknowledges financial support from the Australian Centre for Geomechanics. The authors are grateful to Mr. Frank EE How Tan for his assistance with specimen preparation. AVD acknowledges the support from the School of Civil and Transportation, Faculty of Engineering, Beijing University of Civil Engineering and Architecture.</p>

Reconstruction of the pelvis after traumatically induced bilateral partial hemipelvectomy: a case report

Background Traumatic hemipelvectomy is a catastrophic fracture of the pelvis as a result of high-energy trauma, such as in a car accident. There have been few case reports of traumatic hemipelvectomy because many of these patients die before they are transferred to a hospital. However, an increasing number of patients are being saved and admitted to hospital due to improvements in resuscitation and the emergency response system. Accordingly, there has been a growing body of reports on the management and reconstruction of traumatic hemipelvectomy. Case presentation A healthy 20-year-old Japanese man was trapped beneath a 3-ton steel frame while working on a crane. We describe here a very challenging case of traumatically induced bilateral partial hemipelvectomy with successful reconstruction of our patient’s pelvis using a unilateral anterolateral thigh flap. Conclusion To the best of our knowledge, there have been few reports of bilateral hemipelvectomy and our case is the first to be successfully treated with a unilateral anterolateral thigh flap.

Strain-induced accelerated asymmetric spatial degradation of polymeric vascular scaffolds

Polymer-based bioresorbable scaffolds (BRS) seek to eliminate long-term complications of metal stents. However, current BRS designs bear substantially higher incidence of clinical failures, especially thrombosis, compared with metal stents. Research strategies inherited from metal stents fail to consider polymer microstructures and dynamics––issues critical to BRS. Using Raman spectroscopy, we demonstrate microstructural heterogeneities within polymeric scaffolds arising from integrated strain during fabrication and implantation. Stress generated from crimping and inflation causes loss of structural integrity even before chemical degradation, and the induced differences in crystallinity and polymer alignment across scaffolds lead to faster degradation in scaffold cores than on the surface, which further enlarge localized deformation. We postulate that these structural irregularities and asymmetric material degradation present a response to strain and thereby clinical performance different from metal stents. Unlike metal stents which stay patent and intact until catastrophic fracture, BRS exhibit loss of structural integrity almost immediately upon crimping and expansion. Irregularities in microstructure amplify these effects and can have profound clinical implications. Therefore, polymer microstructure should be considered in earliest design stages of resorbable devices, and fabrication processes must be well-designed with microscopic perspective.

Performance Evaluation of Superhard Coatings in Drilling of Ti-6Al-4V Alloy

In the aerospace industry, titanium (Ti) alloys, especially Ti6Al4V, has been extensively used over other light weight alloys due to their high strength-to-weight ratio. However, the material and production costs have been major obstacles in the adoption of Ti alloys for a wide variety of applications. The machining of Ti alloys is one of the most time consuming and expensive mechanical processes in aerospace manufacturing. Based on previous literature on the topic, coated drills have had some degree of success in the drilling of Ti. To further the work, this paper conducts a comparative study in which Ti6Al4V plates are drilled with super hard coated drills such as Diamond-like-Carbon (DLC), AlMgB14 (BAM) and nanocomposite AlCrSiN. The results are compared with those of an uncoated drill bit. Working with a coating supplier, several variations of BAM coating have been applied and used in our drilling experiments. To evaluate the performance of these drills, scanning electron microscopy and confocal laser microscopy were used to assess the wear progress of each drill qualitatively and quantitatively. In drilling Ti alloys, the primary mechanisms of flank wear are abrasion, microscopic fracture (chipping) and attrition, which result in the detachment of the adhesion layer located at the cutting edge. For all the drills, the predominant wear occurs near the margin. From our drilling experiments, it has been observed that AlCrSiN and BAM drills have survived up to 58 holes and over 80 holes, respectively, while both uncoated and DLC drills have experienced catastrophic fracture at less than 40 holes.