The relationship of weathering cracks to split-line orientation in bone

The essential identity of weathering cracks and split-lines is demonstrated experimentally. Innominate bones, left in the open to weather, developed cracks similar to the split-line orientation typically observed in the same region. These bones were decalcified and intermittent split-lines prepared along their course. The orientation of the split-lines always corresponds to the weathering splits. The same results were obtained with bird and mammal bones collected in the field and showing weathering cracks. The same also applies to cracks in bones of a gorilla which had been partly defleshed and salted down for preservation. Apparently any process which shrinks bone will produce cracks with the same orientation as split-lines. Possibilities for split-line analysis of fossil and archeological bone are opened up by these experiments.     

Detection of cracks in dried spaghetti using transmission images

Cracks are formed during the drying process of spaghetti production, which reduces its commercial value and quality; hence, it is very important to detect the cracks during the manufacturing process of dried spaghetti. However, the presence of mottles, originating from wheat bran, hinders automatic identification of cracks. In this study, we developed a simple method to detect the cracks induced in spaghetti using a digital camera. The cracks and mottles were distinguished by pixelating the transmission light image of the spaghetti and analyzing the image based on the geometric characteristics in the histogram consisting of all the pixels. The method was able to detect the cracks induced in the spaghetti with a rough surface, which was prepared via a bronze die using vacuum extrusion molding, as well as those induced in the spaghetti with a smooth surface, which was prepared using a Teflon die.     

The behaviour of roots encountering cracks in soil

Summary Experimental methods are described for observing the behaviour of roots encountering cracks in soil. The proportions of roots which enter a second soil block after crossing a crack of known width were measured. Soil strength was measured with a penetrometer.Results are presented for the proportions of seminal roots of wheat and primary lateral roots of pea which enter moulded soil of various strengths after crossing cracks. Results are also presented for the proportions of seminal roots of pea, rape and safflower which enter undisturbed soil after crossing cracks.It was found that, in all cases, the proportion of roots penetrating the second soil block decreased with increasing crack width and increasing soil strength. Also, a smaller proportion of thinner roots penetrated the second soil block than thicker roots under similar conditions. Root diameter in the cracks was influenced by both crack width and soil strength, and an empirical equation is presented to describe this effect.     

Cellulases dig deep: in situ observation of the mesoscopic structural dynamics of enzymatic cellulose degradation

Enzymatic hydrolysis of cellulose is key for the production of second generation biofuels, which represent a long-standing leading area in the field of sustainable energy. Despite the wealth of knowledge about cellulase structure and function, the elusive mechanism by which these enzymes disintegrate the complex structure of their insoluble substrate, which is the gist of cellulose saccharification, is still unclear. We herein present a time-resolved structural characterization of the action of cellulases on a nano-flat cellulose preparation, which enabled us to overcome previous limitations, using atomic force microscopy (AFM). As a first step in substrate disintegration, elongated fissures emerge which develop into coniform cracks as disintegration continues. Detailed data analysis allowed tracing the surface evolution back to the dynamics of crack morphology. This, in turn, reflects the interplay between surface degradation inside and outside of the crack. We observed how small cracks evolved and initially increased in size. At a certain point, the crack diameter stagnated and then started decreasing again. Stagnation corresponds with a decrease in the total amount of surface which is fissured and thus leads to the conclusion that the surface hydrolysis "around" the cracks is proceeding more rapidly than inside the cracks. The mesoscopic view presented here is in good agreement with various mechanistic proposals from the past and allows a novel insight into the structural dynamics occurring on the cellulosic substrate through cellulase action.     

Direct visualization of asymmetric behavior in supported lipid bilayers at the gel-fluid phase transition

We utilize in situ, temperature-dependent atomic force microscopy to examine the gel-fluid phase transition behavior in supported phospholipid bilayers constructed from 1,2-dimyristoyl-sn-glycero-3-phosphocholine, 1,2-dipentadecanoyl-sn-glycero-3-phosphocholine, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. The primary gel-fluid phase transition at T(m) occurs through development of anisotropic cracks in the gel phase, which develop into the fluid phase. At approximately 5 degrees C above T(m), atomic force microscopy studies reveal the presence of a secondary phase transition in all three bilayers studied. The secondary phase transition occurs as a consequence of decoupling between the two leaflets of the bilayer due to enhanced stabilization of the lower leaflet with either the support or the water entrained between the support and the bilayer. Addition of the transmembrane protein gramicidin A or construction of a highly defected gel phase results in elimination of this decoupling and removal of the secondary phase transition.     

Diatomaceous earth as a protective vehicle for bacteria applied for self-healing concrete

Crack repair is crucial since cracks are the main cause for the decreased service life of concrete structures. An original and promising way to repair cracks is to pre-incorporate healing agents inside the concrete matrix to heal cracks the moment they appear. Thus, the concrete obtains self-healing properties. The goal of our research is to apply bacterially precipitated CaCO₃ to heal cracks in concrete since the microbial calcium carbonate is more compatible with the concrete matrix and more environmentally friendly relative to the normally used polymeric materials. Diatomaceous earth (DE) was used in this study to protect bacteria from the high-pH environment of concrete. The experimental results showed that DE had a very good protective effect for bacteria. DE immobilized bacteria had much higher ureolytic activity (12–17 g/l urea was decomposed within 3 days) than that of un-immobilized bacteria (less than 1 g/l urea was decomposed within the same time span) in cement slurry. The optimal concentration of DE for immobilization was 60% (w/v, weight of DE/volume of bacterial suspension). Self-healing in cracked specimens was visualized under light microscopy. The images showed that cracks with a width ranging from 0.15 to 0.17 mm in the specimens containing DE immobilized bacteria were completely filled by the precipitation. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to characterize the precipitation around the crack wall, which was confirmed to be calcium carbonate. The result from a capillary water absorption test showed that the specimens with DE immobilized bacteria had the lowest water absorption (30% of the reference ones), which indicated that the precipitation inside the cracks increased the water penetration resistance of the cracked specimens.     

Letter to the editor: Electron diffraction for hydrated crystalline biopolymers: nigeran.

A method is presented to record electron diffraction diagrams on hydrated crystalline biopolymers. It involves quick-freezing of the hydrated crystals, which are maintained at a low temperature throughout the experiment. The technique is illustrated with lamellar single crystals of the linear polysaccharide nigeran. Water of hydration is shown to be in a sheet-like array and its removal is accompanied by development of lateral cracks. This morphological shrinkage coincides with an observed shortening of the b lattice parameter, which is normal to the lateral cracks.     

Formation of Ice Microparticles in the Ovarian Fluid and Homogenates of Unfertilized Russian Sturgeon Eggs during Cooling to –196°C

Cryopreservation of fish and amphibian eggs is still an unsolved problem. The formation of ice crystals inside and outside cells acts as a main detrimental factor during a deep freezing of fish eggs, as well as crystal growth (recrystallization and repeated crystallization). Designing efficient cryoprotective media is necessary in order to avoid egg injury from freezing. Additional components that are present in a cryoprotective medium and reduce the thermomechanical stress and cracks of frozen tissues might increase oocyte survival after freezing–thawing. Natural components of eggs and the ovarian fluid are promising as such additives. The formation of ice microparticles was studied in thin layers (0.2 mm) of the ovarian fluid and components of Russian sturgeon egg homogenates upon their cooling to a liquid nitrogen temperature (–196°C). The processes of freezing, ice cracking, and microparticle formation were observed as the temperature was decreased gradually. The shape and size of ice microparticles were found to depend on the composition of the freezing solution. Certain fractions of egg homogenate were assumed to be suitable as components of a cryoprotective medium.

     

Concrete Crack Restoration Using Bacterially Induced Calcium Metabolism

Concrete structures are prone to develop cracks and cause devastation. Repair and renovation are not enough to ensure complete eradication of crack development. The entire process is costly and laborious. The microbiologically induced calcium carbonated precipitation can be effective in restoring the cracks. The calcium-based nutrients along with specific bacterial strain have been used in the present investigation. The pellets of calcium as per Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy are deposited in the cracks of the concrete over a period of 7 days of incubation. The presence of bacteria in the calcium precipitates as demonstrated by scanning electron microscope provides adequate strength and adhering quality to the pellets. The effective filling of cracks is confirmed with the help ultrasonic pulse velocity test also. Since, elephantine heritage and high sky buildings have high maintenance costs, the use of present technique will cut down the cost and duration of restoration.Supplementary InformationThe online version of this article (10.1007/s12088-020-00916-0) contains supplementary material, which is available to authorized users.     

Characterization of shrinkage cracks in medium black clay soil of madhya pradesh

Summary In a field experiment, the pattern and size of shrinkage cracks were studied under three vegetative covers of wheat crop, grass and cultivated fallow. Both the pattern and size of cracking varied widely. Under wheat crop, the major cracks developed parallel to the rows particularly midway between the two rows of plants. The cracks were few in number and simple in nature. And so was the case under grass where the major cracks developed either in between or around the grass tussocks. However, under cultivated fallow development of too many cracks forming an intricate network showed no definite pattern of cracking. In a soil other than the cultivated fallow, the pattern of cracking appeared to be a function of positioning of the plants rather than of the soil itself.As far as the size of cracks is concerned, the widest and deepest cracks developed under wheat crop and narrowest and shallowest under cultivated fallow. Under grass, the width and depth of the cracks was observed to be intermediate between the two extremes of wheat and cultivated fallow. The size of cracks seemed to depend on the magnitude of water loss from the soil. re]19760713     

Self-Healing Efficiency of Cementitious Materials Containing Microcapsules Filled with Healing Adhesive: Mechanical Restoration and Healing Process Monitored by Water Absorption

Autonomous crack healing of cementitious composite, a construction material that is susceptible to cracking, is of great significance to improve the serviceability and to prolong the longevity of concrete structures. In this study, the St-DVB microcapsules enclosing epoxy resins as the adhesive agent were embedded in cement paste to achieve self-healing capability. The self-healing efficiency was firstly assessed by mechanical restoration of the damaging specimens after being matured. The flexural and compressive configurations were both used to stimulate the localized and distributed cracks respectively. The effects of some factors, including the content of microcapsules, the curing conditions and the degree of damage on the healing efficiency were investigated. Water absorption was innovatively proposed to monitor and characterize the evolution of crack networks during the healing process. The healing cracks were observed by SEM-EDS following. The results demonstrated that the capsule-containing cement paste can achieve the various mechanical restorations depending on the curing condition and the degree of damage. But the voids generated by the surfactants compromised the strength. Though no noticeable improved stiffness obtained, the increasing fracture energy was seen particularly for the specimen acquiring 60% pre-damage. The sorptivity and amount of water decreased with cracks healing by the adhesive, which contributed to cut off and block ingress of water. The micrographs by SEM-EDS also validated that the cracks were bridged by the hardened epoxy as the dominated elements of C and O accounted for 95% by mass in the nearby cracks.     

Can the theory of critical distances predict the failure of shape memory alloys?

Components made from shape memory alloys (SMAs) such as nitinol often fail from stress concentrations and defects such as notches and cracks. It is shown here for the first time that these failures can be predicted using the theory of critical distances (TCDs), a method which has previously been used to study fracture and fatigue in other materials. The TCD uses the stress at a certain distance ahead of the notch to predict the failure of the material due to the stress concentration. The critical distance is believed to be a material property which is related to the microstructure of the material. The TCD is simply applied to a linear model of the material without the need to model the complication of its non-linear behaviour. The non-linear behaviour of the material at fracture is represented in the critical stress. The effect of notches and short cracks on the fracture of SMA NiTi was studied by analysing experimental data from the literature. Using a finite element model with elastic material behaviour, it is shown that the TCD can predict the effect of crack length and notch geometry on the critical stress and stress intensity for fracture, with prediction errors of less than 5%. The value of the critical distance obtained for this material was L?=?90?μm; this may be related to its grain size. The effects of short cracks on stress intensity were studied. It was shown that the same value of the critical distance (L?=?90?μm) could estimate the experimental data for both notches and short cracks.     

Timing of the occurrence of frost cracks in winter

In order to determine the timing of the occurrence of frost cracks, as well as to evaluate the climatic conditions associated with such occurrences, study plots were established in late autumn and early winter. Serial observations were made to identify both the occurrence of new frost cracks and the re-opening of old frost cracks in trees in the study plots until mid-winter or early spring. Field observations were conducted for three winter seasons in different study plots. Most old frost cracks were found to re-open in early winter. However, such re-opening did not occur simultaneously within a very limited period of time, for example, within a single day or night, when air temperature fell suddenly or considerably. Re-opening seemed to occur steadily over the course of several days of continuous subzero temperatures. It has been suggested that freezing of the trunk contributes considerably to the re-opening of old frost cracks. Four frost cracks from 1002 trees were newly formed during the course of this study. The new frost cracks developed both in early winter and mid-winter. It has been suggested that new frost cracks can occur during the same period when most old frost cracks re-open and the air temperature does not fall far below 0°C. However, it remains unclear whether or not there is a tendency for new frost cracks to occur during a particular period.     

Desiccation cracks act as natural seed traps in flood-meadow systems

Desiccation cracks are a natural phenomenon of clay-rich soils that form via soil shrinkage during dry conditions. Our aim was to test the seed trapping potential of such cracks and assess its impact on seed bank formation in a flood-meadow ecosystem. We documented crack patterns on permanent plots and analysed the soil seed content along and adjacent to cracks. Seed translocation via cracks was tested with a mark-recapture experiment, and post-entrapment seed fate was tested with a burial experiment. Most cracks re-opened in the same positions in consecutive dry periods. Along cracks, most seeds were found in 10–20 cm depth, whereas adjacent to cracks most seeds were found in 0–5 cm depth. The majority of seeds found in shallow depths adjacent to cracks belonged to species that were also present in the above-ground vegetation, whereas this rate was always under 50% along desiccation cracks. The mark-recapture experiment gave evidence for vertical seed translocation through desiccation cracks. Post-entrapment seed fate differed between species and burial depth, with a trend towards increasing survival with increasing depth. We conclude that desiccation cracks act as natural seed traps, foster seed bank formation and thus influence plant community dynamics in flood meadow systems.     

The role of the lamellar interface during torsional yielding of human cortical bone

Fragility fractures are a result of alterations in bone quantity, tissue properties, applied loads, or a combination of these factors. The current study addresses the contribution of cortical bone tissue properties to skeletal fragility by characterizing the shear damage accumulation processes which occur during torsional yielding in normal bone. Samples of human femoral cortical bone were loaded in torsion and damaged at a post-yield twist level. The number of microcracks within osteons, interstitial tissue, and along cement lines were assessed using basic fuchsin staining. Damage density measures (number of cracks/mm2) were correlated with stiffness degradation and changes in relaxation. Damaged samples exhibited a wide variation in total microcrack density, ranging from 1.1 to 43.3 cracks/mm2 with a mean density of 19.7 +/- 9.8 cracks/mm2. Lamellar interface cracks comprised more than 75% of the total damage, indicating that the lamellar interface is weak in shear and is a principal site of shear damage accumulation. Damage density was positively correlated with secant stiffness degradation, but only explained 22% of the variability in degradation. In contrast, damage density was uncorrelated with the changes in relaxation, indicating that a simple crack counting measure such as microcrack density was not an appropriate measure of relaxation degradation. Finally, a nonuniform microcrack density distribution was observed, suggesting that internal shear stresses were redistributed within the torsion samples during post-yield loading. The results suggested that the lamellar interface in human cortical bone plays an important role in torsional yielding by keeping cracks physically isolated from each other and delaying microcrack coalescence in order to postpone the inevitable formation of the fatal crack.     

Propagation of surface fatigue cracks in human cortical bone

An understanding of how fatigue cracks grow in bone is of importance as fatigue is thought to be the main cause of clinical stress fractures. This study presents new results on the fatigue-crack growth behavior of small surface cracks (approximately 75-1000 microm in size) in human cortical bone, and compares their growth rates with data from other published studies on the behavior of both surface cracks and many millimeter, through-thickness large cracks. Results are obtained with a cyclically loaded cantilever-beam geometry using optical microscopy to examine for crack growth after every 100-500 cycles. Based on the current and previous results, small fatigue cracks appear to become more resistant to fatigue-crack growth with crack extension, analogous to the way the fracture resistance of cortical bone increases with crack growth. Mechanistically, a theory attributing such behavior to the development of bridges in the wake of the crack with crack growth is presented. The existence of such bridges is directly confirmed using optical microscopy.     

Stress intensity factors for a vertical surface crack in polyethylene subject to rolling and sliding contact.

Pitting wear is a dominant form of polyethylene surface damage in total knee replacements, and may originate from surface cracks that propagate under repeated tribological contact. In the present study, stress intensity factors, KI and KII, were calculated for a surface crack in a polyethylene-CoCr-bone system in the presence of rolling or sliding contact pressures. Variations in crack length and load location were studied to determine probable crack propagation mechanisms and modes. The crack tip experienced a wide range of mixed-mode conditions that varied as a function of crack length, load location, and sliding friction. Positive KI values were observed for shorter cracks in rolling contact and for all crack lengths when the sliding load moved away from the crack. KII was greatest when the load was directly adjacent to the crack (g/a = +/- 1), where coincidental Mode I stresses were predominantly compressive. Sliding friction substantially increased both KImax and KIImax. The effective Mode I stress intensity factors, Keff, were greatest at g/a = +/- 1, illustrating the significance of high shear stresses generated by loads adjacent to surface cracks. Keff trends suggest mechanisms for surface pitting by which surface cracks propagate along their original plane under repeated reciprocating rolling or sliding, and turn in the direction of sliding under unidirectional sliding contact.     

Residual stress due to curing can initiate damage in porous bone cement: experimental and theoretical evidence

Residual stress due to shrinkage of polymethylmethacrylate bone cement after polymerisation is possibly one factor capable of initiating cracks in the mantle of cemented hip replacements. No relationship between residual stress and observed cracking of cement has yet been demonstrated. To investigate if any relationship exists, a physical model has been developed which allows direct observation of damage in the cement layer on the femoral side of total hip replacement. The model contains medial and lateral cement layers between a bony surface and a metal stem; the tubular nature of the cement mantle is ignored. Five specimens were prepared and examined for cracking using manual tracing of stained cracks, observed by transmission microscopy; cracks were located and measured using image analysis. A mathematical approach for the prediction of residual stress due to shrinkage was developed which uses the thermal history of the material to predict when stress-locking occurs, and estimates subsequent thermal stress. The residual stress distribution of the cement layer in the physical model was then calculated using finite element analysis. Results show maximum tensile stresses normal to the observed crack directions, suggesting a link between residual stress and pre-load cracking. The residual stress predicted depends strongly on the definition of the reference temperature for stress-locking. The highest residual stresses (4-7 MPa) are predicted for shrinkage from maximum temperature; in this case, magnitudes are sufficiently high to initiate cracks when the influence of stress raisers such as pores or interdigitation at the bone/cement interface are taken into account (up to 24 MPa when calculating stress around a pore according to the method of Harrigan and Harris (J. Biomech. 24(11) (1991) 1047-1058). We conclude that the damage accumulation failure scenario begins before weight-bearing due to cracking induced by residual stress around pores or stress raisers.     

Stress intensity variations in bone microcracks during the repair process

Microscopic cracks form and grow in compact bone in vivo due to cyclic loading. Their growth can cause stress fractures and has been implicated in the processes of remodelling and adaptation. These cracks are repaired by the actions of BMUs which are mobile resorption cavities. In this work, we studied the interaction between cracks and BMUs by making finite element models representing different stages in the repair process. The tendency of the crack to grow was measured by its stress intensity factor, K. We found that K changes in a complex manner during the repair process, both decreasing and increasing depending on the size of the crack and the type of loading applied. For loading conditions similar to those that exist in vivo, the presence of the BMU can cause K to rise significantly, in some cases by more than 20%, implying a substantial increase in crack growth rate. This information is important for our general understanding of the complexities of the repair process, and especially for the development of theoretical models to simulate damage and repair in bone.     

Measurement of the microstructural fracture toughness of cortical bone using indentation fracture

The purpose of this work is to investigate the use of indentation fracture as a method of measuring toughness at the microscale in cortical bone. Indentation fracture employs sharp indenters to initiate cracks, whose length can be used to calculate the toughness of the material. Only a cube corner indenter tip is found to initiate cracks at a suitable size scale for microstructural measurement. Cracks from 7 to 56 microm in length are produced using loads from 0.05 to 3N. Preliminary data predicts rising toughness with increasing crack length (rising R-curve behaviour) at the microscale. This technique provides a new insight into fracture in cortical bone since it allows the investigator to observe mechanisms and measure toughness at a size scale at which in vivo damage is known to exist.