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.     

Fracture mechanics of bone with short cracks

Tensile fracture experiments were performed upon specimens of wet mature bovine Haversian bone, with short, controlled notches. Stress concentration factors were found to be significantly less than values predicted using a maximum stress criterion in the theory of elasticity. Results were also modeled with the aid of linear elastic fracture mechanics. Agreement of experiment with theory was better in this case, however deviations were seen for short notches. Two mechanisms were evaluated for the behavior: plasticity near the crack tip, and effects of the Haversian microstructure, modelled by Cosserat elasticity, a generalized continuum theory. Plastic zone effects were found to be insignificant. Cosserat elasticity, by contrast, predicted stress concentration factors which better approximated observed values. To explore strain redistribution processes, further experiments were conducted upon notched specimens in torsion at small strain. They disclosed a strain redistribution effect consistent with Cosserat elasticity. These microelastic effects were attributed to the Haversian architecture of bone. The implications of the results are that bone resists the effect of stress raisers such as fatigue microcracks and surgical sawcuts to a much greater extent than anticipated on the basis of its elastic or elastoplastic properties.     

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.     

The behaviour of roots encountering cracks in soil

Summary It is shown that probabilities of root penetration across cracks in soil can be calculated effectively using a mathematical model involving root stress and soil distributions and penetrometer/root stress ratios. Penetration criteria are developed, and it is found that the effective penetrometer/root stress ratios take values of about 4 for crack widths smaller than about 2 mm and about 8 for wider cracks. Root swelling does not appear to contribute significantly to the probability of root penetration through any effect on root buckling stress. Suggestions are made for further work on the effects of soil structure and strength on root behaviour.     

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.     

Microbial healing of cracks in concrete: a review

Concrete is the most widely used construction material of the world and maintaining concrete structures from premature deterioration is proving to be a great challenge. Early age formation of micro-cracking in concrete structure severely affects the serviceability leading to high cost of maintenance. Apart from conventional methods of repairing cracks with sealants or treating the concrete with adhesive chemicals to prevent the cracks from widening, a microbial crack-healing approach has shown promising results. The unique feature of the microbial system is that it enables self-healing of concrete. The effectiveness of microbially induced calcium carbonate precipitation (MICCP) in improving durability of cementitious building materials, restoration of stone monuments and soil bioclogging is discussed. Main emphasis has been laid on the potential of bacteria-based crack repair in concrete structure and the applications of different bacterial treatments to self-healing cracks. Furthermore, recommendations to employ the MICCP technology at commercial scale and reduction in the cost of application are provided in this review.     

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 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.