The high-strain behavior of bone has remained relatively unexplored, however, and its properties under dynamic loads perpendicular to the bone diaphysis are somewhat poorly understood. Bone is an anisotropic material and exhibits different properties depending on its orientation during testing. It is known that bone is weaker under a compressive load in the transverse direction, but. the relationship has not been tested at high strain rates.
The test results were analyzed and compared with the literature on bovine bones tested under various other parameters. Damian Stoddard provided essential supervision and instruction during the Hopkinson bar tests, and helped immensely with both the test itself. Dwight Waddell provided valuable advice and encouragement during the later stages of dissertation preparation. in the fast and effective processing of the bone samples.
The graduate students who helped with the testing also deserve thanks, for their support and effective advice.
INTRODUCTION
- DYNAMIC LOAD CONDITIONS
- DYNAMIC TESTING BACKGROUND
- HIGH STRAIN RATES ON BONE
- POTENTIAL DISEASES IN BONE
- HYPOTHESIS
Although the exact definitions vary somewhat, the general consensus is that there is an additional distinction in the field of dynamic testing. Classification of each dynamic test using strain rate provides important insight into the speed of the test and the type of impact the specimen receives during the test. 3] The comparative difficulty of testing within this range makes experimentation difficult, especially in the context of a standard Hopkinson device.
The historical basis of materials science is rooted in the metallurgical advances made in the Industrial Age, where inventors often used the most advanced metals available for pressure vessels, steam turbines, and other mechanical devices. Given the limitations of bone samples, predictable problems arise in determining which type of bone tissue is being studied in the experiment. The samples used are cut in such a way that there is as little variation as possible in the tissue composition.
Unlike the study by Cloeta, Paul and Ismail, the samples were cut perpendicular to the center of the bone (see Figure 3). This core cutting direction allows testing of bone properties transverse to the longitudinal axis of the bone. There are differences in cortical bone thickness and cell microstructure, but they are composed of the same extracellular matrix and are similar enough to give a good approximation of the mechanical properties.
Testing a bovine bone in the Hopkinson Bar at high strain rate carries some risk of contamination with potentially biohazard material. Anthrax spores can be found in contaminated soil, undercooked meat, or direct exposure through a wound in the skin. This disease is preventable in livestock with good feeding practices, but it is difficult to determine the quality of forage from bone sample sources.
Coli is found in the digestive tract of both humans and domestic animals, disease-causing strains can rarely be transmitted through asymptomatic cattle. It causes abdominal cramps, bloody diarrhea and occasionally life-threatening kidney and blood disease in the elderly, children and the immunocompromised. Bone is a semi-crystalline structure consisting of millions of osteocytes that simultaneously support the load of an animal's body while allowing vital nutrients, blood cells and oxygen to flow in and out of the marrow located in the interior of the bone [14].
To validate this claim, testing horizontal loads in the dynamic range should show that bone is not as strong in that direction.
PROCEDURE
BONE SOURCING
Further tests all removed the marrow from the bone to ensure uniformity and produce valid and appropriate test results. Despite efforts to file the bone samples to a uniform length, the unevenness of the samples caused several tests to yield less applicable data than desired.
TESTING
RESULTS
All four other samples were in a similar range and favored analysis of these results. The stress-strain rate curves of the various samples are all shown in Figure 8 below. There are also differences in the rate of stress increase, with sample 1 showing a significantly slower increase per strain rate.
Most of the deviations in the specimens are due to the variance between their shapes and sizes, again indicating the imperfect machining process. 16] This could be attributed to the nearly frozen state in which the bone samples were kept before the experiment, which is likely. The low stress ceiling is most likely caused by an increase in porosity, due to the aforementioned frozen state of the bone.
Therefore, the behavior of these samples most closely matches old or osteoporotic bones due to the low peak load. It is possible that some water was still trapped in the pores of the microstructure, adding mass that ultimately did not contribute to the compressive properties of the samples. Each curve appears to fit the ceramic model of a rapid increase in strain rate followed by peak stress, although there is no sudden loss of data.
The strain rate varied with the condition and position of each specimen on the Hopkinson strip, with the highest average value coming from specimen 1 at 3576 s-1 and the lowest average value coming from specimen 4 at 2120 s-1. The highest strain rate previously tested in bovine bone specimens was about 2500 s-1, due to small However, the interesting finding of decreased peak stress may be a function of this extremely high strain rate in addition to the anisotropic character of the bone.
The use of longitudinal samples under identical test conditions will clarify the trend, or possibly reverse it. In the comparable region, the tested leg appears to be significantly weaker, with stresses below 50 MP even at 1000 s-1 accounting for the majority of the results.
DISCUSSION
STRESS INVESTIGATION
A potential factor in the lower strength of the tested samples may lie in their processing method. The drill bit used was a metal tube with an internal diameter of 6 mm, with teeth filed into the contact end to help cut into the bone. The bit was attached to a drill which was used on the frozen femur to cut the necessary specimens.
Disruption of the extracellular matrix may have compromised the structural integrity of the sample, either through the effects of thawing and freezing or through the action of biting and gouging the core piece. The finger had difficulty cutting through the cortical region, often failing to find purchase on the bone surface. Increased porosity due to ice formation in the bone was mentioned earlier as another possible cause of the decrease in the maximum stress of the sample.
During core removal, the samples were immersed in non-freezing water, which would have flowed freely into these pores. Any water still trapped within the bone would have encountered the bone matrix connections. While the expansion could have simply followed the channels into which the water originally flowed, it is also likely that additional stress concentrations were introduced into the bone samples, particularly in areas where the pores are narrow and restrictive, such as cortical bone.
In a study of an apparently isotropic glass/graphite/epoxy composite, the material was found to have practically uniform energy absorption in all tests. 21] The bone samples were of course not processed in the same way, but could be expected to have similar energy absorption due to their similarity of origin (ie, all specimens are cored from the femur). Instead, strain rate variance is more indicative of a range of energy absorption that probably contains relevant data.
Alternatively, the variation in porosity depending on the region from which the specimens were obtained would also vary the energy absorption. However, since pultrusion [22] is not an option for processing organic specimens, research involving bone may be forced to work with this energy absorption variance.
RECOMMENDATIONS
This would mean a lower threshold for failure, in addition to the stretch-to-brittle effect that low temperatures often have on materials. Another option is to use bones from other mammals to obtain a more general dynamic bone model or to investigate the strength of certain species. Further testing could involve an intermediate level of deformation, relying on either a polycarbonate tube or a specialized impact mechanism to apply force to the bone samples more slowly.
To model different types of bones, density tests and mineral tests can be performed on the source to investigate the nature of the samples before testing them. As for the samples themselves, the custom drill bit designed to cut into the femur of cattle was effective, but could only produce samples suitable for testing in a single direction. Future studies could machine a rectangular prismatic preparation from the bone to allow testing along the vertical axis and horizontal axis.
Specimens also experienced difficulties in removing pith, as both some of the pith could remain on the samples and the surfaces of some samples became uneven after the pith was filed away. The simplest solution to this problem would be to simply cut away all the marrow-bearing parts of the bone, an action that was not available during the preparation and testing phase. The number of samples was also quite low, relative to the amount of surface area available on the femoral portion.
This is, to some extent, a sign of the difficulty in replacing the part of the core that was broken during the formation of the samples. Improvements to the Hopkinson strip technique can be achieved through the use of one of the intermediate strain rate test devices developed by Cloete et al. These include the specialized striker discussed earlier, but also show the use of a unique wedge rod apparatus that has been developed specifically for the lower strain rate range.
An alternative to compressive testing would be a test of the elastic or torsional properties of bone, as stresses frequently occur at the ends of bone components. There would also be additional difficulties in machining samples to the appropriate dimensions for such a test.
CONCLUSION
