Materials for Biomedical Engineering. Mohamed N. Rahaman

Читать онлайн.
Название Materials for Biomedical Engineering
Автор произведения Mohamed N. Rahaman
Жанр Химия
Серия
Издательство Химия
Год выпуска 0
isbn 9781119551096



Скачать книгу

often shows considerable scatter because the size, position, and orientation of flaws such as microcracks vary from one specimen to another (Section 4.2.5). This has important consequences for design. As failure at any flaw can lead to failure of the entire specimen, the strength that can be safely used in design is often far lower than the average measured strength. A specimen size effect also has to be taken into account because there is a higher probability of a strength‐determining flaw present in a larger volume. Another factor is that the measured strength depends on the testing technique because each technique subjects a different volume of the specimen to a tensile stress. In order to account for all of these factors quantitatively, the design of brittle materials such as ceramics and glasses is often treated by a statistical method called Weibull statistics.

      Weibull statistical analysis employs a weakest link assumption that failure of a single element of the specimen results in failure of the entire specimen. Instead of the more common distribution functions used to describe experimental data, such as the normal distribution, Weibull analysis assumes a function, called the Weibull distribution, to describe the measured strength of brittle materials. One form of the distribution, called the two‐parameter distribution, is

      where, Pf ( σ ) is the cumulative probability of failure, that is, the probability that failure has occurred by a stress σ, m is the Weibull shape parameter, a number often called the Weibull modulus, which is an inverse measure of the width of the distribution, that is, a higher value of m corresponds to a narrower distribution, and σo is the Weibull scale parameter, a measure of the center of the distribution, equivalent to the probability of failure occurring at or below a stress σo is 0.63.

      (4.38)equation

Schematic illustration of Weibull plots for porous bioactive glass (BG) specimens in compression and flexural loading. Data for hydroxyapatite (HA) and beta -tricalcium phosphate ( beta -TCP) specimens with a similar microstructure are shown for comparison.

      Source: From Liu et al. (2013) / with permission of Elsevier.

      These data illustrate features that are significant in the design of brittle materials for use in biomedical applications that are subjected to significant stresses, such as healing large defects in the long bones of the human limbs. At a specific applied stress, the bioactive glass specimens are more reliable in compression than in bending due to their higher m and σo values in compression. This may be attributed to the higher volume of the specimen subjected to a tensile stress in bending. For example, the maximum compressive stress on the human femur in walking or running is estimated at less than ~10 MPa. Based on the data in Figure 4.15, the probability Pf that one of these bioactive glass specimens will fail under a compressive stress of 10 MPa is estimated at less than 10−6, that is, less than one in a million specimens is predicted to fail. On the other hand, for the same stress (10 MPa) in bending, the probability of failure Pf is 0.3, or approximately 1 in 3 specimens will fail.

      4.4.3 Designing with Polymers

      In designing with polymers, the property of viscoelasticity must be taken into account. For applications in which a tensile load is present, for example, data for the tensile strength and creep modulus over the appropriate range of conditions such as stress, time scale, and temperature are relevant. If the polymer degrades in vivo, the effects of environmental conditions should also be taken into account. Designing to avoid yielding of the polymer is often straightforward once the time and temperature dependence of the yield strength is accounted for. In comparison, designing to avoid brittle fracture is more difficult due to the presence of flaws such as microcracks and pores (Section 4.2.5).

      Electrical properties are important for biomaterials used in devices to deliver an electric current or an electrical signal. Electrically conducting metals such as platinum are used as electrodes in cardiac pacemakers and neural probes. On the other hand, electrically insulating materials such as polyurethane are used as coatings to isolate or insulate sensitive electronic devices from surrounding tissues and fluids. Whereas polymers are typically electrical insulators, several polymers have been synthesized recently which show a strong ability to conduct an electrical current.