Answer following questions: a) Explain the difference between an isotropic mater
ID: 1467836 • Letter: A
Question
Answer following questions:
a) Explain the difference between an isotropic material and an anisotropic material. Give one example of abiological material other than bone that is anisotropic and explain how it is anisotropic.
b) It is well documented that humans are typically taller first thing in the morning, but get progressively shorter (by very small amounts) as the day goes on. Based on what you know about the viscoelastic properties of human body tissues, can you explain this phenomenon?
c) Explain how the creep response of a viscoelastic material is different than the stress-relaxation response of that material. Provide an example of one biological material for which creep is a significant issue to deal with and one biological material for which stress-relaxation is a significant issue to deal with.
Explanation / Answer
a)
Isotropic vs Anisotropic
“Isotropic” and “anisotropic” are two contrasting adjectives and nouns used to describe the properties of materials and minerals. Both “isotropic” and “anisotropic” also contain the element of direction in its description.
“Anisotropic” refers to the properties of a material that is dependent on the direction. Another condition that can fit the anisotropic definition is the presence of different properties in different directions. A different chemical bonding in all directions is also a condition for anisotropy.
A mineral can be considered as anisotropic if the mineral allows some light to pass through it. The mineral’s upper polar system allows light to pass through. In truth, it affects the polarization of light. The velocity of light is also different, and there is double refraction (which means that light is split into two ways).
In anisotropic minerals, the consequence of double refraction can lead to either of its two types; uniaxial (meaning one optic axis) or biaxial (two axes).
Anisotropic materials are often found in materials in different fields like computer graphics, chemistry, real-world imagery, physics, geography and geophysics, medical acoustics, material science and engineering, microfabrication, and neuroscience.
On the other hand, an isotropic materias or minerals have properties that have the same or uniform properties in all directions. Isotropic materials are said to be independent in direction or manner. An implication of being an isotropic material or mineral is that the chemical bonds are all identical in all directions.
An isotropic mineral can appear dark or stay dark when light passes through it. The uniform structure of the mineral blocks the light in all directions. In addition, the light doesn’t affect the mineral’s polarization or the direction of light. The velocity of light is in all directions and the index of refraction is everywhere.
b) viscoelastic properties of human body tissues
All materials exhibit some viscoelastic response. In common metals such as steel or aluminum, as well as in quartz, at room temperature and at small strain, the behavior does not deviate much from linear elasticity. Synthetic polymers, wood, and human tissue as well as metals at high temperature display significant viscoelastic effects. In some applications, even a small viscoelastic response can be significant. To be complete, an analysis or design involving such materials must incorporate their viscoelastic behavior. Knowledge of the viscoelastic response of a material is based on measurement.
Some examples of viscoelastic materials include amorphous polymers, semicrystalline polymers, biopolymers, metals at very high temperatures, and bitumen materials. Cracking occurs when the strain is applied quickly and outside of the elastic limit. Ligamentsand tendons are viscoelastic, so the extent of the potential damage to them depends both on the velocity of the change of their length as well as on the force applied.
c)
In fact the name creep comes from the creep test, in which a sample is submitted to a constant stress state and the time-dependent deformation is registered. You may want to investigate the phenomena, for example, by setting a constant strain state and registering the time-dependent stress, this leads to the stress relaxation test.
Viscoelastic materials present a time-dependent reversible stress-strain behavior. You apply a constant stress, the material slowly deforms, you remove the stress, the material slowly returns to the original form (if you have access to a leather couch, you may observe this phenomenon in first-hand) . In order to do so, the material has to keep a memory of the original form. Polymers have physical crosslink points or mechanical entanglement points which gives the material this reference to the original form.
Creep in metals is caused by the irreversible movement of either vacancies, or dislocations, or grain boundaries and none of these movements keep track of the original position, so, creep deformation in metals shows a time-dependent stress-strain deformation behavior, but this time the material does not return to the original form as the stress is removed. We call this behavior viscoplastic .
In summary, the name creep is used for both phenomena because they refer to the same kind of mechanical test, but they refer to different phenomena.
stress relaxation is the observed decrease in stress in response to the same amount of strain generated in the structure. This is primarily due to keeping the structure in a strained condition for some finite interval of time and hence causing some amount of plastic strain. This should not be confused with creep, which is a constant state of stress with an increasing amount of strain.
Since relaxation relieves the state of stress, it has the effect of also relieving the equipment reactions. Thus, relaxation has the same effect as cold springing, except it occurs over a longer period of time. The amount of relaxation which takes place is a function of time, temperature and stress level, thus the actual effect it has on the system is not precisely known, but can be bounded.Stress relaxation describes how polymers relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion.This nonlinearity is described by both stress relaxation and a phenomenon known as creep, which describes how polymers strain under constant stress.Viscoelastic materials have the properties of both viscous and elastic materials and can be modeled by combining elements that represent these characteristics. One viscoelastic model, called the Maxwell model predicts behavior akin to a spring (elastic element) being in series with a dashpot (viscous element), while the Voigt model places these elements in parallel. Although the Maxwell model is good at predicting stress relaxation, it is fairly poor at predicting creep. On the other hand, the Voigt model is good at predicting creep but rather poor at predicting stress relaxation (see Viscoelasticity).