Materials Science

Materials Science tests knowledge of the properties and characteristics of metals, ceramics, polymers and composite materials, with a focus on material characterization techniques, intermolecular forces, and surface chemistry.

Event Format
Teams are allowed to bring one nonprogrammable calcualator and one three-ring binder of notes.

Event supervisors will provide a periodic table and any needed materials and constants.

Students must wear closed-toed shoes, chemical splash goggles, full length pants or skirts, and a lab coat or chemical apron & long-sleeved shirt. Gloves are optional.

The Material Performance section and the Intermolecular Forces section will both be weighted 50% in this event.

Materials Science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates elements of applied physics and chemistry.

Classes of Materials

 * Metals -
 * Ceramics -
 * Polymers -
 * Composites -

Basic Terms
Stress - Force per unit area.

Strain - The amount of deformation an object experiences compared to its original size and shape.

Young's Modulus - A measure of the stiffness of an elastic material.It is equal to the stress over the strain.

Plastic Deformation- Irreversible deformation, as opposed to the reversible elastic deformation.

Yield Strength - The stress at which a material begins to deform plastically.

Creep Rate - The slow permanent deformation under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material.

Viscosity - A measure of a fluid's resistance to gradual deformation by stress.

Surface Tension - The tendency of liquids to resist external force, caused by cohesion.

Contact Angle - The angle where a liquid/vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation.

Ionic Bond - The bonding between a non-metal and a metal that occurs when charged atoms (ions) attract after one loses one or more of its electrons.

Covalent Bond - The chemical bond that involves the sharing of pairs of electrons between atoms.

Crystalline - A solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three spatial dimensions.

Semi-Crystalline - A material composed partially of crystalline and partially of amorphous matter.

Amorphous - Also known as a non-crystalline solid, they lack the long-range order characteristic of a crystalline material.

Atomic Packing Factor - The fraction of volume in a crystal structure that is occupied by atoms.

Young's Modulus
Young's modulus describes a materials resistance to liner strain, like pulling a wire or placing a weight on a column.

$$E &= \frac{A_0\Delta\L}}$$ $$E$$ is the Young's modulus (modulus of elasticity)

$$F$$ is the force exerted on an object under tension;

$$A_0$$ is the original cross-sectional area through which the force is applied;

$$\Delta\L$$ is the amount by which the length of the object changes;

$$L_0$$ is the original length of the object.

The force may be found using Hooke's Law, $$F=-kx$$ where

$$x$$ is the displacement of the spring's end from its equilibrium position.

$$F$$ is the restoring force exerted by the spring on that end.

$$k$$ is a constant called the rate or spring constant.

Yield Strength
The Yield Strength of a material is the force at which the material begins to deform plastically, or the force at which changes are not fully reversible.It is determined through a Stress-Strain curve, such as the one below.




 * 1) True elastic limit
 * 2) Proportionality limit
 * 3) Elastic limit(yield strength)
 * 4) Offset yield strength

For some materials (e.g., metals and plastics), change from elastic to plastic cannot be easily identified. Therefore, an offset method to determine the yield strength of the material tested is used. An offset is specified as a % of strain (for metals, usually 0.2%, for plastics, usually a value of 2%). The stress that is determined from the intersection point when the line of the linear elastic region (with slope equal to the Young's Modulus) is drawn from the offset becomes the Yield Strength by the offset method. For example, in the above image, an ofset of 0.2% is used for the yield strength.

Surface Area/Volume Ratio
As simple as it sounds, The ratio can be found by dividing the Surface area of an object by its volume, both of which can be determined by simple geometry. Typically, as a shape gets bigger, its surface area to volume ratio tends to decrease.

Creep Rate
Creep Rate is "the permanent deformation of material under constant load."

To test the creep rate of a material, a sample put under a constant stress and constant temperature, for which the strain is measured over time.



There are three stages of creep. The first stage is characterized by a high initial strain rate, which slows with time due to work hardening. This leads it to the second stage of creep, in which the strain rate remains almost constant as work hardening and thermal softening remain about equal. In the third and final stage, the strain rate exponentially increases.

Viscosity
Viscosity is "the resistance to flow."

Viscosity is caused by friction between molecules that move at different velocities. When in a tube, for example, the adhesion to the walls causes the outer material to move slower, while the material in the center moves faster. This means that some stress is required for the liquid to move. The more viscous the material, the more stress is needed.

A simple test of viscosity is to simply fill a graduated cylinder with the test liquid, and time how long a steel ball(or some other heavy object) takes to reach the bottom. While not giving an "accurate" measure of viscosity, or a number in terms of viscosity's actual units(which is in Pascals times seconds, $$Pa$$·$$s$$), it does provide a good comparison between liquids, and is a test you may find in the lab portion of the event.

State and National Topics
The following are advanced topics that should only be found on state and national tests.

Fracture toughness
Fracture toughness is the ability of a material containing a crack to resist fracture.

The linear-elastic fracture toughness of a material is determined from the stress intensity factor ($$K$$). It is measured in $$Pa$$·$$\sqrt{m}\$$.

There are three types of tests for fracture toughness, diagrams of which can be found below.



Mode I is the most common, and the following equation can be used to determine the stress intensity factor.

$$K_I$$ can be found by using the following:

$$K_I=\sigma\ \sqrt{\pi\ a \beta\$$

Where $$K_I$$ is the fracture toughness,

$$\sigma\$$ is the applied stress (in $$Pa$$)

$$a$$ is the crack length (in meters)

$$\beta\$$ is a crack length and component geometry factor that is different for each specimen and is dimensionless.

Fatigue Limit
The fatigue limit is the amplitude of cyclic stress required to cause failure. Some metals are able to withstand small stresses for a seemingly infinite number of cycles, while others fail even with small stresses with enough cycles.

The following is an example of a Fatigue Limit test.



The X-axis is a log scale of the number of cycles(of the stress). The Y axis is the amount of stress. The lines show the number of cycles at a given stress for failure to occur.For example, at about 45 ksi, the aluminium fails after 10000 cycles.

At the endurance limit, the steel does not break under even large number of cycles at low stress, while the aluminium does not show this property.

Shear Modulus
The shear modulus is equal to the shear stress over the shear strain, and is a measure of rigidity. It describes a materials resistance to shearing strains, like that of dull scissors.

$$G &= \frac{A\Delta\x}}$$

Where $$G$$ is the Shear Modulus,

$$F$$ is the force,

$$L$$ is the original length of the object,

$$A$$ is the area on which the force acts

$$\Delta\x$$ is the transverse displacement. This diagram shows where those factors are.



Poisson's Ratio
Poisson's Ratio is the measure of how much a material expands(or contracts) when stretched or squeezed. Most objects have a positive Poisson's ratio, meaning they expand when squeezed and contract when stretched. However, some materials do exist with a negative ratio. The value of the ratio is $$-1.0<\nu\ <0.5$$ due to the requirement for the Young's, shear, and bulk modulus to be positive.

$$\nu\ &= -\frac{d\epsilon\ _y}{d\epsilon\ _x}$$

Where $$\nu\$$ is Poisson's ratio,

$$d\epsilon\ _y$$ is transverse strain (negative for (stretching), positive for compression)

$$d\epsilon\ _x$$ is axial strain (positive for tension, negative for compression).



Surface Chemistry
- Surface Tension - Contact Angle

Crystal Types
- Ionic - Covalent - Crystalline/Semi/Amorphuos

FCC
FCC stands for Face-Centered Cubic, which has the same structure as Cubic Close-Packed. The Unit Cell, or repeatable unit, of FCC is a cube with an atom at each corner of the unit cell and an atom situated in the middle of each face of the unit cell.



FCC has an Atomic Packing Factor of 0.74. The Atomic Packing Factor, or APF, is equal to the volume of the atoms in a unit cell over the volume of the unit cell. A packing factor of 0.74 means 74% of the volume can be thought of as containing an atom.

BCC
BCC stands for Body Centered Cubic. The unit cell of BCC is a cube with an atom at each corner and a single atom in the center.



BCC has an APF of 0.68, meaning it is less dense than FCC or HCP.

HCP
HCP stands for Hexagonal Close-Packed. Its structure is the most complex of the three. Its structure is most easily explained through a picture.



HCP has an APF of 0.74, the same as FCC