ME115 - Week 10

Fatigue 2, Ceramics, Glasses

Fatigue Life

# of cycles a component survives at a particular stress. Depends on crack initiation and propagation which should be delayed as long as possible

Fatigue Strength

Maximum stress for which fatigue will not occur for a set number of cycles.

How to improve fatigue life

1. Choose materials that are not sensitive to defects

2. Avoid/reduce surface flaws by improving design (no sharp corners) and manufacturing (polish)

   1. Avoid sharp changes in direction in your design, they are stress raisers

3. Reduce tensile stress levels

4. Create compressive residual stress on the surface

Compressive Residual Stresses

If a metal’s surface carries a residual compressive stress, applied tensile loads must first cancel it out before a crack can open — raising the fatigue threshold and slowing crack growth.

e.g. shot peening or rivets

Fatigue Testing Machine

1. Rotating-bending machine: not common

2. Tensile test: expensive

Crack Growth Rate (Paris’ Law)

da/dN (crack length per cycle), dependent on a range of crack intensity factors (ΔK) since there is a range of stress being applied to the material and it is bent and unbent and the equation for K depends on σ (stress)

Cracks propagate at Kc

Crack Growth Stages

Pre-Stage: K<Kth (intensity threshold), no crack growth

Stage 1: Slow growth

Stage 2: Growth is linear on a log-log scale

Stage 3: Rapidly unstable crack growth until rupture

Ceramic

Solid compounds with elements bonded together in a fixed atomic ratio (determined by chemical formula) by ionic &/or covalent bonds that comprise:

• at least one metal and one non-metal (e.g. Al2O3, MgO)

• at least two non-metals (e.g. SiC, Si3, N4)

These are solid compounds that are formed by applying heat or a combination of heat and pressure (sintering)

General Characteristics of Ceramics

• High melting temps

• High compressive strength, low tensile strength (due to low fracture toughness - also causes them to be brittle)

• Hard and wear-resistant (strong bonding)

• Prone to thermal shock i.e. damage or cracking caused by a sudden, extreme change in temperature

• Generally electrically and thermally insulating - not a lot of freely moving electrons

• Generally non-magnetic

• Chemically stable and oxidation-resistant (many are already made out of oxidized metals)

Crystalline Ceramics

Ceramics that are comprised of crystal lattices. A larger cation (+) usually allows for a higher concentration #, as is seen in the photo

Types of Crystalline Ceramics

1. Oxides: Al2O3, Si2O (silica), FeO, Fe2O3

2. Carbides: SiC, WC, Fe3C (cementite - strengthens steel), Al4C3

3. Nitrides (high hardness): BN, Si3N4, TiN

Processing of Ceramics

Due to high melting temp, they can’t be melted and then molded, thus the process is:

Powder prep → shaping → sintering (applying heat and pressure) → finishing

Glasses

This is a type of ceramic, non-crystalline (short-ranged order only), usually made from silica (Si2O).

Metastable

Glass is stuck in a higher-energy, amorphous state — kinetically trapped rather than thermodynamically stable. Given enough time or heat, glass can slowly crystallize, a process called devitrification.

Glass Vs. Other Ceramics

Most properties are similar to ceramics (brittle, non-conductive, etc.). The difference is that glass is metastable and so it’s not usually crystalline at room temperature, the fact that it is amorphous makes it have a gradual melting temperature, compared to other ceramics that have a very distinct melting temp

Glass Transition Temp (Tg)

Other ceramics do not have this, this is not a fixed value for glass, meaning that over a range of temperatures glass begins to transition into a liquid.

Melting Temp (Tm)

This is a fixed value for other ceramics. However, for glasses it is not, meaning that there is no one temperature where glass stops being solid, it is solid and then it is viscous and then it is liquid, but this happens over a range of temperatures.

This is because glass is amorphous, so there is no need to break strong crystal lattices which means that not a lot of energy is required to turn it into a liquid.

Processing Sheet and Plate Glass