What is thermal shock?

Short Answer:

Thermal shock is the sudden development of stress or cracking in a material due to a rapid change in temperature. When a material is heated or cooled quickly, different parts of it expand or contract at different rates, leading to uneven thermal stresses. If these stresses exceed the material’s strength, cracks or fractures occur.

In simple words, thermal shock happens when a material experiences a sudden temperature difference within itself. This temperature change creates internal stresses because one part of the object heats or cools faster than another. Materials like ceramics and glass are highly sensitive to thermal shock.

Detailed Explanation:

Thermal Shock

Thermal shock refers to the cracking, warping, or breaking of a material when it is subjected to a rapid temperature change. It is a result of the uneven expansion or contraction of different parts of the material due to the temperature gradient.

When a material is exposed to heat, it expands; when cooled, it contracts. However, if the change in temperature occurs too quickly, the outer layers and the inner parts do not have enough time to adjust uniformly. This causes the material to develop thermal stresses. If these stresses become greater than the material’s tensile strength, it leads to cracking or even complete failure.

Thermal shock is a common problem in engineering applications such as turbines, engine components, furnaces, boilers, glassware, and ceramics, which are frequently exposed to high and fluctuating temperatures.

Mechanism of Thermal Shock

The process of thermal shock occurs in the following steps:

  1. Rapid Temperature Change:
    The material is suddenly heated or cooled. This causes a steep temperature gradient between the surface and the interior.
  2. Uneven Expansion or Contraction:
    • When heated, the outer surface expands faster than the inner core.
    • When cooled, the surface contracts faster than the interior.
  3. Development of Thermal Stresses:
    The difference in expansion or contraction between layers creates internal stresses.

    • If the material is brittle (like glass or ceramics), it cannot deform plastically to relieve these stresses.
    • The internal tensile stresses then cause cracks to form.
  4. Fracture or Cracking:
    When the thermal stress exceeds the strength of the material, cracks propagate rapidly, leading to thermal shock failure.

Causes of Thermal Shock

  1. Rapid Heating or Cooling:
    The most common cause of thermal shock is sudden exposure to high or low temperatures, such as quenching a hot object in cold water.
  2. Non-Uniform Temperature Distribution:
    When heat transfer is uneven within the material, it causes temperature gradients that result in internal stress.
  3. High Coefficient of Thermal Expansion:
    Materials that expand or contract significantly with temperature changes are more prone to thermal shock.
  4. Poor Thermal Conductivity:
    Materials with low thermal conductivity (like glass and ceramics) cannot distribute heat evenly, leading to large temperature differences and high internal stresses.
  5. Brittleness of Material:
    Brittle materials, which cannot deform plastically, are more likely to fail under thermal shock because they cannot absorb or redistribute the stress.

Thermal Shock Resistance

The ability of a material to resist failure due to sudden temperature change is called its thermal shock resistance. It depends on several material properties, such as:

  1. Coefficient of Thermal Expansion (α):
    • A lower value of α means the material expands or contracts less, reducing internal stress.
    • Materials with small α values (like quartz) have high resistance to thermal shock.
  2. Thermal Conductivity (k):
    • High thermal conductivity allows rapid heat distribution, reducing temperature gradients.
    • Metals generally have high thermal conductivity and are less affected by thermal shock.
  3. Modulus of Elasticity (E):
    • A lower E value allows the material to deform slightly under stress, reducing the likelihood of cracking.
  4. Fracture Strength (σf):
    • Materials with high fracture strength can withstand higher stresses before cracking.
  5. Toughness:
    • Tough materials can absorb more energy before fracturing, improving thermal shock resistance.

Examples of Thermal Shock

  1. Glassware:
    • A glass tumbler cracked when hot water is poured into it after being in the refrigerator. The outer surface expands quickly while the inside remains cold, causing thermal stress and fracture.
  2. Ceramics in Furnaces:
    • Ceramic linings in furnaces or kilns experience rapid heating and cooling cycles, making them prone to cracking.
  3. Engine Components:
    • Pistons, cylinder heads, and exhaust valves in engines are exposed to fluctuating temperatures that can cause thermal stress and fatigue over time.
  4. Turbine Blades:
    • Gas turbine blades face extreme temperature variations during operation, and poor thermal shock resistance can lead to cracks and failure.
  5. Aerospace Applications:
    • Spacecraft materials face rapid temperature changes between sunlight and shadow, requiring excellent thermal shock resistance.

Prevention and Control of Thermal Shock

  1. Use of Materials with Low Thermal Expansion:
    • Select materials like quartz, fused silica, or certain ceramics that have a low coefficient of expansion.
  2. Improve Thermal Conductivity:
    • Use materials or coatings that distribute heat more evenly. Metals are often used as supports or coatings for ceramics for this reason.
  3. Controlled Heating and Cooling:
    • Avoid sudden temperature changes by preheating or slow cooling processes.
  4. Material Design Modifications:
    • Use thin sections or gradual thickness transitions to reduce temperature gradients.
  5. Use of Composite Materials:
    • Combining materials with complementary thermal properties can help absorb and redistribute stress.
  6. Surface Treatments:
    • Coatings like thermal barrier coatings (TBCs) can help insulate and protect the material from sudden thermal loads.

Effects of Thermal Shock

  • Cracking and Fracture: Sudden temperature change causes surface or internal cracks.
  • Loss of Mechanical Strength: Microcracks reduce the load-bearing capacity of the component.
  • Distortion or Warping: Uneven expansion causes permanent deformation.
  • Failure of Components: Repeated thermal cycling can cause fatigue and eventual failure.
Conclusion

Thermal shock occurs when a material is exposed to rapid temperature change, leading to uneven expansion or contraction and the formation of internal stresses. If these stresses exceed the material’s strength, cracks or fractures occur. Materials with low thermal expansion, high thermal conductivity, and good toughness have better thermal shock resistance. In engineering design, it is essential to consider thermal shock when selecting materials for high-temperature or rapidly changing environments to prevent premature failure and ensure long service life.