1. The crack that starts small

Michael Crichton’s The Andromeda Strain returns as the recalled work because the novel’s tension comes from tiny failures that accumulate until a system is compromised. Thermal fatigue is the same: a microcrack at a corner or a joint does not matter until it does. The job of the designer is to keep the crack from starting and to make sure it grows slowly if it does.

This entry reads about how thermal fatigue manifests in metals, composites, and joints.

2. Metal fatigue

Metals in spacecraft structures, especially aluminum alloys, are ductile and forgiving in many ways, but they are vulnerable to low-cycle fatigue when thermal strains are large:

  • Stress concentrators: holes, notches, welds, and sharp corners initiate cracks.
  • Creep-fatigue interaction: at elevated temperatures, creep deformation adds to fatigue damage.
  • Thermal ratcheting: repeated thermal loading can produce progressive deformation in structures with kinematic constraints.
  • Surface finish matters: scratches and machining marks act as crack starters.

A CTE guide for metals notes that spacecraft structures combine aluminum, titanium, steel, and composites, each with significantly different expansion coefficients, and that CTE data is a primary input for thermal stress analysis.

3. Composite fatigue

Composites can be designed for high stiffness and low expansion, but thermal cycling still damages them:

  • Microcracking: plies with different fiber orientations expand by different amounts, creating matrix cracks parallel to the fibers.
  • Fiber-matrix debonding: repeated stress can break the bond between fiber and resin.
  • Moisture effects: composites that absorb water on the ground can experience internal pressure when the water freezes or boils in vacuum.
  • Outgassing and embrittlement: the polymer matrix may lose volatiles and become brittle.

Research on thermal cycling of spacecraft composites describes temperature ranges from −196 °C to +180 °C for satellite antenna structures and emphasizes that thermal expansion and contraction drive fatigue.

4. Joint fatigue

Joints are usually the weakest part under thermal cycling:

  • Bolted joints: thermal motion can loosen preload, wear contact surfaces, and cause fretting.
  • Adhesive joints: CTE mismatch between adhesive and adherends creates shear stress; the adhesive may crack or debond.
  • Welded or brazed joints: residual stresses from manufacturing add to thermal stresses.
  • Mixed-material joints: aluminum to carbon fiber is a classic high-mismatch pair; titanium fittings are often used at composite joints because titanium’s CTE is closer to CFRP than aluminum’s.

5. Mitigation strategies

The literature suggests several approaches:

  • Match CTEs across joints where possible.
  • Use low-CTE materials like Invar or tailored composites for dimensionally critical parts.
  • Minimize constraints so thermal expansion is not fought.
  • Add flexibility in the form of bellows, flexures, or compliant layers.
  • Control temperature range with thermal design, coatings, and heaters.
  • Design for inspectability so cracks can be found before they become critical.

6. What this changes

  • Thermal fatigue is a joint-heavy problem as much as a material problem.
  • CTE matching, joint design, and thermal control are the main mitigations.
  • The next entry will translate these findings into requirements for the desktop.