Understanding breakage in curly hair

Area of Science:

• Hair science
• Materials science
• Biomechanical engineering

Background:

• African-American hair exhibits a high incidence of breakage (96% in a 2005 study).
• Structural and grooming-induced stresses, not chemical composition, are implicated in increased breakage.
• Understanding differential failure mechanisms of cuticle and cortex is key to preventing hair damage.

Purpose of the Study:

• To investigate crack propagation and fracture mechanisms in African-American hair.
• To elucidate the failure modes of hair under mechanical stress.

Main Methods:

• Controlled mechanical stress applied to hair sections using a high-resolution mechanical stage.
• In-situ monitoring of hair surface using scanning electron microscopy (SEM) during stress application.
• X-ray tomographic microscopy (XRM) used to quantify energy absorption and crack density changes.

Main Results:

• Proposed a multi-step breakage mechanism: cuticle sliding, cuticle-cortex interface failure, intercellular crack nucleation and growth at the cuticle-cortex junction.
• Observed crack propagation towards the hair surface, culminating in breakage upon merging.
• SEM and XRM revealed internal crack formation and propagation as primary fracture drivers.

Conclusions:

• SEM and XRM provided novel insights into hair fracture mechanisms.
• Accumulated mechanical damage and environmental factors create internal cracks, leading to unpredictable breakage.
• A continuous hair care regimen is recommended to mitigate damage and breakage throughout fiber life.