Meaning
Mathematical stress field models calculate elastic stress field intensities near sharp defect tips assuming linear material response and minimal plastic deformation zone size. Linear elastic fracture mechanics defines failure limits using stress intensity factors derived from crack length and nominal applied load. The framework governs structural integrity assessments and crack growth predictions for rigid, brittle polymers operating below glass transition temperatures.
Theory validity ends when plastic yielding surrounding the crack tip exceeds ten percent of total crack length or specimen thickness.
Stress Intensity
Elastic stress equations describe the parabolic stress elevation approaching a sharp crack tip inside a stressed elastic body. Application of linear elastic fracture mechanics allows engineers to calculate critical stress intensity factors where unstable fracture propagation occurs catastrophically. Sharp radii and unblended weld lines act as stress raisers that initiate unstable brittle fracture.
Datasheet fracture toughness values measured via small-scale tests help predict maximum allowable defect dimensions in rigid components.
Plasticity Boundary
Theoretical limits fail when excessive plastic deformation around crack tips invalidates linear elastic assumptions, requiring elastic-plastic fracture mechanics models instead. Tough polycarbonates or modified polyamides yield extensively before crack propagation, rendering stress intensity factor equations inaccurate for thin-wall designs.
Structural Application
Structural calculations based on critical stress intensity factors prevent catastrophic brittle failure in rigid polymer housings under transient impact loads.
