Analysis of fractal structures from the 3D height profile of a polymer fracture surface reveals local changes of crack propagation mechanisms
Source: BAM, Polymer Matrix Composites division
Slow Crack Growth and the related Environmental Stress Cracking (ESC) are relevant damage mechanisms of polymeric materials - especially polyethylene – in various practical applications. ESC occurs often unexpectedly and already at quite low external loads as the result of a complex crack propagation mechanism involving the interaction of the polymer with a surrounding liquid medium under certain local stress conditions. Such stress conditions are often induced by a tiny local damage or defect in the material and critical liquid media are typically aqueous surfactant solutions. Extensive testing is performed worldwide to assess the influence of polymer characteristics such as molecular weight distribution, molecular architecture and semi-crystalline morphology on the resistance against this kind of crack propagation and to qualify the material for specific practical applications, e.g. as pipes or containers.
An important tool to evaluate such laboratory tests is the fracture surface analysis by microscopic methods. An examination of the surface structures left behind on the test specimen by the progressing crack obtained under well-defined conditions and geometries reveals a wealth of information about the underlying processes and mechanisms. In the presented work this fracture surface analysis was applied for the first time beyond routine practice for a statistical quantification of ESC surface structures which result from the so-called craze-crack mechanism, involving the formation of microscopic fibrillar structures which upon failure lead to macroscopic crack propagation. The remains of these fibrils on the fracture surface reflect local properties such as stress conditions or crack propagation rates which vary with space and time of crack growth. The quantitative assessment of the topography of these structures is based on Laser Scanning Microscopy (LSM) providing a three-dimensional height profile (with a lateral resolution up to 70nm) of the fracture surface represented by a matrix of numerical values. This allows for the analysis of the one-dimensional height-to-height fluctuations which up to an upper size limit obey a power-law characterized by a roughness exponent (or Hurst exponent) indicating self-affine or fractal behavior.
Mapping of these measured spatially resolved scaling exponents across the fracture surface reveals an isotropic fracture on a local length scale, but systematic changes on a larger length-scale reflecting stress-state transitions and variations in the crack propagation rate.
This approach provides for the first time insight into specific characteristics of the fracture surface allowing for the clear identification of dominant mechanisms of local deformation and fracture. This deeper understanding of polymer fracture will lead to improved analysis and development of optimized polymer materials used in contact with aggressive media.
Spatially resolved roughness exponent in polymer fracture
Maximilian Thuy, Alexander Spyrantis, Martin Böhning, Ute Niebergall and Robert Maaß
published in:
Physical Review Materials 6, L090601 (2022) DOI: 10.1103/PhysRevMaterials.6.L090601
BAM division Polymer Matrix Composites
BAM division Technical Properties of Polymeric Materials