Citation: Thuy, M.;
Pedragosa-Rincón, M.; Niebergall, U.;
Oehler, H.; Alig, I.; Böhning, M.
Environmental Stress Cracking of
High-Density Polyethylene Applying
Linear Elastic Fracture Mechanics.
Polymers 2022, 14, 2415. https://
doi.org/10.3390/polym14122415
Academic Editor: Filippo Berto
Received: 9 May 2022
Accepted: 12 June 2022
Published: 14 June 2022
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Article
Environmental Stress Cracking of High-Density Polyethylene
Applying Linear Elastic Fracture Mechanics
Maximilian Thuy
1
, Miquel Pedragosa-Rincón
1,†
, Ute Niebergall
1
, Harald Oehler
2
, Ingo Alig
2
and
Martin Böhning
1,
*
1
Bundesanstalt für Materialforschung und—Prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany;
maximilian.thuy@bam.de (M.T.); miquelpedragosar@iqs.url.edu (M.P.-R.); ute.niebergall@bam.de (U.N.)
2
Fraunhofer Institute for Structural Durability and System Reliability LBF, Research Division Plastics,
Schlossgartenstraße 6, 64289 Darmstadt, Germany; harald.oehler@lbf.fraunhofer.de (H.O.);
ingo.alig@lbf.fraunhofer.de (I.A.)
* Correspondence: martin.boehning@bam.de
† With Bundesanstalt für Materialforschung und—Prüfung (BAM) until 31 March 2021.
Abstract:
The crack propagation rate of environmental stress cracking was studied on high-density
polyethylene compact tension specimens under static loading. Selected environmental liquids are
distilled water, 2 wt% aqueous Arkopal N100 solution, and two model liquid mixtures, one based on
solvents and one on detergents, representing stress cracking test liquids for commercial crop protec-
tion products. The different surface tensions and solubilities, which affect the energetic facilitation
of void nucleation and craze development, are studied. Crack growth in surface-active media is
strongly accelerated as the solvents induce plasticization, followed by strong blunting significantly
retarding both crack initiation and crack propagation. The crack propagation rate for static load as a
function of the stress intensity factor within all environments is found to follow the Paris–Erdogan
law. Scanning electron micrographs of the fracture surface highlight more pronounced structures
with both extensive degrees of plasticization and reduced crack propagation rate, addressing the
distinct creep behavior of fibrils. Additionally, the limitations of linear elastic fracture mechanisms
for visco-elastic polymers exposed to environmental liquids are discussed.
Keywords:
crack propagation; environmental stress cracking; fracture toughness; crop protection
products; high-density polyethylene; fracture surface structure; stress intensity factor; craze–crack
mechanism; linear elastic fracture mechanics
1. Introduction
Crack propagation in high-density polyethylene (PE-HD) materials is of critical im-
portance in the industry of packaging and transportation of dangerous goods. Slow crack
growth (SCG) and, in particular, environmental stress cracking (ESC) are complex damage
mechanisms that especially affect PE-HD in container and pipe applications and are known
to be responsible for premature failure due to cracking in these polyolefin materials. For
material developers and packaging or pipe manufacturers, it is therefore of importance to
determine the environmental stress cracking resistance (ESCR) of the packaging material.
Standardized methods include the full-notch creep test (FNCT) [
1
–
3
], Pennsylvania edge
notch test (PENT) [
4
–
6
], or cracked round bar test (CRB) [
7
–
9
]. Within these methods, the
time to failure is commonly the characteristic criterion to assess ESCR. In order to shorten
testing times or to study specific fracture mechanisms, cyclic loading is often conducted
through fatigue testing, as also demonstrated with the CRB test [
10
–
13
]. A different and
widely used standardized method to determine a material’s fracture toughness employs
displacement controlled experiments of notched specimen [
14
]. For fracture mechanic
experiments under static load within environmental media, such as organic liquids or
aqueous surfactant solutions, no standardized methods have been currently defined for
Polymers 2022, 14, 2415. https://doi.org/10.3390/polym14122415 https://www.mdpi.com/journal/polymers
