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The FNCT is widely used to characterize the slow crack growth (SCG) behavior of polyolefinic materials in “inert” media as well as effects of environmental stress cracking (ESC) in which the medium has decisive influence on damage mechanism and time to failure. The test is of greatest importance for pipe and blow molding types of PE-HD - especially for transport and packaging of dangerous goods but also for most other high-performance applications.
Usually the FNCT is applied as a standardized testing method (ISO 16770) using a few universal liquid media, such as solutions of Arkopal N 100.
In our study, selected relevant PE-HD materials are investigated also in real media – practical formulations as well as representative pure chemicals – and influences of temperature and geometry of specimen and notch are explicitly addressed. Furthermore, the investigations comprise also the ESC behavior of PE-HD in media that are sorbed to a significant extent – one example are fuels, such as diesel and biodiesel – based on comparison with samples previously saturated with those media. Thus, also the underlying diffusion controlled sorption process has to be assessed before.
The investigations were performed using a FNCT-device with 12 individual sub-stations, each equipped with individual electronic stress and temperature control and continuous online monitoring of the specimen elongation.
Due to the increasing demand for utilization and improvement of energy efficient materials, especially concerning requirements in lightweight construction and design, polyolefin materials are used extensively and in an increasingly broad range of applications.
Although loaded with stresses under yield stress, preformed voids, inhomogeneities or notches can possibly lead to damage and unexpected failure induced by liquid media in polyolefin materials. For PE-HD, slow crack growth (SCG) as well as environmental stress cracking (ESC) are relevant mechanisms of damage. SCG appears without influence of a surrounding medium or in “inert” media whereas ESC occurs in “active” media, which have decisive influence on damage mechanism and time to failure. To characterize the intrinsic resistance of the material against those damage mechanisms, the Full-Notch Creep Test (FNCT) is widely used and of paramount importance particularly for the assessment of pipe and blow molding types of PE-HD.
In this study, the FNCT – usually applied as a standardized testing method (ISO 16770) using a few universal model liquid media – is extended by investigations of selected relevant PE-HD materials with a variety of properties also in real media. Mainly topical fuels, customary in the market such as diesel and biodiesel are examined. Especially the influence of temperature, and the ESC behavior of PE-HD in media that are sorbed to a significant extent, are addressed. The underlying diffusion-controlled sorption process was assessed before comparing non-saturated specimens with samples previously saturated with sorbing media.
The investigations were performed using a novel FNCT-device with 12 individual sub-stations, each equipped with individual electronic stress and temperature control and continuous online monitoring of the specimen elongation.
In addition to acquired results concerning time to failure and elongation behavior, imaging techniques, such as light microscopy (LM), laser scanning microscopy (LSM) and X-ray computed tomography (CT scan) were utilized to obtain data concerning crack propagation and media migration. Fracture surface analysis (LM, LSM) and quasi in-situ imaging (CT scan) via measurement in different load stages are considered explicitly. Correlations of elongation behavior and data derived from imaging analysis such as fracture plane roughness and height of areas of ductile deformation were obtained.
Due to the increasing demand for utilization and improvement of energy efficient materials, especially concerning requirements in lightweight construction and design, polyolefin materials are used extensively and in an increasingly broad range of applications.
Although loaded with stresses under yield stress, preformed voids, inhomogeneities or notches can possibly lead to damage and unexpected failure induced by liquid media in polyolefin materials. For PE-HD, slow crack growth (SCG) as well as environmental stress cracking (ESC) are relevant damage mechanisms. SCG appears without influence of a surrounding medium or in “inert” media whereas ESC occurs in “active” media, which have decisive influence on damage mechanism and time to failure. To characterize the inherent resistance of the material against those damage mechanisms, the Full-Notch Creep Test (FNCT) is widely used. Therefore, the FNCT is of paramount importance particularly for the assessment of pipe and blow molding types of PE-HD – especially in the field of polymer development and improvement for high-performance applications.
In this study, the FNCT – usually applied as a standardized testing method (ISO 16770) using a few universal model liquid media – is extended by investigation of selected relevant PE-HD materials with a variety of properties also in real media. Mainly topical fuels, customary in the market such as diesel and biodiesel are examined. Especially the influence of temperature, and the ESC behavior of PE-HD in media that are sorbed to a significant extent, are addressed.
The investigations were performed using a novel FNCT-device with 12 individual sub-stations, each equipped with individual electronic stress and temperature control and continuous online monitoring of the specimen elongation.
Additional to acquired results concerning time to failure and elongation behavior, imaging techniques, such as light microscopy (LM), laser scanning microscopy (LSM) and X-ray computed tomography (CT scan) were utilized to obtain data concerning crack propagation and media migration. Correlations of elongation behavior and data derived from imaging analysis were obtained.
The full-notch creep test (FNCT) is widely used to characterize the slow crack growth (SCG) behavior of polyolefin materials in “inert” media as well as effects of environmental stress cracking (ESC) in which the medium has decisive influence on damage mechanism and time to failure. The test is of greatest importance for pipe and blow molding types of polyethylene, high density (PE-HD). Usually the full-notch creep test is applied as a standardized testing method (ISO 16770) using a few universal liquid media, such as solutions of Arkopal N 100. In our study, selected relevant polyethylene, high density materials are investigated also in real media – practical formulations as well as representative pure chemicals – and influences of temperature and geometry of specimen and notch are explicitly addressed. Furthermore, the investigations comprise also the environmental stress cracking behavior of polyethylene, high density in media that are sorbed to a significant extent – examples are diesel and biodiesel – based on comparison with samples previously saturated with those media. Thus, also the underlying diffusion controlled sorption process has to be assessed before. The investigations were performed using a full-notch creep testing device with 12 individual sub-stations, each equipped with individual electronic stress and temperature control and continuous online monitoring of the specimen elongation.
Environmental Stress Cracking (ESC) and Slow Crack Growth (SCG) of PE-HD induced by external fluids
(2020)
High-density polyethylene (PE-HD) is widely used as a packaging material. Typical applications are pipes and containers for storage and transport of dangerous goods. For these applications, the understanding of the craze-crack damage mechanisms slow crack growth (SCG) and environmental stress cracking (ESC) is of importance. Since these mechanisms are considered to be the major causes of failure, their understanding is essential for inspection and release of those materials.
A well-established test method for the assessment of these damage mechanisms is the full-notch creep test (FNCT). It is used in this study for a detailed investigation of crack propagation phenomena in PE-HD container materials under the influence of different fluids such as air, water and aqueous detergent solutions (Arkopal N 100) as well as biodiesel and diesel. Based on the results of the FNCT, a classification scheme of different fluids is proposed, which allows for an assignment of the respective damage mechanisms. Hereby, it is differentiated between (i) inert, (ii) purely surface-active and (iii) additionally sorptive, bulk-active fluids with respect to SCG. If the test fluid changes the intrinsic properties (at the surface or in the bulk), the damage mechanism is addressed to ESC behavior.
In FNCT investigations, stress, temperature and specimen geometry were varied systematically. In addition to the time to failure as common measure for the resistance of a PE-HD type against crack propagation, specimen elongation was considered in detail.
Several imaging techniques were applied for fracture surface analysis of specimens tested in FNCT to gain novel information on SCG and ESC behavior. From height profiles obtained by laser scanning microscopy (LSM) and information on surface structures from scanning electron microscopy (SEM), indicators for the differentiation of the crack propagation mechanisms could be derived. Based on the LSM data, an algorithm for the distinction between ductile shear deformation and brittle crack growth as dominating failure mechanism was developed. Imaging techniques were also used for determination of crack propagation rates, which were related to time-resolved FNCT elongation data. From the time-resolved determination of crack lengths of partly damaged FNCT specimens, an increasing length of craze zone with a progressively propagating crack was revealed for the first time. This relation of crack and craze zones was specified by fracture mechanical considerations.
Characteristics of environmental stress cracking of PE-HD induced by biodiesel and diesel fuels
(2024)
In the context of the increasing effect of carbon dioxide emissions on the global climate biodiesel produced from renewable sources has emerged as a promising contender replacing fossil fuels, especially in long-range transport vehicles, using existing engines and infrastructure.
High-density polyethylene is one of the prevailing materials for pipe and container applications for storage and transport of such fuels, both, from fossil and renewable resources. The contact with the respective fuels raises questions concerning material compatibility as biodiesel exhibits significant differences compared to conventional diesel fuel affecting its sorption and plasticization behavior in polyethylene. In this study, its behavior with respect to environmental stress cracking, considered one of the most frequent damage mechanisms leading to failure of polymer parts and packaging, was evaluated using the well-established Full Notch Creep Test. This
approach allows for a detailed fracture surface analysis using imaging techniques, such as optical and laser scanning microscopy, as well as infrared spectroscopy. Comparing the environmental stress cracking behavior in standard surfactant solutions with that in biodiesel and diesel, respective crack propagation rates, showing different levels of acceleration, were determined and details of the underlying mechanisms could be revealed.
Furthermore, the specific infrared absorption of the biodiesel’s ester functionality allows its semi-quantitative determination on the fracture surface of the tested specimens after failure. Thus, a preferred uptake of sorptive fluids in the fracture zone due to local morphological changes of the polyethylene could be directly evidenced by infrared spectroscopy.