SKZ Sets New Standards in Plastics Testing with Innovative Strain Hardening Test (SHT)
The SKZ Plastics Center uses advanced testing methods for quality assurance of polyolefin plastics. The Strain Hardening Test (SHT) enables precise testing of the stress crack resistance of plastics, in particular pipes made of PE80, PE100 and the improved PE100RC.
The Strain Hardening Test (SHT) allows for accurate testing of the stress crack resistance of plastics. (Photo: SKZ)
Establish environmentally friendly testing required to test advanced polymer pipe materials
The sensitivity of polyethylene to slow crack growth, also known as environmental stress cracking (ESC) or slow crack growth (SCG), is critical to the long-term performance of polymers. Conventional test methods such as the (accelerated) Full Notch Creep Test (aFNCT, FNCT), the (accelerated) Notched Pipe Test (aNPT/NPT), or the Notched Constant Tensile Load (NCTL) test are increasingly proving to be unsuitable for advanced polymer pipe materials due to the long test times. The wetting agents previously used for these tests fall under REACH and are therefore no longer allowed to be sold in Europe. There is an urgent need for alternative, environmentally friendly test methods. The NCTL test has been an accepted test method for geomembranes for some time.
Significant reduction in test time
The SHT not only significantly reduces conventional test times, but also enables accurate estimation of SCG properties by improving reproducibility and accuracy. This results in a significant reduction in test effort and provides rapid measurement results. The test corresponds to a tensile test at a temperature of 80°C. The strain hardening modulus is calculated from the gradient of the neo-Hookean material model between an elongation of 8 and max. 12. The test is based on the assumption that the behavior of the plastic fibrils under deformation corresponds to the behavior of the strain hardening modulus above the natural strain at elevated temperature.
Two standards with very similar test methods
There are two standards for SHT with two very similar test methods: One is ISO 18488 for tube materials: Press plates are made from granules and specimens are punched. The other is DIN EN 17096 for geomembranes: specimens are punched directly from the geomembrane.
"A major step forward in plastics testing
Dr. Andrea Monami, Head of Customer and Project Management, states: "The introduction of the Strain Hardening Test is a significant step forward in plastics testing. This innovative method allows us to evaluate the stress crack resistance of polyethylene pipes more efficiently and accurately, which is crucial for the long-term reliability and performance of these materials.
Ongoing research into the correlation between SHT and other established test methods
The SKZ points out that current standards and regulations, such as DIN EN 1555-1 to DIN EN 1555-4 for gas supply or DIBt approvals for LAU systems, already contain requirements for the strain hardening module. In addition, the correlation between the strain hardening test and other established test methods such as FNCT, aFNCT and NCTL is currently being investigated
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