Test & measurement
Polyamides sit between brittle PMMA and ductile polycarbonate. While polycarbonate excels in toughness, it lacks the stiffness required for structural use. Reinforced polyamides combine good impact resistance with much higher strength and rigidity, making them closer to true structural materials. Yet, even within one family of materials, variations can be striking. Magnesium-catalysed cast polyamide-6, for instance, displays variations of more than 50 per cent in Charpy impact strength depending on its crystallinity and processing conditions. Crystallinity, or how ordered and tightly packed the molecular chains are, can impact toughness in polymers. Crystalline regions resist deformation, while amorphous regions are more disordered and able to absorb energy. As a result, higher crystallinity increases stiffness and strength but lowers impact toughness, since less of the material can deform and dissipate energy before fracturing. The production characteristics of a polymer, such as the mould thickness or cooling rate, can influence crystallinity. Slower cooling or larger sections of polymer can allow more crystalline structure to develop, which often makes polymers more brittle. Controlling both crystallinity and processing conditions enables manufacturers to balance strength, ductility and impact resistance. A polymer’s crystallinity and formulation can directly influence the safe operating window. Polypropylene becomes noticeably more ductile above 0°C, whereas polycarbonate retains impact resistance down to -40 °C before brittle fracture dominates. Low-density polyurea foams, measured at around 140 kg/m³, have absorbed up to 30 per cent more impact energy than thermoplastic polyurethane or ethylene vinyl acetate under 7J impacts.
Tensile impact tests, governed by ISO 8256:2004, involve stretching a sample at high speed until it fails. They are particularly valuable for assessing flexible plastic films, as drop towers can be configured with thermostatic chambers and higher velocity ranges, allowing tests at lower or more elevated temperatures, conditions that a pendulum setup cannot replicate as easily. Material testing systems, such as drop-weight testers fitted with load cells and high-speed data acquisition systems, enable real-time force measurements during impact events, capturing full force-vs-time data points that permit precise calculation of total absorbed energy. From those traces, engineers can identify the ductile-to-brittle transition temperature — the point where a polymer shifts from shattering under impact to deforming. This threshold often determines whether a given polymer is safe for various applications and industries.
Together, these standards form the backbone of impact testing. Each captures a different failure mode, and only by combining them can laboratories understand how polymers are likely to respond under various forces and conditions.
Instrumentation Monthly April 2026
THERMAL TESTING AND AGEING The temperature at which a polymer moves from brittle fracture to ductile deformation depends as much on its internal structure as on the surrounding conditions.
Polymers with higher crystallinity, such as high- density polyethylene (HDPE), tend to become stiffer and stronger but lose impact resistance. Crosslinking can boost strength and toughness at low temperatures, yet it also limits ductility. Adding fillers like glass fibre may improve toughness in certain tests while reducing flexibility. Because formulation and processing alter behaviour, two samples of the same polymer type can behave very differently depending on how they are produced. Therefore, many manufacturers will start with an established polymer family, such as polyethylene, and then adjust formulation and processing parameters to achieve the required balance of strength, toughness and flexibility for the given application.
Polymethyl methacrylate (PMMA) is often known to fail in a brittle manner under impact. Meanwhile, polycarbonate is valued for its toughness across a wide range of temperatures, with Charpy impact values of 20-35 kJ per m² (unnotched) and Izod notched values of 0.6-0.8 J per m.
Results like these illustrate how engineered microstructures outperform standard materials such as unmodified thermoplastic polyurethane or ethylene-vinyl acetate under impact, offering much greater energy absorption.
Laboratories capture the brittle-ductile transitions of these materials by running tests in controlled thermal environments. The resulting data show not just how much energy a material can absorb but also how its mode of failure changes as temperature varies. Brittle fracture is revealed by an abrupt drop in force after peak load, while ductile behaviour is characterised by a more gradual release of energy over a longer period. These distinctions underpin polymer choice in safety-critical roles, from pipelines in polar regions to under-bonnet components in vehicles. Beyond short-term thermal conditioning, long- term ageing studies provide valuable insights into how polymers degrade under extended exposure to heat or cold.
Engineers can use conditioning samples in thermostatic chambers for hours or days to accelerate the effect of chain scission, oxidation and other aging mechanisms. At elevated temperatures, polymers might lose elongation at break and
Continued on page 62... 61
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72
