What creep and relaxation tell you
Polymers, gels, solders, tissues and many coatings keep deforming under a sustained load, and the force they exert relaxes when held at a fixed deformation. That time dependence decides whether a seal keeps sealing, a solder joint keeps its shape, a press fit stays tight, or an implant coating holds up. Indentation measures it locally and quickly, on the actual material rather than a separately made test bar. The nanoindentation page explains the technique and the instrument behind it.
How the tests work
Creep (constant load). The indenter is loaded to a set maximum force and held there. The independent depth sensor records how far it continues to sink. Creep depth, or a creep rate, characterises the material.
Stress relaxation (constant depth). The indenter is loaded to a set point and its position is then held fixed. The load cell records the force decaying over time as the material relaxes, following the principle of ASTM E328.
Both rely on the instrument holding one quantity truly constant while measuring the other. NANOVEA testers use independent load and depth sensors in a closed feedback loop for exactly this.
Choosing the tip for soft materials
Sharp Berkovich tips concentrate stress and suit most polymers and coatings. Very soft materials need a large contact area to give a measurable load: NANOVEA’s hydrogel study used a 3 mm spherical tip at 10 mN, recording ~42 µm of creep in 70 s. On such materials, contact detection is itself a challenge, since surface tension can pull the tip into the gel before any load registers.
Creep in service conditions
Time-dependent behaviour is strongly environment-dependent. Moisture plasticises many polymers. In NANOVEA’s humidity-controlled study, creep depth rose from 36 nm at 25% RH to 105 nm at 75% RH, while hardness fell. Temperature has similar effects, especially near the glass transition. Testing at service conditions avoids relying on room-temperature, room-humidity data.
Comparing materials
Because creep and hardness come from the same indent, they can be compared directly. NANOVEA’s polymer creep study tested eight polymers at 40 mN: the softest, LDPE, crept 798 nm, about seven times more than Delrin, the hardest.
