Polyurethane (PU) elastomers are among the most used rubberlike materials due to their combined
merits, including high abrasion resistance, excellent mechanical properties, biocompatibility, and good
processing performance. A PU elastomer exhibits pronounced hysteresis, leading to a high toughness on
the order of 104 J/m2. However, toughness gained from hysteresis is ineffective to resist crack growth
under cyclic load, causing a fatigue threshold below 100 J/m2. Here we report a fatigue-resistant PU
fiber–matrix composite, using commercially available Spandex as the fibers and PU elastomer as the
matrix. The Spandex fibers are stiff, strong, and stretchable. The matrix is soft, tough, and stretchable.
We describe a pullout test to measure the adhesion toughness between the fiber and matrix. The test
is highly reproducible, showing an adhesion toughness of 3170 J/m2. The composite shows a maximum
stretchability of 6.0, a toughness of 16.7 kJ/m2, and a fatigue threshold of 3900 J/m2. When a composite
with a precut crack is stretched, the soft matrix causes the crack tip to blunt greatly, which distributes
high stress over a long segment of the Spandex fiber ahead the crack tip. This deconcentration of stress
makes the composite resist the growth of cracks under monotonic and cyclic loads. The PU elastomer
composites open doors for realistic applications of fatigue-resistant elastomers
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Stretchable materials of high toughness and low hysteresis
In materials of all types, hysteresis and toughness are usually correlated. For example, a highly stretchable elastomer or hydrogel of a single polymer network has low hysteresis and low toughness. The single network is commonly toughened by introducing sacrificial bonds, but breaking and possibly reforming the sacrificial bonds causes pronounced hysteresis. In this paper, we describe a principle of stretchable materials that disrupt the toughness–hysteresis correlation, achieving both high toughness and low hysteresis. We demonstrate the principle by fabricating a composite of two constituents: a matrix of low elastic modulus, and fibers of high elastic modulus, with strong adhesion between the matrix and the fibers, but with no sacrificial bonds. Both constituents have low hysteresis (5%) and low toughness (300 J/m2), whereas the composite retains the low hysteresis but achieves high toughness (10,000 J/m2). Both constituents are prone to fatigue fracture, whereas the composite is highly fatigue resistant. We conduct experiment and computation to ascertain that the large modulus contrast alleviates stress concentration at the crack front, and that strong adhesion binds the fibers and the matrix and suppresses sliding between them. Stretchable materials of high toughness and low hysteresis provide opportunities to the creation of high-cycle and low-dissipation soft robots and soft human–machine interfaces.
more » « less- NSF-PAR ID:
- 10087741
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 116
- Issue:
- 13
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 5967-5972
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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