Fellow Harvard University Zhigang Suo team: cycle stretching 30,000 times without any pressure! A reinforced gel material, gel material highly resistant to fatigue advent

Stretchable material (such as an elastomer, hydrogel, organogel, and ion gel) in tissue repair, drug delivery, robot, ion electronics, bioelectronics, synthetic biology, and wearable devices are widely used. When these materials are subjected to a load, must be resistant to crack growth to prevent material failures, this capability can be measured by toughness and Γ under monotonic loading threshold Γth under cyclic loading. The picture shows the material toughness – threshold table, the figures represent the same diagonal and toughness threshold materials, such as ceramics, but they are brittle and fatigue resistance. Most materials, such as plastics, metals, elastomers, hydrogels, are in something of toughness, so are located below the diagonal, good toughness, but these materials fatigue. Thus thresholds for these toughening material is usually lower than the toughness of example one to two orders of magnitude, a representative is a natural rubber, its toughness generally in excess of 10000 J / m 2 , but the threshold miserable 50 J / m 2 . 哈佛大学锁志刚院士团队:循环拉伸30000次毫无压力!给凝胶材料加筋,高度抗疲劳凝胶材料问世

The results presented in

Based on the above analysis, Harvard University Professor Zhigang Suo Task Force a can be presented drawability and fatigue resistance while improving the method of gel material . They polydimethyl siloxane (PDMS) fibers embedded in a softer, higher stretchability polyacrylamide (the PAAm) matrix material, by thinning the two covalent bonds with crosslinked, synthetic a composite gel. The toughness of the individual fibers and PDMS PAAm gel is only 365 J / m 2 and 1142 J / m 2 , but Composites Toughness and up to 4136 J / m 2 , and after 30,000 cycles the material no longer continue to expand the crack, exhibits excellent fatigue resistance. Researchers believe that this approach has broad applicability, to open the door to development of high performance gel material. 哈佛大学锁志刚院士团队:循环拉伸30000次毫无压力!给凝胶材料加筋,高度抗疲劳凝胶材料问世

Materials Design Principle

Figure 1. stretchability and high fatigue resistance material design principles.

is flexible and stretchable fibers having the characteristics of high modulus and low threshold, if the base material is more flexible, more stretchable, the low modulus, high threshold. Mild both connected by a covalent bond together to form a composite material, when not deformed, the fibers are uniformly dispersed in the matrix (square marks for the red portion of the base), the composite material cut with a razor blade on a crack, and stress is applied stretching it, the matrix of the large shear deformation occurs, the deformation of the red square to a parallelogram, and a crack becomes blunt, the fiber is a high degree of stretching, the development of cracks in the composite material will be more difficult, the material is completely before the failure, all of the fibers can be evenly share the load, so the composite material has greater toughness and fatigue resistance.

Synthesis and mechanical properties of a composite material

FIG. 2. Tensile properties and good mechanical properties and preparation method Test material fatigue.

To prepare a high stretchability, good composite fatigue resistance, researchers polydimethyl siloxane (PDMS) elastomer as fibers, polyacrylamide (the PAAm) hydrogels as a substrate. Firstly AAM monomer, the photoinitiator preparation of a gel; and then the curing agent Sylgard 184 (weight ratio 10: 1) prepared by mixing a 0.5mm thick PDMS film, and then was cut into a width of 1 mm ~ 2.5 mm fibers, the fibers are arranged in the skeleton, AAM photocuring gel reaction thereon, the composite material finally obtained. Monotone researchers used to test ultimate tensile load of the composite, shear modulus, and toughness, cyclic loading test fatigue behavior of the composite material, as shown in FIG.

Figure 3. The development of the composite cracks.

In order to study the development of cracks in the composite material, the researchers were tested using cyclic tensile test, the tensile load amplitude is 1.725, the energy release rate is 1290 J / m 2 . Found pre-cut or crack after the initial composite thousands of cycles, the crack extends only to the first fiber, after more than 30,000 cycles no crack to continue to expand. In contrast, the toughness of the composite fibers is PDMS only 365 J / m 2 ,Toughness gel matrix of the water is higher, reaching 1142 J / m 2 , but the toughness of the composite material is as high as 4136 J / m 2 , exhibit excellent fatigue properties .

Research on Fatigue Performance Composite Materials

Figure 4. Testing the number of cycles at failure of composites with different amplitudes energy release rate. (A) sample due to fiber breakage and failure in the first cycle, the energy release rate of 4441 J / m2; (b and c) with decreasing amplitude of energy release rate and increase the number of circulating material failure; (D and e) when the amplitude of the energy release rate is sufficiently low, the end of the experiment was not observed failure of the material; GN curve (f) of the composite material.

In order to study the fatigue properties of the material, the cyclic loading researchers tested if the composite matrix softer than the fiber, the fiber material fracture will fail. When the energy release rate is less than the sample 4441 J / m 2 , the fibers in the first cycle occurs in the fracture; composite at a smaller magnitude of energy release rate and can withstand more cyclic loading; energy release rate, and if the amplitude is further reduced in tens of thousands of cycles without breaking the fiber candidates, the composite material does not fail.

Figure 5. stretchability when the substrate is not high, the fatigue behavior of the composite material.

In contrast, researchers have synthesized a matrix and hydrogel can not high stretch material, the composite material found in the energy release rate of 1290 J / m 2 under load after 6414 cycles after the failure, the failure mode is cracking failure kink, forming cracks and kink at the boundary cracking base.

FIG 6. Fatigue Behavior smaller feature sizes composites

Researchers increases the number of fibers, but fiber content remains constant (20.3 wt%), to reduce the wherein the size of the composite material. We found 1290 J / m when the amplitude 2 of the energy release rate, the material after 6432 cycles crack growth to the first fibers, cracks began to appear in the kink 10317 cyclesAnd subsequently propagate upwards.

Fiber – Effect of matrix adhesion strength mechanical properties

FIG fiber 7 – Effect of matrix on the mechanical properties of the adhesive strength. Having weak and strong interfacial adhesion hydrogels and composites (a) notched sample and (b) non-notched samples of stress – elongation curve, * represents rupture; (c) having weak and strong interfacial adhesion hydrogels and toughness of the composite.

To investigate the effect adhesion of the fiber and matrix size on mechanical properties, researchers have synthesized two composite, wherein a covalent crosslinking group in the fiber and the hydrogel-forming, in another set of fibers and hydrogel-forming non-covalent action. It found that two groups of materials exhibit different stress – tensile behavior, toughness strong interfacial adhesion composite is four times weaker interfacial adhesion, and when the weak adhesion of fibers and a matrix, the composite material due to breakage of the matrix will fail. Zhigang Harvard University professor lock synthesized TF one kind good toughness, fatigue-resistant composite gel. They (PDMS as a fiber, to the photocurable acrylamide (AAm) as the hydrogel matrix composite bonded synthesis of a gel material. Fatigue properties of the material were tested in a tensile test cycle, when the tensile load amplitude of 1.725, the energy release rate is 1290 J / M2 when, a pre-cut crack in the composite material after the initial few thousand cycles, the crack extends only to the first fiber, after more than 30,000 cycles no crack continues to expand, the performance excellent fatigue resistance. While the toughness PDMS fiber and the gel matrix is ​​water only 365 J / m2 and 1142 J / m2, but after covalent bonding after the composite toughness as high as 4136 J / m2. If the matrix water stretchability of the gel is not high, the composite material occurred after 6414 cycles after kink crack failure; if the fiber density increases, the material 10After 317 cycles the same cracking and failure as a kink. And adhesive strength fibers and the matrix to be higher, the toughness of the composite material having strong interfacial adhesion is weak interfacial adhesion four times. Original link: https: //www.sciencedirect.com/science/article/abs/pii/S1369702119307606