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Updated: 20 hours 37 min ago

Thanks for your insightful

Tue, 2020-07-07 03:37

In reply to Structure Property Relationship

Thanks for your insightful commnets. Looking forward to your further work on this field!

Abaqus mailing list

Mon, 2020-07-06 15:14

In reply to Can someone post an example of UMAT and VUMAT for the same elasto-plastic problem?

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Good luck

Frank

Structure Property Relationship

Mon, 2020-07-06 10:40

In reply to Oh, sorry, something seems to

Dear Chenghai,

Nice to meet you in this Imechanica Journal Club. I believe your question is something missing but highly desirable in this field from the fundamental perspective.

People are actively studying the impact of topological defects on elasticity, basically correlating polymer topologies to phantom elasticity (e.g. M. Zhong, R. Wang, K. Kawamoto, B. D. Olsen, J. A. Johnson, Quantifying the impact of molecular defects on polymer network elasticity. Science 353, 1264-1268 (2016).) Such kind of study on the fracture of polymers is not well established yet, as far as I know.

If I track the literature correctly, Flory first proposed the concept of polymer topology and introduced the concept of graph theory, for understanding elasticity (Flory, P.J, Network topology and the theory of rubber elasticity. Br. Polym. J. 17, 96–102, 1985).

By tracking this work, my feeling is we need to classify defects first, then understand the impact of each type of topological defects one by one. With some theoretical understanding, we may harness existing chemical synthesis to design hydrogels with controlled defects as a model material system for experimental investigation. I am studying and working in this direction recently. Hopefully, I can talk more once I make some progress in this direction.

This is really a good question and insight.

Best,

Shaoting

Oh, sorry, something seems to

Mon, 2020-07-06 01:21

In reply to  

Oh, sorry, something seems to be wrong for the text editing. Please ignore the unnecessary text.

 

Mon, 2020-07-06 01:18

In reply to Journal Club for July 2020: Fatigue-resistant hydrogels: Principles, Experiments, and Applications

 

Dear Shaoting,

Thanks for this nice review. I'm Chenghai Li, a PhD student from UCSD. I’m curious about the relationship between microscopic molecular structures and macroscopic mechanical properties. For simple microscopic structures like glass, ideal network or ceramics, pioneering people have proposed some clear understanding to relate macroscopic properties to microscopic structures. (e.g. Griffith's paper on glass fracture, Lake-Thomas model for predicting threshold of NR or toughness for a perfect network...) But generally speaking, the microscopic structures become more complex for polymers. (inhomogeneity, chain length distributions, dangling chains, loops...) As shown in Canhui and Zhigang’s JMPS paper, deviation from perfect networks can significantly affect mechanical properties of polymers (e.g. toughness, flaw sensitivity, work of rupture...). My question is that, how to qualitatively and quantitatively establish a more clear relationship between microscopic molecular structures with macroscopic properties for polymers?

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Hi Canhui,

Sun, 2020-07-05 22:32

In reply to Hi Bin, thank you for sharing

Hi Canhui,

Thank yo uso much for your response!  I always have been wondering if I get the main and key points properly/correctly from those high-quality papers......   You paper assured me that long, comprehensive reviews can be very interesting and enlightening, along with the length, depth, and breadth.

Best,

Bin

Brainstorming for the "next"

Sun, 2020-07-05 21:30

In reply to Some random thoughts

Dear Ruobing,

I like your random thoughts a lot, which are sort of brainstorming for the future pathway of this direction.

1, Symptom of fatigue in real applications indeed involves more complex loading conditions, sample geometries, and chemical environments, which eventually turns to be an engineering problem. From my personal perspective, such kind of engineering designs in real applications, in turn, can inspire new scientific challenges. For instance, taking the neuro probe as one example, long-term chronic damage remains a central challenge in the field, which involves micro-motions of the human body as the boundary conditions, scar formation as the foreign body response, and increased impedance as the symptom of fatigue. This engineering challenge may inspire the design of electrically conductive fatigue-resistant materials for prolonged recording or stimulation of neuron cells. The scientific question could be what is the principle for endurant conductive materials, which falls into the design of molecular structures as well.

2, Beyond the applications, I am more interested in the fundamental implications, honestly speaking. I believe you already laid some foundation work to use fatigue characterization as a platform, showing that viscoelasticity does not contribute to the fatigue threshold. When either viscoelasticity or poroelasticity involves, the time scales should be taken into account, thereby the loading frequencies must play roles as well. I am particularly interested in understanding intrinsic fracture energy of soft materials, which has been studied since 1958 but still remains ambiguous. I believe the recent development of experimental techniques for probing chain fracture, and stress mapping may enable new opportunities for an in-depth understanding of fracture of soft materials. 

3, I totally agree with you about this comment. Some applications of hydrogels do not contain long cracks, therefore may not even suffer the issue of fracture or fatigue during its lifetime due to its flaw-insensitivity. However, as a fundamental study at initial stage, we still need standard testing methods (e.g., pure-shear samples with long cracks) to measure the value of fracture energy as its material property. A specimen with unreasonable small cracks will not give a value with a clear physical meaning, thereby it would be challenging for people to compare. 

On the other hand, I also see the strong reason why people in material science seem to be more willing to use the area of stress-strain curve to quantify a material's toughness. This toughness in the unit of J/m^3 is more relevant to real applications where samples are typically unnotched.

Best,

Shaoting

Some random thoughts

Sun, 2020-07-05 18:23

In reply to Journal Club for July 2020: Fatigue-resistant hydrogels: Principles, Experiments, and Applications

Dear Shaoting,

Congratulations on having such a nice overview of an on-going field of hydrogel fatigue and anti-fatigue. Since we have already discussed quite a lot online and offline, I am here with just some random questions and thoughts.

1. While people in the mechanics community usually refer fatigue to the failure under cyclic loads, the word "fatigue" in reality can mean a lot of things, as we previously summarized as "symptoms of fatigue". We have mainly focused on the one symptom, "cyclic fatigue crack growth", for a while, which is of critical importance, and is now more or less well adressed thanks to the efforts by you and other colleagues. On the other hand, there are still challenges in other symptoms of hydrogel fatigue. For example, the property of a hydrogel degrades after long-time loading. To address these challenges, it requires other complex loading conditions, sample geometries, and in vivo tests, as folks already discussed above. In addition, these problems should be related to real applications. Pursuing a "perfect" "fatigue resistant" hydrogels may be too much, and there will be an extent to which the problem becomes more engineering than scientifically interesting. What is your thought for the "next"?

2. In addition to engineering applications, I believe that fatigue of hydrogels, together with all the characterizations and designs surrounding it, forms a great platform for targeting fundamental problems. The long time study and ambiguity on "intrinsic fracture toughness" is one of such example. Viscoelasticity, poroelasticity, and complex rheology accompanying the fracture and failure of hydrogels still have numerous intersting fundamental problems to study. I know you have tried to target some of these problems for a while. What do you see as the challenges or opportunities related to these fundamental studies?

3. In fracture mechanics, many people talk about both crack nucleation and crack growth. Classical fracture mechanics addresses crack growth quite well, but crack nucleation can always be a challenging subject. In hydrogel fracture and fatigue, I think it is interesting to think about crack nucleation, or even, whether we truly need fatigue-crack-growth-resistant hydrogels or not (of course we do!). The flaw sensitivity length or cohesive zone size of a double-network tough hydrogel can be as large as 1 mm. In this case, all we have worried and tested using samples with large initial crack length may be just irrelevant to real applications. So long as we have a great tough hydrogel that resists fatigue damage (degradation over cycles/loading time), we are good to go. Of course all these phenomena are intrinsically connected, but I sometimes reflect on myself whether we are chasing some particular aspect too far.

 

Sincerely,

Ruobing

 

 

Fatigue testing in a psychological environment

Sun, 2020-07-05 11:13

In reply to Dear Shaoting,

Dear Jingda,

Thanks for your great points. Yes, we conducted fatigue characterizations in different biological fluids as well.

In our ex-vivo long-term characterization of the ingestible hydrogel balloon, we used the gastric juice.

In the fatigue characterization of hydrogel adhesion using a 90-degree peeling test, we immersed the sample in a PBS buffer.

In the cartilage-on-flat dynamic fractional test, the interface between hydrogel coating and the cartilage was sprayed a thin layer of simulated synovial fluid as a lubricant to mimic the working environment of knee joints.

Best,

Shaoting

Dear Shaoting,

Sat, 2020-07-04 23:22

In reply to Journal Club for July 2020: Fatigue-resistant hydrogels: Principles, Experiments, and Applications

Dear Shaoting,

    Thank you for such a deep and helpful discussion. And thank you for sharing the two reviews, I usually read each paper from your group.  I agree with you that the developments of the study on hydrogel adhesion and hydrogel fatigue are in a different stage.  In fact, hydrogel adhesive have been studied for many decades. Only strong hydrogel adhesion initiates in recent years. This makes the comparison clear. Hydrogel fatigue is a much younger field (hahaha).  

    If we suppose fatigue resistant hydrogel will be mostly used in biomedical engineering, have you ever thought about the effect of in vivo environment on the fatigue resistance of hydrogels? You did fatigue tests under water in your papers, better than what we did before. Have you ever changed the water environment to a PBS Buffer or an other biological liquid (e.g., gastric juice, very low pH)? These are my random thoughts, may not be fair questions :)   

    Best,

Jingda

Hi Bin, thank you for sharing

Sat, 2020-07-04 22:54

In reply to keep reading-2

Hi Bin, thank you for sharing such an clear and thothough reading note on our review. You definitely hit the points we want to address in the paper. 

best,

canhui

Abaqus mailing list

Fri, 2020-07-03 16:20

In reply to How to include temperature effects on the shear rates in Dr. Huang's crystal plasticity code

Hello

subscribe to and seek assistance from the
ABAQUS mailing list
https://groups.yahoo.com/neo/groups/Abaqus/info
Search the archive of the list before posting in it.

 

Good luck

Frank

Two recent review papers by my colleagues

Fri, 2020-07-03 11:45

In reply to Comprehensive review

Dear Jingda,

I would like to also share two review papers by my colleagues, which particularly shed light on the applications of hydrogels.

Hydrogel machine by Xinyue

Hydrogel bioelectronics by Hyunwoo and Baoyang.

Best,

Shaoting

PVA and applications

Fri, 2020-07-03 10:45

In reply to Comprehensive review

Dear Jingda,

Thanks a lot for your kind words. Also, hearty congratulation on your fantastic talk in EASF! Your questions are challenging but really inspiring. I will try to answer them from my perspective. I would like to hear your thoughts as well.

(1)  You are right. The developments of the study on hydrogel adhesion and fatigue of hydrogels are in a different stage, though both fields are initiated at the same time. Hydrogel adhesion, as a leading technique for translational science, already shows its broad and immediate impacts; while the study on fatigue-resistant hydrogels is mostly focused on the fundamental investigation or material development.

One application for fatigue-resistant hydrogel which I also envision a huge potential for real translations is the ingestible hydrogel pill as a gastro-retentive device, for prolonged psychological signal monitoring, obesity control, drug delivery, and imaging of GI tract. For the groups who have the ability to conduct such kind of in vivo tests, I believe there are huge potentials for fatigue-resistant hydrogels to achieve previously inaccessible performances or functions. 

In addition to the application of ingestible gastro retentive devices, fatigue-resistant hydrogels are crucial for many existing applications of soft materials towards soft machines. The majority of the efforts in soft machines are laboratory work. However, when targeting practical deployment, their long term reliability is the killing factor for their lifetime. If we look back to the development of metals, composites, and plastics, their fatigue studies have been a hot topic and fatigue evaluation has been a standard routine before the commercialization of a product. I feel the development of soft machines has not entered that stage yet, but will bring people's attention to more realistic factors such as lifetime, corrosion, aging.

I also need to point out that many applications of hydrogels actually do not need that high fatigue resistance. For example, in an epidermal wearable device, the deformation of the relevant materials is typically small. When used in human body, on-demand degrading is also more favorable. It really depends on the specific application conditions.

(2) As I mentioned in (1), ingestible hydrogel pills are the new application of PVA hydrogels. If we track literature of PVA hydrogels, mostly are focused on cartilage replacement. I had the experience of visiting Philips, where they used PVA as an ultrasound phantom with complex shapes. Robust coatings for existing biomedical devices seem to be also the interests of doctors. 

If not limited to PVA hydrogels, I actually also see the potential of using hydrogels in applications beyond biomedical engineering. Hydrogels recently also bring interests to the area of sustainable water, energy harvesting, and agriculture. For example, the following paper used hydrogels for photovoltaic cooling (R. Li, Y. Shi, M. Wu, S. Hong, P. Wang, Photovoltaic panel cooling by atmospheric water sorption–evaporation cycle. Nature Sustainability, 1-8, 2020).

(3) We actually present two forms of PVA hydrogels: dry-annealed PVA hydrogel and nanofibrous PVA hydrogel. For the dry-annealed PVA hydrogel, it is isotropic, thereby it displays fatigue-resistance in any directions. However, for the nanofibrous PVA hydrogel, it is anisotropic. The fatigue resistance is high (i.e., 1250 J/m^2) when loading is along the direction of aligned fibrils, but relatively low (i.e., 233 J/m^2) when the loading is perpedicular to the aligned fibrils. Ruobing's work (https://imechanica.org/node/23079) also shows a similar anisotropic property of nanofibrous PVA hydrogels using a different fabrication technique. 

To solve that, we print the nanofibrous hydrogels into mesh-like structures, so that the sample shows fatigue-resistance in both in-plane directions. I believe you show a much-advanced version of 3D printing to achieve fatigue-resistant elastomers. 

Best,

Shaoting

Comprehensive review

Fri, 2020-07-03 01:47

In reply to Journal Club for July 2020: Fatigue-resistant hydrogels: Principles, Experiments, and Applications

Dear Shaoting,

Thank you very much for this wonderful review on fatigue of hydrogels. It is so comprehensive that people can get into this field easily. Since the last meeting on EASF seminar, I am waiting for this review. I want hear your opinion on  some points :

(1) The study on fatigue of hydrogels almost starts  at the same time with the study of hydrogel adhesion. Both are around 2015. Now, we can envision the huge potential of hydrogel adhesion in biomedical engineering. What will be the killer application of fatigue-resistant hydrogels?

(2) PVA hydrogel is a classical material. Thanks to your contribution, people now know it is very fatigue resistant.  Will this discovery find new applications and oppurtunities for PVA hydrogels? What is the status for the study of PVA hydrogels, especially for applications? 

(3) A detailed question: For PVA hydrogels, it is fatigue-resistant in the loading direction because of the obstruction of alligned polymer chains. How about the direction perpendicular to the loading axis? Is that one fatigue prone?

Fatigue characterizations

Thu, 2020-07-02 22:56

In reply to Dear Shaoting,

Dear Canhui,

I think you are hitting a very important point when people try to translate the fundamental studies of fatigue of soft materials into a more practical deployment. 

1, First of all, as you mentioned, at the initial stage of material innovations, we first need standard characterization tests (e.g., pure-shear, single-notch tension, 90-degree peeling) so that we can obtain a material property (e.g., fatigue threshold, interfacial fatigue threshold) favorable for theoretical analysis. 

2, When we try to apply a material into a practical system, we do need characterizations that can closely mimic the real conditions including complex stress states, acidity, or even real psychological environment. For example, when designing ingestible hydrogel pills, we perform cyclic compressive tests on the spherical hydrogel balloon in acid solution, as an ex-vivo test to simulate its real dynamic loading conditions. To further evaluate the material's performance in real conditions, people even prefer in vivo tests to evaluate if the material is working when implanted or ingested. Here, we collaborate with Prof. Gio Traverso for a large animal test to show the hydrogel pill can stay in a pig stomach for 30 days.  

3, Your point of the necessity of different designing principles is indeed correct. More specifically, We first need design principles as a material-level characterization. We also need design criteria as a system-level evaluation. 

I try to tackle this broad and challenging point. I would like to hear your viewpoints as well.

Best,

Shaoting

Dear Shaoting,

Thu, 2020-07-02 21:31

In reply to Journal Club for July 2020: Fatigue-resistant hydrogels: Principles, Experiments, and Applications

Dear Shaoting,

        Thank you for sharing such timely and comprehesive review. Fatigue of hydrogel is indeed important yet challenging for practical use. I believe the progresses made recently have laid the foundations for addressing this mission-critical issue, and the emerging of following up work is expectable.
        Most existing experiments for probing fatigue behavior utilize the tension of thin sheets of samples (pure shear tests). Such configuration makes it easier to corelate experimental results to theoretical analysis, whereas it deviates from practical scenarios to some extent. How do you think about the necessity of different designing principles, provided the fatigue behaviors under more complex stress states may be different from those observed in simple tension.

Best,

canhui

Different nature of crystallization

Thu, 2020-07-02 14:22

In reply to Dear Shaoting,

Dear Junjie,

This is a very important question. There are several intrinsic differences between the crystallinity in natural rubber and PVA hydrogels.

1, The crystallinity in natural rubber is induced by stretch and is reversible, while the crystallinity formed in PVA hydrogels are thermodynamically stable. Please refer to a previous discussion among Jingda, Ruobing, and me. https://imechanica.org/node/22932

2, Due to the nature of stretch-induced crystallization, the crystalline domains are only localized at the crack tip where large deformation is generated. Such localized crystalline domains may not be sufficient to pin the fatigue crack propagation. People used XRD to show the map of crystallinity around the crack tip, confirming its localized crystalline domains. Some results are presented in the paper (S. Trabelsi, P.-A. Albouy, J. Rault, Stress-induced crystallization around a crack tip in natural rubber. Macromolecules 35, 10054-10061, 2002).

3, In addition, the crystallinity in natural rubber is typically below 10% for the extension ratio of 5 (Fig. 6 in B. Huneau, Strain-induced crystallization of natural rubber: a review of X-ray diffraction investigations. Rubber chemistry and technology 84, 425-452, 2011). In contrast, for the fatigue-resistant PVA hydrogels we reported, its crystallinity is as high as 47%. 

Indeed, I also agree that an in-depth understanding of the correlation between fatigue properties and its molecular structures requires future efforts. 

Best,

Shaoting

Dear Shaoting,

Thu, 2020-07-02 13:13

In reply to Journal Club for July 2020: Fatigue-resistant hydrogels: Principles, Experiments, and Applications

Dear Shaoting,

       Thanks for leading the discussion on the topic of fatigue resistant hydrogel and hydrogel adhesion. Your works revealed that the crystalline domains at the crack front of the hydrogel or hydrogel-substrate interface inhibit the crack propagation under fatigue loading. For natural rubber, it will also crystalize at crack front under loading. But why natural rubber still suffers fatigue fracture? How do you think the difference between the two materials?

Best,

Junjie 

 

umat & usdfld

Thu, 2020-07-02 03:45

In reply to Dear Bojan Huljev, I have

you should just relate the number of cycles and increment in cyclic loading to determine life and fatigue damage. you can use both usdfld and umat subroutines. 

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