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Updated: 22 hours 59 min ago

Nice work!!

Wed, 2019-01-02 22:58

In reply to A review on modeling of electro-chemo-mechanics in lithium-ion batteries

Peter,  Congratulations to you, Ying, and Baixiang on the very nice review! 

Hi Tongqing,

Tue, 2019-01-01 23:58

In reply to Hi Qihan,

Hi Tongqing,

Very good question. The bending you mentioned should come from the residual stress in the bulk material. The interfacial layer is probably too thin to cause anything noticable for the size of a common device. I guess the bending should be gone if you can control the drying of the gel during the bonding process?

Macroscopic stress-strain computation in ParaDis

Fri, 2018-12-28 23:19

In reply to ParaDis : Discrete Dislocation Dynamics Simulation

How the macro stress strain are evaluated in ParaDis?

great

Fri, 2018-12-28 19:07

Thank you, Yonggang !

Fri, 2018-12-28 13:22

In reply to 2018 Timoshenko Medal Acceptance Lecture: Academy Family, by Prof. Ares Rosakis

Ares is always nice and thoughtful to our graduate students.

Roy

Hi Qihan,

Thu, 2018-12-27 21:41

In reply to Journal Club for December 2018: Bonding hydrophilic and hydrophobic soft materials for functional soft devices

Hi Qihan,

I have learned a lot form your interesting and inspiring work. My question is would any residual stress exist after the adhension. Some students here found that after adhension of two layers, the bilayer bent a lot. Is the stress good or bad for the adhension?

Tongqing

Congratulations Guoying!

Thu, 2018-12-27 08:37

In reply to Soft wall-climbing robots

Congratulations Guoying!

Hi Ruobing, thanks for

Thu, 2018-12-27 08:35

In reply to Review: fatigue of hydrogels

Hi Ruobing, thanks for sharing this wonderful review, which will shed light on a new exciting field.

Crystallization in NR

Mon, 2018-12-17 13:35

In reply to Nice work

Hi Jingda,

Regarding natural rubber, I recall the crystallization melting temperature is below the room temperature. As a result, crystallization forms when the natural rubber undergoes large stretch (e.g., at the crack tip), but melts when the stretch is released. For comparison, the crystallization of PVA is (largely) thermodynamically stable at room temperature.

People have indeed studied crack deflection in fatigue crack growth of natural rubber. See:

[1] 2010 Le Cam The mechanism of fatigue crack growth in rubbers under severe loading: the effect of stress-induced crystallization

[2] 2011 Saintier Cyclic loadings and crystallization of natural rubber: An explanation of fatigue crack propagation reinforcement under a positive loading ratio

If you cut a rubber band with a notch, and cyclically stretch it, you can observe the rough crack surface growing by eye. I'm fascinated to wonder why the threshold of NR is only 50 J/m2. Perhaps it's because the crystallization domain around the crack front is too small, as Shaoting mentioned. However, this "self-activated" composite effect deserves further development in soft materials.

Best,

Ruobing

notice incidentally that if you define a problem

Mon, 2018-12-17 09:39

In reply to Thanks!

classically in terms of a remote stress and a crack size, the problem is indifferent on E, and hence the DKth formulation is more interesting.  If you reach a value close to a metal in these respects it is even more fascinating.

 

Mon, 2018-12-17 00:07

In reply to Nice work

 

Hi Jingda,

Thanks for your insightful discussion. We thought about the difference between nature rubber with strain induced crystallization and PVA hydrogels. The main difference may come from the different types of crystalline domains. The strain-crystallization in natural rubbers is reversible. The crystalline domains mostly dissapear when releasing the natural rubbers to undeformed state, but the cyrstalline domains form and preserve in undeformed as-prepared PVA samples.

If I refer correctly, Lake [1] measured the fatigue threshold of nature rubber in this paper which is 50 J/m2. They particularly focused on the case with cut growth at small deformations. At small deformations, strain-crystallization is only activated in the region at crack tip. The crack may still propagate by cutting amorphus chains instead of fracturing the crystalline domains at crack tip. The paper also mentioned the other effect coming from ozone attacking, which is not the case in swollen PVA in water either.

It would be very interesting to investigate fatigue behaviors in varous semi-crystalline soft materials in future works.  

For the dc/dN ~G curve at high G zone, it is truly nolinear with m larger than 2 if we fit the curve into Paris Law as I discussed with Mike.

BTW, your papers on fatigue fracure of hydrogels motivate the study in fatigue fracture in hydrogels. Look forward to more interesting results.

 

[1] G. Lake, P. J. J. o. A. P. S. Lindley, The mechanical fatigue limit for rubber. 9, 1233-1251 (1965).

Shaoting

 

Thanks!

Sun, 2018-12-16 21:43

In reply to you are just 1 order of magnitude below a metal!

Thanks!

Nice work

Sun, 2018-12-16 20:59

In reply to Anti-fatigue-fracture hydrogels

Hi Shaoting,

Nice to meet you on imechanica. Very beautiful work. The anti-fatigue mechanism for hydrogels is always our concern since we study the topic. You show the crystalline zone strengthening PVA can achieve this goal. The Ashby plot on the final figure well summarized the recent development in this field. I have 2 short questions after reading the paper:

(1) It is known that nature rubber (NR) can also form crystalline zone after stretched, but its fatigue threshold is only 50 J/m2. Have you ever thought about the reason of the big difference on the fatigue threshold between NR and PVA hydrogel?

(2) What is the dc/dN ~ G curve like in the high G zone in Fig. 3F?

Thank you for sharing this great paper with us.

Jingda

Hi Yang,

Sun, 2018-12-16 16:48

In reply to Photodetachable adhesion

Hi Yang,

Nice work! I think this is the first controllable adhesion ever reported. I would have included it in my review if it were out earlier.

As for the applications, doctors needs to clamp tissues around when they are doing operation. A controllable detachment would be necessary in this situation. And just like building a tower requires temporty structure, building a complex soft device may very well require tempory structure that disassembles afterwards. I'm really looking forward to more exciting works in this direction!

Hi Hang,

Sun, 2018-12-16 16:41

In reply to Adhesion method applied for coating

Hi Hang,

There must be some modification of the surface for bonding to happen. Even topological adhesion modifies the properties near the interface, although it does not require any modification before hand. So I guess the question is not how to achive bonding without modification. Rather, we should ask what is the suitable modification for a particular situation.

you are just 1 order of magnitude below a metal!

Sun, 2018-12-16 13:08

In reply to Thanks for the insightful

Congratulations.  I prefer to speak about Kth instead of Gth because it is more common.

For very low cristallinity you find

kth= Sqrt (114 15 10^3) =1.3 kPa m^1/2 

to high cristallinity

kth= Sqrt (10 1200 10^6) =  Sqrt (12 10^9) = 0.111 MPa m^1/2 

so an increase of 2 orders of magnitude, and just one order of magnitude lower than a metal which by comparison would have kth of the order of 1-5 MPa m^1/2 .....

 

Thanks for the insightful

Sun, 2018-12-16 10:58

In reply to you seem to find a rather linear relationship, unlike classical

Thanks for the insightful discussion. In this work, we particularly focused on the fatigue threshold (the minimal fracture energy at which crack propagation occurs under cyclic loads).To identify this number from experiment, we linearly extrapolated the curve of  dc/dN vs. G  to the intercept with the abscissa, to obtain the critical energy release rate Gc, which gives the fatigue threshold.

Regarding the shape of the curve of dc/dN versus G, the curve shown in this paper was focused on the region with very small value of dc/dN, in which the relation between dc/dN and G approximates linear. However, the overall crack extension curve is truly nonlinear. Particular when dc/dN is moderate high, m is larger than 2. For elastomers, people showed that m =2 when dc/dN is small and m = 4 at moderate dc/dN [1-2].

Regarding the interesting case of m=2, I agree with your argument that the integral is logarithmic with m = 2. Also, I enjoy reading your work on effect of distributions of cracks on the distributions of fatigue life.

[1] A. Gent, P. Lindley, A. J. J. o. A. P. S. Thomas, Cut growth and fatigue of rubbers. I. The relationship between cut growth and fatigue. 8, 455-466 (1964).

[2] G. Lake, P. J. J. o. A. P. S. Lindley, The mechanical fatigue limit for rubber. 9, 1233-1251 (1965).

you seem to find a rather linear relationship, unlike classical

Sun, 2018-12-16 09:14

In reply to Thanks for the comment. Here,

If I read correctly, your da/dN is rather linear with G.  This would be equivalent, in the language of metals crack growth with Paris law, more or less to da/dN=C Kmax^2, which means m=2, the exponential crack growth.  This case is an interesting limit case.  Among the main consequences of having m=2 are

 

1) when you integrate the law between initial size a1 and final size a2, normally for m>>2, the effect of final crack size (which gives then influence of static toughness) is very limited.   With m=2, the integral is logarithmic, and hence static toughness cannot be neglected

 

2) if you have a certain distribution of cracks, normally you would expect a distribution of fatigue lives.  What we found recently is that the case you are invoquing (m=2 ) leads to extremely reduced scatter.  See

 [PDF] researchgate.net

On the distribution and scatter of fatigue lives obtained by integration of crack growth curves: Does initial crack size distribution matter?CiavarellaA Papangelo - Engineering Fracture Mechanics, 2018 - ElsevierBy integrating the simple deterministic Paris' law from a distribution of initial defects, in the 
form of a Frechet extreme value distribution, it was known that a distribution of Weibull 
distribution of fatigue lives follows exactly. However, it had escaped previous researchers …

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