iMechanica - research - Comments

damage modeling

umer virk — Sat, 11 Feb 2012 09:11:53 -0500

hi can you please help me what material model to use to simulate composite impact in ls-dyna

tsai wu failure criteria in ANSYS

yashidanadir — Sat, 11 Feb 2012 08:31:59 -0500

dear friends, i want to know how to get the tsai -wu failure criteria for orthotropic material in ANSYS.after modelling and giving the material properties ,should i give anything special to activate this option ,or will i get automatically this criteria for all the elements from the output file itself.

please help me as my research work couldnt able to proceed.

my mail id is

yashidanadir@gmail.com

ANSYS modelling

prakashn63 — Sat, 11 Feb 2012 07:02:44 -0500

Hi,

I am n graduate student doing my resarch in non-linear finite element modelling of reinforced concrete members strengthen with CFRP using ANSYS. It would be helpful if you can send me the related pdf files to me. My mail ID is prakashn3@gmail.com

Thank you.

N.Prakash

wallster, both and none, ubt the point is another!

Mike Ciavarella — Sat, 11 Feb 2012 02:18:26 -0500

Dear Wallstet

you are referring here to elementary concepts, which drive us in the design. Obviously, rounding corners is usually beneficial, as it relieves the "elastic" stress concentration. Whether it does so with plastic limit, is a lot less obvious, and whether it does so under fatigue, also less obvious. Just imagine one of the classical counterexample of notches being detrimental not being true. In Stephen Fuch's paper, he shows how compressive part of the cycle can induce in the stress concentration certain residual stresses wich are later beneficial.

However, the point is.

1) how to transfer this elementary knowledge to components different in scale, in details of the assembly, under spectrum loading which includes also extreme events like turbolence (of the type of the accident we are talking about). This procedure can be used at design stage, but nobody even dreams of letting a new plane in the market only with this!

2) we also rely, at design stages, on tables of data obtained by, say, NASA, in the 1960's, and we constantly refer to those. But maybe they are too old now? Or maybe we are starting to extrapolate when we are changing the size of the plane? My friends at Airbus lately were concerned that design of A300's (the good one, not the one under attack, A380), return after 10 or 20 years of service, and have LESS cracks than expected, so their life can be prolonged. The worry with A380 is that cracks are emerging too soon!

3) We have always, and only learned by mistakes. Limiting attention to aircraft only (but remember the example I made on railways fatigue,on which by the way we undestand very little even after 200 years...), see wikipedia

Structural failure of the aircraft

Examples of failure of aircraft structures caused by metal fatigue include the De Havilland Comet accidents (1950s) and Aloha Airlines Flight 243 (1988). Now that the subject is better understood, rigorous inspection and nondestructive testing procedures are in place.

Composite materials consist of layers of fibers embedded in a resin matrix. In some cases, especially when subjected to cyclic stress, the layers of the material separate from each other (delaminate) and lose strength. As the failure develops inside the material, nothing is shown on the surface; instrument methods (often ultrasound-based) have to be used to detect such a material failure. In the 1940s several Yakovlev Yak-9s experienced delamination of plywood in their construction.

4) You know the story of Comet? They were using static design with factor of safety 2, and they thought it was more than enough. Later, after 7 accidents, they decided appropriate criteria for fatigue should be taken into account, for the pressurization of the cabin was enough of a cycle of stress, in case of stress concentrations at the windows. Notice that we had had the experience of railyways, but people do not always transfer technology, so the progress is not linear and "monotonous" function...

5) Companies such as Airbus and Boing, not not rely only on design. They want to rely on experience on testing full scale. Tests are done and under various harsch conditions. Yet, no test can cover the entire spectrum of what can happen in life.

So I repeat my question. Can we be near the discovery of something equivalent to when we found out why Comet were falling down one after the other for no apparent good reason?

Michele Ciavarella, Politecnico di BARI - Italy, Rector's delegate.
http://poliba.academia.edu/micheleciavarella

L-shaped members

wallstedt — Fri, 10 Feb 2012 16:05:06 -0500

This is somewhat related to the crack model question: when I was an undergrad I was taught that the inside concave corners of L-shaped members should be rounded to avoid stress concentrations. Later, I heard experts in the field say that rounded corners are not necessary because ductility in the structures will absorb the stress concentrations. So which is it? Can concave corners be sharp or not?

Is elastic vibration theory enough?

Rui He — Fri, 10 Feb 2012 02:22:58 -0500

Hello, Biswajit:

Thanks for your comment. In my opinion, it's suitable to study the "far-field" wave propagation problems (such as the transmission of earthquake waves from one place to other place) using elastic models, however, if a "near-filed" problem (such as the prediction of the motion and bearing capacity of a wind turbine built on the seabed. The soils in the vicinity of the turbine is of great importance, and they will be inelastic when the turbine starts work), in this kind of problem, I think in order to be enough accurate, inelastic effects should be considered in the vibration theory. Do you think so?

Rui

Internal energy -> 1/resonance frequency ?

Biswajit Banerjee — Thu, 09 Feb 2012 18:38:15 -0500

For a simple linear spring-mass system:

Internal Energy: W = 1/2 k d^2 where d = displacement, k = spring stiffness.

Resonance frequency: omega = sqrt(k/m) where m = mass.

Then

omega^2 = 2/(md^2) W

So what you're seeing could be due to changing m and d (or some inelastic effect).

-- Biswajit

On the relevance of elastic vibration theory

Biswajit Banerjee — Thu, 09 Feb 2012 18:19:29 -0500

There is a lot of interest in earthquake mitigation using transformation elastodynamics. The field is in its infancy and we still don't know how useful these ideas will be. Large amplitude elastodynamics is the next obvious step in that direction.

The field of crustal seismology uses elastic (and poroelastic) models but still matches observations to a high degree of accuracy. Inelastic effects do not seem to be first-order effects in many situations. Perhaps you can prove that inelastic effects are first order?

You can also explore the possibility of guiding vibrations aways from structures using "cloaking" concepts. Weak shocks in saturated/partially staurated soils are not well understood aas are their effects on structures connected to those soils (but not necessarily supported). The number of open questions is limited only by our imagination.

-- Biswajit

\omega_n = \sqrt{1/fm}, f

tigerylx — Thu, 09 Feb 2012 04:44:59 -0500

\omega_n = \sqrt{1/fm}, f is flexural stiffness. For multi-freedom system, flexural matrix is the inverse of stiffness matrix. For single freedom system. K = 1/f;

potential energy

E = 0.5 u^2/f

u is displacement.

Hi Ravi, Thanks for the

sinornis — Wed, 08 Feb 2012 20:43:11 -0500

Hi Ravi,

Thanks for the response. Flexural stiffness of a body is inversely proportional to internal strain energy, though, isn't it? So when internal strain energy increases, natural frequency decreases.

What equations describing natural frequency can this relationship be seen in?

Roger

Thank you Professor.

Mubeen — Wed, 08 Feb 2012 10:18:11 -0500

Thank you Professor.

This example is very helpful.

Although FEAP is very famous and popular among people in academia due to the efforts of the developers of FEAP, however there is a dearth of tutorials/examples related to umat/user element routines.

Your web-page is the only source of information related to umat/uel development in FEAP.

Thanks for the help.

FEAP: umatiXX.f and umatlXX.f

Sanjay Govindjee — Wed, 08 Feb 2012 01:12:53 -0500

Have a look at http://www.ce.berkeley.edu/~sanjay/FEAP/feap.html .

There is an example implementation of a user model in version 8.x of the (finite deformation) microsphere model by Miehe, Goektepe, and Lulei.

Prof. Dr. Sanjay Govindjee
University of California, Berkeley

Hello Roger,

ravi_gutti — Wed, 08 Feb 2012 00:09:39 -0500

Hello Roger,

Natural frequency is directly proportional to stiffness of the body. This stiffness inturn is directly proportional to the internal strain energy of the body.

This means when the internal strain energy increases the natural frequency also increases and vice vers.

Anybody correct me if i am wrong...

ravi

Hi Mokarram, Thanks for

Jiangshui Huang — Tue, 07 Feb 2012 21:58:14 -0500

Hi Mokarram,

Thanks for your interest. I have sent the paper to your email address.

Sincerely,

Jiangshui

MPC

Lihua Jin — Mon, 06 Feb 2012 14:54:41 -0500

You may consider the following two ways:

1. You can use different mshing in different parts, and then use multi-point constraint to constrain the displacement in the connecting region of different meshes.

2. You can use bias meshing in the connecting region.