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how can i measure local stress and strain in fatigue crack tip?

It seems that the measurment of local stress and strain in fatigue crack tip will be difficult. some researcher use F.E analysis method,in contrary using of numerical solution. however analytical method also has investigated. what method you can imagine? i need a brain storm in this issue.


Thank you



Wei Hong's picture

I guess you can use optical/image correlation methods to measure the strain field.  However, there is no way to measure a stress field directly.  The methods you mentioned are not measurements.  Are you thinking of measuring or calculating strain/stress?

Kejie Zhao's picture

As I know one way to measure the displacement and strain field at crack tip/dislocation is to use TEM and geometric phase mapping technique. Stress distribution around the crack tip is long-standing problem, especially in the plastic zone. As Prof.Hong mentioned, there may not be any effective way to "measure" it. Alternatively you may be able to use some numerical simulations, molecular dynamics for instance, to study the stress distribution.

Xiaodong Li's picture

Thanks for posting this. Yes, experimentally it is very difficult to measure/map local, nanoscale strain field around a crack tip. One method may help - combined atomic force microscopy and digital image correlation techniques. You may start with the following paper to see how to map the strain at the nanoscale and where the crack forms? Hope this helps. Thank you.

Xiaodong Li, Weijie Xu, Michael A. Sutton, and Michael Mello,
"Nanoscale Deformation and Cracking Studies of Advanced Metal
Evaporated Magnetic Tapes Using Atomic Force Microscopy and Digital
Image Correlation Techniques," Materials Science and
Technology, 22 (2006) 835-844. 

ennio curto's picture


17 through 19 November, 2009

  Introduction Internal stresses are to be considered as the following: 1) Operational strains referring to loads that the material is subject and calculated  2) Residual stresses in the material caused by heat treatments or stresses caused by welding, forging, casting, etc. The new technique is able to measure the applied load and residual stress that are balanced on the surface of the material, and in a relatively large volume, at times even the same size as the entire structures. This stress is part of the metal’s elasticity field and has a three axis spatial orientation.  Description  Elastic oscillations (also called vibrations) of an elastic material consisting of elementary masses alternately moving around their respective balance positions; these movements cause a transformation of the potential energy into kinetic energy. This phenomenon takes place due to reactions (elastic forces) that the aforementioned masses produce in opposition to elastic movements; these reactions are proportional according to Hooke’s Law to the same movements. The elastic waves that are produced propagate according to a fixed speed that depends on how rapidly the elemental masses begin to oscillate.
Elastic waves of this type are called “permanently progressive”, and they propagate at a constant speed which is absolutely independent of the speed with which the elemental masses move during the oscillating motion, and therefore also their respective oscillations.
    Impact with the metallic surface results an elastic deformation energy.  Ed  = Ei – ( Ek +  Ep )
 Ei = Impact energy     Ek = Kinetic energy      
 Ed = elastic deformation energy          Ep = plastic deformation energy + lost energy
 Ed = ½ K dx² =   ½ m ω² dx²       K = constant elastic material (stiffness) 
   Behavior elastic metals, due to new discovery                           Fig. 1                                     Fig.2 The system works through the accelerometer mounted with a magnetic base to generate the acceleration value of the vibrations created by the device impacting on the metal surface. The acceleration value, in combination with other parameters, permits obtaining the exact value of the residual stress or load applied in the desired point. This value will appear on the display directly in N / mm ². For non-magnetic metals, wax or gel will be used to mount the accelerometer.The system doesn’t recognize the compressive from tensile stress.                                                                                           Fig .3  Conclusion  Application of this type of non-destructive method NDT provides the possibility to measure residual stress and the effect of the service load in a very rapid and simple way on any point of the metallic surface. The testing method requires smooth surfaces free of oxides, paint, lubricants and oil. Precision depends on the roughness of the surface. This technology has demonstrated its validity over years of mechanical experimentation and has confirmed its theoretical basis. The new system provides a full-field, large area inspection, in real time to point-by-point inspection too rapid and easy     About residual stresses
The residual stress in a metal doesn’t depend on its hardness, but from the elasticity module  or Young module and from its chemical composition.
The hardness of a metal indicates its ability to absorb elastic or plastic energy, but through it not possible to determine the value of residual stress. In a metal with the same hardness we will have different values of this stress. .
The residual stresses tend to equilibrate themselves in the surface of the material.
The measurement made with all the major methods, X-ray, string gauge (destructive), optical etc. the residual stress is determined between the measuring the displacement of the equilibrium point the reticule crystalline.
The method discovered analyzes the value of frequency and vibratory acceleration generated by an impulse with the subsequent reaction elastic (elastic field) from the metal.
  You will realize the convenience of this technique. 1) Portable system easy to use and very swift.
 2) NDT non-destructive test.
 3) Repeatable in unlimited number of points. 4) All metals type (a-magnetic) 5) Don’t expensive. Effective for welding, hardened treatments, vessels control, bridges, pipes line, aeronautics, NDT inspection for every metal types.  

p.i. Ennio Curto.

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