Metal Fatigue is a phenomenon that imposing small loads on metal materials repeatedly for long term makes metal fatigued and fractured.
Mechanical structures supporting our life consist of a variety of metal products.
Then, will such metal products be NEVER fractured?
Yes, they WILL be fractured anyway.
When and how much does metal fracture happen?
There are 2 main factors to cause metal products to be fractured.
Please see a chart bilow.
1)Corrosion This includes rust as you know.
2)Strength deficiency Fatigue Test is included.
It is said that 80% in causes of fracture accidents happening to mechanical structures is because of metal fatigue.
Serious accidents caused by metal fatigue have happened to familiar things such as a plane,train,bridge and roller coaster in the past.
Here are some examples shown below.
- year 1954
- The world’s first English plane COMET was broken up in midair because of fatigue destruction at the corner of the plane window.
- year 1994
- Seongsu Bridge in Korea collapsed suddenly, which was because fatigue fracture proceeded rapidly due to welding deficiency and traffic increase.
- year 1998
- the German high speed train ICE was derailed due to fatigue fracture of vibration-proof tire.
- year 2007
- A roller coaster of an amusement park in Osaka Japan was derailed due to fatigue fracture of an axle.
An research of metal fatigue has being developed so that such kind of accidents shound never happen.
Then, what do you think is needed to prevent those accidents above?
It is reliable and safety design.
And, what is needed to have such design is FATIGUE STRENGTH !!
And more , it is the best way to use a Fatigue Testing Machine to find out fatigue strength.
Isn’t it high time to see once again if Fatigue Strength data which is a basis of safety design is certain enough or not?
In materials science field, “Giga-cycle fatigue” is a trend nowdays. Why?
Please see a chart below.
Thus, once we aim to the earth-friendly society, it naturally lets us try to use everything which consists of metal products importantly for a lomg time.
However, there is an question”Is it really possible to use them safely?”.
There are still few results in the materials science field regarding to this question.
It is because it takes enormous time.
Actually, it needs extremely long time to perform a fatigue test in the giga-cycle regime(more than 107times).
Nevertheless, we will never achieve the earth-friendly safety society unless the fatigue test in the giga-cycle regime.
Thereupon, a multi-type fatigue testing machine has been developed.
Our Dual Spindle Rotating Benfing fatigue testing machine “GIGA QUAD” made it possible to perform tests for 4specimens simultaneously, which means very speedy!!
Fatigue tests are performed as follows below.
S of S-N curve stands for “Stress”. That means repeatitive load.
N stands for “Number of cycles to failure”.
S describes a vertical axis and N describes a cross axis. Those make a graph below.
The graph mentioned above is an example of S-N curve.
The lins is downward-sloping. We can see from this graph that the more the number of cycles becomes, The smaller stress even causes fatigue fracture.
However, we can find one line that it won’t fracture by getting a load on unlimitedly as long as the load is one constant repeatitive stress. It is called Fatigue Limit.
These are some new theories as follows these days and the research of them has been proceeded.
“Is the fatigue limit which appeared until 107 times real one? “
“Fatigue fracture might actually happen if the test is continued to Giga-cycle regime,that is , further than conventuional regime. Several cases have been found to fracture.
Do you want to create a S-N curve for a given material?
Different from the common metal specimen testing machine, our GIGA QUAD® can do fatigue tests for 4 specimens simultaneously under various environmental conditions (such as Corrosion environment, High temperature environment, etc.) with high efficiency.
For details, you can see how our GIGA QUAD® operates here:
We need S-N curve to make sure of safety and reliability of mechanical structures, and to compare coventional materials with new materials.
■Compare coventional materials with new developing materials
In case you develop the new materials to improve or substitute conventinal materials, S-N curve is utilized.
When you make S-N curve below, you can recognize that developed material is more difficult to break and stronger than conventional one.
Therefore, you can adopt new materials with security.
■Utilize product designs
In case the fatigue limit of S-N curve is not clear, we have to bind the product to excessive safty factor.
For that, the product will get heavy and the cost is higher than expected.
In other words, if you know the clear S-N curve, you will reduce the wight and cut down the price.
■Utilize quality inspection
In case you are served with claims for materials, you utilize S-N curve to quality inspection.
First, you make specimens of the claimed material, and then make S-N curve.
When you compare conventinal material with claimed material, you will make sure whether the material has promlem, or not.
In case the material has problem, you have the chance to renew production technique.
■Evaluate surface modification
GIGA QUAD is suitable for any kind of research and studies for metal fatigue characteristics with surface modification.
In the case of rotating bending, the loading on the surface is maximum, but the loading of the center is zero.Actually, GIGA QUAD is rotating bending, this is why you can perceive that the distance of fatigue resistance between after surface treatment and before that is clear, not for sure axial loading.
When you usually perform rotating bending fatigue test, the specimen is fractured due to the crack occurred on the material surface.
But, in the giga-cycle regime more than 108 times, the specimen sometimes fractures because “Fish-eye” which is generated inside of material (refer to a figure below) causes progress of the crack.
Though there are still many unexplained facts regarding how and when “Fish-eye” is generated, “Fish-eye” seems to be a factor which declines fatigue limit further that is estimated by experiment result to 107 times.