Hi Purnashis,

Have you checked the temperature of specimen in your experiments.

It could be that in higher strain rates the temperature of the specimen increase by adiabatic heating. So, the increase in the fracture stain might be caused by the temperature effect.

Hi Purnashis,

Have you checked the temperature of specimen in your experiments.

It could be that in higher strain rates the temperature of the specimen increase by adiabatic heating. So, the increase in the fracture stain might be caused by the temperature effect.

The strain rate for any materials gives increasing in fracture strain and fracture toughness as it make material more strong and deformation in microstructre of material make it difficult to fracture .

Can you present graphs?

Dear Purnashis,

you know that it is difficult to answer because you did not provide very significant details.

1) What is the alloy chemical composition;

2) What is the specimen stress state;

3) What is the specimen temperature history;

4) What do you mean by increase (How many MPa of stress or % of Strain

5) How many specimens did you test already;

6) What is the range of your strain rate .00001 up to 100 1/s;

7) By the way for aluminium microstructure of an alloy is one of the most important things;

8) What were the test conditions;

9) What was the specimens shape.

Without this info it is extremely difficult as you probably know aluminium alloys are very sensitive to all this parameters.

I am sure when you analyze all this info in application to your Alloy you will answer your question by yourself.

Regards,

Anton

the fracture toughness is related to mean area beneath stress-strain curve. then it can be said that toughness is directly related to strength as well as strain. based on dislocation theory, by increasing the strain rate, the strength of material will increase. then it can be acceptable the the fracture toughness increases with increasing the strain rate. also, Elongation increased monotonously with strain rate in pure aluminum [ Article Effect of strain rate on elongation and serration behavior i...

The are a number of possible explanations for this observation with the probability varying with the alloy and testing parameters as mentioned above. In general, an observation of this type is an indicator of either (1) an environmental effect or (2) the influence of an interstitial solute in the alloy. However, for Al alloys there are 2 additional possibilities because grain boundary flow and fracture can occur at very low strain rates. This is enhanced by precipitate free zones and/or grain boundary segregation of embrittling elements (e.g. Na) or low melting point elements (e.g. Pb) that may enter the alloy with recycled metal. Holroyd and Hardie (Corro. Sci., 1981, p129) examined alloy 7049-T651 and found that when testing in vacuum or dry air that the ductility reduced with increasing strain rate over the range 5e-7 to 1e-3 per second, but the reverse was true if tested in laboratory air or seawater.

Ali Torkabadi Sir,

We are Characterizing 7075 T6 aluminum alloy with Split Hopkinson Pressure Bar, the specimen size Dia=10 mm Thickness =5 mm. As the specimen is Not very thick and we are in the regime of 500-7000 1/s strain rate so temperature effect will not that much predominant. It will not cause Thermal Softening. We are also doing dynamic fracture toughness with the help of Modified Hopkinson Bar dimension (length= 100 mm, width =20 mm thickness =10 mm) .

Regards

Purnashis

Dear Purnashis,

you see after details it is now clear that it is an impact loading. It is completely other field of science rather than quasistatic loading that most of the people (and myself) are doing. And there are other physics underlying. May be there are some additional brittle-ductile transition at that conditions, which will give you that effect? In my opinion you will find the answer after investigation of the fracture surfaces in SEM.

Good luck,

Anton

As the strain rate increases it increases the flow stress or strain hardening of the material , hence the fracture toughness increases due to increased strain hardening.

Dear Purnashis,

Apparently you are using an SHPB setup for the tests. My research team has been doing SHPB tests on armour materials, including aluminum alloys, for quite a long while. I can assure you that in the range you mention (500/s to 7000/s) you will almost surely see an increase in thermal softening effect. If you use thin samples of 5mm thickness (which I do not recommend, as you will have important triaxiality and possibly inertia effects and hence the one-dimensional theory of Hopkinson and Kolsky for signal analysis is less valid), you will probably not reach full adiabatic conditions for a 500 /s strain rate (your bars will act as heat sinks). At 7000 /s you will almost definitely have adiabatic conditions. So thermal softening might be the explanation. This might be validated experimentally (e.g. by measuring outside sample temperature using IR system, or inside sample temperature using an embedded thermocouple), or doing a combined experimental/numerical approach using thermal properties from literature. It is hence essential to determine the evolution of the so-called Taylor-Quinney coefficient as a function of strain rate.

However, it is important to notice that fracture strain and fracture toughness based on compression testing for metals is 'notional' or 'indicative' at best, as the material does not fail due to intrinsic compressive properties (theoretically a metal cannot fail under purely one-dimensional compression loading), but due to the development of parasitic shear stresses (leading to the typical cross-diagonal failure plane). The latter are heavily dependent on sample geometry, friction conditions and load ramp up (speed at which stress equilibrium is reached in the sample). Fracture toughness and fracture strain based on experimental data from SHPB only are hence generally not useful.

Finally, for generic, brittle materials, it can be expected that the fracture toughness increases with increasing strain rate, due to the associated strength increase and often no considerable decrease in failure strain. For ductile metals, unless there is sufficient thermal softening, fracture strain and hence also fracture toughness normally decrease sufficiently with increasing strain rate to overcome the thermal softening effect (which anyway saturates once you reach adiabatic conditions).

A very nice, concise overview of all of the above can be found in the book of Marc Meyers, "Dynamic behavior of materials".

Please consider the temperature of specimen in your experiments. The temperature is an important factor.

In thermodinamic terms the material entropy will be the response of the work capacity of these type of material. So I tnhink that it will depends on the potential energy, the intrinsic bonds, the porosity and the shape if you are working under controled and constant temperature

It seems to me that this statement mistakenly generally

Dear all,

there a good explanation to this fact via the understanding of the Deborah Number which is the ratio of the stressing rate (or tome) over an intrinsic response time (specific for each material). The response of a material will be different depending on wether the experiment time is higher or lower than the intrinsic response time. Regards

Generally, Strain hardening will increase the fracture toughness of the material thereby increasing the fracture strain. I agree with the obserations of Mr. Maysem Aghakinar-- A.V.Sethuraman