Apparatus and method for carrying out in a controlled atmosphere material fatigue tests in a high cycle regime with a controlled strain ratio

1. An apparatus for carrying out, in a controlled atmosphere, material fatigue tests of a specimen in a high cycle regime with a controlled strain ratio, comprising:

a pressure vessel having an outer wall defining an internal chamber,

a load train composed of a first horn and a second horn between which the specimen is to be arranged, wherein said load train has a longitudinal axis and is essentially arranged within said internal chamber of said pressure vessel, and

a converter adapted to apply ultrasonic waves into the load train by exciting the first horn in order to apply a dynamic stress to the specimen in a direction parallel to said longitudinal axis,

wherein internal chamber is divided into a first chamber defining a free volume around at least a portion of said load train including said specimen and a second chamber which is axially aligned with said first chamber along said longitudinal axis, said second horn comprising a base part which is partially and movably engaged into said second chamber to define in said second chamber a second free volume separated from the first free volume of said first chamber, a first gasket being interposed between said base part and a portion of the outer wall of said pressure vessel which is substantially parallel to said longitudinal axis to sealingly separate said first chamber from said second chamber, and first feeding means and second feeding means being provided for feeding said first and second free volumes of said first and second chambers with gases respectively having different gas pressures in order to apply a predetermined static stress to the specimen.

2. The apparatus according to claim 1 , wherein said first horn comprises a base part having a cylindrical portion protruding out of the pressure vessel to be attached to the converter and a shoulder portion located opposite a portion of the outer wall of the pressure vessel which is substantially perpendicular to said longitudinal axis, a second gasket being provided at the interface between said shoulder portion and said portion of the outer wall to sealingly separate said base part of said first horn from said outer wall of said pressure vessel whilst enabling longitudinal oscillations of said first horn.

3. The apparatus according to claim 2 , wherein the first horn, the second horn, and the specimen having a center are dimensioned in resonance with the frequency of the ultrasonic waves coupled by the converter into the load train and in that said first gasket and said second gasket as well as the center of the specimen are located at nodes of displacement of said load train where the stress is at a maximum.

4. The apparatus according to claim 1 , wherein said first and second feeding means each comprise at least a source of gas having a predetermined composition, a control device to define a predetermined gas pressure, a gas conduit and at least one channel provided in said outer wall of said pressure vessel to be in communication with said first free volume of said first chamber or said second free volume of said second chamber respectively.

5. The apparatus according to claim 4 , wherein the first feeding means comprise a first source of gas having a predetermined composition, a control device to define a predetermined gas pressure for the gas taken from the first source of gas, a first gas conduit connected to said first source of gas and to an inlet channel outputting into said first free volume of said first chamber, and an outlet channel in communication with said first free volume of said first chamber and outputting into a second gas conduit connected to said first source of gas, said control device further defining a predetermined gas flow through the first free volume that is larger than a residual leakage flow between the first and second free volumes of the first and second chambers.

6. The apparatus according to claim 1 , wherein said longitudinal axis is substantially vertical, the first horn defining an upper horn and the second horn defining a lower horn.

7. The apparatus according to claim 1 , wherein the gas to be fed in the first chamber or the second chamber is a pure gas.

8. The apparatus according to claim 1 , wherein the gas to be fed in the first chamber or the second chamber is a mixture of different gases components.

9. The apparatus according to claim 1 , wherein the gas to be fed in the first chamber or the second chamber comprises at least hydrogen.

10. The apparatus according to claim 1 , wherein the first and second horns are designed as members having a generally tapering shape from their base part to their tip which is connected to the specimen.

11. The apparatus according to claim 1 , wherein the pressures in the first and second free volumes of said first and second chambers are lower than 1000 bar.

12. The apparatus according to claim 1 , wherein the pressures in the first and second free volumes of said first and second chambers are comprised between 200 bar and 500 bar.

13. A method for carrying out, in a controlled atmosphere, material fatigue tests of a specimen in a high cycle regime with a controlled strain ratio with an apparatus according to claim 1 , comprising:

locating a specimen within the first chamber of the apparatus by fixing the one end of the specimen to the tip of the first horn and fixing the opposite end of the specimen to the tip of the second horn,

feeding gas in the first and second chambers of the pressure vessel such that due to a pressure difference in both first and second chambers a desired static stress is applied to the specimen,

controlling the converter such that a dynamic stress is applied to the specimen by exciting ultrasonic waves coupled from the converter into the load train, and

maintaining a nearly constant preset pressure difference between the first and second chambers by controlling the gas flow to the first and second chambers via said first and second feeding means.

14. The method of claim 13 , wherein a pure gas is used to feed the first and second free volumes of the first and second chambers and the step of maintaining a nearly constant preset pressure difference between the first and second chambers comprises:

controlling an exhaust from the one of the first and second chambers having the lower pressure and increasing a gas flow into the other of the first and second chambers having a higher pressure such that a leakage flow between said first and second chambers is compensated.

15. The method of claim 13 , wherein a gas mixture is used to feed the first and second free volumes of the first and second chambers and the step of maintaining a nearly constant preset pressure difference between the first and second chambers comprises:

maintaining a predetermined gas flow through the one of the first and second chambers having the higher pressure, said predetermined gas flow being larger than a leakage flow between the first and the second chambers in order to control a gas composition in said one of the first and second chambers having the higher pressure.