Turbine airfoil of composite material and method of manufacturing thereof

1. A turbine blade, comprising:

a root;

a tip;

an airfoil extending therebetween with a core section made of composite material;

a protective sheath joined to the core section at least in an area of the airfoil exposed to erosion, the core section of the airfoil is continuous from a location in a vicinity of the root to a location in a vicinity of the tip of the blade and comprises a section which widens from the rotational axis along the length of the airfoil and which ends at the location in the vicinity of the tip of the blade,

wherein the protective sheath comprises separate parts joined to the core section, the separate parts comprising a leading edge cover, a trailing edge cover, a first covering sheet for a pressure side of the airfoil, and a second covering sheet for a suction side of the airfoil; and

an interference fit provided for the protective sheath by the widening section of the core section configured for preventing movement of the protective sheath along the core section in a radially outward direction.

2. The turbine blade of claim 1 , wherein the widening is continuous or step-wise.

3. The turbine blade of claim 1 , wherein the widening includes a lengthening of the airfoil's chord length towards the tip.

4. The turbine blade of claim 3 , wherein the airfoil has a section where the chord length decreases as a function of the radial distance followed by a section where the chord length increases as a function of the radial distance.

5. The turbine blade of claim 3 , wherein the chord length increases continuously as a function of the radial distance from the location in vicinity of the root to the location in vicinity of the tip of the blade.

6. The turbine blade of claim 1 , wherein the protective sheath is joined with the core section by a bonding or adhesive layer.

7. The turbine blade of claim 1 being a blade for a steam turbine.

8. The turbine blade of claim 7 being a last stage blade for the steam turbine.

9. The turbine blade of claim 1 , wherein the root is pyramid-shaped.

10. A method of manufacturing a blade for a steam turbine comprising:

providing a core section of composite material of an airfoil of the blade that continuously extends from a location in a vicinity of a root section of the blade to a location in a vicinity of a tip section of the blade;

surrounding the core section with a protective sheath of a different material than the composite material, wherein the protective sheath is assembled from separate parts joined to the core section, the separate parts comprising a leading edge cover, a trailing edge cover, a first covering sheet for a pressure side of the airfoil, and a second covering sheet for a suction side of the airfoil;

increasing the width of the core section at least at the vicinity of the tip end of the blade; and

providing an interference fit for the protective sheath by the widened core section configured for preventing movement of the protective sheath along the core section in a radially outward direction.

11. The method of claim 10 , wherein the protective sheath is bonded to the core section by an adhesive layer.

12. The method of claim 10 , further comprising joining the leading edge cover, the trailing edge cover, and the second covering sheet of the protective sheath at a first welding seam and a second welding seam, followed by placing the core section surrounded by an adhesive layer into a recess formed by the welded parts.

13. The method according to claim 12 , wherein the first covering sheet is placed on top of the core section which is placed into the recess to completely enclose the core section.

14. The method according to claim 13 , wherein the first covering sheet is joined at a third welding seam and a fourth welding seam which are located at edges of recesses formed on the respective leading edge cover and the trailing edge cover.