Superelastic racket string

Abstract

The present invention relates to a ball game racket with strings that comprise at least one string comprising a superelastic material.

Claims

1. A ball game racket with strings that comprise at least one string comprising a superelastic material, wherein: the superelastic material includes a first phase transition, at which austenite is transformed into martensite, when a tensile stress applied to the superelastic material increases; the superelastic material includes a second phase transition, at which martensite is transformed into austenite, when the tensile stress is reduced; and a tensile stiffness of the superelastic material is based on a prestress applied to the strings that is higher than the tensile stress at which the first phase transition occurs.

2. The ball game racket according to claim 1, wherein the at least one string has a diameter of no greater than 1.1 mm.

3. The ball game racket according to claim 1, wherein the first phase transition of the superelastic material occurs at room temperature at a tensile stress between 250 MPa and 900 MPa.

4. The ball game racket according to claim 1, wherein the second phase transition of the superelastic material occurs at room temperature at a tensile stress between 50 MPa and 700 MPa.

5. The ball game racket according to claim 1, wherein the difference between the tensile stress at which the first phase transition occurs and the tensile stress at which the second phase transition occurs at room temperature is smaller than 350 MPa.

6. The ball game racket according to claim 1, wherein the strings comprise at least one further string made from a non-superelastic material or wherein the entire set of strings consists of strings comprising a superelastic material.

7. The ball game racket according to claim 1, wherein the superelastic material comprises one or a combination of the following alloys: NiTi, NiTiCr, NiTiFe, NiTiCo, NiTiCu, NiTiV, CuZnAl, CuAlNi, FeNiAl, FeMnSi.

8. The ball game racket according to claim 1, further including at least one string made from a non-superelastic material.

9. The ball game racket according to claim 1, wherein the at least one string includes a jacket of the superelastic material surrounding a hollow core.

10. The ball game racket according to claim 1, wherein only a part of a cross-section of the at least one string includes the superelastic material.

11. The ball game racket according to claim 1, wherein, along a longitudinal direction of the at least one string, the at least one string includes portions of the superelastic material interrupted by portions of a non-superelastic material.

12. A ball game racket with strings that comprise at least one string comprising a superelastic material, wherein: the superelastic material includes a first phase transition, at which austenite is transformed into martensite, when a tensile stress applied to the superelastic material increases; the superelastic material includes a second phase transition, at which martensite is transformed into austenite, when the tensile stress is reduced; and a tensile stiffness of the superelastic material is based on a prestress applied to the superelastic material that is lower than the tensile stress at which the first phase transition occurs.

13. The ball game racket according to claim 12, wherein the difference between the tensile stress at which the first phase transition occurs and the prestress at room temperature is greater than 100 MPa.

14. The ball game racket according to claim 12, wherein the at least one string has a diameter of no greater than 1.1 mm.

15. The ball game racket according to claim 12, wherein the first phase transition of the superelastic material occurs at room temperature at a tensile stress between 250 MPa and 900 MPa.

16. The ball game racket according to claim 12, wherein the second phase transition of the superelastic material occurs at room temperature at a tensile stress between 50 MPa and 700 MPa.

17. The ball game racket according to claim 12, wherein the difference between the tensile stress at which the first phase transition occurs and the tensile stress at which the second phase transition occurs at room temperature is smaller than 350 MPa.

18. A ball game racket with strings that comprise at least one string comprising a superelastic material, wherein: the superelastic material includes a first phase transition, at which austenite is transformed into martensite, when a tensile stress applied to the superelastic material increases; the superelastic material includes a second phase transition, at which martensite is transformed into austenite, when the tensile stress is reduced; and a tensile stiffness of the superelastic material is based on a prestress applied to the superelastic material that is lower than the tensile stress at which the second phase transition occurs.

19. The ball game racket according to claim 18, wherein the difference between the tensile stress at which the first phase transition occurs and the prestress at room temperature is smaller than 400 MPa.

Description

(1) The invention is described in the following with reference to the Figures in more detail, in which:

(2) FIG. 1 shows a stress-strain diagram for several nitinol strings having different diameters; and

(3) FIG. 2 schematically shows the phase diagram of nitinol.

(4) In FIG. 1, the stress-strain diagram for four nitinol strings having different diameters is illustrated. The nitinol is superelastic nitinol S/BB having a transition temperature (austenite start temperature) As of 15 C. The measurement of the stress-strain diagram was made at room temperature. As can be very well seen, there is a stiff austenitic range at stresses of about 100 to 500 MPa, at which the nitinol string behaves linearly. At a stress of about 600 to 650 MPa, a phase transition occurs at which austenite transforms into martensite. During this phase transition, the strain of just under 2% increases to over 7% without the stress having to be noticeably increased. As regards higher strains or stresses of about 700 to 900 MPa, there is a second linear range. This is soft martensite.

(5) In the two marked ranges, the nitinol string can be used like a conventional string, wherein the tensile stiffness either essentially corresponds to that of a natural gut string (soft martensite) or is considerably higher (stiff austenite). Such a nitinol string behaves like conventional ball game racket strings in so far as in each of the marked ranges the stress increases in proportion to the strain.

(6) Alternatively, however, the nitinol string can also be used in the range of the phase transition, as will be explained in the following by means of a schematic illustration of the hysteresis in FIG. 2. As apparent from FIG. 2, starting from a strain of 0%, the stress initially linearly increases with increasing strain. When the start of the first phase transition, at which austenite transforms into martensite, is reached at a stress .sub.1, the stress essentially remains constant (.sub.1) with increasing strain. When the strain is further increased after the complete transformation into martensite, a second proportional range in the stress-strain diagram is reached until eventually the yield strength YS and the ultimate tensile strength UTS are reached. However, when the strain is reduced again after the complete transformation into martensite, at first the martensite is not directly retransformed into austenite but firstly part of the stress is reduced within the martensite until again the stress .sub.2 is reached. Only at this stress .sub.2 does the second phase transition occur, at which martensite is transformed into austenite, until the starting point of the hysteresis in the phase diagram is reached again.

(7) When, according to a preferred embodiment, prestress just below the stress at which the second phase transition occurs is applied to the strings of the ball game racket according to the present invention, and when the hysteresis curve is so narrow that the stresses typically occurring within the string during playing the ball game racket are higher than the tensile stress at which the first phase transition occurs, the hysteresis curve schematically illustrated in FIG. 2 can be completely run through during playing the ball game racket. When the ball hits the string bed of the ball game racket with sufficient force, the austenite is transformed into martensite and extreme deformations of the strings can occur. While the ball leaves the string bed or thereafter, the martensite is retransformed into austenite so that the next time the ball hits the spring bed the complete transformation can be run through again.

(8) Even though the above statements have been made with the example of a nitinol string, they analogously apply, of course, also to other superelastic materials, wherein the concrete strains and stresses at which the phase transitions occur can, of course, deviate from the values illustrated here.

(9) The strings according to the present invention can be used as longitudinal and/or transverse strings. The racket can be stringed exclusively with superelastic strings or it can be stringed in combination with conventional strings made from nylon, polyester or natural gut.

(10) Strings made from superelastic materials such as, for example, nitinol can be produced, for example, via wire drawing (in the soft-annealed state). Such strings can be basically produced by means of respective dies so as to be round, angular or of any other shape. Nitinol can be coated with various plastics, such as, for example, PTFE.