1. TORSIONAL FATIGUE
OF WOOD
Zheng Chen

2. AIM OF THIS RESEARCH
• To investigate properties of hardwood (red lauan) and
softwood (Sitka spruce and scots pine) under static and
fatigue torsional load.
• To develop a new suitable technique for research on
torsional fatigue of wood.
– To design rigs suitable for torsional fatigue test of wood.
– To develop new measurement methods for torsional test.

3. RESEARCH METHOD
1 Test method:
• Static loading.
• Cyclic loading under displacement control.
2 Measurement methods for torsional test:
• Load and number of cycles.
• Acoustic emission techniques.
• Thermal imaging.
• Visual observation and a Phantom high speed camera observation
of cracks.
• Optical and scanning electron microscopy.
• Hysteresis loops.
• S-N curve for softwood and hardwood under cyclic torsional load.
3 To correlate the failure mechanism with the structure of
the wood.

4. Rig used for torsional fatigue test

5. The rig used for thermal imaging
test

6. Comparison of S-N curves for hardwood
and
softwood

7. Comparison of twist angle and cycles to
failure for hardwood and softwood

8. Shear stress vs twist angle for hardwood at
different cycle numbers

9. Hardwood
0
0.5
1
1.5
2
2.5
2 50 310(Crack
happen)
889
Cycle number
Hysteresis loop areas for hardwood at
different cycle numbers

10. Hardwood
0
0.5
1
1.5
2
2.5
2 50 310(Crack
happen)
889
Cycle number
Hysteresis loop areas for hardwood at
different cycle numbers

11. Hardwood
0
0.5
1
1.5
2
2.5
2 50 310(Crack
happen)
889
Cycle number
Slopeofthe
Hysteresisloop
Hysteresis loop slopes for hardwood at
different cycle numbers

12. Hardwood
0
0.5
1
1.5
2
2.5
2 50 310(Crack
happen)
889
Cycle number
Slopeofthe
Hysteresisloop
Hysteresis loop slopes for hardwood at
different cycle numbers

13. Shear stress vs twist angle for softwood
at different cycle numbers

14. Softwood
0
1
2
3
4
5
2 52(prior to broken)
Cycle number
Hysteresis loop area of stress-twist angle
of softwood at different cycle number

15. Softwood
0
0.2
0.4
0.6
0.8
1
1.2
2 52(cycle prior to fracture)
Cycle number
Slopeofthe
hysteresisloop
Hysteresis loop slopes for softwood at
different cycle numbers

16. A comparison of hysteresis loops for hardwood
and softwood at the beginning of cycling

17. Hysteresis loop area of twist angle and
stress of hardwood and softwood

18. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

19. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

20. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

21. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

22. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

23. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

24. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

25. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

26. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

27. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

28. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

29. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

30. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

31. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

32. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

33. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

34. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

35. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

36. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

37. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

38. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

39. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

40. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

41. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

42. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

43. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

44. AE cumulative events vs cycle comparing
load vs cycle for both hardwood and
softwood

45. Schematic of crack propagation
in hardwood

46. Crack growth from one grain to
another (Hardwood)

47. Optical micrograph of a cross-
section of hardwood (100×)

48. Optical micrograph of a cross-
section of hardwood (100×)

49. Optical micrograph of a cross-
section of hardwood (100×)

50. Optical micrograph of a cross-
section of hardwood (100×)

51. Optical micrograph of a cross-
section of hardwood (200×)

52. Optical micrograph of a cross-
section of hardwood (100×)

53. Crack in hardwood along grain normal to the
tensile stress component

54. Crack in hardwood along grain normal to the
compressive stress component

55. Fracture in Softwood

56. Optical micrograph of a cross-
section of softwood (100×)

57. Optical micrograph of a cross-
section of softwood (100×)

58. Optical micrograph of a cross-
section of softwood (100×)

59. Optical micrograph of a cross-
section of softwood (100×)

60. Optical micrograph of a cross-
section of softwood (100×)

61. Optical micrograph of a cross-
section of softwood (100×)

62. Optical micrograph of a cross-
section of softwood (200×)

63. Optical micrograph of a cross-
section of softwood (100×)

64. Schematic of crack propagation in a test
piece containing a knot

65. Safe region for torsional loading of
hardwood

66. Thermal imaging result of a
hardwood

67. Thermal imaging result of a
hardwood

68. Thermal imaging result of a
hardwood

69. Thermal imaging result of a
hardwood

70. Monitoring fracture of softwood
using high speed camera

71. Monitoring fracture of softwood
using high speed camera

72. Monitoring fracture of softwood
using high speed camera

73. FEA model for cracking direction of
softwood

74. FEA model for cracking direction of
softwood

75. CONCLUSION
1 The S-N curve results show that with increasing cycle number to failure, the
torsional strength of hardwood reduces faster than that of softwood.
2 The failure modes for both hardwood and softwood under torsional loading
can be determined.
3 The results from acoustic emission measurement show that there is an
increase of AE total counts before the torsional loading begins to drop, which
indicating some microcracking before final failure.
4 The results of thermal imaging show that there is an increase in temperature at
crack initiation site prior to or during failure.
5 Visual and microscopic observation show that the cyclic damage caused by
torsional loading in hardwood is gradual whereas in softwood failure occurs
by sudden crack propagation. The crack growth is along the tangential
direction in hardwood and the radial direction in softwood.
6 FEA model show that the location of stress concentration around a pre-
existing crack determines the direction of the crack development.