TORSIONAL FATIGUE OF WOOD
Zheng Chen
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.
RESEARCH METHOD1 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.
Rig used for torsional fatigue test
The rig used for thermal imaging test
Comparison of S-N curves for hardwood and
softwood
Comparison of twist angle and cycles to failure for hardwood and softwood
Shear stress vs twist angle for hardwood at different cycle numbers
Hardwood
00.5
11.5
22.5
2 50 310(Crackhappen)
889
Cycle number
Hysteresis loop areas for hardwood at different cycle numbers
Hardwood
00.5
11.5
22.5
2 50 310(Crackhappen)
889
Cycle number
Hysteresis loop areas for hardwood at different cycle numbers
Hardwood
0
0.5
1
1.5
2
2.5
2 50 310(Crackhappen)
889
Cycle number
Slop
e of
the
Hys
tere
sis
loop
Hysteresis loop slopes for hardwood at different cycle numbers
Hardwood
0
0.5
1
1.5
2
2.5
2 50 310(Crackhappen)
889
Cycle number
Slop
e of
the
Hys
tere
sis
loop
Hysteresis loop slopes for hardwood at different cycle numbers
Shear stress vs twist angle for softwood at different cycle numbers
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
Softwood
0
0.2
0.4
0.6
0.8
1
1.2
2 52(cycle prior to fracture)Cycle number
Slop
e of
the
hyst
eres
is lo
opHysteresis loop slopes for softwood at
different cycle numbers
A comparison of hysteresis loops for hardwood and softwood at the beginning of cycling
Hysteresis loop area of twist angle and stress of hardwood and softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
AE cumulative events vs cycle comparing load vs cycle for both hardwood and
softwood
Schematic of crack propagation in hardwood
Crack growth from one grain to another (Hardwood)
Optical micrograph of a cross-section of hardwood (100)
Optical micrograph of a cross-section of hardwood (100)
Optical micrograph of a cross-section of hardwood (100)
Optical micrograph of a cross-section of hardwood (100)
Optical micrograph of a cross-section of hardwood (200)
Optical micrograph of a cross-section of hardwood (100)
Crack in hardwood along grain normal to the tensile stress component
Crack in hardwood along grain normal to the compressive stress component
Fracture in Softwood
Optical micrograph of a cross-section of softwood (100)
Optical micrograph of a cross-section of softwood (100)
Optical micrograph of a cross-section of softwood (100)
Optical micrograph of a cross-section of softwood (100)
Optical micrograph of a cross-section of softwood (100)
Optical micrograph of a cross-section of softwood (100)
Optical micrograph of a cross-section of softwood (200)
Optical micrograph of a cross-section of softwood (100)
Schematic of crack propagation in a test piece containing a knot
Safe region for torsional loading of hardwood
Thermal imaging result of a hardwood
Thermal imaging result of a hardwood
Thermal imaging result of a hardwood
Thermal imaging result of a hardwood
Monitoring fracture of softwood using high speed camera
Monitoring fracture of softwood using high speed camera
Monitoring fracture of softwood using high speed camera
FEA model for cracking direction of softwood
FEA model for cracking direction of softwood
CONCLUSION1 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.