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.