Soft Matter Group

Fatigue Behaviour of Carbon Cord /HNBR Composite

Yinping Tao

Supervisors: Prof James Busfield, Dr. Emiliano Bilotti

Innovations in Rubber Design: 7-8 December 2016

Soft Matter Group

Outline

• Reinforcement in future timing belt application

• Load distribution in composite

• Cord behaviour in composite

• Fatigue behaviour comparison between pure cord and cord in rubber• R effect

• Hypothesis 1: Hysteresis and temperature rise during fatigue

• Hypothesis 2: Strain-induced crystallization(SIC)

• Conclusion

2

Soft Matter Group

Timing Belt or Timing Chain?

In a 1.6 litre gasoline engine, for example, the belt drive reduces

fuel consumption by more than 1% and saves up to 1.5 grams

of CO2 per kilometre

Timing Belt in Oil Reduces Friction and Noise, IAA2013

Less friction

less weight

less fuel consumption fewer CO2 emissions

3

Soft Matter Group

SEM image of glass cord

Narrower, Lighter, more Durable Belts

• Installation space in vehicles is becoming even tighter

• Good timing effect• High modulus

• Low creep

• Long life

• Low cost

1��500��

SEM image of carbon cord 4

1��

Soft Matter Group

Sample Preparation and Method

Table 1:Materials used in this study

• Samples were subjected to load-controlled sinusoidal waveform.

• Tensile fatigue tests were performed at different frequencies and R ratios with constant maximum load on pure cord and composite.

min

max

LR

L=

Materials Comment

Carbon cord

supplied in the form of a 6000-filament tow with 80 twists/m; impregnated with HNBR latex and reactive chemicals

HNBR formulation is commercially confidential

5

Soft Matter Group

Outline

• Reinforcement in future timing belt application

• Load distribution in composite

• Cord behaviour in composite

• Fatigue behaviour comparison between pure cord and cord in rubber• R effect

• Hypothesis 1: Hysteresis and temperature rise during fatigue

• Hypothesis 2: Strain-induced crystallization(SIC)

• Conclusion

6

Soft Matter Group

Load-controlled Tension-tension Fatigue

Rubber will take the load immediately and will be highly strained

Cord will take the majority of load

Cord fracture①①①①

②②②②

①①①①

②②②②

7

Soft Matter Group

How Much Load does the Cord Take?

Assuming: equal strain in fibre and matrix during loading (Rule of Mixture).

r fε ε=

�

�

, ,

, ,

f f f f f f

r r r r r r

F A E A

F A E A

ε ε

ε ε

σ ε

σ ε

= =

,

,

5.45f f f

r r r

F E A

F E A

ε

ε

= =

composite loaded in longitudinal

tensile force

The composite fracture strain is determined by the ultimate strain of cord ,c uε ,f u

ε

When , matrix stress-strain behaviour can be viewed as linear, thus ,c f uε ε<

Carbon cord takes 84.5% of load;

HNBR matrix takes 15.5% of load.

,f rF F Load carried in cord and rubber respectively

,f rσ σ

,f rA A

Stress in cord and rubber at a strain before composite fracture

Cross-section area of cord and rubber in composite

,f rE E cord and rubber modulus

8

Soft Matter Group

Importance of Actual Stress in Cord

• Avoid the overestimation of the fatigue life.

• From now on, y-axis of all the S-N plots present here is the actual stress taken by cord in composite material according to load distribution calculation.

• By making these corrections, it enables us make direct comparison between pure cord and cord-in-rubber behaviour.

9

Soft Matter Group

Content

• Reinforcement in future timing belt application

• Load distribution in composite

• Cord behavior in composite

• Fatigue behavior comparison between pure cord and composite• R effect

• Strain-induced crystallization(SIC)

• Hysteresis and temperature rise during fatigue

• Conclusion

10

Soft Matter Group

The Fundamental Difference between the Behavior of Cord in Air and Cord in Matrix

Single breakage of cord in air Multiple breakage of cord in rubber

How many breaks will happen in a certain composite system ?

Stress along fibre ��

= �

��∗

Stress along fibre ��

< �

��∗

Stress along fibre ��

> �

��∗

2

f

c

dL

σ

τ

=: interfacial shear stress.

: fibre strength

fσ : cord strength.

τ

d : cord diameter

*

fσ

Critical fibre length

11Hull, D.; Clyne, T. An introduction to composite materials; Cambridge university press, 1996.

Soft Matter Group

t sF F=

(2 ) (2 )t

F rL r Lπ τ π τ= =

6.04MPaτ =

Measurement- Static Cord Pullout TestcL

982

f

c

dL mm

σ

τ

= =

12

Soft Matter Group

Dynamic Adhesion Test

1mm

Cord morphology after 90 mins at 700 °C

under N2 in tube furnace

13

Soft Matter Group

SEM-EDX : debonding

200��200��

Cl mapping resultSEM image of as-received CF cord

CF CordFrom EDX mapping,

Cl is a characteristic element

detected in adhesive layerHNBR rubber matrix

Rubber-adhesive interface

CF cord-adhesive interface

Adhesive layer

300��

SEM image of ‘untidy’ CF cord Cl mapping result

300��

2�� 2��

SEM image of pullout fracture surface Cl mapping result

Most debonding happens at the adhesive-cord interface

during a static cord pull out testing.14

Soft Matter Group

Outline

• Reinforcement in future timing belt application

• Load distribution in composite

• Cord behaviour in composite

• Fatigue behaviour comparison between pure cord and cord in rubber• R effect

• Hypothesis 1:Hysteresis and temperature rise during fatigue

• Hypothesis 2:Strain-induced crystallization(SIC)

• Conclusion

15

Soft Matter Group

Pure Cord S-N Plot Cord in Rubber S-N Plot

16

Soft Matter Group

Effect of Stress Ratio on Fatigue Life

17

Soft Matter Group

Hysteresis and Heating

18

Soft Matter Group

Examination of SIC on HNBR Latex

• No SIC in HNBR latex

HNBR Latex XRD

• No crystallinity revealed in DSC

19

HNBR latex

Soft Matter Group

Conclusion

• Lc is determined by cord-pullout test.

• S-N curves are obtained for both cord in air and cord in composite.

• Similar R ratio effects take place during fatigue, which might because of hysteresis heating.

• No measurable SIC in HNBR Latex has been observed which could potentially prevent crack propagation in the carbon fibre bundle.

20

Soft Matter Group

• Thank you.

21