50

mm

190 mm

G1 lens measurement area

27mm lens measurement areaP P

Infra-red Technique for Damage Tolerant Sandwich Structures

W.Wang1 –ww2g09@soton.ac.uk-, J.M.Dulieu-Barton1, R.K.Fruehmann1 and C.Berggreen2

1 Faculty of Engineering and the Environment, University of Southampton, UK.2 Department of Mechanical Engineering, Technical University of Denmark, Denmark.

Fluid Structure Interactions Research Group

FSI Away Day 2012

Background•Foam cored composite sandwich structures have been commonly adopted in ship structures and wind turbine blades for their high bending stiffness and strength to weight ratios.•An important failure mode of this structure is the face sheet/core debonding. It can significantly degrade the structural performance and the debonded region may grow further under compression. •The picture on the right shows the typical debonding failure caused by impact damage.Aims•To develop the infra-red technique which can be used as a quantitative, full-field measurement technique to investigate the fracture characterizations under different mode mixities.•To use optical fibre sensor embedded between face sheet and core to accurately capture crack initiation strains.•To develop novel crack arresting device and examine the improved damage tolerance using the developed technique.Thermoelastic Stress Analysis (TSA)•TSA is based on the thermoelastic effect to measure the small temperature change on the surface of a material under dynamic load. •The temperature change (ΔT) can be directly related to the stress change by the following relationships:

01 2( )

p

aTT

C

(1)

T1

T3

T2

•Large sample motion during dynamic loading can degrade accuracy of TSA measurement and can be illustrated below:

Expected result: ΔT = - T1 T2 T1 T3

Background temperature

Specimen temperature before motion

Specimen temperature after motion

TSA result: ΔT = -

Development of Motion Compensation (MC) Method for TSA

1. Motion compensation approach

DIC camera

IR detector

2. Experimental validation

• 27 mm lens (pixel resolution: 0.24mm)

TSA and FEA of Double Cantilever Beam (DCB) Sandwich Structures

Debonded region

Hinge

25 mm

175 mm

Upper face sheet

25

mm

32 mm

30 mm40 mm

TSA (27 mm lens) measurement area

TSA (G1 lens) measurement area

P

P

•Large motion caused by face sheet detachment from core can totally ruin the TSA result, especially for face sheet. •The developed motion compensation method is expected to be used for thermoelastic stress measurement.

X

Y

Conclusions•A DIC based motion compensation method for TSA was developed. The validation test shows that the accuracy of TSA measurement can be improved by this method if large motion exists. •A full-field stress state around the crack tip was established by TSA technique using both 27mm lens and G1 lens. The developed motion compensation method was successfully applied. •The stress distribution around the crack tip obtained by TSA shows a good agreement with the corresponding FEA.

Pixel 1

Pixel 1 temperature change value:

Step 1: Temperature change is measured by infra-red detector under dynamic load.

Sample motion is measured using digital image correlation (DIC) under the same dynamic loading condition as thermoelastic stress measurement.

Linear interpolation the full-field motion results from DIC grid to IR image grid. Rebuild the thermoelastic stress image according to its corresponding motion.

Step 2:

Step 3:

1. Test specimen

1. DCB sandwich specimen dimension and test arrangement

•The above figures indicate that the ΔT/T result after MC are less noise and more accurate.•The stress distribution measured by TSA showsa stress gradient in the upper face sheet from compression to tension. •ΔT/T result obtained by high resolution (G1) lensshows stress distribution in the foam core materialis mainly depends on its geometry. •The stress distribution predicted by FEA gives thesimilar result compared with TSA

Line plot result along x-direction (1mm under

interfacial crack)

•An aluminium specimen is designed to examine the proposed motion compensation method. Theresults indicate that this method can reduce thenoise and improve the accuracy of TSA.

P Composite face sheet

P Crack tip area

Element type: Plane 42

Minimum element size: 0.1mm

2. Stress distribution results

A B

• G1 lens (pixel resolution: 0.03mm)

Comparison of test results with theoretical value along line AB

A B

2. FE model of DCB sandwich specimen

with MC

without MC with MC

without MC

without MC with MC

3. ΔT/T results measured by TSA

with MC

• 27 mm lens

• G1 lens

4.ΔT/T predicted by FEA and comparison with TSA

5. Summary

without MC