THROUGH-THICHNESS STRAIN FIELD MEASUREMENT IN A COMPOSITE/ALUMINIUM ADHESIVE JOINT

Transcription

1 Marie-Pierre MOUTRILLE Katell DERRIEN Didier BAPTISTE Laboratoire d Ingénierie des Matériaux, ENSAM, Paris, France Xavier BALANDRAUD Michel GREDIAC Laboratoire de Mécanique et Ingénierie, Blaise Pascal University, Clermont-Ferrand, France

2 Why composite patches? To repair damaged structures To improve mechanical properties and to prevent damages Experimental analysis of the load transfer mechanism between substrate and composite In situ determination of stress/displacement/strain field in the adhesive joint

3 Composite Patch Geometry

4 Determination of the stress/strain fields in the adhesive Before loading: measurement of residual stresses due to bonding process by X-Ray Diffraction (XRD) - XRD = suitable technique for the measurement of residual stresses and strains - Non destructive, uses coherent domain of a material like strain gage During loading: determination of the displacement/strain field by Digital Image Correlation (DIC) - DIC is an appropriate method to determine displacements on a surface - It is based on the estimation of a shift between two sub-images by using a correlation function.

5 Residual stress determination by XRD - Method Adhesive is a non-crystalline material: aluminium particles have been embedded in the joint during bonding process. Measurement of σ xx and σ yy in the aluminium particles along the joint (y axis) by using Phillips X Pert Goniometer with a CuKα radiation. Comparison with free stress state defined by statistical treatment of stress measurements in free aluminium powder.

6 Residual stress determination by XRD - results Free stress state definition: -3 MPa to 3 MPa Experimental results in aluminium particles embedded in the joint: Residual stresses due to bonding process are negligible

7 Strain determination by DIC - experimental conditions Images captured by: - CCD video camera - long-distance microscope Pictures are taken along the stress transfer length Images treatment : - displacement field determined by CORRELI LM - strain field determined by CORRELI LMT (numerical R. de Borst algorithm) Sample preparation : grey level distribution obtained by spraying black and white paint on the sample surface

8 Strain determination by DIC - experimental conditions Geometry and loading of the specimens: Experimented specimens: Sample number joint nominal joint thickness (mm) real joint thickness (mm) loading force (kn) Loading stress (Mpa) 1 0,5 0,24 10; 15; 20;25 125; 187,5 2 0,5 0,21 10; 15; 20;25 125; 187,5 1 0,4 0,26 10; 15; 20;25 125; 187,5 2 0,4 0,27 10; 15; 20;25 125; 187,5 1 0,5 0,53 125;187,5; 250; 10; 15; 20;25 312,5 2 0,5 Too small (<0,1) 10; 15; 20;25 125;187,5; 250; 312,5

9 Strain determination by DIC - performance of measurement Uncertainty: estimation of the commited error during strain calculation Depends on image quality (gray level repartition). Calculation is based on pictures that are numerically displaced. Shear strain uncertainty is smaller than Resolution in strain : minimal strain whitch can be determined Depends on the evolution of experimental conditions (particulary light conditions). Calculation is based on the capture of several identical pictures at different instants. Shear strain resolution: from to 1, Spatial resolution in strain : distance between two independant measurement Spatial resolution: nearly 80μm

10 Strain determination by DIC - results: displacement field calculated by Correli LMT Adhesive joint: shear strain area

11 Strain determination by DIC - results: shear strain cartography Picture of the adhesive joint / shear strain cartography for 250 MPa loading stress Shear strain cartography: comparison 2D-model/experimental results Y-axis - distance from free edge Y-axis - distance from free edge 2D-model Experimental

12 Strain determination by DIC - results: mean shear strain through the thickness of the adhesive Experimental mean shear strain vs. distance from the free edge at several loading : Mean shear strain in the adhesive for specimen 3

13 Strain determination by DIC - results: comparison experimental / theoretical results Experimental mean shear strain vs distance from the free edge at loading case 312 Mpa (25kN) Mean shear strain through the thickness of the adhesive, specimen 3

14 Strain determination by DIC - results: comparison experimental / theoretical results Experimental shear strain vs distance from the interface aluminium/adhesive at loading case 312 Mpa (25kN) shear strain in the thickness of the adhesive, specimen 3, different distance from free edge

15 Strain determination by DIC - conclusion and perspectives Results from DIC allow: - cartography of shear strain in the adhesive - comparison with theoretical models Perspectives - To test several times the same samples in order to increase reliability of results - To analyse in a more quantitative way experimental results -To implement a more realistic 2-D model which accounts adhesiveoverlap at the free edge - To identify in situ the adhesive mechanical properties