This project is focused on the establishment of a novel data collection methodology that involves high resolution, full-field optical techniques. The aim is to inform a model of the high strain rate behaviour of composite materials. The benefits are:

• Identify local phenomena such as strain gradients or discontinuities.

• Take strain heterogeneities into account.

• The use of a mixed experimental/numerical strategies.

Full-field Optical Techniques for High Strain Rate Behaviour Investigation on Glass Fibre Reinforced

PolymersMarco L. Longana – mll1c09@soton.ac.uk - School of Engineering Sciences

Supervisors – Professor Janice M. Dulieu-Barton, Dr Stavros Syngellakis, Prof. Fabrice PierronIntroduction

Virtual Fields Method (VFM)

Experimental Set-up

Advantages of full field optical techniques:• No mechanical interaction between measurand and sensor: the

measurement process does not modify the system.

• Reduces the number of experiments and needed sensors.

• Allows full-field measurements of deformation and strain.

• All the components of the strain can be determined in a single experiment.

• The limitation are in the hardware, no intrinsic limitation in strain, spatial or temporal resolution in the techniques.

Grid Method (GM)

Undeformed image Deformed image Strain map (εY)

Digital Image Correlation (DIC)

Undeformed image Deformed image Strain map (εX)

A stochastic pattern is applied to the specimen surface. DIC follows the movement of the random spackle structures, tracking the gray value pattern in small neighbourhoods called subsets during the loading of the specimen. It is then possible to identify displacements and strain of the whole specimens surface.

A regularly spaced grid is applied to the specimens surface.The GM works correlating the changes in the phase of the light detected by a certain pixel with a Fourier transformation. The changes of the light phase are directly linked to the displacement. From the displacement the strain can be calculated.

PRINCIPLE OF VIRTUAL WORKS:

The total virtual work done by all the forces acting on a system in static equilibrium is zero for a set of infinitesimal virtual displacements from equilibrium. The virtual work is the work done by arbitrary virtual displacements, that must be consistent with the constraints of the system:

HYPOTHESIS:

1)Equilibrium equations:

2)Constitutive equations:

3)Small strains:

Introducing the constitutive equations (Eq.2) in the expression of the PoVW it is possible to write the VFM:

It is valid for any cinematically admissible virtual field. Each choice of VF gives one equation. As many VFs as unknowns have to be chosen. Solving the system leads to the identification of the constitutive parameters.

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Marine applications

INSTRON VHS:•High-speed, servo-hydraulic tensile

test machine

•Allows to test real scale specimen:

- More representative of the material behaviour

- Allows to observe a wider area of the specimen

•Allows to achieve intermediate strain rates

- Compatible with high speed camera characteristics

Strain rates achievable with Instron VHSUsed cameras:

•Redlake Motion Pro X3

- 7000 frames per second @ 1280 x 300 pixel resolution

•Photron SA-3

- 3000 frames per second @ 1024 x 256 pixel resolution

•Photron SA-1

- 13000 frames per second @ 1024 x 352 pixel resolution

The pulse generator is used to contemporarily trigger the camera and the Instron VHS data-log system, that operates at a sampling frequency of 200 kHz .

The Strain Gauge signal is conditioned with a Vishay 2311 conditioning amplifier and recorded by the tensile test machine.

• Investigate the behaviour of fibre reinforced polymer composites subjected to high strain rates events.

•Gather characterisation data to inform material models.

•This will allow to take into account the high strain rate behaviour while designing.

•This will benefit the design of:

• Primary shock resistant structures

• Blasting and impact absorbing structures

Fluid Structure Interactions Research Group

FSI Away Day 2012