Exploration of novel high strain rate impact tests for composites based on ultra-high speed photomechanics

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment

Abstract

n many areas of engineering, materials suffer deformation at high rates. This is the case when structures undergo impact, crash, blast, etc. Therefore, it is essential for design engineers to have reliable mechanical
models to predict the behaviour of the materials in such applications. This is enhanced by the spectacular progress in numerical simulation which now enables to perform detailed computations of very complex situations. However, robust experimental identification of refined high strain rate deformation models is lagging behind and hinders the delivery of the full potential of numerical simulations.The scientific objective of this project is to explore the development of novel high strain rate mechanical tests in order to overcome the strong limitations of the current techniques relying on impact force measurement, such as the Split Hopkinson Pressure or Kolsky bar. The underpinning novelty here is to exploit the inertial effects developed in high strain rate load. This is possible through the use of state-of-the-art ultra-high speed (UHS) imaging (camera with submicrosecond interframe time) combined with image processing (like digital image correlation) and inverse identification (like the Virtual Fields Method, VFM).This project will look at developing this methodology to identify a damage model for fibre reinforced polymeric matrix composites. This is part of a larger effort within the framework of an EPSRC Fellowship programme http://gow.epsrc.ac.uk/NGBOViewGrant.aspx?GrantRef=EP/L026910/1. The candidate will join a team of half a dozen researchers working on this topic and will benefit from excellent local expertise on UHS full-field deformation measurements and the VFM. This project is both numerical and experimental and of a very exploratory nature. This requires candidates with curiosity and enthusiasm to work at the cutting edge of the current knowledge in this area.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509747/1 01/10/2016 30/09/2021
1789931 Studentship EP/N509747/1 29/09/2016 30/09/2019 Jared Van Blitterswyk
 
Description This work has resulted in the successful development of a new method for testing composite materials that are subjected to high rates of loading. Using ultra-high speed cameras we can track the deformation of the specimen in time during the loading process. This provides direct access to full-field measurements of strain and acceleration. From force equilibirum we can reconstruct the average stress at any cross-section on the sample. When combined with strain measurements provides direct access to the constitutive behaviour of the material. This can be used to measure stiffness and strength with high repeatability at rates of loading that are not achievable with existing test methods. This test method can be extended to measure shear properties, and potentially measure strength under combined loading of tension/compression and shear. This information is not available currently and would be very useful for simulating the response of composite materials subjected to high rates of loading (crash, blash, foreign object strike, etc.). The usefulness and accuracy of these models is directly dependent on the quality of material properties used as inputs. Therefore, the ability to reliably measure stiffness and strength under high strain rate loading is critical for the development of numerical models and the design of composite structures subjected to dynamic loading.
Exploitation Route This test method can be extended to other fields to measure material properties of brittle materials (concrete, ceramics, etc.). Eventually, this work may lead to the development of new testing standards that can be implemented by composite manufacturers/companies who perform material characterisation for structural design (aerospace, civil, naval). The ability to collect quantitative measurements at very high framing rates is invaluable for advancing our understanding of the response/behaviour of materials subjected to high rates of loading.
Sectors Aerospace, Defence and Marine,Transport

URL http://photodyn.org
 
Title Image-Based Inertial Impact (IBII) test 
Description This is a new spalling-type method to test materials at high strain rate, wat we think has the potential to become a successor to the current gold standard, the Kolsky (or Split Hopinson) Bar apparatus. The idea is to impact a thin rectangular test specimen in a free-free condition using a gas-gun launched impactor. The compressive waves travels along the specimen and reflects off the free surface as a tensile wave, allowing for the material's tensile response to be evaluated. A set of black dots is printed onto the specimen previously covered by a layer of white paint, producing a grid-pattern. The transient response of the specimen is captured by recording images of the deforming specimen (and grid) with an ultra-high speed camera (5 MHz). Grey level images are converted into displacement maps using spatial phase shifting and strain and acceleration are computed. Acceleration provides stress from equilibrium while strain provides deformation information, allowing for constitutive parameters to be identified (stiffness, strength, non-linear behaviour). This method provides data of unprecedented detail and quality. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact This very new method has already attracted significant interest from industry and research/defense labs (see collaborations section). We are hoping to attract further funding on the back of this technique in the coming year. We have published a series of journal papers illustrating the power of the method. In an effort to transfer the technique to the scientific community, we have published an extnsive manual containing all information required to set up the test from scratch (gas gun design including blue prints, test specimens preparation procedure, experimental protocol, software for processing images to obtain material parameters). 
URL https://eprints.soton.ac.uk/433431/
 
Description Materials Sciences LLC 
Organisation Materials Sciences LLC
Country United States 
Sector Private 
PI Contribution Tested through-thickness properties of AS4-145/MTM45-1 carbon/epoxy composite at high strain rates, in shear and transverse tension.
Collaborator Contribution Provided an 18 mm thick panel of AS4-145/MTM45-1 carbon/epoxy composite
Impact One journal paper was published, two have been submitted.
Start Year 2017