Combined Blast and Fragment Loading of Sandwich Systems

Lead Research Organisation: University College London
Department Name: Mechanical Engineering

Abstract

Protection of personnel and structures against the threat of an air-blast as a result of hostile actions requires a multi-hazard approach to design involving a complex series of trade-offs that must be balanced against other design constraints. The traditional strategy to counter blast and impact threats with thicker blast shields and armour plating are severely dated. Recent trend has moved towards high-strength, lightweight alternatives that do not compromise cost, performance and safety. The reduced weight also saves energy and fuel. Protection against blast and penetrating fragments is normally accomplished by a multi-layered strategy, often with different material combinations, to increase survivability. As a result, this has revived interests in using a sandwich construction for blast mitigation. The high flexural stiffness-to-weight ratio and strength of a traditional sandwich component can be exploited, in combination with the beneficial effects of fluid-structure interaction, as bases for designing 'all-metal' sandwich structures with improved blast resistances. Driven by these needs significant advances have been made over the last decade to design lightweight blast-resistant sandwich systems that employ novel 'micro-architectured' core topologies. The sandwich technology may also be implemented as structural components, designed to integrate several functions into a single component to reduce overall cost and weight, or as retrofits to existing structures for containing a blast to protect personnel and equipment in safety-critical compartments. However, the performance of these sandwich systems to the secondary effect of fragment impact is not well understood and it is, as yet, unclear how the sandwich systems will perform under the combined threat of blast and fragment impact often encountered during close-in explosions. The challenge in designing a combined blast and impact protection system is further complicated by their competing requirements as often the most efficient protection against each threat is, in general, different. The present proposal outlines a systematic study, by a combination of experiments and modelling, to assess and compare the performances and failure modes of 'all-metal' and 'ceramic/metal' sandwich panels subjected to the combined influence of blast and local impulse(s) imparted by a fragment field. The results from this work will be used to quantify the observed synergism which is essential to the future implementation of an optimal sandwich design for blast mitigation in close range.

Planned Impact

The development of lightweight protective technology, using a sandwich strategy, for blast and impact protection will interest academic, governmental and industrial research communities in equal measures. The three major groups of beneficiaries and how they will, potentially, benefit from the proposed research is as follows: (1) Civilian population The development of lightweight blast and impact resistant sandwich structures, apart from the immediate relevance to the military, has a wider spin-off benefit into other areas of civilian survivability assessment and design. The technology and methodology developed can readily be extended or adopted for civilian applications. For example, the technology can be applied to retrofit key buildings and infrastructure against random acts of terrorism and to protect safety critical installations such as nuclear power-plants. With domestic terrorist threat predicted to rise in the foreseeable future (as indicated in a recent white paper presented to the Parliament in 2009 on 'The UK's Strategy for Countering International Terrorism'), the development of more efficient passive protective measures to safeguard lives and key infrastructures is both timely and challenging. Importantly, the light-weighting technology developed in this project saves energy, fuel and, therefore, contributes positively to the low carbon agenda. (2) Government and Classification Societies In the short to medium term, the project will have immediate benefits to the shipbuilding and ship owning communities for both commercial and defence (military) sectors. The results from this research will have direct input, through the involvement of Lloyd's Register and the Ministry of Defence, into the continual development of design rules and procedures against combined blast and fragment loading. As indicated in the Statement of Support by the Head of Strategic Research Group at Lloyd's Register, the topic of the proposed research is particularly urgent in view of increased pirate activities which demand the need to develop novel structural systems to give better protection to personnel sailing onboard ships . The technology developed is useful to the development of lightweight strategies to protect (a) machinery spaces against explosive loads and small arms attack; (b) large crowded spaces in passenger ships; (c) accommodation spaces against terrorist and pirate attack; and, (d) bulkheads in ships against internal blast and fragmentation. (3) Academic Researchers Despite significant advances in our understanding of the blast performance of metal sandwich systems in the last 10 years, the fundamental physics concerning the interaction of a blast wave and its ensuing fragments on the performance of sandwich panels are unclear and have not been studied systematically. The results from the proposed research will be an important valuable addition to the academic community and will help to further strengthen the technological basis for a sandwich strategy to protect personnel and structures against accidental explosions and/or acts of terrorism.

Publications

10 25 50
publication icon
Christodoulou I (2013) Crack initiation and fracture toughness of random Voronoi honeycombs in Engineering Fracture Mechanics

publication icon
Christodoulou I (2013) Role of specimen size upon the measured toughness of cellular solids in Journal of Physics: Conference Series

publication icon
Shojaei A (2014) Multi-scale constitutive modeling of Ceramic Matrix Composites by Continuum Damage Mechanics in International Journal of Solids and Structures

publication icon
Yuan Y (2013) Deformation and failure of rectangular plates subjected to impulsive loadings in International Journal of Impact Engineering

publication icon
Yuan Y (2016) Large deformation, damage evolution and failure of ductile structures to pulse-pressure loading in International Journal of Solids and Structures

publication icon
Yuan Y (2017) The influence of deformation limits on fluid-structure interactions in underwater blasts in International Journal of Impact Engineering

publication icon
Yuan Y (2013) Energy and momentum transfer to a 'fully-clamped' elastic plate in an air-blast in Journal of Physics: Conference Series

publication icon
Yuan Y (2014) Energy and Momentum Transfer to a Clamped Elastic Plate in an Air-Blast in Applied Mechanics and Materials

 
Description This EPSRC-funded project (EP/I028811/1) of one-year duration involves several strands of work carried out in parallel. Their respective findings are summarised as follows:

(1) A multi-scale visco-plastic constitutive model has been developed which is capable of capturing low to high strain rates and ductile to brittle damage response within a single unified model, never before possible for polycrystalline materials. To capture a wide range of dynamic problems with different energy densities, the deformation and damage mechanisms of polycrystalline materials were classified into four generalised regimes based on the level of stress triaxiality. A continuum damage mechanics (CDM) framework was then introduced to bridge the damage mechanisms at the micro-scale to the degradation of the elastic material properties at the continuum scale. A unified low to high strain rate visco-plastic model was proposed where we show that once the material parameters are set for the lowest and highest strain rates, the predicted mid-range strain rate response an be simulated with acceptable fidelity. The coupled visco-plastic-visco-damage relations were coded into ABAQUS and the damage parameters calibrated against existing experimental data. Our paper shows that the multi-scale damage framework developed here has excellent flexibility and is able to reproduce a wide range of experimental results (see A. Shojaei, G.Z. Voyiadjis and P.J. Tan, Visco-plastic constitutive theory for brittle to ductile damage in polycrystalline materials under dynamic loading, International Journal of Plasticity 48 (2013) 125-151.).

(2) We make use of the damage model developed previously to study combined blast and localised impact loading on monolithic plates. Our simulation shows that there is a critical range of projectile velocities within which synergistic interaction, due to combined blast and impact loading, is a maximum. The range is found to depend on various factors, including projectile nose shape, angle of impact, target plate geometry, its boundary conditions, and location of impact on the plate. For a single blunt projectile, it is predicted that maximum synergy, resulting in a greater level of damage, occurs at projectile velocities corresponding to 1.6 times the ballistic limit of the plate. This is due to interactions between stress waves generated by combined blast and impact loading. Under certain loading conditions, we found that synergistic interactions between combined blast and impact loading is important, leading to premature failure, and needs to be accounted for in design. The results are currently being prepared for submission to an international journal (see A. Shojaei and P.J. Tan, A failure model to capture synergistic interactions during combined blast and impact loading, in preparation.).

(3) We carried out further studies into the structural response of monolithic plates and beams under blast loading. In the first, see Y. Yuan and P.J. Tan, Deformation and failure of rectangular plates subjected to impulsive loading, International Journal of Impact Engineering 59(2013) 46 - 59, we found that (a) the mode II impulsive response can be categorized into 3 distinct types, depending on whether tearing initiates at the support; (b) the central deflection of a rectangular plate deforming in mode I and II (Types 1+2) decreases with aspect ratio for the same non-dimensional impulse I*; (c) a higher I* is needed to cause non-through thickness and through-thickness tearing at the support; (d) mode III response is insensitive to aspect ratio and thickness of the plate; (e) for very thin plates, the critical impulse for transition to mode III is a function of material properties only; and (f) increasing the blast duration delays the transition between deformation modes for plates of the same dimensions and subjected to the same I*. In the second, we looked at the influence of support upon the response of elasto-plastic beams in deep under-water blast, specifically to address issues on energy and momentum transfer. Studies by others found that the fluid-structure interaction (FSI) reduces the pressure acting on the structure due to the motion of the structure itself - this reduces the transmitted impulse if the mass per unit area of the structure becomes smaller. As the aspect ratio L/H of the beam increases, the beneficial effects of FSI is somewhat diminished for a fully-clamped beam compared to a corresponding free-standing one because less impulse is transmitted to the latter due to the earlier onset of cavitation. This work is currently under review, see Y. Yuan and P.J. Tan, The response of clamped elasto-plastic beams to deep under-water blast: implications on energy and momentum transfer, submitted to International Journal of Solids and Structures. We are also extending the latter study to air-blast loading.

(4) In a parallel strand of study, we looked at how random variations in the cell architecture Voronoi honeycombs affects its fracture toughness and the location of initial cell wall fracture. It was found that the pure mode I toughness of a lattice decreases as it becomes more irregular with an overall reduction of up to 25% for completely random lattices. By energy partitioning, we found that axial stretch contributed to less than 10% of the overall deformation of the cracked cell wall regardless of relative density, cell regularity and mode-mixity. The Voronoi lattices, whether regular or irregular, have a greater resistance to mode I than mode II loading. The mode I toughness of the lattices are more sensitive to cell topological variations in the vicinity of the crack tip than mode II. Fracture loci for the lattices are obtained in combined mode I and mode II stress intensity factor space. The study shows there is a 70% chance the critical effective SIF of an irregular lattice will be greater than a corresponding regular one of the same relative density for all mode mixities M > 0. The introduction of T-stress result in a significant decrease/increase in fracture toughness: for example, at B = -1, the knockdown in the effective toughness of the lattice is nearly 75% for mode I and 50% for mode II loadings. This trend reverses with positive T-stresses. The knockdown/enhancement in toughness caused by changes in relative density or cell-regularity is generally insignificant compared to the overall knockdown/enhancement due to the inclusion of a T-stress. Fracture location maps are obtained for lattices with different cell-regularities, mode-mixities, relative densities and T-stresses. We found significant scatter in the initial cell wall fracture location is observed: the majority of failed cell walls occur near the crack tip, although they are also observed at up to five cells away from the crack-tip cell, suggesting a highly discontinuous cracking path that is bridged by many un-cracked ligaments. As mode-mixity changes from mode I to mode II, the clustering of the fractured cell walls shifts relative to the crack plane and is reminiscent of the evolution of the plastic zone shape in fully dense solids from LEFM. The introduction of a T-stress changes considerably the clustering of the fractured cell walls: mode I remains, in general, unaffected, whilst for mode II and mixed mode loadings at high T-stresses, the clustering is reminiscent of that seen in mode I loading. There is no correlation between the calculated fracture toughness for different lattice realisations, with the same regularity and relative density, and the location where cell wall cracking first initiates. This work has been published, see I. Christodoulou and P.J. Tan, Crack initiation and fracture toughness of random Voronoi honeycombs, Engineering Fracture Mechanics 104 (2013), pp. 140 - 161.

This study is currently being extended to look at the role of specimen size upon the measured toughness of lattice material. Parts of this work has been accepted for publication, see I. Christodoulou and P.J. Tan, Role of specimen size and geometry on fracture toughness testing of cellular solids, accepted by J. Phys: Conference Series (2013), International Symposium on Dynamic Deformation and Fracture of Advanced Materials, Loughborough, UK.
Exploitation Route The development of lightweight blast and impact resistant sandwich structures, apart from the immediate relevance to the military, has a wider spin-off benefit into other areas of civilian survivability assessment and design. The technology and methodology developed can readily be extended or adopted for civilian applications. For example, the technology can be applied to retrofit key buildings and infrastructure against random acts of terrorism and to protect safety critical installations such as nuclear power-plants.
Sectors Aerospace, Defence and Marine,Construction,Education,Energy,Security and Diplomacy

 
Description The development of lightweight blast and impact resistant sandwich structures, apart from the immediate relevance to the military, has a wider spin-off benefit into other areas of civilian survivability assessment and design. The technology and methodology developed can readily be extended or adopted for civilian applications. For example, the technology can be applied to retrofit key buildings and infrastructure against random acts of terrorism and to protect safety critical installations such as nuclear power-plants. With domestic terrorist threat predicted to rise in the foreseeable future (as indicated in a recent white paper presented to the Parliament in 2009 on 'The UK's Strategy for Countering International Terrorism'), the development of more efficient passive protective measures to safeguard lives and key infrastructures is both timely and challenging. Importantly, the light-weighting technology developed in this project saves energy, fuel and, therefore, contributes positively to the low carbon agenda.
First Year Of Impact 2013
Sector Aerospace, Defence and Marine,Construction,Energy,Environment,Security and Diplomacy
Impact Types Societal,Policy & public services

 
Description PhD Studentship (sponsors: UCL Impact Award + UK-MOD + Lloyd's Register)
Amount £65,000 (GBP)
Organisation Lloyd's Register 
Sector Charity/Non Profit
Country United Kingdom
Start 10/2010 
End 09/2013
 
Description PhD Studentship (sponsors: UCL Impact Award + UK-MOD)
Amount £66,720 (GBP)
Organisation Ministry of Defence (MOD) 
Sector Public
Country United Kingdom
Start 09/2014 
End 08/2017
 
Description PhD studentship (sponsors: QinetiQ and UCL Impact)
Amount £65,720 (GBP)
Organisation Qinetiq 
Department QinetiQ (Farnborough)
Sector Private
Country United Kingdom
Start 09/2015 
End 08/2018
 
Description PhD studentship (sponsors: UK-MOD, BAE Systems, UCL-Impact award)
Amount £91,360 (GBP)
Organisation BAE Systems 
Department BAE Systems Maritime – Naval Ships
Sector Private
Country United Kingdom
Start 09/2013 
End 08/2017
 
Description PhD studentship (sponsors: UK-MOD, UCL Impact)
Amount £66,720 (GBP)
Organisation Ministry of Defence (MOD) 
Sector Public
Country United Kingdom
Start 09/2014 
End 08/2017
 
Description Invited Speaker - Nanyang Technological University (Singapore) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact Invited speaker at a seminar, School of Mechanical & Areospace Engineering, Nanyang Technological University (Singapore), 18th August 2011.

The presentation stimulates discussion on the dynamic mechanical properties of metal foams applied to energy absorption.

I receive an invitation for joint collaboration work.
Year(s) Of Engagement Activity 2011
 
Description Invited Speaker - National University of Singapore 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact Invited speaker at seminar, Department of Mechanical Engineering, National University of Singapore, 16th August 2012.

Seminar sparked lively debates regarding its potential application

I was invited to give a seminar at the University of Oxford.
Year(s) Of Engagement Activity 2012
 
Description Invited Speaker - University of Oxford 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact Invited speaker at a seminar, Solid Mechanics and Materials Engineering Group, Department of Engineering, Oxford University, 20th February 2013.

Seminar sparked lively discussions on the results and methodology.

I was contacted regarding its potential application to characterising fracture in cancellous bones.
Year(s) Of Engagement Activity 2013