Characterising material effects in blast protection

McDonald, B 2019, Characterising material effects in blast protection, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Characterising material effects in blast protection
Author(s) McDonald, B
Year 2019
Abstract Higher strength grades of modern armour steel have promising applications in the blast protection system of armoured vehicles due to the combination of high strength, good energy absorption capacity and familiar fabrication techniques for vehicle manufacturers. While higher strength grades of armour steel are used regularly for ballistic protection and have been integrated into other areas of a vehicle armour systems, there is limited understanding of the response of this class of materials to localised blast loading and further their performance in a blast protection application is unclear. This thesis produces new knowledge and predictive tools regarding the deformation and fracture response of multiple grades of high strength armour steel subjected to localised blast loading.

The response of four grades of high strength steel to localised blast loading was characterised through an extensive experimental investigation, providing significant new insight into the protective capacity provided by high strength, moderate ductility armour steels. The steels tested include three grades of armour steel: rolled homogeneous armour (RHA), improved rolled homogeneous armour (IRHA) and high hardness armour (HHA) as well as a high strength abrasive resistant steel (ARS) with a transformation induced plasticity (TRIP) strengthening phase. Quadrangular target plates were tested using cylindrical charges of PE4 plastic explosive at stand-off distances (SOD) from the target plate between 13 mm and 50 mm. The wide range of blast loading conditions produced localised bulging of the target plates through to rupture and wide-spread fracture propagation.

Along with a thorough assessment of target plate deformation, the magnitude of blast loading required to rupture the four armour steels was isolated at a 13 mm and 25 mm stand-off distance. The rupture threshold of the armour materials was significantly higher than more ductile mild steel evaluated extensively in the literature. ARS, which possessed the highest rupture threshold of the armour materials, fractured at a charge mass 81% higher than the mild steel. Fractographic analysis showed that the high strength steels investigated initiated rupture via ductile shear fracture, as opposed to tensile tearing which is common in lower-strength steels. Cracks were propagated by a variety of ductile tensile and shear modes as well as a brittle radial crack propagation mode identified for the HHA steel.

For the first time, the significant effect of SOD on the target response under free air blast loading was incorporated into a non-dimensional impulse parameter (NDIP) framework. The SOD impulse correction parameter was formulated to capture the more concentrated spatial distribution of blast loading and the contribution of a transverse shear response mode within the target plate, associated with reductions in SOD. The new SOD dependant NDIP produced significant improvements in the prediction of non-dimensional deflection across a large range of experimental programs and identified a characteristic NDIP at fracture for each armour material, which was unified across the SOD conditions tested. The large body of experimental blast results produced through this test program provides a level of insight into the deformation and fracture behaviour of this class of materials which has not been reported previously in literature.

Comprehensive material characterisation was conducted for the four high strength steels to develop new constitutive models and understanding into the fundamental mechanical properties of each armour material. The plasticity and ductile fracture behaviour of each steel was experimentally characterised across a range of stress states and loading conditions, including elevated temperatures and strain rates. State-of-the-art constitutive models were generated for each armour material, capable of capturing the plasticity and fracture behaviour of 13 unique specimen geometries. Ductile fracture was modelled effectively by the Basaran fracture model, producing a level of fracture characterisation unseen previously for these grades of steel. The Basaran model was calibrated following inverse numerical modelling of each mechanical test and utilised a new time-dependant stress state calibration approach for history dependence in the ductile fracture process. Inverse numerical modelling of the high strain rate tensile experiments identified dislocation drag effects on flow stress at 2700 s-1. A novel two-stage exponential strain rate hardening term was developed and integrated into the constitutive model to capture the dynamic behaviour of each material accurately.

A numerical modelling methodology was developed which significantly improved on the state-of-the-art approach for the prediction of deformation and fracture of the armour materials under localised blast loading. The loading from the explosive charge is modelled in an Eulerian representation and is coupled to a Lagrangian representation of the target plate, which deforms and fractures according to the constitutive models defined for each material. The model predicted the final deformation of target plates within 10% for 39 experimental test conditions and a gave good qualitative reproduction of the 3D scanned deformation profile of the experimentally tested target plates.

The charge mass rupture threshold was predicted within 12% of experiments for both SOD conditions. Analysis of the spatial distribution of blast loading highlighted a significantly higher mesh dependence than for overall impulse transfer and a fine discretisation of the fluid domain was required to accurately capture fracture behaviour. The stress state evolution within the target plate approaching fracture was analysed and a shear failure mode was identified early in the target plate response. This shear mode is produced by the initial impingement of the blast product from close proximity explosive charges and plays a critical role is initiating strain localisation in the 13 mm SOD test conditions.

An extensive numerical modelling study was performed providing a new understanding into the effect of various target plate mechanical properties on the deformation and plastic strain evolution under blast loading. High yield strength was highlighted as the most important target plate property for deformation resistance and minimising plastic strain development. While a high strain hardening capacity showed a smaller influence on deformation resistance, it significantly improved the ability of the material to resist the thermo-mechanical instability governing strain localisation thereby increasing the rupture threshold. The most critical property governing the onset of strain localisation and subsequent fracture of the target plate was found to be thermal softening behaviour (magnitude of strength loss at elevated temperatures).

The results of this study provide guidance for new material developments and key parameters of armour materials. This highly accurate material characterisation and numerical modelling methodologies developed throughout this thesis can provide meaningful predictions of protective capacity without relying on extensive experimental blast testing.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Aerospace Materials
Keyword(s) Blast loading
Armour steel
Numerical modelling
Mechanical testing
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Created: Wed, 26 Jun 2019, 14:59:43 EST by Keely Chapman
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