Lightweight Vehicle Structures that Absorb and Direct Destructive Energy Away from the Occupants

One of the main thrusts in current automotive industry is the development of occupant-centric vehicle structures that make the vehicle safe for the occupants. A design philosophy that improves vehicle survivability by absorbing and redirecting destructive energy in underbody blast events should be developed and demonstrated. On the other hand, the size and weight of vehicles are also paramount design factors for the purpose of providing faster transportation, great fuel conservation, higher payload, and higher mobility. Therefore, developing a light weight vehicle structure that provides a balance between survivability and mobility technologies for both vehicle and its occupants becomes a design challenge in this research. One of the new concepts of absorbing blast energy is to utilize the properties of “softer” structural materials in combination with a damping mechanism for absorbing the destructive energy through deformation. These “softer” materials are able to reduce the shock loads by absorbing energy through higher deformation than that of characteristic of normal high strength materials. A generic V-hull structure with five bulkheads developed by the TARDEC is used in the study as the baseline numerical model for investigating this concept. Another new concept is to utilize anisotropic material properties to guide and redirect the destructive energy away from the occupants along pre-designated energy paths. The dynamic performance of multilayer structures is of great interest because they act as a mechanism to absorb and spread the energy from a blast load in the lateral direction instead of permitting it to enter occupant space. A reduced-order modeling (ROM) approach is developed and applied in the preliminary design for studying the dynamic characterization of multilayer structures. The reliability of the ROM is validated by a spectral finite element analysis (SFEA) and a classic finite element analysis by using the commercial code Nastran. A design optimization framework for the multilayer plate is also developed and used to minimize the injury probability, along with a maximum structural weight reduction. Therefore, the goal of designing a lightweight vehicle structure that has high levels of protection in underbody blast events can be achieved in an efficient way.PHDNaval Architecture & Marine EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135895/1/leaduwin_1.pd

Study of blast effects on structures

Engineers have a duty to the public to preserve life and protect the community and occupants within structure that we build and use. All practicing engineers are obligated to foster the health, safety and wellbeing of the community and the environment. This involves acting on the basis of adequate knowledge and foreseeable risks that pose a potential hazard towards the built environment. The terrorism threat has evolved rapidly in scale and occurrences in recent history and with that the need to create resilient structures. This dissertation endeavours to undertake a study of the global blast loading effects on structures and identify techniques for improved structural resilience of critical elements. Blasts can be delivered by explosive events either deliberate, accidental or through indirect action. A historical review of case studies and blast incidents was undertaken to identify susceptible structures to blast and development of a structural model in order to simulate a credible scenario and understand the blast effects and predicting the design loading. The scope of the dissertation is restricted to the blast pressure disturbance effects interacting with a structure delivered by an external air blast and not considering the secondary effects of a blast incident including thermal and high velocity fragments. Common structural members and materials were used to devise a Finite Element model and simulate against the blast loading cases derived from empirical methods. Since the nature of blast load only lasting for a short time and undergoes constant change Non-Linear Transient Dynamic Analysis approach was well suited to undertaking this type of analysis. Some of the findings include whipping effects due to inertia as the structure accelerating from its initial position to develop resistance against the applied loading even after the applied load has ceased. The global response of a structure due to blast pressure, is generally a consequence of lateral or out-of-plane loading. Longer pressure phase durations tend to result in bending failures while impulsive loads (short pressure phase duration) lead to shear responses. Resilience techniques including steel UC encased in concrete, RC steel plate wraps and RC shear reinforcement lacing have the potential to improve the robustness of structural elements reducing overall displacements and stress responses

Blast analysis of enclosure masonry walls using homogenization approaches

A simple rigid-plastic homogenization model for the analysis of enclosure masonry walls sub- jected to blast loads is presented. The model is characterized by a few material parameters, is numerically inexpensive and very stable, and allows full parametric studies of entire walls subject to blast pressures. With the aim of considering the actual brickwork strength along vertical and horizontal axes, masonry out-of-plane anisotropic failure surfaces are obtained by means of a compatible homogenized limit analysis approach. In the model, a 3D system of rigid infinitely strong bricks connected by joints reduced to interfaces is identified with a 2D Kirchhoff-Love plate. For the joints, which obey an associated flow rule, aMohr-Coulomb fail- ure criterion with a tension cutoff and a linearized elliptic compressive cap is considered. In this way, the macroscopic masonry failure surface is obtained as a function of the macroscopic bending, torque, and in-plane forces by means of a linear programming problem in which the internal power dissipated is minimized. Triangular Kirchhoff-Love elements with linear in- terpolation of the displacements field and constant moment within each element are used at a structural level. In this framework, a simple quadratic programming problem is obtained to analyze entire walls subjected to blast loads. The multiscale strategy presented is adopted to predict the behavior of a rectangular wall supported on three sides (left, bottom, and right) representing an envelope wall in a building and subjected to a standardized blast load. The top edge of the wall is assumed unconstrained due to an imperfect connection (often an inter- layer material is used to prevent damage in the in-fill wall). A comparison with a standard elastic-plastic heterogeneous 3D analysis conducted with a commercial FE code is also pro- vided for a preliminary verification of the procedure at a structural level. The good agreement found and the very limited computational effort required for the simulations conducted with the presented model indicate that the proposed simple tool can be used by practitioners for the safety assessment of out-of-plane loaded masonry panels subjected to blast loading. An ex- haustive parametric analysis is finally conducted with different wall thicknesses, joint tensile strengths, and dynamic pressures, corresponding to blast loads (in kilograms of TNT) ranging from small to large

Homogenized rigid-plastic model for masonry walls subjected to impact

A simple rigid-plastic homogenization model for the analysis of masonry structures subjected to out-of-plane impact loads is presented. The objective is to propose a model characterized by a few material parameters, numerically inexpensive and very stable. Bricks and mortar joints are assumed rigid perfectly plastic and obeying an associated flow rule. In order to take into account the effect of brickwork texture, out-of-plane anisotropic masonry failure surfaces are obtained by means of a limit analysis approach, in which the unit cell is subdivided into a fixed number of sub-domains and layers along the thickness. A polynomial representation of micro-stress tensor components is utilized inside each sub-domain, assuring both stress tensor admissibility on a regular grid of points and continuity of the stress vector at the interfaces between contiguous sub-domains. Limited strength (frictional failure with compressive cap and tension cutoff) of brick-mortar interfaces is also considered in the model, thus allowing the reproduction of elementary cell failures due to the possible insufficient resistance of the bond between units and joints. Triangular Kirchhoff-Love elements with linear interpolation of the displacement field and constant moment within each element are used at a structural level. In this framework, a simple quadratic programming problem is obtained to analyze entire walls subjected to impacts. In order to test the capabilities of the approach proposed, two examples of technical interest are discussed, namely a running bond masonry wall constrained at three edges and subjected to a point impact load and a masonry square plate constrained at four edges and subjected to a distributed dynamic pressure simulating an air-blast. Only for the first example, numerical and experimental data are available, whereas for the second example insufficient information is at disposal from the literature. Comparisons with standard elastic-plastic procedures conducted by means of commercial FE codes are also provided. Despite the obvious approximations and limitations connected to the utilization of a rigid-plastic model for masonry, the approach proposed seems able to provide results in agreement with alternative expensive numerical elasto-plastic approaches, but requiring only negligible processing time. Therefore, the proposed simple tool can be used (in addition to more sophisticated but expensive non-linear procedures) by practitioners to have a fast estimation of masonry behavior subjected to impact

Shock resilience of structural pillars in naval vessels

Although structural pillars are extensively used in commercial vessels, traditionally their use on board UK warships has been discouraged. This is due to the tendency of pillars to "punch through" the deck when subjected to the high impulse loading of shock from underwater explosions (UNDEX). There are however many spaces within naval ships that would significantly benefit from the wide-open spaces created from the use of pillars as opposed to full bulkheads, such as machinery rooms, mooring decks and accommodation flats. This paper re-addresses the question of a shock capable pillar, looking at how a pillar can be designed or mounted to increase its resilience to shock from underwater explosions. It is proposed that the advice against the use of pillars in warships could be unfounded; this is supported by the fact that not all navies reject their use. The results of this study imply that as long as the pillar is sited properly on primary structural members, then pillar buckling should occur long before "punch though"

Shock resilience of structural pillars in naval vessels

Although structural pillars are extensively used in commercial vessels, traditionally their use on board UK warships has been discouraged. This is due to the tendency of pillars to "punch through" the deck when subjected to the high impulse loading of shock from underwater explosions (UNDEX). There are however many spaces within naval ships that would significantly benefit from the wide-open spaces created from the use of pillars as opposed to full bulkheads, such as machinery rooms, mooring decks and accommodation flats. This paper re-addresses the question of a shock capable pillar, looking at how a pillar can be designed or mounted to increase its resilience to shock from underwater explosions. It is proposed that the advice against the use of pillars in warships could be unfounded; this is supported by the fact that not all navies reject their use. The results of this study imply that as long as the pillar is sited properly on primary structural members, then pillar buckling should occur long before "punch though"

Dynamic response of aluminium sheet 2024-T3 subjected to close-range shock wave: experimental and numerical studies

Abstract This present study investigates experimentally and numerically the behaviour of 1 mm thick aluminium 2024-T3 alloy sheets from near field shock waves. A comparison and examination are undertaken with respect to global deformation and plastic damage formation from two different stand-off distances of 4 mm and 50 mm that were exposed to a constant charged mass. A 4-cable instrumented pendulum blast set-up was used to carry out and monitor the blast test. The results of the blast test were subsequently used to simulate the pressure history for different stand-off distances. The simulation involved implementing a user subroutine in ABAQUS/Explicit solver to model non-uniform pressure fields for use in finite element simulation. The results provided a strong alignment of the numerical method when compared with the experimental data. The main outcome of this study is to show the significant effect of the changing damage from highly localised perforation to global deformation when the stand-off distance is changed from 4 mm to 50 mm

Ongoing Research into the Failure of Glass at High Strain-Rates

The failure of glass has been studied extensively by many researchers. However, the focus has previously been on the static to quasi-static, rate-independent behaviour. It is commonly accepted that the strength of glass is sensitive to the applied loading time, however, the amount of research in the field of loading rate dependency seems very limited. Consequently, the effect of loading rates on the strength is sparingly described in the available literature despite its relevance when designing for impact and blast loads. The present paper presents an ongoing research project considering the failure of glass at high strain-rates. It provides a brief review of existing studies showing a strength increase with loading rates relevant for e.g. blast loads. Based on existing experimental work, a numerical model considering different failure models is presented. The different considered failure models are then compared and discussed for their applicability. The paper also includes an outlook of the project, briefly explaining a novel concept for a high strain-rate test setup planned to be built during the summer of 2020