Extreme Dynamic Loading Conditions Research Articles

Toughening static and dynamic damping characteristics of ultra-high performance concrete via interfacial modulation approaches

The damping capacity is one of major concerns for structures under dynamic loads, which significantly affects the safety and service life. Ultra-high performance concrete (UHPC) is a new generation of cementitious material characterized by superior mechanical strength and durability, exhibiting enormous potential in application to innovative structures. However, the weak steel fiber-matrix interface within UHPC leads to insufficient damping performance and poses threats to UHPC structures under extreme dynamic loading conditions. In this paper, various interfacial modulation approaches were investigated to improve static and dynamic damping properties of UHPC, including physical shape and chemical modification of steel fibers, macrofibers and microfibers hybridization. Results show that the interfacial modulation can significantly enhance damping ratio, loss factor and energy dissipation ratio of UHPC, where the chemical modification of steel fibers endows the highest damping ratio. The loss factor and energy dissipation ratio of UHPC reach 0.579 and 0.091 after interfacial modulation, which improved by more than 110% and 100%, respectively. The working mechanisms behind variations of damping performance are attributed to the toughening in interfacial bond and load transfer, leading to improvement in energy dissipating ability of interface. Furthermore, a comprehensive comparison between UHPC and existing vibration damping materials was conducted through multi-criteria analysis from the perspectives of damping performance, mechanical strength, processability and economic benefit, and UHPC with chemical modification of steel fibers exhibits the best overall performance. The findings contribute to inspiring a novel structural vibration control strategy through UHPC with enhanced static and dynamic damping properties for resisting extreme loads.

Elastic Properties of Steel-Cord Rubber Conveyor Belt

Reinforced with steel-cord rubber conveyor belt (SCB), i.e. a unidirectional composite material (CM) with some of the fundamental mechanical properties values of its reinforcement and matrix differing by a factor of ten thousand, is a key infrastructure component of overland minerals transportation industry. Critically, to date no investigative work has been performed on the mechanical behaviour of SCBs subjected to the extreme dynamic-loading conditions of tropical cyclones, hurricanes, and tornados. Determination of a full suite of elastic characteristics of the belt will enable the study of the mechanical behaviour of SCB subjected to these natural hazardous wind events. We investigate tensile and shear moduli of a commercial SCB with the use of modified standard practices and innovative approaches, including tensile testing and methods of torsion of long straight bar and of square plate. We propose a novel design of mechanical tensometer which allows tensile testing of significantly shorter test specimens as compared with the test specimen dimensions per current standards. Analytically, tensile modulus is determined using the rule of mixtures and shear moduli are calculated based on the variational principles. We find that the tensile modulus of SCB can be determined analytically with high confidence. However, analytically derived in-plane shear moduli values can only be considered as a first approximation and need to be verified experimentally. The results of our work improve understanding of stress-strain state and thus can help predict the mechanical behavior of the SCBs under the irregular and extreme dynamic loading conditions of natural hazardous wind events.

Performance of Geogrid Reinforced Concrete Slabs under Drop Weight Impact Loading

Reinforced concrete (RC) structures are often subjected to extreme dynamic loading conditions, mainly caused by effects of impact loading. A countable studies have been carried out on the structural behaviour of RC slabs under static and dynamic loadings. However, it is relatively infrequent to examine the impact behaviour of RC slabs that are embedded with non-conventional reinforcement layouts. Consequently, an experimental study was performed to examine the impact behaviour of geogrid reinforced concrete slabs. A total of six RC slab specimens embedded with different combination of steel and geogrid reinforcement layers was tested under drop weight impact test. The impact response in terms of failure modes, impact energy, impact ductility index and maximum deflection produced at each impact blow were studied. The results showed that, the RC slab specimens provided with geogrid reinforcement layer at both faces of slab specimens along with the conventional reinforcement resisted the crushing of concrete by spreading the impact stress to a larger area. This configuration of reinforcement also helps to withstand for higher impact forces, thereby influencing the enrichment in impact energy and impact ductility index.

Spall Characterization in Epoxy Via Laser Spallation

Background: The strength of materials under extreme dynamic loading conditions, such as in the case of shock wave loading, is assessed from their spallation characteristics. Under laboratory conditions, flyer plate impact, or sometimes laser-induced stress waves, is employed to instigate spall in a material. These methods are often combined with velocity interferometer system for any reflector (VISAR) technique for performing transient measurements. Although the VISAR can record the velocity of extremely fast-moving surfaces, it requires a complex optical setup and a specialized data reduction technique. Objective: In this study, a simpler approach is adopted by extending laser spallation method to determine the spall strength of epoxy, while performing in situ interferometric measurements, directly on top of thick epoxy films. Methods: The glass/epoxy test samples are prepared by transferring an aluminum coating on top of epoxy layers with different thicknesses. Laser-induced stress waves transmit across the substrate/film interface and induce subsurface failure in the epoxy at sufficiently high incident laser energy. The nature and magnitude of the waves are deciphered from the out-of-plane displacement histories of the top reflective sample surfaces, which are recorded by using a Michelson interferometer. Results: The interferometric data reveal the development of two (temporally) well-separated stress waves: an ablation-induced high-amplitude short-duration longitudinal pulse, which is referred to as the primary wave, and a secondary wave, which travels at a comparatively slower speed. The complex constructive interaction of the two waves develops a high-magnitude tensile stress region in the epoxy layer. The spall strength is quantified by superimposing the two stress wave histories associated with the critical energy fluence. Conclusions: The spall depths predicted from spatiotemporal wave travel analyses are in excellent agreement with the experimental observations. The newly adopted methodology estimates the spall strength of epoxy as 260 ± 20 MPa.

Variation of damping behaviour of T105Mn120 castings, used for railway safety systems, as an effect of extreme loading conditions

Abstract Hadfield steel, invented by Sir Robert Hadfield in 1882, has an exceptional hardness and toughness, which recommend it for the construction of executive elements of railway passages and digging equipment. In the former case, the structures are subjected mainly to dynamic deformation, when the texture of austenite is mainly controlled by slip. In a previous work on solution-treated T105Mn120, we observed a marked work hardening tendency during strain sweep cycles, with average rates of 5.3 GPa/μm, which was further augmented by frequency increase, causing storage moduli growth up to 176 GPa in the fifth cycle. In the present paper, the limit frequencies were chosen as 0.5 and 16.66 Hz, replicating the extreme dynamic loading conditions of the parts cast from Hadfield steel at minimum and maximum train wagons, respectively. Dynamic mechanical analysis (DMA) were performed in strain sweep mode, at the above two frequencies, at 233 K and 323K, aiming to reproduce the extreme weather conditions on the ground. The structural changes, observed by electron-scanning microscopy, were corroborated with strain sweep results.

Dynamic Analysis of HSDB System and Evaluation of Boundary Non-linearity through Experiments

<p class="FAIMTextBody">This paper deals with mechanical design and development of high speed digital board (HSDB) system which consists of printed circuit board (PCB) with all electronic components packaged inside the cavity for military application. The military environment poses a variety of extreme dynamic loading conditions, namely, quasi static, vibration, shock and acoustic loads that can seriously degrade or even cause failure of electronics. The vibrational requirement for the HSDB system is that the natural frequency should be more than 200 Hz and sustain power spectrum density of 14.8 Grms in the overall spectrum. Structural integrity of HSDB is studied in detail using finite element analysis (FEA) tool against the dynamic loads and configured the system. Experimental vibration tests are conducted on HSDB with the help of vibration shaker and validated the FE results. The natural frequency and maximum acceleration response computed from vibration tests for the configured design were found. The finite element results show a good correlation with the experiment results for the same boundary conditions. In case of fitment scenario of HSDB system, it is observed that the influence of boundary non-linearity during experiments. This influence of boundary non-linearity is evaluated to obtain the closeout of random vibration simulation results.</p>

A closed-form criterion for dislocation emission in nano-porous materials under arbitrary thermomechanical loading

Our traditional view of void nucleation is associated with interface debonding at second-phase particles. However, under extreme dynamic loading conditions second-phase particles may not necessarily be the dominant source of void nucleation sites. A few key experimental observations of laser spall surfaces support this assertion. Here, we describe an alternative mechanism to the traditional view, namely shock-induced vacancy generation and clustering followed by nanovoid growth mediated by dislocation emission. This mechanism only becomes active at very large stresses. It is therefore desirable to establish a closed-form criterion for the macroscopic stress required to activate dislocation emission in porous materials. Following an approach similar to Lubarda and co-workers, we derive the desired criterion by making use of stability arguments applied to the analytic solutions for the elastic interactions of dislocations and voids. Our analysis significantly extends that of Lubarda and co-workers by accounting for a more general stress state, finite porosity, surface tension, as well as temperature and pressure dependence. The resulting simple stress-based criterion is validated against a number of molecular dynamics simulations with favorable agreement.

The nature of plastic flow in bond zone of explosively welded metals

The physical nature of the formation of wavy interfaces under extreme dynamic loading conditions typical for explosive welding was established. It was shown that such surfaces arise through interaction between the shear and rotational modes of metal plastic flow realized at the macro-, meso-, and microlevels.

Numerical Prediction of the Dynamic Behavior of Two Earth Dams in Italy Using a Fully Coupled Nonlinear Approach

The paper deals with the seismic stability assessment of two existing earth dams in Italy using a fully coupled effective stress nonlinear approach implemented in a finite-element (FE) code. The mechanical behavior of the involved clayey and granular soils is described through advanced elastoplastic constitutive models, calibrated on laboratory and in situ test results. Before the application of the seismic motions, appropriate FE static analyses are performed in both cases to define the initial stress state and the internal variables of the material models. The stability of both dams during dynamic loading is proved by inspection of the cumulated horizontal and vertical displacement time histories of the monitored solid nodes, which become constant immediately after the end of the seismic actions. Moreover, the computed crest settlements induced by the earthquakes are considerably smaller than the service freeboard of the dams. Because the applied seismic actions are characterized by high return periods, the presented results are indicative of a satisfactory dynamic performance of the two embankments during extreme dynamic loading conditions.

Damage characterization of concrete panels due to impact loading by motionless X-ray laminography

Concrete structures are often subjected to extreme dynamic loading conditions due to direct impact. Typical examples of these loading conditions include transportation structures subjected to vehicle crash impact, marine and offshore structures exposed to ice impact, protective structures subjected to projectile or aircraft impact and structures sustaining shock and impact loads during explosions or earthquakes. The assessment and extent of damage to the structure associated with these transient dynamic loading conditions is of paramount importance. Often there are external visual signs of damage after dynamic loading events, however, the extent of damage inside the structure and the determination of its integrity remains obscure. Therefore, one needs to resort to non-destructive evaluation (NDE) techniques not only to detect damage but also to assess the severity of damage. Unfortunately, high-resolution NDE methods to characterize damage in structural materials have been less successful when applied to concrete, partly due to its inhomogeneous microand macrostructure associated with heterogeneities at various length scales that create interferences, such as scattering, attenuation, reflection and diffraction. This paper illustrates the feasibility of using motionless X-ray laminography to detect and characterize damage caused by dynamic loading events. Plain and fabric reinforced concrete panels are subjected to impact loading by a steel projectile of certain initial velocity. The fabric reinforced concrete panels consist of polypropylene fabric attached to the front and back panel. Whereas in plain concrete panels, the damage due to impact loading is clearly visible, in fabric reinforced concrete panels the damage is obscured by the presence of the fabric. Hence, motionless laminography based on reverse geometry X-ray radiography is applied to these fabric reinforced concrete panels to obtain information on the damage evolution through the thickness of the concrete panel. The motionless X-ray laminography concept and its advantage over conventional laminography are briefly described, followed by discussions on the effect of impact loading on plain and fabric reinforced concrete panels.

Rate Dependent Multicontinuum Progressive Failure Analysis of Woven Fabric Composite Structures under Dynamic Impact

Marine composite materials typically exhibit significant rate dependent response characteristics when subjected to extreme dynamic loading conditions. In this work, a strain-rate dependent continuum damage model is incorporated with multicontinuum technology (MCT) to predict damage and failure progression for composite material structures. MCT treats the constituents of a woven fabric composite as separate but linked continua, thereby allowing a designer to extract constituent stress/strain information in a structural analysis. The MCT algorithm and material damage model are numerically implemented with the explicit finite element code LS-DYNA3D via a user-defined material model (umat). The effects of the strain-rate hardening model are demonstrated through both simple single element analyses for woven fabric composites and also structural level impact simulations of a composite panel subjected to various impact conditions. Progressive damage at the constituent level is monitored throughout the loading. The results qualitatively illustrate the value of rate dependent material models for marine composite materials under extreme dynamic loading conditions.

Constitutive laws of materials in dynamics—outline of a programme of testing on small and large specimens for containment of extreme dynamic loading conditions

A sound foundation of stress and/or strain criteria for the mechanical design of fast breeder reactor structures capable of bearing extreme dynamic loading conditions, passes through the experimental determination of dynamic mechanical properties of materials in end-of-life conditions with respect to the damaging processes to which the structures are submitted. Calculation codes must be implemented by constitutive equations describing the dynamic mechanical response of the materials, without the knowledge of which any calculation code is liable to important inaccuracies which provoke the use of high safety coefficients and, often, the uncertainty as to the effective capability of the structures to withstand the accidental loads. The results of a screening programme of dynamic tensile tests performed on AISI 304 and AISI 316 austenitic stainless steels using small specimens (7 mm 2) showed that the dynamic response of such steels at temperatures of 400 and 550°C is not univocal, passing from a substantial dynamic hardening behaviour to a dynamic softening behaviour, probably due to residual microstructural differences caused by the transformation processes. From the discussion of the results obtained, follows the development of a testing programme on small (up to 20 mm 2 cross section) and large specimens (up to 5000 mm 2 cross section) focused on: (i) developing and calibrating dynamic constitutive equations founded on basic physical aspects, for uniaxial and multiaxial loading conditions under particular deformation and loading histories; (ii) verifying in dynamics the existing yielding criteria, hardening rules and failure theories; (iii) investigating by means of a high-load apparatus (5 MN) the influence of near real-size thickness, welding, defects and notches, on dynamic strength and deformation capability of large reactor structures.

Dynamic rupture analysis of reinforced concrete shells

Extreme dynamic loading conditions often require the rupture analysis of reinforced and prestressed-concrete structures. The study presented in this paper extends a method of analysis of dynamic loading conditions which has proven efficient for short- and long-time loads. Another aim is to adapt the method to thin-walled structures. It is not sufficient to work only with plastic rupture and yield surfaces locally which are compared to the elastic distribution of the stress resultants; it is essential to account for the redistribution of the latter. The method proposed consists of discretizing the structure into isoparametric three-dimensional elements with 20 nodes for the concrete and one-dimensional bar elements with three nodes for the steel. The latter can also be handled with a ‘smeared’ two-dimensional membrane element. In compression a three-dimensional non-linear elastic constitutive law is introduced for the concrete, and a triaxial failure surface expressed in the stress invariants is used, determining cracking and crushing. Two- and three-dimensional cracking surfaces in which no components of stress are transmitted are accounted for. The possibility exists that, during the history of loading, cracks can close up again. For steel, a yield criterion is selected. The non-linear analysis is based on the concept of initial stress. Residual loads are calculated using information in Gauss integration points. The ultimate load is reached when the algorithm does not converge. The corresponding failure modes can be interpreted as those for which a state of equilibrium is no longer possible. The equations of motion are discretized in time, using an extension of the linear acceleration method. As in the static case, several iterations are necessary to reduce the residual load vector to a negligible quantity. A built-in circular plate is analyzed for an evenly distributed load. The results with one and several isoparametric elements in width are compared to the solution determined with the classical yield line theory. Cracks in the concrete and yielding of the steel in both directions are properly represented. In the non-linear domain moment redistribution is observed which allows for a substantial increase in the ultimate load. All states of material behaviour are observed in the final stage. A thin-walled shell consisting of a cylinder and a sphere is also examined for a non-symmetric loading involving the interaction of membrane and bending behaviour. A dynamic non-linear analysis is performed for this load, which represents the impact of an airplane on the external shield building of a nuclear power plant. The non-linear analysis does allow for a substantial saving of reinforcement steel compared to the standard design procedure. Conclusions which include pitfalls, shortcomings and suggestions for future research are specified.

Sound steel for rails and structural purposes

Hadfield steel, invented by Sir Robert Hadfield in 1882, has an exceptional hardness and toughness, which recommend it for the construction of executive elements of railway passages and digging equipment. In the former case, the structures are subjected mainly to dynamic deformation, when the texture of austenite is mainly controlled by slip. In a previous work on solution-treated T105Mn120, we observed a marked work hardening tendency during strain sweep cycles, with average rates of 5.3 GPa/μm, which was further augmented by frequency increase, causing storage moduli growth up to 176 GPa in the fifth cycle. In the present paper, the limit frequencies were chosen as 0.5 and 16.66 Hz, replicating the extreme dynamic loading conditions of the parts cast from Hadfield steel at minimum and maximum train wagons, respectively. Dynamic mechanical analysis (DMA) were performed in strain sweep mode, at the above two frequencies, at 233 K and 323K, aiming to reproduce the extreme weather conditions on the ground. The structural changes, observed by electron-scanning microscopy, were corroborated with strain sweep results.