High Rate Impact Loading Research Articles

Dynamic behavior of bridge columns reinforced with shape memory alloy rebar and UHPFRC under lateral impact loads

This paper numerically evaluates the combined effect of ultra-high-performance fiber-reinforced concrete (UHPFRC) jacket and shape memory alloy (SMA) rebar on the dynamic response of a reduced-scale bridge pier under lateral impact loads of different velocities. The performance of the columns is assessed considering the variations of different parameters including the type of SMA rebars, thickness of UHPFRC jacket (tU), impact velocity (Vimp), and axial load ratio (ALR). From the FE simulations, it is found that the SMA rebars have a more positive influence on the lateral strength of the conventional columns with normal concrete (NC) compared to UHPFRC-strengthened columns when subjected to impact loadings at 5 m/s and 15 m/s velocities that represent relatively low- and medium-rate impacts, respectively. However, the positive effect of the SMA rebar is more pronounced on the resistance of UHPFRC-strengthened columns under a high-rate impact loading with Vimp = 25 m/s. In addition, a 60 mm-thick UHPFRC jacket is recognized as the optimal level based on the behavioral trend of the UHPFRC-strengthened columns when they are subjected to impact loads with velocities higher than 10 m/s (representing a medium-rate impact). From the evaluation of the effectiveness of ALR, an absolute positive influence of ALR on the impact resistance of the steel-reinforced columns is found when it ranges between 0.1 and 0.15. However, the increase of ALR beyond a critical value of 0.125 demonstrates a negative influence on the lateral strength of SMA-reinforced UHPFRC columns.

Semi-Crystalline Polymers Applied to Taylor Impact Test: Constitutive, Experimental and FEM Analysis.

Based on mechanical properties of Polyamide 66 (PA66) under complex loading conditions, a Drucker–Prager yield criterion was employed to characterize its yield behavior. Then, a one-dimensional model, which contains a viscoelastic regime and a viscoplastic regime, was introduced and converted into a three-dimensional constitutive model. The three-dimensional model was implemented into a LS-DYNA software, which was used to predict the dynamic response of PA66 under Taylor impact conditions, whose corresponding tests were conducted by gas gun and recorded by high-speed camera. By contrasting the simulation results and these of the corresponding tests, the deformed shapes including the residual length, the maximum diameter and the shape of the mushroom head of the PA66 bars were found to be similar to these obtained from the tests, which verified the accuracy of the three-dimensional constitutive model, and proved that the model was able to be applied to high-rate impact loading conditions.

Chondrocyte viability is lost during high-rate impact loading by transfer of amplified strain, but not stress, to pericellular and cellular regions

Deleterious impact loading to cartilage initiates post-traumatic osteoarthritis (OA). While cytokine and enzyme levels regulate disease progression, specific mechanical cues that elucidate cellular OA origins merit further investigation. We defined the dominant pericellular and cellular strain/stress transfer mechanisms following bulk-tissue injury associated with cell death. Using an invitro model, we investigated rate-dependent loading and spatial localization of cell viability in acute indentation and time-course studies. Atomic force microscopy (AFM) and magnetic resonance imaging (MRI) confirmed depth-wise changes in cartilage micro-/macro-mechanics and structure post-indentation. To understand the transfer of loading to cartilage domains, we computationally modeled full-field strain and stress measures in interstitial matrix, pericellular and cellular regions. Chondrocyte viability decreased following rapid impact (80%/s) vs slow loading (0.1%/s) or unloaded controls. Viability was lost immediately during impact within regions near the indenter-tissue contact but did not change over 7days of tissue culture. AFM studies revealed a loss of stiffness following 80%/sloading, and MRI studies confirmed an increased tensile and shear strain, but not relaxometry. Image-based patterns of chondrocyte viability closely matched computational estimates of amplified maximum principal and shear strain in interstitial matrix, pericellular and cellular regions. Rapid indentation worsens chondrocyte death and degrades cartilage matrix stiffness in indentation regions. Cell death at high strain rates may be driven by elevated tensile strains, but not matrix stress. Strain amplification beyond critical thresholds in the pericellular matrix and cells may define a point of origin for early damage in post-traumatic OA.

Scaled models for failure under impact loading

Small-scale experimentation is recognised to play an important role in the investigation of the behaviour of large-scale structures. Presently however, there exists no known procedure for analysing structures involving damage or failure by means of scaled experiments when high-rate impact loading is involved. The principal problem impeding the scaled approach is that similarity seldom exists for high-rate dissipative problems and consequently dimensional analysis provides an ineffective framework for experimental design and analysis.This paper is concerned with the application of the finite similitude theory which scales experiments by means of space distortion. Although finite similitude suffers many of the limitations of dimensional analysis it has one principal advantage, i.e. it is able to cater for inexact similitude where sources and effects of mismatch can be quantified. This feature is exploited in the work to answer the open question whether it is possible to determine the failure response of structures subjected to high rate loading by performing experimental tests on scaled structures manufactured with different materials.The efficacy of the proposed methodology for scaled experimentation is tested by means of numerical analysis on a number of well-known test cases, which are supported by experimental evidence. In particular the perforation of circular plates and the Taylor bar impacting on a rigid wall are analysed using the commercial finite element software package Abaqus. The Johnson–Cook thermo-viscoplastic constitutive equation together with the Johnson–Cook damage model are used to describe yielding, strain hardening, strain rate effects, thermal softening and material failure. It is revealed in the paper that although exact similitude is not achievable the results of the full-scale model can be predicted to good accuracy by means of scaled experimentation founded on finite similitude theory.

Larger vertebral endplate concavities cause higher failure load and work at failure under high-rate impact loading of rabbit spinal explants.

Vertebral fractures are among the most common of all osteoporosis related fracture types and its risk assessment is largely based on bone quality measures. Morphometric parameters are not yet considered, although endplate thickness and concavity shape were found to be important in fracture prediction in low-rate tests. We hypothesized that, under high-rate impact loading, the shape and size of the central endplate concavity are of key importance for fracture prediction. Therefore, we tested rabbit spinal segment explants in vitro under high-rate impact loading. With a combination of microCT to describe endplate morphometry, high-speed video imaging, and impact force measurement, endplate morphometry was correlated to the mechanical response. We found that endplate concavity shape and volume were important in describing the mechanical response: larger concavities caused higher failure load. We suggest a model for the fracture mechanism under high-rate impact loading, considering the morphometry of the endplates: wider and more voluminous concavities are protective whereas steeper slopes of the concavity edges and increasing bone volume fraction of the central endplate moiety are disadvantageous. Therefore, the shape and size of endplate morphometry are important in vertebral fracture prediction and should be considered included in vertebral fracture risk assessment.

Atomistic Origin of Deformation Twinning in Biomineral Aragonite.

Deformation twinning rarely occurs in mineral materials which typically show brittle fracture. Surprisingly, it has recently been observed in the biomineral aragonite phase in nacre under high rate impact loading. In this Letter, the twinning tendency and the competition between fracture and deformation twinning were revealed by first principles calculations. The ratio of the unstable stacking fault energy and the stacking fault energy in orthorhombic aragonite is hitherto the highest in a broad range of metallic and oxide materials. The underlining physics for this high ratio is the multineighbor shared ionic bonds and the unique relaxation process during sliding in the aragonite structure. Overall, the unique deformation twining along with other highly coordinated deformation mechanisms synergistically work in the hierarchical structure of nacre, leading to the remarkable strengthening and toughening of nacre upon dynamic loading, and thus protecting the mother-of-pearl from predatory attacks.

A methodology to condition distorted acoustic emission signals to identify fracture timing from human cadaver spine impact tests

While studies have used acoustic sensors to determine fracture initiation time in biomechanical studies, a systematic procedure is not established to process acoustic signals. The objective of the study was to develop a methodology to condition distorted acoustic emission data using signal processing techniques to identify fracture initiation time. The methodology was developed from testing a human cadaver lumbar spine column. Acoustic sensors were glued to all vertebrae, high-rate impact loading was applied, load-time histories were recorded (load cell), and fracture was documented using CT. Compression fracture occurred to L1 while other vertebrae were intact. FFT of raw voltage–time traces were used to determine an optimum frequency range associated with high decibel levels. Signals were bandpass filtered in this range. Bursting pattern was found in the fractured vertebra while signals from other vertebrae were silent. Bursting time was associated with time of fracture initiation. Force at fracture was determined using this time and force–time data. The methodology is independent of selecting parameters a priori such as fixing a voltage level(s), bandpass frequency and/or using force–time signal, and allows determination of force based on time identified during signal processing. The methodology can be used for different body regions in cadaver experiments.

Heterogeneous deformation and spall of an extruded tungsten alloy: plate impact experiments and crystal plasticity modeling

The role of microstructure in the dynamic deformation and fracture of a dual phase, polycrystalline tungsten alloy under high-rate impact loading is investigated via experiments and modeling. The material studied consists of pure tungsten crystals embedded in a ductile binder alloy comprised of tungsten, nickel, and iron. The tungsten crystals are elongated in a preferred direction of extrusion during processing. Plate impact tests were conducted on samples oriented either perpendicular or parallel to the extrusion direction. Spatially resolved interferometric data from these tests were used to extract wave propagation behavior and spall strength dependent upon position in the sample microstructure. Finite element simulations of impact and spall in digitally reproduced microstructural geometries were conducted in parallel with the experiments. Finite deformation crystal plasticity theory describes the behavior of the pure tungsten and binder phases, and a stress- and temperature-based cohesive zone model captures fracture at grain and phase boundaries in the microstructure. In results from both experiments and modeling, the grain orientations affect the free-surface velocity profile and spall behavior. Some aspects of distributions of free-surface velocity and spall strength among different microstructure configurations are qualitatively similar between experimental and numerical results, while others are not as a result of differing scales of resolution and modeling assumptions. Following a comparison of experimental and numerical results for different microstructures, intergranular fracture is identified as an important mechanism underlying the spall event.

Mechanics of Taylor impact testing of polycarbonate

The deformation of polymers under high-rate loading conditions is a governing factor in their use in impact-resistant applications, such as protective shields, safety glass windows and transparent armor. In this paper, Taylor impact experiments were conducted to examine the mechanical behavior of polycarbonate (PC), under conditions of high strain rate (∼10 5 s −1) and inhomogeneous deformation. High-speed photography was used to monitor the progression of deformation within the sample. A recently developed three-dimensional large strain rate-dependent elastic–viscoplastic constitutive model which describes the high-rate behavior of glassy polymers was used together with the ABAQUS/Explicit finite-element code to simulate several Taylor impact conditions. The simulation results are compared directly with experimental images for a range in initial rod dimensions and velocities. Final deformed shapes are found to correspond with those obtained experimentally, demonstrating the ability to predict complex inhomogeneous deformation events during very high-rate impact loading scenarios. The dependence of the observed behaviors on the various features of the polymer stress–strain behavior are presented in detail revealing the roles of strain softening and strain hardening in governing the manner in which deformation progresses in a polymer during dynamic inhomogeneous loading events.

Influence of welding parameters on microstructures and mechanical properties of electron beam welded aluminium–lithium plates

The present work focused on the welding characteristics of electron beam welding (EBW) in 8090 Al–Li plates, evaluated in terms of strength or toughness degradation in post-weld impact and bending tests with loading rates of 103, 10-1, and 10-4 s-1. The influence of welding parameters, such as welding power, welding speed, and electron beam focus position, on the post-weld microstructures, porosity, and mechanical properties were examined. Although the joint efficiency for the maximum flexure strength (Fp ), or tensile strength, can be as high as 85–90%, the joint efficiency for the fracture absorption energy (Et ) was only 20–40%, a level usually unsuitable for applications. It was found that changing the welding power and speed by a factor of 3 resulted in significant variation in Et but only minor variation in Fp . Changing the welding focus position had little effect on post- weld mechanical properties. The abundant grain boundary precipitates in the welded specimens were thought to be the main cause of the degraded post-weld properties. Other microstructural factors included the δ′ precipitate and grain sizes. The volume fraction of porosity did not play any decisive role owing to the small size (<0·3 mm), low quantity (1–2%), and spherical shape of the EBW induced pores. Finally, given the same post-weld microstructures, the toughness degradation of the EBW specimens was worst under high rate impact loading.