Impact Load History Research Articles

Nonlinear spring model for dynamic uplift behaviour of suction caisson under impact loading

Suction caissons are widely employed as support structures for various fixed or floating structures. The impact loading in extreme environment is a key condition for marine foundation design. In this paper, a nonlinear spring model is proposed to predict the dynamic behaviour of suction caisson under impact loading, and its reliability is verified by experimental tests and finite element simulations. A mass module is introduced to describe the foundation inertia action, and a novel component consisting of a reverse end bearing spring and a soil plug slider is developed to decouple the overall vertical stiffness of suction caisson. The friction module is updated with t-z springs positioned along the depth to reflect the stress distribution in the soil layer. Afterwards, the influence of impact loading amplitude and loading history are studied. For the impact loadings with different amplitude (Fu), the maximum vertical displacement of caisson is only 0.02D when the uplift loading reaches 0.4Fu, indicating a marked contribution of inertia force. It is recommended to properly consider foundation inertia action under high magnitude impact loading, which can effectively optimize the structure design. In light of this, a design chart for suction caisson applied in engineering practices is presented.

A hybrid FCN-BiGRU with transfer learning for low-velocity impact identification on aircraft structure

In the work process of an aircraft, its external structure frequently encounters some low-velocity impact events, resulting in some barely visible impact damages inside the structure. Therefore, impact localization and reconstruction are critical for the structure’s health monitoring and reliability analysis. However, traditional inversion methods usually perform poorly in identifying random impact loads due to a series of problems. Inspired by the superior model properties of deep learning algorithms, a novel feature learning-based technique for impact load reconstruction and localization is presented. The proposed method consists of two parts, the first of which is utilized to localize the impact loads and is referred to as one-dimensional full convolution network. The other part aims at reconstructing the impact load and is called attention mechanism-full convolutional network-bidirectional gating recurrent unit-multilayer perceptron. Moreover, a transfer learning technique is introduced to optimize the model structure and process in light of the suggested innovative network model. The experimental piece of this research was an aircraft cutting segment whose structural surface was equipped with a number of piezoelectric sensors. These sensors received impact response signals with differences in amplitude from hammer strikes at various positions. In the work, the process of locating first and then reconstructing the impact load history was followed. And the influence of implementing the transfer learning mechanism on the performance of the impact load history reconstruction model was investigated as well. Three experiments and repeated cross-validation verified the efficacy of the proposed method. The findings indicated that the proposed technique could accurately and quickly detect impact loads at various locations. Simultaneously, it also allowed a perfect reconstruction of the impact loads of different impact energies.

Drop-weight impact responses and energy absorption of lightweight glass fiber reinforced polypropylene composite hierarchical cylindrical structures

A high spike force at the beginning of impact and the load fluctuations caused by unstable plateau deformation are disadvantages to avoid for impact resistance structures. Lightweight glass fiber-reinforced polypropylene composite hierarchical cylindrical structures (namely GFRP-CHCS, with a relative density of less than 0.1) have the potential to achieve low initial force and high crushing force efficiency under dynamic conditions. This work explores the drop-weight impact behaviors and energy absorption capacity of GFRP-CHCS. Deformation modes, failure conditions, and impact load history responses are discussed. Results showed that GFRP-CHCS has no initial load peak and achieves gentle response curves under the low-velocity impact when the load speed is less than 10 m/s. Among the experiments where the impact energy was beyond the range of static energy absorption, more than 94% of the impact energy was absorbed or dissipated when the impact energy ranged from 200 J to 300 J. After unloaded, GFRP-CHCS maintained good integrality and recovered a significant deformation which was over 93% initial structural height. With ideal isotropic material properties, numerical and theoretical predictions were conducted regarding the loading velocity effects on structural deformation mode and mean impact response force. This work utilized a thermoplastic composite consisting of GFRP face sheets and polypropylene core. The results have reference significance for the anti-impact design of thermoplastic composite hierarchical structures.

Time-Domain Inversion Method of Impact Loads Based on Strain Monitoring Data

A helicopter deck is the main load-bearing component under the emergency landing conditions for helicopters. However, it is generally difficult to directly obtain the landing load from measurements due to the high randomness of the landing position. As the main design load of the helicopter deck, the emergency landing load is very important to its structural design. A large design load value leads to an overly conservative structural design and affects the control of the ship’s weight and center of gravity, while a small design load may lead to a lack of security and affect the safety of the helicopter and the ship. As a result, the time domain inversion method, which is based on strain monitoring data, is an important and effective method for obtaining the helicopter emergency landing load. In this study, a grillage model experiment was conducted to study the time domain inversion method. The helicopter impact load was simulated by falling body impact, and the impact load history and structural strain response were recorded by sensors. The grillage model impact load was calculated with different inversion methods, including the direct inverse, truncated singular value decomposition (TSVD), and Tikhonov regularization methods. The solution accuracy of different methods and number of sensors needed were compared. The results demonstrated that the Tikhonov regularization method based on four measurement points along with the L-curve determination criterion showed a better performance for capturing the impact load time history features.

Tibial cortical and trabecular variables together can pinpoint the timing of impact loading relative to menarche in premenopausal females.

Though relationships between limb bone structure and mechanical loading have provided fantastic opportunities for understanding the lives of prehistoric adults, the lives of children remain poorly understood. Our aim was to determine whether or not adult tibial skeletal variables retain information about childhood/adolescent loading, through assessing relationships between cortical and trabecular bone variables and the timing of impact loading relative to menarche in premenopausal adult females. Peripheral quantitative computed tomography was used to quantify geometric and densitometric variables from the proximal tibial diaphysis (66% location) and distal epiphysis (4% location) among 81 nulliparous young adult female controls and athletes aged 19-33 years grouped according to intensity of impact loading both pre- and post-menarche: (1) Low:Low (Controls); (2) High:Low; (3) High:High; (4) Moderate:Moderate; (5) Low:Moderate. ANCOVA was used to compare properties among the groups adjusted for age, stature, and body mass. Significant increases in diaphyseal total cross-sectional area and strength-strain index were documented among groups with any pre-menarcheal impact loading relative to groups with none, regardless of post-menarcheal loading history (p < .01). In contrast, significantly elevated distal trabecular volumetric bone mineral density was only documented among groups with recent post-menarcheal loading relative to groups with none, regardless of pre-menarcheal impact loading history (p < .01). The consideration of diaphyseal cortical bone geometric and epiphyseal trabecular bone densitometric variables together within the tibia can identify variation in pre-menarcheal and post-menarcheal impact loading histories among premenopausal adult females.

Impact toughness of closed cell aluminum foam

The impact toughness of closed-cell aluminum foam with various densities was investigated using Charpy impact. The impact load history revealed an elastic region followed by a rapid load drop region. The peak load and impact toughness of aluminum foam increase exponentially with density. The power exponents for the impact toughness test are greater than that of the compressive test. Fracture analysis indicated a mixed-rupture mode of quasi-cleavage and small shallow dimples. It can be attributed to the complex state of stress of notched specimens and elevated impact velocity under impact loading. Copyright © 2018 VBRI Press.

Effect of Impact Load on Splice Length of Reinforcing Bars

Impact loading damage of reinforced concrete (RC) members deteriorates bond strength of reinforcing bars. To understand the effect of strain rate on the bond strength of reinforcing bars in RC beams under impact load, drop hammer test was performed on twenty-four simply supported RC beams with lap spliced bars at the mid-span. The test parameters were reinforcing bar diameter, splice length, drop height, and hammer mass. The dynamic responses including the impact load history, mid-span deflection history, crack distribution, and strain history of reinforcing bar were evaluated. Although the designed bar development length was 31–69% of the requirement of current design codes under static load, the tensile strength of bar splices was greater than the dynamic yield strength when subjected to large impact energy under impact load. On the basis of the test results, existing design equations for the bar development length under static load were modified to consider the impact loading effect on the bond strength. Factors related to the strain rate effect of materials, impact damage, and impact energy loss were proposed. The prediction of the proposed method agreed well with the tensile strength of bar splices under impact load.

Low velocity impact performance of curved steel-concrete-steel sandwich shells with bolt connectors

Abstract The low velocity impact performance of curved steel-concrete-steel (CSCS) sandwich shells with bolt connectors subjected to a hemi-spherical head drop hammer was investigated in this paper. Experimental results including impact force history, displacement history and permanent deformation were analyzed to reveal the influences of concrete core and steel plate thickness as well as shear connectors’ spacing on the impact performances of CSCS shells. The impact process consisted of three stages, i.e., inertial stage, loading stage and unloading stage. Meanwhile, three failure types were observed from the nine specimens subjected to impact load. They were plastic deformation of steel plate without fracture (Failure type I), fracture of steel plate (Failure type II) and penetration of steel plate (Failure type III). The presence of bolt connectors was proven to be an effective way to prevent the detachment of steel plates from concrete core. Moreover, finite element (FE) method was used to simulate the CSCS shells subjected to impact load. The accuracy of the FE model was verified by comparing the impact load history, displacement history and failure type from FE simulations with test results. It turned out that concrete core was the main part to dissipate impact energy, followed by top steel plate and bottom steel plate.

Influence of the stacking sequence on the low-energy impact resistance of flax/PA11 composite

In this paper, the low-energy impact behavior of a fully biobased composite made of bio-sourced polyamide 11 resin reinforced with flax fibers was investigated. Different composite laminates were studied in order to determine the stacking sequence effects on the impact behavior of these composites. Four stacking sequences were manufactured: unidirectional [0°]8, cross-ply [0°/90°]2s, sandwich-like [02°/902°]s and quasi-isotropic [45°/0/−45°/90°]s. A low impact energy of 3.6 J was applied on these laminates by means of a drop weight impact tower. The impact properties of these lay-ups were ascertained by analysing the impact load history, the maximal displacement of the impactor and the absorbed energy. Damage after impact was further assessed by visual inspections, topographic measurements, C-scan and X-ray micro-tomography observations. The results show that impact damage of composite plates is highly influenced by fiber orientation. The impact test data are in good agreement with damage analysis after impact and indicate that stacking plies in the same orientation lead to a larger induced damage, which is responsible for energy dissipation. The quasi-isotropic composite has the smallest induced damage and the highest peak load. Otherwise, the sandwich-like sequence shows the lowest peak load, the highest energy absorption and significant induced damage. Therefore, it is necessary to choose the most suitable lay-up, in terms of impact behavior, for each considered industrial application.

Impact Toughness of Closed-Cell Aluminum Foam

The impact toughness of closed-cell aluminum foam with various densities was investigated. The impact load history revealed an elastic region followed by a rapid load drop region. The peak load and impact toughness of aluminum foam increases exponentially with density. The power exponents for impact toughness test are greater than that for compressive test. Fracture analysis indicated a mixed-rupture mode of quasi-cleavage and small shallow dimples. It can be attributed to the complex state of stress of notched specimens and elevated impact velocity under impact loading.

Design and simulation of innovative foam-filled Lattice Composite Bumper System for bridge protection in ship collisions

In this paper, an innovative Foam-filled Lattice Composite Bumper System (FLCBS) with fiber-reinforced polymer (FRP) skins and a foam-web core is proposed as protective structures for bridge piers against ship collision. FLCBS consists of FRP face sheets, FRP lattice webs and polyurethane (PU) foam cores, which can be manufactured at the same time by vacuum assisted resin infusion process (VARIP). FLCBS exhibits good energy-absorbing and highly designable properties. The design, performance evaluation, manufacture and installation of FLCBS for a real prestressed concrete continuous girder bridge were introduced. A detailed numerical model for performance evaluation was developed with the explicit finite element (FE) software package ANSYS/LS-DYNA. An experimental study of two foam-filled lattice composite panels under low-velocity impact was conducted to verify the numerical model for FLCBS. The corresponding FE models of the composite panels generally showed good agreement, compared to the impact load history and mid-span deflection history of experimental results. Based on the validated numerical model, both head-on collision cases and lateral collision cases with and without FLCBS were investigated. Simulation of different impact angles, velocities and locations were carried out. Numerical results indicate the obvious advantages of FLCBS by comparing peak impact force and impact duration. Significantly decrement of the peak impact force and effectively prolonged impact process indicate the superior performance of FLCBS. The modular fabrication and replacement, easy and time-efficient installation and highly designable structure make FLCBS very attractive as bridge protection in ship collisions.

Impact loading history modulates hip fracture load and location: A finite element simulation study of the proximal femur in female athletes

Sideways falls impose high stress on the thin superolateral cortical bone of the femoral neck, the region regarded as a fracture-prone region of the hip. Exercise training is a natural mode of mechanical loading to make bone more robust. Exercise-induced adaptation of cortical bone along the femoral neck has been previously demonstrated. However, it is unknown whether this adaption modulates hip fracture behavior. The purpose of this study was to investigate the influence of specific exercise loading history on fall-induced hip fracture behavior by estimating fracture load and location with proximal femur finite element (FE) models created from magnetic resonance images (MRI) of 111 women with distinct exercise histories: 91 athletes (aged 24.7 ± 6.1 years, >8 years competitive career) and 20 women as controls (aged 23.7 ± 3.8 years). The athletes were divided into five groups based on typical loading patterns of their sports: high-impact (H-I: 9 triple-jumpers and 10 high jumpers), odd-impact (O-I: 9 soccer and 10 squash players), high-magnitude (H-M: 17 power-lifters), repetitive-impact (R-I: 18 endurance runners), and repetitive non-impact (R-NI: 18 swimmers). Compared to the controls, the H-I, O-I, and R-I groups had significantly higher (11–26%, p < 0.05) fracture loads. Also, the fracture location in the H-I and O-I groups was significantly more proximal (7–10%) compared to the controls. These results suggest that an exercise loading history of high impacts, impacts from unusual directions, or repetitive impacts increases the fracture load and may lower the risk of fall-induced hip fracture.

The measurement of dynamic strain and resonant frequency for three-dimensional solids partially immersed in water using free-edge bonded fiber Bragg grating sensors

In this work, the feasibility of liquid level detection based on structural vibration modes is investigated. We specifically consider impact-induced vibration modes of an aluminum solid partially immersed in a water tank. A free-edge bonded fiber Bragg grating (FBG) sensor and a corresponding high-speed FBG filter-based demodulation system are employed to measure the transient impact responses. To perform more accurate in vacuo finite element analysis (i.e., for the mode shapes, resonant frequencies, and the impact responses), elastic constants of the aluminum solid and impact loading history of a steel ball are obtained beforehand respectively according to the elastic wave theory and with a polyvinylidene fluoride (PVDF)-based technique. Our experimental results show good correspondences of intensities and shifts of the impact-induced resonant modes with liquid levels. The modes of the same type (e.g., double modes in out-of-plane or in-plane dominated modes) are shown to possess similar behaviors in relationships between shifts of the resonant modes and the liquid level. This work also provides related experimental evidence for future researchers interesting in simulation of complicated fluid-structure interaction (FSI) problems for three-dimensional structures.

Simultaneous measurement of dynamic displacement and strain in a single fiber using coarse wavelength-division multiplexing and fiber Bragg-grating filter-based sensing system

Displacement and strain, two of the most important physical quantities in experimental solid mechanics, are seldomly measured simultaneously in a single experimental configuration. In order to provide and improve corresponding sensing techniques, an experimental setup system for simultaneous measurement of dynamic displacement and strain on a flexible cantilever beam using two fiber Bragg gratings (FBGs) in a single fiber is proposed. To realize high-speed multiplexing and demodulation, a configuration incorporating a coarse wavelength-division multiplexing (CWDM) technique and an FBG transmission filter is implemented. The cantilever beam is subjected to steel-ball impact from which the dynamic multipoint displacement/strain sensing performances of the CWDM and FBG filter-based sensing system are demonstrated. Experimental results in temporal and frequency domain are compared with those obtained by the finite element method (FEM) predictions based on identification of the impact-loading history. A noncontact Fotonic displacement sensor and a polyvinylidene-fluoride film (PVDF) strain sensor are also used for comparison. With transient and resonant frequency simulations conducted by the FEM, loading effects of the sensing system are examined. The results obtained in this study indicate that the proposed CWDM and FBG filter-based sensing system is capable of performing simultaneous multipoint displacement/strain measurements in a single fiber with large bandwidth, high sensitivity, and low intensity loss.

Investigation of Transient Behavior of a Cantilever Plate Based on Elastic Impact Loading History Detection Using Polyvinylidene Fluoride Film Sensors

In this paper, we investigate elastic impact loading history detection based on polyvinylidene fluoride (PVDF) film sensors and demonstrate its applications on simulating flexible structural responses or investigating dynamic measurement performances of sensors. A pair of PVDF film sensors, bonded collocatedly on two opposite surfaces of a cantilever plate, is used to extract steel ball-generated elastic impact loadings. Impact detecting performance is first demonstrated by comparing the transient displacement responses of the cantilever plate with finite-element method (FEM) prediction based on the obtained impact loading history. Transient strain responses of the plate, also detected by the PVDF film sensors, are then compared with the FEM computation to discuss the cross-sensitivity effect (i.e., influence of the strain in orthogonal direction) of the PVDF film sensors. In addition to the comparison of the time-domain measurement results, sensing results in frequency domain are discussed with the nodal lines in the plate mode shapes. The experimental results in both time and frequency domain show that the commercial PVDF film sensors have the capability for obtaining elastic impact loading history on flexible structures that can be employed for further dynamic structural or dynamic sensing performance investigation.

Investigating dynamic transient behavior of Timoshenko cantilever beam by theoretical analysis, numerical calculation and experimental measurement

Polyvinylidene fluoride (PVDF) is a piezoelectric polymer that has been used in many sensing applications. To precisely determine the impact loading history, a special technique is developed by attaching the PVDF film on the top and bottom surfaces of a cantilever beam where a dynamic impact is applied directly on the top side. The Laplace transform method is employed to investigate the transient response of the displacement and strain for a Timoshenko cantilever beam subjected to impact loading. The Durbin method is used to perform the Laplace inverse transformation to construct the transient responses for displacement and strain in the time domain. After confirming the correctness of the experimental boundary conditions, the impact loading history obtained from the experimental measurement is used for the Timoshenko and Bernoulli–Euler beam theories to investigate the transient responses for different slender ratios. The strain and displacement of cantilever beams are measured experimentally by PVDF films and fiber optics sensors separately. A three-dimensional finite element cantilever beam model is implemented to perform the numerical calculation of the transient response. By comparing the theoretical analysis and the finite element simulation against measurements from corresponding experiments, this research shows that the transient response of strain and displacement for a cantilever beam can be accurately evaluated using the Timoshenko beam theory if the slender ratio is greater than 50.

Real-Time Impact Monitoring of Composites for Ship Applications

Composites are getting more and more attention in the usage of major structures on ships owing to many advantages such as high specific strength and rigidity. However, composites are sensitive to impact, which indicates that the load-carrying capacity and stability of composite structures will decrease dramatically even for an invisible impact damage. Hence, a real-time impact monitoring system which has already been applied in aviation industry is important for reliability of ships, which inevitably encounter impacts during their lifetime due to different sources. This paper deals with the realization of an operational processing scheme to monitor the impact on composite structures, and estimate the impact location and load history by power distribution approach and system identification method. A PZT (piezoelectric lead zirconate titanate) sensor network was designed and employed to collect sensing signals for monitoring the impact. In the monitoring process, stress wave signals caused by impact were captured, and used to determine the impacted location and load history. Results demonstrate that the proposed method can be successfully applied in identifying the location and load history of impact on composite structures. The impact identification approach is of great significance for marine applications.

Impact source identification in finite isotropic plates using a time-reversal method: experimental study

An impact source identification technique for finite isotropic plates using a time-reversal method with associated signal processing algorithms is investigated experimentally. A series of low velocity impact experiments on an aluminum plate is performed to verify this impact identification method using merely four piezoelectric sensors. Validation of the experimental system for impact source identification using the time-reversal method is first examined. Critical issues concerning the spacing of calibrated points, and associated problems, have been studied in detail. The appropriate spacing of the transfer function is determined and both the impact location and impact loading history are well estimated. The factors which affect the accuracy of impact location detection and force reconstruction are analyzed. The results of the impact identification experiments are repeatable and stable. The time-reversal method is suited to identifying impact events in plate-like structures.

Design Concept for Local Damage of Concrete Plates Subjected to High Velocity Impact

Local damage of concrete structures, i.e. spalling, scabbing and perforation, would occur when concrete structures are impacted by high velocity projectiles. To propose protective design of concrete structures against high velocity impact, failure mechanism of local damage of concrete plates should be investigated. The authors have studied on failure mechanism of the local damage through impact tests and numerical analyses. This study presents a design concept for the local damage of a concrete plate subjected to high velocity impact. In the design procedure, occurrence of scabbing is judged based on comparison between maximum impact load and scabbing resistant capacity of the concrete plate. Impact load history and penetration depth are calculated using the modified theoretical model, and scabbing resistant capacity is estimated by applying a modified formula for punching shear resistant capacity.

Impact fracture behavior at the material of aluminum foam

Abstract In this work, impact tests with drop weights are performed on closed-cell aluminum foam and the experimental results are compared with the simulation results. The finite element software, ABAQUS is used for the impact analysis. For the tests, low velocity impact test equipment from Dynatup conforming to ASTM D 3763 is used. The low velocity impact damage behavior of aluminum foam is studied by comparing the numerical analysis and experimental result data. The numerically estimated damages, impact load history, and dissipated energy for penetration are compared with results of the experiments. Dissipated energy results show good agreement between experiment and simulation with errors of less than 6.7%. The characterization of the aluminum foam on the basis of impact velocity and energy absorption is also investigated. The important factor affecting material penetration for aluminum foam is the impacting velocity as well as the dissipated energy of a material. A good method for examining minimum energy absorption ability is the impact test with lower mass and higher velocity, to avoid bending deformation mode.