Post-impact Behavior Research Articles

Effect of Braiding Angle on the Impact and Post-Impact Behavior of 3D Braided Composites

In this study, the impact and post-impact behavior of three-dimensional (3D) four-directional carbon/epoxy braided composites having different braiding angles were investigated. The same impact energy (45 J) was applied to the specimens. The post-impact mechanical properties of the materials were performed by compression after impact (CAI) testing, and the processes were monitored by the acoustic emission (AE) technique. Results showed that the specimens with larger braiding angle sustained higher peak loads, and smaller impact damage area, mainly attributed to a more compact space arrangement. The CAI strength and damage mechanism were found to be mainly dependent on the axial support of the braiding fiber tows. Increasing the braiding angle of the composites, the CAI strength was reduced, and the damage mode of the composites was changed from transverse fracture to shear one. Combining AE parameters and CAI curves allows one to characterize the failure process, thereby enabling fracture analysis of the materials under study.

Design Guidelines in Nonconventional Composite Laminate Optimization

Nonconventional laminates, as defined in this paper, are laminates where ply angles are not restricted to a finite set: for example 0, 45, and 90 deg. Removing this restriction, structural behavior (for example, postimpact behavior) can be significantly improved. Nonconventional laminates are made of straight or steered fibers leading to constant- or variable-stiffness composites. In traditional composite design, empirical guidelines are imposed, guaranteeing the robustness of the composite. This paper presents a method to take design guidelines into account during nonconventional laminate optimization. The 10% rule is interpreted as a lower bound on the degree of isotropy and formulated as a positive-semidefinite matrix constraint. Other guidelines are interpreted as bounds on ply angle or angle difference, or the number of variables is reduced by fixing variables to user-defined values. Numerical results optimizing a plate under biaxial tension for strength demonstrate the optimizer generates nonconventional laminates obeying all guidelines, performing at least as well as conventional composites. For variable-stiffness laminates, a flat plate under uniaxial compression is optimized for buckling under a stiffness constraint. Applying the 10% rule, almost half of the improvement over conventional laminates is lost compared to the unconstrained case, demonstrating the importance of including design guidelines during nonconventional laminate optimization.

Impact and post impact behavior of fabric reinforced geopolymer composite

Impact and post impact behavior of fabric reinforced geopolymer composites is investigated in this study. Carbon, E-glass and basalt fabrics are incorporated in the geopolymer matrix. The geopolymer composites are tested for impact resistance to an out-of-plane low velocity impact test, followed by post impact performance testing using residual strength measurement on the sample. Impact properties such as impact energy, energy absorption capacity, and damaged area of the composites were examined for fabric reinforced geopolymer composite samples. The drop weight impact tests were performed on composite samples of 15×15cm. The quality of the samples was examined using C-scan in ultrasonic vibration mode and μCT before and after impact test. The post impact behavior of the composite samples was characterized using four point bending tests. Fiber orientation and alignment was observed using SEM. The results obtained showed that E-glass fabric reinforced composites show slightly better performance than carbon reinforced composites. Especially the damaged area is smaller for E glass composites. Basalt fabric reinforced composites have low bending strength and very low impact resistance. The energy absorption in E-glass is due to post-debond fiber sliding phenomena and in carbon fiber to fiber pull out from the composite. The smaller damaged area is attributed to the higher elastic energy absorption of E-glass fibers. The residual strength of carbon and E-glass composites decreased in relation to the size of the visually damaged area, meaning that no invisible damage took place.

Experimental Validation of a Mechanistic Multibody Model of a Vertical Piano Action

The validity and accuracy of a high-fidelity mechanistic multibody model of a vertical piano action mechanism is examined experimentally and through simulation. An overview of the theoretical and computational framework of this previously presented model is given first. A dynamically realistic benchtop prototype mechanism was constructed and driven by a mechanical actuator pressing the key. For simulations, a parameterization based on geometric and dynamic component properties and configuration is used; initial conditions are established by a virtual regulation that mimics a piano technician's procedure. The motion of each component is obtained experimentally by high-speed imaging and automated tracking. Simulated response is shown to accurately represent that of the real action for two different (pressed) key inputs using a single fixed parameterization. Various specialized model features are separately evaluated. Proper simulated dynamic behavior supports the accuracy of the friction representation; this is especially so for softer key inputs which demand a more actively controlled playing technique. The accuracy of the contact model is confirmed by the proper timing and function of the mechanism, as the relationship between components is strongly dependent on the state of compression of the interface between them. The value of including three flexible components is weighed against their significant computational cost. Compared to a rigid fixed ground point target, hammer impact on a compliant string reduces impact force, contact duration, and postimpact hammer velocity to improve accuracy. Flexibility of the backcheck wire and hammer shank also strongly affects postimpact behavior of the mechanism. The sophisticated balance pivot model is seen to be valuable in creating a realistic key response, with compression of felt balance punching and lift-off of the key, very important for achieving the proper key–hammer relationship. Finally, two components unique to the vertical mechanism—the bridle strap and butt spring—are shown to be essential in controlling the hammer for detached key inputs, where the key is released before it has reached the front punching. Accurate postimpact response is important for proper simulation of repeated notes, as well as the “feel” of the action. In general, the results reported can be considered as a validation of the method for constructing and parameterizing a dynamically accurate multibody model of a specific prototype mechanism or system including compliant contacts and flexibility of some components, as well as ad hoc components to cover unusual dynamic behavior.

Inversion of a Capsule Impacting Water: Flip by Resurge Jet

A scale-model blunt-cone capsule intended for ocean splashdown was projected into a water pool to evaluate impact loads and postimpact behavior. In a small region of the speed/angle parameter space, the capsule would reproducibly capsize, flipping forward (pitch-down), despite a pitch-up motion induced at impact. Inspection of high-speed video shows that the resurging central jet of the impact cavity is responsible. Capsize occurs when this jet is energetic enough (for which we develop a simple criterion), and is timed to lift the trailing edge of the vehicle. The same phenomenon was observed on the Apollo capsules, and may be relevant for lifeboat deployment from ships and offshore platforms.

The Influence of Ship Collision Parameters on the Global Nonlinear Dynamic Response of a North-Sea Jack-Up Platform

In the present study, the influence of ship collision parameters related to vessel impact energy and also the impact scenario on the global nonlinear dynamic behavior of a three-leg jack-up platform is investigated. A North-Sea three-leg jack-up platform is studied as a case which is located in water depth of about 105 m. Nonlinear elastoplastic and hyperelastic type spring models are used for ship bow and broad-side impacts. A nonlinear elastoplastic type spud-can-soil interaction model is also applied. Loose sand to medium dense sand profile is considered at the sea-base. The effects of ship collision parameters such as ship mass and velocity, impact direction, hit point on jack-up as well as the spud-can-soil interaction are studied. For the first time, the supply vessel impact energy level far beyond 14 MJ as conventionally applied for ship-jack-up leg collision analysis (i.e., higher energy impact) has been also considered. The findings of this study indicated that the type of bow or broad-side impact as well as the spud-can-soil interaction may have considerable effects on the nonlinear dynamic behavior of the jack-up platform during ship collision. It is also found that ship collision in the direction of incident waves with mass of 5000 tons and impact velocity of 5 m/s displacement may have more profound effect on global dynamic response of the jack-up platform near ultimate collapse. It is also found in this research work that dynamic postimpact behavior of the jack-up platform may be greatly influenced by the combined action of extreme wave and current.

Impact and post impact (CAI) behavior of stitched woven–knit hybrid composites

In this study, the effect of stitch pattern on the impact and post impact (CAI) behavior of eight ply woven–knit hybrid composite plates consisting of 2D woven fabrics as outer layers and rib knitted fabrics as inner layers was investigated. Layers of the woven–knit hybrid composite plates were sewed in circular, square and diamond shape through thickness in order to prevent delamination and slippage between the layers. Impact and post impact testing were carried out in order to determine the impact and post impact resistance of three different stitched composite structures. Results showed that the impact strength of composite laminate increased 5.5%, 11% and 22% in the case of square, diamond, and circular stitching, respectively. It was also found that the woven–knit hybrid composites with square pattern have the highest CAI strength.

Comparison of the Post-Impact Behavior of Tubular Braided and Filament Wound Glass/Polyester Composites under Compression

In this study, different tubular braided and filament wound structures were produced using a Maypole braiding machine with glass fiber and then consolidated with unsaturated polyester resin using the Vacuum Infusion Process (VIP). For experimental tests, half of the samples were impacted in the center with 2.74 J impact energy. Then the compression test was conducted on all of the specimens. Graphs of force-elongation were obtained and the failure works of the specimens were calculated. It was observed that non-impacted filament wound tubular composites have 34% higher failure work than those of their braided counterparts with the same fiber volume fraction (FVF), which is due to the existence of more reinforcing layers in filament wound structure compared with braided composites. The braided composites with the same number of layers as filament-wound ones have higher FVF and higher wall thickness, hence, show higher compression properties. However, the drop of compression properties after the impact load is significantly higher in filament wound composites compared with the braided ones with the same FVF. Therefore it can be concluded that the interlacing structure of fibers in braided composites controls the delamination and restricts the propagation of cracks in the structures after being subjected to impact loads, and causes a less drop in the compression properties of the composites.

The effect of stacking sequence on the impact and post-impact behavior of woven/knit fabric glass/epoxy hybrid composites

In this study, the effect of stacking sequence on the impact behavior of sequentially stacked woven/knit fabric glass/epoxy hybrid composites was investigated. 2D (double layer) fabric was used as a woven fabric and rib and milano fabrics were used as knitted fabrics. Woven and knit fabric layers were sequentially stacked in six different variations to fabricate eight ply woven/knit fabric hybrid composites. The composite laminates were processed by hand lay-up technique using 3mm thick spacers and cured under pressure of 8MPa for 100min. at 100°C, followed by cooling at room temperature under same pressure. The impact and post-impact (CAI) behavior of hybrid composites at various impact energies were investigated. FEA results were compared with experimental results. Results show that specimens having outer layer of woven fabric exhibited better impact properties than that of the specimens having outer layer of knitted fabric. The worst performance in terms of impact resistance was observed in knit/knit composites. FEA analysis was in a good agreement with the experimental results.

Investigation of knitting architecture on the impact behavior of glass/epoxy composites

In this study, the effect of impact and post-impact behavior of E-glass/epoxy composite plates having different knitted fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5–25J) were subjected to the plates consisting of eight layers of Plain [P]8, Milano [M]8, and Rib [R]8 knitted fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression after Impact (CAI) strength of layer fabric samples. The results of these impact and post-impact tests showed that the maximum contact force was observed in the [R]8 fabric and the minimum contact force was observed in the [P]8 fabric and the CAI strength reduced by increasing the impact energy.

Simulation of impact and post-impact behavior of carbon nanotube reinforced polymer using multi-scale finite element modeling

The impact analysis of carbon nanotube reinforced polymer (CNTRP) is performed using 3D finite element model. A multi-scale finite element model is employed consisting of single walled carbon nanotube (CNT), non-bonded interphase region and surrounding matrix. The discrete structure of CNT is modeled at nanoscale, while surrounding polymer is simulated at microscale. Impact and post-impact analysis of neat resin and CNTRP are conducted and the results are compared. The behaviors of CNTRP subjected to high strain rate loading in both axial and transverse directions are evaluated. The results revealed that even small portion of carbon nanotubes improve dynamic behaviors of polymer against impact loading in presence of non-bonded interactions in the interphase region.

Impact and post impact behavior of layer fabric composites

In this study, the effect of impact and post impact behavior of E-glass/epoxy composite plates having different layer fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5J–60J)were subjected to the plates consisting of eight layers of plain weave (1D), double (2D) and triple (3D) layer fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression After Impact (CAI) strength of layer fabric samples. The results of these impact and post impact tests showed that contact force occurring between the impactor and the composite specimen increased and the CAI strength reduced by increasing the impact energy. The objective of this study was to determine the perforation threshold of E-glass/epoxy composite plates having different layer fabrics as plain weave (1D), double (2D), and triple (3D) layer fabrics.

Solution to indeterminate multipoint impact with frictional contact using constraints

This work presents a method for determining the post-impact behavior of a rigid body undergoing multiple, simultaneous impact with friction. A discrete algebraic model is used with an event-driven function which finds impact events. In this work, the indeterminate nature of the equations of motion encountered at impact is examined. Velocity constraints are developed based on the rigid body assumption to address the equations and an impact law is used to determine the impulsive forces. The slip-state of each impact point is then determined and appropriate methods are used to resolve the post-impact velocities. These techniques are applied to a 3-D model of a ball which is forced to impact a corner between the ground and two wall planes. Additionally, a rocking block example is considered. Simulations are presented for 2-D and 3-D cases of the ball example, and a 2-D model of the rocking block problem to demonstrate the effectiveness of the proposed approach.

Response and Damage Tolerance of Composite Sandwich Structures under Low Velocity Impact

The deformation and failure response of composite sandwich beams and panels under low velocity impact was reviewed and discussed. Sandwich facesheet materials discussed are unidirectional and woven carbon/epoxy, and woven glass/vinylester composite laminates; sandwich core materials investigated include four types of closed cell PVC foams of various densities, and balsa wood. Sandwich beams were tested in an instrumented drop tower system under various energy levels, where load and strain histories and failure modes were recorded for the various types of beams. Peak loads predicted by spring-mass and energy balance models were in satisfactory agreement with experimental measurements. Failure patterns depend strongly on the impact energy levels and core properties. Failure modes observed include core indentation/cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core. In the case of sandwich panels, it was shown that static and impact loads of the same magnitude produce very similar far-field deformations. The induced damage is localized and is lower for impact loading than for an equivalent static loading. The load history, predicted by a model based on the sinusoidal shape of the impact load pulse, was in agreement with experimental results. A finite element model was implemented to capture the full response of the panel indentation. The investigation of post impact behavior of sandwich structures shows that, although impact damage may not be readily visible, its effects on the residual mechanical properties of the structure can be quite detrimental.

Thermal Post-Impact Behavior of Closed-Cell Cellular Structures with Fillers – Part I

The study describes the behavior of regular closed-cell cellular structure with gaseous fillers under impact conditions and consequent post-impact thermal conduction due to the compression of filler gas. Two dependent but different analyses types have been carried out for this purpose: (i) a strongly coupled fluid-structure interaction and (ii) a weakly coupled thermalstructural analysis. This paper describes the structural analyses of the closed-cell cellular structure under impact loading. The explicit code LS-DYNA was used to computationally determine the behavior of cellular structure under compressive dynamic loading, where one unit volume element of the cellular structure has been discretised with finite elements considering a simultaneous strongly coupled interaction with the gaseous pore filler. Closed-cell cellular structures with different relative densities and initial pore pressures have been considered. Computational simulations have shown that the gaseous filler influences the mechanical behavior of cellular structure regarding the loading type, relative density and type of the base material. It was determined that the filler’s temperature significantly increases due to the compressive impact loading, which might influence the macroscopic behavior of the cellular structure.

Thermal Post-Impact Behavior of Closed-Cell Cellular Structures with Fillers – Part II

The paper describes the post-impact thermal conduction of regular closed-cell cellular structure with gaseous fillers due to the dynamic compression. Two different but subsequent computational analyses have been carried out for this purpose. To define the behavior of the cellular structure under compressive dynamic loading, a unit volume element of the cellular structure has been analyzed with the explicit finite element code LS-DYNA by considering a strongly coupled interaction of the cellular structure base material with the gaseous pore filler. The resulting deformed cellular structure has then been imported in the finite volume code ANSYS CFX 10.0 for further weakly coupled thermal-structural analyses of post-impact heat conduction through the base material and filler gas. The increased temperature and pressure of the filler gas after compressive impact loading from the initial analyses have been used as initial conditions for the thermal analyses, where only the heat conduction due to the gas compression has been taken into account. This paper considers only the closed-cell cellular structure with two different relative densities and air inside the pores. Computational simulations have shown a low overall temperature increase of the cellular structure due to filler gas compression. The temperature increase of the base material is expected to be higher at lower relative densities. The presented procedure illustrates a convenient approach to solving strongly coupled fluid-structure interaction problems by considering also a weakly coupled thermal-structural solution, which can be used for a wide range of engineering applications.

Impact and post impact behavior of composite sandwich panels

Assessing the residual mechanical properties of a sandwich structure is an important part of any impact study and determines how the structure can withstand post impact loading. The damage tolerance of a composite sandwich structure composed of woven carbon/epoxy facesheets and a PVC foam core was investigated. Sandwich panels were impacted with a falling mass from increasing heights until damage was induced. Impact damage consisted of delamination and permanent indentation in the impacted facesheets. The Compression After Impact (CAI) strength of sandwich columns sectioned from these panels was then compared with the strength of an undamaged column. Although not visually apparent, the facesheet delamination damage was found to be quite detrimental to the load bearing capacity of the sandwich panel, underscoring the need for reliable damage detection techniques for composite sandwich structures.

Lessons Learned from Failures of the Building Envelope in Windstorms

Lessons learned from failures of the building envelope in windstorms are encapsulated in three principal findings. The building envelope is crucial to the performance of buildings in windstorms. Windborne debris is decisive in shaping the performance of the building envelope. Design attention should be given to postimpact behavior of building-envelope systems. Reviews of damage documentation, insurance records, and computer simulations of building failures establish the importance of the building envelope to satisfactory building performance. These reviews also establish the decisive role of windborne debris in causing damage. The imperative for considering the postimpact behavior of building envelope systems is discussed, and innovative glazing products that meet new design criteria are presented. It is concluded that the building envelope should be given status equal to the principal structural frame in terms of design attention.

An Experimental Study of Planar Impact of a Robot Manipulator

An experimental study of planar impact of a robot manipulator with a stationary rigid surface is presented. Using the data collected from a series of experiments, this paper investigates the post-impact behavior for different pre-impact conditions such as the configuration of the robot, the angle and the velocity of impact. A better understanding of the post-impact behavior for various pre-impact conditions can lead to improved control strategies during impacts in robotic manipulation. Potential applications of this study include robotic assembly and contact tasks. Further, this study could be seen as a preliminary experimental work on impact of general kinematic chains.

The Efficacy of the Momentum Balance Method in Transverse Impact Problems

An elastic-plastic contact law that incorporates local permanent deformation in the contact zone has been used to investigate the efficacy of the momentum balance method in transverse impact problems. The momentum balance solution is compared to the numerical solution of the same equations using the contact law. It is shown that the momentum balance method gives very good results for the post impact behavior of both the impacter and the beam. In certain cases, it is also shown to yield satisfactory results for the dynamic behavior during contact. It is demonstrated that the momentum balance method with an appropriate coefficient of restitution obtained from the contact law, is physically the same as using the contact law itself, with the difference that the local deformation is neglected.