Impact Response Research Articles

Transverse impact response of GFRP tubes filled with MWCNT-reinforced syntactic foam

Tubular composites made of glass fiber reinforced polymer (GFRP) have been widely used in the fields of infrastructure and automobile engineering. However, such composites are vulnerable to transverse loading occurrences and their effect on structural integrity. This paper focuses on investigating the response of hollow GFRP tubes and GFRP tubes filled with syntactic foam under transverse impacts. To enhance the stiffness of GFRP tube and improve the brittleness of syntactic foams, ribs were added to strengthen the tube wall, and multiwalled carbon nanotubes (MWCNTs) were used to reinforce the syntactic foam core. The impact response of the tubes was examined by considering the influence of ribs, MWCNTs, and the impact location. Experimental results revealed that the ribs contributed to enhancing the impact resistance of both hollow and filled tubes. However, the impact of ribs on energy absorption was less noticeable for the filled tubes. Incorporating 1.2 wt.% MWCNTs in the foam resulted in significant improvements of 25 % in peak impact force and 43 % in energy absorption for the filled tubes. Changing the impact location from the tube wall to the rib led to a significant increase of 33 % in peak impact force and 18 % in energy absorption for the hollow tubes. In contrast, the impact location had a minimal effect on the overall impact responses of the filled tubes. An analytical model was developed to predict the energy absorption capability of the composite tubes. This model accurately determined the energy absorption contribution of each component within the composite tube. The predicted results demonstrated good agreement with the experimental findings, validating the predictive capability of the model. Furthermore, multiscale finite-element (FE) models were developed to simulate the impact characteristics of foam-filled composite tube specimens. The validated FE model was subsequently utilized to conduct parametric study.

Predicting damage behaviors of composite laminates under multiple low‐velocity impacts

AbstractIn present investigation, experimental and numerical approaches were combined to investigate the distance between impact positions (DBIP) effects upon impact response and damage accumulation regarding carbon/glass hybrid laminates with multiple low‐velocity impacts. Force/energy‐time curves as well as force‐displacement curves with respect to the laminates with three various impact energies at four different impact positions were obtained from experiment data. Then C‐scans were made to quantify the damage of all the impacted specimens. Additionally, an integrated multi‐impact model was created considering both interlaminar delamination and intralaminar damage. Continuum damage mechanics model (CDM) was developed to calculate the impact resistance regarding laminates under various impact loadings with high efficiency. The simulation and experimental results agreed well. Moreover, the qualitative effects of impact positions on the laminates' mechanical responses were summarized utilizing predictive model that validated.Highlights The effect of the DBIP on the impact response and damage accumulation of carbon/glass hybrid laminates subjected to multiple low‐velocity impacts was studied. An integrated multi‐impact model was developed to predict the impact resistance of laminates under various impact loadings with high efficiency. The qualitative effects of impact positions on the laminates' mechanical responses were summarized using a validated predictive model.

Evaluation of the low-velocity impact response of high-performance multi-axial warp-knitted flexible composites

This article aims to investigate the dynamic behavior of high-performance multi-axial warp-knitted flexible composite materials under low-velocity impact tests through experiments and numerical simulations. In this paper, high-performance multi-axial warp-knitted flexible composites were prepared. Three different preparation processes, 175°C-5 min, 185°C-10 min, and 195°C-15 min, were designed for the multi-axial warp-knitted flexible composites. Studied the impact response of different preparation processes, initial impact energy, and punch shapes and diameters on materials. The results showed that the flexible composites prepared by various processes exhibit the same impact response curves in the impact resistance process, while the damage morphology and failure modes of the samples are different. Different initial impact energies caused multiple failure modes in the samples. The material showed penetration damage at high energy impacts and permanent depression damage at low energies. For different punch shapes, the impact resistance of materials to hemispherical punches is better than that of cylindrical punches. Numerical simulations were carried out using the finite element software ABAQUS. The custom material subroutine (VUMAT) based on the Hashin damage criterion model was implemented in the finite element program. The experimental and numerical simulation results agree regarding impact response characteristics. This paper analyzes the composite damage shapes, crack extensions caused by low-velocity impact tests and finite element simulation on multi-axial warp-knitted flexible composites. It provides a valuable reference for failure and structural optimization of multi-axial warp-knitted flexible composites for architectural applications.

Assessment of Pedestrians' Head and Lower Limb Injuries in Tram-Pedestrian Collisions.

Analysis of pedestrians' head and lower limb injuries at the tissue level is lacking in studies of tram-pedestrian collisions. The purpose of this paper therefore to investigate the impact response process and severity of pedestrians' injuries in tram-pedestrian collisions, using the Total Human Model for Safety (THUMS) pedestrian human body model together with the tram FE model. Two full-scale tram-pedestrian dummy crash tests were performed to validate the FE model, and the total correlation and analysis (CORA) score of head acceleration yielded values of 0.840 and 0.734, confirming a strong agreement between the FE-simulated head responses and the experimental head kinematics. The effects of different tram speeds and impact angles on pedestrians' impact response injuries and the differences were further analyzed. The results indicate that direct impact of the lower limb with the tram's obstacle deflector leads to lower limb bone shaft fractures and knee tissue damage. Neck fling contributed to worsened head injury. Coup contusions were the predominant type of brain contusion, surpassing contrecoup contusions, while diffuse axonal injury was mainly concentrated in the collision-side region of the brain. Pedestrians' injuries are influenced by tram velocity and impact angle: higher tram velocities increase the risk of lower limb and head injuries. The risk of head injury for pedestrians is higher when the impact angle is negative, while lower limb injuries are more significant when the impact angle is 0°. This study provides practical guidance for enhancing tram safety and protecting pedestrians.

Numerical investigation of the low velocity impact behavior of CFRP laminates

The impact response and failure mechanism of carbon fiber reinforced composite laminates under low velocity impact was investigated using ABAQUS®. A finite element model was developed based on the progressive damage theory to study the impact response of CFRP laminates with hemispherical impactor. The impact tests were conducted on cross-ply (0/90)2s and quasi-isotropic, QI (0/45/-45/90)s laminates with different impact velocities. From the numerical simulation, the damage initiation and propagation in the CFRP laminates were found in terms of damage location, size and shape. The intra-laminar damages such as fiber and matrix damages were predicted using in-built Hashin criterion of ABAQUS®. The traction separation based cohesive surface modelling approach was utilized for capturing the inter-laminar failure initiation and propagation induced by impact loading. The numerical results of different impact energy levels were analysed for different failure modes for varying impact force, displacement and kinetic energy. The total volume of fiber damage and the total area of delamination were calculated for each laminate sequence. From numerical simulations, it was observed that the delamination was less on cohesive surfaces above the mid-thickness plane due to high compression through the thickness. The fiber damage was found at 65 J of impact energy. The first load drop was observed in the cross-ply laminates due to the delamination between lamina 5 and 6. The study revealed that the QI laminates had higher impact resistance and smaller damage zone than the cross-ply laminates.

FURTHER DEVELOPMENT OF GDEM FOR THE MODELING OF MULTI-SCALE DYNAMIC RESPONSE OF ROCK SUBJECTED TO BLASTING AND IMPACT

The dynamical responses of rock subjected to blasting and impact have been of concern in most underground projects. Due to the size effects and strain-rate enhancement induced by inertial effects, the dynamic responses of rock and underground structures show multi-scale characteristics. Thus, in order to achieve better understanding of multi-scale dynamic responses of rocks, both computation accuracy and numerical efficiency have been taken into account. This is achieved by further development of a continuum-discontinuous element method code, called GDEM, with a new type of dynamic bounding-surface damage model being implemented, thus forming a bounding-surface dynamic plasticity damage-GDEM model (DB-GDEM). A dynamic bounding-surface damage model is adopted, with the strength surface of Holmquist-Johnson-Cook model being employed as bounding surface. The concept of critical state, strainrate hardening, and continuum damage evolution has been taken into account for both brittle and ductile responses of rock. The model performance has been demonstrated by a few numerical simulations, including the Landau blast model, explosions under various confinements, and trapezoidal impulse loading, etc. By comparing modeling results among DB-GDEM model, linear-elastic model in GDEM, and D-P model in GDEM, it is noted that the contribution of plasticity-damage can result in the decay of stress waves and deterioration of dynamic strength. Both continuous and discontinuous mechanical behaviors of rocks have been captured well by the newly developed DB-GDEM model. The main multi-scale dynamic characteristics of rock subjected to blasting and impact, including particle movement, fragmentation of rock mass, stress-wave propagation, and failure models, have been captured. The introduction of bounding-surface plasticity damage model in GDEM would reproduce a more realistic dynamic damage response of rock compared with the original GDEM model that embedded with conventional constitutive models.

Interactions between temperature and nutrients determine the population dynamics of primary producers.

Global change is rapidly and fundamentally altering many of the processes regulating the flux of energy throughout ecosystems, and although researchers now understand the effect of temperature on key rates (such as aquatic primary productivity), the theoretical foundation needed to generate forecasts of biomass dynamics and extinction risk remains underdeveloped. We develop new theory that describes the interconnected effects of nutrients and temperature on phytoplankton populations and show that the thermal response of equilibrium biomass (i.e. carrying capacity) always peaks at a lower temperature than for productivity (i.e. growth rate). This mismatch is driven by differences in the thermal responses of growth, death, and per-capita impact on the nutrient pool, making our results highly general and applicable to widely used population models beyond phytoplankton. We further show that non-equilibrium dynamics depend on the pace of environmental change relative to underlying vital rates and that populations respond to variable environments differently at high versus low temperatures due to thermal asymmetries.

Lessons should be learned: Why did we not learn from the Spanish flu?

COVID-19 has become a global pandemic that has affected millions of people worldwide. The disease is caused by the novel coronavirus that was first reported in Wuhan, China, in December 2019. The virus is highly contagious and can spread from person to person through respiratory droplets when an infected person coughs, sneezes, talks, or breathes. The symptoms of COVID-19 include fever, cough, and shortness of breath, and in severe cases, it can lead to respiratory failure, pneumonia, and death. The Spanish flu, caused by the H1N1 influenza virus, and the COVID-19 pandemic caused by the novel coronavirus SARS-CoV-2 are two of the most significant global health crises in history. While these two pandemics occurred almost a century apart and are caused by different types of viruses, there are notable similarities in their impact, transmission, and public health responses. Here are some key similarities between the Spanish flu and SARS-CoV-2. The Spanish flu pandemic of 1918-1919 stands as one of the deadliest pandemics in human history, claiming the lives of an estimated 50 million people worldwide. Its impact reverberated across continents, leaving behind a legacy of devastation and lessons that, unfortunately, seem to have been forgotten or ignored over time. Despite the advancements in science, medicine, and public health in the intervening century, humanity found itself facing a strikingly similar situation with the outbreak of the COVID-19 pandemic. Additionally, amidst the search for effective measures to combat COVID-19, novel approaches such as iodine complexes, such as Iodine-V has emerged as potential interventions, reflecting the ongoing quest for innovative solutions to mitigate the impact of pandemics. This raises the poignant question: why did we not learn from the Spanish flu?

Unraveling the effects of geometrical parameters on dynamic impact responses of graphene reinforced polymer nanocomposites using coarse-grained molecular dynamics simulations.

Nacre plays an important role in bionic design due to its light weight, high strength, and structure-function integration. The key to elucidate its reinforcing and toughening mechanisms is to truly characterize its multi-layer structure and properties. In this work, the dynamic impact responses of graphene reinforced polymer nanocomposites with a unique brick-and-mortar structure are investigated using coarse-grained molecular dynamics simulations, in which the interfacial coarse-grained force field between graphene and the polymer matrix is derived by the energy matching approach. The influences of various geometrical parameters on dynamic impact responses of the nanocomposites are studied, including the interlayer distance, lateral distance, and number of graphene layers. The results demonstrate that the impact resistance of the nacre-like structure can be significantly improved by tuning the geometrical parameters of graphene layers. It is also found that the chain scission and interchain disentanglement of polymer chains are the main failure mechanisms during the perforation failure process as compared to the stretching and breaking of bonds. In addition, the microstructure analysis is performed to deeply interpret the deformation and damage mechanisms of the nanocomposites during impact. This study could be helpful for the rational design and preparation of graphene reinforced nacre-like nanocomposites with high impact resistance.

Impact Responses and Wave Dissipation Investigation of a Composite Sandwich Shell Reinforced by Multilayer Negative Poisson's Ratio Viscoelastic Polymer Material Honeycomb.

This analysis investigated the impact wave response and propagation on a composite sandwich shell when subjected to a low-velocity external shock, considering hygrothermal effects. The sandwich shell was crafted using face layers composed of functional gradient metal-ceramic matrix material and a core layer reinforced with negative Poisson's honeycomb. The honeycomb layer consisted of a combination of viscoelastic polymer material and elastic material. The equivalent parameters for the functional gradient material in the face layers were determined using the Mori-Tanaka and Voigt models, and the parameters for the negative Poisson's ratio honeycomb reinforcement core layer were obtained through Gibson's unit cell model. Parameters relevant to a low-velocity impact were derived using a modified Hertz contact law. The internal deformations, strains, and stress of the composite sandwich shell were described based on the higher-order shear deformation theory. The dynamic equilibrium equations were established using Hamilton's principle, and the Galerkin method along with the Newmark direct integration scheme was employed to calculate the shell's response to impact. The validity of the analysis was confirmed through a comparison with published literature. This investigation showed that a multilayer negative Poisson's ratio viscoelastic polymer material honeycomb-cored structure can dissipate impact wave energy swiftly and suppress shock effectively.

Low-velocity impact response of the post-buckled FG-MEE plate resting on visco-Pasternak foundation: Magneto-electro-mechanical effects-based interaction analysis

Revealing the coupling of nonlinear behavior and the magneto-electro-mechanical effects in the impact responses of post-bucked magneto-electro-elastic structures contributes to the intelligent development of aircraft structural systems. The low-velocity impact responses of the post-buckled functional gradient magneto-electro-elastic(FG-MEE) plate resting on visco-Pasternak foundation are investigated. In the framework of the von Kármán-type nonlinear model considering post-buckling configurations, the low-velocity impact dynamics model of the post-buckled FG-MEE plate is constructed. The two-step perturbation method is developed to obtain the post-buckling equilibrium path induced by the magneto-electro-mechanical effects. Further, the higher-order form of the two-step perturbation method-Galerkin integral method is proposed for the post-buckled FG-MEE plate resting on visco-Pasternak foundation, acquiring the high-order truncated solutions of displacement, electric potential, and magnetic potential. Ultimately, the snap-through phenomenon of the post-buckled FG-MEE plate under low-velocity impact is captured, and the variation in the degree of coupling between nonlinear behavior and the magneto-electro-mechanical effects is systematically revealed.

The influences of ply interference on ballistic impact response and energy absorption behavior of multilayered woven fabrics

This paper presents a detailed experimental and numerical analysis aiming to investigate the ballistic impact response and energy absorption behavior of multilayered woven fabrics with different ply interference. Different levels of ply interference are provided by controlling the influencing factors such as angle-plying, stitching the constituent plies and spacing the adjacent plies. The three aforementioned designs were evaluated for their energy absorption capacity against ballistic impact with the impact velocities ranging from 300∼400 m/s. The energy absorption mechanism was studied through the combination of high-speed camera and numerical simulations in terms of fabric deformation, and evolution of energy absorption for each ply at different impact velocity. It was found that stitching the aligned systems showed the most pronounced beneficial effect on energy absorption, showing approximate 50 % ∼ 110 % increase than the quasi-isotropic system with a mismatched lay-up angle of 22.5º.This is probably because that thread stitching enables the panel to be more efficient in spreading energy to a more global extent, involving a more defined transverse deformation and a larger area of stress distribution, which is beneficial for energy absorption.

Impact response of filament-wound structure with polymeric liner: Experimental and numerical investigation (Part-A)

Filament wound pipelines and Type IV composite pressure vessels (CPVs) constitute polymeric liners and are extensively used to transport and store petroleum products, hydrogen, and compressed natural gas (CNG). The polymeric liner does not share much pressure load; hence, the composite layers share most of the load. The situation gets worse under transverse impact loads on such structures. For the polymeric liner to be effectively used in pipelines and CPVs, it is crucial to study impact response through testing and computational methods. This article presents experimental and numerical investigations of the transverse low-velocity impact response of filament wound samples. High-density polyethylene (HDPE) liner was adopted, and carbon fiber (T700) continuous filaments with epoxy resin were wound over the liner with several layers. A drop-weight impact loading with 40 J energy has been applied to the fabricated samples. The development of impact damage was assessed using the finite element method, and the damage modes have been discussed. The specimen remains unperforated at the chosen energy level. Though HDPE is ductile, however at impact loads liner damage was encountered, displaying a brittle fracture. At higher strain rates, the material reaches its brittle fracture point sooner, leading to failure. The material breaks as brittle due to its inability to dissipate impact energy quickly, resulting in fracturing instead of deformation. Fiber damage was scarcely seen; however, matrix damage has been the dominant failure mode at the chosen impact energy. Comparisons between the simulation and test findings were made, and they agreed on force-time and force-displacement histories.

Complementary Approaches to Planning a Restored Coastal Wetland and Assessing the Role of Agriculture and Biodiversity: An Applied Case Study in Southern Italy

The European Parliament has recently passed the “Nature Recovery” law to restore degraded ecosystems and prevent natural disasters as part of its “Biodiversity Strategy 2030” and “Green Deal”. In this respect, wetlands can provide a wide range of ecosystem services such as biodiversity conservation, hydrological land protection, provision of products, cultural and recreational benefits, and many others. However, they are still threatened by the expansion of agricultural land, overexploitation of water resources, water pollution, climate change, etc. Wetland conservation, however, is essential and requires coordinated action by managers, policymakers, stakeholders, and scientists. A systemic planning and design process is required to address these complex challenges. This research aims to outline an integrated, comprehensive, and well-structured planning framework for wetland systems that can be applied to different wetland types, in line with institutional wetland policy, governance, and management. The methodological approach developed in this study aims to integrate a longer-term strategy plan with a shorter-term action plan by combining the Yeomans scale of permanence and the Driver–Pressure–State–Impact–Response model. This innovative approach was applied to a specific case study and may guide further wetland planning in the future. The Nominal Group Technique was used, a consensus method aimed at achieving a general agreement and convergence of opinion. An expert group of seven members with different technical backgrounds was engaged and expert consultation was found to be a simple and rapid technique for carrying out wetland planning. The expert judgements were sound, consistent, and did not overlap (i.e., were not redundant). “Pressures” and “Impacts” were identified by the experts and clustered according to corresponding “States” and “Drivers”. Expert scoring allowed the resulting “Responses” to be ranked in terms of their relevance and influence on the development of the wetland strategy and action plan, while a priority order for their implementation was assessed according to the Yeomans scale of permanence. Agriculture was the highest rated ‘Driver’; similarly, Biodiversity (habitats and species) was the ‘State’ with the highest score. Therefore, their combination (agriculture and biodiversity) should be considered as the strategic cornerstone of the whole planning framework. This means designing and implementing a system in which agriculture and nature (in our case a wetland) are allied ecological systems in mutual compensation, according to the way natural elements are embedded in the agricultural system. A collection of factsheets containing the full list of responses considered in the Wetlands Action Plan, with detailed operational actions, is provided in the Appendixes.

The response of glass fibre reinforced PEKK laminates subjected to single and multiple impact loading

This paper presents a study on the impact behaviour of newly-developed composites based on S-glass fibre reinforced PEKK (GF/PEKK) composites subjected to single and repeated impact loading. The laminates, with a thickness of 2 mm and fibre weigh fraction of 60 wt%, were manufactured by stacking eight plies of GF/PEKK woven prepregs using a hot compression moulding technique, and impacted centrally using a drop-weight apparatus. Beginning with a single impact test, these samples were subjected to repeated impacts using the same projectile and energy until full perforation was achieved. During each test, the impact response characteristics, contact force, velocity, displacement and energy absorption with respect to time were measured and recorded. The results show that, for a single impact test, the absorbed energy increases significantly at low impact energies, while repeated impacts lead to a decrease in the values of peak force and stiffness of the composite panels. Here, up to 5 times of energy can be absorbed over multiple low energy impacts compared to that required to perforate the target with a single impact. For multiple impacts at different impact energy levels, accumulation of damaged area increases rapidly around the impact location up to perforation failure (e.g. the rate of damage from 2 mm2 per impact at 1 J case jumping to 25 mm2 per impact at 2 J case, further to 68 mm2 per impact at 3 J case). The rate of damage progression (per impact) appears to be related strongly to the rate of energy absorption. The continuous nature of the absorbed energy and damage progression over multiple impacts suggest that semi-empirical analytical relationships, similar to that used in a fatigue analysis, could be useful in providing a means of assessing impact damage in a laminated structure and predicting the remaining ‘impact fatigue’ life.

Data-driven modelling and evaluation of a battery-pack system’s mechanical safety against bottom cone impact

The exploration of the mechanical responses of the battery pack system (BPS) when subjected to dynamic impact loads is crucial for the safety of power batteries during collisions. To elucidate the mechanical response mechanism of the BPS under bottom impact, in this study, a series of finite element (FE) analysis tests were conducted in simulation environments with various combinations of the top radius of the cone model simulating objects, shell thickness, impact velocity, and impact angle. In the FE simulation, the BPS’s nonlinear model was developed and then validated through modal analysis. Subsequently, five machine learning (ML) models enhanced by optimized algorithms were used to evaluate the mechanical response to the bottom impact of BPS. The training data of the ML model is obtained through the optimal Latin hypercube experimental design based on the FE dynamic simulation process. By comparison between cases, the predictive capabilities of various ML models are examined and the robustness of these models are assessed through adding Gaussian noise to ML models. In conclusion, the results demonstrate that ML methods can effectively evaluate the mechanical response of BPS bottom impact and furnish critical design inputs for manufacturing next-generation batteries with durability and safety limitations.

Numerical study on dynamic responses and residual axial bearing capacity of square CFDST columns under lateral impact

This paper presented a numerical investigation into the dynamic responses and residual axial bearing capacity of square concrete-filled double-skin steel tube (CFDST) columns under lateral impact using ABAQUS. The finite element analysis (FEA) model for lateral impact and post-impact axial compression of square CFDST members was established, where the material nonlinearity and strain rate of steel tubes and concrete were considered and validated by existing experiments and studies. Based on the verified FEA model, the failure mechanism of the impact response process of the square CFDST member was revealed. To further explore the behavior of CFDST under axial compression after lateral impact, the typical axial load-axial displacement curves, axial strains of the steel tube, failure mode, and stress development of different components of the members were analyzed in depth. Parametric studies were conducted to clarify the influence of each parameter (including hollow ratio, slenderness ratio, concrete strength, and steel strength) on the residual capacity of post-impact axial compression in square CFDST members at different impact energies. Results demonstrated a linear correlation between residual axial bearing capacity coefficient and mid-span residual deflection after impact. Finally, a simplified formula for calculating the residual axial bearing capacity of square CFDST columns after lateral impact was proposed, which provided a reference for practical engineering applications.

IMPACT EFFECT ON THE EXISTING AND STRENGTHENED REINFORCED CONCRETE COLUMNS

Reinforced concrete (RC) columns are the main vertical elements in the buildings. Superstructure loads are transferred to the foundation by the columns whose design involves sufficient section sizes with longitudinal and transverse reinforcements. However, RC columns are required to be strengthened when low concrete quality and lack of reinforcement is stated. The most common strengthening technique for the RC columns is jacketing. In this method, strength of the existing column is improved by concreting and installing jacketing reinforcements. Although columns are designed according to several vertical and lateral load combinations, impact loads whose effects may reach very high values at short notice are not regarded. In the scope of this study, impact behavior of the existing and strengthened RC columns is numerically investigated. For this purpose, non-linear incremental dynamic analysis is carried out by Abaqus software that is widely utilized by engineers for such solutions. Acceleration, displacement and impact load values as well as crack patterns are obtained after numerical analysis. The results are comparatively presented for the existing and the strengthened columns. Analysis outputs reveal that proposed finite element models give an opinion about the impact responses of the RC columns. Consequently, it is evaluated that this study may make contribution to the researchers generating non-linear analysis without having to perform impact experiments that require high costs and heavy workloads in the laboratory conditions.

Characterization of low-velocity and low-energy responses of elastic-plastic plate struck by elastic-plastic impactor

This paper presents a study on the characterization of low-velocity and low-energy impact responses of elastic-plastic plate struck by elastic-plastic spherical impactor. The applicability of the existing impact characterization diagram is examined firstly by intensive finite element (FE) simulations for a wide range of complicated impact situations including all three types of contact: indentation, flattening and combined flattening and indentation. The limitations of the existing diagram are found in identifying the type of impact response and predicting the maximum impact force for complicated impact situations. The effects of elastic and plastic deformations of impactor on the identification and prediction by the existing diagram are analyzed. A new linear contact law is presented according to the piecewise linear characteristics of contact stiffness observed from the intensive FE results. An extended impact characterization diagram is then proposed to characterize low-velocity and low-energy impact responses of plate under complicated impact situations with arbitrary three contact types. The extended diagram is validated by the intensive FE simulations and experimental results. It is concluded that the extended impact characterization diagram can identify correctly the plate response type to complicated impacts and predict accurately the maximum impact force in a fast and simple way without the aid of FE models and experimental tests.

Dynamic characterization and damage analysis for the thermoplastic fiber-reinforced epoxy composites exposed to repeated low velocity impact

In the current study, contrary to conventional fiber-reinforced composites, polyamide fiber was used as reinforcement material, and the effects of thermoplastic fiber reinforcement on repeated low velocity impact (LVI) responses of composites were examined. In this regard, polyamide fiber-reinforced composites were fabricated using the vacuum-assisted hand lay-up method (VAHLM) and then exposed to repeated LVI loadings. Experimental tests were performed on the specimens for 100 impacts with a constant velocity of 3 m/s, which is equivalent to 25.2 J, and the impacts of repeated LVI loadings on the dynamic responses such as peak force, energy absorbing/rebounding, total impulse, bending stiffness and contact stiffness were examined. Moreover, the damage mechanisms resulting from the relative damage accumulation depending on the impact numbers were examined. According to the findings, the thermoplastic polyamide fibers absorbed more than 60% of the applied energy, and the absorbed energy increased with ascending impact number. Furthermore, the thermoplastic fiber-reinforced epoxy composites gained stiffness with increasing impact, which was linked to the thermoplastic chain structure. Despite quite a number of impact loadings, no serious damage mechanisms such as fiber breakage, perforation, or penetration were observed, and the specimens maintained their structural integrity. Due to the higher energy absorption of thermoplastics, the utilization of polyamide fibers in composites has been found to be well suited for applications subjected to repeated impacts.