Low-velocity Impact Tests Research Articles

Impact Energy Absorption of Sisal Fiber Reinforced Ferrocement Slab Elements and Its Statistical Relationship with Composite Compressive Strength

This study evaluates the level of energy absorption of sisal fiber reinforced ferrocement (SFRFC) slab elements of size 300 mm × 300 mm × 25 mm) (thickness) through low-velocity impact tests. The slabs are reinforced with 2 and 3 layers of steel weld mesh as primary reinforcement; and the sisal fibers of short and long fiber length, S-FL = 20 mm and L-FL = 40 mm (respectively) of varying volume fraction, (FVf of = 0.25–1.00%) act as secondary reinforcement. The compressive, split-tensile strength and flexural strength of hardened sisal fibrous mortar and plain (control) mortar specimens are determined and presented. The initial impact energy absorption (IIEA) and ultimate impact energy absorption (UIEA) of SFRFC slabs were found to be higher than ferrocement (FC-control) slabs (reinforced with 2 and 3 layers of steel mesh) demonstrating the effective contribution of sisal fibers in improving the performance of ferrocement slabs. When the length of the sisal fibers was increased from 20 mm to 40 mm in SFRFC slab elements (in hybrid combinations of steel mesh of 2–3 layers and fiber volume fraction of 0.25–1%), the impact resistance has increased several-fold over FC-control slabs. The composite compressive strength (CCS) of SFRFC has substantially increased when sisal fibers (of short and long fibers) of fiber volume fraction (FVf = 0.25–1%) are added to FC-control slab elements. Statistical prediction models have been developed separately for three dependent variables, IIEA, UIEA and CCS keeping constant the four independent variables, mesh layers (ML), fiber volume fraction (FVf), fiber length (FL) and mortar compressive strength (MCS), and the predicted values matched with the experimental results, within the acceptable error range. This study has statistically established a significant relationship between UIEA and CCS of sisal fiber reinforced ferrocement (SFRFC) slab elements.

Mechanical Properties of Bio-Based Sandwich Composites Containing Recycled Polymer Textiles.

In this paper, sandwich composites were produced by compression moulding techniques, and they consisted of regenerated cellulose fabric (rayon) and bio-based polypropylene (PP) to form facings, while virgin and recycled polyamide (PA) textiles were used as core materials. To compare the mechanical performance between sandwich composites and typical composite designs, a control composite was produced to deliver the same weight and fiber mass fraction from rayon and PP. To evaluate the influence of recycled textile on the mechanical properties of the composites, a series of flexural, low velocity impact (LVI) and tensile tests were performed. It was found that the incorporation of thicker PA textile enhanced the bending stiffness by two times and the peak flexural force by 70% as compared to those of control. Substitution of a layer of recycled textile for two layers of rayon provided a good level of impact energy absorption capacity (~28 J) and maximum force (~4893-5229 N). The tensile strength of the four sandwich composites was reported to be in the range of 34.20 MPa and 46.80 MPa. This value was 91.90 for the control composite. The 2D cross-section slices of the composite specimens did not show any evidence of fiber tow debonding, fiber bundle splitting, or delamination.

Effect of woven yarn angle on the low‐velocity impact damage response of fibre woven thermoplastic composites

AbstractFibre woven thermoplastic composites (FWTC) are widely used in aerospace and other fields because of their excellent performance. During service, FWTC structures are inevitably subjected to low‐velocity impact (LVI), which can cause invisible damage and eventual failure of the material. At the moment, studies on FWTC mostly focused on the orthogonal woven yarns while there's few reports about the effect of the yarn angle changing on the woven material's LVI damage response. This study aims at the effect of yarn angle changing on the damage behaviour of FWTC. A method for preparation of nonorthogonal prepregs was proposed, by which FWTC laminates with different yarn angles (60°, 75°, and 90°) were prepared for LVI tests. The results show that the maximum impact displacement and the impact duration of the impactor decrease with the decrease of the yarn angle when the FWTC laminate is subjected to LVI, while the maximum impact force shows an increasing trend. This indicates that the smaller yarn angle causes the better load‐bearing capacity of the FWTC laminate under LVI conditions, while the orthogonal FWTC laminate is more ductile. The damage morphology indicated by the impact of the FWTC laminate are matrix cracks and yarn breaks, and the damage area increases with the decrease of yarn angle, where the damage of orthogonal laminate is more serious more concentrated. The results found in this paper can provide useful guidance for engineering applications and failure analysis of FWTC.

Low-velocity impact response and compression behaviour after the impact of 3D-printed CCFR self-sensing honeycomb structures

Continuous-carbon-fibre-reinforced (CCFR) honeycomb structures with self-sensing abilities were fabricated by three-dimensional (3D) printing. Pure polylactic acid (PLA) honeycombs of the same size were also fabricated for comparison. Low-velocity impact (LVI) tests and compression after impact (CAI) tests were conducted. Cone beam computed tomography (CBCT) and scanning electron microscope (SEM) were employed to investigate the corresponding damage mechanisms. The LVI test results showed that the presence of continuous carbon fibre effectively inhibited the generation and propagation of the damage in the matrix during impact, which allowed the CCFR honeycomb structure to have better low-velocity impact resistance, especially for repeated impacts with small energies. In contrast to the PLA honeycomb, the CAI test results showed that the CCFR honeycomb still retained good specific compression strength and specific energy absorption properties even after experiencing impacts. Furthermore, the continuous carbon fibre integrated into the CCFR honeycomb could be used as a sensor element to realize the in-situ structural health monitoring during impact and compression. The 3D-printed CCFR honeycomb structure, investigated in this research, demonstrates remarkable mechanical performance and self-sensing capabilities both during LVI and CAI tests, which highlights the vast potential of 3D-printed CCFR honeycomb for diverse applications.

The effect of multiscale Kevlar/Glass hybridization on the mechanical properties and the damage evolution of 3D orthogonal woven composites under flexural and impact loadings

This work aims to evaluate the effectiveness of inter-ply/intra-tow hybridization in 3D woven composites to achieve an ameliorated impact and flexural performance. In this regard, inter-ply/intra-tow hybrid epoxy composites with Kevlar, E-glass, and ultra-high-molecular-weight polyethylene (UHMWPE) reinforced 3D fabrics are manufactured using the vacuum infusion method. To scrutinize the static and dynamic responses, the produced composites are subjected to flexural and low-velocity impact (50, 100, and 150 J) tests. The findings show that inter-ply hybrid composites exhibit more improved impact and flexural performance than their intra-tow hybrid counterparts. While the elastic moduli and strength values of weft-directional hybrid composites increase, on the contrary, warp-directional counterparts do not display an enhancement in flexural performance. Scanning electron microscopy (SEM) examinations and acoustic emission analysis are conducted together on the bending specimens to understand failure mechanisms leading to observed mechanical responses.

Numerical analysis of the impact behaviour of a composite eco-structure

An explicit numerical model is developed for the impact behaviour of a new composite eco-structure made of a plywood core and flax/epoxy composite skin. A constitutive orthotropic material behaviour is imposed to model the elastic response of plies of plywood core and flax skin. A stress-based modified Hashin-3D damage model with four independent failure criteria in tensile and compressive mode is incorporated for predicting the damage initiation. A strain threshold based exponential method is adopted to predict the damage scaling and its evolution. The above proposed model is implemented through user material subroutine (VUMAT) in ABAQUS Explicit. The developed model is validated with experimental low-velocity impact test results and can accurately predict the stiffness degradation, plateau, and post-peak softening of wooden plies. A good agreement with the better accuracy is achieved between the predictions and the test results.

Effect of nanosilica on low‐velocity impact and compression after impact properties of continuous carbon fiber‐reinforced epoxy composites manufactured by 3D printing

AbstractTo improve the low‐velocity impact (LVI) and compression after impact (CAI) properties of three‐dimensional (3D)‐printed continuous carbon fiber‐reinforced epoxy composites (CFRC), nanosilica (NS) was incorporated into these composites. NS (0.5 and 1 wt%) was dispersed in a liquid epoxy matrix by ultrasonication and mechanical stirring before mixing in a planetary mixer. The addition of NS up to 1 wt% did not render any processing barrier during the extrusion followed by the automated tape placement (ATP) method, adopted in this study. The resulting “hybrid” laminates showed void content <1% and had minimal internal defects as ensured by nondestructive technique (C‐scan) and scanning electron microscopy. The neat and NS‐loaded CFRPs were subjected to an impact energy of 6 J mm−1. The rebound energy increased by ~36% with the addition of 1 wt% NS as compared to unfilled samples, obtained from the LVI test. The CAI results suggest that with 1 wt% NS the residual compressive strength increased by ~13% in comparison with the neat CFRP laminates. After impact, the damage area was reduced significantly, that is, ~62% with the addition of 1 wt% NS in comparison with the neat CFRP laminates.

Low-velocity impact damage detection in cfrp laminates based on ultrasonic phased-array ndt technique

To investigate the delamination characteristics of CFRP laminates in Barely Visible Impact Damage (BVID) state, Low-Velocity Impact Tests (LVIT) and Non-Destructive Testing (NDT) were employed. The energy thresholds of six diverse ply-stacking sequences of laminates in the BVID state were established utilizing visual inspection and contact measurement methods. Ultrasonic phased-array NDT techniques were employed to evaluate quantitatively the magnitude of delamination damage from different viewpoints, including delamination area, shape, and trend variation. Furthermore, the impact resistance of laminates was qualitatively evaluated by analyzing the distinctive delamination damage forms of various specimens to identify the effects of the ply-stacking sequence. The findings of this study demonstrate that reducing the angle disparity between adjacent plies and minimizing the repetition of identical plies can mitigate delamination damage. Additionally, enhancing the ratio of ±45 and 0° plies can improve the flexural stiffness of the structure. Specifically, the ultrasonic phased-array NDT technique was shown to effectively detect delamination damage within the laminate in the BVID state.

Peridynamic simulation for the damage patterns of glass considering the influence of rate-dependence and pre-defects

This study presents a novel rate-dependent bond-based peridynamic model for predicting the mechanical behavior of brittle aluminosilicate glass under quasi-static and dynamic flexure loading. To achieve this, the damage criterion is modified with reference to the concept of strain rate in the classical continuum mechanics (CCM). Additionally, a peridynamic (PD) model with inhomogeneous critical stretch based on the Weibull distribution is proposed to account for the randomly distributed defects in glass and investigate their effect on crack initiation and propagation. The accuracy and rationality of the improved PD model are verified by comparing the calculated strength and fracture modes with the experimental results under quasi-static Ball-On-Ring (BOR) and dynamic low-velocity impact tests. The results show that the rate effect increases the damage threshold of glass, enhancing strength and damage ratio. Notably, flexural failures induced by tensile stress are observed under both static and dynamic loading. The rate-dependent inhomogeneous PD model indicates that glass inhomogeneity reduces strength while increasing crack propagation velocity (CPV). The introduction of pre-existing defects alters the symmetrical crack propagation pattern typically observed in homogeneous materials, resulting in a more randomized crack propagation path. Overall, the results can provide a promising approach for predicting the mechanical behavior of brittle materials.

Low-velocity impact behaviours of sandwiches manufactured from fully carbon fiber composite for different cell types and compression behaviours for different core types

Abstract This study aims to manufacture sandwich composite structures consisting of fully fiber-reinforced composites and then to investigate experimentally and numerically the impact and compressive strength and damage mechanisms of these structures for different cell and core types. To achieve this, firstly, composite sandwich plates with triangular, square and rectangular cell structures were manufactured. Low-velocity impact tests were run on sandwich structures with these three different cell geometries and the effect of cell shape on impact strength was determined. Although the maximum contact force of the square structure under impact load was smaller than the maximum contact force of the triangular and rectangular specimens, the specific contact force was 15.52 % greater than the triangular specimen and 16.29 % greater than the rectangular specimen. Then, the specimens with square, egg box and lattice core types were manufactured and compression tests were performed. The specific peak load value for square specimens was 33.2 % greater than that of egg box specimens and 78.1 % greater than that of lattice specimens. In the numerical study, the experimental results were confirmed by applying the progressive failure analysis with the LS-DYNA finite element program.

Repair of Impacted Thermoplastic Composite Laminates Using Induction Welding.

The lack of well-developed repair techniques limits the use of thermoplastic composites in commercial aircraft, although trends show increased adoption of composite materials. In this study, high-performance thermoplastic composites, viz., carbon fibre (CF) reinforced Polyetherketoneketone (PEKK) and Polyether ether ketone (PEEK), were subjected to low-velocity impact tests at 20 J. Post-impact, the damaged panels were repaired using an induction welder by applying two different methods: induction welding of a circular patch to the impacted area of the laminate (RT-1); and induction welding of the impacted laminates under the application of heat and pressure (RT-2). The panels were subjected to compression-after-impact and repair (CAI-R), and the results are compared with those from the compression-after-impact (CAI) tests. For CF/PEKK, the RT-1 and RT-2 resulted in a 13% and 7% higher strength, respectively, than the value for CAI. For CF/PEEK, the corresponding values for RT-1 and RT-2 were higher by 13% and 17%, respectively. Further analysis of the damage and repair techniques using ultrasonic C-scans and CAI-R tests indicated that induction welding can be used as a repair technique for industrial applications. The findings of this study are promising for use in aerospace and automotive applications.

Barely visible impact damage detection in composite structures using deep learning networks with varying complexities

Visual inspection is one of the most common non-destructive testing (NDT) methods that offers a fast evaluation of surface damage in aerospace composite structures. However, it is highly dependent on human-related factors and may not detect barely visible impact damage (BVID). In this research, low velocity impact tests with different energy levels are conducted on two groups of composite panels, namely ‘reference’ and ‘sensor-integrated’ samples. Then, the results of impact tests, together with C-scan and visual inspection images, are analysed to define the BVID range and create an original image dataset. Next, four different deep learning models are trained, validated and tested to capture the BVID only from the images of the impacted and non-impacted surfaces. The results show that all four networks can learn and detect BVID quite well, and the sensor-integrated samples reduce the training time and improve the accuracy of deep learning models. ResNet outperforms other networks with the highest accuracy of 96.2% and 98.36% on the back-face of reference and sensor-integrated samples, respectively. The proposed damage recognition method can act as a fast, inexpensive and accurate structural health monitoring tool for composite structures in real-life applications.

A novel design method of circular sandwich panels with gradient continuous controllable core based on space-filling design and 2D-Voronoi method

In this paper, a novel method based on the space-filling design and 2D Voronoi method was proposed to design the in-plane gradient continuous controllable honeycomb (GCCH) structures. Low-velocity impact tests were carried out using the sandwich panels with GCCH core layers, which were all made of Aluminium alloy material and fabricated with the wire-cut machine. A concise theoretical solution for loading capacity and denting profile was developed and verified. Three-dimensional finite element models were implemented in ABAQUS/Explicit to model sandwich panels.The FE models were validated with experimental results and, subsequently, the simulation results were further extended to study the impact response and energy absorption characteristics. Results show that the method proposed in this work was effective to design multi-cell structures with graded mechanical properties in the radius direction and approximately homogeneous in the circumferential direction by controlling the distribution of the sites. Sandwich panels with GCCH core layers had better energy absorption properties compared with sandwich panels whose core layers were hexagonal honeycomb with similar cell wall lengths.

Repeated healing of low velocity impact induced damage in orthogrid-stiffened sandwich panel

Herein, we present a new sandwich panel composed of a carbon fiber grid-stiffened shape memory vitrimer (SMV) core. The sandwich panels were fabricated via a pin-guided dry-weaving technology, and their impact responses were evaluated via low-velocity impact testing. The main failure mode observed after the first round of impact was the transverse cracking of the SMV matrix in the sandwich core. The healing efficiency according to the crack initiation energy (CIE) was found to be 76.5% after the first healing cycle. Even after the second healing cycle, the healing efficiency was greater than 72%. From the low-velocity impact tests, reinforcing the pure SMV core with a grid-skeleton enhanced the impact resistance significantly, that is, the crack initiation energy and peak load were increased by 64.0% and 169.0%, respectively. The results also show that smaller bay area leads to higher impact resistance. With the repeated crack healing, increased impact tolerance, and shape memory effect, it is expected that the sandwich panels will have a good possibility for usage in aerospace and automotive applications.

Effect of ageing conditions on the low velocity impact behavior and damage characteristics of aramid-basalt/epoxy hybrid interply composites

Polymer composites reinforced with aramid and basalt fibres are widely used in aviation, automobile, marine and civil structural components. In these applications, composites are often exposed to different environments involving temperature and humidity, which leads to the deterioration of mechanical properties and service life of the structure. This study aimed at investigating the influence of different ageing conditions on the low velocity impact (LVI) performance of 2-D aramid-basalt/epoxy interply composites. Compression molding method of fabrication was employed to prepare the hybrid laminates. LVI coupons were made as per the ASTM D3763 standard and aged in distilled water medium at three different temperatures namely, ambient condition (25 ℃), sub-zero condition (−10 ℃), and humid condition (40 ℃ and 60% relative humidity) until the specimens attain moisture saturation. After ageing, specimens were put through low velocity impact testing at two different impact energies of 10 J and 15 J and results were compared with the pristine specimens. Ultrasonic C-Scan and Scanning Electron Microscopy (SEM) were utilized to analyse the delamination due to impact and micro structural assessment of the pristine and aged specimens. Studies revealed that, specimens aged under sub-zero conditions showed better impact response compared to all other ageing conditions. Furthermore, fibre rupture, matrix decomposition, weakened fibre–matrix interface bonding and delamination were the major damages observed in aged specimens.

Influence of fibre orientation, moisture absorption and repairing performance on the low-velocity impact response of woven flax/Elium® thermoplastic bio-composites

The importance of natural reinforced bio-composite materials with desirable properties such as high modulus-to-weight ratio, good impact resistance, and the ability to be easily repaired is crucial in the industry. A woven flax/Elium® thermoplastic bio-composite is manufactured to challenge these needs. Elium® is the only resin that permits the manufacture of large parts with thermoset-like processes, as Liquid Resin Infusion presented in that work. The paper highlights the influence of stacking fibre orientation, moisture absorption, and repair aptitude on its impact resistance. An instrumented drop tower was needed to conduct low-velocity impact tests at several energies and high-speed image coupled with microscopic observations was used to assess the damage evolution. The impact resistance was improved for the dry (0/90)6 orientation, but moisture absorption decreased its impact peak force by 20%. A three-point bending test was preferred to compression after impact for studying the residual properties after impact. The bio-composites with (±45)6 orientation showed higher impact residual performance than the (0/90)6 orientation, and corresponded to the reduction of maximum bending force by 20% than that of the reference. In addition, a thermo-compression process was applied to repair the impacted plates and conduct multiple impact/repair cycles which showed a significant recovery of stiffness and maximum impact force at 4J on (0/90)6 plates, highlighting their potential for repair in the automotive and marine industries.

Low-velocity impact responses of composite structures incorporating shear thickening fluid tubes under various temperatures

This study proposes a novel approach to enhance the impact resistance of carbon fiber reinforced polymer (CFRP) composite plates by incorporating an additional layer consisting of shear thickening fluid (STF)-filled tubes, resulting in a hybrid composite referred to as STF hybrid composite (STFC). The proposed STFC plates were subjected to low-velocity impact tests under varying impact energies and temperatures to consider realistic operating conditions for practical applications. It was found that the insertion of STF-filled tubes into the composite structure led to lower contact forces during impact and reduced residual velocities of the impact striker at temperatures ranging from 25 to 100 °C, thereby improving the impact resistance of the structure. Specifically, the STFC plates exhibited enhanced energy absorption capabilities, with energy absorption increasing by up to 38.10% at 25 °C under the impact energy of 2 J, potentially mitigating secondary damages within the composite structure. These results suggest that the proposed STFC hybrid composite is a promising solution for applications requiring high energy absorption capabilities.

Low‐velocity impact and self‐healing behavior of CFRP laminates with poly(ethylene‐co‐methacrylic acid) filament reinforcement

AbstractAn experimental study is performed to explore the low‐velocity impact (LVI) and self‐healing behavior of carbon‐fiber‐reinforced‐polymer (CFRP) laminates reinforced by thermoplastic poly(ethylene‐co‐methacrylic acid) (EMAA) filaments. Firstly, differential scanning calorimetry (DSC), thermos‐gravimetric analysis (TGA) and dynamic mechanical analysis (DMA) are used to determine the process parameters. Thereafter, the unreinforced laminates and EMAA reinforced laminates are prepared via phased hot‐processing curing process. The LVI tests with four energy levels (5, 15, 25 and 35 J) are then carried out on the two types of specimens. In these energy cases, the peak‐force values of the reinforced specimens are 2.2%, 1.9%, 2.3% and 7.5% higher than those of the unreinforced ones, while the energy‐absorption‐rate values of the reinforced ones are 6.02%, 3.53%, 4.24% and 2.32% lower. The comparisons demonstrate the impact‐resistance enhancement of EMAA. Moreover, multiple LVI tests are performed on the unhealed and healed specimens. In 15 and 25 J cases, around 6.92% higher peak load and 4.58% lower absorbed energy are observed within the healed specimens, which is attributed to the healing effect. The healing mechanism is investigated by utilizing attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR), scanning electron microscope (SEM) and optical microscope (OM) technologies. It reveals that EMAA flows into the delaminations and cracks during the healing process, and thus accomplishing self‐healing behavior effectively.

Sinusoidally architected helicoidal composites inspired by the dactyl club of mantis shrimp

ABSTRACT The impact region of the dactyl club of mantis shrimp features a rare sinusoidally helicoidal architecture, contributing to its efficient impact-resistant characteristics. This study aims to attain bioinspired sinusoidally architected composites from a practical engineering way. Morphological features of plain-woven fabric were characterized, which demonstrated that the interweaving warp and weft yarns exhibited a sinusoidal architecture. Interconnected woven composites were thus employed and helicoidally stacked to achieve the desired structure. Quasi-static three-point bending and low-velocity impact tests were subsequently performed to evaluate their mechanical performance. Under three-point bending condition, the dominant failure mode gradually changed from fiber breakage to delamination with the increase in the pitch angle. Failure displacement and energy absorption of the helicoidal woven composites were, respectively, 43.89% and 141.90% greater than the unidirectional ones. Under low-velocity impact condition, the damage area of the helicoidal woven composites decreased by 49.66% while the residual strength increased by 10.10% compared with those of the unidirectional ones, exhibiting better damage resistance and tolerance. Also, effects of fiber architecture on mechanical properties were examined. This work will shed light on future design of the next-generation impact-resistant architected composites.

Experimental and numerical analysis of low-velocity impact behavior of wound products using multi-filament winding technique

The wound products are subjected to various impact loads during their service life, which caused invisible damage to the composite layers. In this paper, we investigated the failure behavior of composite layers fabricated using an innovative multi-filament winding (MFW) process under impact load. Firstly, the principle of the MFW technique was introduced. Finite element models were constructed for both novel and conventional techniques, and subsequent low-velocity impact (LVI) tests were performed to acquire the time-force and time-energy data for the CFRP (Carbon Fiber Reinforced Polymer) laminates. The test results were in good agreement with the LVI responses of the composites obtained from the simulations. The failure modes and microscopic damage of the composite layers of the two techniques were analyzed by scanning electron microscopy (SEM). The results showed that under the same impact velocity, three damage modes of fiber fracture, matrix cracking, and delamination were observed for both techniques. However, the composite layers of the MFW technique exhibited more excellent impact resistance properties in terms of maximum contact force, absorbed energy, as well as inter-laminar and intra-laminar damage. The MFW technique exhibits a notable level of winding efficiency and demonstrates exceptional performance of manufactured products, which can meet the increasing demand of winding products over the years.