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Carbon fiber laminates are susceptible to impact damage due to the lack of fibers in the direction of impact. Often such damage can be difficult to find with visual assessment, prompting the need for detection via nondestructive testing techniques. This study quantifies the ultrasonically characterized impact damage diameters in 22 layered CFRP samples from 16 J, 18 J, 20 J, 22 J, and 24 J impact energies. These energies result in damage often defined as barely visible impact damage. This study utilizes x-ray computed tomography as a baseline to verify the results of the ultrasonic testing. Compression after impact test measurements are used in this study to find the ultimate compressive residual strength of the impacted carbon fiber laminates and to investigate the negative correlation between impact energy and residual mechanical properties of the damaged carbon fiber laminates. Many studies focus on the characterization of impact damage from various impact energies and setups via ultrasonic testing or, separately, the effect of the impact setups on the residual strength of the composite. This study strengthens the bridge between the barely visible impact damage quantified via ultrasonic testing and the residual compressive strength of the affected composite. Overall, this paper offers further insights into the relationship between internal damage size obtained using ultrasonic testing and correlates the characterized internal damage geometry to the resultant compressive strength reduction, providing a path for future studies in predictive modeling of the residual strength after impact. [ABSTRACT FROM AUTHOR]

The load‐bearing capacity and failure characteristics of composite structures are closely related to their dimensions. In this study, low‐velocity impact, ultrasonic non‐destructive testing, and compression after impact tests were conducted on composite laminates of varying widths to investigate the influence of size effects. Distinct impact response profiles and compression failure modes were characterized. The results indicate that under impact loading, the delamination threshold load (6700 N ~ 7200 N) and the knee‐point energy (40 J) are not affected by width. However, variations in width lead to differences in indentation depth, impact duration, peak load, delamination characteristics, and energy absorption. Furthermore, increasing impact energy further magnifies these differences. Under compression loading, wider laminates exhibit lower residual strength, with the influence of impact energy on residual strength decreasing as width increases. Importantly, the observed compression failure deformation mode transitions from localized buckling to widespread sub‐layer delamination‐induced compression failure. This suggests that changes in width alter the governing mechanical mechanisms of CAI failure. Consequently, the use of a single‐size laminate to represent both LVI and CAI processes is not recommended. Based on experimental results, the reliability of the equivalent damage method in predicting the residual compression strength of laminates of different widths was confirmed, providing valuable insights for practical engineering applications. Highlights: Assess the variations in the low‐velocity impact process of laminates with different widths.Evaluate the delamination damage of laminates after impacts by ultrasonic C‐scan techniques.Investigate the effect of variation in widths on the compression behavior of laminates after impact.Validate the feasibility of equivalent damage modeling for calculating residual strength. [ABSTRACT FROM AUTHOR]

To investigate the impact and flexure behavior of carbon fiber reinforced polymers/aluminum-honeycomb sandwich panel, low-velocity impact, compression-after-impact (CAI), and three-point flexure tests are conducted carefully. Four kinds of carbon fiber prepregs are selected to make face sheets by hot press preparation. Further, the digital image correlation technique is employed to record the damage evolution under the compression and flexure loads. The results show that the sandwich structure has two stages of impact response, first, the brittle behavior of the upper panel, and then the resistance of both sandwich and lower panel to absorb energy. In the CAI test, the failure position shifts from the ends to the impact cross section, and the compressive strength can be reduced by 40% only by 1 J impact. The strength of in-plane flexure is at least twice greater than that of out-of-plane flexure. The damage and deformation of in-plane flexure are found mainly in the upper panel and sandwich core, and the lower panel bends significantly.

Highlights The present work highlights the benefits of matrix strengthening through the inclusion of hard particles within the resin‐impregnated woven Kevlar mat. Aluminium Oxynitride (ALON) particles are added to epoxy resin by 5, 10, and 15 volume percentages. The test coupons were developed through a hand‐lay‐up technique to estimate the low‐velocity impact resistance. The characterization was performed through EDAX and SEM to ensure the presence of the ALON particles and their homogenous distribution respectively. Low‐velocity testing is preferred to assess the capacity of the materials to rebound the incident energy. The damage assessment was made to estimate the material's stiffness. The compression after impact (CAI) was executed to observe the strength of the material after the impact. The post‐CAI micrographic observation reveals the delamination history. The results manifest that the ALON–rich coupon exhibits higher impact resistance to the scale of 33.33% than that of the ALON‐free coupon. The damage assessment and CAI results annunciate the lesser damage and higher compressive strength of ALON‐rich material respectively. The micrographic study studied after the CAI reveals the delamination and failure behavior. Additionally, explicit numerical assessment was conducted to validate the experimental results. A good agreement is attained between the experimental results and numerical predictions. The enriched stiffness of the synthesized material makes it a perfect candidate for structural application where the frequency of impact loading is high. Development of ALON reinforced Kevlar‐Epoxy composites for low‐velocity impact applications. Evaluating the performance of developed composites under low‐velocity impact loading. Determining the optimal volume percentage of ALON in the composites. Studying the fractography of developed composites. Correlation between experimental and numerical studies. [ABSTRACT FROM AUTHOR]

Flax and carbon fibers reinforced polymer composites have high potentials to be used as sustainable building materials. To understand the impact resistance of these fiber composites, the present article studied the impact resistance of unidirectional flax fiber and carbon fiber reinforced polymer (FFRP and CFRP) composite plates. The impact response, damage resistance, and impact tolerance were investigated using drop hammer impact and compression after impact (CAI) tests. The internal damage of FFRP and CFRP due to impact loads was studied using acoustic emission technology. In addition, the development process of impact damage was analyzed. CFRP plates show a higher specific energy absorption rate than that of FFRP, because of the more serious internal damage and brittle fracture tendencies. Matrix cracking was the primary damage in FFRP and CFRP under impact load. Compared with CFRP, FFRP possesses a higher CAI due to less damage. Further expansion of the matrix crack caused by the original impact occurs in the impacted fiber-reinforced polymer composites (FRP) under a compressive load. [ABSTRACT FROM AUTHOR]

Room temperature drop hammer impact and compression after impact (CAI) experiments were conducted on carbon fiber–epoxy resin (CF/EP) composites to investigate the variation in impact load and absorbed energy, as well as to determine the residual compressive strength of CF/EP composites following impact damage. Industrial CT scanning was employed to observe the damage morphology after both impact and compression, aiding in the study of impact-damage and compression-failure mechanisms. The results indicate that, under the impact load, the surface of a CF/EP composite exhibits evident cratering as the impact energy increases, while cracks form along the length direction on the back surface. The residual compressive strength exhibits an inverse relationship with the impact energy. Impact damage occurring at an energy lower than 45 J results in end crushing during the compression of CF/EP composites, whereas energy exceeding 45 J leads to the formation of long cracks spanning the entire width of the specimen, primarily distributed symmetrically along the center of the specimen. [ABSTRACT FROM AUTHOR]

Boeing and Airbus developed a special testing procedure to investigate the compressive response of laminates that have been impacted (following standards ASTM D 7137 and DIN 65561). This study focuses on both experimental and numerical analysis of Kevlar plates subjected to compression after impact. To ensure high quality and appropriate mechanical properties, the composite plates were manufactured using autoclaving. The DIN 65561 protocol was followed for all three test systems. Initially, ultrasonic C-scanning was performed on all plates before testing to confirm they were free of any significant defects arising from the manufacturing process. Subsequently, low-energy impact testing was conducted at levels ranging from 0 to 8 Joules. Three specimens were tested at each energy level. After the impact, all specimens underwent ultrasonic C-scanning again to assess the internal delamination damage caused by the impactor. Finally, both pristine and impacted specimens were subjected to compressive testing using the special jig specified in DIN 65561. The compressive impact strength results obtained from these tests were plotted against the delamination area measured by C-scanning. These data were then compared to the results obtained from specimens with artificial damage. Semi-empirical equations were used to fit both sets of curves. The same procedure (impact testing, C-scanning, and data analysis) was repeated to investigate the relationship between impact energy and total delamination area. Lastly, finite element modeling was employed to predict the buckling stresses that develop under compression in the impacted systems studied. These modeling approaches have demonstrated good accuracy in reproducing experimental results for CAI tests. [ABSTRACT FROM AUTHOR]

In this article, carbon/kevlar hybrid composites were prepared using Vacuum Assisted Resin Infusion (VARI). The sandwich structure with plain carbon weave fabric as the core layer and plain kevlar weave fabric as the surface layer was selected as reinforcement and the solution of epoxy resin and curing agent was selected as matrix. The low velocity impact experiment and compression after impact (CAI) experiment were carried out on the samples at different hybrid ratio and different temperatures, and the mechanical response on the impact loads were analyzed respectively. The results show that the specimen can limit the delamination defect expansion of the surface kevlar fiber as the carbon fiber used as the core layer, which reduce the delamination defect area and improve the impact resistance of the sample. The toughness of the sample improves the brittle failure mode, that is, the sample still has a certain bearing capacity after the load exceeds the maximum strength, the sandwiched composites presents a positive hybrid effect. With the increase of temperature, the matrix was obviously softened, the macromolecular movement in the system was intensified, the three‐dimensional network macromolecular structure of the matrix and the fiber/matrix interface properties were damaged, and the mechanical properties showed an obvious downward trend. As far as the influences of hybrid ratio and temperature on impact behavior of composite are concerned, it lay a theoretical foundation for the industrial application of carbon/kevlar hybrid composites. Highlights: The sandwiched structure was used to design the composites, which greatly improves the impact properties of the specimens.The impact properties and compression after impact properties at different hybrid ratios and different temperatures were studied.Based on the hybrid effect and temperature effect, the impact failure mechanism of the sample was analyzed. [ABSTRACT FROM AUTHOR]

With the help of the high‐speed camera and self‐designed fixture, it was investigated that the effects of the impactors in a variety of shapes on the mechanical behavior and failure mode of carbon fiber‐reinforced polymer (CFRP) tubes with a range of inner diameter during the low‐velocity impact (LVI) of four impact energies. Then, the three‐dimensional digital image correlation (3D‐DIC) system was carried out to monitor the damage evolution of the LVI‐damaged specimens under compressive and flexural loading. In addition, the residual compressive and flexural strength of all kinds of cases were compared and discussed. The results show that the increase in wall thickness can improve the LVI, compression‐after‐impact (CAI), and flexure‐after‐impact performance. The failure mode of compression changes to brittle fracture failure at the impact circumferential region. The CAI strength of the specimen damaged by a flat impactor is 23.89% less than that of a hemispherical one, but there is little difference in flexural loading. Highlights: Mechanical behavior on in & post LVI of CFRP tubes varying wall thickness and impactor.Due to point‐line contact for flat impactor, two peak forces are in the F‐D curves.LVI property and residual strength are improved with the increase of wall thickness.Compressive failure mode is brittle fracture at the impact circumferential area.Flexure‐after‐impact strength mainly depends on the no‐impact surface of tubes. [ABSTRACT FROM AUTHOR]

The main goal of this study is the comparison of different reinforcement architectures on the low-velocity impact behavior of green composites. The study includes the comparison of unidirectional, basket weave, and twill weave flax/PLA composites, they are subjected to unidirectional tensile tests, drop-weight impact tests, and after-impact compression tests. Results show that the unidirectional composite demonstrates superior tensile strength and initial modulus due to reduced fiber crimp, while basket weave exhibits the highest energy absorption capability and strain capacity attributed to its higher fiber–weight ratio and fiber crimp. Unidirectional composite also shows a larger impacted damage area compared to basket weave and twill weave, attributed to its internal architecture. Residual compressive strength across all composites decreased by 40% compared to the reference sample. However, the reduction in stiffness after impact was different, UD/PLA composite stiffness was reduced by 30% while the reduction in BW/PLA and T/PLA composites was about 20%. [ABSTRACT FROM AUTHOR]

In this study, advancements are presented in the in-situ detection of delamination reinitiation from Barely Visible Impact Damage (BVID) in composite materials, utilizing enhancements in Digital Image Correlation (DIC) techniques during a Compression After Impact (CAI) test. The study measured strain fields in the longitudinal, transverse, and shear directions, focusing specifically on the point of highest out-of-plane displacement to identify the onset of delamination propagation from BVID sites generated at different impact energy levels. By correlating the measured strains with the peak out-of-plane displacement, a unique determination of onset damage reinitiation associated with BVID during CAI testing was achieved. This method introduces a refined in-situ assessment technique for structural integrity, allowing for the early detection of critical damage propagation in composite materials.

Thermoplastic carbon fiber-reinforced plastics (CFRPs) are increasingly utilized in the aerospace industry owing to their beneficial properties and enhanced formability relative to thermoset CFRPs. Despite the extensive use of these materials, studies focusing on compression after impact (CAI) tests with impact energies exceeding 20 J for thermoplastic CFRPs remain scarce. This study examines CAI tests on quasi-isotropic laminates of thermoplastic CFRP, subjected to a low-velocity impact energy of 27.04 J. For comparison, quasi-isotropic laminates of thermoset CFRP were subjected to a low-velocity impact energy of 36.5 J. These tests reveal that the CAI strength of both materials is comparable, notwithstanding the lower fiber volume fraction in the thermoplastic CFRP. Further, this research incorporates a finite element (FE) analysis to investigate the damage mechanisms in thermoplastic CFRP. The FE model, integrating interlaminar damage observed during the impact tests, accurately predicted the relationship between compressive stress and strain, correlating closely with the experimental outcomes. It was observed that both interlaminar and intralaminar damage propagation were constrained until the point of maximum compressive stress. Prior to reaching this maximum, a region of elevated compressive stress in the fiber direction was noted in the 0° layer near the non-impacted side. These findings indicate that the compressive stress in the fiber direction in the 0° layer adjacent to the non-impacted side is pivotal in dictating the final failure, which determines the CAI strength of thermoplastic CFRP laminates. [ABSTRACT FROM AUTHOR]

Carbon fiber‐reinforced polymers have been widely applied in structural parts and components in several sectors, in addition to being constantly used in environments with the presence of humidity and high temperatures, which can affect their density, hardness, and rigidity. In this work, the influence of hydrothermal conditioning on carbon fiber (CF)/epoxy composites was investigated using three types of epoxy resin and two different CF fabric reinforcements, that is, plain weave and eight harness satin (8HS) arrangements. The CF/epoxy composites were subjected to compression after impact (CAI) test by 28 and 40 J energy and then exposed to hydrothermal conditioning for 8 weeks. After the CAI tests, the visual analysis of all composites presented microbuckling mechanisms. The composites tested with 40 J energy absorbed only 2% more moisture compared with the other composites, nonimpacted, and tested with 28 J, indicating that the impact damage did not cause delamination between the layers of the composites, which could facilitate the absorption of water. All composites analyzed showed resistance to CAI even after exposure to humidity, with decreases ranging from 2.8% to 23.8% about the unconditioned specimens. The decrease in CAI also shows the influence of the type of epoxy matrix and the arrangement of the CF in fabrics. [ABSTRACT FROM AUTHOR]

The low-velocity impact (LVI) characteristics and compression after impact (CAI) properties of aircraft composite laminates under the influence of ultraviolet (UV) radiation were studied. Firstly, 0, 30, 60, 90 days of UV radiation experiments were carried out on the specimens to analyze the characteristics of the surface topography. Secondly, LVI experiments, the impact energies of 10, 17, and 25 J, were carried out on specimens to analyze the impact response laws and the characteristics of the impact instantaneous deformation. Finally, through CAI experiments and scanning electron microscope (SEM) techniques to clarify the influence mechanism of UV radiation on the residual compressive strength of specimens. The results showed that with the increase of UV radiation time, the shrinkage and erosion phenomena of the surface matrix and the exposure degree of the surface fibers were gradually increased, and the surface fibers were completely exposed after 90 days of radiation. At the same impact energy, with longer radiation time, the peak force gradually decreased (from 8673.86 to 7851.68 N at 17 J), the absorbed impact energy (from 9.48 to 11.13 J at 17 J) and maximum displacement (from 4.1 to 4.73 mm at 17 J) increased, the depth of pit on the impact surface decreased (from 0.213 to 0.14 mm at 17 J), and the internal damage projection area increased (from 906.68 to 1158.66 mm2 at 17 J). The compression damage of the specimens not exposed to/exposed to UV radiation was mainly fiber cracking/interlayer delamination, respectively. Highlights • Ultraviolet (UV) radiation makes the matrix of surface shrink, the fibers exposure increase. • The Stage II has obvious oscillation phenomenon in four stages of the lowvelocity impact process. • With prolonged UV radiation, the projected area of internal damage expands. • The residual compressive load increases with the increase of UV radiation time. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this work, a new type of epoxy foam‐filled 3D woven spacer composites (FF‐3DWSCs) was designed and manufactured by self‐made 3D loom and foaming technology. The low velocity impact behaviors, residual compressive properties and strengthening mechanisms of epoxy foam were systemically investigated. As the results, the FF‐3DWSCs showed higher impact load of 4000–5200 N than that of 3D woven spacer composites (3DWSCs) (2800–4300 N) at the energy from 10 to 40 J. The residual compressive strength of FF‐3DWSCs (23.85 MPa) was maintained at about 80% after 30 J energy impact. Due to the good compatibility of the epoxy foam and the epoxy resin and the synergistic effect between the pile yarn and the epoxy foam, the interface between epoxy foam and fiber/epoxy was strong. Therefore, the novel FF‐3DWSCs proposed here exhibited excellent impact properties and energy absorption capacity and can be used as a candidate material in the field of high‐performance composites. [ABSTRACT FROM AUTHOR]

Copyright of Engineering Mechanics / Gongcheng Lixue is the property of Engineering Mechanics Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this study, the mechanical properties of non-crimp fabric (NCF) composite laminates under low-velocity impact and compression after impact (CAI) tests were studied by Scanning electron microscopy (SEM) and Digital image correlation (DIC) techniques. The impact response under different impact times, impact angles, and impact distance is studied. Similarly, in CAI test, DIC technique is used to reveal the whole process of NCF composite compression failure, and SEM is used to reveal the microscopic failure form. The experimental results show that the impact damage process of NCF composites has strong directivity. The concrete manifestation is that the internal failure will extend along the paving direction at the failure layer. The peak load generated under 20 J impact energy is about 1/2 of that under 40 J impact energy. The impact distance is one of the important factors affecting the coupling effect of multiple impacts, and the impact angle has little effect on the internal damage extension. The proportion of internal damage area also supports the relevant view, that is, the average difference in the proportion of internal damage area under different impact distance is about 5%, while the average difference in the proportion of internal damage area under different impact angles is about 3%. During the compression process, the main failure mode is shear failure and the failure mode is brittle fracture. The oblique fracture occurs only when the oblique is 45° and the impact distance is large (50 mm). The impact angle has little effect on the residual compression performance of NCF.

ObjectivesIn order to study the effects of impact damage on the compressive strength and failure modes of woven carbon fiber reinforced thick composite laminates, in-plane compression tests are carried out. MethodsThe modified Hashin failure criterion and material degradation model are realized with user-defined subroutines to simulate the failure behaviors of laminates using the ABAQUS/Explicit modelling package. The effectiveness of the numerical model is validated through comparison with experiments aimed at the compressive strength and failure modes. ResultsThe results show that the impact damage reduces the compressive strength of the impacted laminates. The compressive failure mode of the non-destructive specimen is concentrated at the ends of the impacted laminates, while truncated failure occurs across the middle region. The compressive strength decreases with the increase in impact energy, but there is no linear relationship between the compressive strength and impact energy. The evolution of the damage behavior of laminated plates is closely related to the history of compression load. The damage failure of laminates hardly develops when the compression load is below the threshold of the failure load. Otherwise, the damage expands rapidly in the width direction, and compression damage eventually occurs across the whole width direction of the laminate. ConclusionsThe results of this study can provide references for evaluating the impact resistance of woven carbon fiber reinforced thick composite laminates.

Boeing and Airbus developed a special testing procedure to investigate the compressive response of laminates that have been impacted (following standards ASTM D 7137 and DIN 65561). This study focuses on both experimental and numerical analysis of Kevlar plates subjected to compression after impact. To ensure high quality and appropriate mechanical properties, the composite plates were manufactured using autoclaving. The DIN 65561 protocol was followed for all three test systems. Initially, ultrasonic C-scanning was performed on all plates before testing to confirm they were free of any significant defects arising from the manufacturing process. Subsequently, low-energy impact testing was conducted at levels ranging from 0 to 8 Joules. Three specimens were tested at each energy level. After the impact, all specimens underwent ultrasonic C-scanning again to assess the internal delamination damage caused by the impactor. Finally, both pristine and impacted specimens were subjected to compressive testing using the special jig specified in DIN 65561. The compressive impact strength results obtained from these tests were plotted against the delamination area measured by C-scanning. These data were then compared to the results obtained from specimens with artificial damage. Semi-empirical equations were used to fit both sets of curves. The same procedure (impact testing, C-scanning, and data analysis) was repeated to investigate the relationship between impact energy and total delamination area. Lastly, finite element modeling was employed to predict the buckling stresses that develop under compression in the impacted systems studied. These modeling approaches have demonstrated good accuracy in reproducing experimental results for CAI tests.

Room temperature drop hammer impact and compression after impact (CAI) experiments were conducted on carbon fiber–epoxy resin (CF/EP) composites to investigate the variation in impact load and absorbed energy, as well as to determine the residual compressive strength of CF/EP composites following impact damage. Industrial CT scanning was employed to observe the damage morphology after both impact and compression, aiding in the study of impact-damage and compression-failure mechanisms. The results indicate that, under the impact load, the surface of a CF/EP composite exhibits evident cratering as the impact energy increases, while cracks form along the length direction on the back surface. The residual compressive strength exhibits an inverse relationship with the impact energy. Impact damage occurring at an energy lower than 45 J results in end crushing during the compression of CF/EP composites, whereas energy exceeding 45 J leads to the formation of long cracks spanning the entire width of the specimen, primarily distributed symmetrically along the center of the specimen.

The main goal of this study is the comparison of different reinforcement architectures on the low-velocity impact behavior of green composites. The study includes the comparison of unidirectional, basket weave, and twill weave flax/PLA composites, they are subjected to unidirectional tensile tests, drop-weight impact tests, and after-impact compression tests. Results show that the unidirectional composite demonstrates superior tensile strength and initial modulus due to reduced fiber crimp, while basket weave exhibits the highest energy absorption capability and strain capacity attributed to its higher fiber–weight ratio and fiber crimp. Unidirectional composite also shows a larger impacted damage area compared to basket weave and twill weave, attributed to its internal architecture. Residual compressive strength across all composites decreased by 40% compared to the reference sample. However, the reduction in stiffness after impact was different, UD/PLA composite stiffness was reduced by 30% while the reduction in BW/PLA and T/PLA composites was about 20%.

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Yarn reduction is the core technology of near-net forming conical rotating components. This paper presents the influence of yarn reduction defects on the low-velocity impact (LVI) and compression after impact (CAI) damage mechanism of 3D woven composites. Three kinds of specimens with different yarn reduction defects, no yarn reduction (NO-YR), half row-yarn reduction (HIN-YR), and all row-yarn reduction (AIN-YR), were tested using drop weight LVI and CAI equipment. X-ray micro-computed tomography (Micro-CT) and Digital Image Correlation (DIC) technique were used to identify the damage distribution and damage development of the impact of 3D woven composites. Results indicated that the yarn reduction defects can reduce the LVI and CAI mechanical characteristic values of 3D woven composites. The maximum deflection, permanent deformation, and energy absorption rate of HIN-YR and AIN-YR are about 3% and 6% lower than NO-YR, respectively. At the same impact energy, NO-YR damage was the least and AIN-YR damage was the most severe. Furthermore, compared with AIN-YR, HIN has higher CAI strength. More importantly, yarn reduction changed the damage mechanism of the specimens. The final failure of NO-YR is decided by debonded, whereas that of HIN-YR and AIN-YR are mainly influenced by yarn breakage and delamination. [ABSTRACT FROM AUTHOR]

This study aimed to analyze the damage tolerance of a carbon/epoxy composite laminate (AS4/8552) impacted at low velocity, relating residual strength values with the impact energy and delaminated area. AS4/8552 laminated plates [0°3/90°/±45°]S were submitted to low-velocity impact tests with three energy levels: 30 J, 45 J and 74 J. Next, the test specimens were analyzed by X-ray computed tomography to check for cracks and delaminations, and compression after impact (CAI) tests conducted to assess their residual strength. The results indicate that above the minimum energy level of low-velocity impact to cause damage, the residual strength of the laminate may be related to the delaminated area. [ABSTRACT FROM AUTHOR]

This article investigates the effect of alternating the sequence of variable‐energy repeated low‐velocity impact on the residual strength of carbon fiber reinforced polymer (CFRP) laminates. Quasi‐isotropic CFRP specimens were impacted using three impact energy levels, that is, Low (L), Medium (M), and High (H). Every group, out of three groups in total, was impacted consecutively using one of three sequences: LMH, HML, and MLH. Delamination and its growth after each impact was quantified using a phased‐array ultrasonic probe. The impacted specimens were then subjected to compression after impact loading with in situ acoustic emission (AE) and digital image correlation (DIC) measurement. Mechanical results showed that the residual strength of the specimens was not significantly affected by alternating the impacts sequence. However, the AE, sentry function and DIC results revealed that the impact‐induced damage in HML configuration was activated much earlier than MLH, and the damage induced in LMH configuration was activated later than both. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Polymer matrix composites are vulnerable to low-velocity impact, although they have found many applications in high-tech industries. The main objective of this study is to improve compression after impact strength of the carbon/epoxy composites using nanoclay particles. All test samples were fabricated using a vacuum infusion process. The solution blending methodology was applied for better dispersion of the nanoparticles in the resin. The low-velocity impact response of the composite and nanocomposite laminates was investigated at various energy levels of 10, 15, and 20 J. To quantify the impact effect, the residual strength of the impacted specimens was experimentally measured under the compression test. In addition, the effect of nanoclay incorporation through the solution blending methodology on damage mechanisms was determined by the radiographic analyses and optical microscope method. The findings of this research showed a rise, up to 25.15% in the residual compressive strength, and a 7-fold increase in local buckling threshold stress of the carbon/epoxy/nanoclay specimens. [ABSTRACT FROM AUTHOR]

The purpose of this work is to evaluate the reconsolidation of a carbon fiber composite with poly (aryl ether ketone) (PAEK)/carbon fiber laminates after suffering impacts from different energy levels (5, 10 and 30 J) and being restored by reconsolidation. For this comparison, thermal analysis, and compression after impact tests were performed to evaluate the properties of the material before and after the reconsolidation process. According to the found results, it was observed that with small damages it is possible to fully recover the material after new consolidation and for larger damages there is also a partial recovery compared to the base laminate. The material remains thermally stable in all situations, in other words, its thermal properties remain even after impact tests and reconsolidation process. [ABSTRACT FROM AUTHOR]

This research offered an experimental examination of the effect of binder yarns, and 3D woven patterns on impact (Charpy and drop weight impact), and compression after impact (CAI) performance of seven (07) different kinds of 3D woven jute/green epoxy composites. Along with four (04) typical classifications of 3D woven reinforcements i.e., OLL, OTT, ALL, ATT, and three (03) novel (hybrid) 3D reinforcements i.e., H1 (OTT and ATT interlocking pattern), H2 (OTT and ALL interlocking pattern), H3 (OLL warp and weft interlocks “bidirectional”) were also developed on dobby weaving machine. OTT composite displayed the highest amount of impact strength during Charpy impact in both in-plane directions i.e., warp and weft in comparison with others due to the existence of the truly vertical binder yarns which is comparable with H1. While ALL sample exhibited the highest value of maximum load, work done, and energy absorbed during the 3 J and 6 J drop weight impact energies which is nearest comparable with hybrid 3 (H3) composite. H3 composite sample revealed the highest value of compression after impact (CAI) stress and modulus in both energy levels due to the presence of both warp and weft binder yarns in a single 3D structure.

The low-velocity impact and compression after the impact of the composite stiffened panels were carried out after damp-heat aging. The experimental results show that reinforcing the ribs can enhance the impact resistance of test pieces after damp-heat aging. After impacting, the specimens were tested in an axial compression. The results show that the ultimate bearing capacity of the specimen is also affected by different located positions of the impact and different aging times. Compared with the intact specimen, the ultimate load-bearing capacity was reduced to 16.83, 12.10, and 17.10% with the specimen aging for 0, 45, and 90 days, respectively, while the impact position located at the intersection of longitudinal and transverse bars has the greatest influence on the damp-heat aging of specimens.

This article solves one of the central problems of how impact damage affects the strength of polymer composite materials (PCM). Impact damage on sandwich materials varies in the resulting shape and size, thus hampering the numerical assessment of their stress-strain state because these damage characteristics are difficult to predict. Here, we introduced an experimental technique to determine the PCM strength and modeled the impact damage on rectangular plates to illustrate the conclusions drawn. The critical compressive load and stress of the undamaged plates were obtained by the finite element method. The experimental correction function was deduced to evaluate the effect of the damage area and impact energy on the residual strength. Vertical impact testing of the PCM samples was performed. Following the impact on the plate, the depth of the dents and the damage area were measured using the pulse-echo ultrasonic technique. The failure behavior was analyzed layer by layer for individual samples with the help of X-ray computed tomography. For a number of PCM samples, a relationship between the damage size and the impact energy was established. Compression testing of the damaged plates was carried out. The critical buckling loads and the functions of a decrease in the load-bearing capacity of the plates depending on the impact energy value were calculated.

In this study, polyamide fibers, which stand out with their excellent plastic deformation and energy absorption capacity, were used as reinforcement materials, and in‐house manufactured composite specimens were subjected to low‐velocity impact (LVI), compression after impact (CAI) and tensile tests. Within this scope, one and two repeated drop tests were performed under 3 m/s velocity to determine LVI responses and how impact number affects the dynamic properties. CAI tests were also performed at a 1 mm/min crosshead speed, and mechanical properties for non‐impacted, one‐impacted, and two‐impacted specimens were determined. As a result of the outstanding plastic deformation capacity of thermoplastic fabrics, it is concluded that polyamide composites exhibited quite large strains. Furthermore, it was understood from the tensile responses that tensile stresses were carried by the thermoplastic fibers in two different regimes and significantly high toughness was obtained. Moreover, reductions in the maximum compression loads, critical buckling loads and axial stiffness were observed due to degradation in structural integrity after impact loads. Additionally, the utilization of recyclable thermoplastic polyamide fibers as reinforcement material instead of conventional reinforcement materials such as carbon and glass fibers provide more environmentally friendly products. [ABSTRACT FROM AUTHOR]

A novel family of composite laminates with a simplified stacking sequence and double-double layup [±ϕ/±ψ] n has significant potential to reduce weight and increase strength, while facilitating design optimization and simplifying the manufacturing process. In this study, the low-velocity impact response of a double-double (DD) laminate and a quadriaxial (Quad) laminate of equivalent stiffness and thickness were compared. Carbon/epoxy laminates were produced with stacking sequences of [±0/±50]10 and [03/90/±45/02/±45]2S corresponding to double-double and quadriaxial varieties, respectively. Low velocity impact tests were conducted at 74 J of energy and damage areas were examined using X-ray computed tomography. Compressive strength and compression after impact (CAI) strength were equivalent for the two laminates. However, differences in impact damage morphology were observed and are discussed. [ABSTRACT FROM AUTHOR]

An experimental analysis of mechanical behaviour for aluminium-based fibre metal laminates under compression after impact was conducted. Damage initiation and propagation were evaluated for critical state and force thresholds. Parametrization of laminates was done to compare their damage tolerance. Relatively low-energy impact had a marginal effect on fibre metal laminates compressive strength. Aluminium–glass laminate was more damage-resistant than one reinforced with carbon fibres (6% vs. 17% of compressive strength loss); however, aluminium–carbon laminate presented greater energy dissipation ability (around 30%). Significant damage propagation before the critical load was found (up to 100 times the initial damaged area). Damage propagation for assumed load thresholds was minor in comparison to the initial damage size. Metal plastic strain and delaminations are dominant failure modes for compression after impact. [ABSTRACT FROM AUTHOR]

The impact performance and compression after impact characteristics of 2D and 3D fiber metal laminates (FMLs) are investigated both experimentally and numerically. Commercial-grade GLARE3A-3/2-0.3, and a recently developed FML, which incorporates a unique 3D glass fabric, are used in the study. Both FMLs have similar areal densities. The specimens are subjected to impact loading at three energy levels—low, intermediate, and high. The test results indicate that GLARE is slightly more resilient under impact compared to the 3DFML. However, since GLARE is much thinner than the 3DFML, the two-material systems exhibit very different failure modes. GLARE and 3DFML lost up to 62.6% and 41.5% of their original compressive load-bearing capacity, respectively. Robust and accurate finite element models are developed that can predict the damage evolution and failure modes of both FMLs. Knowing the level of reduction in the residual load-bearing capacity of a material resulting from an impact is of practical importance when assessing the service life of materials. However, further exploration would be required to determine how the information obtained through testing relatively small-sized specimens in a laboratory environment can be extrapolated to larger real-life structural components. [ABSTRACT FROM AUTHOR]

Aeronautical composite primary structures must evidence sufficient residual strength in the presence of damage for compliance with damage tolerance requirements. The study of stiffener debonding on panels subjected to compression after impact is performed in that scope. Compression leads to the buckling of the skin between the stiffeners, and thus a complex loading of the bonding between the skin and the stiffener. This paper describes the development of a stiffened specimen for the VERTEX multiaxial test rig as a first step towards the study of the damage tolerance evaluation of stiffened structures, under combined loadings and at the intermediate scale of the test pyramid. By using virtual testing, the specimen was designed to produce the phenomenology of interest as the first damage, i.e., the debonding of the stiffener from the centre. Three samples were manufactured and subjected to low velocity impacts at various locations and energies. Then the three samples were subjected to compression after impact, up to the stiffener debonding, under a post-buckling regime of the skin. Test loading evolution is described with force fluxes and global strains, obtained from in situ stereo-correlation. The different impacts were found to give different types of damage but similar residual strength to compression after impact. [ABSTRACT FROM AUTHOR]

Research into residual strength after lightning strike is increasing within the literature. However, standard test methods for measuring residual compressive strength after lightning strikes do not exist. For the first time, a systematic experimental study is undertaken to evaluate modifications necessary to standard Compression After Impact (CAI) specimen geometry and test jig design to induce specimen failure at the lightning damage region. Four laboratory generated lightning strike currents with peak amplitudes ranging from 25 to 100 kA have been studied. Test set-up modifications were made considering the scale of the lightning damage and its potential proximity to specimen edges. Specimen geometry and anti-buckling guides were adjusted for each peak current to induce specimen failure at the lightning damage. The Compression After Lightning (CAL) strength was 28% lower than the pristine CAI strength even at a relatively low peak current of 25 kA. This study shows that the standard CAI test setup has the potential for CAL application, however, careful modifications are required depending on the peak amplitude of the applied lightning current waveform. [ABSTRACT FROM AUTHOR]

The influence of a dispersed filler located between the layers of a continuous reinforcing filler has been studied on the characteristic of residual compressive strength after impact of carbon fiber reinforced polymer based on SYT-49S high-strength carbon tow filler (People's Republic of China) and VSE‑1212 epoxy matrix. Microstructural studies of the powders of various nature have been presented (the particle sizes have been determined, and the microstructure of these powders has been given) used as a dispersed filler. The following have been presented and analyzed: diagrams obtained as a result of a low-speed impact and C-scans of nondestructive ultrasonic testing after impact of the carbon fiber samples, as well as the results of compression after impact for these carbon fiber samples. [ABSTRACT FROM AUTHOR]

In this paper, carbon fibers (CF) with different electrochemical treatment levels were obtained by changing the parameters of electrochemical treatment, and the effect of electrochemical treatment levels on the surface properties of CF was studied by scanning electron microscope, atomic force microscopy and X‐ray photoelectron spectroscopy. The interfacial shear strength (IFSS) of CF with different electrochemical treatment levels were characterized by single fiber‐composite fragmentation. The effects of electrochemical treatment on the mechanical properties of CF were investigated by monofilament tensile test and composite laminate 0° tensile test. Finally, the carbon fibers were compounded with epoxy resin to prepare composite laminates, whose effect on the macroscopic mechanical properties of the composite, such as short beam shear strength (SBSS) and compression after impact (CAI) strength, was studied. As a result, with the increase of the level of electrochemical treatment, the surface roughness and chemical activity of CF increased first and then decreased, while the surface energy shows a gradually increasing trend. The IFSS of CF‐BX shows a gradually increasing trend, while that of the CF‐SX shows a trend of increasing first and then remaining basically unchanged, while SBSS shows the same trend. The monofilament tensile test and composite laminate 0° tensile test showed that the electrochemical treatment did not have a significant effect on the mechanical properties of CF. The compression after impact test showed that with the enhancement of the electrochemical treatment, the CAI of the composite laminates increased first and then remained roughly unchanged. [ABSTRACT FROM AUTHOR]

The initial delamination defects may arise during the preparation of carbon fiber‐reinforced polymer (CFRP) composite laminates, so its effects need to be considered in the evolution of mechanical properties. In this article, the behaviors of composite laminates with initial delamination defects under impact and compression after impact (CAI) are investigated by numerical method. The simulation framework composed of the micromechanics of failure (MMF) theory and the mixed mode exponential cohesive zone model (ECZM) is constructed. The numerical analysis compares the effects of the interaction between four different initial delamination defects and impact loading on impact response and CAI strength at two impact velocity levels (1.336 and 2.314 m/s). Furthermore, the failure mechanisms of laminates with initial delamination defect are disclosed under CAI loading. The findings indicate that the difference in impact response is minor. The CAI strength is reduced from 250.5/173.5 to 191.5 MPa/139.1 MPa. The sufficient condition for the further significant reduction of CAI strength is that the initial delamination defects merge with the impact‐induced delamination cracks in the width direction of laminates. Moreover, the failure mechanism for CAI condition is mostly matrix damage and interlaminar crack propagation. This work provides a valuable reference for engineering failure analysis of composite laminates. [ABSTRACT FROM AUTHOR]

This research offered an experimental examination of the effect of binder yarns, and 3D woven patterns on impact (Charpy and drop weight impact), and compression after impact (CAI) performance of seven (07) different kinds of 3D woven jute/green epoxy composites. Along with four (04) typical classifications of 3D woven reinforcements i.e., OLL, OTT, ALL, ATT, and three (03) novel (hybrid) 3D reinforcements i.e., H1 (OTT and ATT interlocking pattern), H2 (OTT and ALL interlocking pattern), H3 (OLL warp and weft interlocks "bidirectional") were also developed on dobby weaving machine. OTT composite displayed the highest amount of impact strength during Charpy impact in both in-plane directions i.e., warp and weft in comparison with others due to the existence of the truly vertical binder yarns which is comparable with H1. While ALL sample exhibited the highest value of maximum load, work done, and energy absorbed during the 3 J and 6 J drop weight impact energies which is nearest comparable with hybrid 3 (H3) composite. H3 composite sample revealed the highest value of compression after impact (CAI) stress and modulus in both energy levels due to the presence of both warp and weft binder yarns in a single 3D structure. [ABSTRACT FROM AUTHOR]

A full-scale model for predicting low-velocity impact (LVI) damage and compression after impact (CAI) strength was established based on a subroutine of the material constitutive relationship and the cohesive elements. The dynamic responses of the laminate under impact load and damage propagation under a compressive load were presented. The influences of impact energy and ply thickness on the impact damage and the CAI strength were predicted. The predicted results were compared with the experimental ones. It is shown that the predicted value of the CAI strength is in good agreement with the experimental result. As the impact energy reaches a certain value, the CAI strength no longer decreases with the increase in the impact energy. Decreasing the ply thickness can effectively improve the damage resistance and CAI strength. [ABSTRACT FROM AUTHOR]

In this study, ceramic matrix composite specimens constituting Nextel™720 fibers and alumina matrix were subjected to one-sided thermal shock and subsequent 30-min hold at temperatures of 1200℃, 1350℃ and 1500℃. The plates were then subjected to low velocity impact at 5 J impact energy, followed by edgewise compression with digital image correlation. No noticeable change in mass, roughness and specimen dimensions were observed after the thermal shock and hold event at any of the temperatures for any of the specimens. External and internal damage resulting from the impact event also showed no dependence on the thermal shock temperature. The percentage compressive strength retention after low velocity impact ranged from 61 % to 68 % for all cases considered, again with no trends observed based on shock temperature. However, the compressive modulus for the impacted panels that were exposed to high temperature were higher than the pristine (non-thermal shocked) impacted panels. [ABSTRACT FROM AUTHOR]