High-energy laser impact tests on identical materials and irradiation parameters often show deviating results between different testing institutes. This particularly affects the specular component of the reflection and the temperatures. Even the supposedly easy-to-determine perforation time sometimes shows large variations between different test institutes. For a reliable and reproducible determination of test results, a standardization of the experimental setups is necessary. Test results using two different reference materials are presented. These tests demonstrate that standardizing experimental setups and evaluation methods significantly reduces the deviations of the experimental results. The presented multi-institutional approach aims at gaining data of laser reflection, temperature, and perforation time of laser impacted samples, and at developing an optimized experimental setup based on different existing laboratory equipment and experimental approaches.

The use of drones and unmanned aerial vehicles (UAVs) poses an increasing aerial threat in both military and civilian sectors. High-energy laser (HEL) systems emerge as a promising way to countermeasure these threats with excellent precision and minimal collateral damage. Considering that UAVs and drones are predominantly constructed from lightweight construction materials like carbon fiber-reinforced polymer (CFRP), investigating the vulnerability of this material class becomes crucial, as it directly impacts the resistance of these platforms against high-energy laser countermeasures. This study presents research on HEL interactions with carbon fiber-reinforced polymer (CFRP) composites, exploring the effects of continuous wave (CW) laser powers of up to 120 kW. We established a laboratory environment that meets the demands of operational safety requirements for laser processing of CFRP including a robot-controlled sample exchange and automatic laser beam shut-off. The interaction of the laser with the composite material was evaluated using high-speed images showing the rapid expansion of a smoke cloud, that contains fragments of fibers and a partially ionized plasma. Experiments reveal a decrease in perforation times with increasing laser power, which can be described by a power law. A detailed examination of the damaged samples, visually and via micro-focused computed X-ray tomography, offers insights into heat affected zones and ablation dynamics. Furthermore, compression after impact (CAI) tests assess the residual strength of the irradiated CFRP panels. A decrease of the compressive strength of a factor of about 2 is observed. The outcomes of this research contribute to the understanding of CFRP behavior under extreme laser conditions, providing valuable knowledge of the dynamic interactions between high-energy lasers and complex composite materials, highlighting potential applications for countermeasures in defense technology.

Instrumented impact testing and compression-after-impact testing are important to adequately qualify material behavior and safely design composite structures. However, the stresses to which fiber-reinforced plastic components are typically subjected in practice are not considered in the impact test methods recommended in guidelines or standards. In this paper, a test setup for investigating the impact behavior of composite specimens under plane uniaxial and biaxial preloading is presented. For this purpose, a special test setup consisting of a biaxial testing machine and a specially designed drop-weight tower was developed. The design decisions were derived from existing guidelines and standards with the aim of inducing barely visible impact damage in laminated carbon fiber-reinforced plastic specimens. Several measurement systems have been integrated into the setup to allow comprehensive observation of the impact event and specimen behavior. A feasibility test was performed with biaxially prestressed carbon fiber-reinforced plastic specimens in comparison with unstressed reference tests. The compressive-tensile prestressing resulted in lower maximum contact forces, higher maximum deflections, higher residual deflections and a different damage pattern, which was investigated by light microscopic analysis. Finally, the functionality of the experimental setup is discussed, and the results seem to indicate that the test setup and parameters were properly chosen to investigate the effect of prestresses on the impacts behavior of composite structures, in particular for barely visible subsequent damages.

Carbon fiber-reinforced polymer (CFRP), noted for its outstanding properties including high specific strength and superior fatigue resistance, is increasingly employed in aerospace and other demanding applications. This study investigates the interactions between CFRP composites and high-energy lasers (HEL), with continuous wave laser powers reaching up to 120 kW. A novel automated sample exchange system, operated by a robotic arm, minimizes human exposure while enabling a sequence of targeted laser tests. High-speed imaging captures the rapid expansion of a plume consisting of hot gases and dust particles during the experiment. The research significantly advances empirical models by systematically examining the relationship between laser power, perforation times, and ablation rates. It demonstrates scalable predictions for the effects of high-energy laser radiation. A detailed examination of the damaged samples, both visually and via micro-focused computed X-ray tomography, offers insights into heat distribution and ablation dynamics, highlighting the anisotropic thermal properties of CFRP. Compression after impact (CAI) tests further assess the residual strength of the irradiated samples, enhancing the understanding of CFRP’s structural integrity post-irradiation. Collectively, these tests improve the knowledge of the thermal and mechanical behavior of CFRP under extreme irradiation conditions. The findings not only contribute to predictive modeling of CFRP’s response to laser irradiation but enhance the scalability of these models to higher laser powers, providing robust tools for predicting material behavior in high-performance settings.

We investigated the resistance of carbon fiber-reinforced plastics (CFRP) to thermal radiation when silver nanoparticles are added to the resin. An epoxy based CFRP material filled with up to 10 wt.% silver nanoparticles, with respect to the polymer matrix, shows an increase in time to ignition of up to ∼ 30% when irradiated from one side with an electrical heater with heat fluxes of 35 and 80 kW/m2. Scattering and specular reflectance of infrared light was analyzed before thermal irradiation, showing enhanced scattering of samples with increasing silver particle content, which correlates linearly to the time to ignition. The reaction to fire properties of the CFRP, analyzed by cone calorimetry, are also influenced positively by the silver nanoparticles. The thermal conductivity, as measured by laser-flash-analysis, shows no influence by the nanoparticles. Therefore, the increase in time to ignition, as well as the more homogeneous combustion, are not due to changes in conductivity, but to scattering and reflection effects caused by the nanoparticles. The modified material shows no decline in interlaminar shear strength, representing structural properties. Health and environmental risks by nanoparticle release during production, combustion, and machining of the modified CFRP are also investigated. The preliminary results show no additional hazards.

Commercially available carbon fiber reinforced polymer matrix composite panels (Hexply® M18-1/G939 and 8552/IM7) with different thicknesses (1-6 mm) were exposed to continuous-wave high-energy laser radiation at various laser powers up to 10 kW under static conditions. The perforation times, the size of the damaged volume and the residual compressive strengths are determined and correlated to the irradiation parameters. It is found that for the time of perforation, the damaged volume, which is approximated by a cylinder shaped model, correlates linearly with the laser energy imposed onto the sample surface. This relationship can be used for a prediction of the perforation time for various laser powers, material thicknesses and laser spot diameters. Increasing laser energy results in a decay of residual compressive strength after impact. Visual inspection as well as micro-focused computed X-ray tomography and scanning electron microscopy indicate a small area of thermal damage outside the laser spot. Additionally, infrared spectroscopy characterizes incipient heat damage most sensitively.

This study focuses on observing and analyzing the time to failure of carbon fiber reinforced polymers subject to mechanical loading and one-sided heat flux simulating fire damage. The purpose of this investigation is to understand the rate of thermal degradation and mechanical property loss from fire exposure, resulting in catastrophic failure under simultaneous tensile loading. Composite samples of varying thicknesses and layup patterns are subject to a constant tensile load below the ultimate strength of the material. A thermal load is applied to one side by an infrared band heater, emitting a constant heat flux. The time to failure is monitored to determine how long the material can withstand this combined loading condition. A consistent trend is observed for various heat flux settings. High mechanical loads contribute to a shorter time to failure, and low mechanical loads contribute to a longer time to failure. Similarly, higher heat flux settings result in shorter failure times, and lower heat flux settings result in longer failure times. Temperature profiles are created based on heat flux exposure time and position through the sample thickness, establishing failure criteria for different loading conditions. The resulting trends are observed and extrapolated to create a predictive model using an Arrhenius exponential decay function.

The analytical and numerical analysis of lightning direct effects has mainly been focused on thermoelectric effects of different lightning current components. Mechanical forces created by the electric current flow, by blasting of materials (epoxy resin, metallic protection, coatings) or by shock waves caused by a supersonic expansion of the plasma channel have rarely been taken into account so far. The present publication concentrates on lightning shock waves caused by a rapid temperature rise of the plasma channel due to resistive Joule heating. In this context it is first of all necessary to understand the underlying physical mechanisms of this effect. Furthermore, theoretical models for the interpretation of experimental tests as well as for reliable prediction of the magnitude of the shock wave pressure pulses have to be provided. Finally, the temporal and spatial propagation of the shockwaves and pressures that are needed as input for the simulation of mechanical damage prediction are derived from semi-analytical and numerical computations.

This study focuses on exploring the initial failure of thermally degraded carbon fibre-reinforced polymers. It is the aim of this study to provide deep insight into the damage development and propagation as well as to understand failure mechanisms of thermally degraded composites. Carbon fibre-reinforced polymer panels with different fibre orientations are exposed to heat above maximum operational temperature (up to 200 ℃) for various durations (up to ca. 200 days). Thermal degradation of the material is characterized by scanning electron microscopy and infrared spectroscopy. The onset of the failure in tension is determined by acoustic emission analysis. The results show that the development and propagation of cracks depend on the level of thermal degradation and the fibre orientation relative to the applied load. With increasing thermal degradation, transverse matrix cracking, as the prevailing initiating failure mode, is replaced by crack initiation and propagation in the damaged outermost ply independent of its fibre orientation. Severe thermal matrix degradation is limited to this area, as characterized by infrared spectroscopy. With increasing thermal degradation of the polymer matrix, the onset of crack initiation and propagation is shifted to lower strains. The effects of damage initiation on fracture behaviour are discussed.

The effects of particularly heavy lightning strikes on representative carbon fibre reinforced plastics composite airframe structures, specifically with epoxy matrix systems, which are common within the aerospace industry, have been investigated in this study. The applied action integrals of the lightning strikes significantly exceed the requirements for airworthiness. All tests were performed with conventional prepreg materials and resin transfer moulding/non-crimp fabric materials with high lightning strike current ratings. The pristine panels exhibit major damage zones around the impact points. The results of non-destructive investigations show that the surface damage is predominantly superficial. Only small zones were considerably damaged where extensive repair was necessary. Carbon fibre reinforced plastics panels featuring repair patches were also investigated. Lightning strikes were placed above the scarf and the damage was analysed by various non-destructive investigation methods including micro-computed tomography. In contrast to the pristine panels, the repaired panels reveal different damage behaviour. The damaged zone on the surface was relatively small. In the tapered zone of the patch, electric flashovers between the patch and the base material were observed. Additional microscopy investigations show that these electric sparks also occur inside within the adhesive layers between the patch and the base material. After enhancing the electrical conductivity of the adhesive by adding carbon nanotubes, these difficult-to-detect electric sparks within the layers disappear.

The time to failure of carbon fiber-reinforced polymer composites is investigated during simultaneous compression and one-sided thermal degradation from a constant heat flux. The purpose of this investigation is to determine the rate of thermal degradation and mechanical property loss from fire exposure by observing the time to failure during simultaneous compression loading. A custom compression test fixture is designed and validated for use under thermomechanical loading. Testing is conducted on CFRP samples of varying thicknesses and quasi-isotropic layups. Test conditions are adjusted for a range of heat fluxes and compressive loads. Test parameters are isolated to observe individual impacts on time to failure. In each case, decreasing exponential trends are observed. High heat fluxes and high compressive loads result in shorter failure times. Temperature profiles are created based on heat flux exposure time and the position through the material thickness in order to determine failure criteria for thermomechanical loading. An empirical model is created to extrapolate and predict failure times using a stretched exponential function, based on mechanical preload, heat flux, and material thickness.

This study focuses on understanding and prediction of short-term thermal degradation of polymer matrix composites. One sided irradiation of two commercial composites (HexPly® 8552/IM7 and M18-1/G939) is carried out on specimens of various thickness (2, 4, 6 mm) at different heat fluxes (50 and 80 kW/m2) for various exposure times prior to ignition. The aim is to correlate the amount of the applied thermal energy with the heat damage and the residual mechanical strength. Among the two primary components of each matrix the epoxy resin is observed to degrade faster than the thermoplastic under thermal load, as measured by IR spectroscopy. A correlation is achieved between the interlaminar shear strengths and the relative amount of the residual matrix components. The interlaminar shear strengths and degradation processes are assessed in dependence of the applied energy per volume. The derived relationships and a chemometric analysis of IR spectra, can be used to rapidly estimate mechanical properties, as well as other properties of specimens with unknown thermal preload. Degradation processes are discussed in detail.

Aiming at an increase in failure resistance and damage tolerance of composite T-joints, a novel reinforcement technique in through-thickness direction using metallic arrow-pins has been proposed by the authors in previous studies. In a recent investigation, different options for further improvement of the T-pull performance have been assessed. These include optimized arrow-pin configurations, filler and ply materials and thermoplastic interleaf layers. T-specimens have been manufactured and tested under quasi-static and high-rate dynamic loading conditions to quantify the influence of these measures. Additionally, FE models have been developed in LS-Dyna to predict the performance numerically. Model validation was conducted step by step using material coupon test data, dedicated single-pin pull-out tests and T-pull tests.

This study is concerned with the feasibility of different T-joint designs for application in aircraft composite fuel tanks which may be subjected to hydrodynamic ram loading. In a first step, the response of a hybrid metallic/composite T-joint is compared against a state-of-the-art composite T-joint. In a second step, two different resin systems are compared using a third T-joint design – one state-of-the-art aerospace resin and one novel, toughened system. Experimental work is done under both, quasi-static and under high-rate loading conditions. The hybrid metallic/composite T-joint showed superior post-failure behavior compared to the state-of-the-art composite T-joint. The use of the toughened resin system results in a significant increase of joint strength. The post-failure behavior is however not affected by the resin system.

Carbon-fibre epoxy panels have been subjected to rapid high temperature loads. The effects of temperature, exposure time and moisture content of the panels have been studied. It could be demonstrated that the combination of high moisture content and rapid heating can lead to excessive damage, such as sudden formation of delaminations up to the development of large bubbles on the panel. While it is well known that matrix and fibre–matrix interface strength generally decrease both with water uptake and temperature increase, a more severe damage mechanism has been observed here. The combination of high moisture content and rapid high temperature loads leads to an internal vapour pressure overload that can cause extensive cracks and delaminations. Subsequently, this permanent damage leads to serious changes in mechanical properties. Whereas heating dry panels to 350℃ for 1 h and testing them at room temperature reduced interlaminar shear strength less than 25%, many of the moist panels were fully destroyed within the first minute of heating.

Three different composite T-joint designs were investigated experimentally and numerically for application in fuel-filled wing tanks under hydrodynamic ram (HRAM) loads. The test campaigns covering 0° T-pull and 30° T-bending tests were conducted under quasi-static and high-rate dynamic conditions in order to assess potential strain rate effects on the failure behaviour. In addition to the experimental test campaign, numerical modelling with the explicit finite element code LS-Dyna was conducted with the models being validated against the test results and being applied to ballistic impact simulations of a composite fuelfilled tank structure. While the unreinforced baseline design showed a rather brittle behaviour and poor performance, significant residual strength improvements and structural integrity under HRAM loads could be obtained with a hybrid design with metallic, arrow-shaped z-reinforcements between the composite laminates of skin and spar. A promising macro modelling approach for an efficient representation of the T-joint failure behaviour in large models was derived and successfully applied to structural HRAM simulations.

An experimental and numerical study of the failure behaviour of composite T-joints under quasi-static and high-rate dynamic loading is presented, focusing on the investigation of a novel reinforcement technique in the through-thickness direction using metallic arrow-pins to increase failure resistance and damage tolerance. Specimen manufacturing and testing are described in detail. The test campaigns covering 0° T-pull and 30° T-bending tests were conducted under quasi-static and high-rate dynamic conditions in order to assess potential loading rate effects. The novel concept with the arrow-pin reinforcement showed significantly increased post-damage load levels and energy absorption capability with the pins being pulled out of the laminate under large global deformations. In addition to the experimental test campaign, numerical simulations with the explicit finite element code LS-Dyna were conducted on local, global and macro-modelling level. The models were validated against the test results and applied to ballistic impact simulations of an exemplary composite fuel tank structure under hydrodynamic ram loading, where the novel joint design led to significantly higher damage resistance.

This work provides techniques to separately determine temperature and duration of a thermal pre-load on a polymer matrix composite with a focus on long-term load (max. 400 days) without massive polymer degradation. The aim is a non-destructive, rapid, robust and precise in-service method to characterize incipient heat damage. A commercially available composite 8552/IM7 is investigated. Infrared spectroscopy of the surface and bulk material traces thermal degradation of the polymer. A multivariate (chemometric) data analysis was performed. The reliability of the calculated values for time (±19 d) and temperature (± 12℃) is increased by including other parameters with various degradation velocities such as mass loss of the composite and color changes and binder degradation of a typical top coat. The residual strength of a composite with unknown thermal history can be predicted and thermal loads such as heating in an oven and hot air are compared.

Two commercially available carbon fibre reinforced composites (8552/IM7 and M18-1/G939) were exposed to heat above maximum operational temperature at various durations. Mass loss and mechanical properties were measured over time. A chemical analysis was also performed on these composites. The two primary components of each matrix, the epoxy resin and the thermoplastic, were observed to degrade at different rates under various thermal loading conditions. The epoxy resins degrade predominantly as measured by IR spectroscopy and thermal desorption/gas chromatography mass spectrometry. By using mass loss, strength, and IR spectroscopic data, a correlation was made between strength characteristics of each composite and the relative amount of the two primary matrix components. The developed relationship can be used to estimate rapidly the mechanical properties from the intensity ratio of IR bands characteristic of the two components.

Vorhersage des Langzeitverhaltens kohlenstofffaserverstärkter Kunststoffe (CFK) aus dem Kurzzeitverhalten unter Zugbelastung anhand des Zeit-Temperatur-Verschiebungsprinzips Johannes Wolfrum, Johannes Wolfrum Wehrwissenschaftliches Institut für Werk-, Explosiv-und Betriebsstoffe, Erding, DeutschlandSearch for more papers by this author Johannes Wolfrum, Johannes Wolfrum Wehrwissenschaftliches Institut für Werk-, Explosiv-und Betriebsstoffe, Erding, DeutschlandSearch for more papers by this author Book Editor(s):Prof. Dr.-Ing. Walter Krenkel, Prof. Dr.-Ing. Walter Krenkel Lehrstuhl Keram. Werkstoffe, Universität Bayreuth, Ludwig-Thoma-Str. 36b, 95440 Bayreuth, DeutschlandSearch for more papers by this author First published: 15 April 2009 https://doi.org/10.1002/9783527627110.ch51 AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Summary Vorhersage des Langzeitverhaltens kohlenstofffaserverstärkter Kunststoffe (CFK) aus dem Kurzzeitverhalten unter Zugbelastung anhand des Zeit-Temperatur-Verschiebungsprinzips. Die angewandte Methode beruht auf vier wesentlichen Annahmen: • Gleiches Versagensverhalten bei statischer Kurz-, Langzeit- und bei Ermüdungsbelastung • Anwendbarkeit des Zeit-Temperatur-Verschiebungsprinzips für alle Lastfälle • Anwendbarkeit des Prinzips der linearen Schadensakkumulation bei Ermüdungsbelastung • Linearer Zusammenhang zwischen der Ermüdungsfestigkeit und dem Spannungsverhältnis R Aufgrund der Vielfalt verfügbarer Komponenten für Faser-Kunststoff-Verbunde (FKV) kann nicht automatisch davon ausgegangen werden, dass im Einzelfall alle diese Annahmen zutreffen. Eine generelle Anwendung dieser vielversprechenden Methode setzt umfassende Erfahrungen mit dieser im praktischen Einsatz voraus. In dieser Arbeit wurde daher ihre Anwendbarkeit für eine typische Hochleistungs-CFK-Type aus der Luftfahrt untersucht. Den Vorhersagen wurden teilweise reale Messergebnisse gegenübergestellt, wobei gute Übereinstimmung festgestellt wurde. Verbundwerkstoffe: 17. Symposium Verbundwerkstoffe und Werkstoffverbunde RelatedInformation