Environmental effects on composites durability with regard to fibers, matrix, and interphase

The proposed work is based on aging experiments on composites and their single constituents (fibers, matrix resins, fiber sizings, and interphases). The effects of the aging temperature and the specific materials used were investigated. Prediction models for strength and lifetime were derived. The first chapter focuses on the change of the thermo-mechanical material properties of epoxies. It could be demonstrated that plasticization and physical aging co-occur and oppositional affect the major properties epoxy strength and ductility. The second part focuses on the aging effects on GFRP composites and the importance of the fiber and sizing constitution for the lifetime under environmental degradation. Therefore, a novel fiber aging methodology was developed and implemented to manufacture composites with artificially aged fiber/matrix interphases.Die Arbeit basiert auf Alterungsexperimenten an Verbundwerkstoffen und ihren Bestandteilen (Fasern, Matrixharze, Faserschlichten und Grenzphasen). Die Auswirkungen verschiedener Alterungstemperaturen und Materialien wurden untersucht und Vorhersagemodelle für die Festigkeit und Lebensdauer abgeleitet. Das erste Kapitel befasst sich mit der Änderung der thermomechanischen Eigenschaften von Epoxidharzen. Es konnte gezeigt werden, dass Plastifizierung und physikalische Alterung zusammen auftreten und die Festigkeit und Duktilität entgegengesetzt beeinflussen. Der zweite Teil befasst sich mit der Alterung von GFK und der Bedeutung der Faser- und Schlichtebeschaffenheit für die Lebensdauer. Es wurde eine neuartige Methode zur Faseralterung entwickelt und implementiert, um GFK mit gealterten Faser/Matrix-Grenzphasen herzustellen

Effects of wet aging on the damage progress in multidirectional GFRP composites under fatigue loading

The impact of harsh environments with variable temperatures, high humidity, or direct water contact poses a challenge for lightweight construction materials, such as polymer composites, because of their hydrophilic and temperature-dependent properties. Since the most widely used glass fiber reinforced polymers are known to suffer from the above-mentioned conditions, precise knowledge of their long-term durability is of high interest. In addition to the pure change of mechanical properties due to water absorption or temperature, damage evolution and lifetime performance are important. This is what the present study addresses, bringing together the initiation and development with the effects on fatigue life. In contrast to what is regularly reported in the literature, a lifetime increase was found for a specific multidirectional (quasi-isotropic), epoxy-based laminate after short- and long-term wet-aging. The reasons for this outcome were investigated using interrupted fatigue tests, microscopy, and residual strength analysis. As a result, significant differences in damage initiation and progress could be identified and related to the quasi-static and fatigue result. The main reason for the increase in lifetime is expected to be the distributed energy dissipation in the wet-aged condition. Here, damage occurs simultaneously in several regions, which leads to the fact that the introduced energy is divided among numerous defects at the same time. In the dry case, on the contrary, the growth of the few, individual damages terminates in an early catastrophic failure

Fully-integrated carbon nanotube epoxy film sensors for strain sensing in GFRP

Structural health monitoring of fiber-reinforced polymer composites becomes more important to ensure a safe and reliable operation. This work demonstrates a method for local matrix modification of glass fiber-reinforced polymers with fully-integrated pre-cured carbon nanotube epoxy thin-film sensors enabling a piezo-resistive strain and damage monitoring. The film sensors were manufactured using a manual film applicator, partially pre-cured for 48 h under lab conditions, cut to shape, and placed on dry glass fiber fabrics before infusion in a resin transfer molding process. Three-point bending tests with sensor films under the upper ply, in the middle, and over the lower ply prove the sensor films' ability for localized strain monitoring. Furthermore, detection of critical buckling is possible in structural parts, as demonstrated in compression tests of coupon specimens and stringer components

Effects of hygrothermal ageing on the interphase, fatigue, and mechanical properties of glass fibre reinforced epoxy

Reliability and cost-effectiveness represent major challenges for the ongoing success of composites used in maritime applications. The development of large, load-bearing, and cyclically loaded structures, like rotor blades for wind or tidal energy turbines, requires consideration of environmental conditions in operation. In fact, the impact of moisture on composites cannot be neglected. As a result of difficult testing conditions, the knowledge concerning the influence of moisture on the fatigue life is limited. In this study, the impact of salt water on the fatigue behaviour of a glass fibre reinforced polymer (GFRP) has been investigated experimentally. To overcome the problem of invalid failure during fatigue testing, an improved specimen geometry has been developed. The results show a significant decrease in fatigue life for saturated GFRP specimens. In contrast, a water absorption of 50% of the maximum content showed no impact. This is especially remarkable because static material properties immediately decrease with the onset of moisture absorption. To identify the water absorption induced damage progress, light and scanning electron microscopy was used. As a result, the formation of debondings and cracks in the fibre-matrix interphase was detected in long-term conditioned specimens, although no mechanical loading was applied.German Research Foundation (DFG) within the project number FI 688/4-1 and the Federal Ministry for Economic Affairs and Energy (BMWi) within the AIF project number ZF4563401

Hygrothermal aging history of amine-epoxy resins: effects on thermo-mechanical properties

Epoxy systems are widely used as matrix resins for fiber reinforced polymers (FRP) and, therefore, often have to withstand harsh environmental conditions. Especially in marine and offshore environments, moisture or direct water contact leads to water absorption into the epoxy resin. As a result, the mechanical properties change during application. Since diffusion at room or colder temperatures is slow, industry and academia typically use accelerated aging methods at elevated temperatures for durability prediction. However, as the water-polymer interaction is a complex combination of plasticization, physical aging, and molecular interaction, all of these mechanisms are expected to be affected by the ambient temperature. To reveal the impact of aging time and temperature on the thermo-mechanical properties of an amine-epoxy system, this publication includes various hygrothermal aging conditions, like water bath and relative humidity aging at temperatures ranging from 8°C to 70°C and relative humidity from 20% to 90%. Thus, it is demonstrated via long-term aging, DMTA and FTIR investigations that, e.g., strength, stiffness, strain to failure, and the glass transition temperature (T g ) can differ significantly depending on aging time and temperature. For example, it can be shown that water absorption at cold temperatures leads to the strongest and longest-lasting reduction in strength, although the maximum water absorption amount is lower than at higher temperatures. For the application, this means that strength differences of up to 26% can be obtained, depending on the aging method selected. Furthermore, it can be shown that conventional prediction models, such as Eyring correlation, which consider the mobility of the molecular structure for the prediction of thermo-mechanical properties, can only be used to a limited extent for prediction in hygrothermal aging. The reasons for this are seen to be, in particular, the different characteristics of the water-polymer interactions depending on the aging temperature. While plasticization dominates in cold conditions, relaxation and strong water-molecule bonds predominate in warm conditions

Impact damage detection in glass fibre reinforced polymers via electrical capacitance measurements on integrated carbon fibre bundles

Impact damages are critical for fibre reinforced polymers, as they can lead to large delaminations that strongly influence the structural integrity of the components. Therefore, reliable detection of impact damages through structural health monitoring is desired. This paper demonstrates a method for the detection and size estimation of impact damages in glass fibre reinforced polymers using capacitance measurements on integrated carbon fibre bundles. Therefore, individual rovings of glass fibre fabrics are replaced by carbon fibre rovings. After infusion of the laminate, the electrical capacitance is measured between the carbon fibre bundles. Damage in the form of matrix cracks or delaminations results in incorporated air that changes the permittivity of the material and, therefore, can be detected as a capacitance decrease. Considering the magnitude of capacitance decrease, a size estimation of the impacts is possible. Consequently, the presented method can be used for in-situ monitoring of damages in glass fibre reinforced polymers

General influence of the environmental temperature on the matrix strength under tensile and compressive loading - A comprehensive study on high performance matrices

This comprehensive experimental study investigates the influence of operating temperatures ranging from −50 °C to 256 °C on the mechanical properties of various high-performance thermosets regularly used as matrix polymers for composites. The resins are either crucial for specific industry sectors or challenging to obtain because they are part of commercial prepreg systems. The research focuses on the temperature-dependent tensile and compressive strength of those materials and provides a wide database of mechanical properties. This paper summarizes the mechanical properties on the one hand in terms of a strength versus temperature scale, which allows an easy comparison for engineering applications. And on the other hand, it demonstrates that the thermo-mechanical properties are mainly dependent on the polymer's glass transition temperature. More precisely, three approaches to estimate the temperature dependence of the strength of thermosets with reduced testing effort are applied and presented. While one is based on the assumption that strength depends linearly on the environmental temperature, the others focus on the distance or the ratio of the environmental temperature to the glass transition temperature. This data set and the resulting correlations provide the community with valuable material data and allow to forecast the thermo-mechanical behavior of most thermosets in the future with significantly reduced effort

Influence of sizing aging on the strength and fatigue life of composites using a new test method and tailored fiber pre-treatment : a comprehensive analysis

Given the time-consuming and complex nature associated with the aging of composites, a novel fabric pre-aging method was developed and evaluated for static and fatigue testing. It allows for investigating sizing and interphase-related aging effects. This fast method is independent of the diffusion processes and the composites’ thickness. Moreover, the new methodology offers enhanced analysis of the sizing, interphase, and fiber-related degradation of composites without aging them by conventional accelerated procedures or under severe maritime environments. For validation purposes, fiber bundle, longitudinal, and transverse tensile tests were performed with five different glass fiber inputs. Significant differences in the durability of composites were found for pre-aging and classical aging, respectively. The impacts of degradation of the single constituents on the fatigue life are identified by cyclic testing of untreated, pre-aged, and wet-aged composites. Here, it is evident that the interphase strength is likewise essential for the tension-tension fatigue performance of unidirectional composites, as is the fiber strength itself. In summary, the presented method provides industry and academia with an additional opportunity to examine the durability of different fibers, sizings, and composites for design purposes following a reasonable methodology.Given the time-consuming and complex nature associated with the aging of composites, a novel fabric pre-aging method was developed and evaluated for static and fatigue testing. It allows for investigating sizing and interphase-related aging effects. This fast method is independent of the diffusion processes and the composites’ thickness. Moreover, the new methodology offers enhanced analysis of the sizing, interphase, and fiber-related degradation of composites without aging them by conventional accelerated procedures or under severe maritime environments. For validation purposes, fiber bundle, longitudinal, and transverse tensile tests were performed with five different glass fiber inputs. Significant differences in the durability of composites were found for pre-aging and classical aging, respectively. The impacts of degradation of the single constituents on the fatigue life are identified by cyclic testing of untreated, pre-aged, and wet-aged composites. Here, it is evident that the interphase strength is likewise essential for the tension-tension fatigue performance of unidirectional composites, as is the fiber strength itself. In summary, the presented method provides industry and academia with an additional opportunity to examine the durability of different fibers, sizings, and composites for design purposes following a reasonable methodology

Effects of the hygrothermal aging history on epoxy resins and GFRP composites

As the success of fiber reinforced polymers (FRP) has reached a wide range of industries, sustainability, reliability, and durability are increasingly important requirements. Complex processes occur in the polymeric matrices and the multiscale composites, especially under challenging conditions (humidity, water, temperature). Due to the variety of available constituents (fibers, sizings, resins) and the high time pressure for new products and innovations, long-term property predictions rely mostly on accelerated aging procedures. However, since the available materials can differ considerably in their characteristics, aging effects need to be understood better. Therefore, long-term studies on four epoxies demonstrate the importance of the hygrothermal aging temperature affecting their thermo-mechanical properties by plasticization and physical aging processes. Additional investigations on several glass fiber composites under the same aging conditions reveal that the matrix and interphase dominated properties are likely to develop differently from neat epoxy. The sizing durability is essential here

Strain sensing in GFRP via fully integrated carbon nanotube epoxy film sensors

Structural health monitoring of composite structures enables early damage detection to prevent critical component failure. Surface-mounted strain gauges are commonly applied to monitor the integrity of composite structures in highly loaded areas. However, strain gauges can only measure strain on the structure's surface and are exposed to environmental influences. Within this paper, fully integrated carbon nanotube thin-film sensors for strain and damage sensing in glass fibre reinforced polymers (GFRP) via electrical resistance measurements are presented. Single wall carbon nanotube epoxy thin-films were manufactured using a manual film applicator, partially pre-cured, placed on dry glass fabrics and infused in a resin transfer moulding process. The mechanical properties of the composites and strain sensing capabilities of the integrated sensors were studied for various load cases and different laminate lay-ups. Results of quasi-static and step-wise three-point bending and tensile tests show that the integrated films allow for localised strain measurements in GFRP without significant loss of mechanical properties. Open hole tensile tests proof the ability to monitor local strain and damage in highly loaded areas enabling failure prediction via threshold resistance change values. The proposed thin-films enable a tailored strain and damage monitoring of GFRP offering the possibility for measurements at different material depths, over large sections or selectively in highly loaded areas. The manufacturing process is easily automatable and suitable for large scale manufacturing.Deutsche Forschungsgemeinschaft (DFG