The reduction of fossil fuel dependency requires the expansion of the renewable energy sector. The European Union seeks to cover 20% of its energy needs from renewables by 2020. In order to achieve this goal [1
], the wind energy capacity should be expanded by two orders of magnitude. The EU offshore wind energy capacity is expected to grow by 21% annually [3
The history of wind turbines for electric power generation started in 1988 Cleveland Ohio, USA, 1888 by Charles F. Brush [6
] and in Askov, Denmark in 1889 by pioneer Poul La Cour [7
]. In 1941, electricity production from wind was made using turbines with steel blades built by the company S. Morgan-Smith at Grandpa’s Knob in Vermont in USA. One of the blades failed after only a few hundred hours of intermittent operation (see Figure 1
a). Thus, the importance of the proper choice of materials and inherent limitations of metals as a wind blade material was demonstrated early in the history of wind energy development. The next, quite successful example of the use of the wind turbine for energy generation is the so-called Gedser wind turbine, designed by Johannes Juul, with three composite blades built from steel spars, with aluminum shells supported by wooden ribs, installed at Gedser coast in Denmark in 1956–1957. After the 1970s, wind turbines were mainly produced with composite blades [8
The Gedser turbine (three blades, 24 m rotor, 200 kW, Figure 1
b) was the first success story of wind energy, running for 11 years without maintenance. In this way, the linkage between the success of wind energy generation technology and the application of composite materials became an issue from the beginning: the first turbine, built with steel blades, failed, while the second one, with composite blades, worked for many years.
2. Composite Structures of Wind Turbines: Loads and Requirements
2.1. Overview of Blade Design
Composite materials are used typically in blades and nacelles of wind turbines. Generator, tower, etc. are manufactured from metals. Blades are the most important composite based part of a wind turbine, and the highest cost component of turbines.
A wind turbine blades consists of two faces (on the suction side and the pressure side), joined together and stiffened either by one or several integral (shear) webs linking the upper and lower parts of the blade shell or by a box beam (box spar with shell fairings) (see Schema on Figure 2
]. The flapwise load is caused by the wind pressure, and the edgewise load is caused by gravitational forces and torque load. The flapwise bending is resisted by the spar, internal webs or spar inside the blade, while the edges of the profile carry the edgewise bending. From the point of loads on materials, one of the main laminates in the main spar is subjected to cyclic tension-tension loads (pressure side) while the other (suction side) is subjected to cyclic compression-compression loads. The laminates at the leading and trailing edges that carry the bending moments associated with the gravitation loads are subjected to tension-compression loads. The aeroshells, which are made of sandwich structures, are primarily designed against elastic buckling. The different cyclic loading histories that exist at the various locations at the blades suggest that it could be advantageous to use different materials for different parts of the blade.
A major trend in wind turbine development is the increase in size and offshore placements. Increasing size is motivating by the desire to reduce of the leveraged cost of energy. With increasing size, the weight of the rotor blades increases, so that gravitational loads become design drivers. Also longer blades deflect more, so that structural stiffness (to ensure tip clearance, i.e., to avoid the blade to hit the tower) is of increasing importance. Thus, from a materials perspective, the stiffness-to-weight is of major importance. In addition, with the turbine designed to be in operation for 20–25 years, the high-cycle fatigue (exceeding 100 million load cycles) behavior of composites and material interfaces (bondlines, sandwich/composite interfaces) is of major importance.
2.2. Overview of Manufacturing of Wind Turbine Blades
During the first decades of the wind energy development, wind turbine blades were often produced using the wet hand lay-up technology, in open molds. The glass-fiber reinforcement was impregnated using paint brushes and rollers. The shells were adhesively bonded together/to the spars. This technology was used mainly to produce small and medium size blades (up to 35 and 55 m, respectively). For larger blades, the same technology was used, but the web were inserted and adhesively bonded between two sides, and the plies with more fiber content were used. The disadvantages of the open mold technology are high labor costs, relatively low quality of products and environmental problems. In 1970s, several companies and institutes explored the applicability of filament winding technology, seeking to improve the quality of turbine and to reduce labor costs [13
The introduction of vacuum infusion and prepreg technologies allowed improving the quality of manufacturing [14
]. The prepreg technology, adapted from the aircraft industry, is based on utilizing “pre-impregnated” composite fibers, which already contain an amount of the matrix material bonding them together. Prepreg (widely used, for instance, by the Danish wind turbine producer Vestas) allows the industrial impregnation of fibers, and then forming the impregnated fibers to complex shapes.
The most widely used technology to produce the wind blades, especially longer blades, is the resin infusion technology. In the resin infusion technology, fibers are placed in closed and sealed mold, and resin is injected into the mold cavity under pressure. After the resin fills all the volume between fibers, the component is cured with heat. The resin infusion technologies can be divided into two groups: Resin Transfer Molding (RTM) (resin injection under pressure higher than atmospheric one) and Vacuum Assisted Resin Transfer Molding (VARTM) (or Vacuum Infusion Process) (when resin is injected under vacuum or pressure lower than atmospheric, typically, under a vacuum bag) [15
]. A variation of VARTM called SCRIMP™ (i.e., Seemann Composite Resin Infusion Process) was developed in late 1980s and is quite efficient for producing large and thick parts. Currently, vacuum assisted resin transfer molding (VARTM) is the most common manufacturing method for manufacturing of wind turbine rotor blades. With his method, layers of fabrics of dry fibers, with nearly all unidirectional fibers, aligned in the direction along the length of the blade, are position on mold parts along with polymer foams or balsa wood for sandwich structures (for the aeroshells). In order to form a laminate that is thick by the root and gradually becomes thinner towards the tip, most plies run from the root only partly toward the tip; the termination of a ply is called ply-drop. The fabrics and subsequently covered by a vacuum bag and made air-tight. After the application of vacuum, low-viscosity resin flows in and wets the fibers. After infusion, the resin cures at room temperature. In most cases, wind turbine rotor blades are made in large parts, e.g., as two aeroshells with a load-carrying box (spar) or internal webs that are then bonded together. Sometimes, the composite structure is post cured at elevated temperature. In principle, this manufacturing method is well suited for upscaling, since the number of resin inlets and vacuum suction points can be increased. A challenge with upscaling is however, than quite many layer of dry fabrics must be kept in place and should not slip relative to each other. The composite is quite thick by the root section, typically exceeding 50–60 mm in the consolidated state. In practice, it can be a challenge to avoid the formation of wrinkles at double-curved areas and areas with un-wetted fibers and air bubbles can be entrapped in the bondlines. After manufacturing, the blades are subjected to quality control and manufacturing defects are repaired. Since a large blade represents a large value in materials, increasing sizes means that it becomes less and less attractive to discard blades with manufacturing defects. Thus, with increasing size the requirements towards materials go towards easier processing and materials should preferably be more damage tolerant so that larger manufacturing defects can be tolerated. Figure 3
shows the schematics of the manufacturing of a wind turbine rotor blade by assemblage and bonding of two aeroshells and two shear webs.
The infusion process is usually cheaper that the prepreg process. However, the prepreg composites have more stable, better and less variable mechanical properties than the composites produced by resin infusion. This technology is relatively environmental friendly, and makes it possible to achieve higher volume content of fibers, and to control the materials properties. Further, the prepreg technology allows higher level of automation and better choice of resins.
Lately, the automated tape lay-up, automated fiber placement, two-pieces or segment wind blades, enhanced finishing technologies are expected to come into use to improve quality and reduce costs of the composite blade manufacturing [14
]. A big challenge, in comparison with e.g., automatization of composite structures for aerospace, is the much larger thicknesses and the much larger amount of materials to be places in the molds for wind turbine rotor blades. For some parts of the blades, 3D woven composites represent a promising alternative to producing fiber reinforced laminates. Mohamed and Wetzel [16
] suggested producing spar caps from 3D woven carbon/glass hybrid composites. It was demonstrated that this technology allows producing spar caps with higher stiffness and lower weight, than the commonly used technologies.
2.3. Overview of Blade Damage
Precise information about the range and extent of damages found and repaired in operating wind farms is not generally available, however detailed studies of the composite material and adhesive interface damage found in wind turbine blades subjected to structural testing have existed for some time [17
]. The static loads and cyclic loading applied to the blade structure during full scale testing can result in the damage in the form of failure of various adhesive layers, laminate delamination, debonding at skin/core interfaces and splitting along fibers or in-plane compressive failure as well as gelcoat/skin debonding and cracks in the gelcoat. Obviously, damages in the primary load-carrying laminates (main spar and laminates at the leading and trailing edges) are of major concern. Fortunately composite materials are damage tolerant materials. Still, a major issue is that many of these damage modes are not easily detectable, since the damage do not originate from the external surfaces and may not be visible. For instance, in thick composite parts wrinkles may lead to the formation of compression failure and delamination. Cracks and delamination can also start from processing details such as ply-drops that locally causes a stress concentration. Cracks at e.g., trailing edge bondlines can be seen visually, but it is more difficult to assess how far they extend into the composite structure.
In addition to the various structural loading effects, wind turbine blades can also be subjected to lightning strikes, physical impacts and damaging surface erosion conditions whilst in operation. In certain rare but dramatic cases, a particular event can cause the total failure of a blade almost immediately; a powerful lightning strike or an extreme wind loading that leads to a rotating blade hitting the tower for example. Operators of wind farms take measures to minimize exposure of their structural assets to the full effect of storm conditions when these are forecast. But more commonly, over the course of a normal 25-year service life, it is expected that the composite material in a wind turbine blade will accumulate some signs of damage.
Blades are the most vulnerable parts of a wind turbine with respect to lightning. As every turbine can expect to experience a significant number of strikes during service life [19
], all blades have a lightning protection system to reduce the effect of such strikes when they occur. Despite this it is common to observe scorching damage and cracking around the lightning attraction point of a blade as well as spar rupture, separation and surface tearing in more extreme cases [20
A significant damage form observed in operating turbine blades is caused by (abrasive) airborne particulates impacting and eroding the leading edge, especially towards the tip where velocities are higher. Once established this rough surface will degrade the aerodynamic performance of the blade and reduce power production; if left unrepaired structural damage to the laminate material will soon develop requiring a longer and more complex repair effort [21
Icing is the accumulation of ice on the surface of the blades under particular low temperature weather conditions [23
]. In extreme cases it will stop the operation of the turbine, but before that will disrupt the aerodynamics of the blade and reduce the energy generation as well as unbalancing the load distribution in the system and thus reducing structural fatigue lifetime [24
5. Damage in Operating Wind Turbine Blades: Inspection and Monitoring Tools
Minor damage in the composite can be tolerated if it does not impair the structural performance of the turbine or risk propagating under normal operating conditions. But some forms of damage, once present in the structure, will propagate quickly and reduce the performance of the turbine or even cause an instability that will overload other structural components and potentially cause structural failure of the turbine. Clearly this will halt the operation of the turbine. Once this has occurred the wind farm operator will face a significant cost to make the necessary repairs before the turbine can be put back into operation. Although no strict guidelines exist to determine the criticality of different damages and defects to be found in operating WTBs, all wind farms have an inspection and maintenance procedure for their blades. The justification is that by checking the condition of the structure regularly, it is possible to schedule a series of minor repair tasks that will reduce the financial risk of an unscheduled major repair becoming necessary.
This process requires firstly an inspection of the current condition of all the blades. Previously requiring direct access by maintenance crew using climbing ropes or a crane lift, this task is more commonly done now with high resolution camera images taken from the ground an inspection platform, or mounted on drones [109
]. These images can be handled by computer software that “stiches” all the images together to allow a detailed overview of the entire surface area. Image analysis software can then compare previous inspections to the most recent and highlight any “exceptions” that should be checked by a trained blade engineer. In this way surface cracks (and also occasionally surface dirt and lighting/shading effects) will be registered for more detailed inspection in the follow-up phase.
As mentioned earlier, many severe damages are sub-surfaces and it is not always possible to make a judgement on the severity of the damages via an image. Therefore, a close inspection by a maintenance engineer must also be scheduled. Visiting all the areas highlighted by the initial inspection, the engineer will gather more information on each damage area (perhaps also discovering new ones) and generate a recommendation for each blade’s repair requirement. The inspection is commonly based on the simplest and most robust techniques of visual assessment and manual tap testing. From this effort a job list (and a cost estimate) for the entire wind farm repair will be produced and approved.
It is common for external contractors to bid for the maintenance contract on large wind farms. Although lucrative, it is a competitive business and as the wind industry has grown and matured, access technologies and logistic advances have, together with the competition for contracts, had a lowering effect on the market price. Resisting this is the trend for larger, more complex and damage critical blades being placed in remote offshore wind farms. So maintenance and repair for blades in wind farms is still a significant portion of operating costs. And for operators there is also the consideration of risk around ensuring trustworthy third party inspections and repairs. On-site repairs are routinely documented with an annotated photograph of the pre- and post-repair damage. More analytical evaluation of the repair effectiveness is not conducted.
The idea of introducing a degree of automation into the entire inspection process for wind turbine blades has been investigated for some time [110
]. Sensors mounted in or on the blades provide continuous data remotely to the wind farm operator that can then be used to make best use of the available maintenance manpower. But implementing this in operating wind farms would incur additional expense at the highly price-focused manufacturing stage, where embedded sensors would need to be integrated with the structure. And transporting expensive and fragile measurement and inspection equipment to the harsh operating environment of offshore wind farms has similarly limited the uptake of more advanced NDT hardware. Whereas the robust nature of visual/manual inspection (now streamlined and improved by tele- or drone photography and image analysis software) has been favored.
Advanced inspection tools (such as Ultrasonic scanning and thermography) are routinely used in the industry to provide Quality Assurance and Control from blade manufacturing. And full scale blade test facilities use sensor technology (resistance strain gauges, fiber optics, acoustic emission, digital image correlation, etc.) to provide real-time information about the response of the structure to various load conditions, and warnings about the occurrence and severity of any damage events. So there is no question that there exists inspection and monitoring technology that can be applied. The challenge that has yet to be solved is the relevant integration of robust, low-cost monitoring and inspection technology into the entire lifecycle analysis of the large wind farms that provides incontrovertible evidence of an improved exploitation of the structural asset. The discussion on detection tools is a balance between sensors with high sensitivity (enabling very early detection of small damages) possible requiring many sensors with small spatial coverage to sensors with less sensitivity, and thus much later detection of the damage, but requiring less sensors.
The offshore wind sector will continue to expand over the next several years (under 20 GW in 2016 to over 150 GW in 2030); in parallel to this, sensor (and inspection) technology will continue to become cheaper, more robust, with a higher functionality, and more widely applied and integrated within industrial systems and processes. In addition, the materials and structural design and manufacturing (plus repair procedures) will also advance. It seems inevitable that some combination of sensorised multi-functionality will shortly be incorporated into blades that are specifically designed to take advantage of these new understandings emerging around damage detection and damage tolerance criteria [114
]. The most commonly investigated sensor technologies for permanent on-line monitoring are described below.
Vibration based damage detection systems rely on analysis of the dynamic response of the blades, either during operation or following an applied mechanical input. Damage identification is commonly based on the comparison between an undamaged and a damaged state. Ideally the ambient energy generated by the turbine operation would be used as the excitation source, however under realistic conditions detecting damage with simple analysis is challenging as the same order of modal property variations are generated by environmental effects and noise contamination [115
]. Therefore, more sophisticated methods must be deployed to create a reliable SHM system [116
], and this can limit general application. For vibration based damage detection, more success has been reported using an external shaker or embedded actuator as using these a well distributed excitation is created within the entire structure, and a flat spectrum is generated in the frequency range of interest [117
]. Vibration based techniques have the advantage of being a mature, well-proven and cheap solution with respect to wind turbine gearbox and bearing fault detection systems. Despite the challenges involved in monitoring the more complex blade structure, some successes have been demonstrated. However, structural response changes will only detect relatively large damages in the blades and this limits the usefulness of the technique.
Fiber optics embedded in the blade structure can be used to measure strain. Several fibre Bragg gratings (FBGs) can be multiplexed within a single measurement fibre optic and with high sensitivity and reliability return local point strain measurements [118
]. Combined with their long fatigue life and immunity to electromagnetism, fibre optics are a promising sensor technique for integration within fibre reinforced plastic structures. FBG based systems are the most technologically ready fibre optic measurement system currently with commercial systems reducing prices and the size of installation hardware, as well as improving their robustness. The problem of fragile, bulky and expensive fibre optic measurement hardware is still true for most other (non-FBG) forms of fibre optic systems. For example, distributed sensing based on optical backscatter reflectometry [119
] recognizes changes in density and composition along the entire length of an unmodified optical fibre. Once recognized by the system, this “fingerprint” value will change due to any local variations in strain and temperature allowing measurements to be returned all along the fibre with a high resolution. This gives far more detail than even a heavily multiplexed FBG array can provide. The technique however, is currently limited to static testing as the extremely low levels of natural back scatter means signal to noise ratio is unacceptable in a dynamic situation.
Acoustic emission (AE) involves detecting transient bursts of elastic stress wave energy released by the formation of damage within the structure. Usually achieved with surface-mounted piezoelectric sensors which transform the elastic energy into an electrical waveform which can be processed and analysed. Detectable AE signals occur when a fibre composite material begins to experience local fiber failure, debonding, matrix cracking, delamination and splitting as the structure is placed under load. FRP structures generate huge numbers of such AE signals when loaded above previously reached maximum loads, as well as at lower load levels when previously formed damage is already present in the structure. This means that AE is a simple, useful and intuitive tool for detecting and locating damage during full scale testing in both static and dynamic loadings. The frequency range of useful AE signals is between 100 kHz and 1 MHz which above that used in vibration-based sensing (and in human hearing), but below the range designated as ultrasonic. Acoustic emission monitoring systems are used commercially in rotating machinery, metallic structures, bridge structures, and simple composite structures like pressure tanks. However, in-operation wind turbine blade monitoring is not a commercial activity due to the expensive hardware, the large data sets generated by the high sampling rate, and the attenuation rate for this frequency range in composite materials limiting the sensoric range meaning many sensors would be required to fully instrument each large wind turbine blade.
Acoustic emission is a passive technique that detects energy generated by the structure, in contrast guided wave technology (or acoustoultrasonics) is the terms used when the piezoelectric sensors are instead used as active transducers to generate a pre-defined input signal that can propagate through the structure and be detected by neighboring sensors [114
]. This known “Pitch-Catch” configuration will be altered when the signal interacts with damage or other factors that affect wave propagation between the input and detection points. In this way a network of sensor transducers can monitor any changes occurring over an entire structure. Commercially this technology is successful in simple oil and gas pipeline monitoring, however the complex composite material and dynamic structural environment of the wind turbine blade is proving a challenge. The use of guided wave technology for this application is therefore still in development, but is considered a promising technique.
As pressure is put on to have greener and more sustainable products, the recycling of wind turbine blade have increasingly attracted the interest of wind turbine blade manufacturers and owners. However, recycling blades remains a challenge. The difficulties related to the process are mainly due to the structure of the blade and to the composite materials used.
The structure of the blade, as represented in Figure 2
, is made of several elements, namely shear webs, load carrying beam, leading and trailing edge and aerodynamic shell. Depending on the blade manufacturer, the design and the arrangement of these elements will be different. In general each of these elements is consisting of a specific type of composite and the blade is manufactured as a one-piece component. To separate the different elements, the locations of the elements need to be known and a saw with diamond blade and sufficient water cooling is required. Due to that complex structure, it is difficult to recycle blades into any other application than blade. In addition, the blades to be recycled will be found in various conditions. Decommissioning of wind turbines can be decided as the turbines are reaching end of life, but also at earlier stage if it becomes interesting to replace the turbines by newer models or because the turbines were prematurely damaged. As a result, the quality of the material found in blades and the quality of the blade structure will be varying from blade to blade. The assessment of the blades conditions also represents a challenge. Visual inspection, which is normally used to determine the conditions of blades during inspection, does not reveal the presence of potential sub-surface damages. Finally, the amount of material coming from blades will fluctuate greatly as material will sporadically come from the decommissioning of single turbine or large windfarm. To summarize, the amount of material to be recycled coming from wind turbine blades will be varying in design and material, in quality and quantity. The development of a sustainable recycling solution for blades is therefore very complicated.
The other challenge in recycling blades is related to the composite material used in blades, which are made of a thermosetting matrix and glass fibers or a combination of glass and carbon fibers. Unlike thermoplastics, thermosetting matrix cannot be remolded to form new product. So the options are either to reuse the blade and the composite material elements as they are found in the blade or to transform the composite material into a new source of material. The first option only requires cutting the blade, while in the second option, heavier and more advanced processes need to be use. The first options will necessarily lead to a limited number of possible applications, while the second options will open up to many more. As an example, Figure 10
shows a playground made out of entire sections of blade. The structure of the blade is reused, but the application will be difficult to upscale to an industrial scale solution. Regarding the solutions involving heavier reprocessing, research on composite recycling has focused on processes to separate the fibers from the matrix to reuse them in new polymer composite applications [121
]. To do so, heat is necessary to degrade or dissolve the matrix material. The temperatures used in the recycling processes vary from 280 °C for a supercritical fluids process to 450 °C for a fluidized bed process [122
]. The issue is that the heat treatment will be detrimental to the mechanical properties of glass fibers, which will become extremely brittle [126
]. The glass fibers properties are however not the only challenge to overcome. The recovered fibers should also have not too rough surfaces and should be repositioned in specific directions to deserve the purpose of the new application. Finally the cost of the recovered fibers represents the main barrier for implementing these processes on an industrial scale, as pristine fibers remains less expensive. A simpler transformation of the composite material is to shred it. This solution represents however a significant down-cycling of the composite material, as the resulting shredded composite can only be used as a filler or similar material. Neowa GmbH, is using this technique to recycle glass fiber reinforced thermosetting composites in cement production [127
]. This company is currently the only industrial recycling station in Europe able to process composites. Another solution based on shredded composite was developed by Miljøskærm in Denmark. The company uses agglomerated shredded composite in sound insulation panels [128
]. Given the challenges presented, it seems that a sustainable solution for recycling wind turbine blade will need to be based on a combination of several solutions in order to consider all possible scenarios.
For the reduction of the fossil fuel dependency, the renewable energy, in particularly, wind energy production should be drastically expanded in the next decades. This can be achieved by the installation and use of large and extra-large wind turbines, to be placed in wind parks either off-shore or on-shore. The basic requirements to the performances of such wind turbine can be satisfied only by using advanced, lightweight, highly durable, fatigue resistant and damage tolerant and stiff composite materials.
The most important parts of the turbines, produced from composites, wind turbine blades, are subject to complex, combined impact, static and random cyclic loading. In order to resist these loading over many years and hundreds of millions of loading cycles (on the one side) and to reduce the loads (like gravity, on the other side), the wind blades are built from fiber reinforced polymer composites. While the currently available solutions (in the simplest case, E-glass/epoxy composite) satisfy most of these conditions partially, the necessity for new, better solutions leading to the increased reliability and reduced costs for wind turbines, is apparent. That is why a lot of efforts are put in the development of new, stronger, more damage resistant, faster producible, more environmentally friendly and recyclable composites for wind turbines. Some of the promising directions of development of stronger, more reliable, environmentally friendly and economically producible composites are listed below.
Development of new epoxy resin systems which have low mix viscosity, better wetting of fibers (by modifying either resins or applying special sizing on fibers) and allow low infusion pressure in the vacuum assisted resin transfer molding (VARTM) should lead to the blades with minimum production defects. Further, automated component deposition during VARTM can allow improving the wind blade quality as well. Yet, the increase in size of turbine blades most likely leads to more manufacturing defects. Thus, the development of more damage tolerant materials is desired. Resins with faster cure and lower curing temperature allow reducing the processing time and automating the manufacturing.
Carbon fibers represent a very promising alternative to the traditional E-glass fibers. Other alternatives are high strength glasses, basalt, aramid and natural fibers. Carbon fibers ensure higher stiffness while their disadvantages are higher costs, lower compressive strength and high sensitivity to local defects (e.g., misalignment). In several studies, the combination of carbon and E-glass fibers was recommended as a promising solution, which allows to achieve the combination of higher stiffness (due to carbon fibers) with limited cost increase. With view of resin matrix, thermoplastics have some advantages over traditionally used thermosets, e.g., recyclability. The investigations on the applicability of these groups of materials for wind blade composites have been carried out intensively during the last years.
The strength and durability of wind blades are controlled by damage processes at the microlevel, in fibers, on the fiber/matrix interfaces, between plies. It suggests an idea that if these microscale properties of the materials are enhanced, the strength and lifetime of the composites, and, generally, wind turbines is increased. This can be realized by nanoscale modifications of the material structures, i.e., by introducing nanoscale particles (of the size order 1–10 nm) in the fiber sizing, matrix and interfaces between plies. The materials with nanoengineered matrix (or sizing) and microscale (e.g., carbon fiber) reinforcement can demonstrate in some cases the up to 80% higher fracture toughness and lifetime than the neat composites.