Introduction

Composite materials, especially those that exhibit high strength-to-weight ratios and thermal stability, continue to be of increased interest to the Defense industry. Materials researchers, vehicle designers/manufacturers, and other industry stakeholders are recognizing the potentially significant cost savings (among other benefits) that could result from the lighter weight and correspondingly improved performance and fuel efficiency associated with incorporating composites into vehicle designs and a myriad of other applications. That said, the complex interaction between composite materials and the respective system requires careful design, especially in structural applications. One issue in particular is the failure to design for reparability, while striving to minimize up-front costs, which could ultimately burden the customer with high system life-cycle costs [1]. This article discusses contemporary issues and considerations associated with structural composite composition, failure, inspection, and repair.

Composite Composition: A Closer Look

A structural composite consists of matrix material and load-carrying reinforcement material. Matrix materials include plastic, ceramic, metal, and glass. Reinforcement materials include fiber, chopped fibers, flakes, particles, and whiskers.

The most common composite matrix materials are some form of plastic. Plastics can be categorized in two groups: thermoplastic and thermoset. Thermoplastics can be further categorized into their degree of crystallinity (i.e., amorphous, semi-crystalline, or crystalline). Amorphous polymers consist of completely random-ordered chains. Crystalline polymers consist of completely ordered three-dimensional chains, further oriented with weak interfacial bonds, as illustrated in Figure 1. Semi-crystalline polymers reside somewhere in between amorphous and crystalline polymers. Amorphous polymers are impact-resistant and elastic while crystalline polymers are harder, stiffer (more brittle), and more thermally stable than amorphous polymers. The degree of crystallinity depends on several factors, including viscosity and the number of side groups of the polymer chains. The weak interfacial bonds between chains of a thermoplastic fade with the application of heat. As the chains ebb, the material softens, allowing it to be reformed. Common thermoplastic materials include polyetheretherketone (PEEK) and polyphenylene sulfide (PPS).

A thermoset polymer is one large cross-linked molecule that is held together with strong covalent bonds. If one were to heat a cured thermoset, the cross-links would prevent displacement of the chains and the material would degrade. Consequently, thermosets cannot be reshaped like thermoplastics after curing, and repairs require a liquid state catalyzer for cross-linking the repair material into the cured material. Commonly used thermoset materials include epoxy, polyester, silicone, and bismaleimides.

Typically, thermoplastics exhibit increased toughness when compared with thermosets. However, it is important to note that the increased addition of fillers or fibers to thermoplastics reduces its impact strength to levels comparable to thermosets [2]. A comparison of thermoplastic and thermoset properties is provided in Table 1.

Table 1:  Qualitative Comparison of Current Thermoplastics and Thermosets [3]

Composite Failure

Composite failures can largely be attributed to a material fracture or physical damage. Composite fractures typically result from material breakage at a fundamental level. The interface between the fiber and matrix is crucial for stress transfer. Fracture damage can be the result of breakage of atomic bonds, fiber breakage, debonding between the fiber and matrix, and delamination [4, 5]. In this context, damage refers to the distributed irreversible change that results from external physical or chemical loading. The most common damage types include core damage, intralaminar matrix cracks, delamination, and fiber fracture [6]. Delamination, one of the more common forms of fracture damage, is the separation of plies caused by interlaminar cracking between two adjoining plies in a laminate [4].

Inspection Techniques
The extent of composite material damage cannot be easily assessed by visual inspection alone. For example, low-energy impacts often leave no visible marks on the surface, yet can result in extensive underlying delaminations, commonly referred to as barely visible impact damage (BVID). Accordingly, there are several nondestructive inspection (NDI) techniques for composites, such as tap testing, ultrasonic inspection, X-ray inspection, and thermography, which can be used to detect hidden damage.

Tap testing (which is performed manually or with a special tap hammer) is one of the simplest methods that can be used in the field on composites with up to five or six plies. Such testing is often performed by tapping the surface of a structure and discriminating good areas from bad by analyzing differences in sound resonance. Experienced testers need only their ears to discern the good areas, which tend to reverberate and ring from the bad areas, which tend to thud [7]. Alternatively, special tap hammers can be used in noisy environments, such as an airfield, to provide greater objectivity. In either case, the tester must have a good understanding of the underlying structure, as the difference in tones from internal doublers and stiffeners could lead to a false interpretation [8].

One of the most popular NDI techniques for composites is ultrasonic inspection [9, 10]. Pulse-echo ultrasonic equipment can be used to determine the depth of defects. However, this equipment does not work well when inspecting composites with a core material. Alternatively, through-transmission ultrasonics overcome the core limitation of pulse-echo ultrasonics but require access to both sides of the item under inspection [11]. Additionally, through-transmission equipment is typically fixed in a dedicated facility and not generally field deployable.

X-ray equipment can provide the most detailed information to inspectors [11]. However, the major limitation with this type of equipment is detecting delaminations parallel to the image plane. Similar to ultrasonic equipment, X-ray equipment is also generally fixed in a dedicated facility and requires well-trained inspectors [8].

Repair

Repairing a damaged composite component to its original mechanical properties is extremely challenging. The characteristics of the repaired composite are generally never the same as the original. Consequently, there are typically three tradeoffs to consider when implementing a composite repair: strength, stiffness, and weight. For example, if attempting to match the original strength of a composite, the resulting repair is often heavier and stiffer.

Further repair options are heavily influenced by the constituent materials, fiber orientations, core material, laminate thickness, number of lamina, and designed strain level. Key factors for implementing a successful repair include surface preparation, adhesive choice, repair materials, and processing conditions. The major requirements and considerations for typical repairs are summarized in Table 2.

Table 2:  Requirements and Considerations for Repair of Composites [6]

Surface Preparation
Moisture in the materials (parent composite and repair) and air (humidity) can lead to a degradation of properties, especially when subject to high heat cure cycles. In sandwich structures, the skin and core can rupture due to high steam pressure from the presence of moisture during the cure process [12, 13]. Most adhesive formulations require a low relative humidity (>40% ) repair environment. Moisture absorbed by adhesives (typically during application) can result in porous bondlines during high-temperature cure cycles [12, 13].

The ability of the adhesive to maintain contact with the solid parent structure—termed wettability—is largely dependent on surface preparation. Typical surface preparations include blasting, sanding, and chemical treatments. Additionally, laser surface preparation is a promising newer technique that can remove virtually all surface contaminates [14]. The type of laser is crucial, as the bulk material properties can be inadvertently affected. An integrated laser for both damage removal and surface preparation could provide a great benefit for bonded composite repairs, most notably the reduction of human variability [5].

Adhesive Choice
Adhesive choice is also a crucial element for successful composite repair. The repair patch must have a strong and durable bond to the parent composite throughout the remaining lifetime. The adhesive choice will depend on the repair patch and parent composite materials, operating environment, geometry, accessibility (ability to remove the part), and manufacturing facility.

Typically, high-temperature adhesives are brittle and stiff at low temperatures, and low-temperature adhesives are too weak or degrade at high temperatures. Adhesives capable of withstanding a high-temperature environment generally require higher cure temperatures. High-temperature curing, requires caution, as it is possible to create even more damage through overheating of the undamaged parent materials. This curing also poses a higher risk of damage, such as a ruptured core or delamination, due to pressure, which occurs as any absorbed moisture is converted into steam. Typical characteristics of common composite repair adhesives are provided in Table 3.

Table 3:  Typical Characteristics of Adhesive Types [13]

Adhesive Fillet

With many composite repairs, adhesive joints result in peel stress (moment) concentrations at the end of the overlapping repair patch. When subjected to high peel stresses, the composite will likely fail in the transverse direction, as there is little to no load-carrying between lamina. Both the stress concentration and the transverse stress distribution in the composite can be significantly reduced by filleting the adhesive joint. The maximum transverse stress location in an adhesively bonded metal doubler, with and without an adhesive fillet, for a thermal and tensile load is illustrated in Figure 2. The ability of an adhesive to be used in a fillet is a major consideration for structural composite component repair. For such applications, high-viscosity adhesives are a highly desirable characteristic. And once cured, the exposed fillets and bondlines of a repair are typically protected with the use of a sealer.

Figure 2:  Maximum Transverse Stress Locations in the Composite for a Tensile Load and a Thermal Load [15].

Thermoset Adhesives
The most common adhesives for composite repairs are thermosets, and the specific choice of adhesive depends largely on the constituent material. The cross-linking of a thermoset, and therefore the quality of the bond, are largely dependent on cure process settings. The rate and degree of cross-linking can be tailored through the combined use of accelerators and temperature.

With similar properties to constituent composite materials, epoxy is the most common family of adhesives. Epoxy adhesives can be altered with a variety of additives, such as viscosity modifiers, flexibilizers, and tougheners. Further, epoxy adhesives are supplied as a film and one- or two-part curing liquid (or pastes). One-part epoxies require an elevated temperature for curing (250–350°F) whereas two-part epoxies are capable of curing and cross-linking at room temperature due to the chemistry of a curing agent [13].

Adhesives for Thermoplastic Composites
A major advantage of thermoplastic composite repair is the composites’ higher processing temperatures. Thermoset and amorphous thermoplastic adhesives generally require lower cure temperatures than semi-crystalline thermoplastics, mitigating concerns for temperature-induced damage of the parent material. The ability of thermoplastic materials to be melted and reshaped, in conjunction with their mechanical and thermal properties, makes their use appealing for future repair technologies.

Because thermoplastic materials have a lower surface energy than thermosets, they are more difficult to wet for an adhesive bond [2]. Accordingly, the surface treatment requires methods to alter the chemistry and surface geometry (roughing) for adequate adhesive bonding [16].

Surface treatments for thermosets are often inadequate for thermoplastics, requiring the repair of thermoplastics to adopt less-conventional surface preparation techniques. Examples of such include plasma etching, flame treatment, laser treatment, and corona discharge treatment. Plasma treatment provides surface etching on an atomic level via inert gases. Flame etching oxidizes the surface by passing a gas flame over the surface. Corona treatment is performed through electrical discharge of one or more high-voltage electrodes through the thermoplastic [2].

Adhesive-Bonded Repairs
Adhesive-bonded repairs, such as the scarf repair, are generally considered the best alternative in terms of strength, stiffness, and weight trade-offs [6, 8]. Thin, lightly loaded laminates and sandwich structures are generally limited to adhesive-bonded repairs. This limitation arises from stress concentrations induced by holes for mechanical fasteners. Accordingly, adhesive joints must be designed to sustain loading shear, such as the illustrated shear stress distribution of the various bonded joint configurations in Figure 3. Generally, adhesive bonds for composite materials are incapable of withstanding peel loading, tension, and cleavage [13].

Figure 3:  Various Bonded Joint Configurations.  (a), (b), and (c) Shear, (d) Scarf, (e) Tension, (f) Peel [13].

The doubler repair is the most basic adhesive-bonded repair. The doubler repair patch is applied to one or both sides of the parent composite and is typically referred to as a single or double lap. The doubler patch overlay must be long enough to distribute elastic forces through the adhesive [6]. The adhesive shear stress for a skin-doubler specimen is illustrated in Figure 4. For these types of sandwich structures, the damaged core is removed, a replacement core is fixed in the void, core splice adhesive bonds the replacement core to undamaged core, film adhesive bonds the core to ply (transverse direction), and a doubler patch is bonded over the replacement core and parent lamina. When implementing a doubler repair, the use of a vacuum to apply pressure for an adhesive cure is not recommended as it can result in voids and increased porosity, leading to a poor bondline [13].

Figure 4:  Adhesive Shear Stress for a Skin-Doubler Specimen.  E, Tensile Modulus of Adherends; G, Shear Modulus of Adhesives [13].

The two primary aerodynamic composite structure repair methods are referred to as stepped and scarf. In a stepped repair, each ply is cut down individually, leaving a stepped pattern of decreasing area as depth increases. Care is required when removing the individual plies to avoid damaging fibers in undamaged areas from grinding and sanding. Surface preparation is also crucial for adhesive bonding. Ply patches are oriented in the parent composite’s ply fiber orientation. It is generally preferred to use precured patches of the same material. However, precured patches are not always suitable (e.g., as with a complex surface), requiring precautions to avoid overheating.

The primary drawback to the stepped repair is matching the repair patch to the parent ply’s void. Mismatches result in sharp edges with stress risers. Additionally, stiff edges resulting from patch plies and/or adhesive can lead to peel mode failures and damage to the underlying part.

Stress discontinuations in the stepped repair can be mitigated with a scarf repair (also referred to as a tapered repair). With a scarf repair, the damaged area is removed at a constant cutting angle, called the scarf angle. Low-peel stress can be achieved by using an extremely small scarf angle. This angle is typically limited to approximately 2° to 3°, resulting in a negligible peel stress compared with the shear stress [6, 17].

Bolted Repairs

The bolted doubler repair is commonly used for repairs requiring strength. Plates, typically steel or titanium, are bolted on both sides of the damaged area as illustrated in Figure 5. While simplistic in its implementation, there are advantages and disadvantages to the bolted doubler repair method. The bolted doubler repair method requires less training as drilling and fastening require little specialized expertise; however, knowledge of the materials is crucial, and proper drilling, sealing and corrosion prevention are common areas of concern. For example, both heat and vibration from drilling can cause undesirable effects to the undamaged composite. Further, bolted repairs are generally not feasible for thin laminates, aerodynamic applications, weight-balanced components, and applications with low-observability (signature) requirements [8].

Figure 5:  Bolted Doubler Patch Repair on a Thick Skin Composite [8].

As with bolted designs for metal, bolted designs for composites should aim for low bearing stresses. Accordingly, when designing the optimal bolt pattern, the designer must know the underlying fiber layup pattern. A typical bolted repair requires at least 12.5% of the fibers in the 0°, ±45°, and 90° direction (a maximum of 37.5%) [1]. Fasteners should be selected to prevent corrosion; for carbon/epoxy composites, they are typically made of titanium or a nickel-copper alloy [11]. The patch should be sealed to the parent composite, followed by inserting and sealing the fasteners in the wet condition [6].

Thermoplastic Composite Repairs
Similar to thermoset composites, thermoplastics can be repaired using adhesives and mechanical fasteners. The ability for thermoplastics to be melted and reformed allows for fusion bonding and thermo-reforming. A fusion bond is generated through high heat and pressure, and thermo-reforming involves the removal of the part for re-processing on the original mold [2]. Both repair techniques are suitable for repairing cracks and delaminations in thermoplastics.

Welding thermoplastics with a heated tool (co-consolidation) is limited by the ability to control the melt, as the viscosity of a thermoplastic is a function of temperature. Elaborate tooling and fixtures are required for parts that cannot be removed and returned to their original mold. Resistance heating is another method for welding carbon composites that can be performed using an appropriate power supply. The natural electrical resistance of carbon fibers can generate a sufficient amount of heat, when subjected to a voltage, to facilitate a weld. In fact, the U.S. Air Force concluded resistance heating can be employed as a viable repair method for on-aircraft, in-field repairs using a heater ply of APC-2 unidirectional prepreg tape with polyetherimide (PEI) film, as illustrated in Figure 6. The amorphous thermoplastic PEI is preferred over PEEK because of its lower processing temperatures, as the high-temperature curing of PEEK film during the repair process damages the parent composite [18].

Figure 6:  Configuration for Amorphous Thermoplasic Bonding via Resistance Welding [18].

Similar welding repair methods include induction and ultrasonic welding. As with resistance welding, induction welding also leverages the composite material properties to generate heat. The conductive and ferromagenetic materials in the composite facilitate the weld by absorbing electromagnetic energy to generate heat. With ultrasonic welding, frictional heat generated from high-frequency ultrasonic acoustic vibrations facilitates the weld. Ultrasonic repair welds are limited by the beam size. Large repairs require multiple passes of the beam and yield variable results [18].

Repair Verification
Composite repair verification training is becoming increasingly important as the number of skilled repair technicians is not meeting the demands of increased composites use. Verification of repair quality is also growing increasingly expensive and more difficult to perform with the growing complexity of composite components. As with many structural repairs, it is seemingly impossible to know the strength of a repair without breaking it. Therefore, it is crucial to make companion test coupons for every adhesive repair. The long-term durability of a bonded repair requires periodic inspection throughout the remainder of the component’s lifetime.

References: