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The fragility of fiber-reinforced plastics: How does high strength encounter delamination?
In materials science, delamination refers to the occurrence of layered failure in materials. This mode of failure affects not only fiber-reinforced plastics but also a variety of materials such as laminated composites and concrete. Delamination often results from layered structures created during processing, and is a common problem in plastics and metals manufactured using technologies such as roll-formed steel or 3D printing.
In laminated composites, the bonds between the layers often fail first, causing separation between the layers.
The structure of fiber-reinforced plastics usually consists of a high-strength reinforcing material (such as carbon fiber or glass fiber) and a weaker polymer matrix (such as epoxy resin). In this structure, when a load perpendicular to the reinforcement layer is applied, or when a shear load is applied, the polymer matrix is prone to cracking, and even the reinforcing fibers may debond from the polymer matrix.
To further understand the impact of delamination, let’s turn to another common application: reinforced concrete. In this material, when the near-surface corrosion of metal reinforcement occurs, delamination of the concrete will ensue. As the oxidized metal increases in volume, it will create stresses when restrained by concrete; when these stresses exceed the strength of the concrete, cracks will form and propagate, causing the surface concrete to separate.
Improper handling during processing can lead to material delamination. If the surface of the concrete peels off due to improper construction, the risk is even higher.
In solid materials, failure of layered structures due to improper handling is not an isolated problem, especially in additive manufacturing techniques (such as fused deposition processes) where the gap between the cooling thermoplastic layer and the cooling substrate Temperature differences can also cause the layers to separate.
Detection method
Exploring effective ways to detect delamination is an important task in materials science research. Currently, a variety of non-destructive testing techniques are available to detect delamination in structures, including visual inspection, percussion inspection, ultrasound, radiography, and infrared imaging. Visual inspection can be used to detect delamination on the surface and edges of materials, but it cannot detect delamination hidden inside the material through cutting.
Tapping detection determines the presence of delamination based on sound changes by tapping the material. A bright ringing sound usually indicates good bonding of the materials, while a dull sound may mean something is wrong.
In the application of laminated composite materials, detecting the clarity of sound is a simple and effective method to evaluate the condition of the material. However, the subjectivity of this detection method depends on the hearing and judgment ability of the examiner, and design changes of the part may also affect the pitch of the sound, thereby affecting the detection conclusion.
Layered resistance test method
In the process of solving layering problems, resistance testing methods are also extremely important. The American Society for Testing and Materials (ASTM) provides a series of standards, including coating adhesion tests, that are used to evaluate coatings and their adhesion to the substrate and resistance to delamination.
The flexure test is an important method for testing the fracture toughness between fabrics and is frequently used in unidirectional fiber-reinforced polymer laminates.
In addition, interlaminar shear strength testing is also used to measure the bond strength between fibers and matrix. Under various loading conditions, these tests can help evaluate the potential for delamination and guide material improvements.
Not only that, for natural materials such as wood, interlaminar shear strength also plays an important role. For example, the structure of a floor panel makes it susceptible to vibration in response to deformation.
Future challenges
As technology advances, researchers will face increasing challenges to improve the strength and durability of materials. How to overcome inherent material limitations and achieve more stable fiber-reinforced plastics? This will be a long-term research topic and a question worthy of our deep thought.
