Plasticity Stress Limits For Cfrp: How Much Is Too Much?

Carbon fibre reinforced plastics (CFRP) are high-strength, lightweight composites that are increasingly being used in place of metals and glass fibre reinforced plastics (GFRP). CFRP is a popular engineering material due to its superior stiffness and strength-to-weight ratio. The effects of cyclic loading on the fatigue of CFRP have been widely studied, and the material's fatigue life and response to stress are important considerations in its application. The plasticity of CFRP is influenced by factors such as temperature, strain rate, stacking sequences, and ply orientation.

Characteristics and Values of CFRP

The effects of cyclic loading on CFRP fatigue

Carbon fibre reinforced plastics (CFRP) are high-strength, low-weight composites that are increasingly being used in place of metals and glass fibre reinforced plastics (GFRP). CFRP is a popular engineering material because of its superior stiffness and strength-to-weight ratio. However, when used in engineering structures, CFRP is often subjected to cyclic loading, which can detrimentally affect its service life and damage tolerance. This phenomenon is known as fatigue.

Fatigue in CFRP is a complex process that is influenced by various factors, including the strain rate, stacking sequence, ply orientation, temperature, and fibre properties. The effect of cyclic loading on CFRP fatigue has been the subject of numerous studies, which have shown that the tensile strength of CFRP laminates is sensitive to the strain rate. For example, at a strain rate of 100 s−1, the failure of the matrix occurred instantaneously, indicating a brittle failure process. In contrast, when the strain rates were 1 s−1 and 10 s−1, the stress-strain curve exhibited pseudo-plasticity.

The fatigue properties of CFRP have also been compared to those of other materials, such as polymers, metals, and foams. CFRP has been found to have superior specific stiffness and specific strength, making it a desirable material for engineering applications. However, the fatigue life of CFRP can be significantly affected by environmental factors such as temperature variations, submersion, and aging. For example, the fatigue life of CFRP can be reduced by matrix cracks, fibre matrix debonding, and fibre fracture, which can occur at different stages of the fatigue process.

Additionally, the test methods for cyclic fatigue loading have been summarized in several standards, such as ASTM D6115, which is a standard test method for mode I fatigue delamination growth onset of unidirectional fibre-reinforced polymer-matrix composites. Test development for cyclic-fatigue delamination propagation is an active area of research, with organizations such as ASTM and the European Structural Integrity Society (ESIS) conducting round-robin tests to investigate the effects of cyclic loading on CFRP fatigue. These tests have yielded valuable insights into the fatigue behaviour of CFRP and its potential applications in various industries.

The importance of environmental factors on CFRP fatigue

Carbon fibre reinforced plastics (CFRP) are high-strength, low-weight composites that are increasingly being used in place of metals and glass fibre reinforced plastics (GFRP). CFRP is favoured due to its strength-to-weight ratio and its resistance to environmental degradation, which makes it a good candidate for corrosion-resistant applications.

Engineering structures are often subjected to cyclic loading, which causes material fatigue and affects the service life and damage tolerance of components and joints. This is a concern for CFRP applications as the endurance of the composite varies depending on factors such as the type of fibres, resin, and stacking sequence. The fatigue life (N) of a component is defined as the total number of stress cycles required to cause material failure.

The fatigue performance of CFRP is influenced by environmental factors such as temperature variations, moisture, and exposure to corrosive solutions. These harsh service environments can accelerate the accumulation of fatigue damage, leading to the untimely failure of the composite. Mechanical loading, thermal gradients, chemical ingress, and other ambient conditions also contribute to the rate at which fatigue damage occurs in CFRP.

The dynamic mechanical properties of CFRP materials are affected by factors such as temperature, fibre direction, and stacking sequences. The tensile strength of CFRP laminates is sensitive to the strain rate, with the peak stress of composites being influenced by the strain rate. Therefore, it is important to consider the environmental factors that CFRP will be subjected to when designing fatigue-resistant structures and components.

The effect of strain rate on CFRP tensile properties

Carbon fibre reinforced plastics (CFRP) are high-strength, low-weight composites that are increasingly being used in place of metals and glass fibre reinforced plastics (GFRP). CFRP is used in a variety of industries, including aviation, aerospace, automotive, and shipbuilding, due to its excellent properties, including light weight, high strength, and corrosion resistance.

When CFRP is applied, it is often subjected to complex dynamic loadings, especially in the event of collisions. Therefore, the effect of strain rate on the tensile properties of CFRP is critical for design and product development.

Research has shown that the tensile strength of CFRP is sensitive to the strain rate. For instance, the peak stress of composites was found to be sensitive to the strain rate, with a threefold increase in peak stress when the strain rate increased from 1370 s−1 to 6066 s−1. The tensile strength of CFRP was also found to be related to the fiber direction, with the sensitivity of the tensile strength to the strain rate dependent on the fiber orientation.

The strain rate effect was also found to be related to the stacking sequences and ply orientations. The experimental results showed that the strain rate effects of the cross-ply laminates and quasi-isotropic-ply laminates were lower than those of the unidirectional-ply laminates. Additionally, the failure strain of the 45° specimen was much higher than that of the 0° specimen, indicating that the properties of CFRP laminates were highly influenced by stacking sequences and ply orientation.

In conclusion, the effect of strain rate on the tensile properties of CFRP is a critical factor in the design and product development of CFRP structures. The tensile strength of CFRP is sensitive to the strain rate, and the strain rate effect is related to the stacking sequences, ply orientations, and fiber direction.

The effect of temperature on CFRP mechanical properties

Carbon fibre reinforced plastics (CFRP) are high-strength, low-weight composites that are increasingly being used in place of metals and glass fibre reinforced plastics (GFRP). CFRP has the advantages of high strength, light weight, and excellent fatigue resistance, and has been widely used in aerospace, automotive, civil infrastructure, and other fields.

However, when CFRP is exposed to high temperatures, its mechanical properties are affected. The softening and decomposition of the resin reduce the strength of the resin itself and weaken the bonding effect of the fibres, resulting in a rapid reduction in the strength of CFRP composites. This phenomenon is further exacerbated when the temperature exceeds the glass transition temperature (Tg) and the decomposition temperature (Td), where the tensile strength of CFRP tendons decreases dramatically.

Research has shown that the tensile strength and elastic modulus properties of CFRP degrade significantly beyond 300 °C due to the decomposition of the resin. At 200-300 °C, longitudinal cracks appear on the surface of the tendons, and the CFRP tendons separate into several bunches of fibres. When the temperature exceeds 500 °C, only the carbon fibres in the CFRP tendons remain due to the complete decomposition of the resin, resulting in a loss of the majority of the mechanical properties of the CFRP tendons.

The effect of temperature on the mechanical properties of CFRP is an important consideration in engineering applications, especially in terms of fire resistance. The degradation of mechanical performance at high temperatures is a significant challenge for engineers, and further research is needed to address this issue.

Stress-strain models for FRP-confined concrete

The stress-strain behaviour of concrete confined by fibre-reinforced polymer (FRP) composites has been the subject of extensive research. The performance of FRP-confined concrete in circular columns has been widely studied, and the efficiency of the available models is considered satisfactory. However, the case of rectangular reinforced concrete (RC) sections with FRPs is more complex, and the mechanism has not been adequately described.

Several studies have focused on modelling the conditions during the failure of FRP-confined concrete, including ultimate compressive strength and the corresponding ultimate axial strain. Other studies have simulated the overall behaviour of the stress-strain curve. The existing models for FRP-confined concrete implicitly assume a constant confining stiffness and are, therefore, only applicable to concrete confined by linearly elastic materials with constant stiffness until rupture, such as FRP.

A new axial and lateral stress-strain model for confined concrete has been developed, which is applicable to circular and rectangular concrete columns retrofitted by FRP jackets. This model predicts the ultimate strength and ultimate strain and depicts the entire stress-strain diagram for FRP-confined concrete specimens. The model assumes a simple linear relationship between the confining stress and lateral strain. The axial strain is applied to the concrete incrementally, and the lateral strain and confining stress are then evaluated by solving the equations governing the lateral-to-axial strain relation and confining stress-axial strain relation.

Another theoretical stress-strain model has been developed to better understand and simulate the behaviour of CFST columns. This model consists of four main components:

• Interaction between the steel tube and concrete, taking into account the de-bonding effect
• An accurate hoop strain equation
• A passively confined concrete model considering stress-path dependence
• A three-dimensional stress-strain model for the steel tube

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