Probabilistic assessment of pin joint strength in CFRP laminates
Introduction
Advanced composite materials such as carbon fiber reinforced plastics (CFRP) are being expanded in their application in not only aerospace but also other industries because of their excellent properties. However, they generally have better scattering properties than metal materials. In addition, there is much less experience using composites as structural materials than metals. In current design and analysis methods, design parameters such as load conditions and material strengths are regarded as deterministic values, but originally, they were uncertain values. The load is ordinarily assumed as the maximum load expected in service (worst-case load). The strengths are set to design tolerances that can be regarded as the lower bound of the strengths. Furthermore, the load is multiplied by a safety factor. The safety factor is greater than unity (1.5 for manned aircrafts) and is intended to compensate for the uncertainty in loading conditions, imperfections in analysis, etc. The method based on deterministic values and safety factor have been highly refined over a long history, but a large part of the accumulated history or experience has been compiled from metallic structures; aluminum structures in the aerospace industry and composite materials are entirely different materials. Therefore, the designs of structures made from composites tend to be conservative despite their high mean values of specific strength and stiffness and good fatigue resistance.
To obtain greater efficiency in the design of structures made from composites, there have been attempts to introduce a probabilistic or reliability-based design method [1], [2], [3]. This method optimizes structures on the basis of their probability of failure or reliability and should be able to more fully utilize the good mechanical properties of composite materials. In addition, the probabilistic view enables quantitative assessment of the effects of design parameters. This quantitative information is useful for determining the most effective way to improve structures.
Joint structures are necessary in practical use, as they allow us to re-access internal structures and replace damaged components easily. They are not only the elements of a structure but also critical for the strength assessment of the structure resulting from induced stress concentration. Composite-made joints have often been the subject of many studies both experimentally and analytically [4], [5], [6]. The present study focused on the strength of a pin joint made from CFRP laminate and applied a method combining damage propagation analysis using the finite element method and the Monte Carlo method [7] to estimate the stochastic properties of the joint strength. The effects on the strength of a joint caused by scattering of its hole diameter were considered. These effects were compared to that resulting from the compressive strength in the fiber direction of the lamina, which is inherent to the material itself.
Section snippets
Bearing test
The bearing test evaluates the properties of composite joints [8], [9]. Fig. 1 shows a simplified image of the test. The CFRP specimen has a hole, and a tensile load is applied through the loading pin. Extensometers were equipped on both edges of the specimen to measure displacements. The dimensions of the specimen are shown in Fig. 2 and Table 1. PYROFIL TR50S/#1053I prepreg (Mitsubishi Rayon Co., Ltd.) was used in this study to fabricate the laminates and its fundamental properties are
Conclusion
In this study, sensitivity and probabilistic strength analyses for pin joints in CFRP laminates were performed. A 2D FE model of the bearing test system was adopted both to achieve accuracy and conserve resources, and damage progression analysis was performed on the model according to combinations of the factors, the diameter of a hole in a specimen D and compressive strength in fiber direction of a lamina Lc. The response surface approximation for the behavior of the FE failure analysis was
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