Unidirectional Fiber Reinforced Polymer Laminates Research Articles

Enhanced thermographic inspection of woven fabric composites by [formula omitted]-space filtering

Infrared thermography is a well-known non-destructive testing technique for detecting defects in unidirectional fiber reinforced polymer laminates. When applied to woven fabric reinforced polymers, however, the weave structure causes strong disturbances and patterns in the background of the thermal images, making accurate and reliable defect assessment a challenging task. In this paper, the concept of k-space filtering is introduced for an improved evaluation of thermographic images obtained from woven fabric composites. An algorithm is introduced to automatically decompose a thermographic image into an image which contains the structured thermal background related to the weave pattern, and a residual image representing other features, e.g. defects. The proposed k-space filtering approach is demonstrated on thermographic data from various woven fabric composites with different weave patterns and defect scenarios, clearly showing an enhanced performance in terms of defect detection and sizing.

On the importance of finite element mesh alignment along the fibre direction for modelling damage in fibre-reinforced polymer composite laminates

A finite element mesh, aligned along the fibre direction, has often been purported to yield more accurate results in the modelling of damage in composite structures constructed from unidirectional fibre reinforced polymer laminates. However, there has been a lack of a systematic assessment of this approach. This issue is addressed through modelling a selection of test cases; (i) end notched tension (ENT) coupons, (ii) simple tensile/compressive specimens, (iii) open-hole tension (OHT), (iv) low velocity impact (LVI) and (v) compression-after-impact (CAI) of laminated plates. In each case, two models were constructed, one using a uniform mesh, aligned with the global coordinate system, and independent of the fibre direction, and the other where each ply mesh was aligned along the fibre direction. In both cases the local material axes in each ply were correctly represented. Results show that a finite element mesh aligned along the fibre direction plays an important role in the prediction of damage, particularly in the presence of a crack. However, when matrix crack paths are not established a priori, or may not be the dominant damage mode, fibre mesh alignment is unnecessary. Rather, long-established approaches of refined meshes, robust damage models and well-defined material data and boundary conditions, are shown to be sufficient requirements.

Strength and Modulus Degradation of Carbon Fiber-Reinforced Polymer Laminates from Fiber Misalignment

Fiber-reinforced polymer (FRP) laminates are being used as external reinforcement for strengthening concrete members. The performance of unidirectional FRP laminates is highly dependent on fiber orientation with respect to applied load direction. In the case of fabrication by manual layup, it is possible to have fiber plies installed with improper orientation. In this project, the degradation of strength and modulus of carbon FRP laminates from fiber misalignment was investigated experimentally using tensile coupons. The specimens consisted of one and two plies of unidirectional carbon FRP impregnated with a two-component epoxy. The misalignment angles varied from 0 to 40° for the one-ply samples, and from 0 to 90° for one ply of the two-ply samples. The size effect on the strength and modulus was investigated for one-ply specimens with misalignments of 5 and 10°. For these specimens, the ply width was maintained constant and the length was varied to that the aspect ratio ranged between 2 and 8. It was concluded that misalignment affects strength more than elastic modulus. However, provided that mechanical parameters are related to the cross-sectional area of laminate with fibers continuous from end to end of the coupon, the degradation of strength can be accounted with a knock-down factor that is independent of misalignment angle.