A sensor for visualizing the fatigue load cycles was designed, fabricated, and tested. The sensor is made of a glass/carbon hybrid composite and utilizes the delamination length at the glass/carbon interface as an indicator for fatigue cycles. Appropriate design parameters were obtained by performing finite element analysis on the delamination development at the interface between the glass and carbon layers. Hybrid sensors with different carbon layer thicknesses were manufactured, attached to glass/epoxy substrates, and tested under fatigue loading. The predicted results based on the Paris law for crack extensions in one configuration are compared with the experiments for a different configuration to illustrate the efficacy of the approach.

A new method of creating in-plane combined tension-shear stress states using only tensile loading is proposed. Thin-ply angle-ply carbon/epoxy laminates are sandwiched between unidirectional glass layers to eliminate any stress concentration around the samples ends and gripping zone. Use of the thin plies successfully suppresses early occurrence of other modes of failure i.e. matrix cracking and free-edge delamination, so the first mode of failure is fibre failure in the angle-ply carbon sub-laminate. Compared with other methods of creating combined tension-shear stresses, the proposed technique has a simpler geometry, is significantly easier and cheaper to manufacture and test. Therefore, it provides more repeatable low-scatter experimental results. The obtained experimental results showed that the presence of in-plane shear stresses did not have a significant impact on the tensile fibre-direction failure strain of the tested carbon/epoxy laminate, suggesting that even at high shear stresses, the longitudinal tensile strength of the carbon/epoxy laminate is not significantly reduced.

This paper introduces novel structural health monitoring (SHM) sensors to improve the detection of low energy impact damage in laminated composites. The sensor is a purposely designed thin-ply hybrid composite, composed of a layer of unidirectional S-glass/epoxy and another layer of unidirectional ultra-high modulus (UHM) carbon/epoxy. The sensor was incorporated onto both the impacted face and back of a substrate plate made from unidirectional T800 carbon/MTM49-3 epoxy prepregs with the stacking sequence of [45/0/90/-45]_4S. A series of drop tower tests were conducted on the composite plates with and without the attached hybrid sensing layer, with two different in-plane dimensions and varying energy levels ranging from 3 J to 124 J. The results indicate that the sensors functioned satisfactorily and provided direct correlations between visible and internal hidden damage detected by C-scan. The sensor can be optimized by selecting appropriate material properties and adjusting it to the in-plane dimensions of the substrate.

This paper investigates the fatigue behaviour of pseudo-ductile Quasi-Isotropic (QI) interlayer hybrids with un-notched and open-hole configurations. Two different types of QI pseudo-ductile hybrids were evaluated; HighC, with carbon to glass thickness ratio of 0.29, that is made of thin-ply M46JB-carbon/epoxy and thin-ply Xstrand-glass/epoxy prepregs, and LowC, with carbon to glass thickness ratio of 0.19, that is made of thin-ply T300-carbon/epoxy and standard-ply S-glass/epoxy prepregs. The hybrid configurations were loaded at 4 Hz in tension–tension fatigue without any initial damage and at different percentages of the pseudo-yield stress (σ_py) at which damage initiates. It was observed that there is no stiffness reduction, after 100,000 cycles, for a stress level of 80 % and 50 % of the σ_py for the un-notched and open-hole laminates, respectively. By increasing the stress level to 90 % and 70 % of the σ_py for the un-notched and open-hole laminates, respectively, there is a gradual stiffness reduction due to the appearance of matrix cracking and delamination in LowC, but no gradual reduction and no visible damage were observed for HighC. The final failure is more brittle and happens at a lower number of cycles for HighC compared with LowC. Different damage extents were observed for the investigated laminates before the final sudden failure due to variables such as the ply thickness, the cyclic energy release rate and the interfacial fracture toughness.

Despite the key advantages of Fiber Reinforced Polymer (FRP) composites, they are susceptible to Barely Visible Impact Damage (BVID) under transverse loadings. This study investigates BVID in two quasi-isotropic carbon/epoxy laminates under quasi-static indentation and Low-Velocity Impact (LVI) loadings using Acoustic Emission (AE). First, the evolution of interlaminar and intralaminar damages is studied by analyzing the AE signals of the indentation test using b-value and sentry function methods. Then, the specimens are subjected to the LVI loading and the induced damages are compared with the indentation test and the percentage of each damage mechanism is calculated using Wavelet Packet Transform (WPT). In consistent with the mechanical data, ultrasonic C-scan and digital camera images of the specimens, the AE results show a considerable similarity between the induced BVID under quasi-static indentation and LVI tests. Finally, the obtained results show that AE is a powerful tool to study BVID in laminated composites under quasi-static and dynamic transverse loadings.