In this paper, experiments have been performed and finite element models have been developed for studying the influence of low-velocity impact damage on the four-probe electrical resistance of carbon fiber-reinforced polymer matrix laminates. Sixteen-ply and 32-ply AS4/3501-6 laminates with quasi-isotropic layup were analyzed. Electrical resistance was evaluated using a four-step procedure. First, finite element models were created in Abaqus Finite Element Analysis (FEA) for simulating low-velocity impact using a quasi-static loading approach. Second, matrix rupture in the inside plies was evaluated, and delamination analysis was performed at the corresponding interfaces to determine delamination patterns. Third, four-probe electrical finite element models were developed in Abaqus FEA for specimens before and after impact using the concept of effective conducting thickness and the delamination patterns obtained from the delamination analysis. Effects of the low-velocity impact delamination on four-probe top and oblique electrical resistance were studied. Electrical resistance predictions were compared to the experimental data. Both top and oblique resistance planes were sensitive to presence of delamination with the oblique resistance measurement being more sensitive as compared to the top resistance measurement. In addition, the resistance of the 16-ply specimens was more greatly affected by the delamination compared to the 32-ply specimens. The proposed analysis can be utilized for design of carbon fiber-reinforced polymer matrix composites with optimized damage sensing capabilities.

In this paper, finite element models were developed in ABAQUS for studying damage in carbon fiber-reinforced polymer matrix (CFRP) specimens based on the 4- probe electrical resistance method. The concept of effective conducting thickness was leveraged in the development of the electrical finite element models. The 4-probe electrical models were created assuming a linear current-voltage relationship and the damage response was recorded for CFRP materials with varying thickness and layup. Damage was modeled with prescribed shape and location in order to represent failure modes of fiber breakage, matrix cracking, and delamination. Fiber breakage and matrix cracking were identified using the top resistance measurement plane. The top resistance measurement was generally found to be more sensitive to fiber breakage compared to matrix cracking. It was demonstrated, however, that several factors affect the sensitivity of the response to damage including: (1) orientation of the fibers relative to the line of current probes, (2) location of the damage, and (3) type of damage. The material properties were then theoretically optimized such that Ohmic response was achieved throughout the full thickness of the CFRP specimen. With the use of optimal electrical conductivity, the model demonstrated improved sensitivity to fiber breakage near the top and bottom surfaces of the specimen.

In this paper, finite element models were developed in ABAQUS for studying the influence of simulation-based low-velocity impact damage on the 4-probe electrical resistance of carbon fiber-reinforced polymer matrix (CFRP) laminates. The specimens in the current study were 16- and 32-ply AS4/3501-6 laminates with quasi-isotropic layup. Electrical resistance in the CFRP laminates was evaluated using a 4-step procedure. First, FE models were created in ABAQUS for simulating low-velocity impact using a quasi-static loading approach. Second, the stress results were imported into Matlab, and delamination analysis was performed using approach developed by de Moura and Marques [1]. The delamination predictions were plotted onto a discretized grid for each interface between plies with dissimilar fiber orientations. Third, 4-probe electrical FE models were developed in ABAQUS for specimens before and after impact using the concept of effective conducting thickness. The electrical conductivity was reduced in the through-thickness direction for damaged elements in order to represent the impact-induced delamination. Forth, a direct current of 10mA was applied to the electrical models in order to determine the influence of the simulation-based lowvelocity impact delamination on 4-probe electrical resistance using top and oblique measurement planes. For the top measurement plane, source and sensing electrodes were placed in a single line on the top surface of the specimen. For the oblique measurement plane, positive source and sensing electrodes were placed on the top surface, whereas the negative source and sensing electrodes were located on the bottom surface of the specimen. Generally, the oblique resistance measurement was more sensitive to the impact damage compared to the top resistance measurement. In addition, the resistance of the 16-ply specimens was more greatly affected by the delamination compared to the 32-ply specimens. Electrical resistance predictions were compared to the experimental data.

The effects of ultraviolet (UV) light and moisture environments on the properties of continuous fiber-reinforced thermoplastic composites were investigated in this study. Materials included carbon fiber-reinforced polyphenylene sulfide (CF/PPS), glass fiber-reinforced polyphenylene sulfide (GF/PPS), glass fiber-reinforced polypropylene (GF/PP), and glass fiber-reinforced polyethylene terephthalate (GF/PET). Fiber orientation was noted to affect the response of the materials to UV and moisture environments. For CF/PPS, UV exposure led to enhanced unidirectional tensile and flexure properties, and quasi-isotropic compression strength, but reduced in-plane shear properties of cross-ply laminates and the short-beam shear strength of unidirectional laminates. Exposure to moisture had minimal effect on CF/PPS laminates. UV light caused a slight decrease in tensile strength and strain of unidirectional samples of GF/PPS and a slight increase in these properties with various degrees in quasi-isotropic samples. Exposure to moisture was noted to cause a reduction in most of the mechanical properties with various degrees in unidirectional and quasi-isotropic GF/PPS laminates. For GF/PP, exposure to UV and moisture decreased the tensile and compressive strengths of unidirectional and quasi-isotropic laminates. However, a notable increase in flexure strength was observed in both laminates after UV exposure. The GF/PET quasi-isotropic laminates showed reduced tension, compression, and flexure strengths but increased impact resistance and short-beam strength under UV exposure, while moisture decreased in-plane shear properties of cross-ply laminates and increased the glass transition temperature of unidirectional laminates. Vertical burn and surface flammability and smoke generation tests showed high flame resistance of CF/PPS and GF/PPS composites, and less so for GF/PP and GF/PET composites. The findings highlight the various responses of the materials to extreme environments.

A new fully automated experimental setup has been developed to study the response of carbon fiber reinforced polymer (CFRP) composites subjected to a high-intensity pulsed electric field and low-velocity impact. The experimental setup allows for real-time measurements of the pulsed electric current, voltage, impact load, and displacements on the CFRP composite specimens. The setup includes a new custom-built current pulse generator that utilizes a bank of capacitor modules capable of producing a 20 ms current pulse with an amplitude of up to 2500 A. The setup enabled application of the pulsed current and impact load and successfully achieved coordination between the peak of the current pulse and the peak of the impact load. A series of electrical, impact, and coordinated electrical-impact characterization tests were performed on 32-ply IM7/977-3 unidirectional CFRP composites to assess their ability to withstand application of a pulsed electric current and determine the effects of the pulsed current on the impact response. Experimental results revealed that the electrical resistance of CFRP composites decreased with an increase in the electric current magnitude. It was also found that the electrified CFRP specimens withstood higher average impact loads compared to the non-electrified specimens.