Degradation in fatigue behavior of carbon fiber–vinyl ester based composites due to sea environment

This study focuses on the effect of confined and one sided sea water confinement on the cyclic fatigue behavior of carbon fiber reinforced vinyl ester composites that serve as facings materials for naval sandwich structures. Experimental results for facings yielded failures under much lower number of cycles when fatigued under immersed conditions surrounded by sea water than in air. Water penetrates the matrix resin through diffusion and fiber/matrix interface by capillary action through micro-cracks or inter-layer delaminations. During fatigue loading, its inability to drain during the downward (compressive) cyclic loading and the near incompressibility of water induces an internal pore water pressures which dominates the progressive failure mechanism. Sea water induced fatigue degradation data and resulting microstructure changes are obtained using high resolution X-ray micro-tomography along with the implications for marine composites.     

Pulse-modulation eddy current probes for imaging of external corrosion in nonmagnetic pipes

Since the nonmagnetic pipe is normally utilized in corrosive and hostile environment, it is prone to the external corrosion which occurs on the outer surface of the pipe and severely undermines the structural integrity and safety. Although Pulsed Eddy Current technique (PEC) is currently preferred for detection and evaluation of subsurface defects in tubular conductors, it is subject to technical drawbacks. In light of this, Pulse-modulation Eddy Current technique (PMEC) is intensively investigated in the paper for enhancement of the evaluation sensitivity to external corrosion and accuracy of corrosion imaging. Closed-form expressions of the PMEC response and its sensitivity to external corrosion in tubular conductors are formulated via the Extended Truncated Region Eigenfunction Expansion (ETREE) modeling. Following simulations for analysis and comparison of field signals and evaluation sensitivities of PMEC and PEC, experiments of PMEC for evaluation and imaging of external corrosion are carried out. Through theoretical and experimental investigation, it has been found that regarding the evaluation and imaging of external corrosion in nonmagnetic pipes, the PMEC-based probe have higher sensitivity and imaging accuracy than that based on PEC. The superiority of PMEC to PEC in inspection of tubular conductors is further identified.     

Characterising the loading direction sensitivity of 3D woven composites: Effect of z-binder architecture

Three different architectures of 3D carbon fibre woven composites (orthogonal, ORT; layer-to-layer, LTL; angle interlock, AI) were tested in quasi-static uniaxial tension. Mechanical tests (tensile in on-axis of warp and weft directions as well as 45° off-axis) were carried out with the aim to study the loading direction sensitivity of these 3D woven composites. The z-binder architecture (the through-thickness reinforcement) has an effect on void content, directional fibre volume fraction, mechanical properties (on-axis and off-axis), failure mechanisms, energy absorption and fibre rotation angle in off-axis tested specimens. Out of all the examined architectures, 3D orthogonal woven composites (ORT) demonstrated a superior behaviour, especially when they were tested in 45° off-axis direction, indicated by high strain to failure (∼23%) and high translaminar energy absorption (∼40 MJ/m³). The z-binder yarns in ORT architecture suppress the localised damage and allow larger fibre rotation during the fibre “scissoring motion” that enables further strain to be sustained by the in-plane fabric layers during off-axis loading.     

Diode-pumped medium-aperture-size square Nd,Y:CaF2 rod amplifier for Inertial Confinement Fusion laser drivers

We demonstrate, for the first time, an 12 mm × 12 mm 0.5%Nd,5% Y:CaF₂ crystal rod having a uniformly-distributed fluorescence spectrum and capable of operating as an amplifying medium at high repetition frequencies. A small gain of 2.7 is experimentally achieved at repetition frequency of 10 Hz for a pump center wavelength of 802 nm, power and absorption efficiency, 61.2 kW and 63.7%, respectively. Spatial-uniformity degradation of the output near-field beam distribution is observed, which should be attributed to the inhomogeneity of Nd,Y:CaF₂ crystal. For a pump power of 61.2 kW, the stored energy of Nd,Y:CaF₂ amplifier is 3.73 J. When the input energy is 50 mJ, the output laser energy is 1.4 J of extraction efficiency up to 37.53% after four-pass amplification.     

Using thin-plies to improve the damage resistance and tolerance of aeronautical CFRP composites

Thin-ply composites are currently receiving specific attention from researchers due to their capabilities to delay matrix cracking. In this paper, the aim is to design a hybrid laminate that contains both thin- and normal plies. The objective is to improve the tolerance of normal plies by adding thin-plies to the composite in different configurations. Two alternatives were designed, tested, and compared to the specimens made of traditional plies. Impact and compression after impact tests were conducted on each configuration at different impact energies. After being impacted, the specimens were c-scanned to define the delamination pattern. Results showed that surrounding each normal ply with two thin-plies improved the delamination threshold by 15% as compared to the specimens made all of normal plies. Under compression, 15% improvements in the compression after impact strength were obtained. By using thin-plies, the size of each individual delamination was reduced, resulting in small threads instead of peanut delaminations.     

In vitro Streptococcus mutans biofilm formation on surfaces of chlorhexidine-containing dentin bonding systems

This in vitro study evaluated the influence of chlorhexidine diacetate (CDA) when blended within dentin bonding systems (DBSs) on Streptococcus mutans (S. mutans) biofilm formation.One commercially available 0.2% wt CDA-containing DBS (Peak Universal Bond) and five experimental 0.2% wt CDA-containing DBS formulations (experimental Adper Scotchbond 1XT plus experimental resins, R₂, R₃, R₄, R₅) were assessed vs their no-CDA containing counterparts. Twenty-eight DBSs disks were prepared for each group (6.4 mm×1.0 mm) and cured for 80 s at 800 mW/cm² in a nitrogen atmosphere. A modified Drip-Flow Reactor was used to grow S. mutans biofilms on specimen surfaces for 24 h and adherent, viable biomass was evaluated using a tetrazolium salt assay (MTT). Two specimens from each of the tested materials were processed with LIVE/DEAD stain and observed using laser confocal microscopy (CLSM) while two disks from each group were examined by using scanning electron microscopy (SEM).MTT assay, CLSM and SEM observations showed that CDA addition decreased, increased or did not change S. mutans biofilm formation. The lowest biofilm formation was obtained with Peak Universal Bond and R₅ (with and without CDA).It may be concluded that the chemical composition of DBSs determines their ability to promote or hamper biofilm formation. Therefore, CDA addition may be helpful in modulating biofilm formation provided that DBS formulation is tuned and optimized.     

Effects of compressive stresses on torsional fatigue

Rolling contact fatigue (RCF) is the dominant failure mode in properly installed and maintained ball and roller element bearings. Lundberg and Palmgren in their seminal publication indicated that this failure is due to the alternating component of shear stress. Thus, torsional fatigue experiments have been used to predict the RCF behavior of bearing materials. In non-conformal contacts, due to Hertzian pressure the contact experiences large compressive stresses. Hence, it is critical to take into account the effect of these large compressive stresses in torsional fatigue to better simulate RCF conditions. This paper presents an investigation of torsional fatigue of bearing steels, while the effects of combined compressive stress and its relevance to material behavior in rolling contact fatigue is examined. An MTS test rig was used to investigate the fatigue life of several bearing steels and their failure mechanisms were evaluated through fractography. Then the effects of compressive stresses on torsional fatigue were investigated. A set of custom designed clamp fixtures were designed, developed and used to apply Hertzian pressures of up to 2.5 GPa on the torsion specimens. The experimental results indicate that at high cycle fatigue, a combination of shear and biaxial compression, by application of Hertzian contact, is more detrimental to fatigue life than shear alone; however, as expected it has little to negligible effects in the low cycle fatigue regime. Also the failure mode changes such that fracture planes form a cup and cone pair with multiple internal cracks as opposed to helical planes observed in pure torsion which are formed by a single crack. A 3D finite element model (using ABAQUS) was developed to investigate the fatigue damage accumulation, crack initiation, and propagation in the material. The topology of steel microstructure is modeled employing a randomly generated Voronoi tessellation wherein each Voronoi cell represents a material grain and the boundaries between the cells are assumed to represent the weak plane in the steel matrix. Continuum damage mechanics (CDM) was used to model material degradation during the fatigue process. A comprehensive damage evolution equation is developed to account for the effect of mean stress on fatigue. The model predicts the fatigue lives and crack patterns successfully both in presence and absence of compressive stresses.     

Priority load control algorithm for optimal energy management in stand-alone photovoltaic systems

In stand-alone PV System facilities no grid connection exists, therefore the solar generator and battery bank have to be carefully sized in order to supply the energy demand for a given period of time. Batteries are considered as a weak component of the system, comprising an important part of the total cost and are usually replaced one or two times during PV system lifetime. A priority load control algorithm has been developed in order to gain an optimal energy management over system loads and the battery storage, and therefore provides a better energy management efficiency and guarantee the energy supply for critical loads. This will increase the reliability of the system and the end-user satisfaction. This article describes a stand-alone PV system model used for the development of a priority load control algorithm and explains and implements the algorithm. The results of several test scenario simulations are shown and discussed.     

A process model for the compaction and saturation of partially impregnated thermoset prepreg tapes

Automated manufacturing of composite materials is key to reducing cost and improving consistency in part quality. Modeling of manufacturing processes can identify influential material, geometric, and process variables, while providing a framework for their optimization and control. A process model has been formulated to describe the compaction and saturation of partially impregnated thermoset prepreg tapes during their placement, using a roller compaction and vacuum consolidation. The roles of various parameters that characterize the initial prepreg tape state and the final prepreg tape state are investigated. The model yields the degree of resin saturation in the tape with changes in its thickness after being compacted, due to an external pressure. The results estimate the degree of dry fiber region within the tape, which can provide vacuum pathways for air or volatile removal before consolidation and cure. The model can be adapted for different material constitutive relationships of tape compaction and saturation.     

Thermochemical investigation of Sm–Mg alloys

The paper focuses on the thermochemical behaviour of the binary Sm–Mg alloys. The enthalpies of formation of the Sm–Mg intermetallic compounds have been determined at 300 K by high temperature direct synthesis calorimetry. The following results in kJ/mole of atoms are reported: −18.5±2 (SmMg), −16.0±2 (SmMg₂), −12.0±2 (SmMg₃), −8.5±2 (SmMg₅) and −5.5±2 (Sm₅Mg₄₁). The results are compared with the earlier experimental value obtained by vapour pressure.