Emmanuel Ayorinde

Fatigue of Foam and Honeycomb Core Composite Sandwich Structures: A Tutorial

This article is intended to be a tutorial on the subject of fatigue of foam and honeycomb core composite sandwich structures. First, several different analytical models for predicting the fatigue life of sandwich composites are presented. Then representative publications which have reported on the major failure modes in sandwich beams under dynamic fatigue loading are summarized, along with several related publications dealing with static and impact loading. Papers dealing with the effects of loading frequency, environmental factors, and block loading on the fatigue life of sandwich composites are discussed. Finally, recent research on different types of non-destruction evaluation (NDE) techniques employed for failure investigations during fatigue testing of sandwich structures is reviewed. Conclusions and generalizations that can be drawn from the literature are presented along with discussions of areas in which further research is needed.

Design of test specimens for the determination of elastic through-thickness shear properties of thick composites from measured modal vibration frequencies

Recommendations are developed and presented regarding the most desirable test plate aspect ratio (length-to-thickness ratio) to use during the non-destructive measurement of elastic constants of thick composite plates by modal vibration testing. The effects of transverse shear were studied over the range of practical plate aspect ratios (length to thickness ratios) and mode numbers for three specific types of composite materials ranging from planar isotropic (E-glass/polyester sheet molding compound) to moderately orthotropic (unidirectional graphite/epoxy) to highly orthotropic (unidirectional pitch/epoxy). Frequency predictions from previously developed Rayleigh-type two-dimensional (2-D) analytical models with and without through-thickness (transverse) shear effects were compared with those from 2-D and 3-D finite-element analyses. Reasonable agreement between the models was observed, and results were used to determine the aspect ratios at which transverse shear effects become important enough to be taken into consideration during vibration tests. It was also found that the avoidance of undesirable in-plane modes must be factored into the process of selecting the proper plate aspect ratio.

Development of damage in some polymeric foam-core sandwich beams under bending loading

The damage mechanisms in sandwich plates with polymer foam cores are investigated. There has been a long-standing controversy on what is the weakest element of sandwich plates, the skin-core bonding or the foam core. Under four-point bending of a sandwich plate, we found that in all cases cracks are nucleated inside the polymer foam core as monitored by a high-speed camera. Several separated small cracks develop in the core; then the small cracks are connected and form the main crack that propagates through the core. We complemented the optical recording by the acoustic emission study which provides an additional insight in the fracture process. In addition, the influence of sub-zero temperature of the ultimate load of each material is examined at discrete values of sub-zero temperature. The results revealed that both materials become increasingly brittle with decreasing temperature.

Basic NDE of some nano composites

The influence of the nano-sized particulate or slender structures admixed into the material of regular composite structures of various architectures is being enthusiastically studied in many places across the world, but the study is yet in its infancy because there are so many aspects to be investigated. This work is basically on foam-cored structural sandwich composites, and even here there are many variables involved, for example the nano-enhancer can be introduced into the reinforcement, the matrix or the foam. The focus of the work is on some possible effects of the presence of the nano-materials on the NDE process in testing these composites. Acoustic emission is emphasized in these studies, as it appears to hold promise for non-destructively testing materials of this nature, and basic and standard mechanical test methods are employed.Copyright

Characterization of Fatigue Behavior of Composite Sandwich Structures at Sub-Zero Temperatures

This chapter summarizes recent studies of the flexural fatigue characteristics of foam core carbon/epoxy and glass/epoxy composite sandwich beams over the temperature range from 22°C to −60°C. Core shear was found to be the dominant fatigue failure mode for the test specimens over this temperature range. Significant increases in the useful fatigue life with brittle type core shear failure were observed at low temperatures by comparison with the corresponding room temperature behavior. Fatigue failure at the subzero temperatures was catastrophic and without any significant early warning, but the corresponding failures at room temperature were preceded by relatively slow but steadily increasing losses of stiffness. Two different approaches were used to investigate stiffness reductions during fatigue tests, and both approaches led to the same conclusions. Static finite element analyses confirmed the experimentally observed locations of fatigue crack initiation.

Application of acoustic emission technology to the characterization and damage monitoring of advanced composites

This chapter gives some highlights of the research done by the author in the area of acoustic emission technology for the Navy under the much valued guidance and support of program manager Dr. Yapa S. Rajapakse over a number of years. Although the authors ONR research covered a wider base, such as low velocity impact, static and dynamic loadings with damage studies of monolithic and foam-cored sandwich composites, and application of some other NDE methods, the acoustic emission NDE method was widely applied, and it merits a dedicated mention. It is not possible, in such a short paper, to describe every investigation undertaken over the period of time, but an attempt is made to outline a representative number of cases which together present an informed cross-sectional view of work done on this topic. Control samples of monolithics like aluminum and steel were used in some of the studies for the sake of comparisons, but focus was centered on composite materials. It is well known that composite materials are being increasingly utilized in ship structures, and in fact generally in both the civilian and military sectors of the economies across the world. The need for rapid, low-cost, non-destructive and reliable methods for obtaining mechanical property data for materials and evaluating damage in them has boosted this kind of research.

Determination of Impact Behavior of ABS From Acoustic Emission, Ultrasound and Optics

ABS (Acrylonitrile Butadiene Styrene) is an industrially-important and widely used amorphous thermoplastic on which billions of dollars are spent annually in USA. Its applications cover impact-mitigation, infrastructural and laboratory piping systems, sports goods, musical instruments, automotive trim components and bumper bars, medical devices, enclosures, protective headgear, marine craft, luggage, domestic appliances, toys, consumer goods, edgings for industrial goods, etc. Its use to contain impact damage is primary; hence continued research in this area is warranted.The novelty and contribution of this work lies in its employment of more deeply insightful parameters and methods of characterization in acoustic emission and ultrasonics, as well as advanced optical microstructural characterization with sophisticated instrumentation, on one hand, and the wider correlations and conclusions now made possible by these means.In the work which this paper reports, the impact response behavior of ABS material under various levels of impact energy was investigated using results obtained from the mechanical test, and parameters obtained from non-destructive test methods such as Acoustic Emission (AE), Ultrasonics, and Optical Inspection.The ABS plates were impacted by a hemispherical steel projectile in a drop-weight impact tester. Two AE sensors were placed on the surfaces of ABS plates during the impact tests. After the impact tests were completed, ultrasonic C-Scan investigation of the damaged areas was also carried out, and sections were inspected under the microscope. Correlations between damage areas and various parameters of the non-destructive diagnostic test methods utilized were explored. ABS is one of the most highly impact-resistant materials utilized in industry. Its characterization under impact is therefore very important, in order to devise ways of enhancing properties that would make the material or structures made from it, better in service.In this work, plate samples of rectangular shape were subjected to central impacts from a spherical impactor released from various heights. It is of interest to know how the impact propagates through the plate thickness, and how the microstructure is affected from point to point, both laterally and depth-wise. The issues of energy transfer and dissipation are significant in terms of the effectiveness of the material as an impact deadener.Three non-destructive methods are utilized in this work for comprehensiveness and effectiveness. The AE approach is broadly divisible into two — classical, and transient. The former has dozens of descriptive parameters per each of the three dimensions, while the latter, which is based on the waveform and its several possible transforms, adds even many more possibilities. Thus, characterization in AE is particularly rich, and, when a sufficient number of appropriate parameters are utilized, has a very high probability of correctly depicting what is really going on in the material or structure under inspection. The ultrasonic scan reveals in color-code the variation of the material homogeneity throughout the scanned space, which, in each case, covered the whole plate. This normally provides a good picture of damage and its intensity variation in the test piece. The microstructure of selected parts of the test pieces before and after impact was inspected with a violet laser microscope. In this instrument, reflecting light from the white light source is detected with a color CCD camera. This camera obtains color information at the peak (focal point) detected with the laser light source on a pixel basis, thus enabling a real color examination, which SEMs cannot do. This instrument also uses a pin hole confocal optical system which enables high accuracy measurement and high definition examination by eliminating reflecting light from points other than the peak.The results obtained showed clear relationships between energy and geometrical and material metrics of damage through the damage zone and shed more light on possible pathways to the desired enhancement of impact resistance in this case.Copyright

Simplified Procedure to Estimate the Wind Turbine Blade Aerodynamic Loadings Without Using CFD

The aerodynamic loadings that act on the blade of a horizontal axis wind turbine change as a function of time due to the instantaneous change of the wind speed, the wind direction and the blade position. The new contribution in this study is the introduction of a simplified non CFD based procedure for the calculation of all the aerodynamic loadings acting on a wind turbine blade. The premise of the current simplified model is that (a) the forces can be modeled by a set of point loads rather than distributed pressures, and (b) the magnitudes of these point loads can be estimated using the below load formulas, (c) an interpolation scheme needed to have all computed forces and moments as a function of the blade lengthwise x. Considering a 14m blade length and utilizing a time dependent set of parameters such as angle of attack, material and air density, wind and blade speed, flow angle, yaw, pitch angles, the centrifugal forces (along x-direction of the blade length), the cross-sectional forces (Fy and Fz) and the twisting moment of the blade (about the x-direction) were calculated for each of all the given time steps. After that the authors explain how to interpolate the calculated loadings (forces and twisting moment) and the right formulas to compute the aerodynamic load vector (the right side of the dynamic equations of motion).Copyright

Plate Vibration and Geometry Study of Impact-Control ABS

Acrylonitrile-Butadine-Styrene (ABS) has been receiving much attention as a cushion against impact and vibration, on account of its very favorable mechanical properties, including elevated impact strength, stiffness and tensile strength, as well as outstanding formability. The geometry of deployed samples does influence dynamic performance. In this work, the methods of analysis, experiment and numerical computation have been applied to explore different modal parameters of rectangular ABS specimens with completely free boundary conditions. Experimentally, Pulse 15.1 software was used to investigate the modal parameters while the specimens were numerically modeled in Abaqus/Standard 3D using C3D20R (second order 20-node quadratic brick) element types with the Lancsoz Eigensolver method. Parametric explorations over the geometry space enabled useful trends to be identified with respect to vibration and impact applications. Experimental and numerical results were found to compare very favorably.Copyright

Investigation of some circular membrane-patch acoustic meta-materials

A great need exists for vibration and noise control with thinner and smaller devices, especially at lower frequencies than those at which the usual magnetic and electrical metamaterials operate. Acoustic metamaterials have been under development for such purposes. Some of the present authors have experimentally and numerically investigated circular acoustic metamaterials made of lexan plate and silicon membranes. In this work, the authors extend the investigation to the use of some other materials and relative see if and how well such a construction may be utilized for meta-material applications.Copyright