The failure of engines on jet aircrafts during the past few years has prompted the National Transportation Safety Board (NTSB) to issue an “urgent” recommendation to increase inspections of the engines on U.S. aircraft. Such uncontained engine failures are particularly dangerous, because flying engine parts could puncture fuel or hydraulic lines, damage flight surfaces or even penetrate the fuselage and injure passengers. At issue is older engines found on small number of jets, and the safety and economic impact damage and fracture risk can have on aircraft engines. For example, high-pressure turbine blades are commonly removed from commercial aircraft engines that had been commercially flown by airlines. These engines were brought to the maintenance shop for refurbishment or overhaul. The blades were removed and inspected for damage. The damage was cataloged into three modes of failure, which are thermal-mechanical fatigue (TMF), Oxidation/Erosion (O/E), and Other (O). These show the complexity of damage in turbine engines and the different mechanisms associated with cause of damage. Hence, life prediction of turbine engine is crucial part of the management and sustainment plan to aircraft jet engine. Fretting is often the root cause of nucleation of cracks at attachment of structural components at or in the vicinity of the contact surfaces. Previous effort presented a model to predict fretting fatigue in turbine engine, which is one of the primary phenomena that leads to damage or failure of blade-disk attachments. The influence of thermal effect and temperature fluctuation during engine operation on fretting fatigue damage were investigated. Leveraging these existing capabilities, the present effort focuses on modeling another important damage mechanism in turbine engine blades, which is erosion at high temperatures. Thus a reaction-diffusion model is implemented in addition to the thermo-mechanical one. The model provides a mean to investigate erosion initiation and propagation in turbine engine blades.

Square tubes are primarily used in automotive structures to absorb energy in the event of an accident. The energy absorption capacity of these structural members depends on several parameters such as tube material, wall thickness, axial length, deformation modes, locking strain, crushing stress, etc. In this paper, the work presented is a continuation of research conducted on exploring the effects of the introduction of cellular core in tubular structures under axial compressive loading. Here, the crushing response of composite cellular core tube was numerically studied using ABAQUS/Explicit module. The energy absorbing characteristics such as deformation or collapsing modes, crushing/ reactive force, crushing stroke, and energy curves were discussed. The composite cellular core tube shows promise for improving the crashworthiness of automobiles.

Slug flow is a major problem to the structural integrity and production equipment in offshore production platforms. Pressure oscillations due to the alternation of liquid and gas phases in slug flow regime can cause fatigue on the structural components of the platform. Also, the intermittent high flow rates can cause adverse effects on the production equipment. A 28-foot pilot scale model was constructed to simulate the riser on offshore platforms. Three pressure sensors were attached to the model to monitor and record pressures in the riser during operations. A PID control strategy was utilized to regulate the pressure oscillations in the system by use of a linear actuated valve. Similarity between the pressure signals in the pilot scale model is qualitative when compared to actual pressures observed in an offshore riser system. A MATLAB ® GUI was designed to allow for manipulation of the valve and allow for instant graphing of data for real time visualization of the pressure signals. Pressure oscillations during slug flow with “no control” vary greatly and result in natural vibrations of the designed system. By pinching down on the choke valve to a designated opening, the back pressure in the riser increased, thereby slowing down the liquid slugs. However, an increase in the magnitude of the higher frequency oscillations can have adverse effects on the system. With the implementation of an active control, such as a linear actuated valve, a better control of back pressure on the riser and reduction in the magnitude of the higher frequency oscillations on the system is achieved.

The low carbon, nitrogen enhanced SS 304 L(N) stainless steels are one of the most potential candidates for the structural members in chemical industries and powerplants operating at hostile environments of temperature and corrosion. In service, the structural members fabricated using welding process, when subjected to a combination of mechanical load and elevated temperature can fail by fatigue. The Welding of Austenitic stainless steels using Tungsten Inert gas (TIG) is often limited by the depth of weld penetration, which can be achieved during a single pass. This necessitates for the use of multiple passes resulting in weld distortion and generation of residual stress. The Use of an electronegative flux (Activating flux) during the TIG welding (A-TIG) is known to enhance the weld penetration, thereby reducing the number of passes. The present study evaluates the fatigue crack growth in stainless steel weldment (304L(N) welds) joined using conventional Multipass TIG welding and Activated flux TIG welding at 673K. Compact Tension (C(T)) specimens having a width of 50.8 mm and a thickness of 4 mm were extracted from the location of heat-affected zone (HAZ) and weld metal (WM) for A-TIG and MP-TIG configurations. From the micro-structural evaluation of A-TIG welds, it is noted that high heat input in a single pass has favored the formation of coarse equiaxed grains along the weld center. The use of multiple passes at reduced heat input has resulted in the formation of finer grains, with the orientation of grains changing along each weld pass interface. This finer randomly oriented grains has resulted in increasing crack path resistance through the MP-TIG welds compared to A-TIG welds. Thus from a view point of fatigue crack growth, due to the presence of fine grains, conventional Multi-pass weld is superior compared to A-TIG, but in cases where there is a creep or creep-fatigue combination, the A-TIG weld may prove to be useful.

Healing technology for metallic materials is an important subject in terms of long-term reliability and durability of structural members, a healing technology to heal fatigue crack by applying heat treatment at annealing temperature level has been discovered. In this study, the influences of plasticity-induced crack closure on healing were evaluated by obtaining the crack opening load during the pre-crack introduction and evaluating the fatigue crack propagation characteristics before and after the healing heat treatment, using compact tension specimens made of carbon steel with different test conditions. As a result, the specimen with high crack opening load showed high healing effect and were able to heal up to 95% of the pre-crack length. This suggested that the residual compressive stress due to the plasticity-induced crack closure accelerates the solid-state diffusion bonding during the crack healing process and this leads to the improvement of the healing effect.

Light weight honeycomb structures lend themselves to important applications in aerospace. These range from aerodynamic and structural components such as wing edges, flaps, rotor blades and engine cowlings, to aircraft interior structures such as overhead luggage bins, compartment liners, bulkheads and the monument structures found in galleys and lavatory areas. Often the honeycomb is formed into a composite ply sandwich with fiberglass face sheets bonded to the honeycomb core. These panels are cut to shape using CNC routers and specially designed cutting tools. However, the quality of the cuts generated even with these special tools leaves much to be desired. The low stiffness of the structure leads to imperfections such as fraying of the cut face sheet edges and the generation of flags along the cut honeycomb edge. These impact the ease of assembly and often require manually intensive reworking to mitigate. The cutting of honeycomb structures and sandwich panels is challenging due to low stiffness, anisotropic mechanical properties and a high proportion of interrupted cutting due to the air voids that are present. The cutting mechanics are not well understood at this time. This paper presents findings from the study of cutting of honeycomb sandwich panels using high speed videography and correlates these with results of geometric modeling of the engagement between the cutter and workpiece. The study includes the impact of the trajectory of the tool path through the cell structures on the generation of flagging. It also reports on the effects of two different cutting tool geometries and the introduction of a lead angle on the size and structure of the flags generated. These findings present the case for a research regime similar to the one completed for solid metals, into modeling the mechanics behind machining honeycomb structures. This will help manufacturers using these materials to make better choices in the tools, cutting parameters and machining strategies that they employ in their process planning.

In many engineering applications, such as civil, mechanical and aerospace, large displacements and rotations may occur within the working composite structures, due to the extreme loading conditions that may occur during service. This afflicts the equilibrium states of the structures and could change them, eventually, in a catastrophic manner. Therefore, it may be necessary to predict the nonlinear stress conditions of the laminated structures through numerical simulation, in order to prevent the failure of the entire system. To take into account these conditions, a geometrical nonlinear analysis has to be performed. The nonlinear framework proposed in this work is based on the Carrera Unified Formulation (CUF). CUF is a hierarchical formulation that considers the order of the structural model as an input of the analysis, so that no specific formulations are needed to obtain any refined model. The possibility to generate high-order structural elements makes possible to analyze any loading cases, including the post-buckling situation. Furthermore, this methodology allows to evaulate of the full three-dimensional stress tensor in laminated structures. In fact, as CUF is able to calculate the stiffness matrix in an automatic manner, there is no need to include any simplification to evaluate the out-of-plane components of the stress tensor.

Thin walled members such as square tubes are commonly used in vehicle’s frontal chassis to provide protection and damage attenuation to the passenger cabin in the case of impact loading. These structural members undergo progressive deformation under axial loading. The type of deformation mode is critical as it defines the overall configuration of force-displacement curve. There are different types of deformation modes for square tube under axial loading. Likewise, cellular structure exhibit distinct deformation modes under in-plane loading. The work presented here investigates the effects of partial or discrete bonding of cellular core structure on deformation modes of square tubes under axial loading. The results show that discrete bonding of cellular core with the tube has significant effect on progressive deformation of tubes and therefore, presents an opportunity to re-configure force-displacement curve for improved protection of automobile structures under impact loading.

In this study, a kind of meta-surface was designed for the improvement of nonlinear ultrasonic guided wave detection by creating bandgaps. It is composed of aluminum alloy cylinders arranged in a periodic pattern bounded on an aluminum plate. By artificially adjusting the height of the cylinders, the meta-surface can open up bandgaps over desired frequency ranges. Guided waves within the bandgap cannot propagate through the meta-surface and will be mechanically filtered out. To perform non-destructive evaluation (NDE) of structural components with fatigue cracks, the guided waves generated by a piezoelectric wafer active sensor (PWAS) propagate into the structure, interact with the crack, acquire nonlinear features, and are picked up by the receiver PWAS. In an ideal case, the waves excited by the transmitter PWAS should only contain signals at the fundamental frequency. However, due to the inherent nonlinearity of the electronic instrument, the generated signals are often mixed with weak superharmonic components. And these inherent higher harmonic signals will adversely affect the identifiability of nonlinear characteristics in the sensing signals. The bandgap mechanism and the wave vector dispersion relationship of the meta-surface are investigated using the modal analysis of a finite element model (FEM) by treating a unit structural cell with the Bloch-Floquet boundary condition. In this way, the meta-surface is carefully designed to obtain bandgaps at the desired frequency ranges. Then, a FEM harmonic analysis of a chain of unit cells is performed to further explore the bandgap efficiency. Finally, a coupled field transient dynamic FEM is constructed to simulate the improved nonlinear ultrasonic guided wave active sensing procedure with the bandgap meta-surface. The proposed method possesses great potential for future SHM and NDE applications.

In this study, the uncertain free vibration of structures with hybrid (random and fuzzy) parameters is investigated. The non-deterministic Young’s modulus of structural elements is modeled as random variables while the uncertain mass density is represented by fuzzy parameters with associated membership functions. By implementing the α -sublevel strategy, the stochastic fuzzy eigenvalue problem is rigorously transformed into a series of stochastic interval analysis of natural frequencies. In this context, the Unified Interval Stochastic Sampling (UISS) is adopted such that the statistical characteristics of the upper and lower bounds of natural frequencies at each α -sublevel can be calculated. Subsequently, the membership functions of mean and standard deviations are respectively established for the extreme bounds of eigenvalues. Benefiting from the stochastic samples offered by the UISS method, the probability profiles of the uncertain natural frequencies at each α -sublevel can be identified by utilizing either parametric or nonparametric statistical inference methods. The accuracy and effectiveness of the proposed approach is demonstrated by analyzing a practically motivated 3D structure with comparison to the results obtained by the conventional Monte Carlo simulation method.

The 7050-T7451 aluminum alloy has been widely used in the aerospace industry. Due to its chemical composition, this alloy has high levels of mechanical properties that allow the production of low-weight aircraft structural components. However, these alloys are thermally treatable and are not able to bear manufacturing processes involving heat. Because of the importance of their applications, studies based on the development of solid state welding process would be desirable aiming to find an alternative to generate welded joints for this kind of components. In this work, an investigation concerning the behavior of the 7050-T7451 aluminum alloy during Friction Stir Welding (FSW) was carried out. The profile of longitudinal residual stresses of plates welded by the FSW process was obtained using the ultrasonic method through critically refracted longitudinal waves (LCR). Two different frequencies were employed, 3.5 MHz and 5 MHz. The measurements were performed in the longitudinal direction of the welded joint at different distances from the center line of the weld. The magnitude and distribution of residual stresses found with this method are consistent with literature review, reaching 150MPa on the center of the weld.

Mechanical metamaterials are man-made materials in which the mechanical properties are mainly defined by their structures instead of the properties of each component. Periodic cellular structures consisting of honeycomb, tetrahedral, 3D Kagome and pyramidal truss arrangement of webs or struts have recently attracted a lot of attention since they have a broad range of applications including structural components, energy absorption, heat exchangers, catalyst support, filters and biomaterials. In addition, lattice structures such as the octahedral are being investigated since they are structurally more efficient than foams of a similar density made from the same material, and the ease with which these structures can now be produced using 3D printing and additive manufacturing. This research investigates the mechanical behavior and anisotropy in octahedral lattice structures of two different relative densities fabricated out of Acrylonitrile butadiene styrene (ABS) using Stratasys FDM 360mc and Dimension sst 1200es 3D printers. The machines were used to print octahedral lattice structured parts with struts 1.00 mm in diameter followed by parts with struts 2.6 mm in diameter and tested in compression in three mutually perpendicular directions. The compressive stress-strain behavior of the lattice structures observed is typical of cellular structures which include a region of nominally elastic response, yielding, and plastic strain hardening to a peak in strength, followed by a drop in flow stress. It was found that not only is the stiffness and strength of the as fabricated parts anisotropic but they, in addition to failure, are also a function of the relative density/strut diameter of the structure.

Slender, lightweight structures are demanded to meet efficiency targets or to enhance vehicle system performance characteristics. Yet, when subjected to static stress for load-bearing purposes, the flexible structural members may buckle. Furthermore, additional dynamic excitations may activate adverse snap-through responses in such post-buckled components, which accelerates fatigue and failure. The severe nonlinearity associated with these phenomena challenges traditional forms of analysis and necessitates studious experimental methods for conclusive system characterization and model validation. This research builds upon state-of-the-art analytical and experimental strategies to examine the complex forced, dynamic behaviors of built-up structures that contain one or more post-buckled members. An analytical modeling and solution formulation is reviewed that is uniquely amenable to the study of multistable structures and permits experimentally-observable measures of impedance to be identified. Through theoretical and experimental studies, the efficacy of the impedance measures is evaluated towards their usefulness in identifying the onset of dynamic bifurcations in the multistable structural dynamics. For moderate amplitudes of input energy, the analysis is found to provide qualitatively accurate prediction of the drive point impedance changes observed prior to dynamic bifurcations from low to high amplitude of displacement.

Despite the significant progress in the development of modern alloys, low alloy steels continue to be the materials of choice for large structural components at elevated temperature for extended periods of time. The resistance of these alloys to deformation and damage under creep and/or fatigue at elevated temperature make them suitable for components expected to endure decades of service. The material 2.25Cr-1Mo is commonly applied in boilers, heat exchanger tubes, and throttle valve bodies in both turbomachinery and pressure-vessel/piping applications alike. It has an excellent balance of ductility, corrosion resistance, and creep strength under moderate temperatures (i.e., up to 650°C). In the present work, a life prediction approach is developed for situations where the material is subjected conditions where creep and fatigue are prevalent. Parameters for the approach are based on regression fits in comparison with a broad collection experimental data. The data are comprised of low cycle fatigue (LCF) and creep fatigue (CF) experiments. The form of the life prediction model follows the cumulative damage approach where dominant damage maps can be used to identify primary microstructural mechanism associated with failure. Life calculations are facilitated by the usage of a non-interacting creep-plasticity constitutive model capable of representing not only the temperature- and rate-dependence, but also the history-dependence of the material. For the inelastic response, both the Garofalo and Chaboche models for creep and plasticity are employed, respectively.

Life prediction of turbine engines is crucial part of the management and sustainment plan to aircraft jet engine. Fretting is one of the primary phenomena that leads to damage or failure of blade-disk attachments. Fretting is often the root cause of nucleation of cracks at attachment of structural components at or in the vicinity of the contact surfaces. It occurs when the blade and disk are pressed together in contact and experience a small oscillating relative displacement due to variations in engine speed and vibratory loading. It is a significant driver of fatigue damage and failure risk of disk blade attachments. Fretting is a complex phenomenon that depends on geometry, loading conditions, residual stresses, and surface roughness, among other factors. These complexities also go beyond the physics of material interactions and into the computational domain. This is an ongoing effort, and the Author has been working on computationally modeling the fretting fatigue phenomenon and damage in blade-disk attachment. The model has been evolving in the past few years, and it has been addressing various fretting conditions. The present effort includes the thermal effect and temperature fluctuation during engine operation, and it models the effects of blade to disk attachment’s thermal conditions and its influence on fretting fatigue damage. It further extends the earlier model to include a coupled fatigue damage model. It allows modeling higher speeds and longer durability associated with blade disk attachments. Finally, to demonstrate its capabilities and taking advantage of experimental validation model, the most recent numerical simulations will be presented.

Piping supports and restrains are required to follow the design requirements as mentioned in ASME B&PV Code, Section III, Subsection NF. One of the requirements indicates the necessity of calculating the critical buckling stresses for the members that are subjected to a compressive loading. This paper discusses the prescribed requirements in the Code that specifically address the considerations of the stability and buckling load capacities of linear piping restraints (i.e., struts). The finite element modeling of various strut geometries and the results of the buckling analyses of a slender (slenderness ratio Kl/r greater than or equal to 100) structural members using various finite element solution techniques are presented herein. Specifically, three types of finite element analysis are conducted in an effort to define the critical buckling load for the subject structural member, and include the traditional linear (Eigen value) Euler method; the nonlinear, second order large deformation method; and finally, the nonlinear large deformation method that incorporates nonlinear elastic-plastic material behavior. These techniques are employed for a hollow cylindrical structural member (i.e., a strut assembly) with varying cross sections along its length. Finite element model consists of three dimensional hexahedral elements in combination with beam elements for the general purpose a finite element solver ANSYS. The critical buckling load is calculated in each case, thereby predicting the load at which instability will occur in the structural member. The results obtained from the aforementioned techniques are then compared both numerically and qualitatively with an appropriate explanation of the purpose and usefulness of each particular result with respect to the intent of the ASME B&PV Code, Section III, Subsection NF requirements. The results show significant variations (as expected) based on differences in the assumptions and techniques employed in the respective analyses.

Structural elements with thin-walled open cross-sections are common in metal and composite structures. These thin-walled beams have generally a good flexural strength with respect to the axis of greatest inertia, but a low flexural stiffness in relation to the second principal axis and a low torsional stiffness. These elements generally have an instability, which leads to a flexural-flexural-torsional coupling. The same applies to the vibration modes. Many of these structures work in a nonlinear regime, and a nonlinear formulation that takes into account large displacements and the flexural-flexural-torsional coupling is required. In this work a nonlinear beam theory that takes into account large displacements, warping and shortening effects, as well as flexural-flexural-torsional coupling is adopted. The governing nonlinear equations of motion are discretized in space using the Galerkin method and the discretized equations of motion are solved by the Runge-Kutta method. Special attention is given to the nonlinear oscillations of beams with low torsional stiffness and its influence on the bifurcations and instabilities of the structure, a problem not tackled in the previous literature on this subject. Time responses, phase portraits and bifurcation diagrams are used to unveil the complex dynamic.

With the success of post-graduate programs in a wide range of individual sustainable development subjects such as environmental sustainability, sustainable manufacturing, infrastructure sustainability, etc. it was recognized that there was a void in undergraduate opportunity related to these areas. Consequently, the time seems appropriate for offering a relevant baccalaureate program to create a pipeline of students educated from a systems perspective in sustainable engineering practices that might feed into postgraduate programs, as well as fill a need in government and industry. This paper presents how undergraduate research supports engineering education linked to sustainable practices and influenced the development of a new degree program in Sustainable Systems Engineering (SSE) at Metropolitan State University of Denver (MSU Denver). Two projects are discussed which represent an approach using sustainable systems methodology: one in the area of new sustainable structural systems and the other in development of water filtration devices to be used in Sustainable Community Development (SCD) projects. Additionally, the research on novel multi-composite structural members for new construction as well as retrofits that could be used in conjunction with solar heating technology was used as a pilot instrument in engineering courses to emphasize to students the application of sustainable engineering practices in design and holistic problem solving. This paper describes the curriculum development of the SSE program that was driven in part by these experiences. Discussed is the contribution of this applied learning approach as a contributing influence to a number of courses in the new program such as Mathematical Modeling, Structural Modeling, Humanitarian Engineering, and Sustainable Systems Design. Moreover, it discusses how these courses have been designed to incorporate elements of Undergraduate Research as part of the learning experience.

In this paper, some of the useful properties of guided Lamb waves are investigated, using analytical, numerical and experimental methods, in an effort to provide the knowledge base required for the development of viable nondestructive evaluation (NDE) systems for composite structures. An analytical frame-work that considers propagating, nonpropagating and evanescent Lamb waves to analyze the scattering of an incident ultrasonic Lamb wave at delaminations and disbonds is presented. In addition to the delamination problem, the interaction of Lamb waves with a step discontinuity, which can be found at common stiffeners, is investigated using a similar analytical approach. A near-field solution is obtained, and power flows through the cross-sections of the plate are evaluated. The agreement between experimental, numerical and theoretical results is shown to be excellent in certain frequency ranges, providing guidance for the design of reliable and effective inspection systems for modern aeronautical and aerospace structural components.

Thin walled axial members are typically used in vehicles’ side and front chassis to improve crashworthiness. Extensive work has been done in exploring energy absorbing characteristics of thin walled structural members under axial compressive loading. The present study is a continuation of the work presented earlier on evaluating the effects of presence of functionally graded cellular structures in thin walled members. A functionally graded aluminum cellular core in compact form was placed inside a steel square tube. The crushing behavior was modeled using ABAQUS/Explicit module. The variables affecting the energy absorbing characteristics, for example, deformation or collapsing modes, crushing/ reactive force, plateau stress level, and energy curves, were studied. An approximate 35% increase in the energy absorption capacity of steel tube was observed by adding aluminum graded cellular structure to the square tube. The aluminum graded structure crushed systematically in a layered manner and its presence as core supported the steel square tube side walls in transverse direction and postponed the local (tube) wall collapse. This resulted in composite tube undergoing larger localized folds as compared to highly compact localized folds, which appeared in the steel tube without any graded core. The variation in deformation mode resulted in increased stiffness of the composite structure, and therefore, high energy absorption by the structure. Further, a relatively constant crushing force was observed in the composite tube promoting lower impulse. This aspect has a potential to be exploited to improve the crashworthiness of automobile structures.