Redundant Load Paths Research Articles

Offset-stoichiometric reflowable composite bonding method with adhesive for mitigating strict faying surface tolerances

Inherent susceptibility of adhesive bonds to miniscule quantities of contamination can cause undetectable weakened bonds. For this reason, the Federal Aviation Administration (FAA) places strict regulations on adhesively bonded joints in primary aircraft structures. To meet certification requirements aircraft manufactures resort to redundant load paths in the form of fasteners which inherently add weight to the structure and increase manufacturing time. In prior work, a secondary bonding technique called AERoBOND was developed, which utilized off-stoichiometric epoxy-matrix resins to facilitate reflow and diffusion of the resin within the joint interface during a secondary bonding/cure process, thus achieving a bond similar to a co-cured joint. However, the AERoBOND process required tight spatial tolerances between the two parts being joined. This study examined the utilization of conventional adhesive with the AERoBOND method to act as a filler in the joint line, effectively reducing the need for tight tolerances on the joining parts and serving as a flexible alternative for existing manufacturing processes. Ultrasonic inspection, optical microscopy, and ASTM International standard tests were performed to analyze the joints for defects and to quantify the mode-I and mode-II interlaminar fracture toughness and short beam strength of the proposed methodology with varying manufacturing parameters. The comprehensive results indicate that the AERoBOND+ method with and without surface preparation performs comparably to co-cured and conventional, adhesively bonded joints when secondary cured in an autoclave with 791 kPa of pressure. As an example, the AERoBOND+ panel without surface preparation bonded with 791 kPa of pressure (referred to as AB+3 throughout paper) had a mode-I fracture toughness (GIc) of 0.643 kJ/m2 and a mode-II fracture toughness (GIIc) of 4.000 kJ/m2 in the non-precracked condition and 4.218 kJ/m2 in the precracked condition at the adhesive-to-prepreg interface. These results were 96 %, 142 %, and 217 %, respectively, of a co-cured baseline panel (referred to as C1 throughout paper).

Ultrasonic inspection during autoclave cure of reflowable-interface composite joints

In structural bonds, the interface between adherend and adhesive is nearly two-dimensional making the interface susceptible to minute quantities of contamination, which can cause weak bonds. Regulatory organizations such as the Federal Aviation Administration (FAA) often require redundant load paths in secondary-bonded primary-structures due to uncertainty in bond performance. To address this issue, the NASA Convergent Aeronautics Solutions (CAS): Adhesive Free Bonding of Composites (AERoBOND) project is investigating reformulated aerospace epoxy-matrix resins to enable reflow and diffusion of the resin at the joint interface during a secondary bonding and cure process that can eliminate the material discontinuity at the interface. Implementing in-situ process monitoring enables assessing the bond quality during processing rather than waiting for post-fabrication mechanical testing to be completed. In this paper, an in-situ ultrasonic inspection system is used to monitor the joining of three composite laminates assembled using the AERoBOND technique. For each panel, the amplitude of the wave reflection at the joint was measured throughout the cure cycle. The results indicate the timing and extent of reflow and cure of the epoxy resin at the joint. Factors indicating end of cure were ascertained and a metric was developed to qualitatively predict acceptable mode-II fracture toughness based on the ultrasonic amplitude at the joint line during cure. In addition, since the inspection system scans an area of the joint, high-resolution localized results can be obtained across the joint.

Investigations on MBSE modelling and dynamic performance assessment of an electrical trimmable horizontal stabilizer actuator

With the development of power-by-wire technology for more electric aircraft, the electro-mechanical actuator (EMA) has the advantages to replace the conventional hydraulic servo actuator in some aerospace flight controls. Conventional hydraulically powered trimmable horizontal stabilizer actuation (THSA) system is nowadays developed to be electrically supplied. Given their safety-criticality, no-back mechanism and redundant load paths are utilized to meet the flight control requirements. However, rare literatures have introduced these functions and addressed the virtual prototyping activities from system-level point of view. This paper proposed such a model of a THSA system with dual electric power sources and fault-tolerant mechanical load paths. The nonlinear effects of components are considered with realism, and system-level simulation test is conducted to support the model-based system engineering (MBSE) approach. The models are developed with a power view instead of a pure signal view. Focusing on the friction effect and compliance effect with backlash or preload, some improved and novel approaches are adopted for these crucial components and validated via experimental results. Meanwhile, the implemented system-level model enables injection of crucial faults. Finally, the simulation of the proposed model shows that it is an efficient resource to investigate the actuator’s dynamic performance, to virtually prove that the actuator meets the fail/safe constraint, and to demonstrate the soundness of the fault monitoring functions.

Interlaminar mechanical performance of latent-cure epoxy joints

Because of uncertainty in bond strength of adhesively bonded composite structures for air flight, redundant load paths, such as mechanical fasteners or other crack arresting features, are required for certification. However, these redundant load paths add extra weight to the structure, induce significantly greater fabrication time and cost, and introduce local stress concentrations (in the case of mechanical fasteners) from which damage can more readily initiate. Structures with co-cured joints are more predictable, require much fewer redundant load paths, and are therefore more desirable. A secondary bonding technique for polymer-matrix composites that aims to produce co-cure-like joints with a continuous interface is briefly presented. A brief progression of mechanical testing results is reported and compared to actual co-cured joint performance. The purpose of the work is to determine the feasibility of this technology to produce a secondary bond equivalent in performance and predictability to that of a co-cured material. Joint performance was measured using six ASTM standard tests to measure interlaminar tensile and shear strength as well as mode-I and mode-II interlaminar fracture toughness at ambient temperature. After 50 process iterations, the final overall average joint performance exceeded 80% of the co-cured performance and in some tests had outperformed the co-cured specimens. The results suggest a high degree of feasibility of the technology to be pursued for future development with the intent to increase performance as well as reliability.

Latent cure epoxy resins for reliable joints in secondary-bonded composite structures

In high-performance polymer matrix composite assemblies, adhesive bonding is generally superior to mechanical fastening in structural performance and manufacturing efficiency. However, adhesive bonds are susceptible to minute levels of contamination accumulated during assembly that can lead to unpredictable, weak bonds. Current methods of measuring bond strength are all destructive mechanical tests. To overcome these challenges, redundant load paths (e.g., mechanical fasteners) are often implemented in secondary-bonded, primary-structures, which can greatly reduce structural performance. This study investigated reformulated aerospace epoxy matrix resins with stoichiometric offset to inhibit cure of the matrix resin prior to assembly. Inhibited resins can reflow and mix across the joint interface , which eliminates the material discontinuity and forms a homogenous joint with reliable fracture properties . The goal of this study was to develop and demonstrate secondary composite assemblies that are mechanically and microscopically indistinguishable from a co-cured composite joint. This article describes the development of latent epoxy resins, the fabrication of test articles, and the mechanical properties measured from experimental joints compared with conventional, co-cured laminates. Methods of in-line quality control using and infrared spectroscopy and post-assembly forensics are also described. The final mode-II fracture toughness measured from precracked AERoBOND specimens was similar to that measured from co-cured laminates indicating that later cure epoxy materials could be a suitable replacement for secondary bonding. • Methods enable rapid assembly of thermoset composites with the simplicity of secondary bonding and reliability of co-cure. • Epoxy based resins were formulated with offset stoichiometry: epoxy rich and hardener rich. • End-notched flexure tests indicated that joints with offset stoichiometry matched or exceeded co-cured joint performance. • Infrared spectroscopy and ultrasonic inspection were viable means of inline quality control before and after assembly.

Determining the endurance limit of AISI 4340 steels in terms of different statistical approaches

In engineering applications, fatigue phenomenon is a key issue and needs to be analyzed in the beginning of design phase in case of any component exposed to alternating loading on operation otherwise catastrophic fatigue failure may cause. Component can be designed with safe-life, fail-safe, and damage tolerant approach based on whether redundant load path and damage sensitive. Before starting analyzing the structure, material allowable data needs to be presented in a reliable way to predict fatigue life of components. SN curves with presented confidence levels are the robust approach to make a prediction on safe life of a structure in terms of fatigue. In this point, there are so many approaches to determine fatigue limit of materials and issue shall be handled by statistical manner. In literature, different staircase and curve fitting methods were presented to estimate endurance limit of materials and some reliability manuscript published. In this paper, fatigue limit of AISI 4340 steel will be investigated through most convinced staircase and curve fitting approaches and their reliability will be queried.

Experimental investigation of structural system capacity with multiple fatigue cracks

Few experimental data sets exist in the literature to support the development and evaluation of digital twins predicting structural degradation. The literature is especially sparse for system tests where multiple failures occur and interact. In this work, a laboratory-level experiment is conducted to mimic many of the properties of larger and more complex marine structures with redundant load paths, failure interaction, and component-to-system level integration. In the experiment, such properties are reflected by a hexagon tension specimen with four propagating fatigue cracks tested under displacement-controlled loading. The applied loading cycles and corresponding crack lengths are recorded as the major time-varying data of degradation, with the resisting force at maximum extension used as the system capacity. A novel computer vision method is used to measure the crack length. Strain gauges are also used to monitor the structure’s status. The experimental data is presented and analyzed in this paper. The resulting data sets can be used to evaluate the performance of different digital twin updating approaches.

Dynamic Response Analysis of Composite Tether Structure to Airborne Wind Energy System under Impulsive Load

The tether structure plays the role of transferring the traction force of an airborne wind energy system (AWES) to the fixed or mobile ground system with less motion and maintains the flying airborne system as a critical component. The implementation of a geometrically tailored tether design in an AWES could avoid unwanted snap-through failure, which can be take place in a conventional tether structure under impulsive loading. This concept relies on the redundant load path of the composite structure composed of tailored length and strength. In this study, the dynamic response of this composite tether structure to airborne wind energy systems, such as a kite wind power system, was analytically investigated. Also, for very long tether applications, an approximate model of the tether response was developed, which resulted in a dramatic reduction of computational efforts while preserving the accuracy quite well compared to the exact solution.

Performance-based seismic design framework for RC floor diaphragms in dual systems

Floor diaphragms play several roles in the seismic response of dual systems: support vertically spanning components, transfer lateral forces to walls and frames, provide restraint to columns and walls, tie the structure together, and enable redundant load paths for lateral forces. In many buildings after events as recent as the 2010-2011 Christchurch earthquakes, floor diaphragms were unable to perform one or more of these functions, leading to extensive damage and collapse. Research consistently highlights three underlying causes: a failure to ensure the integrity of the load path, underestimation of in-plane forces, and poorly understood interactions with walls, supporting beams and RC moment frames. Most recent and ongoing research has focused on modifying the prescriptive code provisions, and indeed many codes–but not all- have been consequently updated. Such prescriptive rules, however, are not helpful for assessing existing buildings, comparing alternate means and methods, or in displacement-based design (DBD) for which a performance-based design (PBD) framework is necessary. This paper proposes a new performance-based framework for the seismic design of reinforced-concrete (RC) floor diaphragms with or without precast elements. The floor diaphragm performance limit states (LS) are re-defined in terms of the observed failure modes (FM). Results from prior research on these failure modes are used to select damage measures (DM, e.g. ‘crack width’) for pairs of FM and LS. Expressions for DMs in terms of engineering demand parameters (EDP, e.g., ‘strain’) are derived from experimental results or from first principles. EDPs are the basic output from numerical analysis in PBD; this paper comments on the suitability of different analytical approaches.

Finite Element Modeling and Experimental Characterization of Enhanced Hybrid Composite Structures for Improved Crashworthiness

In this work, two hybrid composite structures were designed, modeled, and tested for improved resistance to impact. They were inspired by bistable composite structures, which are structures composed of two parts: a so-called “main link” and a so-called “waiting link.” These links work together as a mechanism that will provide enhanced damage tolerance, and the structure exhibits a bistable stress/strain curve under static tension. The function of the main link is to break early, at which point the waiting link becomes active and provides a redundant load path. The goal of the current study was to design, manufacture, and test a similar concept for impact loading and achieve greatly improved impact resistance per unit weight. In the current project, the main link was designed to be a brittle composite material (in this case, woven carbon/epoxy) exposed to impact, while the waiting link was chosen to be made with a highly nonlinear and strong composite material (in this case, polyethylene/epoxy), on the opposite surface. Hence, the structure, if proven successful, can be considered an enhanced hybrid concept. An explicit finite element (FE) commercial code, LS-DYNA, was used to design and analyze the baseline as well as two proposed designs. The simulations' methodology was validated with results published in the literature, which reported tests from linear fiber-reinforced composites. The plots were obtained via the ASCII files generated from the FE code, processed using matlab®, and compared to experimental impact tests. An instrumented drop-weight testing machine performed impact tests, and a high-speed camera validated the specimens' displacement under impact. It is shown that the FE model provided qualitative behavior very consistent with the experiments but requires further improvements. Experimentally, it is shown that one of the two enhanced hybrid models leads to up to a 30% increase of returned energy/weight when compared to its baseline and, therefore, is worthy of further investigations.

Laser Ablative Surface Treatment for Enhanced Bonding of Ti-6Al-4V Alloy

Adhesive bonding offers many advantages over mechanical fastening, but requires certification before it can be incorporated in primary structures for commercial aviation without disbond-arrestment features or redundant load paths. Surface preparation is widely recognized as the key step to producing robust and predictable adhesive bonds. Surface preparation by laser ablation provides an alternative to the expensive, hazardous, polluting, and less precise practices used currently such as chemical-dip, manual abrasion and grit blast. This report documents preliminary testing of a surface preparation technique using laser ablation as a replacement for the chemical etch and abrasive processes currently applied to Ti-6Al-4V alloy adherends. Surface roughness and surface chemical composition were characterized using interference microscopy and X-ray photoelectron spectroscopy, respectively. A technique for fluorescence visualization was developed which allowed for quantitative failure mode analysis. Wedge crack extension testing in a hot, humid environment indicated the relative effectiveness of various surface treatments. Increasing ablation duty cycle reduced crack propagation and adhesive failure. Single lap shear testing showed an increase in strength and durability as laser ablation duty cycle and power were increased. Chemical analyses showed trends for surface chemical species, which correlated with improved bond strength and durability.

Basic Principal and Design Methodology of Progressive Storey Collapse Resistance in a New Type RC Twin-Frame Structure

According to earthquake hazard, normal frame structures could hardly avoid failure of ‘Strong Beam and Weak Column’, weak layer and even progressive storey collapse under strong earthquakes, a main reason is the lack of second seismic fortification line in frame structure. Based on the design thought of multiple seismic fortification lines, reinforced concrete twin-frame structure is initiated in this paper. Major feature is increases of redundant constraints and load paths due to added outer limb-columns could take effect as the second seismic fortification line. Twin-frame structure could not only fulfill the demand of ‘three levels, two stages’ in seismic code, but also have been designed through the third stage of ‘resist progressive collapse’, using key member method to promote safety storage and deformability of outer limb-columns, using removal key member method to verify vertical ultimate load of frame beams in weak layer. Though twin-frame structure could not completely avoid plastic hinges in column ends, progressive storey collapse will not appear in this kind of structure. Twin-frame structure is a new structure system which could be applied in constituting new projects as well as strengthening and rebuilding existing projects, further research can be done on this structure system.

Analytical Investigation of the Toughening Potential of a Failure Tailoring Concept

A parametric study has been performed to determine the upper bounds of toughening benefit achievable using an approximate model for the response of one-dimensional, tailored, flexible composite-material tethers with progressive failure. The tailoring concept is achieved through judicious arrangement of redundant load paths with unequal length and strength and has been proposed, modeled, and experimentally verified in previous research. The model is recast in this work using nondimensional material-independent parameters that are varied over ranges of interest in advanced composites. The effects of each parameter on the response of the tailored member are discussed. An increase of nearly two orders of magnitude in toughness over an untailored member is found to be attainable by taking complete advantage of the energy dissipated by the failure of each load path within the bounds of the study. Challenges to implementation of the tailored member as approximated by the model are also discussed.

Energy-Dissipating Composite Members with Progressive Failure: Impulsive Response and Experimental Verification

An experimental investigation of response for tailored one-dimensional energy-dissipating composite members with progressive failure subject to quasi-static and impulsive tensile loading is presented. The tailoring concept relies upon a progressive failure sequence of redundant load paths of tailored strength and length to induce a yield-type response. In a prior publication the authors presented the concept development and analytical modeling of response under quasi-static loading. In this paper, an experimental verification is provided using a universal testing machine for members under quasi-static uniaxial loading. Furthermore, the model is extended to impulsive loading, and a custom-design drop test setup is developed to provide an experimental validation of analytical response. The results obtained confirm the hypothesized progressive failure sequence of redundant load paths, thereby validating the failure tailoring concept, as well as the accuracy and predictive power of the developed models of response in terms of both the number of partial failures induced for a given loading and the increased energy dissipation capability. Potential aerospace applications include snap-resistant tensile structures, for example space tethers, towing and cargo restraint lines, crashworthy helicopter troop seat stroke control straps, and aircraft emergency arrest gear.

Energy-Dissipating Composite Members with Progressive Failure: Concept Development and Analytical Modeling

A new concept for tailored, one-dimensional tensile composite members with sequential, progressive failure and yield-type response is presented. The concept is developed as a solution to a constrained arresting problem. Flexible composites, obtained by reinforcing an elastomeric matrix material with high-performance elastic fibers, are used to create a structure with redundant load paths and progressive failure. Three analytical models are developed for the prediction of response under stroke-controlled loading. The response of the tailored configuration is compared with that of conventional members of identical length, total cross-sectional area, and material properties. Analytical results confirm the predicted progressive failure sequence and yield-type response, as well as the increased energy-dissipation capability.

Structural analyses, sensitivity analyses, and independent confirmatory analyses of the IGNITOR magnet system

Ignitor has been the first experiment proposed and designed to reach ignition conditions in magnetically confined plasmas. It is a compact, high field machine based on the physics developed primarily by the Alcator series of experiments. Each refinement of IGNITOR has drawn from a common set of "tools": Bitter plate copper magnets; bucking/wedging interactions of the CS and TF coils; and passive and active preload mechanisms to offset vertical tension in the inner leg and tension in the horizontal leg. Elements of the IGNITOR design are statically indeterminant. Examples of multiply redundant load paths used in other reactor designs are cited. Despite a large body of historical work confirming the structural feasibility of IGNITOR, if still is erroneously perceived as more challenging than other tokamak designs. Past US analyses are recalled: US contributions from independently derived models are described and are compared with the latest IGNITOR project analyses. Differences in modeling philosophy are presented, and results are compared. Recent US analyses employ non-linear path dependent friction of a 2 coil segment. Sensitivity studies of fit-up tolerances and, uncertainties in material properties are presented. While there are some small differences in results, these analyses independently confirm the benefits of the major structural elements used in IGNITOR.

Cracks and Structural Redundancy

This paper proposes a way to use finite element models to determine the effect of fatigue cracks in ship structures. Cracks of different lengths are modeled and the maximum nodal stress at the crack tip is used to estimate the stress intensity level. The calculated stress intensity factor is substituted into fracture mechanics equations to calculate the rate of crack growth and the critical crack length. The main advantage to calculating the stress intensity factor from an appropriate finite element model is that load redistribution effects are included. Since the ship structure is redundant, load shedding and multiple load paths can significantly reduce the propagation rate and thereby improve the safety of the structure. Including the crack in the finite element model also makes it possible to study the crack's impact on the stress levels in surrounding structure. The method is independent of the cause of structural failure, hence this kind of analysis can be used to investigate the effects of cracks created by fatigue, collision, or grounding damage.

A general theory for frequency domain structural synthesis

Fundamental issues in frequency domain structural synthesis are addressed. A new derivation of the operative synthesis equation which is based on a congruent transformation of the pre-synthesis frequency response matrix is developed. That this transformation is intrinsic to the theory is evident in the form of this operative equation found in virtually all prior references to the theory. The structure of the operative equation facilitates its straightforward incorporation into a finite element analysis environment. The new formulation provides an exact and orderly treatment of both “coupling”, the joining of previously uncoupled structures, and “modification”, the creation of redundant load paths in a structure, and makes plain their equivalence with respect to the derivation and application of the theory. The new formulation encompasses previously addressed capabilities of the theory, and provides new capabilities. A primary feature of the new formulation is the provision for general indirect synthesis, the inclusion of intermediate or interconnecting structural elements of any type. Lumped elements as well as distributed (finite) elements can be included, either between substructures in a coupling operation, or as redundant load paths in a single structure, in a modification. The application of graph theory to structural synthesis gives rise to Boolean and non-Boolean mapping matrices which organize the coupling force sign conventions. The new theory provides a thorough accounting for the role of graph theory in structural synthesis, and a fundamental understanding of the relationship between a given structural element and its mapping matrix is achieved.

Semi-rigid Composite Steel Frames

Semi-rigid, composite steel frames are a new type of structural system being developed to better utilize the composite floor slabs and flexible connections present in common AISC Type 2 frames. This new structural system extends the beneficial aspects of composite action to the negative moment region of continuous beams by providing slab reinforcement across column lines. The resultant system offers significant gains in stiffness and strength not only in the members themselves but also at the connections. Because the connections benefiting most from this action are the relatively flexible ones (not rigid), the name "semirigid composite frames" (SRCF) has been coined. The economies in materials and erection cost associated with composite and mixed construction are widely recognized.1 Composite construction generally results in (1) reductions of steel area needed to support a given load, (2) an increase of overload capacity over non-composite sections, (3) reductions of construction depths and (4) an increase on the safety of the system by providing redundant load paths. Typically, composite action is used to increase the capacity of beams designed as simple spans, but its effect on the overall frame strength and stability is ignored.

Fatigue Notch Factors for Structural Details

The use of fatigue notch factors is proposed for specifying the relationship between the allowable stress range and the number of loading cycles. This is done by premultiplying the stress range with a fatigue notch factor which accounts for the severity of the detail’s stress concentration. The method simplifies the manner in which fatigue specifications are written, and it conveys the idea of classification of details by severity of stress concentrations; it is most suitable for use in computer-aided analysis and design. The proposed equations for redundant load path structures give stress range values closer to those corresponding to the lower confidence limit at two standard deviations of the mean than the rounded off values in the specifications. The proposed equations for nonredundant load path structures are set at a uniform 5.5 standard deviations below the mean.