Damage Tolerance Of Structures Research Articles

An approach based on biological algorithm for three-dimensional shape optimisation with fracture strength constrains

This paper presents an innovative approach to shape optimisation of three-dimensional damage tolerant structures for residual static strength. In this approach, a new and simple method, which we termed FAST (Failure Analysis of Structures), for estimating the stress intensity factor for cracks at a notch, as well as an extension of the biological algorithm was employed to study the problem of optimisation with fracture strength as the design objective. This method is ideal for use in structural optimisation as accurate results are obtained without the need to explicitly model a crack, i.e. it is only necessary to model the uncracked structure. Methodology and software to automate damage tolerance calculations were developed using CAD and FAST software. The numerical examples illustrate the use of the damage tolerance optimisation software and prove that stress optimised geometries are not necessarily optimised for residual static strength or fatigue life.

Three-dimensional structure optimal design for extending fatigue life by using biological algorithm

This paper presents some applications of a new structural shape optimization procedure for maximizing fatigue life or inspection intervals for damage tolerant structures. In this approach, a new and simple method, which we termed FAST (Failure Analysis of Structures), for estimating the stress intensity factor for cracks at a notch, as well as an extension of the biological algorithm was employed to study the problem of optimization with fatigue life as the design objective. Research by the authors has demonstrated that the optimum shape for minimizing stress is not necessarily the optimum shape for static strength or fatigue life of a damage tolerant structure. The examples are presented that highlight this difference. The optimal shapes for stress are compared with optimized shapes found for static strength with different crack lengths. These are also compared with optimized shapes found for maximum fatigue life. The choice of initial crack size was found to have a significant effect on the optimal shapes for the structures presented.

Response of Composite Fuselage Sandwich Side Panels Subjected to Internal Pressure and Axial Tension

The results from an experimental and analytical study of two composite sandwich fuselage side panels for a transport aircraft are presented. Each panel has two window cutouts and three frames and utilizes a distinctly different structural concept. These panels have been evaluated with internal pressure loads that generate biaxial tension loading conditions. Design limit load and design ultimate load tests have been performed on both panels. One of the sandwich panels was tested with the middle frame removed to demonstrate the suitability of this two-frame design for supporting the prescribed biaxial loading conditions with twice the initial frame spacing of 20 inches. A damage tolerance study was conducted on the two-frame panel by cutting a notch in the panel that originates at the edge of a cutout and extends in the panel hoop direction through the window-belt area. This panel with a notch was tested in a combined-load condition to demonstrate the structural damage tolerance at the design limit load condition. Both the sandwich panel designs successfully satisfied all desired load requirements in the experimental part of the study, and experimental results from the two-frame panel with and without damage are fully explained by the analytical results. The results of this study suggest that there is potential for using sandwich structural concepts with greater than the usual 20-in.-wide frame spacing to further reduce aircraft fuselage structural weight. in the airframe industry. Structural trade studies for

A Statistical Approach to the Prediction of Brittle Fracture in Heat-Affected Zones of A707 Steel Welds

This article presents a statistical approach to the prediction of the variations in fracture toughness of the heat-affected zones in A707 steel welds. Fractographic analyses were first used to determine the inclusion size distributions from the ductile fracture regions that precede the onset of cleavage fracture. The inclusion size distributions were shown to be well characterized by the three-parameter Weibull distribution. Based on the inclusion size distribution, a modified version of the model by Lin, Evans, and Ritchie was used to estimate the statistical variability of the crack-tip opening displacement (fracture toughness). The modification involves the use of a polynomial function in the determination of the dimensionless stress parameter, in the Hutchinson-Rice-Rasengren (HRR) field solution. This enables a fully analytical solution to be obtained for the critical driving forces associated with specified failure/survival probabilities under elastic-plastic fracture conditions. The comparison showed that the modified model prediction was in good agreement with measured crack-tip opening displacement (fracture toughness) data. Finally, the implications of the results are discussed for the probabilistic design of damage tolerant structures in which cleavage fracture can occur, following an initial stage of ductile tearing by microvoid coalescence.

Design of structures for optimal static strength using ESO

This paper presents a modified evolutionary structural optimisation (ESO) algorithm for optimal design of damage tolerant structures. The proposed ESO algorithm uses fracture strength as the design objective. The formulation outlined here can be used for shape optimisation of structures and allows for cracks to be located along the entire structural boundary. In this work we use an approximate method for evaluating the stress intensity factors associated with the cracks. This extended ESO algorithm is illustrated using the problem of the optimal shape design of a `cutout' in a rectangular plate under biaxial loading and the design of a shoulder fillet under uniaxial tension. It is found that this method reduced the maximum stress intensity factor for the optimum shape and produced a near uniform level of fracture criticality around the boundary. It is also shown that the shapes optimised for stress and fracture strength may differ. This highlights the need to explicitly include fracture parameters in the design objective function. The results agreed well with those reported in the literature using a biological method.

Cyclic Response of Unbonded Posttensioned Precast Columns with Ductile Fiber-Reinforced Concrete

A precast segmental concrete bridge pier system is being investigated for use in seismic regions. The proposed system uses unbonded posttensioning (UBPT) to join the precast segments and has the option of using a ductile fiber-reinforced cement-based composite (DRFCC) in the precast segments at potential plastic hinging regions. The UBPT is expected to cause minimal residual displacements and a low amount of hysteretic energy dissipation. The DFRCC material is expected to add hysteretic energy dissipation and damage tolerance to the system. Small-scale experiments on cantilever columns using the proposed system were conducted. The two main variables were the material used in the plastic hinging region segment and the depth at which that segment was embedded in the column foundation. It was found that using DFRCC allowed the system to dissipate more hysteretic energy than traditional concrete up to drift levels of 3–6%. Furthermore, DFRCC maintained its integrity better than reinforced concrete under high cyclic tensile-compressive loads. The embedment depth of the bottom segment affected the extent of microcracking and hysteretic energy dissipation in the DFRCC. This research suggests that the proposed system may be promising for damage-tolerant structures in seismic regions.

Development of New Damage Tolerant Alloys for Age-Forming

The suitability of age forming for the shaping of damage tolerant structures is investigated by formulating and testing new alloy-age forming combinations. The alloy formulation process is driven initially by modelling of strength and semi-quantitative understanding of other microstructure-property relations. Using this a range of Al-Cu-Mg-Li-(Zr-Mn) based alloys predicted to provide yield strengths in aged condition comparable with incumbent 2024-T351 alloy for lower wing skins are selected. It is shown that several of these new alloys after artificial aging representative of age-forming have proof strength (PS), fatigue crack growth resistance (FCGR) and toughness that are comparable or better than 2024-T351. UTS to PS ratios of the new alloys are lower than 2024-T351.

Bayesian Updating of Damage Size Probabilities for Aircraft Structural Life-Cycle Management

A statistical approach to estimating the probabilistic distribution of composite damage sizes using aircraft service inspection data has been investigated. Bayesian updating methods were implemented to revise baseline composite damage size distributions using damage size data from the Federal Aviation Administration's Service Difficulty Reporting System. Updating was performed on the Boeing 757 and 767 wing composite trailing-edge devices, elevators and rudders, with the results demonstrating that the assumed baseline damage size estimates are conservative in nearly all cases. Component failure probabilities were recalculated using the updated damage size distributions, and these results show an overall improvement in reliability for the damage mechanisms analyzed. The results of the analysis demonstrate that an inspection and maintenance program that reports damage characteristics can be used to monitor the reliability of damage tolerant structures on a quantitative statistical basis. Recommendations are also made for improving current inspection data reporting systems, which would enhance the ability to gather detailed information on the characteristics of each structural damage event.

Weld‐bonding: the best or worst of two processes?

Weld‐bonding combines the physical force‐based process of welding with the chemical force‐based process of bonding or, more properly, adhesive bonding. When done properly, the claim is that a hybrid process results which offers the best of both processes; the high joint efficiency, resistance to diverse and complex loading, and temperature tolerance of welding; the load‐spreading, stress concentration‐softening, and structural damage tolerance of adhesive bonding. And, beyond these individual process attributes, there are claims, or at least predictions, of synergistic benefits in the form of improved energy absorption and fatigue life for demanding applications. However, it is difficult to find reliable data in the open literature to support these real or potential benefits. Furthermore, complications in performing the hybrid process in practice place an even greater premium on process control than normal. This paper explores the question, “Is it all worth it?” The paper delves into the theory underlying weld‐bonding, the facts concerning the process including pluses and pitfalls, and considers where the process could or should go from here.

In situ monitoring in real time of fatigue-induced damage growth in composite materials by acoustography

There are growing concerns about the effects of accidental impact damage on the structural integrity of aerospace composites and about the possible growth of the damage due to in-service fatigue. There has been some success in the use of established methods (ultrasonic C-scan, thermography, X-rays) to monitor damage development during fatigue experiments by interrupting a test and removing the specimen for damage inspection but this stop-and-restart test procedure is far from satisfactory. Real-time damage monitoring in composite materials during fatigue has now become possible by the emergence of a new ultrasonic imaging technology, acoustography. The successful integration of acoustography and a servo-hydraulic fatigue test machine has resulted in a new measurement system which can be used for the in situ monitoring in real time of damage growth in composite specimens during long-term fatigue tests. Results are presented which show damage-area growth during fatigue cycling under high compressive loads. After an initial small enlargement (stage 1), damage grows at a constant rate (stage 2) until the third stage is reached when there is further growth at an increasing rate to final failure. However, a ‘fatigue limit’ has also been observed. At stresses below this fatigue limit, a zero damage-growth régime has been found in studies of >10 6 fatigue cycles. The results obtained have important implications for the understanding of the effects of damage on fatigue life and for the design of ‘safe’ damage-tolerant structures.

Simulation framework for risk assessment of damage tolerant structures

Damage tolerant structures rely on safety by virtue of damage detection during periodic inspections and follow-up repairs. Aging structures and life extension programs need continuous monitoring for safe operation. Assuring a certain maximum risk level demands defining an inspection program for these structures in which the damage initiation, growth, detection, and tolerance are all probabilistic in nature. Fewer inspections increase the probability of failure. Over-inspecting leads to increased life cycle costs and downtime periods. This paper presents a simulation framework which can synthesize the probabilistic variations to strike a balance between desired safety levels and affordable inspection programs over a life span of a generic damage tolerant engineering structure. This novel framework can absorb virtually any mathematical or numerical model for representation of service life events; which in turn helps quantitatively assess risk, perform sensitivity analysis and resolve optimization issues related to maintenance decisions.

Incorporating interlaminar fracture mechanics into design

In laminated composite structures, interlaminar failures or delaminations have continued to be a predominant life-limiting failure mode. The main concern for designers has focused on the damage tolerance of structures in the presence of delamination damage from impact events. The design approach has been to utilize existing stress/strain-based design criteria to design the part, then to impact the part consistent with the threat and to take a knock-down in design loads in the presence of damage. However, delaminations also occur during service in areas where high interlaminar stresses are present. These are generally at required discontinuities in the design, such as cut-outs, holes, joints or ply-drops. The application of interlaminar fracture mechanics as a design tool for optimizing these regions is still in its infancy and is adopted by very few original equipment manufacturers. This paper addresses this issue and presents the more recent developments in incorporating interlaminar fracture mechanics into design. The paper discusses the improvements in global-local modelling to integrate local design features with global stiffness and load considerations and vice versa. It will address the current methods for characterizing interlaminar fracture behaviour and show examples of how predictions can be made to predict the life of a component.

Approximations in Optimization of Damage Tolerant Structures

Approximations for structural response and design sensitivities signie cantly reduce the computational cost of structural optimization. To be useful, the approximation must be accurate and computationally efe cient. Damage tolerant design puts even more stringent computational demands on optimization procedures and approximations duetothelargestructuralchangesinherentindamage.Arobustapproximationisevaluatedandextendedtoinclude design sensitivities. It is implemented in an optimization procedure and tested. It is shown that this approximation is accurate and efe cient for damage tolerant optimum design of complex structural models. Last, a qualitative measure is developed and tested that estimates the computational savings using these approximations.

Approximations in optimization of damage tolerant structures

Approximations in optimization of damage tolerant structures

Damage modes in dental layer structures.

Natural teeth (enamel/dentin) and most restorations are essentially layered structures. This study examines the hypothesis that coating thickness and coating/substrate mismatch are key factors in the determination of contact-induced damage in clinically relevant bilayer composites. Accordingly, we study crack patterns in two model "coating/substrate" bilayer systems conceived to simulate crown and tooth structures, at opposite extremes of elastic/plastic mismatch: porcelain on glass-infiltrated alumina ("soft/hard"); and glass-ceramic on resin composite ("hard/soft"). Hertzian contacts are used to investigate the evolution of fracture damage in the coating layers, as functions of contact load and coating thickness. The crack patterns differ radically in the two bilayer systems: In the porcelain coatings, cone cracks initiate at the coating top surface; in the glass-ceramic coatings, cone cracks again initiate at the top surface, but additional, upward-extending transverse cracks initiate at the internal coating/substrate interface, with the latter dominant. The substrate is thereby shown to have a profound influence on the damage evolution to ultimate failure in the bilayer systems. However, the cracks are highly stabilized in both systems, with wide ranges between the loads to initiate first cracking and to cause final failure, implying damage-tolerant structures. Finite element modeling is used to evaluate the tensile stresses responsible for the different crack types. The clinical relevance of these observations is considered.

Scaling crucial to integrated product development of composite aerospace structrues. Part 1

I dedicate this manuscript to the memory of Ernest F. Dost, who passed away on 18 September 1995. He had numerous achievements prior to his tragic death at the young age of 34. In less than a decade, he helped solve many difficult problems supporting composite structural design, manufacturing and maintenance as an aeronautics engineer at Boeing. Ernie had a critical role on the integrated product development team that performed the work described in both parts of this article. Part 2 will highlight Ernie's important contributions to the field of composite impact, as related to structural damage tolerance.

Application of a p-version finite element code to analysis of cracks

A commercially available finite element analysis computer package, i.e., the MECHANICA®-APPLIED STRUCTURE code of the RASNA Corporation, has been used to generate stress intensity solutions for structural damage tolerance analysis applications. A building block approach has been implemented in developing a data reduction technique for using the finite element code. Through two sets of numerical examples, it is demonstrated that stress intensity solutions for the center crack panels (two-dimensional), and the almond shaped crack (three-dimensional), matched very well with known solutions available in the literature

Engineering application of fracture mechanics to aircraft structure damage tolerance estimation

Engineering application of fracture mechanics to aircraft structure damage tolerance estimation

Near-threshold fatigue crack propagation of several high strength steels

In support of Douglas Aircraft Co. structural damage tolerance analysis, crack propagation rates were measured in humid air from 10 −8 to 10 −3 mm/cycle of several high strength martensitic steels at stress ratios of R = 0.1 and R = 0.5. Increasing the stress ratio was found to decrease the threshold stress intensity. Increasing the tensile yield material strength in these quench and tempered steels resulted in a nearly linear inverse relationship with the threshold stress intensity. The threshold stress intensity appeared to be independent of the fracture toughness of the alloy steel.