Damage Tolerance

The THI4 gene of Saccharomyces cerevisiae encodes an enzyme of the thiamine biosynthetic pathway. The plant homolog thi1, from Arabidopsis thaliana, is also involved in thiamine biosynthesis; but was originally cloned due to its capacity to complement DNA repair de®cient phenotypes in Escherichia coli. Here, the behavior of a thi4 disrupted strain was examined for increased sensitivity to treatment with the DNA damaging agents ultraviolet radiation (UV, 254 nm) and methyl methanesulfonate (MMS). Although the thi4 null mutant showed a similar level of survival as the wild-type strain, a higher frequency of respiratory mutants was induced by the two treatments. A similar phenotype was seen with wildtype strains expressing an antisense THI4 construct. Further analysis of respiratory mutants revealed that these were due to mutations of mitochondrial DNA (mtDNA) rather than nuclear DNA, consisting of r À petite mutants. Moreover, the frequency of mutations was unaffected by the presence or absence of thiamine in the growth medium, and the defect leading to induction of petites in the thi4 mutant was corrected by expression of the Arabidopsis thi1 gene. Thus, Thi4 and its plant homolog appear to be dual functional proteins with roles in thiamine biosynthesis and mitochondrial DNA damage tolerance.

This paper presents a methodology for predicting the thresholds of multiple site damage and widespread fatigue damage in fuselage lap slices. Widespread fatigue damage is a type of multiple cracking that reduces the airframe residual strength to a level below the damage tolerant requirement. The MSD threshold refers to the point in the lifetime of an airplane when two adjacent

The paper addresses the damage tolerance of sandwich structures, where the prevention and limitation of delamination failure are highly important design issues. Due to the layered composition of sandwich structures, face-core interface delamination is a commonly observed failure mode, often referred to as peeling failure. Peeling between the sandwich face sheets and the core material drastically diminishes the structural integrity of the structure. This paper presents a new peel stopper concept for sandwich structures. Its purpose is to effectively stop the development of debonding/delamination by rerouting the delamination, and to confine it to a predefined zone in the sandwich structure. The suggested design was experimentally tested for different material compositions of sandwich beams subjected to three-point bending loading. For all the tested sandwich configurations the suggested peel stopper was able to stop face-core delamination and to limit the delamination damage to restricted zones.

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T3 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges . In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.

The effects of nanoclay inclusion on cyclic fatigue behavior and residual properties of carbon fiber-reinforced composites (CFRPs) after fatigue have been studied. The tension-tension cyclic fatigue tests are conducted at various load levels to establish the S-N curve. The residual strength and modulus are measured at different stages of fatigue cycles. The scanning electron microscopy (SEM) and scanning acoustic microscopy (SAM) are employed to characterize the underlying fatigue damage mechanisms and progressive damage growth. The incorporation of nanoclay into CFRP composites not only improves the mechanical properties of the composite in static loading, but also the fatigue life for a given cyclic load level and the residual mechanical properties after a given period of cyclic fatigue. The corresponding fatigue damage area is significantly reduced due to nanoclay. Nanoclay serves to suppress and delay delamination damage growth and eventual failure by improving the fiber/matrix interfacial bond and through the formation of nanoclay-induced dimples.

Aircraft is symbol of a high performance mechanical structure, which has the ability to fly with a very high structural safety record. Aircraft experiences variable loading in service. Rarely an aircraft will fail due to a static overload during its service life. For the continued airworthiness of an aircraft during its entire economic service life, fatigue and damage tolerance design, analysis, testing and service experience correlation play a pivotal role. The present study includes the stress analysis and damage tolerance evaluation of the wing through a stiffened panel of the bottom skin with a landing gear cutout. Wing bottom skin experiences tensile stress field during flight. Cutouts required for fuel access and landing gear opening and retraction in the bottom skin will introduce stress concentration. Fatigue cracks will initiate from high tensile stress locations. An integral stiffened panel consisting a landing gear cutout is considered for the analysis. Stress analysis will identify the maximum tensile stress location in the panel. In a metallic structure fatigue manifests itself in the form of a crack which propagates. If the crack in a critical location goes unnoticed it could lead to a catastrophic failure of the airframe. A critical condition will occur when the stress intensity factor (SIF) at the crack tip becomes equal to fracture toughness of the material. SIF calculations will be carried out for a crack with incremental crack lengths using MVCCI method. Analytical evaluation of the crack arrest capability of the stiffening members ahead of the crack tip will be carried out.

Materials in rotating machinery are typically subjected to vibratory loading from a number of sources which, in turn, is superimposed on mean stresses which result primarily from steady-state centrifugal loads. In addition, components subjected to vibratory stresses can sustain damage during manufacturing, break-in cycles, or during service such as from foreign objects, fretting, or other types of wear. The combination of vibratory and ‘steady’ stress levels can, for certain load levels, produce low cycle fatigue damage in addition to the damage produced from the high frequency (HCF) vibratory loading since the ‘steady’ stresses are actually low cycle fatigue (LCF) which results in one cycle for every startup and shutdown operation. Design for HCF is generally based on a Goodman diagram which takes into account the vibratory as well as the steady stress amplitudes for fatigue runout or fatigue under a given number of cycles. It does not, however, take into account the combined effects of LCF and HCF. In this investigation, the combined effects are demonstrated analytically by numerical examples which consider both the initiation and propagation phases of fatigue. In addition to the analysis of LCF/HCF interactions, considerations which must be accounted for in design are reviewed in light of a number of failures of components in service in U.S. Air Force fighter engines. A critical assessment of the concepts embedded in the use of the Goodman diagram is presented. Comments on the limitations on the use of a Goodman diagram for design are provided. Some suggestions are offered for the improvement of the design methodology for HCF which involve both damage tolerance considerations and methods for assessing and improving the margin of safety.

During the past decades, increasing demand in aircraft industry for high-performance, lightweight structures have stimulated a strong trend towards the development of refined models for fibre-metal laminates (FMLs). Fibre metal laminates are hybrid composite materials built up from interlacing layers of thin metals and fibre reinforced adhesives. The most commercially available fibre metal laminates (FMLs) are ARALL (Aramid Reinforced Aluminium Laminate), based on aramid fibres, GLARE (Glass Reinforced Aluminium Laminate), based on high strength glass fibres and CARALL (Carbon Reinforced Aluminium Laminate), based on carbon fibres. Taking advantage of the hybrid nature from their two key constituents: metals (mostly aluminium) and fibre-reinforced laminate, these composites offer several advantages such as better damage tolerance to fatigue crack growth and impact damage especially for aircraft applications. Metallic layers and fibre reinforced laminate can be bonded by classical techniques, i.e. mechanically and adhesively. Adhesively bonded fibre metal laminates have been shown to be far more fatigue resistant than equivalent mechanically bonded structures.

Composites Part A: Applied Science and Manufacturing publishes original research papers, review articles, case studies, short communications and letters from a wide variety of sources dealing with all aspects of the science and technology of composite materials, including fibrous and paniculate reinforcements in polymeric, metallic and ceramic matrices, aligned eutectics. reinforced cements and plasters, and 'natural' composites such as wood and biological materials. The range of applicable topics includes the properties, design and manufacture of reinforcing fibres and particles, fabrication and processing of composite materials and structures, including process science and modelling, microstructural characterization of composites and their constituent phases, interfaces in composites, prediction and measurement of mechanical, physical and chemical properties, and performance of composites in service. Articles are also welcomed on economic and commercial aspects of the applications of composites, design with composites and case studies. All articles published are subject to rigorous peer review and a high standard is set for both content and presentation. The Editors aim to conduct the review procedure with the minimum of delay so that prompt publication ensues.

Weight reduction and improved damage tolerance characteristics were the prime drivers to develop new family of materials for the aerospace/aeronautical industry. Aiming this objective, a new lightweight Fiber/Metal Laminate (FML) has been developed. The combination of metal and polymer composite laminates can create a synergistic effect on many properties. The mechanical properties of FML shows improvements over the properties of both aluminum alloys and composite materials individually. Due to their excellent properties, FML are being used as fuselage skin structures of the next generation commercial aircrafts. One of the advantages of FML when compared with conventional carbon fiber/epoxy composites is the low moisture absorption. The moisture absorption in FML composites is slower when compared with polymer composites, even under the relatively harsh conditions, due to the barrier of the aluminum outer layers. Due to this favorable atmosphere, recently big companies such as EMBRAER, Aerospatiale, Boing, Airbus, and so one, starting to work with this kind of materials as an alternative to save money and to guarantee the security of their aircrafts.

In this paper, a review of engineering coating for engine application is presented. Issues relating to dimensional stability, tribological properties, lubrication, coefficient of friction, hot hardness, amenability for honing, surface roughness and topography, residual stress, adherence, damage tolerance and resistance, pores density and conditions and cost performance are discussed. There exist advantages and limitations of conventional materials systems and techniques such as chemical-vapor-deposited diamond-like carbon (DLC) coating, plasma sprayed metal matrix composite coating, tribologically functional ceramic coatings, etc. Nano-grains of a crystalline phase hold promise to solve several such problems present in conventional coatings. In addition, surface-related problems are addressed for high performance engines and hydrogen powered automotive engines. D

The attainment of both strength and toughness is a vital requirement for most structural materials; unfortunately these properties are generally mutually exclusive. Although the quest continues for stronger and harder materials, these have little to no use as bulk structural materials without appropriate fracture resistance. It is the lower-strength, and hence higher-toughness, materials that find use for most safety-critical applications where premature or, worse still, catastrophic fracture is unacceptable. For these reasons, the development of strong and tough (damage-tolerant) materials has traditionally been an exercise in compromise between hardness versus ductility. Drawing examples from metallic glasses, natural and biological materials, and structural and biomimetic ceramics, we examine some of the newer strategies in dealing with this conflict. Specifically, we focus on the interplay between the mechanisms that individually contribute to strength and toughness, noting that...

This paper presents a study of the effects of ply clustering on polymer-based laminated composite plates subjected to a drop-weight impact loading. The tools used to define the impact configurations, as well as the experimental results obtained, are described in detail. These tools are simplified analytical models for the description of the impact behavior and of the damage thresholds that result in a significant reduction on the structure stiffness and strength, caused by delamination. The results obtained demonstrate that the analytical tools are useful to define the impact configurations, to obtain a preliminary understanding of the effects of each parameter that can influence the response, and to interpret the experimental results. It is concluded that ply clustering reduces the damage resistance of the structure. However, the damage tolerance assessed by the compression after impact tests is unaffected by ply clustering.

In this work, the results of an experimental study for assessing the effects of Ultrasonic Impact Treatment on the fatigue resistance of Friction Stir Welded aluminum alloy panels are presented. Although the significant compressive residual stress introduced on the material by ultrasonic impact treatment (UIT) was expected to cause retardation in the crack growth rate, this was only noted at low initial DJ values. At high DJ values, the effect of UIT practically diminishes. The phenomenon was attributed to the relaxation/ redistribution of the residual stresses with fatigue damage. This provides an alarming situation where damage tolerance design relies on models where only the initial residual stress profile is taken into account without knowledge of the potential re-distribution of the residual stresses caused by the fatigue damage accumulation. The findings of this work also indicate that any FCG tests performed can only be considered as case-specific and conclusions can only be drawn for the case studied.

The deformation and failure response of composite sandwich beams and panels under low velocity impact was reviewed and discussed. Sandwich facesheet materials discussed are unidirectional and woven carbon/ epoxy, and woven glass/vinylester composite laminates; sandwich core materials investigated include four types of closed cell PVC foams of various densities, and balsa wood. Sandwich beams were tested in an instrumented drop tower system under various energy levels, where load and strain histories and failure modes were recorded for the various types of beams. Peak loads predicted by springmass and energy balance models were in satisfactory agreement with experimental measurements. Failure patterns depend strongly on the impact energy levels and core properties. Failure modes observed include core indentation/ cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core. In the case of sandwich panels, it was shown that static and impact loads of the same magnitude produce very similar far-field deformations. The induced damage is localized and is lower for impact loading than for an equivalent static loading. The load history, predicted by a model based on the sinusoidal shape of the impact load pulse, was in agreement with experimental results. A finite element model was implemented to capture the full response of the panel indentation. The investigation of post impact behavior of sandwich structures shows that, although impact damage may not be readily visible, its effects on the residual mechanical properties of the structure can be quite detrimental.

Composites fabricated by VARTM technology with the use of single-ply non-crimp 3D orthogonal woven preforms 3WEAVE Ò find fast growing research interest and industrial applications. It is now well understood and appreciated that this type of advanced composites provides efficient delamination suppression, enhanced damage tolerance, and superior impact, ballistic and blast performance characteristics over 2D fabric laminates. At the same time, this type of composites, having practically straight in-plane fibers, show significantly better in-plane stiffness and strength properties than respective properties of a ''conventional" type 3D interlock weave composites. One primarily important question, which has not been addressed yet, is how the in-plane elastic and strength characteristics of this type of composites compare with respective in-plane properties of ''equivalent" laminates made of 2D woven fabrics. This 2-part paper presents a comprehensive experimental study of the comparison of in-plane tensile properties of two single-ply non-crimp 3D orthogonal weave E-glass fiber composites on one side and a laminate reinforced with four plies of plain weave E-glass fabric on the other. Results obtained from mechanical testing are supplemented by acoustic emission data providing damage initiation thresholds, progressive cracks observation, full-field surface strain mapping and cracks observation on micrographs. The obtained results demonstrate that the studied 3D non-crimp orthogonal woven composites have considerably higher in-plane ultimate failure stresses and strains, as well as damage initiation strain thresholds than their 2D woven laminated composite counterpart. Part 1 presents the description of materials used, experimental techniques applied, principal results and their mutual comparisons for the three tested composites. Part 2 describes in detail the experimentally observed effects and trends with the main focus on the progressive damage: detailed results of AE registration, full-field strain measurements and progressive damage observations, highlighting peculiarities of local damage patterns and explaining the succession of local damage events, which leads to the differences in strength values between 2D and 3D composites.

Two woven fabric laminates, one based on basalt fibres, the other on E-glass fibres, as a reinforcement for vinylester matrix, were compared in terms of their post-impact performance. With this aim, first the non-impacted specimens were subjected to interlaminar shear stress and flexural tests, then flexural tests were repeated on laminates impacted using a falling weight tower at three impact energies (7.5, 15 and 22.5 J). Tests were monitored using acoustic emission analysis of signal distribution with load and with distance from the impact point. The results show that the materials have a similar damage tolerance to impact and also their post-impact residual properties after impact do not differ much, with a slight superiority for basalt fibre reinforced laminates. The principal difference is represented by the presence of a more extended delamination area on E-glass fibre reinforced laminates than on basalt fibre reinforced ones.

Woven textile composites are gaining popularity, judging from the many reports. However, in many instances they are confined to thermoset matrices. In view of many favorable circumstances, thermoplasticbased systems are slowly gaining recognition, which can be easily attributed to their unique properties, viz. better damage tolerance, ease of handling, recyclability, etc. However, reports usually refer to materials derived from fabrics, either preimpregnated or otherwise. Here, a combination of weaving and thermoplastic elements was studied through the utilization of continuous fiber impregnated thermoplastic prepreg (COFIT) to produce a woven system. The effect of sample cutting direction on the prepreg plain weave properties was examined. Correlations between different specimen geometries and weave characteristics were also noted. In general, the specimen cutting directions give rise to various differences in weaving characteristics, which in turn influence the properties obtained.

Damage is inflicted in a series of carbon fiber/epoxy composite specimens using a simulated lightning strike generator in the effort to understand the fundamental damage response of this material form. The strikes up to 50,000 A and 28,000 V are inflicted on both pristine specimens and specimens containing a Hilok stainless steel fastener. Damage area is evaluated via ultrasonic scanning, and advanced optical microscopy is used to gain further understanding in the morphology of damage. Subsequent mechanical testing to assess the residual tensile and compressive strength and modulus of the material is performed according to ASTM standards. Results show that residual tension strength counter intuitively increases after the infliction of damage, while residual compressive strength is much more dramatically and negatively affected. Furthermore, the presence of the fastener influences dramatically both the state of damage in the specimen and its residual strength by spreading throughout the thickness rather than limiting it to the specimen surface.

A fuselage representative carbon fibre-reinforced multi-stiffener panel is analysed under compressive loading. An intact and pre-damaged configuration is loaded into the postbuckling region and further on until collapse occurs. An analysis tool is applied that includes an approach for predicting interlaminar damage initiation and degradation models for capturing interlaminar damage growth as well as inplane damage mechanisms. Analysis of the intact panel configuration predicts collapse due to fibre fracture in the stiffeners close to the panel clamps, which agrees well with the results from experimental testing. The pre-damaged configuration was proposed containing Teflon-coated layers to generate the initial debonds in the skin-stiffener interface. The outcome of the simulation of this configuration shows that crack growth is not predicted to occur, which agrees with the observations of the experiment. A parametric study is conducted to investigate the effect of the skin-stiffener debond parameters such as length, width and location on crack growth and the collapse behaviour of the panel. It is found that the sensitivity of the panel design to the damage parameters is highly dependent on the postbuckling mode shape or displacement pattern, and particularly the extent to which this influences the conditions at the crack front. More broadly, the analysis tool is shown to be capable of capturing the critical damage mechanisms leading to structural collapse of stiffened composite structures in the postbuckling region.

This paper describes the testing and failure analysis of wing relevant skin-stringer panels containing defects. Five panels containing two types of defect; embedded defects (representative of inclusions introduced during fabrication) and impact damage (representative of the damage from dropped tools), were tested under monotonic compression loading. The defects were located at two sites; within the bay between stringers and beneath the stringer foot. All the panels buckled at approximately the same applied strain (22780 m1) and buckling mode; three-half waves in each bay. The undamaged panel failed at an applied strain of 26261 m1 and the failure was attributed to skin/stiffener debonding. The presence of defects led to a reduction in the strength ranging from 7% (bay impact) to 29% (foot impact). Fractographic evidence showed that the effect of the defects on the panel strength was related to how they influenced skin/stiffener debonding. For foot defects, impact was worse than an embedded defect, whilst for bay damage this effect was reversed. The rate and extent of damage growth differed with defect type; the results indicate that modelling impact damage as a single plane embedded defect is of limited validity and may be non-conservative. q

The high cycle fatigue (HCF) performance of turbine engine components has long been improved by the introduction of a surface layer of compressive residual stress, usually by shot peening. However, credit is not generally taken for the improved fatigue performance in component design. Laser shock processing (LSP) and low plasticity burnishing (LPB) provide impressive fatigue and damage tolerance improvement by introducing deep or through-thickness compression in fatigue critical areas, but have been applied primarily to improve existing, rather than new, designs. This paper describes a design methodology to allow credit to be taken for beneficial residual stresses in component design to achieve a required or optimal fatigue performance.

Reliability quantification is a critical and necessary process for the evaluation and assessment of any inspection technology that may be classified either as a Nondestructive Evaluation (NDE) or Structural Health Monitoring (SHM) technique. Based on the sensitivity characterization of NDE techniques, appropriate processes have been developed and established for the reliability quantification of their performance with respect to damage/flaw detection in materials or structures. However, in the case of SHM-based methods, no such well-defined and general applicable approaches have been established for neither active nor passive sensing techniques that allow for their accurate reliability quantification.

The objective of this study is to characterize the sensitivity of active sensing acousto- ultrasound-based SHM techniques with respect to damage detection, as well as to identify the parameters that influence their sensitivity. With such an understanding, it is believed that adequate quantitative methods could then be established to enable the practical use of acousto- ultrasound SHM methods in the aerospace and mechanical engineering communities.

In order to evaluate the sensitivity of a pre-selected active sensing acousto-ultrasound SHM system, both numerical simulations and experiments were performed on thirty aluminum coupons each outfitted with a pair of Lead-Zirconate-Titanate (PZT) based piezoelectric sensors/actuators. A damage index versus damage size relationship was investigated numerically and experimentally to assess the applicability of the traditional NDE linear regression framework for Probability of Detection (POD) for an active sensing SHM system. The results of the study show that the position of each sensor-actuator pair with respect to a known damage location and the damage growth pattern are the two most critical parameters influencing the reliability of the same SHM system applied to identical structural components under the same environmental conditions.

This Part 2 paper presents results of comparative experimental study of progressive damage in 2D and 3D woven glass/epoxy composites under in-plane tensile loading. As Part 1, this Part 2 work is focused on the comparison of in-plane tensile properties of two non-crimp single-ply 3D orthogonal weave E-glass fibre composites on one side and a laminate reinforced with four plies of E-glass plain weave on the other. The damage investigation methodology combines mechanical testing with acoustic emission registration (that provides damage initiation thresholds), progressive cracks observation on transparent samples, full-field surface strain mapping and cracks observation on micrographs, altogether enabling for a thorough characterisation of the local micro- and meso-damage modes of the studied composites. The obtained results demonstrate that the non-crimp 3D orthogonal woven composites have significantly higher in-plane strengths, failure strains and damage initiation thresholds than their 2D woven laminated counterpart. The growth of transverse cracks in the yarns of 3D composites is delayed, and they are less prone to a yarn–matrix interfacial crack formation and propagation. Delaminations developing between the plies of plain weave fabric in the laminate at certain load level never appear in the 3D woven single-ply composites.

Surface enhancement technologies such as shot peening, laser shock peening (LSP), and low plasticity burnishing (LPB) can provide substantial fatigue life improvement. However, to be effective, the compressive residual stresses that increase fatigue strength must be retained in service. For successful integration into turbine design, the process must be affordable and compatible with the manufacturing environment. LPB provides thermally stable compression of comparable magnitude and even greater depth than other methods, and can be performed in conventional machine shop environments on CNC machine tools. LPB provides a means to extend the fatigue lives of both new and legacy aircraft engines and ground-based turbines. Improving fatigue performance by introducing deep stable layers of compressive residual stress avoids the generally cost prohibitive alternative of modifying either material or design.

While previous researchers have conducted their study on the relative impact performance of composite structures from a force or an energy standpoint only, this proposed Composite Structure Impact Performance Assessment Program (CSIPAP) suggests a multi-parameter methodology to gain further insight in the impact behavior of composite structures. These are peak and critical force; critical and dissipated energy; contact duration and coefficient of restitution (COR), which is direct indication of effective structural stiffness; and residual stiffness (normalized contact duration) which yields a plot that bears a striking resemblance with the normalized Compression After Impact (CAI) strength. Using a determinate impactor/target system as baseline configuration, the program is applied toward the understanding of the role played in an impact event by fundamental impactor and target parameters. The equations previously derived for the prediction of the force-energy and residual stiffness curves are shown to apply to the configurations tested, thus confirming their general validity. A modification to the existing effective structural stiffness formulation, which does not account for impactor characteristics, is proposed, and it comprises the impactor material, size and mass characteristics.

The paper proposes a new approach for shape optimisation with fatigue life as the design objective. Conventional designs often incorporate stress optimisation that aims at reducing stress concentrations around a structural boundary by minimising the peak stress. However, this is only an effective and sufficient measure for an 'ideal' or 'flaw-less' structure. It is a well-known fact that flaws (cracks) are inevitably present in most structures. This emphasises the need to investigate the influence of cracks on optimised shapes. Numerical modelling of cracks using the Finite Element Method requires a fine mesh to model the singularity at crack tips, which makes fracture calculations computationally expensive. Furthermore, for a damage tolerance based optimisation, numerous cracks are to be considered at various arbitrary locations in a structure, and fatigue life evaluation needs to be repeated for each crack at every iteration. This makes the optimisation process extremely computationally inefficient for practical purpose. Moreover, the lack of information concerning crack size, orientation, and location makes the formulation of the optimisation problem difficult. As a result, there has been inadequate research to consider fracture parameters, such as fatigue life, in the optimisation objective. To address this, the paper presents an approach for the shape optimisation of damage tolerant structures with fatigue life as the design constraint.

Methods have been developed to describe the fatigue initiation and propagation mechanisms in flat panels as well as mechanically fastened joints and to determine the residual strength of large flat panels. Glare shows excellent crack growth characteristics due to the mechanism of delamination and fibre bridging. The fatigue insensitive fibres restrain the crack opening and transfer load over the crack in the metal layers. During the initiation phase fibre bridging does not occur and the behaviour is dominated by the metal initiation properties. Mechanically fastened joints introduce additional effects such as secondary bending, load transfer and aspects related to the fastener installation. The residual strength of Glare is dependent on the amount of broken fibres and the delamination size and can be described with the R-curve approach. The impact resistance of Glare is related to the aluminium and glass/epoxy properties and is significantly higher than the impact resistance of mono...

This paper investigates the residual torsional strength of cylindrical T300-carbon/epoxy tubular specimens damaged by low-velocity impacts. A total of 24 specimens were subjected to a 7 J transverse impact under various torsional preloads. First, the torsional strength of four different lamination sequences is studied. Later it is compared with the residual torsional strength (RTS) of tubes impacted under different torsional preloads. FEM models were developed to investigate the effect of the impact-induced delaminations on the torsion-after-impact strength. The Acoustic Emission (AE) technique was used to study the damage propagation during the torsional loading. Results show that, even if the absorbed energy during impacts is the same, the residual torsional strength of the laminates is highly affected by the torsional preload.

Bioinspired architectures are effective in enhancing the mechanical properties of materials, yet are difficult to construct in metallic systems. The structure-property relationships of bioinspired metallic composites also remain unclear. Here, Mg-Ti composites were fabricated by pressureless infiltrating pure Mg melt into three-dimensional (3-D) printed Ti-6Al-4V scaffolds. The result was composite materials where the constituents are continuous, mutually interpenetrated in 3-D space and exhibit specific spatial arrangements with bioinspired brick-and-mortar, Bouligand, and crossed-lamellar architectures. These architectures promote effective stress transfer, delocalize damage and arrest cracking, thereby bestowing improved strength and ductility than composites with discrete reinforcements. Additionally, they activate a series of extrinsic toughening mechanisms, including crack deflection/twist and uncracked-ligament bridging, which enable crack-tip shielding from the applied stress and lead to "Γ"-shaped rising fracture resistance R-curves. Quantitative relationships were established for the stiffness and strengths of the composites by adapting classical laminate theory to incorporate their architectural characteristics.

The paper addresses the dynamic modeling of degrading and repairable complex systems. Emphasis is placed on the convenience of modeling for the end user, with special attention being paid to the modeling part of a problem, which is considered to be decoupled from the choice of solution algorithms. Depending on the nature of the problem, these solution algorithms can include discrete event simulation or numerical solution of the differential equations that govern underlying stochastic processes. Such modularity allows a focus on the needs of system reliability modeling and tailoring of the modeling formalism accordingly. To this end, several salient features are chosen from the multitude of existing extensions of Petri nets, and a new concept of aging tokens (tokens with memory) is introduced. The resulting framework provides for flexible and transparent graphical modeling with excellent representational power that is particularly suited for system reliability modeling with nonexponentially distributed firing times. The new framework is compared with existing Petri-net approaches and other system-reliability modeling techniques such as reliability block diagrams and fault trees. The relative differences are emphasized and illustrated with several examples, including modeling of load sharing, imperfect repair of pooled items, multiphase missions, and damage-tolerant maintenance. Finally, a simple implementation of the framework using discrete event simulation is described.

NOTATION α = Non-linear shear strain parameter σ 11 = Stress in the fibre direction σ 12 = In-plane shear stress σ 22 = Stress in the direction normal to the fibres G 12 = In-plane shear modulus S = Allowable in-plane shear strength S T = Allowable transverse shear strength X T = Allowable fibre tension strength X C = Allowable fibre compression strength Y T = Allowable matrix tension strength Y C = Allowable matrix compression strength Abstract: With the aim of gaining a better understanding of the non-linear behaviour of advanced aerospace composite structures under dynamic loadings, this paper provides an overview of the state of the available analysis tools and reports on the progress of an investigation designed to study the low velocity impact characteristics of such structures. The paper further details the experimental set up and procedures of a drop weight impact test system and the equipment used as well as the specifications of its online data acquisition system. In conclusion, as an indicative verification of the ongoing work, a brief comparison of the test results with the numerical data, obtained from a simulation of the test in a number of explicit non-linear finite element environments, is provided.

The paper is aiming at high performance tools which reconcile the contradiction objectives of accuracy and speed and therefore are suited for application already in the design phase of composite structures. By that means they will contribute significantly to a modern Concurrent/Integrated Engineering Process. TRAVEST is a pre-and postprocessing program for finite element solvers which computes the full three-dimensional state of stress from results of a simple shell analysis using a formulation of the extended 2D-method for FE-discretisations with eight-and six-noded elements, respectively. A new theory has been developed, which allows for using the extended 2D-method in conjunction with fournoded elements. This would allow for much wider application in industry. For thick-walled composite structures a hierarchical 3D composite element has been developed, implemented into B2000 and compared to existing elements. All transverse stress components show very much improved quality. For 3D reinforced composites a new failure model has been developed. It is an extension of a physically based criterion which is applicable to unidirectional layers only. The model is implemented into TRAVEST and is also suited for use in B2000 in conjunction with the 3D composite element. The composite damage tolerance analysis code CODAC was improved by implementing a Rayleigh-Ritz procedure for residual strength analysis. Comparison with in-house experiments revealed good quality of the results. The postbuckling behaviour of composite panels was simulated using commercial finite element software and compared to in-house tests. The verified model constitutes the basis for development of a fast postbuckling algorithm which will be based on a hybrid reduced basis technique.

The attainment of both strength and toughness is a vital requirement for most structural materials; unfortunately these properties are generally mutually exclusive. Although the quest continues for stronger and harder materials, these have little to no use as bulk structural materials without appropriate fracture resistance. It is the lower-strength, and hence higher-toughness, materials that find use for most safety-critical applications where premature or, worse still, catastrophic fracture is unacceptable. For these reasons, the development of strong and tough (damage-tolerant) materials has traditionally been an exercise in compromise between hardness versus ductility. Drawing examples from metallic glasses, natural and biological materials, and structural and biomimetic ceramics, we examine some of the newer strategies in dealing with this conflict. Specifically, we focus on the interplay between the mechanisms that individually contribute to strength and toughness, noting that...

Low plasticity burnishing (LPB) is a surface enhancement method that produces a deep layer of compressive residual stress with minimal cold working and an improved surface finish. Extensive fatigue testing, performed on numerous metal alloys in simulated environmental conditions, demonstrates that LPB significantly improves fatigue strength of highly stressed components. LPB is a flexible process, capable of being implemented on a wide variety of CNC machine tools. A product-specific LPB process was developed and applied to the modular neck taper junction of a Ti-6Al-4V total hip prosthesis (THP). LPB produced a compressive residual stress field with an improved surface finish, which enhanced component fatigue strength and resistance to fretting damage. X-ray diffraction (XRD) residual stress measurements, made before and after LPB application, are shown. High cycle fatigue (HCF) results obtained on LPB-processed hip stems are shown along with baseline data for unprocessed hip stems. HCF tests demonstrate complete elimination of fretting fatigue failures in the LPB processed area of the taper junction and a substantial increase in overall THP fatigue strength.

Aging structures and life extension programs in civil and mechanical engineering are defining new challenges for design and maintenance management engineers. Damage tolerant structures rely on safety of damage detection during scheduled inspections and follow-up repairs. Accepting a certain maximum risk level demands the definition of an inspection and maintenance program. Fewer inspections increase the probability of failure while over-inspection will lead to an increase in life-cycle costs and reduced operation times. All governing variables describing aging structures, such as crack initiation, crack growth, damage detection and damage tolerance are of probabilistic nature. This paper presents a simulation framework, based on Monte-Carlo techniques, which assesses the failure risk of generic engineering structures taking into account scheduled inspection and repair programs.

Impact behaviour and post impact compressive characteristics of glass±carbon/epoxy hybrid composites with alternate stacking sequences have been investigated. Plain weave E-glass and twill weave T-300 carbon have been used as reinforcing materials. For comparison, laminates containing only-carbon and only-glass reinforcements have also been studied. Experimental studies have been carried out on instrumented drop weight impact test apparatus. Post impact compressive strength has been obtained using NASA 1142 test ®xture. It is observed that hybrid composites are less notch sensitive compared to only-carbon or only-glass composites. Further, carbon-outside/glassinside clustered hybrid con®guration gives lower notch sensitivity compared to the other hybrid con®gurations.

In this paper the ultimate strength characteristics of dented steel plates under axial compressive loads are investigated using the ANSYS nonlinear ÿnite element code. The e ects of shape, size (depth, diameter), and location of the dent on the ultimate strength behavior of simply supported steel plates under axial thrust are studied. A closed-form formula for predicting the ultimate compressive strength of dented steel plates are empirically derived by curve ÿtting based on the computed results. The results and insights developed in the present study will be useful for damage tolerant design of steel plated structures with local denting.

An overview of the High-Performance Corrosion-Resistant Materials (HPCRM) Program, which was cosponsored by the Defense Advanced Research Projects Agency (DARPA) Defense Sciences Office (DSO) and the U.S. Department of Energy (DOE) Office of Civilian and Radioactive Waste Management (OCRWM), is discussed. Programmatic investigations have included a broad range of topics: alloy design and composition, materials synthesis, thermal stability, corrosion resistance,

Aging structures and life extension programs in civil and mechanical engineering are defining new challenges for design and maintenance management engineers. Damage tolerant structures rely on safety of damage detection during scheduled inspections and follow-up repairs. Accepting a certain maximum risk level demands the definition of an inspection and maintenance program. Fewer inspections increase the prob- ability of failure while over-inspection will lead to an increase in life-cycle costs and reduced operation times. All governing variables describing aging structures, such as crack initiation, crack growth, damage detection and damage tolerance are of probabilistic nature. This paper presents a simulation framework, based on Monte-Carlo techniques, which assesses the failure risk of generic engineering structures taking into account scheduled inspection and repair programs.

Environmental problems are motivating recycling actions and reuse alternatives for materials with main focus for the application of those renewable and biodegradable materials such as lignocellulosic fibers. Composites reinforced with such fibers are being considered by several industrial sectors, not only from the environmental safety, but also from economic considerations and improved properties. This paper, which is continuation of the work (Part I) by the authors’ use of the recycled polyethylene and used jute fabrics, presents evaluation of its toughness measured by the impact energy using both Izod and Charpy methods. Fabric content used is up to 40 wt. %. It is found that the incorporation of both types (new and used) of jute fabric significantly increased the impact energy of composites, with higher values associated with the new jute fabric. Fractographic analysis revealed that weaved configuration of the jute fibers and their low interfacial resistance with the matrix are responsible for the observed impact performance of these composites.

The en vi ron men tal du ra bil ity of car bon nanotube (CNT)-mod i fied car bon-fi bre-re in forced poly mers (CFRPs) is in ves ti gated. The key prob lem of these new-gen er a tion com pos ites is the mod i fi ca tion of their poly mer matrix with nanoscaled fill ers. It was re cently dem on strated that the dam age tol er ance of these ma te ri als, as mani fested by their frac ture tough ness, im pact prop er ties, and fa tigue life, can be im proved by add ing CNTs at weight frac tions as low as 0.5%. This im prove ment is mainly at trib uted to the in cor po ra tion of an ad di tional in ter fa cial area be tween the CNTs and the ma trix, which is ac tive at the nanoscale. How ever, this ad di tional in ter face could have a neg a tive ef fect on the en vi ron men tal du ra bil ity of the afore men tioned sys tems, since it is well known that the mois ture ab sorp tion abil ity of a ma trix is en hanced by the pres ence of mul ti ple in ter faces, which serve as an in gress route to wa ter. To ex am ine this prob lem, CNT-mod i fied CFRPs were ex posed to hydro ther mal load ings. At spec i fied in ter vals, the com pos ites were weighted, and the wa ter up take vs. time was re corded for both the mod i fied and a ref er ence sys tems. The elec tri cal con duc tiv ity of the com pos ites was regis tered at the same time in ter vals. Af ter the en vi ron men tal ex po sure, the interlaminar shear prop er ties of the con di tioned com pos ite sys tems were mea sured and com pared with those of un mod i fied com pos ites, as well as with the shear prop er ties of un ex posed lam i nates.

300M steel is widely used in landing gear because of its ultra high strength with high fracture toughness, but is vulnerable to both corrosion fatigue and stress corrosion cracking, with potentially catastrophic consequences. Plating and shot peening surface treatments currently used to extend life are only partly effective. A surface treatment is needed that will mitigate foreign object damage (FOD), corrosion fatigue and stress corrosion cracking. This paper describes the use of low plasticity burnishing (LPB) to improve damage tolerance and to mechanically suppress stress sensitive corrosion failure mechanisms.

Owing to a lack of microstructure, glassy materials are inherently strong but brittle, and often demonstrate extreme sensitivity to flaws. Accordingly, their macroscopic failure is often not initiated by plastic yielding, and almost always terminated by brittle fracture. Unlike conventional brittle glasses, metallic glasses are generally capable of limited plastic yielding by shear-band sliding in the presence of a flaw, and thus exhibit toughness-strength relationships that lie between those of brittle ceramics and marginally tough metals. Here, a bulk glassy palladium alloy is introduced, demonstrating an unusual capacity for shielding an opening crack accommodated by an extensive shear-band sliding process, which promotes a fracture toughness comparable to those of the toughest materials known. This result demonstrates that the combination of toughness and strength (that is, damage tolerance) accessible to amorphous materials extends beyond the benchmark ranges established by the...