Emerging Trends in Damage Tolerance Assessment:A Review of Smart Materials and Self-Repairable Structures

The discipline of damage tolerance assessment has experienced significant advancements due to the emergence of smart materials and self-repairable structures.This review offers a comprehensive look into both traditional and innovative methodologies employed in damage tolerance assessment.After a detailed exploration of damage tolerance concepts and their historical progression,the review juxtaposes the proven techniques of damage assessment with the cutting-edge innovations brought about by smart materials and self-repairable structures.The subsequent sections delve into the synergistic integration of smart materials with self-repairable structures,marking a pivotal stride in damage tolerance by establishing an autonomous system for immediate damage identification and self-repair.This holistic approach broadens the applicability of these technologies across diverse sectors yet brings forth unique challenges demanding further innovation and research.Additionally,the review examines future prospects that combine advanced manufacturing processes with data-centric methodologies,amplifying the capabilities of these‘intelligent’structures.The review culminates by highlighting the transformative potential of this union between smart materials and self-repairable structures,promoting a sustainable and efficient engineering paradigm.

Eukaryotic DNA damage tolerance and translesion synthesis through covalent modifications of PCNA

In addition to well-defined DNA repair pathways, all living organisms have evolved mechanisms to avoid cell death caused by replication fork collapse at a site where replication is blocked due to disruptive covalent modifications of DNA. The term DNA damage tolerance （DDT） has been employed loosely to include a collection of mechanisms by which cells survive replication-blocking lesions with or without associated genomic instability. Recent genetic analyses indicate that DDT in eukaryotes, from yeast to human, consists of two parallel pathways with one being error-free and another highly mutagenic. Interestingly, in budding yeast, these two pathways are mediated by sequential modifications of the proliferating cell nuclear antigen （PCNA） by two ubiquitination complexes Rad6-Rad18 and Mms2-Ubc13-Rad5. Damage-induced monoubiquitination of PCNA by Rad6-Rad18 promotes translesion synthesis （TLS） with increased mutagenesis, while subsequent polyubiquitination of PCNA at the same K164 residue by Mms2-Ubc13-Rad5 promotes error-free lesion bypass. Data obtained from recent studies suggest that the above mechanisms are conserved in higher eukaryotes. In particular, mammals contain multiple specialized TLS polymerases. Defects in one of the TLS polymerases have been linked to genomic instability and cancer.

DAMAGE TOLERANCE ANALYSIS ON HOLLOW AXLES OF HIGH SPEED MOTOR TRAINS

According to the rules of UIC515-3, the service loads of the axles are defined, which include some different loads cases as follows： the static loads; the impact loads resulted from running through the rail joints and unevenness rails; the loads through curves and from braking. Through the calculating and analysis, the stress distribution of the hollow axles is obtained for 200 km/h high speed motor trains used in China. At the same time, the fatigue crack growth of hollow axles is studied, and the initial surface cracks of 2 mm depth caused by hard objects strike or the other causes are discussed. On the basis of the linear elastic fracture mechanics theory, the stress intensity factor of the crack of the geometry transition outside the wheel seat is also studied. Associated with fatigue crack propagation equation and the corresponding crack propagation threshold, the crack propagation characteristics under different shapes are calculated. Then the running distances are educed with different shapes propagating to the critical length, and the estimation of the residual lives about hollow axles which are the reference values of examine and repair limit of the hollow axle is given.

Evaluation of Impact Damage Tolerance in Carbon Fabric/epoxy-matrix Composites by Electrical Resistance Measurement

Impact damage tolerance is provided in intensity design on composites. The compression intensity of impacted composites requires more than 60% of its original intensity. The influence of impact on compressive intensity and electrical resistance of carbon fabric/epoxy-matrix composites was studied in this paper. The experimental results shows that impact can cause damage in composites, degenerate compressive intensity, and increase resistance. The electrical resistance change rate was used as an evaluation indicator of impact damage tolerance of composites. Impact damage, which results from the applying process of composites, can be identified in time by electrical resistance measurement. So, the safety performance of composites can also be improved.

EXPERIMENTAL STUDY ON DAMAGE TOLERANCE BEHAVIOUR OF COMPOSITE PANELS WITH SOFTENING STRIPS

In this paper an experimental study on damage tolerance behaviour of composite panels with softening strips is carried out. A prediction method of residual strength of panels with softening strips is proposed. The comparison between estimated and experimental results shows that the prediction method can be applied to design. In this paper the failure mechanisms are described.

A Review of Structural Durability and Probabilistic Damage Tolerance

The main purpose of this paper is providing a reference for further research. According to the papers and reports on structural durability and probabilistic damage tolerance, the present paper summarized the pro- gress of the theoretical considerations and engineering application. Several models used in structural durability and probabilistic damage tolerance are reviewed. The characteristics and problems of these methods are ana- lyzed. A new kind of combined analysis model on structural durability and damage tolerance are also introduced. New progress of analysis theory and numerical methods on structural reliability are discussed, such as the re- sponse surface method and numerical method combining neural networks and Monte Carlo simulation. The analy- sis shows that these methods can improve computational efficiency significantly and maintain high computational accuracy. Finally, some prospects of the key research directions are discussed.

Review of the development of the probabilistic damage tolerance assessment of life-limited parts in compliance with the airworthiness regulations

Probabilistic damage tolerance is a critical method to understand and communicate risk and safety.This paper reviews recent research on the probabilistic damage tolerance design for life-limited parts.The vision of the probabilistic damage tolerance assessment is provided.Five core parts of the probabilistic damage tolerance method are introduced separately,including the anomaly distribution,stress processing and zone definition,fatigue and fracture calculation method,probability of failure(POF)calculation method,and the combination with residual stress induced by the manufacturing process.The above currently-available risk assessment methods provide practical tools for failure risk predictions and are applied by the airworthiness regulations.However,new problems are exposed with the development of the aeroengines.The time-consuming anomaly distribution derivation process restricts the development of the anomaly distribution,especially for the developing aviation industries with little empirical data.Additionally,the strong transient characteristic is prominent because of the significant temperature differences during the take-off and climbing periods.The complex loads then challenge the fatigue and fracture calculation model.Besides,high computational efficiency is required because various variables are considered to calculate the POF.Therefore,new technologies for the probabilistic damage tolerance assessment are provided,including the efficient anomaly distribution acquisition method based on small samples,the zone definition method considering transient process,and stress intensity factor(SIF)solutions under arbitrary stress distributions combined with the machine learning method.Then,an efficient numerical integration method for calculating failure risk based on the probability density evolution theory is proposed.Meanwhile,the influence of the manufacturing process on residual stress and the failure risk of the rotors is explored.The development of the probabilistic damage tolerance method can meet the requirement of the published airworthiness regulation Federal Aviation Regulation(FAR)33.70 and guide the modification or amendment of new regulations to ensure the safety of the high-energy rotors.

Damage Tolerance Design of Artificial Mechanical Heart Valve Holder

To study the conservative life of the artificial mechanical heart valve holder,CATIA software was used for modeling,and ABAQUS finite element analysis was used to analyze the stress of the valve holder in the complex environment of the human body and the residual thermal stress during production and processing.The damage tolerance method based on fracture mechanics analyzes the maximum initial crack size that the heart valve holder can tolerate under the premise that the structural safety life is longer than the life of the implanted patient.The results show that the maximum initial crack size is only tens of microns;the traditional S/N life analysis can only obtain non-conservative life,and the damage tolerance method is the basic requirement for analyzing the life and quality control of artificial mechanical heart valves.

Fatigue research of metro car body based on operational loads

Purpose–The principle of infinite life design currently directs fatigue resistance strategies for metro car bodies.However,this principle might not fully account for the dynamic influence of operational loads and the inevitable presence of defects.This study aims to integrate methods of service life estimation and residual life assessment,which are based on operational loads,into the existing infinite life verification framework to further ensure the operational safety of subway trains.Design/methodology/approach–Operational loads and fatigue loading spectra were determined through the field test.The material test was conducted to investigate characteristics of the fracture toughness and the crack growth rate.The fatigue strength of the metro car body was first verified using the finite element method and Moore–Kommers–Japer diagrams.The service life was then estimated by applying the Miner rule and high-cycle fatigue curves in a modified form of the Basquin equation.Finally,the residual life was assessed utilizing a fracture assessment diagram and a fitted curve of crack growth rate adhered to the Paris formula.Findings–Neither the maximum utilization factor nor the cumulative damage exceeds the threshold value of 1.0,the metro car body could meet the design life requirement of 30 years or 6.6 million km.However,three out of five fatigue key points were significantly influenced by the operational loads,which indicates that a single fatigue strength verification cannot achieve the infinite life design objective of the metro car body.For a projected design life of 30 years,the tolerance depth is 12.2 mm,which can underscore a relatively robust damage tolerance capability.Originality/value–The influence of operational loads on fatigue life was presented by the discrepancy analysis between fatigue strength verification results and service life estimation results.The fracture properties of butt-welded joints were tested and used for the damage tolerance assessment.The damage tolerance life can be effectively related by a newly developed equation in this study.It can be a valuable tool to provide the theoretical guidance and technical support for the structural improvements and maintenance decisions of the metro car body.

Developing nacre-inspired laminate-reticular 2024Al/B_(4)C composites with high damage resistance by adjusting compositional wettability

To address the issue that B_(4)C ceramics are difficult to be wetted by aluminum metals in the composites,TiB_(2)was introduced via an in-situ reaction between TiH_(2)and B_(4)C to regulate their wettability and interfacial bonding.By pressure infiltration of the molten alloy into the freeze-cast porous ceramic skeleton,the 2024Al/B_(4)C-TiB_(2)composites with a laminate-reticular hierarchical structure were produced.Compared with 2024Al/B_(4)C composite,adding initial TiH_(2)improved the flexural strength and valid fracture toughness from(484±27)to(665±30)MPa and(19.3±1.5)to(32.7±1.8)MPa·m^(1/2),respectively.This exceptional damage resistance ability was derived from multiple extrinsic toughening mechanisms including uncracked-ligament bridging,crack branching,crack propagation and crack blunting,and more importantly,the fracture model transition from single to multiple crack propagation.This strategy opens a pathway for improving the wettability and interfacial bonding of Al/B_(4)C composites,and thus produces nacre-inspired materials with optimized damage tolerance.

A brief history of the DNA repair field

The history of the repair of damaged DNA can be traced to the mid-1930s. Since then multiple DNA repair mechanisms, as well as other biological responses to DNA damage, have been discovered and their regulation has been studied. This article briefly recounts the early history of this field.

Tensile and fracture properties of Ti-62A alloy plate with different microstructures

Ti-62A alloy plates with three different types of microstructure, fully equiaxed, bimodal, and Widmanstatten, were obtained by various heat treatments to investigate the effects of microstructure on the tensile and fracture properties at room temperature. The results reveal that Widmanstatten microstructure exhibits good damage tolerance behavior considering strength, fracture toughness, and fatigue crack growth behavior, while the bimodal microstructure shows good comprehensive properties considering the plasticity synthetically. Optical microscopy (OM) and scanning electron microscopy (SEM) microstructure analyses on fracture and fatigue crack path demonstrate that the dependence of mechanical properties and fatigue crack growth behavior on microstructural feature are attributed to the α lamellae width and the α colony size. © The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2012.

Preliminary Study on Evaluation System of Capability of Composites to Withstand Impact

This paper summarizes the authors’ experimental study on the characterization system of composite behavior to withstand impact. The content includes: (1)The dent depth is the best parameter describing the impact damage state. (2) There exists the knee point phenomenon for damage resistance behavior (i.e. the relationship between impact energy or contact force and dent depth) and damage tolerance behavior (i.e. the relationship between dent depth and compressive failure strain or stress) of composite laminates. (3) The physical meaning of the knee point phenomenon is that the failure mechanisms change of damaged composites to fiber breakage in the first front plies from matrix crack and delamination. Some suggestions on the characterization system of composite behavior to withstand impact were proposed.

A method of predicting visual detectability of low-velocity impact damage in composite structures based on logistic regression model

This paper proposed a new method for quantitative assessment of visual detectability of damage based on logistic regression,using the Probability of Detection(POD)as a criterion.Experiments were performed to establish the massive hit/miss data of visual inspection.Authoritative investigations verified the reliability of the data.The prediction function concluded comprises more than one flaw size parameters,including the depth and diameter of the dents.The results show that the depth and diameter of the dents are pivotal for the evaluation of detectability;the type of detection,the detection distance,and the qualifications of personnel are critical external factors to be considered.This function,with an accuracy rate of nearly 85%,is capable of predicting the visual detection probability of impact damage under various detection environments,which will provide a reference for the damage tolerance design of composite materials and field maintenance in the NonDestructive Testing(NDT)field.

Acorn size and tolerance to seed predators:the multiple roles of acorns as food for seed predators,fruit for dispersal and fuel for growth

Fitness of parents and offspring is affected by offspring size.In oaks(Quercus spp.),acorns vary considerably in size across,and within,species.Seed size influences dispersal and establishment of oaks,but it is not known whether size imparts tolerance to seed predators.Here,we examine the relative extent to which cotyledon size serves as both a means for sustaining partial consumption and energy reserves for developing seedlings during early stages of establishment.Acorns of 6 oak species were damaged to simulate acorn predation by vertebrate and invertebrate seed predators.Seedling germination/emergence and growth rates were used to assess seedling performance.We predicted that if cotyledons are important for dispersal,acorns should show tolerance to partial seed consumption.Alternatively,if the cotyledon functions primarily as an energy reserve,damage should significantly influence seedling performance.Acorns of each species germinated and produced seedlings even after removing>50%of the cotyledon.Seed mass explained only some of the variation in performance.Within species,larger acorns performed better than smaller acorns when damaged.Undamaged acorns performed as well or better than damaged acorns.There was no pattern among individual species with increasing amounts of damage.In some species,simulated invertebrate damage resulted in the poorest performance,suggesting alternative strategies of oaks to sustain damage.Large cotyledons in acorns may be important for attracting seed dispersers and sustaining partial damage,while also providing energy to young seedlings.Success of oak establishment may follow from the resilience of acorns to sustain damage at an early stage.

Principles and methods for determining calendar life and corrosion tolerance of mechanical parts

In order to facilitate the determination of the calendar life of mechanical parts,the author summarized his relevant research papers that had studied for many years as eight distinct topics:(A).The principle and method for compiling the equal-damage temperature and humidity spectrum;(B).The principle for preparing the high concentration medium solution;(C).The principle and method for determining the metal calendar life;(D).The principle and method for determining the protective coating calendar life;(E).The principle and method for determining the total calendar life;(F).The principle of the reliability processing;(G).The principle and method for determining the corrosion damage tolerance;(H).The principle and matching design method to ensure that the total fatigue life and the total calendar life of the mechanical part were safe simultaneously.The above contents established a complete set of theoretical systems and methods for determining the calendar life.

Nanofibril Organization in Silk Fiber as Inspiration for Ductile and Damage-Tolerant Fiber Design

Ductile and damage-tolerant fibers(DDTFs)are desired in aerospace engineering,mechanical engineering,and biomedical engineering because of their ability to prevent the catastrophic sudden structural/mechanical failure.However,in practice,design and fabrication of DDTFs remain a major challenge due to finite fiber size and limited processing techniques.In this regard,animal silks can provide inspirations.They are hierarchically structured protein fibers with an elegant trade-off of mechanical strength,extensibility and damage tolerance,making them one of the toughest materials known.In this article,we confirmed that nanofibril organization could improve the ductility and damage-tolerance of silk fibers through restricted fibril shearing,controlled slippage and cleavage.Inspired by these strategies,we further established a rational strategy to produce polyamide DDTFs by combining electrospinning and yarn-spinning techniques.The resultant polymeric DDTFs show a silk-like fracture resistance behavior,indicating potential applications in smart textile,biomedicine,and mechanical engineering.

Spark plasma sintering of damage tolerant and machinable YAM ceramics

Single-phase Y_(4)Al_(2)O_(9)(YAM)powders were synthesized via solid-state reaction starting from nano-sized Al_(2)O_(3) and Y_(2)O_(3).Fully dense(99.5%)bulk YAM ceramics were consolidated by spark plasma sintering(SPS)at 1800℃.We demonstrated the excellent damage tolerance and good machinability of YAM ceramics.Such properties are attributed to the easy slipping along the weakly bonded crystallographic planes,resulting in multiple energy dissipation mechanisms such as transgranular fracture,shear slipping and localized grain crushing.

3D-printed Lunar regolith simulant-based geopolymer composites with bio-inspired sandwich architectures

Over time,natural materials have evolved to be lightweight,high-strength,tough,and damage-tolerant due to their unique biological structures.Therefore,combining biological inspiration and structural design would provide traditional materials with a broader range of performance and applications.Here,the application of an ink-based three-dimensional(3D)printing strategy to the structural design of a Lunar regolith simulant-based geopolymer(HIT-LRS-1 GP)was first reported,and high-precision carbon fiber/quartz sand-reinforced biomimetic patterns inspired by the cellular sandwich structure of plant stems were fabricated.This study demonstrated how different cellular sandwich structures can balance the structure–property relationship and how to achieve unprecedented damage tolerance for a geopolymer composite.The results presented that components based on these biomimetic architectures exhibited stable non-catastrophic fracture characteristics regardless of the compression direction,and each structure possessed effective damage tolerance and anisotropy of mechanical properties.The results showed that the compressive strengths of honeycomb sandwich patterns,triangular sandwich patterns,wave sandwich patterns,and rectangular sandwich patterns in the Y-axis(Z-axis)direction were 15.6,17.9,11.3,and 20.1 MPa(46.7,26.5,23.8,and 34.4 MPa),respectively,and the maximum fracture strain corresponding to the above four structures could reach 10.2%,6.7%,5.8%,and 5.9%(12.1%,13.7%,13.6%,and 13.9%),respectively.

“Wood-nacre”:Development of a Bio-inspired Wood-Based Composite for Beam and 3D-Surface Elements with Improved Failure Mechanisms

Following the natural structure of the nacre,the material studied consists of a multitude of hexagonal tiles that are glued together in an offset manner with a ductile adhesive.This so-called“wood nacre”consists of macroscopic tiles of birch wood veneer with a thickness of 0.8 mm and a size of 20 or 10 mm in diameter in order to mimic the aragonite tiles and the ductile PUR-adhesive corresponds to the layers of collagen in between.E-modulus(MOE),bending strength(MOR)and impact bending strength of the samples were determined and compared with reference samples of birch laminated wood.The hierarchical layered structure of the tiles does not cause any relevant loss in stiffness.Like nacre,“wood nacre”also shows tough fracture behaviour and a high homogenization effect.However,strain hardening and high fracture toughness of the natural model could not be fully achieved.The reason for this is the insufficient ratio between the strength and stiffness of the veneer layers and the adhesive.By adjusting the size of the tiles,increasing the strength and surface roughness of the veneers,e.g.by densification,and using more ductile adhesives that can be applied in smaller layer thicknesses,it should be possible to better reproduce the natural ratios of nacre and thus achieve a significant improvement in the material properties of“wood nacre”.In addition to the mechanical properties,the high potential of the new material lies in the possibility of producing 3D shell-shaped elements for lightweight wood hybrid construction.