Underlying Failure Mechanisms Research Articles

Failure mechanism and heat treatment effect of 3D-printed bio-inspired helicoidal CF/PEEK composites

The bio-inspired helicoidal structure has displayed a unique combination of lightweight, high strength, and high impact resistance. This study presents an experimental and numerical investigation of the failure mechanism and heat treatment effect of 3D-printed helicoidal composites by adopting a helicoidal laminate stacking configuration found in the dactyl club in Homarus americanus. The helicoidal specimens were additively manufactured with short carbon fiber reinforced Polyether ether ketone (PEEK) composites and their mechanical performance was characterized through quasi-static three-point bending. The results showed that the flexural strength, flexural modulus, and absorbed energy of the helicoidal composites with the twisted crack pattern were 6.9%, 23.9%, and 5.9% higher than those of the quasi-isotropic composites with the flat crack pattern. Stress analysis was then carried out to investigate the underlying failure mechanism of helicoidal materials. The stress component of σ33 and σ13 in helicoidal specimens showed a milder distribution than that of quasi-isotropic specimens, which plays a major role in the resulting fracture-resistant behavior. Further results showed that the helicoidal specimens under heat treatment of 250 °C over 6 h achieved 24.9% and 20.9% enhancement in flexural strength and modulus. The cracking surface area was increased with extensive translaminar twisted cracks observed with a scanning electron microscope. Experiment results show that heat treatment can exert a significant influence on the crack path of the CF/PEEK helicoidal specimens. Meanwhile, the increased degree of crystallization and crosslinking at the interface could inhibit the initiation of delamination failure.

Synthetic Fault Mode Generation for Resilience Analysis and Failure Mechanism Discovery

Abstract Traditional risk-based design processes seek to mitigate operational hazards by manually identifying possible faults and devising corresponding mitigation strategies—a tedious process which critically relies on the designer’s limited knowledge. In contrast, resilience-based design seeks to embody generic hazard-mitigating properties in the system to mitigate unknown hazards, often by modelling the system’s response to potential randomly generated hazardous events. This work creates a framework to adapt these scenario generation approaches to the traditional risk-based design process to synthetically generate fault modes by representing them as a unique combination of internal component fault states, which can then be injected and simulated in a model of system failure dynamics. Based on these simulations, the designer may then better understand the underlying failure mechanisms and mitigate them by design. The performance of this approach is evaluated in a model of an autonomous rover, where cluster analysis shows that elaborating the faulty state-space in the drive system uncovers a wider range of possible hazardous trajectories and failure consequences within each trajectory than would be uncovered from manual mode identification. However, this increase in hazard information gained from exhaustive mode sampling comes at a high computational expense, highlighting the need for advanced, efficient methods to search and sample the faulty state-space.

In situ consolidation of carbon fiber PAEK via laser-assisted automated fiber placement

Automated fiber placement (AFP) of thermosetting composites has been one of the leading manufacturing techniques for aerospace structures over the last decade. Thermoplastic composites (TPCs) offer several advantages including a high toughness, recyclability, weldability, and potential for rapid out-of-autoclave manufacturing. High-power heating sources and flexible compaction rollers can be used to melt and consolidate TPC tapes via AFP to form large structures in a layer-by-layer manner as used in additive manufacturing. This paper investigates the in-situ consolidation AFP of TPC tapes (ICAT) toward their process optimization and revealing mechanisms that govern the ICAT process. To this end, a laser-assisted AFP system was utilized to fabricate short beam strength samples from carbon fiber reinforced low-melt polyaryletherketone (LM-PAEK™) at various processing parameters. Analysis of results revealed how ICAT process parameters affect interlaminar bonding, crystallinity, and void content in manufactured parts. Multiphysics finite element analysis (FEA) was also carried out to reveal mechanisms that control void, crystallinity, and bonding in ICAT. Finally, a comparison of post-fracture specimens made via ICAT with traditional compression molding was used to explain the underlying failure mechanisms in ICAT coupons. This study shows that at AFP rates of up to 0.15 m/s, high interlaminar shear strengths, up to 60 MPa, are possible to achieve with low void content (<1%) and high crystallinity (>20%), proving that ICAT is potentially a viable process for manufacturing the next generation of aerospace composites.

Enabling Si-Dominant Anodes with Focus on Binder

Since current chemistries in lithium ion batteries (LIBs) including graphite anode reached their energy density limits, a promising candidate on anode side is the abundantly available silicon with its enormous capacity, but showing several failure mechanisms based on its extensive swelling (>300%). This leads to excessive solid electrolyte interphase (SEI) formation and electrode delamination during cycling, which can be reduced by the modification of the active material itself, e.g. carbon-embedded silicon (Si/C), but still these materials show the same failure mechanisms in a less pronounced manner. Regarding the costs and availability, the next generation anode is desired to be based on metallurgical silicon, where one promising path to enable Si-dominant anodes is the design of advanced binder systems to maintain a stable network around the active material during cycling. However, in most studies new binders are mainly tested in electrodes with low active material content and low loadings while lacking of full cell data, which makes it impossible to estimate the capability and influence of the binder to mitigate the failure mechanisms in full cell operation.In this work, several modifications of the binder are combined with commercial Si/C material and bare micro-sized silicon with 100% silicon as active material. The electrodes are tested in half coin cells and small full pouch cells with ~60 mAh capacity. To examine the performance in detail and understand the underlying failure mechanisms, post-mortem analysis was conducted.The already commonly used polyacrylic acid (PAA) can be further adapted to the respective anode active material (AAM) via partial neutralization with e.g. LiOH to tune the interaction with the AAM and reduce the first cycle losses due to the reaction of the carboxylic groups with lithium. Nevertheless, the influence of PAA/LiPAA on the failure mechanisms and maintenance of the electrode, especially on full cell basis, are not yet fully understood. In this work, the effect of the partial neutralization on the full cell performance of a NMC811-Si/C cell is shown with the anode consisting of 91.5% Si/C material, 8% binder and 0.5% conductive additive (single-wall carbon nanotubes SWCNTs). The full cells show 3.5% higher FCE with 70% neutralized LiPAA in comparison to acidic PAA in the anode, while maintaining the same cycle life.

Dynamic response of aluminium matrix syntactic foams subjected to high strain-rate loadings

This paper presents experimental work to characterise the dynamic behaviour of aluminium matrix syntactic foams subjected to compression, Split Hopkinson Pressure Bar and terminal ballistic impact tests as well as blast loading. Numerical models have also been developed to simulate the dynamic response of the composite foams. The effect of strain-rate on their compressive crush behaviour has been investigated, given that the rate-dependent characteristics of these materials are required for designing dynamically loaded structures. Characterisation of the behaviour of the foam under high strain-rate loadings and the identification of the underlying failure mechanisms were also undertaken to evaluate their effective mechanical performance. The results show that the aluminium syntactic foam is sensitive to strain-rate in terms of initial stiffness, peak stress and plateau stress and show a pronounced high-rate dependence at a strain rate above 1000 s-1. The concrete damage plasticity model with rate-dependent features were used to simulate the dynamic behaviour of the foams, with the failure modes being captured. The model was verified and validated against the experimental results, and predictions were made for the normal and oblique ballistic impact response. Overall, the level of agreement between the numerical simulations and the experimental results is encouraging.

Feasibility Analysis on the Support of Rock Slopes Against Flexural Toppling Failure Using the DDA Method-A Case Study

As a typical and complex failure type of rock slopes, flexural toppling failure has increasingly received attention, mainly focused on its failure mechanism and introducing a proper numerical method to reproduce the failure phenomena. However, there are few studies related to the supporting mechanism against flexural toppling failure. This study attempted to make a preliminary analysis on the research gap. For this purpose, an anti-dip layered rock slope supported with anchor cables that failed owning to flexural toppling was taken as an example, and the numerical method discontinuous deformation analysis (DDA) was taken as the major analytical tool. Firstly, the rationality of DDA was verified by correctly simulating the failure state of the anti-layered rock slope and the underlying failure mechanism was studied. The results indicated that the lack of the support from slope toe is the crucial factor inducing the flexural toppling failure, which posed a great obstacle to the further excavation of the open-pit mine. Secondly, by introducing the anchor cable support to the slope toe in the DDA model, the interaction mechanism between the anchor cables and the rock slope was investigated. the results indicated that the supporting system of the ordinary anchor cables is very sensitive to the inhomogeneous flexural toppling deformation. The tension values of the anchor cables are very inhomogeneous, and once some of anchor cables beyond their tension limit, the anchor cables failed, which will greatly threaten the safety of the adjacent anchor cables and even the stability of the whole slope. Meanwhile, a novel anchor cable called constant-resistance-large-deformation (CRLD) anchor cable was introduced for a comparative study using DDA. just replacing the anchor cables of the original slope with the CRLD anchor cables, the slope can reach stable state eventually even without the slope toe being enhanced. In addition, the CRLD anchor cables were able to support the slope well against the inhomogeneous deformation, because they can provide high constant resistances and withstand large deformations. This study can provide a deeper insight into the support mechanism for the snit-layered rock slope against flexural toppling failure and introduce a novel supporting method for such slopes.

Determination of Cyclic Load Limits for Plasma‐Sprayed Copper Tracks on Material Extrusion‐Based Printed Surfaces

Herein, copper particles are deposited on additively manufactured surfaces and investigations are performed to determine the mechanical properties and the impact of copper tracks on the surfaces. The basic investigation covers quasistatic tests, namely, tensile and three point bending, for three different printing orientations and two infill variations (+45°/−45° and 0°/90°), which shows no remarkable differences. In addition, a copper track is sprayed via atmospheric pressure plasma spraying (APPS) onto the polymeric samples and characterized regarding hardness and electric conductivity. Furthermore, a specific application for the copper track on the polymer substrate is recreated by a cyclic three point bending with a novel sample geometry (T‐shaped). The sprayed copper track has 60% of the hardness and 40% of the indentation modulus of bulk copper. Depending on the substrates’ topography, the electric conductivity varies from 7% to 18% of bulk copper. The lifetime of the copper track (i.e., conductivity) is strongly dependent on the deformation and the fracture of the polymer underneath. The underlying failure mechanism is triggered either by the topography of the polymer substrate or as a consequence of the damage in the copper track, leading to superficial cracks in the polymer surface.

Ultrasonic characterization of complete anisotropic elasticity coefficients of compressed oral solid dosage forms

In compacted materials, elastic anisotropy coupled with residual stresses could play a determining role in the manifestation of various types of defects such as capping and lamination, as it creates shear planes/bands and temporal relaxation. This internal micro-structure leads to time-delayed flaw initiation/formation, crack tip propagation under residual stresses, and ultimately product quality failures. Thus, their accurate characterization and variations are useful for understanding underlying failure mechanisms and to monitor variations in materials, processes and product quality during production prior to onset of failure. The extraction of tablet anisotropic elasticity properties is a challenging task, especially for commercial tablets with complex shapes, as shape often prevents the use of traditional destructive techniques (e.g., diametric compression testers) to produce accurate measurements. This study introduces and applies an ultrasonic approach to extracting the complete transverse isotropic elastic properties of compressed oral solid dosage forms to a commercial tablet product. A complete set of waveforms and the constitutive matrix for the compacted materials are reported. In addition, a perturbation analysis is carried out to analytically relate propagation speeds in various directions to the elastic coefficients. The proposed characterization approach is non-destructive, rapid, easy, and reliable in evaluating tablet anisotropy.

Earth pressure in narrow cohesive-fictional soils behind retaining walls rotated about the top: An analytical approach

There is currently a lack of an available design approach to estimate the earth pressure in narrow backfills behind retaining walls rotated about the top (RT). The considerations of some significant factors, primarily load transfer mechanisms (soil arching effect and horizontal shear stress in soils), failure mechanisms (shape and number of slip surfaces) and soil cohesion are often neglected for brevity in routine design. Such simplifications may lead to significant deviations from reality. This paper first uses the finite element limit analysis (FELA) technique to identify the underlying failure mechanisms and load transfer mechanisms. The results observed in FELA models indicate that active rotation of walls about the top develops one curved slip surface, which can be approximated by the log-spiral function. Under the soil arching effect, the upper intermediate passive zone with major principal stress rotation trajectory and the lower active zone with minor one can be defined. The arched differential element method (ADEM) is then introduced to formulate the earth pressure calculation. The results from newly published tests, existing analytical approaches, and FELA are compared to validate the accuracy of the proposed approach in both purely-frictional and cohesive-frictional soils. Parametric studies are further conducted to thoroughly understand the earth pressure problems, considering the effects of sensitive design variables (e.g. aspect ratio, soil strength parameters, and wall-soil interface friction angle). The analytical approach presented here would be a great extension to the design guidelines for the retaining structures with narrow backfills.

Anisotropic Tensile Characterisation of Eucalyptus nitens Timber above Its Fibre Saturation Point, and Its Application.

Plantation-grown Eucalyptus nitens (E. nitens) has been grown predominantly for the pulp and paper industry. In this study, the suitability of E. nitens as a structural material is examined using static tensile tests in a universal testing machine. The anisotropic tensile behaviour of 240 Eucalyptus nitens small clear wood samples with a diversity of grain angles was examined in both dry and wet conditions. The samples had a highly anisotropic tensile characterisation in the context of both a low moisture content (MC = 12%) and a high moisture content (MC > its fibre saturation point, FSP). The results showed that, in a high moisture content condition, the wood showed a lower failure strength and more ductility at all grain angles than in a low moisture content condition. The underlying failure mechanism of Eucalyptus nitens timber in tension was determined in detail from the perspective of the microstructure of wood cellulose polymer composites. The mean tensile failure strengths perpendicular and parallel to the fibre direction were, respectively, 5.6 and 91.6 MPa for the low MC and 3.8 and 62.1 MPa for the high MC condition. This research provides a basis for using E. nitens as a potential structural tensile member. The moisture modification factors of Eucalyptus timber at a mean level are higher than those of the traditional construction material, Pinus radiata, implying that E. nitens is promising as a material to be used for tensile members in water saturated conditions.

Failure Modes of Platinized pn+-GaInP Photocathodes for Solar-Driven H2 Evolution.

The long-term stability for the hydrogen-evolution reaction (HER) of homojunction pn+-Ga0.52In0.48P photocathodes (band gap = 1.8 eV) with an electrodeposited Pt catalyst (pn+-GaInP/Pt) has been systematically evaluated in both acidic and alkaline electrolytes. Electrode dissolution during chronoamperometry was correlated with changes over time in the current density-potential (J-E) behavior to reveal the underlying failure mechanism. Pristine pn+-GaInP/Pt photocathodes yielded an open-circuit photopotential (Eoc) as positive as >1.0 V vs the potential of the reversible hydrogen electrode (RHE) and a light-limited current density (Jph) of >12 mA cm-2 (1-sun illumination). However, Eoc and Jph gradually degraded at either pH 0 or pH 14. The performance degradation was attributed to three different failure modes: (1) gradual thinning of the n+-emitter layer due to GaInP dissolution in acid; (2) active corrosion of the underlying GaAs substrate at positive potentials causing delamination of the upper GaInP epilayers; and (3) direct GaAs/electrolyte contact compromising the operational stability of the device. This work reveals the importance of both substrate stability and structural integrity of integrated photoelectrodes toward stable solar fuel generation.

Dynamic compressive properties and underlying failure mechanisms of selective laser melted Ti-6Al-4V alloy under high temperature and strain rate conditions

In this study, high speed impacting tests are systematically conducted on a split Hopkinson pressure bar device to investigate the strain rate and temperature dependence of dynamic compressive properties of Ti-6Al-4 V (TC4) fabricated by selective laser melting (SLM), the ranges of strain rate and temperature are 2000–6000/s and 25–650 ℃, respectively. The results reveal that the yield strength and ultimate compressive strength of the SLM-TC4 alloy increase with the increasing strain rate and the decreasing temperature, showing obvious strain rate and temperature sensitivities. The high speed impacting load intensifies the texture of the SLM-TC4 alloy significantly. Adiabatic shear band (ASB) is more likely to evolve at higher temperatures and strain rates, submicron equiaxed grains formed in the ASB and the surrounding area are mainly ascribed to the combination of dynamic recrystallization, deformation-induced twinning and transverse α-lath splitting. Within the ASB, grains with {0001} pole orientation are rotated by approximately 45° with respect to the shear direction, indicating that the recrystallized grains are able to reorient themselves to accommodate to the shear deformation. The findings in this work provide a theoretical basis to understand the deformation behavior and mechanism of SLM-TC4 alloy under impacting loads, thus is helpful to widen the application of SLM technique and products. • SLMed TC4 alloy has significant temperature and strain rate sensitivities. • Strain hardening and thermal softening cause instable plastic deformation of SLMed TC4. • Ultrafine grains within ASBs mainly due to the deformation twinning, α -lath splitting and DRX. • Dynamic impact loading enhances the texture intensity of the deformed samples. • Recrystallized grains within ASBs can reorient themselves to accommodate to the shear deformation.

A Study on Decisive Early Stages in White Etching Crack Formation Induced by Lubrication

The reliability of rolling bearings is affected by white etching crack (WEC) or white structure flaking (WSF) failures, causing tremendous commercial burdens for bearing manufacturers and operators. The research for the underlying failure mechanism has attracted interest from a large scientific community over decades. Despite the significant amount of efforts, a root cause of white etching cracking is still missing. Amongst other factors, lubricant chemistry is considered to be essential in WEC formation. The authors aim to elucidate this key parameter by provoking white etching crack formation on a FE8 bearing test rig using a well-described set of chemicals in high- and low-reference lubricants. Scanning electron microscopy and energy dispersive X-ray analysis prove the presence of a patchy tribofilm on the surface of bearing washers, leading most likely to a higher frictional torque at the early stages of operation when the low reference oil is used. Secondary neutral mass spectrometry (SNMS) shows a hydrogen containing tribofilm in the shallow subsurface of about 30 nm depth, suggesting that hydrogen proliferating into bearing material may subsequently facilitate crack propagation via dislocation pileups, leading to premature bearing failure.

Adaptive stochastic morphology simulation and mesh generation of high-quality 3D particulate composite microstructures with complex surface texture

Particulate composite materials have a broad range of potential applications in engineering and other disciplines. Accurate modeling of their microstructures and fast generation of the finite element meshes play a vital role in investigating many micromechanical phenomena and improving understanding of the underlying failure mechanisms. Due to the exceedingly intricate multiscale internal structures that they possess, the modeling and meshing of their microstructures still remain difficult in general. In this work, we present a computational framework and methodology for the representation, simulation, and mesh generation of 3D stochastic microstructures of particulate composites. Towards this goal, we propose a multi-level multiscale scheme that allows for capturing the multiscale structures of particulate composite materials at both the coarse and fine scales. A briging scale approach based on heat kernel smoothing is also presented to seamlessly link the coarse and fine scales. In addition to the microstructural modeling of particulate composite materials, we also develop an adaptive curvature-based surface and volume mesh generation algorithm for particulate composite microstructures with complex surface texture. Following the implementation of the morphology and mesh generation algorithm, a series of numerical examples are presented to demonstrate the capability and potential of the proposed method.

Unveiling Liquation and Segregation Induced Failure Mechanism in Thick Dissimilar Aluminum Alloy Electron-Beam Welds

This study presents new findings on the underlying failure mechanism of thick dissimilar electron-beam (EB) welds through a study on the AA 2219-AA 5083 pair. Contrary to the prior studies on EB welding of thin Al alloys, where liquation in the grain boundaries (GBs) in the partially melted zone (PMZ) was not observed, the present investigation for thick EB welds reports both liquation and increased segregation of Cu in the PMZ. The work is thus directed towards understanding the unusual observation in the PMZ of thick EB weld through investigation of the microstructural variation across the various regions of the produced weld. The microstructural results are correlated with the mechanical properties of the weld, i.e., hardness variation and tensile response. Results of this investigation suggest that unlike the convention that EB welding produces sound dissimilar Al welds, the weld performance for thick EB Al welds is affected by the heat input, the associated cooling rates, and most importantly by the base material thickness. Extensive liquation and Cu segregation induced failure in the PMZ on the AA 2219 side of the dissimilar weld. The underlying failure mechanism is explained through a heat-transfer analysis. Beyond a certain plate thickness, the heat transfer changes from two to three-dimensional. As a result, retarded cooling promotes liquation and Cu segregation in thick EB welds.

Corrosion fatigue behavior of electron beam melted iron in simulated body fluid

Pure iron is very attractive as a biodegradable implant material due to its high biocompatibility. In combination with additive manufacturing, which facilitates great flexibility of the implant design, it is possible to selectively adjust the microstructure of the material in the process, thereby control the corrosion and fatigue behavior. In the present study, conventional hot-rolled (HR) pure iron is compared to pure iron manufactured by electron beam melting (EBM). The microstructure, the corrosion behavior and the fatigue properties were studied comprehensively. The investigated sample conditions showed significant differences in the microstructures that led to changes in corrosion and fatigue properties. The EBM iron showed significantly lower fatigue strength compared to the HR iron. These different fatigue responses were observed under purely mechanical loading as well as with superimposed corrosion influence and are summarized in a model that describes the underlying failure mechanisms.

Effect of binder jetting microstructure variability on low cycle fatigue behavior of 316L

Despite the growing interest for Binder Jet Additive Manufacturing (BJAM), variability in the mechanical properties hinders its industry wide adoption. In order to reduce the variability, the underlying failure mechanisms should be understood. In the present work, a stainless steel 316 L alloy powder is used to generate BJAM parts with different microstructures via changing the powder particle size distribution and post-processing heat treatment. The effects of the microstructural features such as porosity, grain size, secondary phases and annealing twins on fatigue failure mechanisms are analyzed. The results show that the primitive porosity lines generated during BJAM process are the major cause of fatigue crack initiation and mechanical variability. These lines led to the formation of elongated pores during subsequent sintering, causing a reduction in the fatigue life and an increase in the data scattering. It is shown that a minimum level of green density prior to sintering is necessary to reduce the occurrence of this networked porosity. Moreover, the use of HIP treatment removed the process induced porosity and reduced scattering of the fatigue life but did not impact the average fatigue life significantly. The benefit of porosity reduction and pore roundness after HIP treatment appears to be greatly offset by grain coarsening and formation of annealing twins during HIP.

Strength enhancement of CFRP joints composed of partially unmolded laminates manufactured using Vacuum-Assisted Resin Transfer Molding (VARTM)

ABSTRACT Partially unmolded CFRP joints have shown the potential to improve the joining efficiency in adhesive joints. In this work, four different joints were investigated, namely, Original Laminated Joint (OLJ) where the mating parts form butt joints, Overlapped Joints, where mating parts overlapped with different lengths (e.g., 20 mm (O20) and 40 mm (O40)), and a specially designed Multiple-Covers Joint (MLJ). All joints were prepared using the Vacuum-Assisted Resin Transfer Molding (VARTM) process. Tensile and bending specimens were prepared to evaluate the joint’s mechanical performance. The O40 achieved the highest ultimate failure load, with a 56% increase over the OLJ specimens. It also achieved the highest bending strength and modulus compared to other joints and even the standard bending samples. The percentage increase in the strength for the proposed joints ranged from 58% to 68% over the OLJ samples. Fractography showed a delamination failure mode for the OLJ and O20 joints, while a mixture of delamination and fiber breakage failure modes was observed for the O40 and MLJ joints. Finite elements modeling (FEM) of the tensile tests were developed. The results showed good agreement with the experimental ones. Furthermore, FEM results clarified some of the experimental observations and the underlying failure mechanisms and damage propagation.

Image-based and in-situ measurement techniques for the characterization of the damage behavior of additively manufactured lattice structures under fatigue loading

For a safe application of additively manufactured (AM) lattice structures in e.g. load-bearing implants, a comprehensive understanding of fatigue processes is required. With additional systematic investigations of the underlying fatigue behavior, the AM lattice structure design as well as a reliable estimation of their lifetimes can be facilitated. Therefore, nondestructive testing methods, digitalization of measurement techniques as well as signal processing can be combined with experimental routines in order to aquire more information about the fatigue process. In this paper, different designs of Ti6Al4V lattice structures with cubic unit cells were fabricated by electron beam melting. To investigate the fatigue process as well as the underlying fatigue behavior the lattice structures were tested in cyclic bending as well as torsion tests. The digital image correlation technique, temperature field measurement and potential drop method were implemented into the test rigs and tested for the characterization of the fatigue damage behavior as well as failure mechanisms. To distinguish the failure behavior in dependence of manufacturing related imperfections a microstructural analysis was performed. Therefore, some experiments were stopped in dependence of the measured potential and examined in micrograph analysing. The successful application of the measurement techniques allows an insight into the damage behavior as well as underlying failure mechanisms of 3D printed lattice structures under fatigue loading. Moreover, the identified results provide the basis for the validation of subsequent numerical simulations using local damage approaches.

Artificial intelligence in prognostics and health management of engineering systems

Prognostics and health management (PHM) has become a crucial aspect of the management of engineering systems and structures, where sensor hardware and decision support tools are deployed to detect anomalies, diagnose faults and predict remaining useful lifetime (RUL). Methodologies for PHM are either model-driven, data-driven or a fusion of both approaches. Data-driven approaches make extensive use of large-scale datasets collected from physical assets to identify underlying failure mechanisms and root causes. In recent years, many data-driven PHM models have been developed to evaluate system’s health conditions using artificial intelligence (AI) and machine learning (ML) algorithms applied to condition monitoring data. The field of AI is fast gaining acceptance in various areas of applications such as robotics, autonomous vehicles and smart devices. With advancements in the use of AI technologies in Industry 4.0, where systems consist of multiple interconnected components in a cyber–physical space, there is increasing pressure on industries to move towards more predictive and proactive maintenance practices. In this paper, a thorough state-of-the-art review of the AI techniques adopted for PHM of engineering systems is conducted. Furthermore, given that the future of inspection and maintenance will be predominantly AI-driven, the paper discusses the soft issues relating to manpower, cyber-security, standards and regulations under such a regime. The review concludes that the current systems and methodologies for maintenance will inevitably become incompatible with future designs and systems; as such, continued research into AI-driven prognostics systems is expedient as it offers the best promise of bridging the potential gap.