Extending the service life of ageing infrastructure, transportation structures, and processing and manufacturing plants in an era of limited resources has spurred extensive research and development ...in structural health monitoring systems and their integration. Even though piezoelectric transducers are not the only sensor technology for SHM, they are widely used for data acquisition from, e.g., wave-based or vibrational non-destructive test methods such as ultrasonic guided waves, acoustic emission, electromechanical impedance, vibration monitoring or modal analysis, but also provide electric power via local energy harvesting for equipment operation. Operational environments include mechanical loads, e.g., stress induced deformations and vibrations, but also stochastic events, such as impact of foreign objects, temperature and humidity changes (e.g., daily and seasonal or process-dependent), and electromagnetic interference. All operator actions, correct or erroneous, as well as unintentional interference by unauthorized people, vandalism, or even cyber-attacks, may affect the performance of the transducers. In nuclear power plants, as well as in aerospace, structures and health monitoring systems are exposed to high-energy electromagnetic or particle radiation or (micro-)meteorite impact. Even if environmental effects are not detrimental for the transducers, they may induce large amounts of non-relevant signals, i.e., coming from sources not related to changes in structural integrity. Selected issues discussed comprise the durability of piezoelectric transducers, and of their coupling and mounting, but also detection and elimination of non-relevant signals and signal de-noising. For long-term service, developing concepts for maintenance and repair, or designing robust or redundant SHM systems, are of importance for the reliable long-term operation of transducers for structural health monitoring.
Organ injury and impairment are commonly observed in patients with acute heart failure (AHF), and congestion is an essential pathophysiological mechanism of impaired organ function. Congestion is the ...predominant clinical profile in most patients with AHF; a smaller proportion presents with peripheral hypoperfusion or cardiogenic shock. Hypoperfusion further deteriorates organ function. The injury and dysfunction of target organs (i.e. heart, lungs, kidneys, liver, intestine, brain) in the setting of AHF are associated with increased risk for mortality. Improvement in organ function after decongestive therapies has been associated with a lower risk for post‐discharge mortality. Thus, the prevention and correction of organ dysfunction represent a therapeutic target of interest in AHF and should be evaluated in clinical trials. Treatment strategies that specifically prevent, reduce or reverse organ dysfunction remain to be identified and evaluated to determine if such interventions impact mortality, morbidity and patient‐centred outcomes. This paper reflects current understanding among experts of the presentation and management of organ impairment in AHF and suggests priorities for future research to advance the field.
•Process and test operator actions yield significant scatter in fracture test data.•Intrinsic material scatter from manufacture is important for damage-tolerant design.•Delaminations from distributed ...defects in composites are difficult to quantify.•Digital tools can significantly reduce scatter in fracture testing and data analysis.
Fracture mechanics data from fiber-reinforced polymer-matrix (FRP) composites are required for damage-tolerant design. Quantifying scatter is essential for determining design limits. Scatter is affected by the “human factor”, i.e., process and test operator actions. Intrinsic scatter from manufacture, processing and environment shall be preserved; extrinsic scatter from specimen preparation, test set-up, measurement resolution, and analysis minimized. Automated processes yield fairly consistent scatter, but additive manufacturing of FRP composites has not reached the same maturity level yielding more defects or weak interfaces. The potential of digital technology for reducing scatter and quantifying single and multiple delamination propagation are also discussed.
•Identification of microscopic damage mechanisms from acoustic emission signals.•Fiber-reinforced polymer-matrix specimens with single dominant damage mechanism.•Single dominant damage mechanisms ...active in limited volume or at selected time/load.•Extrapolating from microscopic damage mechanisms to structural integrity.
Acoustic emission (AE) analysis for identifying microscopic damage mechanisms in fiber-reinforced polymer (FRP) composites has a long history. Recently, combining unsupervised pattern recognition with Finite Element Modelling (FEM) of signal generation, propagation, and sensor transfer function yielded source mechanism identifications of located sources for laboratory-scale FRP composite specimens. Designing FRP specimens yielding one specific damage mechanism that dominates the damage behavior, at least for selected stages of the damage development, allows for validating the pattern recognition-simulation approach for identifying the damage mechanisms. The challenge of using identified source mechanisms of located AE signals for structural integrity assessment is also discussed.
Acoustic Emission (AE) and Guided Ultrasonic Waves (GUWs) are non-destructive testing (NDT) methods in several industrial sectors for, e.g., proof testing and periodic inspection of pressure vessels, ...storage tanks, pipes or pipelines and leak or corrosion detection. In materials research, AE and GUW are useful for characterizing damage accumulation and microscopic damage mechanisms. AE and GUW also show potential for long-term Structural Health and Condition Monitoring (SHM and CM). With increasing computational power, even online monitoring of industrial manufacturing processes has become feasible. Combined with Artificial Intelligence (AI) for analysis this may soon allow for efficient, automated online process control. AI also plays a role in predictive maintenance and cost optimization. Long-term SHM, CM and process control require sensor integration together with data acquisition equipment and possibly data analysis. This raises the question of the long-term durability of all components of the measurement system. So far, only scant quantitative data are available. This paper presents and discusses selected aspects of the long-term durability of sensor behavior, sensor coupling and measurement hardware and software. The aim is to identify research and development needs for reliable, cost-effective, long-term SHM and CM with AE and GUW under combined mechanical and environmental service loads.
Quasi-static or cyclic loading of an artificial starter crack in unidirectionally fibre-reinforced composite test coupons yields fracture mechanics data-the toughness or strain-energy release rate ...(labelled G)-for characterising delamination initiation and propagation. Thus far, the reproducibility of these tests is typically between 10 and 20%. However, differences in the size and possibly the shape, but also in the fibre lay-up, between test coupons and components or structures raise additional questions: Is G from a coupon test a suitable parameter for describing the behaviour of delaminations in composite structures? Can planar, two-dimensional, delamination propagation in composite plates or shells be properly predicted from essentially one-dimensional propagation in coupons? How does fibre bridging in unidirectionally reinforced test coupons relate to delamination propagation in multidirectional lay-ups of components and structures? How can multiple, localised delaminations-often created by impact in composite structures-and their interaction under service loads with constant or variable amplitudes be accounted for? Does planar delamination propagation depend on laminate thickness, thickness variation or the overall shape of the structure? How does exposure to different, variable service environments affect delamination initiation and propagation? Is the microscopic and mesoscopic morphology of FRP composite structures sufficiently understood for accurate predictive modelling and simulation of delamination behaviour? This contribution will examine selected issues and discuss the consequences for test development and analysis. The discussion indicates that current coupon testing and analysis are unlikely to provide the data for reliable long-term predictions of delamination behaviour in FRP composite structures. The attempts to make the building block design methodology for composite structures more efficient via combinations of experiments and related modelling look promising, but models require input data with low scatter and, even more importantly, insight into the physics of the microscopic damage processes yielding delamination initiation and propagation.
Automated tape placement with in-situ consolidation (ATPisc) is a layer-wise manufacturing process in which the achievement of proper interlayer bonding constitutes one of the most challenging ...aspects. In the present study, unidirectional carbon fiber reinforced thermoplastic laminates were produced following different manufacturing protocols using ATPisc. The interlayer bonding of the laminates produced was characterized by mode I fatigue fracture tests with double cantilever beam (DCB) specimens. Independent of the manufacturing approach, the laminates exhibited multiple cracking during DCB testing, which could not be evaluated simply following standard methods. Thus, various data analysis methodologies from literature were applied for the quantitative assessment of the fracture behavior of the laminate. The examination of the evolution of the damage parameter φ and the effective flexural modulus throughout testing enabled a better understanding of the damage accumulation. The Hartman-Schijve based approach was revealed to be a convenient method to present fatigue crack growth curves of laminates with multiple delaminations. Moreover, a preliminary attempt was made to employ a 'zero-fiber bridging' methodology to eliminate the effect of additional damage processes on the fatigue crack growth that resulted in large-scale, partially massive fiber bridging.
Electrophoretic deposition (EPD) of carbon nanotubes (CNTs) on carbon fibers has been implemented as a continuous process on laboratory-scale. The interfacial adhesion and fracture toughness of the ...carbon fibers in an epoxy composite is assessed by a modified single-fiber push-out test. A detailed energy analysis yields the different energy contributions in the push-out process. A comparison between CNT-deposited, as received and oxidized carbon fibers (passing through the EPD process without CNT) indicates that interfacial adhesion and fracture toughness are not affected by the different fiber treatments. Interfacial friction after fiber debonding, however, is significantly changed. This is confirmed by finite element simulation which has to include friction for reproducing the essential features of the load–displacement plots from fiber push-out. Scanning electron micrographs indicate little interaction between CNT and carbon fibers, but point to changes in surface roughness of CNT-deposited and oxidized fibers after push-out. Therefore, the cyclic loading–unloading fiber push-out test seems well suited to investigate the micromechanical behavior of carbon fiber composites and to discriminate and quantify the different energy contributions to the total load–displacement curves.
A recently proposed mandrel laminate peel (MLP) test for quantification of delamination propagation in Fiber-Reinforced Polymer (FRP) composites is compared with a climbing drum laminate peel (CDLP) ...test, and the standard quasi-static Mode I fracture test with Double Cantilever Beam (DCB) specimens. MLP and CDLP both are applicable to thin laminates, for which the Mode I DCB test is not suitable. MLP and CDLP, however, do not yield delamination initiation values. Delamination propagation resistances from the three tests performed with different types of FRP, one partly and one fully cured epoxy laminate, and one thermoplastic laminate, all with unidirectional fiber lay-up agree within about 20%. Reduction of the diameter of the climbing drum (100 mm for the standard climbing drum peel test for adhesives) indicates a minimum diameter on the order of 50 mm for the CDLP test. Additional tests with selected laminates investigated potential effects of specimen width. It is concluded that FRP laminate specimens 20 mm wide and 180 mm long are sufficient for consistent results from CDLP and MLP tests. The CDLP test yielded less scatter (around 10%), i.e., better repeatability than the MLP (round 13%) and the Mode I DCB test (around 17%). Hence, the CDLP test is considered advantageous for industrial application, also due to the simplicity of the test set-up and of the data analysis.
Observations of a nanometer-scale nodular morphology on differently prepared surfaces of thermosets have frequently been interpreted as a sign for an inhomogeneous molecular network, which would ...result in an inhomogeneous modulus distribution within those thermosets. In order to test this hypothesis, the Peak-Force Tapping atomic force microscopy (AFM) mode was used on fracture surfaces and ultramicrotome cuts of epoxy and other polymers using differently sharp AFM probes.
The nodular morphology is quite likely caused by an AFM artifact, which also seems to cause an apparently inhomogeneous modulus distribution; a variation in the tip–sample contact area could explain this effect. Smooth surfaces are necessary in order to reduce the contribution from this artifact. Ultramicrotome cutting currently seems to be the most appropriate surface preparation technique for the measurement of modulus distribution at the nanometer scale.
All investigated materials seem to be homogeneous on a scale on the order of 10 nm–1000 nm. If modulus inhomogeneities are present, their amount or their lateral size is too small to be unambiguously measurable with this technique. From this data, it seems unlikely that epoxy exhibits an inhomogeneous molecular network.
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