Invisible Damage Research Articles

Low-velocity impact and compression-after-impact behaviors of twill woven carbon fiber/glass fiber hybrid composite laminates with flame retardant epoxy resin

Composite structures suffer from significant residual strength reduction as a result of invisible damage caused by impacts, which can cause severe harm without any prior warning. Fibers play a crucial role in improving the impact resistance of composite materials by providing strength, toughness, and energy absorption. This work explores the dynamic response and residual compressive strength of composite laminates reinforced with twill woven carbon fiber, glass fiber, and carbon fiber /glass fiber hybrid, subjected to low-velocity impact (LVI) and compression-after-impact (CAI) testing. First, the performance of woven carbon fiber composite laminates infused with flame retardant epoxy resin and general epoxy resin was studied. The results indicated a lower load-bearing capacity and elongation of the former. To enhance the impact resistance of the laminates, we explored the effects of incorporating glass fibers into the woven carbon fiber composite laminates. Our findings demonstrated that the introduction of glass fibers alteres the impact damage mode of the laminates and effectively improved their impact resistance and CAI strength. Further, several techniques were adopted to characterize the corresponding damage morphologies and failure mechanisms of the woven carbon fiber composite laminates, which provides valuable insights for their structural design and performance improvement.

Invisible damage identification and danger warning for steel box girders using the metal magnetic memory method

Locating invisible damage and detecting the critical stress state leading to structural damage or failure are essential tasks in structural diagnosis. In this study, a four-point bending static loading experiment was conducted on two steel box girder members—one nondestructive, whereas the other had a butt weld—based on the metal magnetic memory (MMM) method. The correlation between the buckling failure position of the steel box girders and distribution characteristics of the magnetic signals was analyzed. The critical stress states of the elastic and elastic–plastic or elastic–plastic and plastic stages were statistically described using characteristic magnetic parameters. The results showed that the extremum position of the magnetic signal variationΔHpywell correlated with the location of the stress concentration. TheΔHpyextremum could locate the invisible damage position and predict the failure position, while the extremum of the average valueΔHpyaand integral value AofΔHpyvalue could accurately diagnose the critical stress state of steel box girders in the elastic and elastic–plastic or elastic–plastic and plastic stages. The proposed magnetic charge model could explain the distribution characteristics of the magnetic signals on the butt weld. These results are expected to provide a basis for effectively evaluating invisible damage using the MMM method.

Fibroblast Growth Factor 23: Potential Marker of Invisible Heart Damage in Diabetic Population.

Two-dimensional speckle-tracking echocardiography (2DSTE) detects myocardial dysfunction despite a preserved left ventricular ejection fraction. Fibroblast growth factor 23 (FGF23) has become a promising biomarker of cardiovascular risk. This study aimed to determine whether FGF23 may be used as a marker of myocardial damage among patients with diabetes mellitus type 2 (T2DM) and no previous history of myocardial infarction. The study enrolled 71 patients with a median age of 70 years. Laboratory data were analyzed retrospectively. Serum FGF23 levels were determined using a sandwich enzyme-linked immunosorbent assay. All patients underwent conventional echocardiography and 2DSTE. Baseline characteristics indicated that the median time elapsed since diagnosis with T2DM was 19 years. All subjects were divided into two groups according to left ventricular diastolic function. Individuals with confirmed left ventricular diastolic dysfunction had significantly lower levels of estimated glomerular filtration rate and higher values of hemoglobin A1c. Global circumferential strain (GCS) was reduced in the majority of patients. Only an epicardial GCS correlated significantly with the FGF23 concentration in all patients. The study indicates that a cardiac strain is a reliable tool for a subtle myocardial damage assessment. It is possible that FGF23 may become an early diagnostic marker of myocardial damage in patients with T2DM.

Temporal frames, temporal focus, and behavioral expectations: The persuasive impact of near and distant threats

BackgroundUltraviolet (UV) photography and photoaging visuals make hidden sun damage visible to the naked eye, granting the potential to create messages that vary in temporal dimensionality. As UV photos depict immediate skin damage, the photo communicated that exposure in sun causes invisible damage to the young truck driver (near temporal frame) and visible damage (e.g., wrinkles) to the old truck driver (distant temporal frame). ObjectiveThe current study examines the moderating effects of loss/gain frames and temporality variables on the relationship between temporal framing and sun safe behavioral expectations. MethodU.S. adults (N = 897) were assigned to a 2 (near/distant temporal frame) × 2 (gain/loss frame) between-participants experiment. ResultsThe loss frame triggered greater fear compared to the gain frame, this fear forms an indirect path where loss frames increase fear and fear increases changes in sun safe behavioral expectations. Participants exposed to the distant frame had increased behavior expectations if either of the two temporality variables (CFC - future or current focus) were low. Participants with low temporality indicators (i.e., CFC - future, current focus, or future focus) exposed to the gain frame had increased behavior expectations. ConclusionsThe findings demonstrate the potential utility of temporal frames as a tool for designing strategic health messages.

Quantitative monitoring of impact damage to composite structures using blade coated MXene sensors

Low velocity impacts usually cause invisible damage of composite structure，which is hard to be found and will cause catastrophic failure. In this work, the blade-coated MXene sensor was used to monitor impact damage of composite laminates. For the purpose of impact monitoring, an experiment was designed where composite laminates were subjected to controlled energy impacts. A circular sensor with a diameter of 100 mm was arranged on the back of the laminate. And ultrasonic C-scan was used to examine delamination damage of laminates after each impact. The experimental results show that the sensor identifies the onset of impact and the generation of damage. And the relative resistance changes of the sensor have a directly linear proportional relationship to the delamination size. There is also a functional relationship between the electrical response of the sensor and the impact energy. The findings are of great significance for the application of composite structure, and can effectively avoid the safety hazards caused by invisible impact delamination damage.

Mechanical responses of CFRP/PVC foam sandwich plate impacted by hailstone

Mechanical responses of sandwich plate with carbon fiber reinforced polymer (CFRP) laminate as face-sheets and polyvinyl chloride (PVC) foam as core, which is promising in the lightweight design of high-speed maglev trains, was studied for the case of hailstone impact. Nine 500 × 500 mm sandwich plate specimens of three face-sheet thicknesses 1, 2 and 3 mm and a constant core thickness 30 mm were fabricated and tested under the impact of ice spheres of 50 mm in diameter propelled by the first-stage light gas gun. Three impact speeds, i.e. 30, 60 and 90 m/s, of the ice spheres and three incident angles, i.e. 0°, 30° and 45°, were investigated. For all cases, no macro failures were observed, but invisible damages, i.e. delamination in the upper face-sheets and indentation of the foam core, were found by high resolution ultrasonic testing. The experimental results were then used to calibrate the corresponding numerical model, based on which a thorough parametric analysis was performed to show the full dynamical responses of CFRP/PVC foam sandwich panels under hailstone impact. It is found that the foam cushion could effectively protect the rear face-sheet and increasing the ratio of span to thickness of the panel could reduce the damage degree due to the change of failure mode from local to global.

Revealing and simulating the effect of hidden damage on local and full-field deformation behaviour of cast aluminium

Pores have been the main focus of quality assurance in castings. Latest research has shown that in aluminium castings pores can form only if there is existing entrainment damage, i.e., pores are merely the visible parts of the entrainment damage, and usually invisible damage is much more extensive. However, its effect on deformation behaviour has not been previously established or observed in-situ. This work applies 2D Digital Image Correlation (DIC) to an in-situ full-field stress-strain analysis of tensile samples with a non-conventional heterogeneous stress distribution. The observations reveal that the effect of hidden damage extends far beyond its impact on fracture behaviour and is responsible for initiating local strain concentrations during deformation. By extracting local stress-strain data, FE simulations have been performed to mimic the effect of local hidden damage on the heterogeneous stress-strain field. SEM and FIB-SEM analysis has been applied to investigate the cause for the strain concentrations. The combined results show that hidden damage in the form of oxide films is not only responsible for premature fracture, but also affects the deformation behaviour of tensile samples by introducing dispersed strain concentrations.

The research of buckling and post-buckling behavior of bidirectional stiffened composite panel under discrete impact

PurposeIn this paper, an attempt is made to obtain buckling loads, ultimate bearing capacity and other required structural characteristics of grid structure panels. The numerical method for post-buckling behavior analysis of panels involving multiple invisible damages is also presented.Design/methodology/approachIn this paper, two bidirectional stiffened composite panels are manufactured and tested. Multiple discrete invisible damages are introduced in different positions of the stringers, and the experimental and simulation investigation of buckling and post-buckling were carried out on the damaged stiffened panels.FindingsThe simulation load–displacement curves are compared with the experimental results, and it is found that the simulation model can well predict the occurrence of buckling and failure loads. The strain curve shows that the rate of strain change at the damaged site is greater than that at the undamaged site, which reflects that the debond is more likely occurred at the damaged site. The simulation verifies that the panel is usually crushed due to matrix compression and fiber–matrix shear.Originality/valueIn this paper, post-buckling tests and numerical simulations of bidirectional stiffened composite panels with impact damage were carried out. Two panels with four longitudinal stringers and two transverse stringers were manufactured and tested. The buckling and post-buckling characteristics of the grid structure are obtained, and the failure mechanism of the structure is explained. This is helpful for the design of wall panel structure.

Electrospun short nanofibers to improve damage resistance of carbon fiber composites

AbstractCarbon composites are sensitive to matrix cracking, delamination, and fiber‐matrix debonding induced by external transverse loading. Such invisible damages demand frequent non‐destructive testing (NDT) owing to their tendency to propagate in brittle composites. Application of carbon composites in safety critical structures have urged researchers to design for superior damage resistance. Bulk modification of matrices through nanoparticles is one such technique that exploits high surface area and mechanical properties of nano‐reinforcements to engineer desired interfaces and improve mechanical properties. This study adopts the same technique to investigate effect of electrospun nylon 6 short nanofiber addition on damage resistance of carbon fiber/epoxy composites. Different concentrations (0.05, 0.1, 0.2, and 0.4 wt% of epoxy) of short nanofibers were prepared to modify epoxy and fabricate carbon laminates. Quasi‐static indentation tests confirmed improvement of 8.7, 8.8, and 53% in peak force, displacement and elastic toughness of carbon composites at optimum nanofiber concentration (0.05 wt%). External damage area marginally improved though directional damage growth was suppressed. Delaminated area reduced by 12.6% at optimum nanofiber concentration. Suppression of compressive fiber failure and enhanced interlaminar bonding were credited to offer superior performance. In general, development of nanofiber‐rich zones declined the load bearing response above optimum concentration.

Monitoring the behaviour of seven storeyed RC frame subjected to reversed cyclic loading by nonlinear NDT

Large-scale structures are more prone to risk than small-scale structures due to their increased weight, stiffness and ductility parameters. The main focus of this investigative study is on the structural monitoring of a building using the non-destructive method of non-linear ultrasonic pulse velocity. A single-bay seven-storeyed frame model subjected to reverse cyclic loading of varying amplitudes is monitored by non-linear ultrasonic technique based on third harmonic wave generation. The critical nodes are identified in two different ways: the points of possible plastic hinge formation (PPPHF) through the theoretical background of plastic hinge analysis and the highly sensitive regions in structures (HSR) through numerical analysis. The common nodes of PPPHF and HSR are considered to monitor the structure. The invisible primary structural damage in the initial stage and the visible structural damage up to ultimate load are observed using non-linear ultrasonic waves. The fundamental frequency value and the third harmonic frequency value are observed and processed in MATLAB in the form of a Fast Fourier Transform (FFT) algorithm. The relationship between the condition of the structure and the ratio of third harmonic parameter, load and displacement is compared. In the initial stage of loading, only slight change or no variation in third harmonic ratio is observed at the observation points indicating material disintegration. This varies consistently up to the development of visible cracks and the same varies inconsistently on further loading after the appearance of visual cracks.

Electroluminescence Analysis and Grading of Hail Damaged Solar Panels

We analyzed more than 4000 electroluminescence images of hail damaged solar panels from a cluster of houses in Texas. We enhanced the images for ease of analysis and classified the defects within each solar cell into categories of glass breakage, installer damage, inactive substrings, crack severity, interconnect wire problems, and whether the damage was likely caused by hail. From these statistics, we quantified each panel into five levels of hail damage for insurance claims, and four levels of overall quality for potential resale pricing. We share here some statistics regarding the defects with the hope that the data is useful to others attempting to predict the invisible damage to systems based just on the easily observable glass breakage statistics.

Analysis of low-energy impact damage to epoxy resin film based on surface damage characteristics

For fiber reinforced epoxy resin composites, when low-energy impact events occur, although the surface damage of the structure is invisible, a large area of delamination might have appeared inside. In order to detect these less noticeable impact damages, epoxy resin is intended to be used as an indicative coating to show obvious surface characteristics when the impact damage occurs. On that basis, the main purpose of this study is to establish the correspondence between the damage patterns of the epoxy resin film and the low-energy impact. First, a three-dimensional progressive finite element model was established and the impact damage mechanism of the film was analyzed. Then both numerical simulation and experimental investigation were conducted for the low-energy impact response of the film, and they were in good agreement with each other. The results showed that the damage characteristics of the epoxy resin film were clearly visible and changed regularly with the impact, as reflected by the fact that the damage area, the number of cracks and the crack distribution obviously depended on the magnitude of the impact energy. Therefore, the established simulation model can be used to predict and obtain systematic damage patterns of the film and offers the potential to evaluate the invisible impact damage of composite structures based on the regular damage patterns if the film is used as the indicative coating.

A CNN-integrated percussion method for detection of FRP–concrete interfacial damage with FEM reconstruction

Reinforced concrete (RC) structures are commonly strengthened using externally bonded fiber-reinforced polymer (FRP) sheets. The bond between the FRP and concrete is a crucial factor affecting the strengthening effect, and debonding along the FRP–concrete interface is usually accompanied by the fracture of the underlying concrete. Therefore, it is necessary to identify the interface damage of FRP-to-concrete joints and conduct mechanical analysis. However, debonding is invisible damage that occurs inside the underlying FRP layer, which makes damage detection more difficult. To this end, this study fuses a percussion method with a deep learning framework to address the detection of such invisible lesions. Meanwhile, the visualization study provides guidance for later maintenance work. To further illustrate the hazard of the identified lesions, three-dimensional reconstruction for finite element modeling (FEM) with detected damage information based on percussion is proposed to elucidate the mechanical degradation caused by the fracture of underlying concrete. Lastly, the results of this study demonstrate that the detection, visualization, and FEM reconstruction of FRP–concrete interface damage using percussion signals has considerable application potential and is worthy of further study.

Effect of poly ether ether ketone particles on v‐notched shear and drop weight impact behavior of carbon/epoxy composite

AbstractDue to the intrinsic brittleness, low velocity impact (LVI) causes internal invisible damages that drastically degrade the structural qualities of carbon/epoxy composites. The addition of pre‐formed immiscible thermoplastic filler in thermoset matrix offers various advantages over miscible phase separated thermoplastic filler. In this work, poly ether ether ketone (PEEK) micro particles were used to improve the toughness of the carbon/epoxy composite. Mechanical stirring was used to disperse the particles. Composite was fabricated using hand lay‐up process followed by compression molding. The effect of PEEK particles was assessed using in‐plane v‐notched rail shear characteristics, drop weight impact, and compression after impact strength. The addition of PEEK particles significantly improved the in‐plane shear strength and damage tolerance of the carbon/epoxy composite, according to the results.

Nonlinear lamb wave imaging method for testing Barely Visible Impact Damage of CFRP laminates

The traditional linear ultrasonic technique may not provide a clear indication of the underlying damage in some cases of invisible or barely-visible impact damage. Considering nonlinear ultrasonic methods have been used for the detection of defects such as closed crack or delamination, a new nonlinear Lamb wave imaging method for detecting the Barely Visible Impact Damage (BVID) of Carbon Fiber Reinforced Polymer (CFRP) laminates was proposed in this work. First, the testing platform of nonlinear Lamb wave was built, and the test specimen with different impact damages was prepared. Second, the Lamb wave modes that meet the requirements of nonlinear testing were excited, and then, the correlation between the relative non-linear coefficient and status of low-energy impact damage were investigated. Lastly, the scanning imaging methods were developed, and the Damage Index (DI) for imaging impact damage was defined. The research results indicated that the DI based on the relative non-linear coefficient has higher detection sensitivity for low-energy impact damage, and the BVID and the extension of macro damage can be displayed intuitively by use of the new nonlinear Lamb wave imaging method.

Study on Nonlinear Lamb Wave Test for Invisible Impact Damage on CFRP Laminates

The impact damage imposed on carbon fiber–reinforced polymer (CFRP) materials used in aircraft fuselage may seriously affect flight safety. An ultrasonic testing method can be used to inspect for damage; however, in some cases of invisible or barely visible impact damage, linear ultrasound may not provide a clear indication of the underlying damage. Accordingly, a nonlinear Lamb wave technique was developed in this study to detect invisible impact damage (IID). First, a nonlinear Lamb wave testing platform was set, as well as damage areas with different impact energies. Second, the anisotropic propagation of Lamb waves was studied to determine the wave mode and the distribution of the transducers, and the linear parameters of the Lamb waves were determined. Last, three types of characteristic parameters of nonlinear Lamb waves were obtained for damage detection. As revealed from the results, the linear ultrasonic parameters of A0 mode Lamb waves can be applied to the detection of macro surface cracks, and the frequency shift, relative nonlinearity coefficient (RNC), and fluctuation coefficient of RNCs are highly sensitive to the detection of IID. Thus, a combination of nonlinear S0 Lamb waves and linear A0 Lamb waves can be used for IID and macro surface crack detection, respectively.

A Possibility for Quantitative Detection of Mechanically-Induced Invisible Damage by Thermal Property Measurement via Entropy Generation for a Polymer Material.

Entropy generation from a mechanical and thermal perspective are quantitatively compared via molecular dynamic (MD) simulations and mechanical and thermal experiments. The entropy generation values regarding mechanical tensile loading—which causes invisible damage—of the Polyamide 6 (PA6) material are discussed in this study. The entropy values measured mechanically and thermally in the MD simulation were similar. To verify this consistency, mechanical and thermal experiments for measuring entropy generation were conducted. The experimentally obtained mechanical entropy was slightly less than that calculated by MD simulation. The thermal capacity is estimated based on the specific heat capacity measured by differential scanning calorimetry (DSC), applying the assumed extrapolation methods. The estimated entropy generation was higher than the aforementioned values. There is a possibility that the entropy-estimating method used in this study was inappropriate, resulting in overestimations. In any case, it is verified that entropy increases with mechanical loading and material invisible damage can be qualitatively detected via thermal property measurements.

Enrichment process of lanthanum as a nonessential trace element in leaf cells of lettuce (Lactuca sativa L.)

Rare earth elements (REEs) as nonessential trace elements are enriched in living organisms and threaten their health. To early detect and reduce REE enrichment in living organisms, scientists are focused on clarifying the enrichment process of REEs in living organisms and its risks. However, the enrichment process of REEs in edible plant cells has remained unclear. Herein, by using interdisciplinary methods and techniques, the enrichment process of lanthanum (La(III)) in the leaf cells of lettuce (Lactuca sativa L.) was investigated. (1) When La(III) exposure dose is 0.5–5 μmol/L, La(III) is enriched outside the plasma membrane (PM). In this zone, La(III) is bound to vitronectin-like protein (VN) to form La–VN complexes; (2) When La(III) exposure dose is 5–20 μmol/L, besides the zone outside the PM, La(III) is also enriched on the PM and bound to arabinogalactan proteins (AGPs) to form La–AGPs complexes; (3) When La(III) exposure dose is 20–140 μmol/L, besides the zone outside and on the PM, La(III) is enriched inside the PM; (4) When La(III) exposure dose is 60–140 μmol/L, malondialdehyde content (an important indicator of invisible damage) significantly increases. Thus, as La(III) exposure dose increases, La(III) gradually migrates from outside the PM to the PM and inside the PM, enriching in these zones in turn. The enriched La(III) will cause invisible damage to lettuce leaf cells and even enter human bodies along food chains. These results provide references for investigating the enrichment process of REEs in plants and its environmental risks, and finding strategies to early detect and reduce REE enrichment in plants.

On low-velocity impact behaviour of composite laminates: Damage investigation and influence of matrix and temperature

The present manuscript aims to provide an overview of the phenomenon of the low-velocity impact behaviour of composite materials at room and in extreme temperature conditions, by comparing the results obtained on different fibre-matrix combinations and giving a comprehensive review of the impact behaviour and evolution of damage of marine and aerospace composites. The latter was chosen to reduce the environmental impact of plastic wastes. Experimental impact tests up to complete penetration and at different increasing impact energy levels, were carried out by a modular falling weight tower.The ability of different Non Destructive Techniques NDT in detecting and evaluating barely-visible and invisible impact damage on composite laminates was tested. The aspect related to the damage is, in fact, crucial for these materials because of their inhomogeneity and anisotropy.The conventional and largely used UltraSound technique, US, was adopted to investigate the delamination caused by low-velocity impact loads. The results were compared with Electronic Speckle Pattern Interferometry, ESPI, as well as with theoretical and semiempirical formulations for the delamination prediction.

Experimental investigation of axial compression behavior after low velocity impact of glass fiber reinforced filament wound pipes with different diameter

The GRP (Glass Reinforced Polymer) composite pipes produced by filament winding (FW) technique are used in many applications such as the transmission of pressurized gases, liquids, oil, and natural gas transmission. These composite pipes may be subjected to low velocity impacts during production and/or service for various reasons. The impact on the composite pipes may lead to invisible damages such as matrix crack, fiber damage, and inter-layer separation. In this study, pipes with three different diameters (Ø54, Ø72, and Ø96 mm) were produced to investigate the damage caused by low velocity impact and to determine the effect of these damages on the strength losses of the composite pipe. The pipes were subjected to low velocity impact test according to ASTM D 7136 with 1.5, 2, 2.5, and 3 m/s impact velocities. In order to examine the effects of impact damages on the strength of pipes, Compression after impact (CAI) tests were performed on pre-damaged pipes according to ASTM D 7137 and Force-Displacement and Stress-Strain relations were obtained. The results of impacted samples were compared with the behavior of undamaged samples. After the experiments, macro/micro damage analyses of GRP pipes were performed by optical microscope and SEM imaging.