Sandwich structures work much like an I-beam stretched into a panel: two stiff outer skins take bending stresses while a lightweight core holds them apart and resists shear. In civil engineering, this architecture promises high strength-to-weight ratios, built-in thermal insulation, and excellent energy absorption. Fiber-reinforced polymer face sheets add corrosion resistance, cutting maintenance and life-cycle costs compared with steel or reinforced concrete. Curved versions of these panels open additional design freedom for architecturally ambitious roofs and facades, yet their static bending behavior has remained surprisingly underexplored, with most prior research focused on vibration and aerospace applications.

The research team fabricated singly curved panels using vacuum infusion molding, a process that begins by cutting foam into an arc, wrapping it with a layer of ±45° glass-fiber fabric, and then sequentially assembling the bottom face sheet, internal web cores, and outer face sheet on a curved wooden mold before resin infusion. After eight hours of curing, the panels were demolded and trimmed. Three core configurations were compared: conventional polyurethane foam, grid-scored recycled PET foam, and recycled PET foam reinforced with a continuous lattice of glass fiber webs. Each panel had a span of 400 millimeters, a width of 200 millimeters, a 25-millimeter-thick core, and five ring webs spaced 200 millimeters apart horizontally.

Curvature proved to be a decisive variable. Panels were tested at three radii of 300, 580, and 1150 millimeters, corresponding to central angles of 60, 30, and 15 degrees. For the polyurethane foam series, the panel with the largest radius failed at 7.49 kilonewtons through brittle compressive fracture of the upper face sheet beneath the loading plate. Reducing the radius to 300 millimeters cut the ultimate load to 5.29 kilonewtons and the bending stiffness to 0.73 kilonewtons per millimeter, a 29.4 percent loss in capacity and a 26.3 percent loss in stiffness. Tighter curvature intensified membrane-bending coupling and radial compression, triggering premature debonding between face sheet and core before the panel could fully mobilize its bending resistance.

The grid-scored PET foam panels, in which load transfer relies on discontinuous resin-rich channels, fared worse under the same conditions. Their initial stiffness values of 0.66 and 0.77 kilonewtons per millimeter lagged well behind the lattice-reinforced equivalents, and stress concentrations at the resin columns, the foam-resin interface, and the face-core boundary interacted destructively during loading. At the smallest radius, interfacial delamination began at only 3.49 kilonewtons, and the panel ultimately failed at 4.37 kilonewtons before collapsing to 1.50 kilonewtons as shear cracks spread through the foam. The discrete grid could not bridge cracks or redistribute stress once local damage started, so degradation was rapid and brittle.

By contrast, the lattice-reinforced recycled PET foam panels delivered the best performance across every curvature. The largest-radius specimen reached an initial bending stiffness of 1.09 kilonewtons per millimeter and a peak load of 12.4 kilonewtons, while the moderate-radius panel achieved the highest ultimate load of the entire experimental campaign at 13.3 kilonewtons. Even the tightest-radius panel sustained 11.0 kilonewtons. Crucially, after the upper face sheet fractured in compression, the loads did not collapse entirely; they dropped only partially and then recovered, for example from 13.3 to 11.5 kilonewtons, because the continuous lattice webs acted as internal bridges that kept transferring load between the separated face sheets. Compared with the grid-scored design, ultimate loads rose by 71.5 to 151.7 percent.

Digital image correlation provided a strain-map view of these mechanisms in real time. In large-radius panels, strain concentrated in the upper face sheet under the loading plate, confirming compression-dominated failure. As radius shrank, shear strain localization migrated into the foam core near the load point and the face-core interface, signaling the shift toward debonding and core shear failure. The lattice-reinforced panels showed markedly more continuous strain distributions along the curved section than their grid-scored counterparts, evidence that the internal webs diffused stress, restrained local deformation, and delayed unstable crack propagation through the thickness of the core.

To go beyond what experiments alone could reveal, the team built a three-dimensional finite element model in ABAQUS incorporating Hashin damage criteria for the composite laminates, a crushable foam plasticity model with volumetric hardening for the PET core, and a bilinear cohesive zone model for the face-core interface. The simulations reproduced the measured stiffness and peak loads with errors below 10 percent, and predicted failure modes matched the laboratory observations, including interfacial debonding captured through cohesive damage variables. Deflection fields at a 4.0 kilonewton load level also agreed closely with the DIC measurements, validating the model as a trustworthy design tool.

The parametric study then isolated the levers that matter most. Raising the PET foam density from 80 to 150 kilograms per cubic meter increased ultimate load by 29.2 percent but bending stiffness by only 18.9 percent, because global stiffness is governed mainly by the axial stiffness and separation of the face sheets rather than the core. Fiber layup orientation strongly affected initial stiffness, with a (0°,90°) laminate reaching 2.15 kilonewtons per millimeter against 0.85 for an all ±45° stack, yet ultimate loads stayed within a narrow band around 13.7 to 14.1 kilonewtons. Lattice geometry emerged as the most powerful knob: thickening the longitudinal webs from 1.2 to 3.6 millimeters lifted ultimate load from 13.69 to 15.00 kilonewtons, and tightening longitudinal web spacing raised peak load to 16.43 kilonewtons, a gain of roughly half over the baseline configuration.

Across all variations, the dominant failure sequence remained interfacial delamination followed by compressive failure of the upper face sheet, but the lattice consistently delayed that sequence and preserved residual capacity afterward. The findings carry practical weight for sustainable construction: recycled PET foam diverts plastic waste, requires less production energy than traditional core materials, and adds thermal insulation, while GFRP skins resist corrosion and reduce reinforcement demands on existing structures. By demonstrating that a simple internal lattice transforms a recycled-core curved panel from brittle and debonding-prone into a progressively damage-tolerant structural element, the study offers designers a clear, simulation-backed recipe for lighter, greener, and more resilient civil infrastructure.

Subject of Research: Experimental and numerical investigation of the bending behavior, failure modes, and design parameters of curved GFRP sandwich panels with recycled PET foam cores and internal lattice reinforcement

Article Title: Bending behavior of curved sandwich panels comprising GFRP face sheets and recycled PET foam cores: Experimental investigation and numerical simulation

Article References: Xie, H., Man, J., Zhang, Z., Fang, H., Wang, Z., & He, P. (2026). Bending behavior of curved sandwich panels comprising GFRP face sheets and recycled PET foam cores: Experimental investigation and numerical simulation. Case Studies in Construction Materials, 25, Article e06525. https://doi.org/10.1016/j.cscm.2026.e06525

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06525

Keywords: GFRP, recycled PET foam, curved sandwich panels, lattice reinforcement, bending behavior, face-core debonding, finite element simulation, Hashin damage criteria, digital image correlation, sustainable construction, foam core density, civil infrastructure

The research team, led by Huan Xu and Juanjuan Ren with colleagues including Yanquan Wu, Wengao Liu, Jun Ye, and Shijie Deng, approached the problem through a staged, progressive optimization strategy. They prepared thirteen concrete mixtures based on the mix design method specified in the Chinese railway standard TB/T 3275, using ordinary Portland cement, Class F fly ash, ground granulated blast-furnace slag, manufactured sand, and natural crushed stone. In the first stage, polyvinyl alcohol fiber content varied from 0.05 to 0.5 percent by volume; in the second, basalt fiber was added at 0.1 to 0.4 percent on top of the best PVA dosage; in the third, nano-silica was introduced at 1 to 4 percent by mass of cementitious material. This incremental design allowed the researchers to trace the incremental contribution of each component, moving from single-fiber to hybrid-fiber to ternary fiber-and-nanoparticle systems.

Each fiber brings distinct strengths and trade-offs. Polyvinyl alcohol fibers are lightweight, non-conductive, and economical, with an elastic modulus of 40 gigapascals and a tensile strength of 1,830 megapascals, making them compatible with the track circuits that steel fibers would interfere with. Basalt fibers are stiffer still, with an elastic modulus of 87.2 gigapascals and tensile strength of 2,180 megapascals, offering superior load transfer across cracks, though excessive single-fiber use can increase brittleness. Nano-silica, with a specific surface area of 300 square meters per gram and an average particle size near 20 nanometers, acts at an entirely different scale: it physically fills capillary pores, accelerates cement hydration through its pozzolanic activity, and reacts with calcium hydroxide to generate additional calcium-silicate-hydrate gel, densifying the matrix and improving fiber-matrix interfacial bonding.

The first challenge was workability, and here the trade-offs became apparent. All thirteen mixtures satisfied the standard’s slump requirement of no more than 200 millimeters, but fiber and nanoparticle additions steeply reduced fluidity. Increasing PVA fiber content from 0.05 to 0.5 percent cut slump by between 16.1 and 58.7 percent relative to the reference mix, largely because the hydrophilic hydroxyl groups on PVA absorb free water and the flexible fibers intertwine during mixing. Basalt fiber partially mitigated the loss, with one hybrid mixture showing a slump 34.4 percent higher than its PVA-only counterpart, likely because basalt’s lower hygroscopicity reduces PVA agglomeration. Nano-silica caused the steepest decline, with the 3 percent nano-silica mixture reaching only 62 millimeters of slump, a consequence of its enormous surface area adsorbing free water and superplasticizer molecules.

Mechanical testing after 28 days of standard curing revealed a consistent pattern: properties rose and then fell with increasing dosage, reflecting an optimum beyond which fiber agglomeration creates weak zones. The best ternary mixture, designated P2B1N3 with 0.2 percent PVA, 0.1 percent basalt fiber, and 3 percent nano-silica, achieved a cube compressive strength of 60.5 megapascals, a splitting tensile strength of 5.2 megapascals, and a flexural tensile strength of 7.0 megapascals, representing gains of 33.3, 30.7, and 52.2 percent respectively over the unmodified reference concrete. The flexural improvement was particularly striking, nearly 6.5 times the gain achieved by PVA fiber alone, underscoring how nanoparticle-driven matrix densification multiplies the effectiveness of hybrid-fiber bridging.

The heart of the study lay in fracture mechanics. Notched three-point bending beams were tested under closed-loop displacement control on an MTS 810 system, while a 2D digital image correlation system tracked crack initiation and propagation on the specimen surface. The load versus crack mouth opening displacement curves displayed three classic stages: linear elastic deformation, stable crack propagation, and unstable failure. The crack initiation load of the ternary mixture reached 5.11 kilonewtons, 40.4 percent above the reference, and its peak load reached 7.8 kilonewtons, a 22.3 percent increase. Applying the double-K fracture criterion, the researchers found that crack initiation fracture toughness rose by 36.7 percent and unstable fracture toughness by a remarkable 75 percent compared with the reference concrete, while fracture energy determined by the RILEM work-of-fracture method climbed 77.6 percent and the ductility index improved 45.4 percent.

Digital image correlation revealed how fundamentally the modified mixtures changed crack behavior. The unmodified reference specimen showed textbook brittle fracture: its crack length leapt from 3.29 millimeters before peak load to 43.48 millimeters at peak, and the specimen failed with a through crack nearly perpendicular to the loading point. In contrast, the hybrid-fiber mixtures displayed distributed strain fields, delayed strain localization, and crack tip opening displacements that grew far more gradually. At the peak load stage, the ternary mixture’s crack length was 28.93 millimeters, and horizontal strains near the notch tip spread over a broad region rather than concentrating along a single line, evidence that fibers and nanoparticles were sharing the burden of resisting deformation across multiple scales.

X-ray computed tomography provided the mesoscopic explanation. Scanning cylindrical core samples at a voxel size of approximately 45 micrometers, the team found that PVA fiber alone actually increased volumetric porosity, from 1.49 percent in the reference to 2.04 percent, because fiber incorporation entrains air and weakens local matrix-aggregate contact. Adding basalt fiber brought porosity back down to 1.45 percent, and the ternary mixture achieved the lowest porosity of all, 1.40 percent, along with the smallest coarse-pore fraction: only 3.21 percent of pores exceeded 500 micrometers and 0.91 percent exceeded 1,000 micrometers, compared with 5.00 and 1.01 percent in the reference. Layer-by-layer analysis showed the ternary specimen had the most uniform pore distribution with no high-porosity weak layers, and a normalized multi-indicator comparison confirmed an inverse association between coarse-pore content and mechanical-fracture performance.

The authors are candid about the limits of their work. The study examined 28-day laboratory-scale specimens only, used one CT specimen per representative mixture, and did not include complete single- and binary-component control groups, so the independent and interaction effects of the three components could not be statistically separated. Mechanisms such as interfacial transition zone refinement and improved fiber-matrix bonding are inferred from prior literature rather than directly characterized. Future work will need to address early-age shrinkage and thermal cracking, freeze-thaw resistance, sulfate attack, coupled environmental-loading conditions, cost-effectiveness, and ultimately full-scale track slab tests. Nevertheless, the consistency across macroscopic fracture parameters, DIC-based crack evolution, and CT-derived pore structure provides a compelling, cross-scale case that the PF-BF-NS ternary system, with stage-dependent and functionally complementary roles, offers a robust materials-level foundation for optimizing the durability and safety of the concrete slabs that carry high-speed trains across China and beyond.

Subject of Research: Fracture performance and toughening of nano-silica-modified hybrid fiber reinforced track slab concrete for high-speed railway ballastless tracks

Article Title: Fracture performance of nano-modified hybrid fiber reinforced concrete for strengthening and toughening of high-speed railway track slab

Article References: Xu, H., Ren, J., Wu, Y., Liu, W., Ye, J., & Deng, S. (2026). Fracture performance of nano-modified hybrid fiber reinforced concrete for strengthening and toughening of high-speed railway track slab. Case Studies in Construction Materials, 25, Article e06524. https://doi.org/10.1016/j.cscm.2026.e06524

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06524

Keywords: track slab concrete, hybrid fiber reinforcement, polyvinyl alcohol fiber, basalt fiber, nano-silica, fracture toughness, double-K fracture criterion, digital image correlation, X-ray computed tomography, fracture energy, crack propagation, high-speed railway

The motivation is stark. Composite materials such as carbon fibre reinforced plastics and glass fibre reinforced plastics dominate industries from aviation to wind turbine manufacturing because of their exceptional strength-to-weight ratios, yet their end-of-life story is grim. Recycling these composites is expensive and energy-intensive, and enormous volumes of decommissioned wind turbine blades already pile up in landfills worldwide. Even composites made with natural fibres typically fall short of true sustainability because their binding matrices are synthetic polymers that resist degradation. The Mauritian team set out to close that gap by making both components—the fibre and the matrix—fully biodegradable, drawing on a locally abundant plant species that also offers carbon-offsetting benefits while it grows.

The path from leaf to composite began with mechanical extraction. Fibres were harvested from Pandanus utilis leaves, stripped of their cuticle and epidermal layers using a conventional fibre decorticator, and oven dried at 60 degrees Celsius for 24 hours. The researchers then applied mercerization, an alkali treatment with sodium hydroxide at concentrations of 2.5 and 3.0 percent, to prepare the fibre surfaces for bonding with the PLA matrix. Fourier transform infrared spectroscopy confirmed that the treatment worked at the molecular level: characteristic peaks associated with lignin and hemicellulose—those at roughly 1239 and 1730 wavenumbers—flattened noticeably after treatment, while peaks tied to adsorbed water at 1640 and 3400 wavenumbers also diminished. In practical terms, the alkali bath dissolved much of the lignin and hemicellulose that interferes with adhesion, enriched the fibre in cellulose, and reduced its tendency to draw in moisture.

Composite specimens were fabricated by hand lay-up in aluminium-faced moulds, with chopped fibres of 4, 5 and 6 centimetres randomly arranged between two layers of PLA filament, then melted in an oven at 250 degrees Celsius for one hour, compressed, and cooled gradually to prevent cracking. Fibre loadings of 5, 10 and 15 percent by weight were tested against the pure polymer. Differential scanning calorimetry showed textbook PLA behaviour: a glass transition between roughly 50 and 70 degrees Celsius, crystallization peaks near 120 degrees, melting at 171.2 degrees, and thermal decomposition onset around 275 degrees—evidence that the reinforced material remains thermally stable across ordinary service conditions.

The physical tests revealed a familiar trade-off in biocomposites. Water absorption, measured over a 24-hour immersion following the ASTM D570-98 standard, was negligible for pure PLA but climbed to between 2 and 2.75 percent in the composites, rising consistently with fibre content—a statistically significant effect driven by the hydrophilic nature of natural fibres and by microscopic voids at imperfect fibre-matrix interfaces. Fibre length, by contrast, made no statistical difference. The soil burial test, in which specimens spent 30 days in open soil teeming with aerobic bacteria, told a similar story: specimens with 15 percent fibre lost up to 1.6 percent of their mass, compared with only 0.25 percent for plain PLA, confirming that the material genuinely degrades in a biological environment rather than merely fragmenting.

Mechanically, the sweet spot was unambiguous. Both tensile and flexural performance peaked at a fibre loading of 10 percent by weight, where stress distributes more evenly through the structure. The best flexural result—around 270 newtons of load capacity—came from a specimen with 4-centimetre fibres at 10 percent loading, more than double the 110 newtons that plain PLA could bear. Beyond that optimum, at 15 percent fibre content, performance dropped sharply as fibre-to-fibre crowding reduced matrix dispersion and left insufficient adhesive contact, generating stress concentrations and defects. Analysis of variance confirmed that fibre content, though not fibre length, significantly influenced the strength of the unexposed specimens.

The study’s most sobering finding concerns marine conditions, a critical consideration for a material intended for maritime applications. When a full set of reinforced specimens was submerged in seawater for 30 days before tensile testing, the pattern of results inverted: strength now fell with increasing fibre content, dropping from a high of 1400 newtons at 5 percent fibre to a low of 200 newtons at 15 percent. The researchers attribute this to capillary water penetration that progressively undermined the fibre-matrix interface, compounded by the slow hydrolytic degradation of the PLA matrix itself—a reminder that biodegradability, the material’s central virtue, is also its principal vulnerability in wet service environments.

To see failure coming before it happened, the team turned to digital image correlation, a contactless optical technique that tracks a speckled pattern on the specimen surface through a calibrated camera system during tensile loading. The resulting von Mises strain maps revealed localized hot spots of concentrated strain that reliably predicted where each specimen would ultimately fracture in a brittle mode. These hot spots traced back to manufacturing imperfections—randomized void formation, incomplete fibre-matrix adhesion and minor misalignments of the fibres—demonstrating how internal defects, invisible to the naked eye, orchestrate the failure of a composite long before its average material properties would suggest.

Finally, the researchers built a finite element model of the dog-bone tensile specimen in LS-DYNA, meshing it into nearly 75,000 elements with longitudinally aligned fibre bundles and boundary conditions mirroring the physical test. The simulation showed maximum tensile forces of 1400 newtons for the pristine composite and 600 newtons for the seawater-exposed model—at a displacement of 1.5 millimetres, figures that closely matched the experimental data for equivalent specimens. Crucially, the model confirmed that sea exposure cuts the material’s tensile load-bearing capacity roughly in half. With manufacturing defects addressed and fibre loading optimized near 10 percent, the authors conclude, Pandanus-based composites could offer a genuinely sustainable, high strength-to-weight alternative for everyday structural applications—crafted from a tree that grows, quite literally, along the shoreline where these materials may one day serve.

Subject of Research: Development and characterization of biodegradable Pandanus utilis fibre-reinforced PLA composites

Article Title: Biodegradable Pandanus Utilis fibre-reinforced PLA composites: characterization, mechanical behaviour, and fracture analysis

Article References: Dabee, C., Casarejos, E., & Ramful, R. (2026). Biodegradable Pandanus Utilis fibre-reinforced PLA composites: characterization, mechanical behaviour, and fracture analysis. Journal of Materials Science: Polymers, 1(1), Article 17. https://doi.org/10.1007/s44493-026-00019-0

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00019-0

Keywords: Pandanus utilis, PLA, biodegradable composites, natural fibre composites, mercerization, tensile strength, flexural strength, water absorption, soil degradation, digital image correlation, finite element analysis, seawater exposure