Bekir Aktaş

Resonance-induced center section cracking is a critical failure mechanism in conventional mono-block concrete railway sleepers. Traditional designs frequently utilize mid-span cross-sectional "necking" to achieve static material economy; however, this geometric transition acts as a root cause for dynamic fatigue and structural collapse under high-frequency cyclic track loading. To address this vulnerability and mitigate resonance-induced center section failures, this study proposes a proactive preventive action by developing novel sleeper topologies based on Modal Topology Optimization (MTO). To strictly isolate the influence of geometric topology on dynamic response, a triaxial experimental evaluation was conducted on 18 distinct geometries using 1:10 scaled semi-functional cementitious prototypes fabricated via indirect additive manufacturing (3D-printed molds). Multi-axial dynamic characterization was executed using Experimental Modal Analysis (EMA) and contactless Vibro-Acoustic Modal Testing (VMT), comparing 14 innovative topologies against four conventional baselines (B58, B70, B07, B320). The findings demonstrate that optimizing stiffness and modal mass distribution elevates the triaxial resonance frequencies, shifting the resonance modes away from the dominant operational excitation band. Notably, the C-23-230 model, featuring a convex mid-section and short-length topology, elevated the vertical fundamental resonance frequency, enhanced the vertical modal damping ratio, and suppressed the vertical Complex Mode Indicator Function (CMIF) peak amplitude by up to 87%, 71%, and 68% compared to conventional baselines, respectively. By systematically shifting resonance bands away from dominant operational excitation zones and minimizing dynamic energy amplification, the proposed stiffness-modal mass optimization mitigates the risk of structural fatigue, potentially extending the operational service life of railway infrastructure.

The lateral stability of ballasted railway tracks is strongly influenced by ballast bed geometry, particularly in curved sections subjected to significant lateral forces. This study investigates the relationship between ballast profile geometry and the global lateral resonance (GLR) frequency using an integrated framework that combines three-dimensional ballast profiling with experimental and operational modal analyses. Field investigations were conducted on eight curved track sections comprising forty track segments, deliberately selected to represent a wide range of ballast topology configurations. Lateral dynamic characteristics were identified using Experimental Modal Analysis (EMA) with controlled lateral hammer excitation and train-induced Operational Modal Analysis (OMA). In parallel, ballast profiles were digitized using a LiDAR–GNSS-based three-dimensional scanning workflow. The resulting point cloud data were processed using a custom Python-based algorithm to automatically extract ballast topology parameters. Regression-based analyses revealed statistically meaningful relationships between ballast topology parameters and GLR frequency for both EMA- and OMA-derived results, with coefficients of determination of approximately R² = 0.70 under heterogeneous field conditions. To address residual variability and capture regime-dependent behavior, a clustering-based modeling approach was introduced, significantly improving agreement with GLR frequencies (R² = 0.97). The combined analyses indicate that track segments exhibiting relatively weak lateral resistance are generally associated with low compaction density, disproportionate increases in total ballast width leading to dominant dead-mass effects, and insufficient outer shoulder width to provide effective lateral confinement. Different clusters were observed within the same geometric curve, indicating locally varying ballast topology and dynamic response. Although the scanning captures only ballast geometry, the identified GLR frequencies inherently reflect the combined influence of geometry and unmeasured condition-related factors, providing an integrated basis for assessing lateral resistance under field conditions.

Bu çalışmada, rijitlik-kütle dağılımını iyileştirmeyi amaçlayan, orta kesitte genişleyen bir profile sahip ve iki adet düşey altıgen boşluk içeren yeni bir demiryolu traversi geometrisi incelenmiştir. Referans B320 tipi travers ile yaklaşık eşit hacim sağlayacak şekilde tasarlanan O-20-230 prototipleri, aynı üç boyutlu yazdırma parametreleri kullanılarak 1:10 ölçekle üretilmiş ve birbirine yakın kütle değerleri elde edilmiştir. Deneysel modal analizlerle frekans yanıt fonksiyonları (FRF) elde edilmiş, modal parametreler Karmaşık Mod Gösterge Fonksiyonu (Complex Mode Indicator Function, CMIF) analizi ile desteklenen En Küçük Kareler Karmaşık Frekans Düzlemi (Least Squares Complex Frequency-domain, LSCF) yöntemi kullanılarak belirlenmiştir. Sonuçlar, O-20-230 geometrisinin temel düşey rezonans frekansını yaklaşık %37 artırdığını, modal sönümleme oranının yaklaşık %43 yükseldiğini ve orta kesitte ölçülen FRF pik genliğinin yaklaşık %45 azaldığını göstermektedir. Global ölçekte CMIF pik değerinin yaklaşık %12 azalması, iyileşmenin özellikle kritik orta kesitte yoğunlaştığını ve yapı genelinde kontrollü bir modal etkinlik azalması sağlandığını ortaya koymaktadır. Elde edilen dinamik sonuçlar mutlak saha performansını temsil eden değerler olarak değil, aynı ölçek ve malzeme koşulları altında topolojik farklılıkların göreli etkisini gösteren karşılaştırmalı göstergeler olarak değerlendirilmelidir. Bulgular, kütle ve hacim büyük ölçüde korunmasına rağmen geometrik tasarım yoluyla optimize edilen rijitlik-kütle dağılımının rezonans frekanslarını baskın uyarım bantlarından uzaklaştırma ve yapısal sönümleme verimliliğini artırma potansiyeline sahip olduğunu göstermektedir.

Understanding how asphalt concrete (AC) responds to temperature variations is vital for predicting pavement performance and ensuring long‐term durability. Conventional methods are often time‐consuming and require bulky equipment. As a nondestructive alternative, experimental modal analysis (EMA) offers a promising approach. However, EMA applications remain limited—particularly for cylindrical specimens commonly used in laboratory compaction or obtained via field coring. These geometries pose challenges due to high material damping, AC’s heterogeneous internal structure and complex symmetric/asymmetric mode shapes that complicate interpretation and demand careful selection of excitation and response points. This study introduces a simplified yet robust setup that isolates a distinct, temperature‐sensitive mode shape on halved Marshall specimens. EMA was performed from −10 to + 60°C in 5°C increments, using high‐resolution frequency response function (FRF) and coherence measurements. The results revealed a thermally driven softening response, marked by a ~ 28% reduction in average resonance frequency, pronounced damping growth, and spectral broadening, all reflecting the viscoelastic degradation of stiffness and energy retention capacity in AC. These findings confirm that when properly configured, EMA offers a practical and reliable nondestructive method for evaluating temperature‐driven stiffness degradation in AC, directly applicable to pavement design, monitoring, and performance assessment.

Composite railway sleepers are increasingly adopted as durable, low-maintenance, and sustainable alternatives to conventional materials, offering enhanced long-term performance and adaptability under demanding service conditions. In parallel, 3D printing has become an innovative and cost-effective research tool for developing scaled railway components. This study investigates, for the first time, the dynamic behavior of scaled, semi-functional P90-type composite railway sleeper prototypes fabricated by additive manufacturing. The 1:13.33-scale ABS specimens were tested using Experimental Modal Analysis (EMA) in vertical, lateral, and longitudinal orientations under free-free boundary conditions. A detailed Finite Element Model (FEM) was developed in ANSYS, incorporating measured unit weight, accelerometer mass, and elastic support stiffness. Iterative back-analysis was performed to align FEM predictions with EMA-measured fundamental resonance frequencies, yielding excellent correlation at E = 2595 MPa and G = 1120 MPa, with a maximum individual mode deviation of 0.959 % and a total three-mode deviation of 1.601 %. The validated FEM demonstrates partial similitude, a principle long applied in aerospace and mechanical engineering to achieve dynamically consistent results using scaled prototypes. Overall, this study establishes a preliminary methodological foundation for future modal topology optimization (MTO) and full-scale correlation studies, serving as a case-study framework for extending similitude-based dynamic characterization of railway sleeper systems. The proposed methodology can also be broadly applied to investigate and enhance the dynamic behavior of structural components in civil, mechanical, and materials engineering.

Bu çalışmada, taşınabilir cihazlar aracılığıyla demiryolu hatlarında kurp yarıçapının belirlenmesine yönelik düşük maliyetli ve pratik bir yöntem geliştirilmiştir. GNSS ve jiroskop verileri, mobil cihaz sensörlerinden zaman senkronlu olarak toplanmış ve Python ortamında analiz edilmiştir. Ölçümler, Adana Belemedik–Pozantı hattında bir drezin ve bir yolcu treni kullanılarak gerçekleştirilmiştir. Zaman uyumsuzluğu nedeniyle GNSS ve açısal hız verileri interpolasyon yoluyla senkronize edilmiş; Z eksenine ait açısal hız ve hız verileri kullanılarak kurp yarıçapları hesaplanmıştır. Veriler, farklı alçak geçiren filtre (AG) kesim frekanslarıyla işlenmiş ve bu parametrelerin ölçüm doğruluğu üzerindeki etkileri değerlendirilmiştir. En uygun sonuçlar, drezin için 0,2 Hz ve yolcu treni için 0,4 Hz frekanslarında elde edilmiştir. Yirmi farklı kurpta yapılan analizlerde, referans ölçümlere göre ortalama sapma drezin için %2,6; yolcu treni için %2,2 olarak hesaplanmıştır. En yüksek sapma %12,5'e ulaşırken, bazı kurplarda sapma %0,01 düzeyine kadar düşmüştür. Geçiş eğrisi ve gelişim uzunluğu sınırlı olan kurplarda, mobil cihaz verileri ile referans ölçümler arasında sapmaların nispeten arttığı; geçiş sürekliliği bulunan ve daha uzun geometrik gelişime sahip kurplarda ise daha yüksek düzeyde örtüşme sağlandığı gözlenmiştir. Sonuçlar, önerilen yöntemin demiryolu hat geometrisinin gerçek zamanlı izlenmesinde düşük maliyetli, erişilebilir ve uygulanabilir bir teknik çözüm sunduğunu ortaya koymaktadır.

Enhancing both the dynamic damping and static strength of ordinary Portland cement (OPC) concrete simultaneously is a significant challenge. Geopolymer concrete (GPC), particularly fly ash-based GPC, offers a promising alternative. This study explores the relationship between damping and strength in heat-cured, low-calcium fly ash-based GPC using sodium silicate (SS) and sodium hydroxide (SH) activators. The findings reveal that SS activators demonstrate stronger positive correlations between damping and strength compared to SH activators. Microstructural analysis indicated that increasing SS dosage from 55 kg/m³ to 98 kg/m³ resulted in a 17% increase in dynamic damping ratios and a 39% increase in static compressive strength. These results highlight the potential of GPC to surpass OPC concrete in applications requiring both enhanced damping and strength, offering a dual benefit not typically achievable with OPC. The study contributes to a deeper understanding of GPC's capabilities, paving the way for its broader adoption in construction projects.