Finite element analysis of plasticity - induced fatigue crack closure using contact elements

To predict crack opening stresses, contact elements are incorporated into the elastic-plastic finite element analysis of fatigue crack closure. A code using ANSYS Parametric Design Language (APDL) is developed. In spite of the fact that the use of contact elements in modelling crack surface contact and crack closure is inherently natural, efforts to incorporate them in the finite element analysis of crack closure are not widespread. The traditional method of modelling crack closure is based on placing truss elements at the crack surface nodes. In the present research, contact elements are used to model crack surface contact. While the load is applied incrementally, crack opening stress is determined by monitoring the state of the contact elements. The results of two-dimensional plane strain finite element analyses are in good agreement with previous work reported in the literature. Instead of finding crack opening stress at every load cycle, an algorithm to find crack opening stress at predetermined load cycle intervals is developed. With the developed algorithm it was possible to analyze crack closure behaviour during a larger number of load cycles with less execution time. The algorithm that is implemented is parametrically analysed. The effect of load increment is investigated. Since crack opening stress is not evaluated at every cycle, the effect of how often opening stresses are determined is another issue that is investigated. As a result of the simulation with a relatively high number of cycles, it was possible to observe the final stabilization in the crack opening stress values that follows a decay after the initial plateau

Drag force and jet propulsion investigation of a swimming squid

Gökçen, Gökhan (Dogus Author)In this study, CAD model of a squid was obtained by taking computer tomography images of a real squid. The model later placed into a computational domain to calculate drag force and performance of jet propulsion. The drag study was performed on the CAD model so that drag force subjected to real squid was revealed at squid's different swimming speeds and comparison has been made with other underwater creatures (e.g., a dolphin, sea lion and penguin). The drag coefficient (referenced to total wetted surface area) of squid is 0.0042 at Reynolds number 1.6x106 that is a %4.5 difference from Gentoo penguin. Besides, jet flow of squid was simulated to observe the flow region generated in the 2D domain utilizing dynamic mesh method to mimic the movement of squid's mantle cavity

Characterization of the damage mechanism of composites against low velocity ballistic impact using computed tomography (CT) techniques

Baykara, Tarık (Dogus Author)Low velocity impact damages are particularly considered as risky and dangerous since the detection of the damage from the surface is almost impossible for extensive evaluation. The mechanisms of penetration and damage for E-glass reinforced polyester resin composite which was hit by a low velocity projectile has been investigated using computed tomography (CT) scanning techniques. Laminated, 11mm thick plates with [0°/90°] ply architecture with 18 layers were impacted by a 9x19mm FMJ Round Nose 8g bullet with a speed of 380m/s. Since the low velocity impact of the bullet is well below the ballistic limit only a partial penetration has occurred and the kinetic energy is fully absorbed following some local matrix cracking, fiber fractures, delamination and friction-erosion. Computed Tomography (CT) technique is found to be a powerful technique that the full extent of the damage progression in 3-D can be visualized through the interfaces of the composite laminates

Kowa Üç Boyutlu Non-Midriyatik Retinal Kamera ve Heidelberg Retina Tomografi si III ile Ölçülen Optik Sinir Başı Parametrelerinin Sağlıklı Kişilerde Karşılaştırması

Purpose: To compare the optic nerve head (ONH) parameters obtained by KOWA three dimensional (3D) non-mydriatic stereoscopic retinal camera (NMRC) and Heidelberg retinal tomograph III (HRT) in healthy subjects. Materials and methods: In this study, 72 eyes of 36 healthy subjects were enrolled. Kowa 3D NMRC and HRT III examinations were performed to each subject on the same day. Nine ONH parameters, including vertical cup disc ratio, cup area, disc area, rim area, height variation contour, cup volume, rim volume, maximum cup depth, and mean cup depth values of both devices were compared and correlated. Results: Kowa 3D NMRC and HRT revealed similar values for vertical cup disc ratio, cup volume, and rim volume (p>0.05). However Kowa 3D NMRC signifi cantly overestimated the HRT determined cup area, disc area, height variation contour, and rim area (p0.05). Conclusion: Most of ONH parameters including vertical cup disc ratio are similar when measured by Kowa 3D NMRC and HRT III. Kowa 3D NMRC is a potential tool for monitoring ONH parameters.Amaç: Kowa üç boyutlu (3D) non-midriyatik retinal kamera (NMRK) ve Heidelberg retinal tomografi III (HRT) ile elde edilen optik sinir başı (OSB) parametrelerinin sağlıklı kişilerde karşılaştırılması. Gereç ve yöntemler: Bu çalışmaya 36 sağlıklı bireyin 72 gözü alındı. Kowa 3D NMRK ve HRT III ölçümleri her kişiye aynı günde yapıldı. Her iki cihazın vertikal çukurluk disk oranı, çukurluk alanı, disk alanı, rim alanı, yükseklik değişkenlik kontürü, çukurluk hacmi, rim hacmi, maksimum çukurluk derinliği ve ortalama çukurluk derinliği değerlerini içeren 9 OSB parametresi kıyaslandı ve korelasyonlarına bakıldı. Bulgular: Kowa 3D NMRK ve HRT vertikal çukurluk disk oranı, çukurluk hacmi ve rim hacmi için benzer değerler bulundu (p>0.05). Bununla birlikte HRT tarafından belirlenen çukurluk alanı, disk alanı, yükseklik değişkenlik kontürü ve rim alanı Kowa 3D NMRK ile daha yüksek ölçüldü. Heidelberg retina tomografi si ile belirlenen ortalama çukurluk derinliği ve maksimum çukurluk derinliği Kowa 3D NMRK ile daha düşük saptandı. İki cihaz arasında vertikal çukurluk disk oranı, çukurluk alanı, disk alanı, çukurluk hacmi, maksimum çukurluk derinliği ve ortalama çukurluk derinliği için istatistiksel olarak anlamlı, yüksek seviyeli korelasyon bulundu (r=0.66-0.83, p0.05). Sonuç: Vertikal çukurluk disk oranını da içeren çoğu OSB parametresi Kowa 3D NMRK ve HRT III ile ölçüldüğünde benzerdir. Kowa 3D NMRK, OSB parametrelerinin monitörizasyonunda potansiyel bir cihazdır