Dynamic force measurement techniques Split Hopkinson Pressure Bar testing of low acoustic impedance materials used as armor interlayer materials

Thesis (Master)--Izmir Institute of Technology, Mechanical Engineering, Izmir, 2012Includes bibliographical references (leaves: 50-52)Text in English; Abstract: Turkish and Englishx, 52 leavesGoreTM PolarchipTM heat insulating Teflon and Dow ChemicalsTM Voracor CS Polyurethane were characterized in this study by conducting compression tests at various strain rates. Quasi-static compression tests were done with a Shimadzu AG-X conventional test machine while two different modified Split Hopkinson Pressure Bar (SHPB) systems were used for dynamic compression tests. Since dynamic testing of soft materials with classical SHPB is problematic due to low signal levels and relatively higher signal to noise ratio, impact end of transmitter bar was modified with insertion of piezoelectric force transducers through the SHPB tests of Teflon, thus enabling the direct measurement of force on specimen. High strain tests of Polyurethane involved oscillations in both incident and transmitter bar signals. To overcome this, EPDM rubber pulse shaper was used through the SHPB tests of Polyurethane. Experimental results were used in numerical study as material model parameters and SHPB tests of both materials were simulated in LS-DYNA. Experimental study concluded strong strain rate dependency in both Teflon and Polyurethane, depicting an increase in maximum stress with the increase in strain rate. Numerical study showed a good correlation with experiments in terms of bar stresses and damage behavior of specimens, offering a solution to more complex problems that can be encountered in future studies

January 1958

My dear Brother: It does not require a great stretch of the imagination for me to hear you mutter: Didn\u27t I get one of these yellow sheets only a month ago? Why doesn\u27t he get back to work instead of cluttering up my mail? Well, thereby hangs a tale. A few weeks ago I spent an illuminating, and somewhat dismaying, evening with brother Theophilus in the Middle West. He has a good congregation of about five hundred members. It is an average parish, with the usual complement of saints and sinners, with the lines between them shifting back and forth. Theophilus has worked hard and the Lord of the Church has blessed his hands and his voice

Tekrarlı yarımküre çekirdekli sandviç yapıların penetrasyon davranışı: Deneysel ve sayısal bir çalışma

Thesis (Doctoral)--Izmir Institute of Technology, Mechanical Engineering, Izmir, 2018Includes bibliographical references (leaves: 109-114)Text in English; Abstract: Turkish and EnglishIn this study, penetration behavior of novel core structure consisting hemispherical and cylindrical parts was investigated. Core units were manufactured with deep drawing method in two thicknesses to have monolithic form without any sort of assembly method or element. Produced specimens were then subjected to penetration tests at low and intermediate velocities against blunt, conical and hemispherical tipped indenters using special fixtures and apparatuses on conventional testing equipment. Effect of heat treatment on penetration behavior was investigated to observe whether residual stresses arise from manufacturing process changes the penetration behavior. Confinement effects were studied experimentally with a special fixture, allowing tested specimen to be radially confined with other core units as in an armor structure. Finally, experimental work was finished by conducting a case study where core units were subjected to spherical projectile impact up to impact velocities of 180 m.s-1 in a composite sandwich structure. Results show that each indenter geometry showed unique deformation characteristics in testing of both core units and both of the core geometries were able to hold a steel spherical projectile with mass of 110 g without full perforation at impact velocity of 180 m.s-1. Details of experimental results were presented in each chapter. Study also included modeling parts where core units were numerically produced with residual stresses and strains and good correlation was noted where thickness was compared with actual measurements on core units. Test conducted on single core structure in as-received and heat-treated condition were also repeated in numerical environment, where numerical study exhibited good correlation on both forcedisplacement curves and deformation of core units with tests. Correlation achieved with experimental study has led into further investigations of strain rate and micro-inertia where behavior of core units was studied at numerical impact velocities of 300 m.s-1. Results show that both strain rate and micro-inertia increase the local maximums and average of force levels. Effect of strain rate and micro-inertia is clearly distinguished for a threshold displacement level where micro-inertia is further dominant on behavior.Bu çalışmada, yarıküre ve silindirik kısımlardan oluşan yeni bir çekirdek yapının delinme davranışı incelenmiştir. Çekirdek yapılar monolitik yapıda olup herhangi bir montaj yöntemi ya da elemanı olmadan derin çekme metodu ile iki farklı kalınlıkta üretilmiştir. Üretilen numuneler daha sonra düz, konik ve yarıküresel uçlu delicilerle konvansiyonel test ekipmanları ve özel fikstürler kullanılarak düşük ve orta hızlarda delinme testlerine maruz bırakılmıştır. Üretim metodundan gelen kalıntı gerilme ve birim şekil değştirmelerin delinme davranışına etkilerinin olup olmadığı da ısıl işlemin delinme davranışına etkileri ile araştırılmıştır. Sınırlandırma şartlarının delinme davranışına etkisi de özel bir fikstür yardımıyla deneysel olarak incelenmiş ve test edilen numunenin bir zırh yapısındaki gibi çekirdek yapılarla etrafının radyal olarak sarılması sağlanmıştır. Son olarak deneysel çalışma bir vaka çalışması ile tamamlanmış ve çekirdek yapılar kompozit bir sandviç yapı içerisinde 180 m.s-1 çarpma hızlarına kadar küresel projektil çarpmasına maruz bırakılmıştır. Sonuçlar her delici geometrisinin her çekirdek yapı için kendine has deformasyon karakteristiğini gösterdiğini ve iki çekirdek geometrisinin de 110 g ağırlıklı küresel projektili 180 m.s-1 çarpma hızlarına kadar tam delinme olmaksızın tutabildiğini göstermiştir. Deneysel sonuçlar her bölümde sunulmuştur. Çalışma ayrıca modelleme çalışmalarında da çekirdek yapıların kalıntı gerilme ve birim şekil değiştirmeler ile nümerik olarak modellenmesini içermiş, kalınlık değerlerinde gerçek ölçümlerle uyum yakalanmıştır. Üretildiği gibi ve ısıl işlemli tekli çekirdek yapıda tamamlanan testler nümerik ortamda da tekrarlanmış, nümerik sonuçlarda hem kuvvet-yer değiştirme hem de hasar şekillerinde deneysel çalışmayla uyum görülmüştür. Elde edilen bu uyum saesinde ileri araştırmalarla delinme davranışıdaki birim şekil değiştirme ve mikro-atalet etkileri de 300 m.s-1 nümerik çarpma hızlarında incelenmiştir. Sonuçlar hem birim şekil değiştirme hızının hem de mikro ataletin lokal ve ortalama kuvvet değerlerini arttırdığını ortaya koymuştur. Birim şekil değiştirme hızı ve mikro-atalet etkilerinin farkı sınır bir yer değiştirme değerinden sonra ayrıştırılır hale gelmiş ve mikro-atalet etkilerinin baskın olduğu görülmüştür.TUBITAK for the grant number 112M141 and IYTE for grant number 2013IYTE1

Tekrarlı yarımküre çekirdekli sandviç yapıların penetrasyon davranışı: Deneysel ve sayısal bir çalışma

Thesis (Doctoral)--Izmir Institute of Technology, Mechanical Engineering, Izmir, 2018Includes bibliographical references (leaves: 109-114)Text in English; Abstract: Turkish and EnglishIn this study, penetration behavior of novel core structure consisting hemispherical and cylindrical parts was investigated. Core units were manufactured with deep drawing method in two thicknesses to have monolithic form without any sort of assembly method or element. Produced specimens were then subjected to penetration tests at low and intermediate velocities against blunt, conical and hemispherical tipped indenters using special fixtures and apparatuses on conventional testing equipment. Effect of heat treatment on penetration behavior was investigated to observe whether residual stresses arise from manufacturing process changes the penetration behavior. Confinement effects were studied experimentally with a special fixture, allowing tested specimen to be radially confined with other core units as in an armor structure. Finally, experimental work was finished by conducting a case study where core units were subjected to spherical projectile impact up to impact velocities of 180 m.s-1 in a composite sandwich structure. Results show that each indenter geometry showed unique deformation characteristics in testing of both core units and both of the core geometries were able to hold a steel spherical projectile with mass of 110 g without full perforation at impact velocity of 180 m.s-1. Details of experimental results were presented in each chapter. Study also included modeling parts where core units were numerically produced with residual stresses and strains and good correlation was noted where thickness was compared with actual measurements on core units. Test conducted on single core structure in as-received and heat-treated condition were also repeated in numerical environment, where numerical study exhibited good correlation on both forcedisplacement curves and deformation of core units with tests. Correlation achieved with experimental study has led into further investigations of strain rate and micro-inertia where behavior of core units was studied at numerical impact velocities of 300 m.s-1. Results show that both strain rate and micro-inertia increase the local maximums and average of force levels. Effect of strain rate and micro-inertia is clearly distinguished for a threshold displacement level where micro-inertia is further dominant on behavior.Bu çalışmada, yarıküre ve silindirik kısımlardan oluşan yeni bir çekirdek yapının delinme davranışı incelenmiştir. Çekirdek yapılar monolitik yapıda olup herhangi bir montaj yöntemi ya da elemanı olmadan derin çekme metodu ile iki farklı kalınlıkta üretilmiştir. Üretilen numuneler daha sonra düz, konik ve yarıküresel uçlu delicilerle konvansiyonel test ekipmanları ve özel fikstürler kullanılarak düşük ve orta hızlarda delinme testlerine maruz bırakılmıştır. Üretim metodundan gelen kalıntı gerilme ve birim şekil değştirmelerin delinme davranışına etkilerinin olup olmadığı da ısıl işlemin delinme davranışına etkileri ile araştırılmıştır. Sınırlandırma şartlarının delinme davranışına etkisi de özel bir fikstür yardımıyla deneysel olarak incelenmiş ve test edilen numunenin bir zırh yapısındaki gibi çekirdek yapılarla etrafının radyal olarak sarılması sağlanmıştır. Son olarak deneysel çalışma bir vaka çalışması ile tamamlanmış ve çekirdek yapılar kompozit bir sandviç yapı içerisinde 180 m.s-1 çarpma hızlarına kadar küresel projektil çarpmasına maruz bırakılmıştır. Sonuçlar her delici geometrisinin her çekirdek yapı için kendine has deformasyon karakteristiğini gösterdiğini ve iki çekirdek geometrisinin de 110 g ağırlıklı küresel projektili 180 m.s-1 çarpma hızlarına kadar tam delinme olmaksızın tutabildiğini göstermiştir. Deneysel sonuçlar her bölümde sunulmuştur. Çalışma ayrıca modelleme çalışmalarında da çekirdek yapıların kalıntı gerilme ve birim şekil değiştirmeler ile nümerik olarak modellenmesini içermiş, kalınlık değerlerinde gerçek ölçümlerle uyum yakalanmıştır. Üretildiği gibi ve ısıl işlemli tekli çekirdek yapıda tamamlanan testler nümerik ortamda da tekrarlanmış, nümerik sonuçlarda hem kuvvet-yer değiştirme hem de hasar şekillerinde deneysel çalışmayla uyum görülmüştür. Elde edilen bu uyum saesinde ileri araştırmalarla delinme davranışıdaki birim şekil değiştirme ve mikro-atalet etkileri de 300 m.s-1 nümerik çarpma hızlarında incelenmiştir. Sonuçlar hem birim şekil değiştirme hızının hem de mikro ataletin lokal ve ortalama kuvvet değerlerini arttırdığını ortaya koymuştur. Birim şekil değiştirme hızı ve mikro-atalet etkilerinin farkı sınır bir yer değiştirme değerinden sonra ayrıştırılır hale gelmiş ve mikro-atalet etkilerinin baskın olduğu görülmüştür.TUBITAK for the grant number 112M141 and IYTE for grant number 2013IYTE1

Dynamic force measurement techniques Split Hopkinson Pressure Bar testing of low acoustic impedance materials used as armor interlayer materials

Thesis (Master)--Izmir Institute of Technology, Mechanical Engineering, Izmir, 2012Includes bibliographical references (leaves: 50-52)Text in English; Abstract: Turkish and Englishx, 52 leavesGoreTM PolarchipTM heat insulating Teflon and Dow ChemicalsTM Voracor CS Polyurethane were characterized in this study by conducting compression tests at various strain rates. Quasi-static compression tests were done with a Shimadzu AG-X conventional test machine while two different modified Split Hopkinson Pressure Bar (SHPB) systems were used for dynamic compression tests. Since dynamic testing of soft materials with classical SHPB is problematic due to low signal levels and relatively higher signal to noise ratio, impact end of transmitter bar was modified with insertion of piezoelectric force transducers through the SHPB tests of Teflon, thus enabling the direct measurement of force on specimen. High strain tests of Polyurethane involved oscillations in both incident and transmitter bar signals. To overcome this, EPDM rubber pulse shaper was used through the SHPB tests of Polyurethane. Experimental results were used in numerical study as material model parameters and SHPB tests of both materials were simulated in LS-DYNA. Experimental study concluded strong strain rate dependency in both Teflon and Polyurethane, depicting an increase in maximum stress with the increase in strain rate. Numerical study showed a good correlation with experiments in terms of bar stresses and damage behavior of specimens, offering a solution to more complex problems that can be encountered in future studies

The effect of strain rate on the mechanical behavior of Teflon foam

The quasi-static (1 × 10−3, 1 × 10−2 and 1 × 10−1 s−1) and high strain rate (7200 and 9500 s−1) experimental and high strain rate numerical compression deformation of a Gore Polarchip™ CP7003 heat insulating Teflon foam was investigated. High strain rate tests were conducted with the insertion of quartz crystal piezoelectric transducers at the end of the transmitter bar of a compression Split Hopkinson Pressure Bar (SHPB) set-up in order to measure the force at the back face of the specimen. A fully developed numerical model of the SHPB test on Teflon was also implemented using LS-DYNA. The simulation stresses showed close correlations with the experimentally measured stresses on the bars. The developed model successfully simulated the high strain rate loading. The damage initiation and progression of experimental high strain rate tests were further recorded using a high speed camera and found to be very similar to those of the simulation high strain rate tests.TÜBİTAK for the grant # 106M35

Crushing and energy absorption characteristics of combined geometry shells at quasi-static and dynamic strain rates: Experimental and numerical study

The quasi-static and dynamic crushing response and the energy absorption characteristics of combined geometry shells composed of a hemispherical cap and a cylindrical segment were investigated both experimentally and numerically. The inelastic deformation of the shells initiated with the inversion of the hemisphere cap and followed by the axisymmetric or diamond folding of the cylindrical segment depending on the loading rate and dimensions. The fracture of the thinner specimens in dynamic tests was ascribed to the rise of the flow stress to the fracture stress with increasing strain rate. The hemisphere cap absorbed more energy at dynamic rates than at quasi-static rates, while it exhibited lower strain rate and inertia sensitivities than the cylinder segment. For both the hemisphere cap and the cylinder segment, the inertial effect was shown to be more pronounced than strain rate effect at increasing impact velocities. © 2014 Elsevier Ltd.Scientific and Technical Council of Turkey (TUBITAK) (112M141

Dynamic crushing and energy absorption of sandwich structures with combined geometry shell cores

Dynamic crushing and energy absorption characteristics of sandwich structures with combined geometry shell cores were investigated experimentally and numerically. The effect of strain rate on the crushing behavior was presented by the crushing tests at quasi-static, intermediate and high strain rate regimes. It was shown that absorbed energy increased with increasing impact velocity. The effect of confinement on crushing behavior was shown by conducting confined experiments at quasi-static and dynamic rates. Higher buckling loads at lower deformation were observed in confined quasi-static crushing due to additional lateral support and friction provided by confinement wall. By using fictitious numerical models with strain rate insensitive material models, the effect of inertia and strain rate on crushing were shown. It was observed that, increase in impact velocity caused increase in inertial effects and strain rate effects were nearly independent from the impact velocity. The effects of multilayering were also investigated numerically.Scientific and Technical Council of Turkey (TUBITAK-112M141

Effect of heat treatment on the blast loading response of combined geometry shell core sandwich structures

The effect of heat treatment on the dynamic crushing and energy absorption behavior of combined geometry shell cores (hemisphere and cylinder) of sandwich structures were investigated both experimentally and numerically. The applied heat treatment on the combined geometry shell cores relieved the stress caused by deep drawing, diminishing the peak transmitted forces. The verified numerical models of the as-received and heat-treated combined geometry shells were used to model blast loading of various sandwich configurations and the additional sandwich configurations of reversing the cylindrical side of the cores to the impacted side. Both the applied heat-treatment and the reversing process decreased the magnitude of the force transmitted to the protected structure. The applied heat treatment increased the arrival time of blast force wave to the protected structure, while the reversing resulted in opposite.Scientific and Technical Council of Turkey (TUBITAK 112M141

Mechanical interlocking between porous electrospun polystyrene fibers and an epoxy matrix

An epoxy matrix filled with nonwoven mats of porous polystyrene (PS) fibers processed by an electrospinning was compression tested at quasi-static (1 × 10–3 s–1) and high strain (315 s–1) rates. The electrospun PS fibers with a diameter between 6 and 9 μm, accommodated spherical pores on the surface with the sizes ranging from 0.1 to 0.2 μm. The filling epoxy matrix with 0.2 wt % PS fibers increased the compressive elastic modulus and compressive strength over those of neat epoxy resin. The microscopic observations indicated that the surface pores facilitated the resin intrusions into the fiber, enhancing the interlocking between resin and fibers, and increased the deformation energy expenditure of the polymer matrix