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Abstrak :
[Teknologi struktur berdinding tipis banyak digunakan pada desain struktur automotif, pesawat terbang, kapal laut, gerbong kereta api, bangunan gedung dan industri, dan lain-lain. Salah satu grup struktur berdinding tipis adalah struktur baja ringan dimana tebalnya sekitar 0.373 mm sampai 6.35 mm. Umumnya rasio lebar terhadap tebal disetiap bagian elemennya sangat besar. Kelompok baja ini sering disebut group cold-formed steel section. Ide dari suatu struktur dengan menggunakan baja ringan adalah mendapatkan kekuatan maksimum dari berat material seminimum mungkin dan bila digabungkan dengan strategi perencanaan yang inovatif dan tepat maka dapat diwujudkan untuk berbagai keperluan. Pengaruh bentuk geometri penampang sangat besar pengaruhnya terhadap perilaku dan kekuatannya dalam mendukung beban. Adanya perubahan bentuk sedikit saja dari penampangnya maka kekuatan elemen struktur tersebut akan berbeda sama sekali termasuk juga perilaku tekuknya. Keuntungan penggunaan cold-formed steel section meliputi antara lain desain yang lebih ekonomis dapat dicapai untuk beban yang relatif ringan dan bentangan pendek, pengurangan berat struktur sehingga diperoleh perbandingan kekuatan dengan berat yang tinggi serta dapat digunakan langsung sebagai lantai, atap, dinding, conduit, dan lain-lain. Model kerusakan yang dialami oleh struktur berdinding tipis khususnya dibagian yang tertekan adalah mengalami kerusakan berupa tekuk setempat (local bukcling) kemudian akan berkembang menjadi local plastic hinge failure mechanisms (plastic collapse). Secara teknis pengembangan model analisis kekuatan struktur baja ringan yang dibuat dengan proses pembentukan dingin ini dapat diarahkan untuk memperkecil kelemahan disain metode konvensional pada struktur baja dimana dalam metode konvensional, disain kekuatan struktur baja biasanya ditentukan dengan memanfaatkan faktor keamanan (safety factor) sehingga besar kecilnya faktor keamanan yang digunakan dalam disain akan berpengaruh pada berat disain struktur baja yang dihasilkan. Untuk mengurangi ketergantungan penggunaan faktor keamanan dalam disain, maka perlu dikaji dan dikembangkan suatu metode evaluasi kekuatan yang tidak tergantung pada penggunaan faktor keamanan. Metode analisis disain yang dikembangkan didalam penelitian ini tidak menggunakan faktor keamanan sehingga dapat digunakan untuk mendisain struktur baja yang mampu mendukung beban kerja dengan berat yang seringan mungkin sehingga diperoleh struktur yang efektif dan efisien dari segi teknis dan ekonomi. Dalam penelitian ini, suatu metode cut-off strength digunakan sebagai dasar untuk memprediksi kekuatan profil baja ringan berpenampang persegi (SHS) akibat interaksi beban tekan memusat dan momen lentur. Metode cut-off strength diilustrasikan dalam bentuk dua kurva beban-defleksi plastis dan elastis, dimana nilai beban diperpotongan kedua kurva tersebut diasumsikan sebagai kekuatan teoritis profil baja SHS terhadap interaksi tekan memusat dan momen lentur. Kurva plastis dibentuk berdasarkan persamaan beban-defleksi yang dikembangkan melalui analisis kesetimbangan energi model mekanisme kerusakan plastis profil baja SHS. Sementara itu, kurva elastis dibentuk berdasarkan persamaan beban-defleksi yang dikembangkan melalui analisis non-linear elastic profil baja SHS dengan mempertimbangkan efek local buckling pada penampang profil yang tertekan. Metode analisis ini diimplementasikan dengan menggunakan perangkat lunak (software) komputer. Untuk mengukur ketelitian model analisis disain yang dikembangkan dalam penelitian ini, hasil prediksi kekuatan profil baja SHS diverifikasi menggunakan data kekuatan aktual yang dihasilkan melalui pengujian sejumlah profil baja SHS akibat interaksi beban tekan memusat dan momen lentur sampai rusak. Hasil verifikasi menunjukkan bahwa rasio data prediksi analitis dan eksperimental masih tersebar dalam batas-batas toleransi yang umum digunakan yaitu ± 20 %. Dalam penelitian ini juga dipelajari parameter dimensi dan material dasar profil baja SHS yang dapat mempengaruhi kekuatannya, dimana parameter tersebut terdiri dari variasi rasio lebar tumpuan beban, tinggi web dan radius kelengkungan terhadap tebal (n/t, hw/t dan r/t), panjang bentang profil (l) dan tegangan yield material (σyield). Hasil kajian menunjukkan bahwa profil baja SHS lebih kuat mendukung interaksi beban tekan memusat dan momen lentur sejalan dengan peningkatan parameter rasio n/t dan hw/t sedangkan akibat peningkatan parameter r/t dan panjang bentang (l) menyebabkan kekuatan profil baja SHS dalam menerima interaksi beban tekan memusat dan momen lentur akan lebih melemah. Disamping itu terindikasi bahwa pengaruh variasi tegangan yield terhadap kekuatan profil baja SHS akibat interaksi beban tekan memusat dan momen lentur tidak terlalu berarti.;

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Thin-walled structure technology is often used in structural design of aircraft, automotive, ship, train, building of industries, etc. A light weight steel structure is one of thin-walled structure group where its thickness ranges inbetween 0.373 mm to 6,35 mm. Generally speaking that the ratio of width to thickness is very large and this group of steel is called cold-formed steel section. The idea of using this light weight steel section in the design is to obtain a high strength to weight ratio and an innovative design strategy, which can be used in various needs.The form of geometry and cross section of the light weight steel structures affects significantly to the strength and local buckling behavior. The adventage of using the cold-formed steel sectionc is to obtain the economical design and reducing of structural weight. The economical design can be obtained especially for the relative minimum weight and short span. Effect of reducing weight can result in high strength to weight ratio. With reference to this advantages, the light weight steel structures can also directly be used for the design of floor, roof, wall, conduit, etc. In the design of light weight steel structure, its compressive elements tend to undergo local buckling that can be subsequently develop to local plastic hinge failure mechanisms, at collapse. Technically, the development of strength analytical model of a thin-walled steel structure produced by cold-forming process in this research is aimed at reducing the weakness of a conventional method to analyze the strength of the structure. In the conventional method, the strength design of the steel structure is usually determined using a safety factor where the value of this safety factor will affect the weight of the steel tructure. In order to reduce the dependency of using safety factor in the design, it has been attempted to develop another method of designing the strength of the steel structure, which does not depend on the application of a safety factor. Based on this method, the strength analysis used can certainly result in the design steel structure, which is lighter and still able to carry applied loads. Thus, the main objective of developing this analytical method is intended to obtain the steel structural design, which is still effective and efficient in terms of technical and economical points of view. In this research program, a method of cut-off strength is used as a basis of predicting the strength of a square hollow steel section affected by the interaction of concentrated-compressive load and bending moment. The method cut-off strength is illustrated in the form of two plastic and elastic curves of load-deflection, where the value of load at an intersection of these curves is assumed to be the theoretical strength of the steel section with respect to the interaction of concentrated-compressive load and bending moment. The plastic curve is established according to a load-deflection equation that is developed through the analysis of energy equilibrium on the plastic failure mechanisms model of the steel section. Meanwhile, the elastic one is established according to another load-deflection curve that is developed through the analysis of non linear elastic concept of the steel structure. This analytical method is implemented by using computer software. In order to measure the accuracy of the design of this analytical model developed in this program, its predicted results are verified using actual strength data obtained from a number of tests on the square hollow steel sections subjected to the interaction of concentrated-compressive load and bending moment to failure. Verification results indicate that the ratio of the analytical-predicted data and experimental one is still scattered within tolerable limits of ± 20 %. This research also studies parameters of dimension and basic material of the steel SHS, which can influence its strength, where these parameters consist of variation of load bearing length, web height and radius with respect to thickness (n/t, hw/t and r/t), span length of the section (l) and yield strength material (σyield). The assesment results show that the steel SHS under insteraction of concentrated-compressive load and bending moment are stronger along with the increasing of the parameter of n/t and hw/t. Meanwhile, due to the increasing steel section of radius ratio (r/t) and span length (l) the strength of steel SHS tend to decrease. Bisides, it has also indicated tha the effect of various yield strength with respect to the strength of the steel SHS under insteraction of concentrated-compressive load and bending momento is not too significant.;Thin-walled structure technology is often used in structural design of aircraft, automotive, ship, train, building of industries, etc. A light weight steel structure is one of thin-walled structure group where its thickness ranges inbetween 0.373 mm to 6,35 mm. Generally speaking that the ratio of width to thickness is very large and this group of steel is called cold-formed steel section. The idea of using this light weight steel section in the design is to obtain a high strength to weight ratio and an innovative design strategy, which can be used in various needs.The form of geometry and cross section of the light weight steel structures affects significantly to the strength and local buckling behavior. The adventage of using the cold-formed steel sectionc is to obtain the economical design and reducing of structural weight. The economical design can be obtained especially for the relative minimum weight and short span. Effect of reducing weight can result in high strength to weight ratio. With reference to this advantages, the light weight steel structures can also directly be used for the design of floor, roof, wall, conduit, etc. In the design of light weight steel structure, its compressive elements tend to undergo local buckling that can be subsequently develop to local plastic hinge failure mechanisms, at collapse. Technically, the development of strength analytical model of a thin-walled steel structure produced by cold-forming process in this research is aimed at reducing the weakness of a conventional method to analyze the strength of the structure. In the conventional method, the strength design of the steel structure is usually determined using a safety factor where the value of this safety factor will affect the weight of the steel tructure. In order to reduce the dependency of using safety factor in the design, it has been attempted to develop another method of designing the strength of the steel structure, which does not depend on the application of a safety factor. Based on this method, the strength analysis used can certainly result in the design steel structure, which is lighter and still able to carry applied loads. Thus, the main objective of developing this analytical method is intended to obtain the steel structural design, which is still effective and efficient in terms of technical and economical points of view. In this research program, a method of cut-off strength is used as a basis of predicting the strength of a square hollow steel section affected by the interaction of concentrated-compressive load and bending moment. The method cut-off strength is illustrated in the form of two plastic and elastic curves of load-deflection, where the value of load at an intersection of these curves is assumed to be the theoretical strength of the steel section with respect to the interaction of concentrated-compressive load and bending moment. The plastic curve is established according to a load-deflection x equation that is developed through the analysis of energy equilibrium on the plastic failure mechanisms model of the steel section. Meanwhile, the elastic one is established according to another load-deflection curve that is developed through the analysis of non linear elastic concept of the steel structure. This analytical method is implemented by using computer software. In order to measure the accuracy of the design of this analytical model developed in this program, its predicted results are verified using actual strength data obtained from a number of tests on the square hollow steel sections subjected to the interaction of concentrated-compressive load and bending moment to failure. Verification results indicate that the ratio of the analytical-predicted data and experimental one is still scattered within tolerable limits of ± 20 %. This research also studies parameters of dimension and basic material of the steel SHS, which can influence its strength, where these parameters consist of variation of load bearing length, web height and radius with respect to thickness (n/t, hw/t and r/t), span length of the section (l) and yield strength material (σyield). The assesment results show that the steel SHS under insteraction of concentrated-compressive load and bending moment are stronger along with the increasing of the parameter of n/t and hw/t. Meanwhile, due to the increasing steel section of radius ratio (r/t) and span length (l) the strength of steel SHS tend to decrease. Bisides, it has also indicated tha the effect of various yield strength with respect to the strength of the steel SHS under insteraction of concentrated-compressive load and bending momento is not too significant., Thin-walled structure technology is often used in structural design of aircraft, automotive, ship, train, building of industries, etc. A light weight steel structure is one of thin-walled structure group where its thickness ranges inbetween 0.373 mm to 6,35 mm. Generally speaking that the ratio of width to thickness is very large and this group of steel is called cold-formed steel section. The idea of using this light weight steel section in the design is to obtain a high strength to weight ratio and an innovative design strategy, which can be used in various needs.The form of geometry and cross section of the light weight steel structures affects significantly to the strength and local buckling behavior. The adventage of using the cold-formed steel sectionc is to obtain the economical design and reducing of structural weight. The economical design can be obtained especially for the relative minimum weight and short span. Effect of reducing weight can result in high strength to weight ratio. With reference to this advantages, the light weight steel structures can also directly be used for the design of floor, roof, wall, conduit, etc. In the design of light weight steel structure, its compressive elements tend to undergo local buckling that can be subsequently develop to local plastic hinge failure mechanisms, at collapse. Technically, the development of strength analytical model of a thin-walled steel structure produced by cold-forming process in this research is aimed at reducing the weakness of a conventional method to analyze the strength of the structure. In the conventional method, the strength design of the steel structure is usually determined using a safety factor where the value of this safety factor will affect the weight of the steel tructure. In order to reduce the dependency of using safety factor in the design, it has been attempted to develop another method of designing the strength of the steel structure, which does not depend on the application of a safety factor. Based on this method, the strength analysis used can certainly result in the design steel structure, which is lighter and still able to carry applied loads. Thus, the main objective of developing this analytical method is intended to obtain the steel structural design, which is still effective and efficient in terms of technical and economical points of view. In this research program, a method of cut-off strength is used as a basis of predicting the strength of a square hollow steel section affected by the interaction of concentrated-compressive load and bending moment. The method cut-off strength is illustrated in the form of two plastic and elastic curves of load-deflection, where the value of load at an intersection of these curves is assumed to be the theoretical strength of the steel section with respect to the interaction of concentrated-compressive load and bending moment. The plastic curve is established according to a load-deflection x equation that is developed through the analysis of energy equilibrium on the plastic failure mechanisms model of the steel section. Meanwhile, the elastic one is established according to another load-deflection curve that is developed through the analysis of non linear elastic concept of the steel structure. This analytical method is implemented by using computer software. In order to measure the accuracy of the design of this analytical model developed in this program, its predicted results are verified using actual strength data obtained from a number of tests on the square hollow steel sections subjected to the interaction of concentrated-compressive load and bending moment to failure. Verification results indicate that the ratio of the analytical-predicted data and experimental one is still scattered within tolerable limits of ± 20 %. This research also studies parameters of dimension and basic material of the steel SHS, which can influence its strength, where these parameters consist of variation of load bearing length, web height and radius with respect to thickness (n/t, hw/t and r/t), span length of the section (l) and yield strength material (σyield). The assesment results show that the steel SHS under insteraction of concentrated-compressive load and bending moment are stronger along with the increasing of the parameter of n/t and hw/t. Meanwhile, due to the increasing steel section of radius ratio (r/t) and span length (l) the strength of steel SHS tend to decrease. Bisides, it has also indicated tha the effect of various yield strength with respect to the strength of the steel SHS under insteraction of concentrated-compressive load and bending momento is not too significant.]

Abstrak :
ABSTRAK Guna mendapatkan performa yang optimal selama trayektori terbangnya, nosel roket didesain pada tekanan ambient yang rendah. Akan tetapi, tekanan ambien desain ini dipilih setinggi mungkinuntuk mencegah terjadinya separasi aliran di dalam nosel selama operasi pada sea-level. Tetapi, pada bagian clivergen, aliran akan terseparasi dari dinding selama tekanan ruang bakar, pc, belum mencapai nilai nominalnya. Separasi aliran pada nosel roket sangatlah tidak diharapkan, karena separasi aliran tak simetris dapat menyebabkan timbulnya gaya lateral, yang disebut side-load, yang dapat merusak nosel dan menggagalkan misi secara keseluruhan. Oleh karena itu, separasi aliran dan prediksi teoritisnya telah dan masih menjadi subyek beberapa eksperimen dan studi teoritis. Pada nosel over-ekspansi, aliran terseparasi dari nosel pada rasio tekanan tertentu (tekanan dinding terhadap tekanan ambien). Model dan hipotesis prediksinya telah dikembangkan, baik secara fisik ataupun empiris. Meskipun beberapa keberhasilan korelasi separasi telah dicapai, beberapa ketidakpastian tetap terjadi, yang layak untuk dikaji. Pada penelitian ini, sebuah model dikembangkan untuk mengkaji aliran separasi pada nosel tipe bel. Profil kontur nosel diperoleh dengan sirnulasi program yang dibangun menggunakan software MathCAD dan Matlab menggunakan metoda karakteristik (MOC). Model nosel 2-dimensi tipe bel MLN diperoleh dengan MathCad, yang kemudian divariasikan pada daerah upstream dan downstream throat nosel. Sedangkan model nosel bel MLN axis-simetri dan model nosel bel ideal diperoleh dengan Matlab. Semua model nosel divariasikan pada daerah throat menggunakan standar JPL, Rao dan ONERA. Desain nosel yang terbentuk kemudian divalidasi menggunakan perangkat lunak komersial Fluent, dengan hasil yang memuaskan. Pada penelitian ini juga dirancang kontur nosel bel axis-simetri menggunakan MOC dan axis-simetri parabolik. Usaha ini dilakukan untuk memberikan perbandingan antara nosel 2D dan axis-simetri yang digunakan pada aplikasi roket sebenamya. Semua kontur nosel yang dihasilkan dibandingkan dengan teori isentropik nosel pada rasio ekspansi terhadap fluida kerjanya dan bilangan Mach keluar nosel. Akurasi nosel untuk menghasilkan bilangan Mach yang diinginkan juga dicek. Medan aliran yang terbentuk oleh program, juga dicek menggunakan Fluent. Prediksi fluent digunakan untuk memverivikasi asumsi aliran isentropik yang digunakan untuk menghasilkan bilangan Mach yang diharapkan. Keserasian data yang diperoleh pada rasio ekspansi dan bilanganMach yang diperoleh, mengindikasikan bahwa program yang dibangun cukup akurat. Pola separasi pada masing- masing nosel, dperoleh dengan melakukan komputasi dinamika fluida (CFD). Dengan memvariasikan rasio tekanan (number of pressure ratio/NPR) masing-masing pada NPR=7,825; NPR=3.13; NPR=3.0; NPR=2.5; NPR=2.2; NPR=2.0; NPR=l .8; dan pada NPR=1.6, maka pola separasi untuk masing-masing nosel dapat diperoleh. Untuk membandingkan pola separasi aliran tersebut, dirancang sebuah sistem terowongan angin mini yang didesain hingga kecepatan 2 Mach. Sistem dilengkapi dengan sistem schlieren untuk menangkap gambar separasi dari dalam nosel. Sebuah kamera kecepatan tinggi (high speed camera) digunakan untuk menangkap pola separasi yang terjadi dalam nosel. Pola aliran separasi FSS dan RSS teramati dalam pengujian. Dari titik pola aliran tak-simetri, dapat dihitung besarnya side-load yaang di derita oleh nosel. Nilai yang diperoleh cukup besar, sekitar 20% dari gaya dorong yang dihasilkan pada NPR tersebut atau sekitar 4% dari gaya dorong pada NPR penuh.

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ABSTRACT In order to get an optimum performance over the whole flight trajectory, the nozzles are designed for an intermediate ambient pressure. However, this design ambient presstue is chosen high enough to prevent flow separation inside the nozzle during steady-state operation. at sea-level. But in the divergent part of these nozzle, the flow separates from the wall as long as the chamber pressure, pc, has not yet reached its nominal value. Flow separation in rocket nozzle is considered undesirable, because an asymmetry in the tlow separation can caused dangerous lateral forces, the so-called side loads, which may damage the nozzle. Therefore, flow-separation and its theoreticalpredietion have been and still are the subject of several experimental and theoretical studies. In over expanded rocketnozzle, the flow separated fromithe nozzle wall at a certain pressure ratio of wall pressure to ambient pressure. This flow separation and its theoretical prediction have been the subject of several experimental and theoretical studies in the past decades, and models and hypotheses for its prediction have been developed, either physically motivated or purely empirical. Despite the apparent success in correlating separation, several uncertainties still remain which are worthy of investigation. In this research, a model was developed to observe flow separation and its pattern on bell type nozzle. Two types of ironic will conducted, cortical and contour nozzle. The profile of contour nozzle acquired by MathCAD® and Matlab® source code program using method of characteristics (MOC). The 2-D MLN bell type nozzle constructed by MathCad output, then varied in upstream and downstream throat area. The MLN axysimetric constructed by Matlab output program. Both types of nozzle varied by curvature radius of throat using JPL, ONERA standard, and Rao method. The validity of nozzles design checked by commercial CFD software, Fluentm, and satisfied. In this research was also designed an axysimmetric bell contour nozzle using method of characteristic (MOC) and axysimmetric parabolic. This effort conducted to give a comparativeness between 2-dimensioanl nozzle to axysirmnetric one that used in real rocket application. The contours were compared to theoretical isentropic area ratios for the selected fluid and desired exit Mach number. The accuracy ofthe nozzle to produce the desired exit Mach number was also checked. The tlowtield of the nozzles created by the code were independently checked with the commercial Computational Fluid Dynamics (CFD) code FLUENT. FLUENT predictions were used to verify the isentropic flow assumption and that the working fluid reached the user-defmed desired exit Mach number. Good agreement in area ratio and exit Mach number were achieved, verifying that the code is accurate. The flow pattern separation for each nozzle obtained by CFD software simulation. The simulation conducted by varied the number of pressure ratio (NPR): NPR=7,825; NPR=3.13; NPR=3.0; NPR=2.5; NPR.=2.2; NPR=2.0; NPR=1.8; dan pada NPR=1.6 for each nozzle, therefore the pattern can plotted. To compares those flow pattern, a blowdown wind tunnel designed until Mach 2. The system completely with schlieren system for captming separation image in the nozzle as a result of wind tunnel. A high speed camera assigned to capture flow pattern separation. FSS and RSS flow pattern was captured through this experiment. From tmsymrnetrical flow point, the side-load obtained. These side-load is about 20% from thrust tha can produced by the NPR where fhis value obtained or about 4% from full thrust (maximum NPR).

Abstrak :
ABSTRAK Berbagai penelitian dan para peneliti terdahulu terhadap pertumbuhan butir baja terfokus pada kondisi isothermal, sehingga berbagai tinjauan terhadap topik ini terdapat dalam berbagai literatur. Sedangkan berbagai aplikasi proses material, seperti canal panas pengecoran atau tempa berlangsung dalam kondisi non-isoternal. Prediksi pertumbuhan butir mempergunakan persamaan yang didapat secara empris dalam kondisi anil isothermal sehingga terjadi fluktuasi dalam besar butir dan sifat mekanis produk baja. Penelitian ini dilakukan untuk mengevaluasi dan mendapatkan pertumbuhan butir austenit dalam kondisi dimana pertumbuhan butirnya setelah dilakukan deformasi canal satu pass, dalam kondisi pendinginan kontinyu. Pendekatan yang digunakan adalah memberikan regangan deformasi canal panas antara O,3-0,4 dengan temperatur pemanasan awal l200°C, dan temperatur deformasi antara 900-1100C dengan kecepatan pendinginan antara 7-12 C/detik dalam rentang waktu rata-rata 30 detik setelah deformasi, kemudian didinginkan cepat ke temperatur ruang. Kecepatan pendinginan direkayasa dengan memasukkan benda iji ke dalam heating jacket dan pendinginan cepat dilakukan dengan water jetspray. Hasil eksperimen menunjukkan bahwa pertumbuhan butir austenit baja setelah proses canal panas dapat digambarkan sebagai fungsi kecepatan pendinginan. Besar butir austenit semakin menurun dengan meningkatnya kecepatan pendinginan. Kinetika pertumbuhan butir austenit non-isotermal didapat dengan melakukan modifikasi matematis persamaan pertumbuhan butir isotermal dengan memasukkan faktor inverse kecepatan pendinginan berpangkat m. Model modfikasi ini dilakukan iterasi dengan hasil eksperimen dan didapat model empris dengan nilai amat mendekati hasil eksperimen, dengan hubungan besar butir austenit yang berbanding terbalik dengan kecepatan pendinganan berpangkat m (1/Cr), dan penambahan konstanta B. Didapat konstanta kecepatan pendinginan m hampir tidak terpengaruh oleh komposisi baja yaitu sekitar 12 sedangkan konstanta B meningkat dari 3,0 x10'° sampai 8 x l0'° dengan peningkatan prosentase Nb, C atau N dalam baja. Model ini dievaluasi dengan perhitungan penumbuhan butir austenit hasil perhitugan matematis berdasarkan persamaan isothermal dan metode additivity. Didapati bahwa model modifikasi memilih nilai besar butir austenit yang amat mendekati perhitugan matematis , dengan nilai konstanta yang relatif sama dengan model matematis . Didapat bahwa perhitungan dengan model empiris non isotermal memiliki deviasix rendah terhadap nilai eksperimen (4-l5%). sehingga lebih tepat untuk memprediksi pertumbuhan butir austenit kondisi non-isotermal.

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ABSTRACT Many reviews in the literatures by many previous investigators on the steel grain growth mostly focused for the isothermal condition. At the same time, many of the materials processing such as hot-rolling, casting, and forging take place under non-isothermal conditions. Grain growth prediction uses empirically obtained formula in an isothermal annealing condition; in this instant, there are possibilities that the fluctuation in the predicted grains size and thus in the mechanical properties will occur. The main purpose of this investigation is to evaluate and to obtain austenite grains growth in a non-isothermal condition. The grain growth of three compositions of HSLA-Nb steel, i.e. 0.019; 0.037; and 0.056 wt.% Nb, was examined after single-pass-hot-rolling process under continuous cooling condition. The materials were hot-rolled about 0.3-0.4 at an initial temperature of 1200C, deformation temperature of 900-1100C, cooling rate of 7-12K/s in an average time period of 30 second after deformation, and the quenched to room temperature. Cooling rate was achieved by putting the specimen into a heating jacket and quenching was performed by using a water jetspray. The results show that the austenite grain growth was obtained by modifying isothermal grain growth relation with respect to the inverse factor of cooling rate to the power of m. This modification model was irerated by using experimental data and results in an empirical model with the value very close to the experimental data, in which the austenite grain size inversely proportional to the cooling rate power m (1/Cr) and an additional content of B. It was also found that the cooling rate m was almost not affected by steel composition, which is around 12, whereas the constant of B increases from 3.0 x 10 to 8 x 10 with the increase of Nb, C, or N content in the steel. The model was evaluated by using the austenite grain growth calculation based on isothermal and addivity methods. This model results in the same value as the calculation model with the same constant. The austenite grain growth calculated by modified empirical model was found has small deviation compare to the experiments value (4-15%). Hence, the model is appropriates to be applied to predicts the non-isothermal austenite grain growth after deformation in hot rolling process.

Abstrak :
ABSTRAK Guna mendapatkan performa yang optimal selama trayektori terbangnya, nosel roket didesain pada tekanan ambient yang rendah. Akan tetapi, tekanan ambien desain ini dipilih setinggi mungkin untuk mencegah terjadinya separasi aliran di dalam nosel selama operasi pada sea-level. Tetapi, pada bagian divergen, aliran akan terseparasi dari dinding selama tekanan ruang bakar, pc, belum mencapai nilai nominalnya. Separasi aliran pada nosel roket sangatlah tidak diharapkan, karena separasi aliran tak simetris dapat menyebabkan timbulnya gaya lateral, yang disebut side-load, yang dapat merusak nosel dan menggagalkan misi secara keseluruhan. Oleh karena itu, separasi aliran dan prediksi teoritisnya telah dan masih menjadi subyek beberapa eksperimen dan studi teoritis. Pada nosel over-ekspansi, aliran terseparasi dari nosel pada rasio tekanan tertentu (tekanan dinding terhadap tekanan ambien). Model dan hipotesis prediksinya telah dikembangkan, baik secara fisik ataupun empiris. Meskipun beberapa keberhasilan korelasi separasi telah dicapai, beberapa ketidakpastian tetap terjadi, yang layak untuk dikaji. Pada penelitian ini, sebuah model dikembangkan untuk mengkaji aliran separasi pada nosel tipe bel. Profil kontur nosel diperoleh dengan sirnulasi program yang dibangun menggunakan software MathCAD dan Matlab menggunakan metoda karakteristik (MOC). Model nosel 2-dimensi tipe bel MLN diperoleh dengan MathCad, yang kemudian divariasikan pada daerah upstream dan downstream throat nosel. Sedangkan model nosel bel MLN axis-simetri dan model nosel bel ideal diperoleh dengan Matlab. Semua model nosel divariasikan pada daerah throat menggunakan standar JPL, Rao dan ONERA. Desain nosel yang terbentuk kemudian divalidasi menggunakan perangkat lunak komersial Fluent, dengan hasil yang memuaskan. Pada penelitian ini juga dirancang kontur nosel bel axis-simetri menggunakan MOC dan axis-simetri parabolik. Usaha ini dilakukan untuk memberikan perbandingan antara nosel 2D dan axis-simetri yang digunakan pada aplikasi roket sebenamya. Semua kontur nosel yang dihasilkan dibandingkan dengan teori isentropik nosel pada rasio ekspansi terhadap fluida kerjanya dan bilangan Mach keluar nosel. Akurasi nosel untuk menghasilkan bilangan Mach yang diinginkan juga dicek. Medan aliran yang terbentuk oleh program, juga dicek menggunakan Fluent. Prediksi fluent digunakan untuk memverifikasi asumsi aliran isentropik yang digunakan untuk menghasilkan bilangan Mach yang diharapkan. Keserasian data yang diperoleh pada rasio ekspansi dan bilanganMach yang diperoleh, mengindikasikan bahwa program yang dibangun cukup akurat. Pola separasi pada masing- masing nosel, dperoleh dengan melakukan komputasi dinamika fluida (CFD). Dengan memvariasikan rasio tekanan (number of pressure ratio/NPR) masing-masing pada NPR=7,825; NPR=3.13; NPR=3.0; NPR=2.5; NPR=2.2; NPR=2.0; NPR=l .8; dan pada NPR=1.6, maka pola separasi untuk masing-masing nosel dapat diperoleh. Untuk membandingkan pola separasi aliran tersebut, dirancang sebuah sistem terowongan angin mini yang didesain hingga kecepatan 2 Mach. Sistem dilengkapi dengan sistem schlieren untuk menangkap gambar separasi dari dalam nosel. Sebuah kamera kecepatan tinggi (high speed camera) digunakan untuk menangkap pola separasi yang terjadi dalam nosel. Pola aliran separasi FSS dan RSS teramati dalam pengujian. Dari titik pola aliran tak-simetri, dapat dihitung besarnya side-load yaang di derita oleh nosel. Nilai yang diperoleh cukup besar, sekitar 20% dari gaya dorong yang dihasilkan pada NPR tersebut atau sekitar 4% dari gaya dorong pada NPR penuh.

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ABSTRACT In order to get an optimum performance over the whole flight trajectory, the nozzles are designed for an intermediate ambient pressure. However, this design ambient presstue is chosen high enough to prevent flow separation inside the nozzle during steady-state operation. at sea-level. But in the divergent part of these nozzle, the flow separates from the wall as long as the chamber pressure, pc, has not yet reached its nominal value. Flow separation in rocket nozzle is considered undesirable, because an asymmetry in the tlow separation can caused dangerous lateral forces, the so-called side loads, which may damage the nozzle. Therefore, flow-separation and its theoreticalpredietion have been and still are the subject of several experimental and theoretical studies. In over expanded rocketnozzle, the flow separated fromithe nozzle wall at a certain pressure ratio of wall pressure to ambient pressure. This flow separation and its theoretical prediction have been the subject of several experimental and theoretical studies in the past decades, and models and hypotheses for its prediction have been developed, either physically motivated or purely empirical. Despite the apparent success in correlating separation, several uncertainties still remain which are worthy of investigation. In this research, a model was developed to observe flow separation and its pattern on bell type nozzle. Two types of ironic will conducted, cortical and contour nozzle. The profile of contour nozzle acquired by MathCAD® and Matlab® source code program using method of characteristics (MOC). The 2-D MLN bell type nozzle constructed by MathCad output, then varied in upstream and downstream throat area. The MLN axysimetric constructed by Matlab output program. Both types of nozzle varied by curvature radius of throat using JPL, ONERA standard, and Rao method. The validity of nozzles design checked by commercial CFD software, Fluentm, and satisfied. In this research was also designed an axysimmetric bell contour nozzle using method of characteristic (MOC) and axysimmetric parabolic. This effort conducted to give a comparativeness between 2-dimensioanl nozzle to axysirmnetric one that used in real rocket application. The contours were compared to theoretical isentropic area ratios for the selected fluid and desired exit Mach number. The accuracy ofthe nozzle to produce the desired exit Mach number was also checked. The flowfield of the nozzles created by the code were independently checked with the commercial Computational Fluid Dynamics (CFD) code FLUENT. FLUENT predictions were used to verify the isentropic flow assumption and that the working fluid reached the user-defmed desired exit Mach number. Good agreement in area ratio and exit Mach number were achieved, verifying that the code is accurate. The flow pattern separation for each nozzle obtained by CFD software simulation. The simulation conducted by varied the number of pressure ratio (NPR): NPR=7,825; NPR=3.13; NPR=3.0; NPR=2.5; NPR.=2.2; NPR=2.0; NPR=1.8; dan pada NPR=1.6 for each nozzle, therefore the pattern can plotted. To compares those flow pattern, a blowdown wind tunnel designed until Mach 2. The system completely with schlieren system for captming separation image in the nozzle as a result of wind tunnel. A high speed camera assigned to capture flow pattern separation. FSS and RSS flow pattern was captured through this experiment. From unsymmetrical flow point, the side-load obtained. These side-load is about 20% from thrust that can produced by the NPR where this value obtained or about 4% from full thrust (maximum NPR).