The aim of the work presented was to investigate the effects of upper bainite on impact toughness in quench and tempered low alloy plate steels. The experimental research included construction of CCT diagrams by dilatometry, verification of phases by optical microscopy (OM), Vickers hardness, scanning electron microscopy (SEM), transmission electron microscopy (TEM) on precipitates extracted by carbon replica and by electrolytic means and finally impact testing of Charpy specimens with mixed bainite:martensite microstructures. Bainite was formed in High Chromium Low Molybdenum (HCrLMo) and in High Molybdenum Low Chromium (HMoLCr) steel samples by isothermal annealing within the bainite C-curve of the respective CCT diagrams. The isothermal kinetics of the upper bainite transformation was modelled with the Johnson Mehl Avrami Kolmogorov (JMAK) model. Avrami exponents of 1.4 and 1.3 were obtained for the HCrLMo and HMoLCr steels respectively which indicated linear growth with a considerable lengthening rate of laths and negligible thickening. The measurably slower growth kinetics in the HMoLCr steel as observed in the JMAK model and the higher hardenability with reference to its CCT diagram, suggested a strong Mo alloying element effect. The stronger effect of Mo compared to Cr was attributed to a solute drag like effect. The effect of upper bainite in a tempered martensitic matrix was investigated for the following amounts of bainite; 0%, 10%, 25%, 60%, 75%, 90% and 100%. The impact toughness of the mixed bainite:martensite samples was evaluated against the toughness of 100% bainite and 100% martensite. It was demonstrated that upper bainite reduces the total absorbed impact energy by an adverse effect on crack nucleation energy and crack propagation energy. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / Unrestricted

The main purpose of this thesis is the systematic, experimental investigation of how a soft interlayer affects the impact response and energy dissipation mechanisms of all-polymer multi-laminates. An instrumented, intermediate impact velocity experimental setup with strain rates on the order of 100 s-1, is used to assess the impact mechanics of three-layered samples consisting of a poly(methyl methacrylate) (PMMA) front, polymer interlayer or adhesive, and polycarbonate (PC) back layer. Instrumentation of the gas gun is achieved with a shock accelerometer measuring contact force and optical displacement sensors recording deflection. Previous impact research utilizing instrumented gas guns by Levy and Goldsmith, and Delfosse et al. have measured contact force, but did not record simultaneous out-of-plane displacement. Signals acquired are temporally aligned allowing for insight into the response of the multi-laminate during impact, which is inaccessible with typical gas guns. Impact testing is completed on bonded and unbonded sample configurations, with two thermoplastic polyurethane and four polyacrylate interlayers. Quantitative metrics from force and displacement signals, along with post-impact damage observations, are used to compare impact performance between configurations and impact velocities (12 and 22 m/s). In general, the presence and bonding of an interlayer increases impact resistance by mitigating and localizing the impact load. The interlayers are characterized at various strain rates in tension, compression, and shear adhesion. In tension, all interlayers display rate dependence, non-linearity, and hysteretic behavior showing varying degrees of increasing energy dissipation with strain rate. Several trends between sample fracture and energy absorption mechanisms, quasi-static and low rate interlayer response, and metric results are established and discussed. / Master of Science

O presente trabalho visou determinar a tenacidade à fratura dinâmica, KID, primeiramente pelos conceitos da Mecânica da Fratura Elástica Linear (MFEL) e posteriormente pela Mecânica da Fratura Elasto-Plástica (MFEP), JID, em materiais que em operação podem estar sujeitos a impactos em diferentes temperaturas. Os materiais estudados, de uso na indústria aeronáutica, foram uma liga de titânio Ti6Al4V (norma SAE AMS 4911), na condição recozida e uma liga de aço inoxidável PH 15-5 (norma SAE AMS 5659), na condição H1000. Os corpos de prova pré-trincados e entalhados foram retirados nas orientações L-T e L-S, a fim de que fossem determinadas e comparadas a relação tenacidade/densidade ou tenacidade específica dos materiais estudados. Em seguida os corpos de prova foram ensaiados sob condições de carga dinâmica em uma máquina de ensaio Charpy instrumentado, marca Instron-Wolpert PW30, conforme a norma ASTM-E23, com velocidade de carregamento de 5,52 m/s e nas temperaturas de 23 e 400ºC. Os valores das tenacidades, posteriormente comparados e correlacionados, foram obtidos de acordo com as expressões matemáticas mencionadas na literatura. A razão entre as energias estimadas de iniciação (Ei) e de propagação (Ep), (Ei/Ep), foi obtida a partir dos gráficos de carga-deslocamento x tempo. A determinação e caracterização dos aspectos macro e microscópicos da fratura foram realizadas através de microscopia ótica e de varredura. Em seguida, confrontaram-se os valores e os aspectos preponderantes dos mecanismos de fratura apresentados por cada material. / The present work aimed to evaluate the dynamic fracture toughness, KID, firstly using the Linear Elastic Fracture Mechanics parameter, (LEFM), and secondly using the Elasto-Plastic Fracture Mechanics (EPFM), JID. The materials used in this work are from aeronautic grade and are subjected to in service impact loads and temperature variation. The materials are a titanium alloy Ti6Al4V (standard SAE AMS 4911), in the annealed condition and a PH 15-5 stainless steel (standard SAE AMS 5659), H1000 condition. Both precracked and notched specimens were taken in the L-T and L-S directions, for the evaluation and comparison of the toughness/density ratio, i. e., the specific dynamic fracture toughness of the materials studied. Therefore, the specimens were tested under dynamic load using an Instron-Wolpert PW30 Instrumented Charpy Equipment, following the ASTM-E23 standard, with load speed of 5,52 m/s at 23 and 400ºC. The fracture toughness values were compared using mathematical expression from literature. The ration between the initiation (Ei) and propagation (Ep) energies, (Ei/Ep), was obtained from the load-displacement x time. The characterization of the macro and microscopic aspects of the fracture mechanisms were carried out using optical microscope and scan electronic microscope. The fracture toughness values and the fractographic observations were correlated and compared for the two materials studied.

O presente trabalho visou determinar a tenacidade à fratura dinâmica, KID, primeiramente pelos conceitos da Mecânica da Fratura Elástica Linear (MFEL) e posteriormente pela Mecânica da Fratura Elasto-Plástica (MFEP), JID, em materiais que em operação podem estar sujeitos a impactos em diferentes temperaturas. Os materiais estudados, de uso na indústria aeronáutica, foram uma liga de titânio Ti6Al4V (norma SAE AMS 4911), na condição recozida e uma liga de aço inoxidável PH 15-5 (norma SAE AMS 5659), na condição H1000. Os corpos de prova pré-trincados e entalhados foram retirados nas orientações L-T e L-S, a fim de que fossem determinadas e comparadas a relação tenacidade/densidade ou tenacidade específica dos materiais estudados. Em seguida os corpos de prova foram ensaiados sob condições de carga dinâmica em uma máquina de ensaio Charpy instrumentado, marca Instron-Wolpert PW30, conforme a norma ASTM-E23, com velocidade de carregamento de 5,52 m/s e nas temperaturas de 23 e 400ºC. Os valores das tenacidades, posteriormente comparados e correlacionados, foram obtidos de acordo com as expressões matemáticas mencionadas na literatura. A razão entre as energias estimadas de iniciação (Ei) e de propagação (Ep), (Ei/Ep), foi obtida a partir dos gráficos de carga-deslocamento x tempo. A determinação e caracterização dos aspectos macro e microscópicos da fratura foram realizadas através de microscopia ótica e de varredura. Em seguida, confrontaram-se os valores e os aspectos preponderantes dos mecanismos de fratura apresentados por cada material. / The present work aimed to evaluate the dynamic fracture toughness, KID, firstly using the Linear Elastic Fracture Mechanics parameter, (LEFM), and secondly using the Elasto-Plastic Fracture Mechanics (EPFM), JID. The materials used in this work are from aeronautic grade and are subjected to in service impact loads and temperature variation. The materials are a titanium alloy Ti6Al4V (standard SAE AMS 4911), in the annealed condition and a PH 15-5 stainless steel (standard SAE AMS 5659), H1000 condition. Both precracked and notched specimens were taken in the L-T and L-S directions, for the evaluation and comparison of the toughness/density ratio, i. e., the specific dynamic fracture toughness of the materials studied. Therefore, the specimens were tested under dynamic load using an Instron-Wolpert PW30 Instrumented Charpy Equipment, following the ASTM-E23 standard, with load speed of 5,52 m/s at 23 and 400ºC. The fracture toughness values were compared using mathematical expression from literature. The ration between the initiation (Ei) and propagation (Ep) energies, (Ei/Ep), was obtained from the load-displacement x time. The characterization of the macro and microscopic aspects of the fracture mechanisms were carried out using optical microscope and scan electronic microscope. The fracture toughness values and the fractographic observations were correlated and compared for the two materials studied.