Hybrid experimental-numerical determination of the loading path to fracture in TRIP780 sheets subjected to multi-axial loading

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 104-110).The development of ductile fracture models of metals requires reliable measurements of the stress and strain histories up to the onset of fracture in multi-axial experiments. In the present work, a hybrid experimental-numerical approach is taken to determine the loading path in various fracture experiments on TRIP780 steel sheets. In most mechanical experiments on sheet metal, the localization of plastic deformation precedes the onset of fracture. After the beginning of necking, the stress fields within the specimen gage section become non-uniform and of three dimensional nature. Consequently, the stress history prior to fracture can no longer be estimated based on the force history measurements using simple analytical formulas. A detailed finite element analysis of each experiment is required to identify the local stress and strain fields. The results of the hybrid experimental-numerical analysis of a fracture experiment depend strongly on the chosen constitutive model. Here, an extensive bi-axial experimental program comprising more than 20 distinct loading conditions is performed to characterize the monotonic large deformation behavior of the TRIP780 steel. It is found that an anisotropic quadratic yield function along with a non-associated flow rule can accurately describe the inelastic behavior of the TRIP material.(cont.) A first series of fracture experiments is carried out on three types of full-thickness fracture specimens. This experimental program characterizes the onset of fracture for stress states between uniaxial tension and equi-biaxial tension. An effort is made to quantify and minimize the errors affecting the hybrid experimental-numerical analysis of those experiments. Inaccuracies affecting the stress triaxiality and plastic strain histories to fracture are evaluated by comparing surface strains measured by Digital Image Correlation (DIC) and computed by Finite Element Analysis (FEA). A second series of fracture experiments is carried out on a newly designed butterfly-shaped specimen, which allows for multi-axial testing under combinations of normal and tangential loads. Experiments for four different loading conditions are performed and used to analyze the onset of fracture for stress states ranging from pure shear to transverse plane strain tension.by Matthieu DunandS.M

Ductile fracture at intermediate stress triaxialities : experimental investigations and micro-mechanical modeling

Thesis: Sc. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages [239]-256).Accurate predictions of the onset of ductile fracture play an increasingly important role in the design of lightweight sheet metal structures. With the development of virtual prototyping practices, most transportation vehicles are now computer-engineered in great detail before launching their mass production, thereby requiring reliable models for plasticity and fracture. This thesis reports on a comprehensive investigation into the effect of stress state on the onset of ductile fracture of an Advanced High Strength Steel (AHSS), covering development of new experimental procedures, material characterization and phenomenological as well as micro-mechanical modeling of the onset of fracture. Based on an extensive multi-axial experimental program, the anisotropic plasticity of the present material is described by a non-associated quadratic anisotropic model. Comparison of model predictions to experimental results reveals that the proposed model provides better predictions than associated isotropic or anisotropic quadratic models. Moreover, a structural validation is presented that demonstrates the higher prediction accuracy of the non-associated plasticity model. A hybrid experimental-numerical approach is proposed to investigate the dependence of the onset of fracture to stress state. The experimental program covers the complete range of positive stress triaxialities, from pure shear to equibiaxial tension. It includes different full thickness specimens as well as multi-axial fracture experiments where combinations of tension and shear loadings are applied to a newly developed butterfly-shaped specimen. Loading paths to fracture are determined for each experiment in terms of stress triaxiality, Lode angle parameter and equivalent plastic strain and show a non-monotonic and strong dependence of ductility to stress state. The extensive fracture characterization is used to evaluate the predictive capabilities of two phenomenological and physics-inspired fracture models (the Modified Mohr-Coulomb and a shear-modified Gurson model) that take the effect of the first and third stress tensor invariants into account in predicting the onset of fracture. Finally, a micro-mechanical model relating the onset of fracture to plastic localization into a narrow band at the micro-scale is developed. The effect of stress state on localization is investigated numerically by means of a 3D void-containing unit cell submitted to well-controlled and proportional loadings in the macroscopic stress state. Based on simulation results, an analytical localization criterion is proposed which defines an open convex envelope in terms of the shear and normal stresses acting on the plane of localization and correlates well with experimental results.by Matthieu Dunand.Sc. D

Press forming a 0/90 cross-ply advanced thermoplastic composite using the double-dome benchmark geometry

A pre-consolidated thermoplastic advanced composite cross-ply sheet comprised of two uniaxial plies orientated at 0/90° has been thermoformed using tooling based on the double-dome bench-mark geometry. Mitigation of wrinkling was achieved using springs to apply tension to the forming sheet rather than using a friction-based blank-holder. The shear angle across the surface of the formed geometry has been measured and compared with data collected previously from experiments on woven engineering fabrics. The shear behaviour of the material has been characterised as a function of rate and temperature using the picture frame shear test technique. Multi-scale modelling predictions of the material’s shear behaviour have been incorporated in finite element forming predictions; the latter are compared against the experimental results

High-Strain Rate Tensile Characterization Of Graphite Platelet Reinforced Vinyl Ester Based Nanocomposites Using Split-Hopkinson Pressure Bar

The dynamic response of exfoliated graphite nanoplatelet (xGnP) reinforced and carboxyl terminated butadiene nitrile (CTBN) toughened vinyl ester based nanocomposites are characterized under both dynamic tensile and compressive loading. Dynamic direct tensile tests are performed applying the reverse impact Split Hopkinson Pressure Bar (SHPB) technique. The specimen geometry for tensile test is parametrically optimized by Finite Element Analysis (FEA) using ANSYS Mechanical APDL®. Uniform stress distribution within the specimen gage length has been verified using high-speed digital photography. The on-specimen strain gage installation is substituted by a non-contact Laser Occlusion Expansion Gage (LOEG) technique for infinitesimal dynamic tensile strain measurements. Due to very low transmitted pulse signal, an alternative approach based on incident pulse is applied for obtaining the stress-time history. Indirect tensile tests are also performed combining the conventional SHPB technique with Brazilian disk test method for evaluating cylindrical disk specimens. The cylindrical disk specimen is held snugly in between two concave end fixtures attached to the incident and transmission bars. Indirect tensile stress is estimated from the SHPB pulses, and diametrical transverse tensile strain is measured using LOEG. Failure diagnosis using high-speed digital photography validates the viability of utilizing this indirect test method for characterizing the tensile properties of the candidate vinyl ester based nanocomposite system. Also, quasi-static indirect tensile response agrees with previous investigations conducted using the traditional dog-bone specimen in quasi-static direct tensile tests. Investigation of both quasi-static and dynamic indirect tensile test responses show the strain rate effect on the tensile strength and energy absorbing capacity of the candidate materials. Finally, the conventional compressive SHPB tests are performed. It is observed that both strength and energy absorbing capacity of these candidate material systems are distinctively less under dynamic tension than under compressive loading. Nano-reinforcement appears to marginally improve these properties for pure vinyl ester under dynamic tension, although it is found to be detrimental under dynamic compression

Advances in Plastic Forming of Metals

The forming of metals through plastic deformation comprises a family of methods that produce components through the re-shaping of input stock, oftentimes with little waste. Therefore, forming is one of the most efficient and economical manufacturing process families available. A myriad of forming processes exist in this family. In conjunction with their countless existing successful applications and their relatively low energy requirements, these processes are an indispensable part of our future. However, despite the vast accumulated know-how, research challenges remain, be they related to the forming of new materials (e.g., for light-weight transportation applications), pushing the boundaries of what is doable, reducing the intermediate steps and/or scrap, or further enhancing the environmental friendliness. The purpose of this book is to collect expert views and contributions on the current state-of-the-art of plastic forming, thus highlighting contemporary challenges and offering ideas and solutions

Special Issue of the Manufacturing Engineering Society (MES)

This book derives from the Special Issue of the Manufacturing Engineering Society (MES) that was launched as a Special Issue of the journal Materials. The 48 contributions, published in this book, explore the evolution of traditional manufacturing models toward the new requirements of the Manufacturing Industry 4.0 and present cutting-edge advances in the field of Manufacturing Engineering focusing on additive manufacturing and 3D printing, advances and innovations in manufacturing processes, sustainable and green manufacturing, manufacturing systems (machines, equipment and tooling), metrology and quality in manufacturing, Industry 4.0, product lifecycle management (PLM) technologies, and production planning and risks

Semi-Rigid Moment-Resisting Behavior of Multiple Tab-and-Slot Joint for Freeform Timber Plate Structures

Historically based on the transfer of ancestral know-how, the art of carpentry changed considerably at the beginning of the 21st century with the arrival on the market of engineered wood products (EWP) and computer numerically controlled machines (CNC). The development of these wood products was initially started for the needs of aeronautics : lightweight panels but with high structural capacity. It is also in this field that automated multi-axis milling machines and the accompanying computing tool, computer-aided design and manufacturing (CAD / CAM), have begun to be developed to produce more complex mechanical parts. It was also in the early 2000s that several teams of researchers in the field of computer graphics but also in architecture were interested in complex free forms. One of the many challenges is still the discretization with planar elements of doubly curved surfaces oriented freely in space. The encounter of these `` algorithms '' with the high-performant wood panels was ineluctable. However, the assembly of polygonal panels in space to form a structure is not trivial. Optimization efforts are made so that the topology in itself favors the distribution of forces to tend towards a behavior of shells. But the problem is sometimes insoluble for very irregular shapes alternating negative, zero and positive Gaussian curvatures. To ensure the transmission of the axial, shear and bending internal forces induced by vertical or horizontal external loads, whether symmetrical or not, the performance of the assemblies used to connect these panels by their edges becomes paramount. In wood construction, this is a topic currently being studied by a few research laboratories including EPFL for innovative wood-to-wood joints with multiple tab and slots (MTSJ). The main difficulty lies in how to connect thin wood panels with the best stiffness and strength. In the recent past, the assembly of wood parts between them, often with metal fasteners to provide a certain ductility, was assimilated to a hinge. With the new standards like the Eurocodes, the notion of semi-rigidity is now essential to obtain a realistic response of structures to both ultimate and service limit states. The solution proposed at IBOIS enters into this new context. It allows, without glue or metallic connectors, to interconnect thin engineered wood panels, ensuring both locator and connector features and the transfer of multidirectional forces. This thesis focuses on the semi-rigid moment resisting behavior of the through tenon variant (TT) with closed slots. The numerous experiments conducted during this study demonstrated the ability of this variant to compete with screws to resist a moment around the angularly connected edges. The joint is directly generated during the multiaxial cutting of laminated veneer lumber(LVL) panels which include a certain percentage of cross-layers. From a structural point of view, the local mechanical study under a bending moment was inspired by a wood-to-wood joint in service in the ancestral Japanese shrines, the "Nuki" joint. Analyzed by Japanese researchers as it historically participates in the resilience of their ancestral constructions to typhoons and earthquakes, it activates embedment mechanisms of wood in rotational partial compression (RPC) to resist the moment resulting from the external loads. It is these mechanisms that are taken into account for through tenon variant. ..

EXPERIMENTS AND ANALYSIS OF ALUMINUM TUBE HYDROFORMING

This is a thesis on the development of an experimental table-top sized tube hydroforming machine at the University of New Hampshire. This thesis documents the design of the machine and the exploration of the forming envelope of the device via finite element modeling of the forming process. Several experiments on Al-6061-T4 tubes were used to evaluate the plastic behavior and strain limits of the tube in the axial and circumferential (hoop) directions. Two of these material tests, the uniaxial tension test and the ring hoop tension tests, were simulated with finite element models to refine the Al-6061-T4 plasticity curve, including the extrapolation of the hardening curve beyond the point of ultimate tensile stress. 2D and 3D finite element models of the hydroforming process were also used to evaluate potential tube materials, outer diameters, and wall-thickness for future experiments and research efforts

Tube and Sheet Metal Forming Processes and Applications

At present, the manufacturing industry is focused on the production of lighter weight components with better mechanical properties and always fulfilling all the environmental requirements. These challenges have caused a need for developing manufacturing processes in general, including obviously those devoted in particular to the development of thin-walled metallic shapes, as is the case with tubular and sheet metal parts and devices.This Special Issue is thus devoted to research in the fields of sheet metal forming and tube forming, and their applications, including both experimental and numerical approaches and using a variety of scientific and technological tools, such as forming limit diagrams (FLDs), analysis on formability and failure, strain analysis based on circle grids or digital image correlation (DIC), and finite element analysis (FEA), among others.In this context, we are pleased to present this Special Issue dealing with recent studies in the field of tube and sheet metal forming processes and their main applications within different high-tech industries, such as the aerospace, automotive, or medical sectors, among others