This investigation focuses on mode I delamination propagation in a unidirectional (UD) carbon fiber reinforced polymer (CFRP) composite laminate. Delamination propagation in this type of material may be accompanied by fiber bridging, a phenomenon where fibers from one face of the delamination cross over to the other face, such that the fibers are simultaneously pulled from both faces, thus, bridging the delamination. This increases the material’s apparent resistance to further propagation of the delamination. This phenomenon occurs mostly in beam-type test specimens commonly used to characterize failure of composite materials, but does not generally occur in structures with the exception a few structures such as rotor blades. The aim of this investigation is to quantify the effect of fiber bridging for quasi-static and fatigue testing of DCB specimens so as to eliminate it from the fracture and fatigue delamination propagation properties.
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Keywords: Keynote
EXAMINING SUB-GRAIN DRIVING FORCES FOR SMALL CRACK GROWTH [Keynote]
High energy X-ray diffraction microscopy (HEDM) techniques and micro-computed tomography were combined with in-situ cyclic loading to examine the evolution of sub-grain-level fatigue crack growth within a Ni-base superalloy at room temperature. A focused-ion beam notch was introduced within the specimen to concentrate damage within the characterized microstructure region of interest. The test specimen was subjected to fatigue cycling with pauses for periodic micro-computed tomography and HEDM measurements to characterize the sporadic growth of the crack front and grain-level strains ahead of the crack front. The HEDM data was used to instantiate a crystal plasticity finite element model and compared to experimentally determined grain-level strains, sub-grain reorientation, and crack path.
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MIXED FINITE ELEMENT METHOD FOR FRACTURE MODELING OF PIEZO- AND FERROELECTRIC MATERIALS WITH STRAIN GRADIENTS (FLEXOELECTRICITY) [Keynote]
Following the continuous miniaturization of the microelectromechanical systems (MEMS), a size-dependent phenomenon of flexoelectricity starts to play an essential role at the micro- and nanoscale. Direct flexoelectricity is an electromechanical coupling of strain gradients, which are inversely proportional to the length scale, and electric field. Due to the application of strain gradients, the centrosymmetry of the unit cells is broken, allowing a wider choice of dielectrics to be used in applications. In the proposed research, the nonlinear ferroelectric material behavior is further enhanced with strain gradients and applied to fracture problems with naturally occurring gradients of electromechanical fields near the crack tip. Or in another way, it is the incorporation of the remanent strains and polarization into the flexoelectric formulation. Our solution demonstrates the strong influence of the gradients on the ferroelectric domain switching behavior, leading to modified electromechanical fields close to the crack tip compared to the well-known ferroelectric problems.
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TOWARDS PRACTICAL SIMULATION OF STEEL FRACTURE IN STRUCTURAL AND EARTHQUAKE ENGINEERING APPLICAITONS [Keynote]
Over twenty-five years ago, unexpected fractures occurred in dozens of welded steel frame buildings during the 1994 Northridge earthquake. This prompted major research programs to (1) develop more reliable connection details for new buildings, (2) develop strategies to repair and retrofit damaged buildings, and (3) to assess the seismic safety of buildings. Efforts to develop alternative designs for new and retrofitted steel connections primarily relied on large-scale laboratory testing of connection subassemblies. Since then, significant advancements have been made in nonlinear finite element techniques to simulate the inelastic behavior of structural components and systems, although challenges remain to reliably simulate fracture, particularly for seismic design, where structural components are designed to undergo large-scale yielding. This presentation will summarize research on continuum-based fracture mechanics, where cyclic void growth models are used to assess ductile fracture initiation and propagation under large scale cyclic yielding. The models are implemented through finite element analyses and validated through a series of tests on notched axial bars, compact tension specimens, and large scale steel subassembly tests of braces and column base connections. Applications to use detailed finite element models to calibrate macro-scale models for incorporating fracture limit states in overall structural system response are also described.
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WARP3D: OPEN SOURCE SOFTWARE FOR 3D NONLINEAR FRACTURE MECHANICS [Keynote]
The WARP3D project (warp3d.net) provides all source code, extensive documentation and ready-to-run executables on Windows, Linux and macOS for researchers and practitioners worldwide. Developed by a group at the University of Illinois starting in the late 1990s to support academic research, WARP3D capabilities focus on modeling nonlinear fracture processes primarily in metals from the microscale to structural components. This presentation describes the origin and several key capabilities of the code.
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CRACK TIP TRANSFORMATION ZONE MORPHOLOGY IN SMA MATERIALS WITH TRANSFORMATION SOFTENING [Keynote]
The pseudoelastic effect due to martensitic transformation in polycrystalline shape memory alloys is simulated with a phenomenological constitutive model based on a kinematic hardening framework with a gradient enhancement to regularize moving austenite-to-martensite boundaries that arise in softening materials. This constitutive modeling framework introduces a length scale within the theory which has yet to be deteremined experimentally. Calculations are presented for the evolution of the transformation zone around a stationary crack tip. The calculations uncover the interplay between the length scale associated with the size of the transformation zone around the crack tip and the material length scale inherent to the consistutive model. The calcualtions show that localized “fingers” or “needles” of deformation emenate from the transformation zone at a specific level of the applied stress intensity, which provide a comparison to experimental observations that then can be used to quantify the sie of the material length scale.
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ADVANCES IN NECKING-ASSISTED CONTROLLED FRAGMENTATION BY COMPOSITE COLD DRAWING [Keynote]
Fracture of materials has been regarded as the major danger to structures and is to be avoided in design, manufacture and maintenance. However, the application of classical cold drawing technique to advanced composites consisting of brittle semiconductor/glass/2D materials and ductile polymers prone to necking enables controlled fragmentation of the target component, resulting in structured patterns in micro- down to nano- scales. The controlled fragmentation can thus be taken advantage of to produce microstructures in large scale. Mechanism of controlled fragmentation and key parameters for tuning fragment size are revealed through theoretical modeling, experiment and finite element analysis. Effects of the addition of a sacrificial layer/capping layer on fragment size to improve capability of the cold drawing technique will also be discussed.
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