Snap Back Research Articles

Inline Monitoring of continous Ultrasonic Welding Processes of Thermoplastic Composites via a custom polyCMUT based Ultrasound Array

Ultrasonic welding (UW) of thermoplastic composites (TCs) is an emerging technology in the field of composite joining techniques in the aerospace sector. Through a mechanical oscillator, ultrasound at a frequency of 20kHz is induced into the material via a welding horn, where microscopic friction and damping effects melt the thermoplastic. Under further pressure the weld area cools down, permanently joining both parts together. Like all joining processes in the aerospace industry the resulting joints need to be tested for their quality and structural integrity. The traditional testing method using water-coupled ultrasound includes extra steps. This process could be considerably improved by assessing the quality of the weld directly after or even during the welding process, allowing for immediate rework or discard of the parts in question. Ultrasound is still the best solution for this quality assessment, being inexpensive, well understood, and able to create B-Mode images, allowing a look into the cross-section of the weld. However, there are several major problems: To increase the system complexity as little as possible it is necessary to attach the ultrasound unit next to the welding equipment, and as close to the welding horn and compactor as possible to save space and keep the end- effector manoeuvrable. This brings problems for classic piezoelectric ultrasonic arrays: The low welding frequency and its resonance modes reach into the lower resonance modes of the piezoelectric sensors leading to immense noise, hiding any potential echo from the welding zone. Classic piezoelectric crystals are also very brittle and can suffer damage from sustained exposure to this violent environment. The authors present a novel solution: a custom-made polymer-based capacitive micromachined ultrasonic transducer array (polyCMUT). polyCMUTs are tiny drums with two electrodes. One on the bottom and the other suspended over a cavity sandwiched between two layers of polymers. By applying a DC-bias an electrical field is created and the membrane is set under tension. If then an AC voltage is applied, the strength of the electric field decreases, allowing the membrane to snap back into its original position. If done at the resonance frequency of the membrane, a strong ultrasonic signal is created. To receive this signal the polyCMUT is charged with a DC-bias, allowing it to receive the echo of the transmitted signal by measuring the changing capacitance. Not only is the polymer robust and inexpensive to fabricate, the general architecture of CMUTs also allows a design where the first mode of resonance is the actual mode the CMUT is operating in. By designing for a resonance frequency over 5 MHz all noise from the initial welding process is ignored, leading to a working pulse echo imaging system. The array is then mounted onto a PEEK block attached to the compactor unit of the welding end-effector. This publication is intended to present initial results, the design process of the custom array and the tests leading there.

Enabling TEL capacity across complexity

Embedding technology is now a necessary part of higher education teaching and learning policy and practice, owing to cascading effects of technological advancements and the pandemic-driven disruption of traditional teaching and learning modes (Rapanta et al., 2020, 2021). Well-designed technology enhanced learning (TEL) spaces and activities can provide experiences that are authentic, learner-centred, flexible, and equitable (Cochrane et al., 2017; Dunn & Kennedy, 2019).
 
 Despite growing supportive evidence of these benefits, university teaching and learning systems often retain legacy structures and practices where technology-enhanced, creative and flexible activities are not easily integrated or well supported (Bridges et al. 2023). These issues may be exacerbated by the perceived comfort – by both academics and institutions – offered by a ‘snap back’ to pre-pandemic settings (Bryant 2022). As institutions rethink the ways in which they enact TEL (García-Morales et al., 2021; Rapanta et al. 2021), understanding and navigating potential barriers and enablers of innovative TEL design (Bone 2022) becomes important. Within this presentation, I draw and reflect on recent projects to consider how academic development programs might facilitate TEL initiatives that are targeted and sustainable.
 
 For individual academics, approaches to teaching and learning can vary, with flow-on effects into the ways they teach, and the learning outcomes of their students (Trigwell & Prosser, 1997, 2004). In challenging, high-pressure environments, academics who teach may not have the capacity to adapt their approaches to a rapidly changing context (Bone 2021). Indeed, impacts of the pandemic highlight that academic development during times of crisis needs to be more holistic and provide adequate support for innovative change (Bone et al. 2021; Sumer et al., 2021; Mulder et al., 2022; Bone et al. in review). Enacting such holistic academic development in higher education institutions, which are highly complex (Knight 2001) but increasingly fragmented and siloed (Becher & Trowler, 2001), requires approaches to design and delivery of programs and supports that mimic and respond to this complexity (Bone & Ross 2019), and are responsive to the intents and priorities of both academics and management, and the learning needs of students.
 
 Embedding technologies into this complex curriculum environment in ways that are sustainable and equitable requires approaches that both build incentives and drivers from leadership (top-down), and reward and harness the existing enthusiasm and capacity of academics (bottom up) (Bone 2022). Building networked approaches to enable curriculum design, development and innovation can involve communities of practice, mentoring or other knowledge-sharing activities across disciplines and roles (Bone et al., 2023). Enacting these across institutional boundaries can bring together those interested in specific aspects of TEL (Narayan et al., in press) who may also feel isolated within traditional hierarchical institutional structures (Bone et al., in press).
 
 Knowledge sharing and community building has clear potential to drive positive change. As a TEL community, we must continue to advocate for teaching, learning and development spaces that emphasise collaboration and collegial knowledge-sharing, and to push for greater recognition as we work together to build the future of higher education teaching and learning.

Effect of overheat and direct flow loading on the MEMS bistable flow sensor

We present the findings from an experimental study of a MEMS flow sensor in which an initially curved, double-clamped bistable microbeam is the primary sensing element. Our research explores how the overheat ratio, direct flow loading, and turbulence-induced vibration affect the sequential snap-through (ST) buckling and snap-back (SB) release of an electrostatically actuated beam heated by an electric current. The sensor is fabricated from highly doped single-crystal silicon using a silicon-on-insulator (SOI) wafer. Positioned at the chip’s edge, the microbeam is exposed to airflow, enabling concurrent dynamic response measurements with a laser Doppler vibrometer and a video camera.Our research demonstrates that the overheat ratio can be significantly lower for this sensing principle than conventional thermal sensing elements, pointing to the potential for substantial energy savings. We also emphasize the significant impact of flow angles and vibrations on the critical ST and SB voltages, which are vital for the flow sensor’s output. Additionally, we introduce the first direct experimental observation of the beam profile’s time history during the snap-through/snap-back transition.The potential impact of this research lies in developing more robust MEMS flow sensors with enhanced sensitivity and a better understanding of their response to environmental factors, which could have broader applications in fields such as aerospace, environmental monitoring, and industrial process control.

Geographies of COVID‐19

Geographies of COVID‐19

Scale effects in GFRP‐bar reinforced concrete beams

AbstractGlass fiber‐reinforced polymer (GFRP)‐reinforced concrete (RC) can be defined as a cementitious material in which the reinforcing secondary phase consists in corrosion‐resistant GFRP rebars. For this next‐generation structural material, experimental flexural tests highlight how the postcracking response is strongly affected by the amount of GFRP area together with the structural size‐scale. In this work, the cohesive/overlapping crack model (COCM) is adopted to describe the transition between cracking and crushing failures occurring in GFRP‐RC beams by increasing the beam depth, the reinforcement percentage, and/or the concrete compression strength. Within this nonlinear fracture mechanics model, the tensile and compression ultimate behaviors of the concrete matrix are modeled through two different process zones that advance independently one of another. Moreover, this model is able to investigate local mechanical instabilities occurring in the structural behavior of GFRP‐RC beams: tensile snap back and snap‐through, which are due to concrete cracking and reinforcement bridging action, and the compression snap‐back generated by the unstable growth of the crushing zone. In this context, the application of the COCM highlights that the ductility, which is represented by the plastic rotation capacity of the GFRP‐RC beam only when the reinforcement can slip, decreases as reinforcement percentage and/or beam depth increase. In this way, rational and quantitative definitions of hyperstrength and brittle compression crushing behaviors can be provided as a GFRP percentage depending on the beam depth.

Uncertainty in debonding of layered structures with adhesive layers

The impact of uncertainty on the stochastic debonding process in adhesively bonded layered structure is examined in this paper. The emphasis and focus of this work are put on the effect of explicitly modeling the adhesive layer within the stochastic-structural framework. A pull-out test of a single composite strip adhesively bonded to a rigid substrate is considered as a model of the layered configuration. The stochastic analysis includes physical uncertainty attributed to 10 different input parameters that are represented by random fields. Three main methodologies are integrated. The first one combines a high-order modeling of the deformable adhesive layer and the modeling of its two interfaces by the cohesive interface approach. This enables the analysis to look into the stress fields in the adhesive layer and the tractions that develop across its two interfaces. The second methodology quantifies the impact of uncertainty on the structural response by the nonlinear perturbation based stochastic finite elements method. Following the highly nonlinear character of the problem and the snap back folds involved with the response, the third methodology is integrated by means of an extended pseudo-arc-length procedure that is formulated to solve the nonlinear problem. The stochastic analysis points at the different role played by each component of the structure and each uncertain parameter in the stochastic structural response. It also describes the way these uncertainties evolve along the structural process. The results of the high order model are compared to those of a simplified model in which the adhesive layer and its two interfaces are replaced by a single cohesive interface representing the entire bonding layer. The comparison points at significant differences between the two modeling approaches, evident both in the expected (or deterministic) behavior of the structure and in its stochastic scattering measures.

On Sampling Rate Limits in Bistable Microbeam Sensors

Bifurcation-based sensors, implementing stability boundaries monitoring in bistable microstructures, may manifest higher sensitivity and robustness when compared to their statically operated counterparts. In the operation of these devices, two key questions arise: 1) how to reliably identify the critical events associated with the transition between two stable states; 2) what is the maximal sampling rate allowing to identify these critical events in a periodically actuated device. Motivated by the potential implementation as a bifurcation-based flow sensor, in this work we experimentally explore the transient snap-through (ST) and snap-back (SB) dynamics of a bistable initially curved ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$500 \,\mu \text{m}$ </tex-math></inline-formula> long and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5\, \mu \text{m}$ </tex-math></inline-formula> wide) electrostatically actuated single-crystal Si microbeam. We employ a threshold-based approach for the ST and SB events detection and identify two mechanisms determining the maximal sampling frequency of the device: a phase lag between the voltage signal and the mechanical response and free decaying vibrations following the buckling event. In accordance with our experimental results and consistently with the reduced-order model predictions for the bistable beam operating at atmospheric pressure, at the actuation signal frequency of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 0.6$ </tex-math></inline-formula> KHz, which is still much lower than the beam’s natural frequency of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 110 $ </tex-math></inline-formula> KHz, the free oscillations engendered by one critical event (ST or SB) interfere with the consecutive one, preceding its emergence. On the other hand, an inherent phase lag ultimately hinders the abrupt ST and SB transitions at these actuating frequencies. [2021-0091]

Experimental study on complete stress-strain characteristics of two kinds of green concretes

The current study explores the empirical whole stress-strain curves detected from the axial compression test of the green concretes manufactured with two kinds of no-cement binders. The first one is the binder of alkali-activated material (AAM) with ground granulated blast furnace slag blending with Class F fly ash (FFA) and the second is a new hydraulic SFC binder produced by mixing ternary powders of slag (S), FFA (F), and circulating fluidized bed combustion fly ash (C). The performances of two green concretes were conducted and compared with that of the plain ordinary Portland cement (OPC) concretes with the equivalent 28-day compressive strengths graded at 30, 40, and 50 MPa. Experimental results showed that the AAM concretes had the energy absorption capacity lower than those of the SFC and OPC concretes because the descending parts of the whole stress-strain curves of the AAM concretes referred to the snap back mode different from the strain softening modes of those of the SFC and OPC concretes with the compressive strength graded at 30 or 40 MPa. Among three concretes with equivalent strength grades, the AAM concretes had the lowest elastic Young

Ways that Animal Wings Produce Sound

There are at least eight ways that wings potentially produce sound. Five mechanisms are aerodynamic sounds, created by airflow, and three are structural sound created by interactions of solid surfaces. Animal flight is low Mach (M), meaning all animals move at less than 30% of the speed of sound. Thus in aerodynamic mechanisms the effects of air compressibility can be ignored, except in mechanism #1. Mechanism #1 is trapped air, in which air approaches or exceeds Mach 1 as it escapes a constriction. This mechanism is hypothetical but likely. #2 is Gutin sound, the aerodynamic reaction to lift and drag. This mechanism is ubiquitous in flight, and generates low frequency sound such as the humming of hummingbirds or insect wing tones. #3 is turbulence-generated atonal whooshing sounds, which are also widespread in animal flight. #4 are whistles, tonal sounds generated by geometry-induced flow feedback. This mechanism is hypothetical. #5 is aeroelastic flutter, sound generated by elasticity-induced feedback that is usually but not always tonal. This is widespread in birds (feathers are predisposed to flutter) but apparently not bats or insects. Mechanism #6 is rubbing sound (including stridulation), created when bird feathers or insect wings slide past each other. Atonal rubbing sounds are widespread in bird flight and insects; tonal stridulation is widespread in insects. #7 is percussion, created when two stiff elements collide and vibrate, and is present in some birds and insects. Mechanism #8 are tymbals and other bistable conformations. These are stiff elements that snap back and forth between two conformations, producing impulsive, atonal sound. Tymbals are widespread in insects but not birds or bats; insect cuticle appears predisposed to form tymbals. There are few examples of bat wing sounds: are bats intrinsically quiet, or just under-studied? These mechanisms, especially Gutin sound, whooshes, and rubbing (#2, #3, and #6) are prominent cues in ordinary flight of all flying animals, and are the 'acoustic substrate' available to be converted from an adventitious sound (cue) into a communication signal. For instance, wing sounds have many times evolved into signals that are incorporated into courtship displays. Conversely, these are the sounds selected to be suppressed if quiet flight is selected for. The physical mechanisms that underlie animal sounds provides context for understanding the ways in which signals and cues may evolve.

Flow Velocity Gradient Sensing Using a Single Curved Bistable Microbeam

An approach for gas flow velocity measurement at two different closely located points using a simple device incorporating only one thermal sensing element, is introduced and the feasibility to determine different velocities is experimentally demonstrated. An electrostatically actuated initially curved bistable microbeam heated by an electric current and convectively cooled by airflow is switched between two stable locations through the snap-through (ST) and snap-back (SB) buckling mechanisms. The velocities of an air flow at these positions are obtained by measuring the critical ST and SB values of the actuation voltage. In the experiments, our $500~\mu \text{m}$ long, $2~\mu \text{m}$ wide single-crystal Si beam with $\approx \, 2.5\,\mu \text{m}$ nominal initial elevation demonstrated sensitivity of $S_{\text {ST}} \approx 0.25$ V/(m/s) and $S_{\text {SB}} \approx 0.84$ V/(m/s) at the ST and SB points, respectively. In the present device, the distance between the two measurement points is $\approx 10~\mu \text{m}$ . Our experimental results indicate that the suggested approach can be used for the velocity gradient measurements. The sensing principle relying on a single bistable sensing element opens new opportunities for measurement of gas flow velocity and velocity gradients at scales significantly smaller than the state-of-the-art multi hot-wire sensors.

Utilization of Industry 4.0 Related Equipment in Assembly Line Balancing Procedure

In recent decades, production in high-volume/low-variety batches is replaced with low-volume/high-variety production type. This type of production demands excessive flows of both material and information. Recent advances in information and communication technologies (ICT), together with the concept of cyber-psychical system (CPS) enable the concept of Industry 4.0 (I4.0). In this paper, the performance of I4.0 related equipment implementation is presented in iterative assembly line balancing (ALB) process of a gearbox assembly line. Largest candidate rule method through spreadsheet simulation was used for tasks reallocations, with the objective to minimize the cycle time when the number of stations is fixed. Utilization of human analysts using snap back method for manual data gathering process still shown advantage over I4.0 equipment utilization in manual ALB. The assembly process is performed in the learning factory environment, and it is considered as very close to real industry process. The major conclusion is that I4.0 is excellent in process data monitoring and product tracking, but activities to be performed to effectively exploit I4.0 is demanding for task reallocations during the balancing procedure. Nevertheless, future enhancements of I4.0 system are listed to bridge this gap and to increase I4.0 system usefulness in the manual assembly line balancing process.

Winning the Race for Survival: How Advanced Manufacturing Technologies Are Driving Business-Model Innovation

The shock brought by the pandemic is liberating firms to make numerous experiments. Some will fail, while others will succeed. This creates a tipping point for a long overdue conversation about new business models in advanced manufacturing and supply chains. As organizations settle into either a new normal or plan their return to pre-pandemic operations, the question rises as to whether firms will “snap back” to normal or settle on some new equilibrium. We may be on the precipice of “Operational Darwinism”, wherein mere reductions in costs may not be enough to compete against leaders who make manufacturing a rapid and key part of their digital innovation edge. In this paper, we explore what new business models might look like and how they are likely to interact with both existing operating and business models in the context of the new normal brought about by the pandemic.

Bow shocks, bow waves, and dust waves – II. Beyond the rip point

Dust waves are a result of gas-grain decoupling in a stream of dusty plasma that flows past a luminous star. The radiation field is sufficiently strong to overcome the collisional coupling between grains and gas at a "rip-point", where the ratio of radiation pressure to gas pressure exceeds a critical value of roughly 1000. When the rip point occurs outside the hydrodynamic bow shock, a separate dust wave may form, decoupled from the gas shell, which can either be drag-confined or inertia-confined, depending on the stream density and relative velocity. In the drag-confined case, there is a minimum stream velocity of roughly 60 km/s that allows a steady-state stagnant drift solution for the dust wave apex. For lower relative velocities, the dust dynamics close to the axis exhibit a limit cycle behavior (rip and snap back) between two different radii. Strong coupling of charged grains to the plasma's magnetic field can modify these effects, but for a quasi-parallel field orientation the results are qualitatively similar to the non-magnetic case. For a quasi-perpendicular field, on the other hand, the formation of a decoupled dust wave is strongly suppressed.

Towards Understanding of Polymorphism of the G-rich Region of Human Papillomavirus Type 52.

The potential to affect gene expression via G-quadruplex stabilization has been extended to all domains of life, including viruses. Here, we investigate the polymorphism and structures of G-quadruplexes of the human papillomavirus type 52 with UV, CD and NMR spectroscopy and gel electrophoresis. We show that oligonucleotide with five G-tracts folds into several structures and that naturally occurring single nucleotide polymorphisms (SNPs) have profound effects on the structural polymorphism in the context of G-quadruplex forming propensity, conformational heterogeneity and folding stability. With help of SNP analysis, we were able to select one of the predominant forms, formed by G-rich sequence d(G3TAG3CAG4ACACAG3T). This oligonucleotide termed HPV52(1–4) adopts a three G-quartet snap back (3 + 1) type scaffold with four syn guanine residues, two edgewise loops spanning the same groove, a no-residue V loop and a propeller type loop. The first guanine residue is incorporated in the central G-quartet and all four-guanine residues from G4 stretch are included in the three quartet G-quadruplex core. Modification studies identified several structural elements that are important for stabilization of the described G-quadruplex fold. Our results expand set of G-rich targets in viral genomes and address the fundamental questions regarding folding of G-rich sequences.

Snap back testing of unbonded post-tensioned concrete wall systems

Unbonded Post-Tensioned (UPT) precast concrete systems have been shown to provide excellent seismic resistance. In order to improve understanding of the dynamic response of UPT systems, a series of snap back tests on four UPT systems was undertaken consisting of one Single Rocking Wall (SRW) and three Precast Wall with End Columns (PreWEC) systems. The snap back tests provided both a static pushover and a nonlinear free vibration response of a system. As expected the SRW exhibited an approximate bi-linear inertia force-drift response during the free vibration decay and the PreWEC walls showed an inertia force-drift response with increased strength and energy dissipation due to the addition of steel O-connectors. All walls exhibited negligible residual drifts regardless of the number of O-connectors or the post-tensioning force. When PreWEC systems of the same strength were compared the inclusion of further energy dissipating O-connectors was found to decrease the measured peak wall acceleration. Both the local and global wall parameters measured at pseudo-static and dynamic loading rates showed similar behaviour, which demonstrates that the dynamic behaviour of UPT walls is well represented by pseudo-static tests. The SRW was found to have Equivalent Viscous Damping (EVD) between 0.9-3.8% and the three PreWEC walls were found to have maximum EVD of between 14.7-25.8%.

Coexisting crystal and liquid-like properties in a 2D long-range self-consistent model

A two-dimensional class of mean-field models serving as a minimal frame to study long-range interaction in two space dimensions is considered. In the case of an anisotropic mixed attractive-repulsive interaction, an initially spatially homogeneous cold fluid is dynamically unstable and evolves towards a quasi-stationary state in which the less energetic particles get trapped into clusters forming a Bravais-like lattice, mimicking a crystalline state. Superimposed to this, one observes in symplectic numerical simulations a flux of slightly more energetic particles channeling through this crystalline background. The resultant system combines the rigidity features of a solid, as particles from a displaced core are shown to snap back into place after a transient, and the dynamical diffusive features of a liquid for the fraction of channeling and free particles. The combination of solid and liquid properties is numerically observed here within the classical context. The quantum transposition of the model may be experimentally reached using the latest ultracold atoms techniques to generate long-range interactions.

Eyelid metrics assessment in patients with chronic ocular graft versus-host disease

PurposeTo assess eyelid metrics in the setting of chronic ocular graft versus-host disease (oGVHD), and to further correlate them with hematological and ocular characteristics. MethodsProspective case-control study conducted at a single tertiary-referred Center. The following eyelid tests were performed in oGVHD patients and control subjects: vertical lid pull; anterior/lower distraction; lateral/medial distraction; distance between lateral canthal angle and orbital rim; margin reflex distances (MRD) 1 and 2; duration of tarsus exposure; snap back. Correlations of eyelid metrics with hematological and ocular parameters in the oGVHD group were performed. ResultsTwenty-seven patients with oGVHD and 27 healthy matched subjects were finally included. Significantly higher values of vertical lid pull, anterior/lower distraction, lateral/medial distraction, and distance between lateral canthal angle and orbital rim were found in the oGVHD group compared to controls (always p < 0.02). Conversely, MRD1 was lower in oGVHD patients compared to controls (p < 0.001). Duration of tarsus exposure and snap back test were pathological in a higher percentage in oGVHD group compared to controls (respectively 66.7 and 59.3% vs 33.3 and 25.9%; p < 0.005). Vertical lid pull test was significantly higher in oGVHD patients with superior limbic keratoconjunctivitis (13.8 ± 2.4 vs 10.9 ± 2.4, p = 0.010). Ocular GVHD patients with subtarsal fibrosis had a higher percentage of pathological values for duration of tarsus exposure test (77.3 vs 20.0%; p = 0.014). ConclusionsThe present study highlighted for the first time higher eyelid laxity in oGVHD patients. This intriguing association may add a further piece to the puzzle of clinical features occurring in the setting of oGVHD.

Full-scale experimental assessment of the dynamic horizontal behavior of micropiles in alluvial silty soils

The paper deals with an extensive experimental campaign of dynamic tests performed on full-scale vertical injected and not injected micropiles embedded in alluvial soils. The experimentation includes ambient vibration and impact load tests to investigate the dynamic behaviour of the soil-micropile system in very small and small-medium strain ranges, as well as snap back tests to investigate the evolution of the post-elastic response of the system. Micropiles are instrumented with strain gauges along the shaft, accelerometers and displacement transducers at the head. Modalities and results of the different performed tests are presented, focusing on the role of high pressure injections on the dynamic behaviour of the system, on the development of non-linear soil-pile phenomena, and, finally, on the evaluation of effectiveness of the different testing procedures to capture the dynamic response of soil-micropile systems.Ambient vibration test revealed to be a versatile method to identify the dynamic properties of soil-micropile systems, although it requires demanding post-processing techniques of data. Impact load test is faster to perform and easier to post process. Free vibration test proved to be relatively simple to perform and able to catch the dynamic non-linear behaviour of soil-micropile systems.

Snap‐back free shorted‐anode super‐junction TCIGBT

A novel structure called the shorted-anode super-junction trench clustered insulated gate bipolar transistor (SA-SJ-TCIGBT) is proposed and demonstrated through numerical simulations in 1.2 kV, field-stop technology. This device is based on the SJ-TCIGBT concept. In the SA-SJ-TCIGBT structure, due to the introduction of a segmented n+-anode, the device can operate in both forward conducting mode and freewheeling diode mode without any snap back in the current–voltage characteristics. In comparison to the SJ-TCIGBT structure, the proposed device shows significant improvement in trade-off relationship between forward voltage drop and switch off energy losses. Simulation results show that 25% decrease in switching energy losses can be achieved. Moreover, the tail current is effectively reduced without any increase in the overshoot voltage. Detailed two-dimensional modelling of the structure shows that significant amount of excess electrons are extracted through the SA structure during turn-off process.

Anisotropic nonlocal damage model for materials with intrinsic transverse isotropy

This paper presents the theoretical formulation and numerical implementation of an anisotropic damage model for materials with intrinsic transverse isotropy. Crack initiation and propagation are modeled by phenomenological damage evolution laws, controlled by four equivalent strain measures. The latter are constructed so as to distinguish the mechanical response of the material in tension and compression, along the direction perpendicular to the bedding plane and within the bedding plane. To avoid mesh dependency induced by softening, equivalent strains are replaced by nonlocal counterparts, defined as weighted averages over a neighborhood scaled by two internal length parameters. Finite Element equations are solved with a normal plane arc length control algorithm, which allows passing limit points in case of snap back or snap through. The model is calibrated against triaxial compression tests performed on shale, for different confinements and loading orientations relative to the bedding plane. Gauss point simulations confirm that the model successfully captures the variation of uniaxial tensile strength with respect to the bedding orientation. Finite Element simulations of three-point bending tests and compression splitting tests show that nonlocal enhancement indeed avoids mesh dependency, and that the axial and transverse dimensions of the damage process zone are scaled by the two characteristic lengths. Results further show that the damage process zone is direction dependent both in tension and compression. The model can be used for any type of textured brittle material; it allows representing several concurrent damage mechanisms in the macroscopic response and interpreting the failure mechanisms that control the damage process zone.