Process Zone Research Articles

Strain‐Controlled Low‐Cycle Fatigue Behavior of a Superalloy Considering Extensive Temperature Range and Stress Concentration

ABSTRACTThis study aims to investigate the low cycle fatigue (LCF) behavior and fatigue life prediction method of an advanced solid solution–strengthened nickel‐based superalloy at various temperatures and stress concentrations. Cylindrical specimens as well as plates with three types of holes (including straight holes and inclined holes with different orientations) were designed and machined for fatigue tests. The test results revealed fatigue life decreases with the temperature increasing. Meanwhile, the fatigue lives of the plate specimens have decreased differently and are strongly dependent on the geometry and orientation of the holes. The key damage parameter related to the fatigue life was found on the strain gradient path. A criterion based on strain gradient is defined as the boundary of the damage process zone. The results show that the prediction was in good agreement with the experimental data, for the majority of the data are within a scatter band of ±2.

Investigating the effect of dynamic laser beam oscillations in remote fusion cutting process

The latest research on laser beam fusion cutting has revealed significant improvements in process productivity and cut quality through the use of dynamic beam shaping techniques. While many studies have investigated dynamic beam shaping for proximity cutting, the influence of laser beam oscillations on the remote fusion cutting process remains unexplored. The present work aims to study the effect of dynamic beam shaping on the remote fusion cutting process through analytical modeling, experimental investigations, and in situ high-speed monitoring. Initially, an analytical model based on thermodynamic analysis was developed to assess the influence of circular oscillations on the process zone. This model facilitates the evaluation of process performance from an energetic perspective, providing an estimate of the maximum achievable cutting speed for the remote fusion cutting process across various operating conditions. A significant increment in process productivity could be achieved through beam oscillations. Furthermore, based on theoretical findings, the effect of circular laser beam oscillations superimposed on the processing feed direction was experimentally investigated using a 1 mm thick AISI304 stainless steel material. A 6 kW fiber laser was utilized, alongside a high-speed camera-based system for in situ process monitoring. The experimental results demonstrate a significant increase in the process productivity under dynamic beam shaping conditions, consistent with theoretical findings. Specifically, the maximum achievable cutting speed could be increased from 0.13 to 0.20 m/s. Furthermore, the cut quality of produced samples was evaluated in terms of kerf morphology and profile.

A rate-dependent crack bridging model for dynamic fracture of CNT-reinforced polymers

Carbon nanotubes (CNTs) improve the fracture toughness of polymer-based matrix composites by bridging the crack growth path. This research presents a finite element (FE) rate-dependent crack bridging model of Mode-I dynamic crack growth in CNT-reinforced polymers accounting for rate of loading and rapid crack growth. Two distinct CNT bridging and matrix cracking damage mechanisms are taken into account in the fracture process zone (FPZ) accounting for the dissipation of fracture energy. A viscoelastic-viscoplastic material model is adapted for the matrix phase to capture the strain rate effects. The crack in the matrix phase is modeled using cohesive zone elements characterized by experimental data. A rate-dependent traction-separation law obtained from CNT pull-out simulation at relevant crack opening speeds is used to simulate the CNT bridging in the FPZ. Considering the given traction-separation law as a constitutive equation, the CNTs are replaced with non-linear spring elements to facilitate the FE simulation of crack bridging with numerous CNTs in the FPZ. The proposed rate-dependent FE model for crack bridging enables the study of the effect of key CNT factors such as length, orientation, waviness, volume fraction, and agglomeration on fracture energy dissipation at various crack speeds. The developed model can simultaneously consider the interactive effects of all CNT parameters which is useful for considering all processing-induced uncertainties in analyzing the effects of CNTs on the dynamic fracture toughness of nanocomposites.

Anisotropic Fracture Evolution and Size Effect of the Fracture Process Zone in Laminated Shales Derived from Digital Image Correlation

Anisotropic Fracture Evolution and Size Effect of the Fracture Process Zone in Laminated Shales Derived from Digital Image Correlation

Spatiotemporal mapping of auditory onsets during speech production.

The human auditory cortex is organized according to the timing and spectral characteristics of speech sounds during speech perception. During listening, the posterior superior temporal gyrus is organized according to onset responses, which segment acoustic boundaries in speech, and sustained responses, which further process phonological content. When we speak, the auditory system is actively processing the sound of our own voice to detect and correct speech errors in real time. This manifests in neural recordings as suppression of auditory responses during speech production compared to perception, but whether this differentially affects onset and sustained temporal profiles is not known. Here we investigated this question using intracranial EEG recorded from seventeen pediatric, adolescent, and adult patients with medication-resistant epilepsy while they performed a reading/listening task. We identified onset and sustained responses to speech in bilateral auditory cortex and observed a selective suppression of onset responses during speech production. We conclude that onset responses provide a temporal landmark during speech perception that is redundant with forward prediction during speech production and are therefore suppressed. Phonological feature tuning in these "onset suppression" electrodes remained stable between perception and production. Notably, auditory onset responses and phonological feature tuning were present in the posterior insula during both speech perception and production, suggesting an anatomically and functionally separate auditory processing zone that we believe to be involved in multisensory integration during speech perception and feedback control.Significance Statement Specific neural populations in the auditory cortex preferentially respond to the onset of speech sounds. These "onset responses" aid in perceiving boundaries in continuous speech. We recorded neural responses from patients with intracranial electrodes during a speaking and listening task to investigate the role of onset responses in speech production. Onset responses were present in the auditory cortex during listening, but absent during speaking. On the other hand, onset responses were observed in the insula during both conditions, suggesting a different functional role for the insula in auditory feedback processing. These findings extend our knowledge of how different parts of the brain involved in feedback control operate during speech production by identifying two functionally and anatomically distinct patterns of activity.

Strategic Procurement Practices and Performance of Export Processing Zone Firms in Machakos County, Kenya.

Export Processing Zones (EPZs) were established in Kenya to boost industrialization, attract foreign investment, and drive economic growth. However, despite significant government efforts, the performance of EPZ firms has been below expectations. This study investigates whether strategic procurement practices could improve the performance of EPZ firms in Machakos County, Kenya. Strategic procurement, which aligns procurement with organizational goals and emphasizes long-term value creation, was the focus. The study specifically explored the impact of strategic sourcing, strategic partnerships, strategic inventory management, and strategic contract management on the performance of EPZ firms in Machakos County. Using a descriptive research design, all 99 EPZ firms in Machakos County were surveyed through a census approach. Data was collected via structured questionnaires, with a pilot study conducted to ensure content validity and clarity. Both qualitative and quantitative data were analyzed, with results presented in tables and charts. The findings showed that the four strategic procurement practices adopted improved the performance of export processing zone (EPZ) firms in Machakos County. Strategic sourcing for instance significantly improved the operational efficiency and cost-effectiveness of EPZ firms. Firms that developed strategic partnerships with suppliers experienced enhanced product quality and supply chain reliability. Effective inventory management practices led to reduced waste and better resource allocation, positively impacting overall performance. Proper contract management was associated with improved compliance and reduced legal disputes, contributing to better performance metrics. The study concluded that adopting strategic procurement practices substantially enhances the performance of EPZ firms in Machakos County. By aligning procurement strategies with organizational goals and focusing on long-term value, firms can achieve greater efficiency, reliability, and cost savings. Policymakers should promote the adoption of strategic procurement practices within EPZs to boost economic performance.

Photodiode-based process monitoring for the ultrashort-pulsed laser structuring of the diffusion media for fuel cells

In recent years, the incorporation of process monitoring systems has become an integral part across several laser material processing applications. This is due to the feasibility of inline evaluations, which enable the elimination of downstream quality checks that are typically time-consuming and costly. Simultaneous to recent sensor developments, ultrashort-pulsed (USP) laser material processing has become a promising technology for creating complex microstructures in a wide variety of materials due to its high precision and negligible thermal input. So far, process monitoring has yet to be established in USP applications as it is unknown which sensors are suitable, which parameters can be observed, and which process-relevant information can be detected within the data. Within this work, a state-of-the-art photodiode-based process monitoring system was used to collect spectral data from the process zone and extract relevant information from the process. The laser structuring of the two-layered diffusion media used in polymer electrolyte membrane fuel cells was selected as the use case. This structuring process is challenging due to production-related fluctuations in the material thickness and the surface quality. Within this study, information was acquired from a sensor system employing three photodiodes. Through the application of explorative data analysis techniques and machine learning, AI models were created, focusing on determining the focus position of the laser beam and identifying the transition between the two layers. The developed models were capable of determining the initial focus position with varying accuracies. The best performance was achieved by a convolutional neural network using spectrograms as input data, while the worst accuracy was achieved by a ridge regression model using manually selected features. Furthermore, a clustering model was used, demonstrating the capability to detect the layer transitions within a specified tolerance.

Growth of thermoplastic shear band

Shear localization is a fundamental and widely observed non-equilibrium phenomenon in metallic materials subjected to impulsive loadings. Despite extensive research over several decades, the evolution of shear bands still remains poorly understood. In this study, we propose a two-stage model to describe the evolution of thermoplastic shear band, where the effects of strain hardening, strain-rate hardening, and thermal softening are involved. Our analysis reveals that the growth of thermoplastic shear band is derived by the competition between momentum diffusion and thermal diffusion processes, which is controlled by the solid-state equivalent of the Prandtl number. By incorporating these factors into our model, we find that the presence of hardening effect retards the evolution of shear bands, resulting in a wider shear band and increased dissipation of energy. Theoretical calculation results of shear bandwidth, shear band strain, propagation velocity, process zone geometry and dimensions, critical dissipation energy, shear band toughness, and shear band spacing are well consistent with available experimental data. Additionally, we explore the potential transition to turbulence controlled by Reynolds number within shear bands and investigate the influence of the Taylor-Quinney coefficient on the evolution of shear bands. These findings are expected to offer valuable insights into the mechanism of shear band evolution.

Experimental investigations on the formation of boundary layers in glass-based additive manufacturing of fused silica fibers by Laser Glass Deposition

Laser glass deposition is an additive manufacturing method to produce individualized glass components. This process uses a CO2 laser with a defocused beam as a heat source to additively melt-fused silica filaments. The fiber filament is fed laterally in the process zone and is deposited layer-by-layer to form 3D structures. The formation of boundary layers in conventional 3D-printing methods is a usual byproduct of the process. In this paper, the boundary layer formation of deposited fused silica filaments is investigated in detail by means of varying different process parameters such as, laser power, feed rate, laser spot diameter, and printing strategy. This involves examining both thin-walled and thick-walled test specimens. Quality characteristics like the surface roughness and the optical transmission are analyzed for the printed specimen. Finally, fully transparent structures with surface roughness below 100 nm and a transparency of 90% could be printed boundary layer-free and without post-processing.

Finite element modeling for cohesive/adhesive failure of adhesive structures with a thermosetting resin

In this study, we established a novel method based on the finite element method (FEM) for predicting the strength of adhesive structures. Viscoplasticity, void growth, and cohesive zone model were introduced into the FEM to create a nonlinear damage growth (NDG) model. This model was used to comprehensively analyze the process zones within the adhesive layer. Furthermore, the embedded process zone approach was used to develop an interface constitutive law that averages the mechanical response of the adhesive layer. This modified Ma–Kishimoto (MMK) model can accurately represent the adhesive layer as an interface element and is computationally efficient. Furthermore, the study obtained the necessary interface properties for the MMK model from the NDG model, creating a numerical material test that can approximate the effect of the process zone. To validate the proposed method, single-lap shear tests were performed, and the accuracy of the predicted strength and deformation field was evaluated. The damage evolution in the NDG model and the MMK model were compared, and the scope of application of the MMK model was discussed. The results of this study can be used as a reference for the failure mechanism of thermosetting adhesives and establishment of design indices for adhesive structural strength.

Experimental study of type-I crack propagation in rock monitored by fiber Bragg grating

The occurrence of rock mass engineering disasters can be reduced significantly by obtaining the key information in the process of rock internal fracture accumulation and evolution through advanced monitoring means and taking certain preventive measures. Through fiber Bragg grating monitoring tests of the type-I crack propagation in rock, the spectral characteristics of the fiber Bragg grating in the process of rock failure were analyzed, the bandwidth expansion was extracted to characterize the local tensile strain during the crack propagation process, and the damage variable based on the bandwidth expansion was calculated to describe the type-I crack propagation process. The research results showed the following: (1) The fiber Bragg grating generated a non-uniform local tensile strain during type-I crack propagation. The bandwidth expansion of the wave spectrum was positively correlated with the local tensile strain. The change in the characteristics of the wave spectrum could provide the local real strain in the process of crack propagation. (2) During the steady propagation of the type-I crack, the bandwidth increased obviously, while the number of acoustic emission events was relatively small, and most of them were in the process zone. The gentle increase in the bandwidth corresponded to the unstable growth stage of the type-I crack, which could be used as a precursor of rock sample instability. During loading, the maximum tensile strain of the type-I crack was 6 %, while the maximum shear strain reached 1.8 %, primarily attributed to tensile failure. The average value of fracture toughness KIC, based on the equivalent linear elastic fracture mechanics model, was determined to be 1.21 MPa·m1/2, and the average value of fracture energy Gf was calculated as 57.7 N/m. The variation of peak reflectivity in fiber Bragg grating was associated with the closure of pre-existing defects and the propagation of microcracks in rocks, resulting in unstable shear strain at crack extension sites. (3) The damage variable D defined by the bandwidth broadening began to become concentrated and accumulated in the crack steady growth stage, and it increased slowly in the crack intensification and evolution stage. The grating was sensitive to changes when it was pasted to the surface at 90°, but it could easily fracture or become unable to capture spectral signals in time. Based on a comprehensive comparison, the monitoring effect was the best when the grating was pasted at 60° relative to the crack.

Mode II fracture properties of parallel neosinocalamus affinis bamboo strand lumber

This study investigates the Mode II fracture properties in the RL and TL planes of parallel neosinocalamus affinis bamboo strand lumber. Eighty End-Notched Flexure (ENF) specimens were designed, varying pre-crack lengths and specimen widths. Digital Image Correlation (DIC) was employed to analyze strain variations during the development of the fracture process zone (FPZ) and crack propagation stages. The Compliance Based Beam Method (CBBM) and equivalent crack length calculations were used to determine the strain energy release rate (GIIC) of PNABSL. R curves and crack growth rate curves revealed that crack propagation in the RL and TL planes. The results indicated that Mode II fracture propagation in both the RL and TL planes of PNABSL predominantly exhibited self-similar cracking, though the fracture surfaces were rough and uneven, with noticeable fiber bridging. The study found that Mode II fracture propagation was less stable in the RL plane compared to the TL plane, which is attributed to the easier fracturing of thick-walled fiber cells in the RL plane. The critical strain energy release rate for PNABSL was determined, offering valuable insights into its fracture behavior.

A novel mixed mode fracture criterion for functionally graded materials considering fracture process zone

A new mixed mode I/II fracture criterion based on strain energy release rate theory is proposed for investigating fracture behavior in cracked functionally graded materials. In this study, the crack is assumed to be located along the material gradation and within the brittle side of the material. For the first time in this type of material, the fracture process zone and the energy dissipated in this region are considered to enhance the prediction accuracy of the proposed criterion. The mechanical and fracture properties of the damage zone are characterized using the microcrack density parameter. A novel method for manufacturing functionally graded materials is introduced, offering numerous advantages by controlling the gel time of the resin. The critical mixed mode and pure mode II fracture toughness of the fabricated specimens are determined through three-point and four-point bending tests. A comparison of the experimental data with the fracture limit curve demonstrates the effectiveness of the newly proposed criterion for investigating mixed mode fracture in functionally graded materials

A proposal for the combined analysis of bone quantity and quality of human cortical bone by quasi-brittle fracture mechanics

Quasi-brittle fracture mechanics is used to evaluate fracture of human cortical bone in aging. The approach is demonstrated using cortical bone bars extracted from one 92-year-old human male cadaver. In-situ fracture mechanics experiments in a 3D X-ray microscope are conducted. The evolution of the fracture process zone is documented. Fully developed fracture process zone lengths at peak load are found to span about three osteon diameters. Crack deflection and arrest at cement lines is a key process to build extrinsic toughness. Strength and toughness are found as size-dependent, not only for laboratory-scale experimental specimens but also for the whole femur. A scaling law for the length fracture process zone is used. Then, size-independent, tissue fracture properties are calculated. Linear elastic fracture mechanics applied to laboratory beam specimens underestimates the tissue toughness by 60%. Tissue fracture properties are used to predict the load capacity of the femur in bending within the range of documented data. The quasi-brittle fracture mechanics approach allows for the assessment of the combined effect of bone quantity and bone quality on fracture risk. However, further work is needed considering a larger range of subjects and in the model validation at the organ length scale.

Dynamic tensile intralaminar fracture and continuum damage evolution of 2D woven composite laminates at high loading rate

The intralaminar tensile failure of 2D woven composites under dynamic tensile load was investigated in this paper. Compact tension samples were tested at high loading rate with an electromagnetic Hopkinson bar system. The strain field was obtained with high-speed imaging and digital image correlation, and the J-integral method was employed to obtain the fracture toughness and corresponding R-curve. The continuum damage evolution of intralaminar failure was then analyzed by tracking the opening near the initial crack tip. It is found that the dynamic intralaminar fracture toughness is decreased by 51% compared to the quasi-static condition, the continuum damage evolution and its dependence on loading rate have been reported as well. The failure mechanisms were studied with thermal imaging and scanned electron microscopy, shorter fibre pull-out length and thinner failure process zone may be responsible for the reduced toughness at high loading rate.

Accuracy of the geometric shape of the hole in the longitudinal section during honing

The wide application of honing as a finishing treatment of internal cylindrical surfaces for cylinder-piston systems, used in some structures, is caused by high accuracy measured in tenths of a micrometer, and high productivity of the process. The most important indicator of reliable operation of cylinder-piston systems are high requirements for the geometric accuracy of holes. Due to the lack of sufficient theoretical justification for the selection of honing parameters ensuring the accuracy of the geometric shape of the hole in the longitudinal section, the authors proposed a model for the formation of errors in the geometric shape of the hole. The model is built on the kinematic characteristics of the process including the ratio of the honing stone dimensions, the length of the hole, the stroke of the honing head, the ratio of the speeds of translational and rotational movements, and the force action in the processing zone, which changed due to the presence of an overrun of the honing stone. To obtain analytical dependencies ensuring the minimisation of form deviations, the conditions for stock removal for the points of the machined surface were considered, the value of which was taken proportional to the path of movement, and the pressure value. For this purpose, graphs of the distribution functions of displacements and pressure changes were constructed depending on the coordinate of the point location on the generating line of the hole being machined. Using the obtained analytical dependencies, the potential occurrence of a shape error in the form of a saddle shape was found, the dominant factor influencing the value of which is the value of the honing stone overrun. At the same time, it was identified that the ratio of the speeds of translational and rotational movements has an insignificant effect on the violation of the form in the longitudinal section.

The influence of cutting mode elements on the technological parameters of the process of milling blanks of titanium alloy thin-walled parts

The purpose of a rational mechanical processing mode remains an urgent task of pre-production engineering. Known recommendations and methods for selecting this mode are focused on the processing of solid blanks and do not take into account the fact that when processing thin-walled blanks, the temperatures in the processing zone and the surface layer of the blank differ. The study is aimed at identifying patterns in changing the parameters of the milling process of thin-walled blanks depending on the mode elements, as well as developing recommendations for selecting this mode. The authors performed numerical simulation of technological parameters of the milling process of solid and thin-walled blanks made of titanium alloy under various modes. The cutting speed, cutting depth and feed per cutter tooth were varied. The cutting force, power and densities of heat sources and the temperature in the surface layer of the blank, in the contact zones of the cutter tooth with the blank and the chips with the front surface of the tooth were calculated. It has been found that when milling thin-walled blanks, the temperature field differs significantly from that formed when processing solid blanks due to low heat removal from the unprocessed surface. Increasing the feed per tooth by 45% leads to an insignificant decrease in temperatures in the cutting zone (by 5...12%). Increasing the cutting speed by 25%, on the contrary, leads to an increase in temperatures by 5...10%. Increasing the cutting depth leads to an increase in the temperature in the chip-tooth contact zone by 1.5times and to an increase in the temperature in the tooth-blank contact zone.

Numerical and experimental crack-tip cohesive zone laws with physics-informed neural networks

The cohesive zone law represents the constitutive traction versus separation response along the crack-tip process zone of a material, which bridges the microscopic fracture process to the macroscopic failure behavior. Elucidating the exact functional form of the cohesive zone law is a challenging inverse problem since it can only be inferred indirectly from the far-field in experiments. Here, we construct the full functional form of the cohesive traction and separation relationship along the fracture process zone from far-field stresses and displacements using a physics-informed neural network (PINN), which is constrained to satisfy the Maxwell-Betti's reciprocal theorem with a reciprocity gap to account for the plastically deforming background material. Our numerical studies simulating crack growth under small-scale yielding, mode I loading, show that the PINN is robust in inversely extracting the cohesive traction and separation distributions across a wide range of simulated cohesive zone shapes, even for those with sharp transitions in the traction-separation relationships. Using the far-field elastic strain and residual elastic strain measurements associated with a fatigue crack for a ZK60 magnesium alloy specimen from synchrotron X-ray diffraction experiments, we reconstruct the cohesive traction-separation relationship and observe distinct regimes corresponding to transitions in the micromechanical damage mechanisms.

Hot deformation behavior and microstructural evolution of high-carbon high-strength low alloy steel

This study investigates the hot deformation behavior, microstructural evolution, and processing map of a high-carbon high-strength low alloy steel subjected to hot compression tests in the temperature range of 850 °C–1150 °C and strain rates from 0.01 s⁻1–10 s⁻1. A highly accurate flow stress constitutive equation has been established and the activation energy of the experiment steel is 462.6 kJ/mol. Processing maps were generated to identify unstable zones, low-power dissipation zones, and optimum processing zones, accompanied by detailed discussions on corresponding microstructures and mechanical properties. The recommended hot deformation parameters at a true strain of 0.9 are temperatures from 944 °C to 1050 °C and strain rates from 0.01 s⁻1 to 0.52 s⁻1 according to the processing map. The effects of deformation temperature and strain rate on the microstructure were then investigated. It is revealed that reducing deformation temperature and increasing strain suppress the recrystallization process and the growth of recrystallized grains; while VC precipitation is suppressed under high temperatures and high strain rates.

Influence of bedding on fracture toughness and failure patterns of anisotropic shale

The initiation and propagation of hydraulic fractures are closely related to the fracture ability of rocks. Such processes in shale reservoirs are, to a certain extent, controlled by bedding. However, the control mechanism of bedding on the anisotropy of fracture toughness and fracturing behavior remains unclear. In this study, a series of numerical notched semi-circular bend (NSCB) tests are conducted using the discrete element method (DEM) to investigate the influence of bedding properties on the anisotropy of fracture toughness and fracture patterns. Based on the DEM framework, a novel simulation method is proposed to accurately identify two key fracture indicators, the fracture process zone (FPZ) and crack tip opening displacement (CTOD), to reveal the fracture driving mechanism. The results show that the fracture toughness of shale is negatively correlated with bedding angles β but positively correlated with bedding spacing and bedding strength. Both the bedding strength and spacing significantly influence the fracture pattern of the specimens with β = 0°–60°, whereas the specimen with β = 90° is scarcely affected by the bedding planes. The evolution of the CTOD and FPZ in shale exhibits distinct phased characteristics. Due to the strong suppression effect of low-angle bedding planes on pre-peak crack deflection, the CTOD and FPZ exhibit opposite trends with respect to bedding angles before and after the peak load. This study facilitates the understanding of the fracture propagation process of anisotropic shale and could provide guidance for hydraulic fracturing design in shale reservoirs.