Interrupted Tests Research Articles

Ultrasonic nonlinear evaluation of tensile plastic damage in Nickel based single crystal superalloy

The tensile interruption tests of Nickel-based single crystal (NBSX) superalloys at room temperature were carried out to artificially introduce controllable levels of plastic damage, and linear and nonlinear ultrasonic nondestructive methods were used to evaluate it. The results indicate that ultrasonic nonlinear parameter β shows hypersensitivity to early tensile plastic damage compared with traditional linear parameters. Transmission electron microscope (TEM) and X-ray diffraction (XRD) measurements of dislocation string length and dislocation density were conducted to calculate the theoretical predicted values of β by ultrasonic nonlinearity dislocation monopole model, and they are in well agreement with experimental results. In addition, the actuation of slip systems and evolution of dislocation configuration were observed under optical microscope (OM) and TEM. From the perspective of element distribution and energy, it is revealed that the plastic deformation mechanism of the NBSX superalloys at room temperature is anti-phase boundary (APB) dislocation pairs shear γ′ phase. The measurement results of macro mechanical properties manifested that accumulative tensile plastic deformation lead to increase of strength and decrease of plasticity.

Study of Artificial Line for Short‐Line Fault Interruption Test

We developed a novel artificial line circuit that compensates for the time delay of the transient recovery voltage (TRV) for a short‐line fault (SLF) interruption test and an analysis system that automatically extracts each parameter on the SLF line characteristics from the measured waveform with high precision. This new circuit and system can drastically improve the time efficiency for the adjustment and calculation of the prospective line‐side TRV condition, which is carried out before the main test, as compared to conventional methods. We also investigated the effect of the injection current and current transformer (CT) characteristics on the SLF line characteristics. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Experimental investigation of the modified butterfly notch shear specimen with PMCs

An experimental study of the modified butterfly shear sample was undertaken for polymer matrix composites using three quasi-isotropic IM7/977-3 laminates. This study used 3D digital image correlation (DIC) to observe and quantify the severe shear strain concentrations developed by compressively loading the modified butterfly geometry laminates. Interrupted X-Ray computed tomography (X-Ray CT) was used to understand the failure mechanisms present in the region of shear strain. By choosing to conduct interrupted testing, the progressive nature of the damage could be observed. At ultimate failure, extensive damage in the region of high shear strain was observed using post testing inspection with X-Ray CT. The mechanical testing was accompanied by numerical simulations for each laminate. The damage predicted by the numerical simulations was shown to be consistent with the damage observed with the post failure inspection via X-Ray CT. Stress components caused by free-edge effects were also quantified, and shown to have a minimal impact on the failure mode in the region of interest.

δ-ferrite dynamic recrystallization behavior during thermal deformation in Fe–32Mn–11Al-0.9C low density steel

The thermal deformation behavior of Fe–32Mn–11Al-0.9C low density steel was investigated by Gleeble-3500 thermomechanical simulator through single pass compression test with deformation temperature of 900–1100 °C and strain rate of 0.01–1.0 s−1. The true stress vs. true strain curves and flow stress model under different deformation conditions were obtained. Dynamic recrystallization (DRX) behavior of δ-ferrite occurred before dynamic softening of austenite under specific deformation conditions which was studied by interrupted compression tests. The results suggest that the DRX behavior of δ-ferrite suppresses that of austenite and refines δ-ferrite grain. The DRX behavior of δ-ferrite becomes more obvious with increasing strain rate which is attributed to the increase in driving force, whereas it weakens with increasing deformation temperature owing to the advance of dynamic softening of austenite.

Dynamic and Static Strain Aging in a High‐Manganese Steel

Carbon‐containing high‐manganese steels are susceptible to strain aging, which influences the mechanical properties during and after deformation. To understand the strain aging behavior of austenitic Fe‐22Mn‐1.3Al‐0.3C high‐manganese steel, dynamic as well as static strain aging are examined. Dynamic strain aging, occurring during deformation, is quantified by tensile tests at different strain rates. The strain localization during straining is analyzed by digital image correlation. A decrease in strain rate increases the tensile strength and influences the onset of a serrated flow. Static strain aging is quantified by interrupted tensile tests, combined with digital image correlation and small‐angle neutron scattering (SANS). The yield strength increases due to static strain aging at room temperature within the period of days. The samples show extended yield point (yield point elongation) and the effect strengthens at higher pre‐deformation. With SANS, evidence of carbon‐manganese ordering (C–Mn ordering) during aging is obtained and correlated.

A Study on Early Stages of White Etching Crack Formation under Full Lubrication Conditions

The appearance of White Etching Cracks (WEC), not covered by the ISO 281 modified failure rate calculation, leads to difficulties in predicting bearing reliability. This uncertainty in bearing applications leads to a worldwide activity in order to understand and prevent this situation since the WEC failure mode deviates from the traditional Rolling Contact Fatigue (RCF) mode. Plenty of factors have been found to influence this phenomenon over the years, however the precise initiation of the WEC is still under debate. In order to understand the initiation and analyze the temporal evolution, interrupted tests on the same material were performed under conditions that were known to lead to WEC formation and RCF. To avoid the added complexity of boundary lubrication, a Deep Groove Ball Bearing (DGBB) test rig under full lubrication (Elastohydrodynamic Lubrication, EHL) was chosen. Within a standard operating mode, named Mode 1 (RCF), the bearings are solely subjected to a radial load. By suspending the tests at different time steps, a continuous progress of changes in the subsurface material structure seen as equiaxed grains with low dislocation densities, identified as ferrite, is observed. The bearings did not fail up to load cycles of 109. In contrast, a Mode 2 Electrical Charged Contact Fatigue (ECCF) test provoked the early formation of cracks and crack networks, first without WEA, then later with WEA. It became obvious when comparing Mode 1 (RCF) with Mode 2 (ECCF) that Mode 2 (ECCF) achieves far fewer load cycles until failure occurs.

Refining reproductive risk for FMR1 premutation carriers in the general obstetric population.

Female FMR1 premutation (FMR1 PM) carriers for fragile X syndrome (FXS) are at risk to have a child with FXS based on their CGG repeat size and AGG interruption number. Studies examining this risk in unselected populations of female PM carriers are lacking. This retrospective cohort study analyzed carrier status, CGG repeat length, AGG interruption result, and reproductive risk refinement in a population of female patients who underwent routine carrier screening for FXS. A total of 1536 PM carriers (0.43%) were identified, 95% of whom had between 55 and 90 CGG repeats. A number of 1334 carriers underwent AGG interruption testing. The majority had at least one AGG interruption and received a lower reproductive risk for FXS following AGG interruption testing (89% and 85%, respectively) as compared to their risk calculated based on CGG repeat size alone. The average change in risk across the population following AGG interruption testing was -3.4%, with a range from -50.8% to 48.9%. This article describes the range of CGG repeats and AGG interruptions in an unselected population of female PM carriers and suggests that most carriers would benefit from AGG interruption testing to refine their reproductive risk of having a child with FXS.

Twin density gradient induces enhanced yield strength-and-ductility synergy in a S31254 super austenitic stainless steel

Gradient structure (GS), as a typical heterostructure, is arousing great interest for an improved synergy between the strength and ductility which are mutually conflicting. Recently, a novel design of GS is proposed by taking the density of twins in grains, instead of common grain size, as a gradient variable, showing the key role in strain hardening by the nano-scale twin boundaries. Following this idea, here, a deformation twin-density GS was produced by means of the technique of surface mechanical attrition treatment in a S31254 super austenitic stainless steel. To be specific, the GS consisted of a central coarse-grained (CG) core, with two sides sandwiched by the gradient-structured layer (GL), where the density of deformation twins appears gradient in grains along the depth towards the CG core. The tensile tests show that as compared to CG counterpart, yield strength in GS increases 80% to 0.5 GPa, along with comparable ductility of 36%. The interrupted tensile tests show the presence of mechanical hysteresis loops during each unload-reload cycle, indicative of the generation of hetero-deformation-induced (HDI) stress during tensile deformation. Furthermore, both the HDI stress and HDI strain hardening account for a large proportion of global flow stress and forest hardening. The deformation twins and their evolutions, with the emphasis on their interaction with the dislocations, are investigated in detail by means of EBSD and TEM observations to correlate the mechanical properties. The present results shed light on the crucial role of deformation twins in the twin-density gradient for the synergistic enhancement of both strength and ductility.

Notch strength and notch fracture mechanisms of a cast Mg-Gd-Y alloy

This study aims to elucidate the mechanical properties and fracture mechanisms of cast Mg-6Gd-3Y-0.4Zr (wt.%) (GW63) alloy in the presence of geometrical discontinuities induced by notches. Notch strength is evaluated by room-temperature tensile tests using circumferentially notched bars of different notch acuity. Finite element calculations suggest that notch fracture is controlled by the von Mises stress and effective plastic strain at the notch tip. Through fractographic observations and EBSD analysis on samples subjected to interrupted tests, a cleavage micro-cracking behavior uncommon for conventional Mg alloys is found to dominate notch fracture. Grain-sized cleavage microcracks tend to occur along the basal plane of grains favorably oriented for basal slip, which is explained by crack nucleation at co-planar slip bands blocked by the grain boundary according to Smith and Barnby's theory. Failure is initiated by the accumulation and coalescence of such microcracks in the notch vicinity. The unexpected cleavage micro-cracking behavior is mainly ascribed to the RE solutes that suppress {10–12} twinning, and is responsible for the limited deformation capacity and notch strength of the cast-T6 GW63 alloy with coarse grains and significant strengthening due to precipitation.

Desorption Kinetics of Legacy Soil Phosphorus: Implications for Non-Point Transport and Plant Uptake

Soil phosphorus (P) solubility and kinetics partly control dissolved P losses to surface water and uptake by plants. While previous studies have focused on batch techniques for measuring soil P desorption kinetics, flow-through techniques are more realistic because they simulate P removal from the system, akin to runoff, leaching, and plant uptake. The objectives were to measure soil P desorption by a flow-through technique at two flow rates and several batch methods, and utilize both for understanding how flow rate impacts the thermodynamics and kinetics of soil P desorption. Desorption obeyed first-order kinetics in two different phases: an initial rapid desorption phase followed by a gradual release. Desorption was limited by equilibrium and the kinetics of physical processes as demonstrated by an interruption test. Dilution-promoted desorption occurred with increasing cumulative volume, which increased desorption rate via first-order kinetics. The batch tests that simulated cumulative solution volume and time of flow-through were similar to the flow-through results; however, the batch methods overestimated the desorption rates due to less limitations to diffusion. Fast flow rates desorbed less P, but at a greater speed than slow flow rates. The differences were due to contact time, cumulative time, and solution volume, which ultimately controlled the potential for chemical reactions to be realized through physical processes. The interaction between these processes will control the quantity and rate of desorption that buffer P in non-point drainage losses and plant uptake.

Finite element simulation of real cavity closure in cast Ti6Al4V alloy during hot isostatic pressing

The healing behavior of shrinkage cavity inside the cast Ti6Al4V alloy during hot isostatic pressing (HIP) was investigated experimentally by interrupted hot isostatic pressing tests. The X-ray micro computed tomography was used to record the morphology changes before and after hot isostatic pressing. The two-dimensional geometry obtained by the microCT scan was used in simulation to study the evolution of the real shrinkage cavity during hot isostatic pressing. Shrinkage cavities, shrinkage porosity and small gas pores can be effectively eliminated under proper HIP conditions. The two-dimensional morphology in the simulation results agrees well with the experimental results. This study reveals that plastic deformation, creep and diffusion are the main mechanisms of cavity closure during hot isostatic pressing. In addition, the simplified elliptical pores with aspect ratios at different positions were used to replace the real pores to further study the factors affecting the position of dimples after HIP by simulation. It is found that the position of the dimples mainly depends on the aspect ratio of the elliptical pore and the distance between the pore surface and the external surface of the geometric model.

Effects of Al-Si coating structures on bendability and resistance to hydrogen embrittlement in 1.5-GPa-grade hot-press-forming steel

Hot-press-forming (HPF) steels have attracted great attention as automotive reinforcement parts, but are exposed to the potential risk of hydrogen embrittlement (HE) because H introduced easily during HPF processes is hardly de-trapped through the solidified coating. In particular, H seriously deteriorates bendability, which is one of the main properties to be considered. In this study, the Al-Si coating structures were modified to improve H emission by increasing H diffusivity. The effects of coating structures on bendability and H desorption were investigated by interrupted bending tests, H-permeation tests, and thermal desorption analyses according to elapsed time after H-charging. Immersion in an Al-10%Si bath and the subsequent HPF process (930 °C for 6 min) produced a 33 μm-thick multiple coating structure composed of Fe2Al5, FeAl, and ferrite layers. On the other hand, the reduced Al-Si adhesion amount from the dip bath and the increased time and temperature (950 °C for 30 min) produced a 30 µm-thick body-centered-cubic (BCC)-based coating structure composed of FeAl and ferrite layers. The BCC-based crystal structure, reduced Al content in the FeAl layer, and coarsened ferrite grains effectively enhanced H diffusivity and suppressed H-induced degradation. Moreover, the softened FeAl and thick ferrite layers improved bendability by allowing the large strain accommodation of bending deformation. Thus, this work proposes an optimal Al-Si coating design that enhances both bendability and resistance to H-induced degradation for secure HPF steel applications.

Evolution of structure of cohesive granular ensembles in compression

A small amount of cementation between particles drastically changes the mechanical response of a granular ensemble. Through a series of interrupted uniaxial compression tests inside the x-ray computed tomograph (XCT) on fine and coarse grained cemented sand specimens, a quantification of the behavior at both the continuum and particle scales is made. At the continuum length scale, displacements and porosity fields are examined. While at the particle scale, the coordination number, particle and contact fabric tensors, directional distribution of particles and contact normals are extracted and analyzed. Particle level phenomenology—at different stages of loading—such as breakage of bonds, formation of new contacts, manifest themselves at the ensemble scale in the form of bifurcation zones, which coalesce into split zones through the specimen. A new paradigm for the fabric tensor is also provided using principal component analysis. The results of the continuum and particle scale analysis are used to provide insights on the compression behavior of cemented sands.

Investigation of non-classical creep behavior of Inconel 617 alloy at 700 °C and 800 °C through interrupted tests and microstructural characterizations

The ‘non-classical creep behavior’ of the Inconel 617 alloy consisting of the miniscule primary and secondary creep regimes followed by a prolonged tertiary creep regime, has been investigated through interrupted tests at 700 °C/275 MPa and 800 °C/95 MPa, having nearly similar time to rupture (∼3500 h) to understand the role of microstructural evolution. A steady state tertiary creep region present at 800 °C/95 MPa, is found to be absent at 700 °C/275 MPa. The microstructural changes including formation of γ′ and fine secondary carbide precipitates, and twin boundary generation observed on interrupting the creep tests at both the aforementioned conditions after varying durations, have been investigated using optical, scanning and transmission electron microscopy, electron back scattered diffraction (EBSD) and thermal analysis. The formation of ample fine γ′ precipitates along with the fine secondary carbides in the samples subjected to creep tests at 700 °C with interruptions after varying fractions of time to rupture lead to increase in hardness, which is found to be lower for similar tests at 800 °C due to the absence of γ′ precipitates. Moreover, the EBSD analysis has confirmed a higher length fraction ratio of twin boundary (TB) to high angle grain boundary (HAGB) in the samples exposed to the steady state tertiary creep regime (almost 95% of total rupture time) at 800 °C. The increased TB to HAGB length fraction ratio is responsible for improving the creep resistance by delaying the damage accumulation, as the TBs are typical coincident site lattice boundaries with lower energy and greater packing density.

Mechanical behavior and stability of dispersed retained austenite in thermomechanically rolled and isothermally-treated TRIP-aided multiphase steel

The effect of thermomechanical processing and cooling conditions on the microstructure and mechanical properties during multi-step interrupted tensile tests was investigated in 0.24C–1.55Mn-0.87Si-0.40Al multiphase steel. Two temperature variants of isothermal bainitic holdings at 450 and 350 °C were applied. The tensile tests were interrupted at strain levels of 5, 10, 15%. The microstructure was assessed using light and SEM microscopy. EBSD analysis was performed to determine the retained austenite (RA) distribution and size. Detailed calculations concerning the mechanical performance were performed including the Jaoul-Crussard's analysis. Multiphase steels with the high retained austenite fraction (ca. 15%) of different γ phase sizes below 4 μm in a ferritic-bainitic matrix (with an average size of <6 μm) have been obtained. The research showed that during martensitic transformation of RA, the grain undergoes fragmentation. It has been shown that for the proper use of the TRIP effect, it is necessary to produce the dispersed RA of different sizes in the structure instead of striving for maximum fragmentation, which is a commonly used practice. Obtaining too small grains in the microstructure blocks their transformation during deformation due to their too high stability. Obtaining grains of one size leads to the intensification of the TRIP effect at a given stage of deformation only, reducing the plasticity in the others. The corresponding different mechanical stability of the γ phase ensures a gradual martensitic transformation and beneficial strain hardening progress.

Effect of thermal cycling and vibration on cracking in Sn-3.0Ag-0.5Cu solder bump

In present study, the effects of thermal cycling and vibration on cracking in Sn-3.0Ag-0.5Cu (SAC305) solder bump (interconnection between flip chip and printed circuit board) were studied. The thermal cycling test was set in 2 conditions as “TC180C” and “TC140C” that differed in temperature cycle i.e. 15 °C to 180 °C and 15 °C to 140 °C, respectively. While, the vibration test was also set in 2 conditions as “VB20G”and “VB05G” that differed in vibration acceleration ranges i.e. 20G and 5G respectively. Crack initiation and propagation in SAC305 solder bump were investigated by interrupted tests using scanning electron microscope. Results of the thermal cycling tests indicated that cracks initially developed at the edge of solder bump and subsequently propagated along solder/intermetallic compounds layer interface for TC180C condition. While crack propagated to bulk solder for TC140C. And crack initiation cycle differed between test conditions that were 40 cycles and 60 cycles for TC180C and TC140C, respectively. Crack initiation and propagation were accelerated by temperature range. The lower number of crack initiation cycle for TC180C condition, which was operated under a large temperature range, would be induced by higher thermal strain that could accelerate crack initiation in SAC305 solder bump. Results of the vibration tests showed that crack initiation and propagation were found only under VB20G condition, the number of crack initiation cycle was 12 cycles at X vibration axis. While any crack was not found for VB05G condition. The crack initiation and propagation in the SAC305 solder bump under VB20G condition would be induced by higher mechanical vibration acceleration. Whereas, damage accumulation in the SAC305 solder bump would not be induced by lower mechanical vibration acceleration under VB05G condition.

Impact of liquid metal embrittlement inner cracks on the mechanical behavior of 3rd generation advanced high strength steel spot welds

Controlled degradation of the welding parameters is used in this work to produce homogeneous advanced high strength steel (AHSS) resistance spot welds with significant liquid metal embrittlement (LME) inner cracks and study their impact on mechanical behavior. For the first time, the effect of LME inner cracks on damage development is monitored using interrupted cross tension and tensile shear test. Despite their potential effect on the stress state at the notch tip, the observed LME inner cracks seldom contribute to the final fracture path of the welds, leaving the load bearing capacity and failure type of the welds mostly unaffected in tensile shear or even increased in cross tension.

Failure sequence determination in sandwich structures using concurrent acoustic emission monitoring and postmortem thermography

This study provides a novel approach for damage classification and failure sequence evaluation in sandwich panel composite materials under flexural loading condition merely by using acoustic emission monitoring in interrupted mechanical tests and postmortem lock in thermography analysis. The studied sandwich panels consist of glass fiber reinforced phenolic skins and Nomex honeycomb core which are used extensively in the aerospace industry. While one of the standard flexural samples is mechanically tested up to global failure, the other specimens are loaded till a certain load level and the tests are interrupted. Acoustic emission hits are recorded throughout the mechanical tests by using piezoelectric sensors, and are classified using a well-established clustering algorithm. The damaged samples are then investigated via lock in thermography technique to identify the hidden failure progress inside the sandwich structure. Acoustic emission monitoring is successfully used to mark the change of mechanical response under flexural loading condition. The results of acoustic emission analysis indicated four different clusters associated with four major failure modes occurring due to mechanical loading. Correlating failure progress observed in thermography results with the fraction of acoustic emission clusters registered during mechanical tests helped to attribute each class of acoustic emission hits to a specific failure type. Upon using the interrupted test methodology together with the lock-in thermography technique, one can reliably identify the damage types and their sequence of manifestation during flexural tests. The results show that successful analysis of visually hidden failure progress in sandwich panel composites under out of plane loading condition can be attained by using structural health monitoring techniques and quasi static tests.

Laboratory- and Semi-Industrial-Scale Thermomechanical Processing of TRIP-Aided Steel with Acicular Ferrite

The modification of the deformation and cooling methods resulting in the obtainment of acicular ferrite promotes an increase in the proportion of retained austenite (RA) and a corresponding increase in mechanical properties in Si-Al TRIP-aided steel. The effect of controlled thermomechanical processing in laboratory- and semi-industrial scales on the possibility of obtaining acicular ferrite and a high fraction of retained austenite was investigated. The steel was hot deformed in three steps: at 1050, 900 and 750 °C to introduce dislocations into the hot-deformed pancake austenite. Next, slow cooling in a ferritic transformation region was performed, followed by isothermal holding of steel at 450 °C. The interrupted tensile tests at the strain levels of 5, 10 and 15% were performed to investigate the mechanical properties response and the stability of the obtained retained austenite. Light and scanning electron microscopy, XRD and EBSD analyses were performed to assess microstructural features. The produced material showed a multiphase microstructure containing acicular ferrite and 10% of retained austenite. The microstructures obtained in both production methods were slightly different due to high temperature inertia in the semi-industrial process.

Transfer layer evolution during friction in HIPIMS W–C coatings

The investigation of the evolution of transfer layer in the contact area in W–C coating/steel ball system during interrupted friction tests in humid air revealed that fast oxidation occurs simultaneously with water vapor dissociation and carbon hydrogenation. These mechanochemical reactions produced a mixture of hydrogenated carbon, ferritungstate with minor amounts of single tungsten (sub-)oxides and iron oxides forming transfer layer preferentially adhered to steel ball. The steady coefficient of friction (COF) was attributed to the development of a thin continuous tribo-film with higher hardness and stiffness in the central zone of the wear track facing the thickest zone of transfer layer on the ball. Despite direct experimental confirmation is necessary, tribo-film was proposed to consist of ordered rehydrogenated carbon film. Friction would be controlled by shear interactions between transfer layer and thin tribo-film on top surface of W–C coating. The effect of humidity on COF can be explained by a control of oxidation, dissociation and hydrogenation reactions and subsequent influence on the composition of transfer layer and tribo-film in the wear track. The existence of phases identified in transfer layers during friction were confirmed via modelling of parallel and mutually interacting mechanochemical reactions based on the minimization of free Gibbs energy. The ability to apply equilibrium thermodynamical calculations to dynamical processes in friction contact was related to very fast reactions among microscopic asperities eliminating kinetical factors necessary for thermodynamical equilibrium at macroscale. Based on the experimental observations and modelling, three stage model of transfer layer evolution was proposed to explain run-in and steady periods on friction curves.