Bulk Cracks Research Articles

3D characterization of closure of surface and internal fatigue cracks in nodular cast iron using Digital Volume Correlation of laboratory X-ray tomography images

Digital volume correlation (DVC) of laboratory X-ray tomography images has been used to characterize the closure of artificial internal fatigue cracks in nodular graphite cast iron. DVC allows to produce crack opening displacement (COD) map from 3D tomography images at the sub-voxel scale thanks to the graphite nodules which can be used as internal natural markers. Fatigue samples containing controlled internal defects have been produced in a nodular cast iron by diffusion bonding plates containing laser-machined sharp notches. Specimens with internal cracks and surface cracks were respectively characterized by laboratory tomography at different loads during fatigue crack propagation tests. The local stress intensity factors (SIFs) have been extracted from the measured displacement fields by using Williams series, based on those results the closure levels of internal cracks and surface cracks were comparatively analyzed and discussed. Crack closure levels were found to be linearly related to the location of the surface crack front, and the level of internal crack closure was comparable to that of the surface crack in bulk.

Modeling the interaction between intra and interlaminar cracking in FRP composites using phase-field regularized interface fracture

Fiber-reinforced polymer composites (FRPCs) exhibit a complex range of failure mechanisms, including matrix cracking, fiber breakage, fiber–matrix debonding, and delamination. Among these failure modes, interlaminar fracture or delamination across the interfaces between the different layers (plies) is commonly observed in experiments. Hence, the fracture behavior of these interfaces plays a critical role in determining the overall fracture performance of the FRPCs. In this work, our aim is to model the effect of interface on the final failure of FRPC laminates. Specifically, the present work focuses on modeling the complex interactions between the bulk FRPC damage and interfacial delamination in the carbon fiber reinforced composites (CFRP) within the realm of phase-field fracture. In the present formulation, the phase fields smear the sharp interface between FRP plies as regularized cohesive zones and also describe the bulk crack surface density eventually allowing interaction between bulk damage and interfacial delamination. For the composite’s constitutive description, the FRPC is treated as a homogenized material, with effective mechanical properties depending on the properties of the individual matrix and fibers with their respective volume fractions. The elastic matrix is assumed to be isotropic, while the fibers are characterized by their orientations. Further, an anisotropic modification is considered in the fracture energy of the FRPC to promote crack growth along the fiber direction. The model is implemented in a commercial finite element package and several numerical examples are presented to illustrate the complex crack path behavior in CFRP. Specifically, crack growth and interactions are simulated when there is a mismatch in the fiber orientation between the adjacent plies, resulting in dissimilar material properties on either side of the interface. The utilization of the regularized interface phase field model in an anisotropic framework to account for the mismatch in fiber orientation and hence material properties between the laminas makes it capable of predicting complex modes of fracture in FRPC laminates as demonstrated in the numerical simulations. We have also compared the model predictions with the experiments to validate their accuracy.

Properties of composite sintered modified fluidized bed incineration fly ash as cement admixture

As a hazardous solid waste, circulating fluidized bed incineration fly ash faces the risk of heavy metal leaching and excess dioxin content, and thermal treatment plays an important and increasingly evident role in its environmentally sound disposal. This paper summarizes numerous studies and proposes the composite sintering of circulating fluidized bed incineration fly ash with limestone powder. The operation of the composite sintering on the physicochemical properties of circulating fluidized bed incineration fly ash and the performance of the cement prepared as cement admixture were studied. The results show that the composite sintering can decompose more than 92.7% of chlorine and 99.9% of dioxins, increase the activity of volcanic ash to 79%, and produce new minerals - Ye'elimite and Larnite, However, the content of heavy metals will increase due to the enrichment. The prepared cement can solve the bulk cracking caused by aluminum, reduce the water demand by more than 3.6%, and increase the 3 d compressive strength by 19.09% at 5% admixture and 50.2 MPa at 28 d. Besides, there is no risk of excessive leaching of heavy metals. The content of fly ash from circulating fluidized bed incineration in cement increases from 0.52% to 7.14% after composite sintering. It is proved that composite sintering is beneficial to the enhancement of the resource recovery capacity of hazardous solid waste in the cement industry. In this experiment, chlorine remains the most critical factor limiting its application, and the removal of chlorine should be the focus of the subsequent experiments to expand its resource recovery pathway.

Dynamic crack penetration vs. deflection at material interfaces and the role of rate dependent strength and toughness

This work presents a numerical investigation into the dynamic crack penetration vs. deflection at a material interface, for materials exhibiting strain rate dependent damage evolution. The rate dependence of damage manifests as an increased strength and fracture energy (or toughness) at higher strain rates. For this purpose, a strain rate dependent continuum damage mechanics (CDM) based model is considered, which scales the material point softening damage law as a function of the strain rate. The investigations consider two materials, viz. the bulk and the interface, both modeled via the foregoing rate dependent damage model. Attention is focused on a mode I dynamic bulk crack impinging the interface at right angles, for the case where the interface is more strain rate sensitive than the bulk. The model is calibrated and validated with experimental data, and found to predict well the penetration/deflection behavior of a dynamic crack at an interface. Despite this, the predicted crack trajectory approaching the specimen boundary appears to be affected in some cases. The model is then used to conduct a sensitivity analysis for a wide range of bulk and interface properties (moduli, strengths, and fracture energy). The analyses reveal that dynamic crack impingement at the interface is accompanied by sharp changes in the local strain rates, thus altering the local bulk and interface strengths and toughnesses. So, a weak interface can become stronger and tougher than the bulk, locally and instantaneously, if it exhibits a higher strain rate sensitivity than the bulk. This implies that when damage evolution is rate dependent, there is an increased tendency for crack penetration. Due to the local and instantaneous rate dependence of strength and toughness, the previously established criteria in terms of the quasistatic property ratios are shown to be inapplicable. Alternatively, modeling is deemed inevitable for which the main source of uncertainty comes from the boundary conditions, which are shown to significantly affect the predicted cracking behavior.

Coating delamination analysis of diamond/Ti and diamond/Ti-6Al-4V systems using cohesive damage and extended finite element modeling

Diamond coatings are known to improve the wear resistance properties of substrate materials. This paper aims at analyzing damage behavior of brittle diamond coating deposited on two different ductile substrates, namely pure-Ti and Ti-6Al-4V alloy, subjected to uniaxial tensile loading. In-situ tensile tests were conducted on diamond/Ti and diamond/Ti-6Al-4V coated samples to identify the stress-strain response and the damage behavior of the coating. Surface imaging was performed using scanning electron microscope (SEM). The cross sections were made with the focused ion beam (FIB) gun installed in SEM to observe the coating failure modes. Energy dispersive x-ray and x-ray diffraction analyses were carried out to determine composition. The SEM analysis revealed micro-crystalline morphology of the coating and FIB cross-sections showed diamond coating top-layer, TiC interlayer and substrate at the bottom. The XRD and EDX findings confirmed the regions observed in FIB cross-section via chemical analysis. The in situ tensile tests revealed that the coating had undergone failure through crack initiation, fragmentation and delamination. The bucking of the coating under transverse compressive strain of the substrate was also observed. The Ti-6Al-4V alloy caused early cracking and delamination of the coating owing to its higher yield strength and higher rate of stress transfer to the coating. Two-dimensional finite element models were developed to simulate various damage modes, such as cracking, buckling and delamination. Extended finite element method and cohesive damage modeling were exploited to observe cracking and buckling, respectively. Numerical results revealed that the coating was more prone to failure when Ti-6Al-4V, a high yielding less ductile, substrate was used instead of a low yielding highly ductile Ti substrate. Moreover, buckling was found to be the major reason of delamination than bulk cracking. The coating is, therefore, likely to delaminate more severely under the compressive (buckling) than the tensile (bulk cracking) mode of deformation.

Addressing Unfavorable Influence of Particle Cracking with a Strengthened Shell Layer in Ni-Rich Cathodes.

Ni-rich layered materials are widely accepted as pivotal cathode materials to realize low-cost high-energy-density batteries. However, they still suffer from the intrinsic mechanically induced degradation due to the large lattice deformation. Here, we fabricate a strengthened shell layer on polycrystalline secondary particles to address the unfavorable influence of particle cracking instead of suppressing their bulky pulverization. This tough layer, constructed via welding LiNi0.8Co0.1Mn0.1O2 primary particles with a Nb-based ceramic, increases Young's modulus of the particles 2.6 times. This layer allows the particles work properly with the intact spherical morphology even after bulk cracking. It seems that this tough skin stops the bulky flaws growing into perforated fissures and keeps the electrodes from quick polarization. This approach demonstrates that, besides addressing the intrinsic challenges directly, appropriate particle engineering is another efficient way to exploit the potentials of Ni-rich cathodes and power batteries made out of them.

Control over MoSe2 formation with vacuum-assisted selenization of one-step electrodeposited Cu-In-Ga-Se precursor layers.

Cu(In,Ga)Se2 (CIGS) absorbers are prepared by direct current electrodeposition process followed by selenization of precursors. Selenization of electrodeposited layers is performed in a tubular furnace at 550°C in elemental selenium atmosphere using Ar as carrier gas. The effect of evacuation of tube prior to the selenization on the formation of CIGS absorbers is studied. Characterization of CIGS absorbers reveals that the samples selenized without prior evacuation found to have excess MoSe2 formation at the CIGS/Mo interface leading to bulk cracks in underlying Mo back contact compared to their counterparts. Although the fabricated solar cells using the absorbers, prepared with and without evacuation, are observed to be photoactive, the cells from vacuum-based selenization showed improvement in performance compared to the cells from non-vacuum selenization. The process is further being improved to enhance the efficiency, which can pave way towards environmentally friendly low-cost solar cells.

A Soft Resistive Acoustic Sensor Based on Suspended Standing Nanowire Membranes with Point Crack Design

AbstractAn artificial basilar membrane (ABM) is an acoustic transducer that mimics the mechanical frequency selectivity of the real basilar membrane, which has the potential to revolutionize current cochlear implant technology. While such ABMs can be potentially realized using piezoelectric, triboelectric, and capacitive transduction methods, it remains notoriously difficult to achieve resistive ABM due to the poor frequency discrimination of resistive‐type materials. Here, a point crack technology on noncracking vertically aligned gold nanowire (V‐AuNW) films is reported, which allows for designing soft acoustic sensors with electric signals in good agreement with vibrometer output—a capability not achieved with corresponding bulk cracking system. The strategy can lead to soft microphones for music recognition comparable to the conventional microphone. Moreover, a soft resistive ABM is demonstrated by integrating eight nanowire‐based sensor strips on a soft trapezoid frame. The wearable ABM exhibits high‐frequency selectivity in the range of 319–1951 Hz and high sensitivity of 0.48–4.26 Pa−1. The simple yet efficient fabrication in conjunction with programmable crack design indicates the promise of the methodology for a wide range of applications in future wearable voice recognition devices, cochlea implants, and human–machine interfaces.

Healing cracks in selective laser melting by 3D laser shock peening

Selective Laser Melting (SLM) of Ni-based superalloys such as CM247LC is prone to weld-cracking. This paper investigates how to suppress cracks by repeatedly applying Laser Shock Peening (LSP) during the building phase of SLM. Samples made of CM247LC were processed with different LSP parameters, and the influence on bulk crack density has been quantified. It was observed that for all chosen conditions, a significant decrease of up to 95% could be achieved, demonstrating the potential of the new hybrid 3D LSP method in improving SLM processability of alloys sensitive to cracking.

Local Crack-Programmed Gold Nanowire Electronic Skin Tattoos for In-Plane Multisensor Integration.

Sensitive, specific, yet multifunctional tattoo-like electronics are ideal wearable systems for "any time, any where" health monitoring because they can virtually become parts of the human skin, offering a burdenless "unfeelable" wearing experience. A skin-like, multifunctional electronic tattoo made entirely from gold using a standing enokitake-mushroom-like vertically aligned nanowire membrane in conjunction with a programmable local cracking technology is reported. Unlike previous multifunctional systems, only a single material type is needed for the integrated gold circuits involved in interconnects and multiplexed specific sensors, thereby avoiding the use of complex multimaterials interfaces. This is possiblebecause the programmable local cracking technology allows for the arbitrary fine-tuning of the properties of elastic gold conductors from strain-insensitive to highly strain-sensitive simply by adjusting localized crack size, shape, and orientations-a capability impossible to achieve with previous bulk cracking technology. Furthermore, in-plane integration of strain/pressure sensors, anisotropic orientation-specific sensors, strain-insensitive stretchable interconnects, temperature sensors, glucose sensors, and lactate sensors without the need of soldering or gluing are demonstrated. This strategy opens a new general route for the design of next-generation wearable electronic tattoos.

Phase field modeling of interfacial damage in heterogeneous media with stiff and soft interphases

A new interfacial cracking model in the phase field framework is proposed. The developed method is able to capture the effects of both stiff and soft interphases on the fracture behavior of heterogeneous materials. A dimensional-reduced model based on a rigorous asymptotic analysis is adapted to derive the zero thickness imperfect interface models from an original configuration containing thin interphase. Then, the energetic approach is used to describe the material degradation both on the interface and in bulk within the context of the phase field method for fracture. This technique allows to effectively model the competition between the interface and bulk cracking. Furthermore, a unilateral contact condition is also proposed to ensure the physical crack propagation patterns in the case of spring imperfect interface. The complex cracking phenomena on interfaces such as initiation, delamination, coalescence, deflection are successfully predicted by the present method. The numerical implementation using a staggered algorithm provides an extremely robust approach. The performance of the proposed framework is demonstrated through numerical examples involving complex microcracking of both stiff and soft interfaces in complex microstructures.

Interface delamination vs. bulk cracking along wavy interfaces

This paper analyzes the competition between bulk cracking and interface delamination along a wavy interface. The simulations embed cohesive elements throughout the entire mesh of conventional elements, such that crack paths emerge as a natural outcome of the simulations. The shape of interface is described using a sinusoidal wave form, with amplitude-to-wavelength ratios ranging from zero to one half. Asymptotic bimaterial crack tip fields are imposed at distances far greater than either the interface wavelength or cohesive rupture zone size, thus generating results for brittle fracture near asperities much smaller than the parent crack length. Results are presented for the far-field energy release rate that drives cracking (both along the interface and into the bulk), as a function of the interface amplitude, the position of crack tip, and the relative toughness of the interface and the bulk. Regime maps are illustrated that indicate the crack path as a function of the bulk-to-interface toughness ratio, along with the associated far-field energy release rate needed to drive failure.

Effect of slag cement on the properties of ultra-high performance concrete

This study investigates the effects of replacing a portion of Portland cement with slag cement (0%, 25%, 50% and 65% by mass) on the properties of plain ultra-high performance concrete (UHPC) mixes. It is observed that slag cement substantially enhances the flowability of the UHPC mixes and reduces the superplasticizer dosage. Mechanical properties are evaluated for mixes with similar air void characteristics, indicating that early age strength is reduced while there is more strength gain in the later age. This early age strength suppression is consistent with semi-adiabatic hydration heat evolution. The presence of slag cement reduces the maximum temperature rise. Linear deformation measurement on duplicate sealed specimens reveals four distinct stages for the shrinkage development and slag cement increases the shrinkage strain in the steady state, associated with the pozzolanic reaction. Simultaneous measurement of moisture uptake, mass loss and relative dynamic modulus of elasticity (RDM) on UHPC mixes for F-T durability shows that the cumulative moisture uptake and mass loss, when normalized with respect to the paste content, are almost negligible compared with a regular concrete mix. This reconciles the capillary suction dominated surface scaling mechanism. No internal bulk cracking is detected due to the dense matrix restricting the ingress of external moisture and the amount of freezable pore water.

Defects Mechanics; Three-Body Problem of Defects Interaction among Crack, Dislocations and Twin Boundary in Magnesium

Dynamics and statics of defects interaction among crack, dislocations and twin boundary (TB) observed in magnesium were investigated using molecular dynamics and elasticity with the complex stress functions to clarify the effect of long-range elastic stress field. An atomic model containing a crack parallel to (10-11) TB was gradually elongated under KI-mode tension by molecular dynamics simulations. Changing the distance between the crack and the TB, four kinds of crack propagation manners were observed, one of which showed the path transition from the crack to the TB itself by shielding effect of piled-up dislocations around the crack tip. The stress intensity factor of the nanosized crack in bulk is 0.28 MPam1/2, which is smaller than that of crack on the TB. The shielding effect due to the piled-up dislocations drastically decreases stress concentration around the crack tip and the stress intensity factor diminishes down to the 0.22, and thus the crack nucleated from the void nucleation and coalescence on the TB was propagated instead. The elastic stress distributions obtained by the superposition of some complex stress functions suggest that the stress field around the crack tip is disturbed by the localized stress due to the TB in the case of crack closest to TB and also by the back stress due to the piled-up dislocations in the case of crack far from TB.

Characteristic analysis of surface damage and bulk micro-cracks of SiR/SiO2nanocomposites caused by surface arc discharges

Anti-pollution flashover coating made by room temperature vulcanized (RTV) silicone rubber (SiR) has been widely used in electrical equipment insulation. However, it faces many outstanding problems; for example, the erosion caused by surface arc discharges. This paper presents the characteristic analysis on surface damage and bulk microcracks of SiR/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposites caused by surface arc discharges in order to better understand the suppression mechanism of nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> particles. The specimens were prepared by adding nano-sized SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> particles into neat RTV SiR with different weight ratio and the experiments were performed according to IEC 60587. During the experiments, a high-speed camera was used to record the discharge processes. After the experiments, the eroded parts of each specimen were dyed by methylene blue solution. Also, a high-power microscope was used to observe the bulk micro-cracks. Then, an image processing technology was used for processing the surface damage images, bulk cracks images, and discharge light images to analyze the effects of adding nano fillers in SiR. The results show that as the content of nano-SiO2 fillers increases, the distribution ranges of black solid residues and dry-band traces caused by surface arc discharges tend to decrease. It indicates the increasing resistance to surface damage. Moreover, the depth of bulk micro-cracks and their area ratio show a decreasing tendency. It reveals the decreasing propagation of bulk micro-cracks. In addition, the reducing total luminance of discharge light shows the decrease of discharge intensity. Therefore, at the certain weight ratios, the addition of nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> particles can not only improves the resistance to surface damage, but also inhibits the propagation of micro-cracks tracking in SiR bulk.

Drop failure modes of Sn–3.0Ag–0.5Cu solder joints in wafer level chip scale package

To reveal the drop failure modes of the wafer level chip scale packages (WLCSPs) with Sn–3.0Ag–0.5Cu solder joints, board level drop tests were performed according to the JEDEC standard. Six failure modes were identified, i.e., short FR-4 cracks and complete FR-4 cracks at the printing circuit board (PCB) side, split between redistribution layer (RDL) and Cu under bump metallization (UBM), RDL fracture, bulk cracks and partial bulk and intermetallic compound (IMC) cracks at the chip side. For the outmost solder joints, complete FR-4 cracks tended to occur, due to large deformation of PCB and low strength of FR-4 dielectric layer. The formation of complete FR-4 cracks largely absorbed the impact energy, resulting in the absence of other failure modes. For the inner solder joints, the absorption of impact energy by the short FR-4 cracks was limited, resulting in other failure modes at the chip side.

Effect of hydrophobic surface treatment on freeze-thaw durability of concrete

The effect of surface treatment using silanes on the frost durability is investigated on both laboratory and field specimens in an accelerated laboratory test. Measurements include moisture uptake during the pre-saturation and F-T stages, cumulative mass loss and internal bulk cracking under frost salt/water exposure. It is found silane treatment substantially reduces surface scaling, but cannot prevent bulk moisture uptake or the occurrence of the internal frost damage when concrete is insufficiently air entrained. Salt scaling is dominated by the capillary suction process in the thin surface region under freezing which can be curtailed by the pore lining effect from silanes creating a hydrophobic barrier to the ingress of external liquid. This in turn suppresses ice growth in the surface region, evidenced by the complete elimination of sub-freezing dilation in a length-change measurement of small-scale concrete specimens with surface treatment. However, internal frost damage is controlled by the universal degree of pore saturation which in turn is dependent on the bulk moisture uptake.

Simulation of failure of air plasma sprayed thermal barrier coating due to interfacial and bulk cracks using surface-based cohesive interaction and extended finite element method

This article describes a method of predicting the failure of a thermal barrier coating system due to interfacial cracks and cracks within bulk coatings. The interfacial crack is modelled by applying cohesive interfaces where the thermally grown oxide is bonded to the ceramic thermal barrier coating. Initiation and propagation of arbitrary cracks within coatings are modelled using the extended finite element method. Two sets of parametric studies were carried out, concentrating on the effect of thickness of the oxide layer and that of initial cracks within the ceramic coating on the growth of coating cracks and the subsequent failures. These studies have shown that a thicker oxide layer creates higher tensile residual stresses during cooling from high temperature, leading to longer coating cracks. Initial cracks parallel to the oxide interface accelerate coating spallation, and simulation of this process is presented in this article. By contrast, segmented cracks prevent growth of parallel cracks which can lead to spallation.

Spontaneous exfoliation of a drying gel.

Wet starch cracks when it dries inhomogeneously, while hot glass cracks when it cools non-uniformly. In both cases, differential shrinkage induced by drying/cooling from the surface causes superficial cracks to grow perpendicular to the surface in different patterns. In contrast with these observations of bulk cracking in brittle materials, when a soft and homogeneously swollen polymer gel dries, differential strains lead to the peeling of a thin layer that spontaneously tears away from the bulk. Continued drying leads to the process repeating itself, forming a peeled-layered structure. The emergent thickness of the exfoliated layer is a function of both the geometry of the original gel and the physical parameters associated with the drying rate and external temperature. We characterize the experimental conditions under which layer peeling can arise, and use simulations to corroborate these observations. Finally, a minimal theory explains the scaling of the peel thickness, consistent with our experiments.

Creep and Environmental Effects on the High Temperature Creep-Fatigue Behavior of Alloy 617

Abstract Alloy 617 is the leading candidate material for an intermediate heat exchanger (IHX) of the very high temperature reactor (VHTR), expected to have an outlet temperature as high as 950°C. Acceptance of Alloy 617 in Section III of the ASME Code for nuclear construction requires a detailed understanding of the creep-fatigue behavior. Strain-controlled low cycle fatigue (LCF) tests including hold times up to 1800 s at maximum tensile strain were conducted at 950°C and a total strain range of 0.3 % in air and in a simulated VHTR impure helium. For similar test conditions, the creep-fatigue behavior was globally similar in both environments. The fatigue resistance decreased when a hold time was added at peak tensile strain, consistent with the observed change in fracture mode from transgranular to intergranular. Further increases in the tensile hold time, beyond 180 s, were not detrimental to the creep-fatigue resistance. The influence of oxidation on the fatigue crack initiation and propagation is discussed. Bulk damage in the form of grain boundary cracking was present in the interior of the creep-fatigue specimens. This bulk cracking was quantified and found to be similar for hold times of up to 1800 s consistent with the saturation in failure lives.