Microcrack Surface Research Articles

Multiscale modeling of diffuse damage and localized cracking in quasi-brittle materials under compression with a quadratic friction law

The diffuse damage and localized cracking of quasi-brittle materials (i.e., rocks and concretes) under compression can be delineated by a matrix-microcrack system, wherein a solid matrix phase is weakened by a large number of randomly oriented and distributed microcracks, and the macroscopic cracking is formed by a progressive evolution of microcracks. Several homogenization-based multiscale models have been proposed to describe this matrix-microcrack system, but most of them are based on a linear friction law on the microcrack surface, rendering a linear strength criterion. In this paper, we propose a new quadratic friction law within the local multiscale friction-damage (LMFD) model to capture the plastic distortion due to frictional sliding along the rough microcrack surface. Following that, a macroscopic Ottosen-type nonlinear strength criterion is rationally derived with up-scaling friction-damage coupling analysis. An enhanced semi-implicit return mapping (ESRM) algorithm with a substepping scheme is then developed to integrate the complex nonlinear constitutive model. The performance of LMFD model is evaluated compared to a wide range of experimental data on plain concretes, and the robustness of ESRM algorithm is assessed through a series of numerical tests. Subsequently, to effectively describe the localized cracking process, a regularization scheme is proposed by combining the phase-field model with the established LMFD model, and the discretization independent crack localization is numerically verified.

Fracture mechanical properties of sandstone with pre-fabricated cracks under freeze–thaw cycles

This paper evaluates the effect of freeze–thaw (F-T) cycles on the fracture mechanical properties of rocks with pre-fabricated cracks through experimental and numerical simulation. Three-point bending tests are conducted to determine the fracture characteristics of cyan sandstone with varying pre-fabricated crack heights. Subsequently, the failure characteristics of cyan sandstone with pre-fabricated cracks under F-T cycles are examined. A numerical model using parallel bond modeling (PBM) is established and validated against experimental results to further investigate the fracture behavior of sandstones with pre-fabricated cracks. Results demonstrate that both axial peak force and fracture toughness (KIC) linearly decrease with increasing freeze–thaw (F-T) cycles, and the axial peak force reduces with the rise in pre-fabricated crack height (Hc). The deformation damage process is categorized into four stages: compression-density, elastic deformation, pre-peak destabilization, and post-peak destabilization damage. Energy evolution is closely tied to microcrack initiation and propagation, with the majority of energy dissipating as microcrack surface energy. Both strain energy and dissipation energy decrease linearly with increased F-T cycles. These findings offer valuable insights for investigating the fracture mechanism of fractured rock mass in cold regions.

Microscale characterization of an anti-cracking stone base course filled with cement stabilized macadam

Large stone base course filled with cement stabilized macadam (LSFC) is a promising solution for addressing reflection cracks in semi-rigid base asphalt pavements due to its superior crack resistance. However, challenges associated with balancing strength and crack resistance, and mechanizing high-efficiency construction have hindered broader adoption. To address these issues, Wang et al. [1] proposed an optimized mix proportion design for an anti-cracking stone base course filled with cement stabilized macadam (SFC). In this study, various microscopic techniques were employed to explore the anti-cracking mechanism of SFC on a microscopic scale, and compared with the conventional cement stabilized macadam (CSM). These techniques included examining the structural morphology and pore distribution using polarizing microscope (PM) and nuclear magnetic resonance (NMR), analysing the elemental composition and hydration reaction in non-aggregate zones using energy dispersive spectroscopy (EDS), and testing the mechanical properties using atomic force microscope (AFM). The findings revealed, compared with CSM, SFC exhibits a more tightly interlocked coarse aggregate skeleton, which, in turns, resulting in weak interfacial transition zones (ITZs) with a high concentration of pores and defects, weak bonding, and a loose structure, potentially exceeding 20 µm in thickness. The tightly interlocked coarse aggregate skeleton in SFC effectively limits material shrinkage but also leads to localized high stress and weak interfacial damage within the vulnerable ITZ region, converting shrinkage strain energy into microcrack surface energy, thereby delaying the onset of cracking and ultimately enhancing the cracking resistance of the base.

Macro-micro damage model of the effect of freeze–thaw on jointed rocks considering compaction deformation

Jointed rock mass contains microscopic cracks, joints and other defects. Moisture migration during freeze–thaw cycles affects crack initiation and propagation of defects of different scales. The change of defects in turn affects the mechanical and deformation properties of jointed rock mass, especially in the compaction deformation stage. Considering the effects of freeze–thaw cycles on compaction deformation, plastic deformation, macroscopic damage and microscopic damage, a macro–micro damage constitutive model of jointed rock mass under freeze–thaw actions was proposed under the framework of thermodynamics. Based on the internal state variables (ISV) theory, the Helmholtz free energy of jointed rock mass under freeze–thaw actions was obtained by taking compaction deformation, macroscopic damage and microscopic damage as internal state variables. The compaction, elasticity, plasticity, and damage stages were analyzed respectively, and the macro–micro damage model of jointed rock considering compaction deformation were deduced by regarding the total strain as three parts (compaction deformation, elastic deformation and plastic deformation). The validity of the proposed model was verified by compression tests on rock samples with different induced crack inclination angles that have undergone different amounts of freeze–thaw cycles. The model considers the moisture migration mechanism of freeze–thaw damage and the friction and slip mechanism of microcrack surfaces. The effect of moisture migration on the compaction deformation during freeze–thaw cycles is significant, and the proposed model can accurately simulate the full stress–strain curves (compaction-elasticity-plasticity-damage) of jointed rock mass under freeze–thaw actions, which provides quantitative guidance for reducing freezing damage by attenuating moisture migration.

A new micromechanical damage model for quasi-brittle geomaterials with non-associated and state-dependent friction law

In this paper, we shall present a new micromechanical damage model for capturing the mechanical behaviors of quasi-brittle geomaterials under a wide range of compressive stress. Two essential coupled non-linear mechanisms are taken into account: plastic deformation related to frictional sliding along the microcrack surfaces, and microscopic damage induced by the initiation and propagation of microcracks. Within the framework of thermodynamics and Mori–Tanaka homogenization scheme, free energy and thermodynamic forces, including the local stress applied on the microcracks and damage driving force are deduced. The main novelty of this work lies in presenting a new physically based non-associated and state-dependent friction law to capture the plastic distortion, in which the friction coefficient is no longer a constant but a state-dependent variable varying with the surface asperities of microcracks. The relationship between the friction coefficient and mean local stress, as well as microscopic damage is subtly defined. Intriguingly, a macroscopic strength criterion is derived as an inherent part of the proposed model without rotation of the principal stress. Moreover, semi-analytical stress–strain–damage relationships of the proposed model can be derived under conventional triaxial compression. These up-scaling analyses provide a powerful tool for parameter calibration. For application, the semi-implicit return mapping algorithm with plasticity-damage decoupling correction procedure (SRM-PDDC) is adopted for numerical implementation. To demonstrate the performance of the micromechanical model, we compare the model predictions with experimental data of quasi-brittle rocks in literature as well as the results obtained by Zhu et al. (2016). Finally, the applicable condition of the SRM-PDDC algorithm is discussed in detail.

Monitoring of breathing cracks of RC beams based on nonlinear vibro-acoustic modulation technique

ABSTRACT For challenges of early microcrack monitoring of reinforced concrete (RC) structures, a nonlinear vibro-acoustic modulation (VAM) based study was carried out, aiming at investigating the influence law between the excitation signal form and the nonlinear effect of sensing signals through both finite element analysis (FEA) and experimental validation. The mechanism of the nonlinearity in sensing signals coming from microcrack surface vibration collisions due to the breathing effect was found which is the theoretical basis of the VAM-based damage detection. The results show that the nonlinear modulation effect is the highest at the resonant frequency of the structure. The side frequency (SF) amplitude of VAM-based sensing signal in the RC beams with microcracks (damage degree between 0% and 70%) is obvious, but when the cracks continue developing, the effective contact area decreases sharply during the crack opening and closing process due to the nonlinear contact between interfacial crack surfaces. As the width of microcrack is altered from 0 nm to 100 nm, the normalised nonlinear coefficient β is reduced from 0.4 to 0.06 in the approximate form of power function, which can be used to characterise the damage development of the early cracks in RC structures.

Drilling mechanical hydrogen extraction at mining enterprises of Ukraine

Purpose. Increasing the technical and economic indicators of well mechanohydraulic production due to the selection of the type and optimal formulation of drilling flushing fluids. In today's conditions, the lack of energy carriers requires involving a wider range of natural phenomena in energy production, looking for reliable ways and methods of increasing the efficiency of production. The use of energy also forces the introduction of resource-saving, environmentally safe geotechnologies in the energy-intensive technological processes of mining, transportation, crushing and beneficiation of minerals in the conditions of powerful mining enterprises of Ukraine. The methods. The study of the features of the creation and implementation of a progressive complex hydraulic program for the construction of wells was carried out using modern methods of analytical analysis and experimental research, in particular by using general principles of mathematical and physical modeling, methods of processing research results in the environment of МАТHCAD, control and measuring devices and materials. Findings. Development and operational implementation of justified resource-saving ecologically safe geotechnologies and environmental protection strategies. Flushing fluid treated with direct current can be used to increase the technical and economic indicators of the well drilling process. The originality. Dependencies have been established that describe the process of formation under the influence of a rock-destructive tool of individual rock particles that are exposed to the action of the boundary layer, and others that fall into the zone of action of the laminar sublayer. A dependency has been established that describes the process of leaving parts of the sludge that are not carried away by the flow of washing liquid, which, in turn, leads to its further grinding and increased wear of the rock-crushing tool. Practical implementation. Data were obtained on the adsorption reduction of hardness, which is a consequence of the increase in the affinity of the collapsing rock to the washing liquid. This is reflected in the intensive binding of water (hydrate) shells on the inner surface of microcracks or in their mouths. The parameters for improving the efficiency of iron ore extraction in the conditions of deposits of Ukraine are given and substantiated, in particular, a careful approach to the process of regulating the pH level of cleaning agents, taking into account the geological and technical conditions of drilling operations and the intended purpose of wells

Flat-end mill machining analysis of processed CrMnFeCoNi high-entropy alloys

ABSTRACT High-entropy alloys are emerging materials and are widely studied at the synthesis and characterization levels. Their machinability study and product realization are still at a nascent stage. Conventional machining operations are necessary to ensure part assembly and surface quality. Hence, end-mill machining is carried out to check the machinability of the most ductile equiatomic and non-equiatomic CrMnFeCoNi high-entropy alloy (HEA) with a green manufacturing concept. Theoretical and experimental analyses examined the effect of percent composition of an element in a high-entropy alloy and machining parameters (spindle speed, depth of cut and feed per tooth) on the cutting forces, chip morphology, machined surface topography, surface micro-hardness and tool wear mechanism. Kistler dynamometer installed in CNC vertical milling machine is used with AlTiN-coated tungsten carbide tool to record the cutting forces and moments. The microstructure anisotropy induced by low vacuum induction melting of varying compositions of elements in high-entropy alloys influenced the machining results. Feed force values were higher for the non-equiatomic HEA sample than for the equiatomic CrMnFeCoNi HEA. SEM and EDS mapping has revealed microhole and microcrack surfaces after machining. Equiatomic CrMnFeCoNi HEA has shown the highest microhardness of 984 Hv than the non-equiatomic HEA sample.

Multiscale Pore Structure Characteristics and Crack Propagation Behavior of Coal Samples from High Gas Seam.

Investigation on the pore-fracture features and crack propagation behavior of coal is necessary to prevent coal mine disasters. The pore structure features of coal samples taken from high gas seam were obtained by mercury injection porosimetry (MIP) and gas adsorption methods. The process of deformation and failure for coal samples under three-point bending conditions were obtained. The results demonstrate that the adsorption pores with diameter less than 100 nm are the most developed and their surfaces are the roughest (the average surface fractal dimension Ds is 2.933). The surface of micro-cracks is smoother (Ds is 2.481), which is conducive to gas seepage. It may be the explanation for that 14-3# coal seam is a high gas seam, while there was almost no gas outburst accident so far. At the initial stage of crack propagation, the main crack on the coal sample expanded along the direction of the natural cracks. In the process of crack propagation, the surface fractal dimension of the main crack increased, suggesting that the bending degree of the main crack enhanced. The brittle characteristics of coal samples can be reflected by the ratio of the dissipated energy to the accumulated energy.

Variational modeling of hydromechanical fracture in saturated porous media: A micromechanics-based phase-field approach

This paper presents a novel variational phase-field model for different fracture processes in fully saturated porous media. As a key feature, the model employs a micromechanics-based theory for the description of brittle-tensile and compressive–ductile fracture. As such, the field variables are linked to physical mechanisms at the microcrack level, with damage emerging as the consequence of microcrack growth. Similarly, plasticity emerges as a consequence of the frictional sliding of closed microcracks. In this way, the evolution of opening microcracks in tension leads to (mode I) brittle fracture, while the evolution of closed microcracks in compression/shear leads to (mode II) ductile fracture. These failure mechanisms are coupled to fluid flow, resulting in a Darcy–Biot–type hydromechanical model. Therein, in the tensile regime, plasticity naturally vanishes, while damage is driven by poroelastic energy, accounting for the pressure in fluid-filled opening microcracks. On the other hand, in the compressive/shear regime, the plastic driving force naturally follows as a Terzaghi-type effective stress in terms of the local stress field acting on the microcrack surfaces, while damage is solely driven by the frictionally blocked free energy. As another important feature, the model includes a non-associative frictional plasticity law. Nevertheless, a thermodynamically consistent variational framework is employed, for which different energetic principles are discussed. Finally, the numerical simulations show that the model captures relevant hydromechanical coupling effects in benchmark problems, including mechanically induced shear fracture and hydraulically induced tensile fracture.

Advances in nonlinear ultrasonic detection of microcracks

<p indent="0mm">A large number of key components are widely operated under extreme conditions of high temperature and high pressure in power, nuclear energy, aerospace and other important industrial fields. Microdefects are inevitably generated in these key components during the process of manufacturing. Due to the periodic loading and thermal stress, microcracks are generally initiated from these microdefects at the location with stress concentration. It has been found that the initiation and propagation of fatigue microcracks usually occur prior to 70%–90% of fatigue life. It is difficult to observe the evidence before the formation of macrocracks, which may result in catastrophic accidents. Thus, the risk of sudden failure of key components increases greatly. Therefore, the accurate detection of microcracks is crucial for guaranteeing the safe operation of key components. Based on the diffraction, reflection and transmission, traditional ultrasonic testing method is merely validated to effectively detect macroscopic defects within half a wavelength. The nonlinear ultrasonic testing method is found to be sensitive to microcracks, as high order harmonics, sub-harmonics, DC components and frequency mixing waves can be generated due to the strong nonlinear interaction microcracks and ultrasonic waves. With the outstanding effectiveness to detect microcracks, the nonlinear ultrasonic testing method has attracted much attention in recent years. In this work, the research on detection of microcracks using nonlinear ultrasonic testing method is systematically reviewed in terms of theoretical models, numerical simulations and experimental measurements. Firstly, according to the breathing effect and sliding mechanism of microcrack surfaces, we present several theoretical models of nonlinear interaction between ultrasonic waves and microcracks, namely contact acoustic nonlinearity (CAN), bi-linear stiffness model, nonlinear spring model, Hertz contact theory, and rough contact surface model. The advantages and disadvantages of these theoretical models are discussed comparatively. Two mechanisms are mainly considered for the nonlinear interaction of microcracks and ultrasonic waves, namely the macroscopic opening and closing of crack surfaces and the microscopic contact of rough microcracks surfaces. Secondly, we introduce numerical simulation studies on the nonlinear ultrasonic generation induced by microcracks. The applicability is analyzed for several simulation methods, such as finite element method, spectral finite element method, finite difference method, boundary element method and local interaction simulation approach, and the generation and propagation of nonlinear ultrasonic waves are analyzed as well. The effect of microcrack geometries on the nonlinear ultrasonic waves is then summarized. The microcrack geometries are considered mainly clouding the microcrack length, width, depth, density and direction. Furthermore, we present the experimental measurements on the generation of nonlinear ultrasonic waves induced by microcracks and on the localization and imaging of microcracks based on the nonlinear ultrasonic waves. The generation of high order harmonics, sub-harmonics, DC components and frequency mixing waves was observed in the specimens with microcracks. The nonlinear ultrasonic detection is presented to evaluate fatigue microcracks with various microcrack length, width, depth, density and direction. Based on this phenomenon, nonlinear ultrasonic mixing method, transducer array imaging method and nonlinear ultrasonic phased array method are introduced to localize the microcracks. Finally, further studies on evaluation of microcracks using nonlinear ultrasonic waves are proposed, including the comprehensive physical model for actual microcracks in engineering components, robust detection method, and quantitative evaluation of microcracks and service life.

The effect of boron and zirconium on the microcracking susceptibility of IN-738LC derivatives in laser powder bed fusion

The effect of boron (<0.01 to 0.03 wt%) and zirconium (<0.01 to 0.07 wt%) on the microcracking susceptibility of the γ’-strengthened Ni-base superalloy IN-738LC during laser powder bed fusion (LPBF) was studied using custom designed powder grades. It was found that both elements have a strong effect on the microcracking susceptibility, the microcracks are located at high angle grain boundaries based on EBSD measurements and crack density increases with the content of both elements. High crack density in the material with high boron and zirconium content corresponds to a large fraction of intergranular decohesion facets exhibiting a dendritic morphology on the fracture surface, typical for solidification cracking. Investigation of the fracture surface chemistry by X-ray photoelectron spectroscopy (XPS) indicates that considerable amounts of B and Zr are present in oxide state. Auger electron spectroscopy (AES) confirms that both elements are segregated to the intergranular decohesion facets on the fracture surface. Thin layers of B- and Zr-containing oxide on the microcrack surfaces were indicated by atom probe tomography (APT) as well. Hence, it is suggested that the cracking susceptibility of the studied alloying system is caused by formation of B- and Zr-containing oxide at high-angle grain boundaries during solidification.

Distribution of microcrack surface density in the human otic capsule

Background The bony otic capsule is comprised of highly mineralized and dense compact bone. It is rarely remodelled and degenerative changes, therefore, accumulate around the inner ear. It is also a predilection site for the pathological remodelling seen in otosclerosis. Morphometric studies have documented increased numbers of dead osteocytes and microcracks in the human otic capsule. Microcracks may disrupt the lacuno-canalicular network and cause osteocyte apoptosis ultimately breaking up the perilabyrinthine bone signalling pathways and dynamics. This may be important to understand the pathogenesis of remodelling diseases like otosclerosis. Aims/Objectives This study describes the spatial and regional distribution of microcrack surface density in relation to the inner ear and compares it to that previously recorded for otosclerosis. Material and methods Forty-two temporal bones and five ribs were used. All samples were undecalcified, bulk stained in basic fuchsin and plastic embedded. Unbiased stereology was used to estimate the true surface density of microcracks (mm2/mm3) in perilabyrinthine bone. Results The surface density of microcracks accumulates around the inner ear spaces, particularly in the lateral window regions, and increases with age. Conclusions and significance This study documents the spatial and temporal association between microfractures and otosclerosis in the otic capsule.

Experimental and Numerical Investigation of the Micro-Crack Damage in Elastic Solids by Two-Way Collinear Mixing Method.

This study experimentally and numerically investigated the nonlinear behavior of the resonant bulk waves generated by the two-way collinear mixing method in 5052 aluminum alloy with micro-crack damage. When the primary longitudinal and transverse waves mixed in the micro-crack damage region, numerical and experimental results both verified the generation of resonant waves if the resonant condition was satisfied. Meanwhile, we found that the acoustic nonlinearity parameter (ANP) increases monotonously with increases in micro-crack density, the size of the micro-crack region, the frequency of resonant waves and friction coefficient of micro-crack surfaces. Furthermore, the micro-crack damage in a specimen generated by low-temperature fatigue experiment was employed. It was found that the micro-crack damage region can be located by scanning the specimen based on the two-way collinear mixing method.

Combination of eddy current and heat treatment for crack healing and mechanical-property improvement in magnesium alloy tube

The continuous eddy current pulse treatment (ECPT) combined with heat treatment was employed to heal the microcracks in spin formed Mg alloy tubes and improve their mechanical properties in this study. The results show that all the microcracks in different tube specimens were almost healed after different continuous ECPT schemes up to 15 cycles. The schemes with less cooling intervals exhibited better healing effect and increased the strength and elongation of Mg alloy tubes more obviously. After aging treatment, the strength improvement of the specimens with ECPT was more remarkable than that of the specimens without ECPT, and the elongation decrease of the specimens with ECPT was less evident than that of specimens without ECPT due to the segregation of RE elements on the crack surface. Besides, after solution treatment, the strength reduction and ductility improvement of the specimens with ECPT were more pronounced than that of the specimens without ECPT owing to the notable decrease of dislocation density of the specimens with ECPT. Both narrowed cracks induced by ECPT and the segregation of precipitates in the vicinity of microcrack surface during aging treatment contributed to the maximum strength in the as-spun specimens with ECPT followed by aging treatment.

Destruction of cement stone modified with a plasticizer and a nanoscale aluminum oxide

It is well known that hydration and hardening of Portland cement are associated with complicated physical and chemical processes, in particular the interaction of clinker minerals with water to form new hydrate phases of different composition. Sometimes the presence of various substances can affect these processes leading to self-destruction of the hydrated cement stone. The morphology and composition of damaged cement stone modified with an additive and with a nanoscale aluminum oxide were studied. It is shown that in the presence of the ‘Linamix PC’ plasticizer and a nanoscale aluminum oxide, well-crystallized Ca(OH)2 plates with an average thickness of 1.5 microns and a length and width equal to 300–400 microns are formed. Also, a network of thread-like ettringite with a diameter of less than 1 micron on the surface of microcracks of cement stone is formed. Most likely the cement stone sample could be damaged by the forces of crystallization pressure during the growth of both Ca(OH)2 and ettringite crystals.

Effect of Continuous Damage Accumulation on Ignition of HMX‐Based Polymer Bonded Explosives under Low‐Velocity Impact

AbstractThe ignition phenomena of polymer‐bonded explosives (PBXs) due to hotspot formation from grit/friction, shear band formation, and friction between microcrack surfaces have not yet been clarified. Microcracks are known to play a critical role in PBX ignition under low‐velocity impacts. Further, microcrack evolution determines whether or not a specimen ignites because of complex mechanical‐thermal‐chemical coupled processes. Although many studies have investigated the multiphysics processes of ignition under single impacts, few have investigated multiple‐impact scenarios that are more likely in reality. The present study focuses on the influence of continuous damage accumulation during multiple impacts on the safety assessment of octahydro‐1,3,5,7‐Tetranitro‐Tetrazocine (HMX)‐based PBXs. The Visco‐SCRAM model combined with restart technology is applied to conduct simulations with the standard confined Steven test. By controlling the first impact velocity, the damage accumulation could be designed. When the first impact velocity increases from 5 to 20 m/s, the second impact velocity threshold decreases from 34 to 22 m/s and specific mechanical energy decreases from 600 to 240 m2/s2. Multiple impacts change the temperature distribution and ignition position. Further, different projectile shapes influence multiple impacts differently. Owing to continuous damage accumulation, the second impact velocity threshold decreases for flat and pin projectiles. Dimensionless analysis reveals that the trends of critical dimensionless parameters for flat and pin projectiles are similar, whereas oval projectiles show a different response.

Microcrack surface density in the human otic capsule: An unbiased stereological quantification.

Bone is continuously remodeled to repair and strengthen degenerative bone with accumulating dead osteocytes and microfractures. Inner ear osteoprotegerin (OPG)-mediated inhibition of otic capsular bone remodeling causes excessive perilabyrinthine bone degeneration. Consequently, microcracks accumulate around the inner ear. Microcracks cause osteocyte apoptosis and may disrupt the canalicular network connecting osteocytes. Despite their linear microscopic appearance, microcracks are three-dimensional disruption planes and represent surface areas inside a tissue space. With an elevated microcrack burden the number of disconnected osteocytes is expected to increase. This may prove relevant to ongoing research in otic focal pathologies like otosclerosis. Therefore, an unbiased quantification of the microcrack surface density (mm2 /mm3 ) in the human otic capsule is essential. In this study unbiased stereology was applied to undecalcified bulk stained human temporal bones to demonstrate its feasibility in describing the three-dimensional reality behind two dimensional observations of microcracks. A total of 28 human temporal bones and five ribs were bulk stained in basic fuchsin, serially sectioned and hand-ground to a thickness of 80-120 μm. Both horizontal and vertical sections were produced and compared. This study showed that surface density of microcracks was significantly higher around the inner ear compared to ribs. Furthermore, no significant difference in microcrack surface density between horizontal and vertical sections in the temporal bone was demonstrated.

Combining alloy and process modification for micro-crack mitigation in an additively manufactured Ni-base superalloy

The additive manufacturing (AM) of the γ` precipitation strengthened Ni-base superalloys still remains a challenge due to their susceptibility to micro-cracking. Post-processing, such as HIPing, has been shown to heal the micro-cracks but it remains desirable to prevent the micro-cracking from even occurring. Numerous studies highlighting potential mechanisms for micro-cracking exist but few solutions have been demonstrated. The intent of this study was to identify the micro-crack mechanisms and demonstrate how process and alloy modifications can reduce the micro-cracking. The micro-crack surfaces exhibit a dendritic appearance that is indicative of solidification cracking. Additionally, Gleeble experiments, simulating the L-PBF induced Heat Affected Zone (HAZ), were conducted below the γ` solvus temperature and reveal the existence of grain boundary liquation, indicative of liquation cracking. Two cracking mechanisms are thus coexisting during Laser Powder Bed Fusion (L-PBF) of CM247LC. Based on experimental evidence, reduction in the solidification interval of CM247LC was investigated as a candidate for micro-crack mitigation and a new alloy was developed. As Hf is found to have a significant influence on the freezing range of the alloy, a new CM247LC without Hf was produced and tested. The study also involved two separate and distinct processing conditions to highlight the importance of melt pool geometry on micro-crack density. Samples fabricated with the Hf-free CM247LC, CM247LC NHf, in combination with optimized processing conditions exhibit a reduction in crack density of 98 %. This study demonstrates the importance of both processing conditions and alloy chemistry on micro-cracking in L-PBF fabricated γ` hardening Ni-base superalloys.

Dynamic Mechanical Damage and Non-Shock initiation of a New Polymer Bonded Explosive during Penetration.

Complexities of heating mechanisms make it difficult to investigate the safety of a polymer bonded explosive (PBX) charge of earth-penetrating-weapons (EPWs) during penetration. In this paper, the dynamic damage and non-shock initiation of PBX1314 (60 wt % hexahydro-1, 3, 5-trinitro-1, 3, 5-s-triazine (RDX), 16 wt % aluminum, 24 wt % hydroxy-terminated polybutadiene (HTPB)) during penetration is investigated through experiments and simulations. In the experiments, steel projectiles filled with PBX1314 are launched to penetrate concrete targets. In the results, non-shock initiations occur on the tail surface of PBX1314 along with mechanical damage of the tail and middle part of PBX1314. A dynamic damage and initiation model is proposed to characterize the effects of microcracks on the mechanical and thermal responses of PBX1314. Investigation based on the model suggests that microcrack interfacial friction plays significant roles in damage, heat generation and localization in PBX1314. A non-shock initiation criterion is developed based on macroscale variables in PBX1314. Numerical simulations of the penetration experiments are performed by using the proposed model and criterion. The mechanical damage and non-shock initiation of PBX1314 in the experiments are successfully predicted. The simulation results indicate that the tail of PBX1314 impacts the projectile repeatedly during penetration. Finally, the initiation criterion is satisfied because of frictional heat localization near microcrack surfaces and initiation is activated in the tail of PBX1314.