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Fracture of crystalline silicon (c-Si) solar cells in photovoltaic modules is a big concern to the photovoltaics (PV) industry. Cell cracks cause performance degradation and warranty issues to the manufacturers. The roots of cell fractures lie in the manufacturing and integration process of the cells and modules as they go through a series of elevated temperature and pressure processes, involving bonding of dissimilar materials, causing residual stresses. Evaluation of the exact physical mechanisms leading to these thermomechanical stresses is highly essential to quantify them and optimize the PV modules to address them. We present a novel synchrotron X-ray microdiffraction based techniques to characterize the stress and fracture in the crystalline silicon PV modules. We show the detailed stress state after soldering and lamination process, using the synchrotron X-ray microdiffraction experiments. We also calculate the maximum tolerable microcrack size in the c-Si cells to sustain the residual stress after lamination. We further demonstrate the effect of these residual stresses on the cell fractures using the widely accepted fracture (4-point bending) tests. These test results show that the soldering and lamination induced localized residual stresses indeed reduce the load-carrying capacity of the c-Si cells.
The field of in situ nanomechanics is greatly benefiting from microelectromechanical systems (MEMS) technology and integrated microscale testing machines that can measure a wide range of mechanical properties at nanometer scales, while characterizing the damage or microstructure evolution in electron microscopes. This article focuses on the latest advances in MEMS-based nanomechanical testing techniques that go beyond stress and strain measurements under typical monotonic loadings. Specifically, recent advances in MEMS testing machines now enable probing key mechanical properties of nanomaterials related to fracture, fatigue, and wear. Tensile properties can be measured without instabilities or at high strain rates, and signature parameters such as activation volume can be obtained. Opportunities for environmental in situ nanomechanics enabled by MEMS technology are also discussed.
Achieving high fracture toughness and maintaining high strength at the same time are main goals in materials science. In this work, scale-bridging fracture experiments on ultrafine-grained chromium (UFG, Cr) are performed at different length scales, starting from the macroscale over the microscale (in situ SEM) down to the nanoscale (in situ TEM). A quantitative assessment of the fracture toughness yields values of ∼3 MPa m1/2 in the frame of linear elastic fracture mechanics (LEFM) for the macrosamples. The in situ TEM tests reveal explicitly the occurrence of dislocation emission processes involved in energy dissipation and crack tip blunting serving as toughening mechanisms before intercrystalline fracture in UFG body-centered cubic (bcc) metals. In relation to coarse-grained Cr, in situ TEM tests, in this work, demonstrate the importance of strengthening grain boundaries as promising strategy in promoting further ductility and toughening in UFG bcc metals.
Post-irradiation plastic strain spreading in ferritic grains is investigated by means of three-dimensional dislocation dynamics simulations, whereby dislocation-mediated plasticity mechanisms are analyzed in the presence of various disperse defect populations, for different grain size and orientation cases. Each simulated irradiation condition is then characterized by a specific “defect-induced apparent straining temperature shift” (ΔDIAT) magnitude, reflecting the statistical evolutions of dislocation mobility. It is found that the calculated ΔDIAT level closely matches the ductile-to-brittle transition temperature shift (ΔDBTT) associated with a given defect dispersion, characterized by the (average) defect size D and defect number density N. The noted ΔDIAT/ΔDBTT correlation can be explained based on plastic strain spreading arguments and applicable to many different ferritic alloy compositions, at least within the range of simulation conditions examined herein. This systematic study represents one essential step toward the development of a fully predictive, dose-dependent fracture model, adapted to polycrystalline ferritic materials.
Effects of microstructure constituents of α2-Ti3Al/γ-TiAl lamellae, β-Ti grains and γ grains, with various volume fractions on room-temperature ductility of γ-TiAl based alloys have been studied. The ductility of the alloys containing β phase of about 20% in volume increases to more than 1% as the volume fraction of γ phase increases to 80%. However, γ single phase alloys show very limited ductility of less than 0.2%. The present results, thus, confirmed the significant contribution of β phase to enhancement of the room-temperature ductility in multi-component TiAl alloys.
Linear elastic moduli of solids with similar chemical compositions usually vary fairly insignificantly. However, for a broad class of apparently similar materials, their higher-order (nonlinear) moduli may differ by many times or even by orders of magnitude. Besides their large magnitude, nonlinear effects often demonstrate qualitative/functional features inconsistent with predictions of the classical theory of nonlinear elasticity based on consideration of weak lattice (atomic) nonlinearity. The latter is mostly applicable to ideal crystals and flawless amorphous solids, whereas the presence of structural heterogeneities can drastically modify the acoustic nonlinearity of materials without appreciable variation in the linear elastic properties. Despite often rather nontrivial/nonstraightforward relationships between microstructural features of the material and the resultant “nonclassical” acoustic nonlinearity, the extremely high structural sensitivity makes utilization of nonlinear acoustic effects attractive for a broad range of diagnostic applications that have been emerging in recent years in various areas—from seismic sounding and nondestructive testing to materials characterization down to the nanoscale.
A fracture analysis is developed for crack initiation sequences occurring during sharp indentation of brittle materials. Such indentations, generated by pyramidal or conical loading, generate elastic and plastic deformation. The analysis uses a nonlinear elements-in-series model to describe indentation load–displacement responses, onto which lateral, radial, cone, and median crack initiation points are located. The crack initiation points are determined by extension and application of a contact stress-field model coupled to the indentation load, originally developed by Yoffe, in combination with crack nuclei coupled to the indentation displacement to arrive at an explicit fracture model. Parameters in the analysis are adapted directly from experimental fracture and deformation measurements, and the analysis outputs are directly comparable to experimental observations. After adaptation, crack initiation loads and sequences during indentation loading and unloading of glasses and crystals are predicted by the model from material modulus, hardness, and toughness values to within about 25% of peak contact load. This work is dedicated to George M. Pharr IV on the occasion of his 65th birthday in recognition of his contributions to indentation mechanics.
Plasticity and fracture of materials at the nanoscale levels can deviate significantly from the same phenomena in bulk properties, which may have important implications if they are to be used in real-world engineering systems. Nanoscale metal–metal multilayered materials provide a model material system platform to understand plasticity and fracture based on dislocation interactions with microstructural features. Recently, there is a growing trend to understand the fracture of multilayered materials to see the interactions between the crack and multilayered interface through novel experimentation techniques. In this review, we will introduce the rationale, the current microfracture methods to test and analyze the multilayer fracture behavior and the challenges faced in performing them. Four examples of in situ fracture techniques are highlighted in this work through tensile testing of film on a substrate: microfracture clamped beam bending technique across the multilayers and delamination along the multilayered interface.
The stability of dynamic fracture is a fundamental and challenging problem in the field of materials science. The grain size effect on dynamic fracture instability in polycrystalline graphene under tear loading is explored via theoretical analysis and molecular dynamics simulations. The fracture stability phase diagram in terms of grain size and crack propagation velocity is obtained, and three regions of crack propagation are identified: stable, metastable, and unstable. For grain size above 2 nm, there exists a critical velocity beyond which fracture instability occurs, and this critical velocity depends linearly on grain size. Decreasing grain size leads to reduced characteristic time for correction of crack path deflection, which plays a dominant role in dynamic fracture instabilities. However, when grain size is below 2 nm, there does not exist a critical velocity for steady propagation of cracks due to discontinuous effects. Our results also provide a valuable insight into dynamic fracture of polycrystalline graphene as well as other 2D and quasi-2D materials.
Metal matrix composites (MMCs) have great potential to replace monolithic metals in many engineering applications due to their enhanced properties, such as higher strength and stiffness, higher operating temperature, and better wear resistance. Despite their attractive mechanical properties, the application of MMCs has been limited primarily due to their high cost and relative low fracture toughness and reliability. Microstructure determines material fracture toughness through activation of different failure mechanisms. In this paper, a 3D multiscale modeling technique is introduced to resolve different failure mechanisms in MMCs. This approach includes 3D microstructure generation, meshing, and cohesive finite element method based failure analysis. Calculations carried out here concern Al/SiC MMCs and focus on primary fracture mechanisms which are correlated with microstructure characteristics, constituent properties, and deformation behaviors. Simulation results indicate that interface debonding not only creates tortuous crack paths via crack deflection and coalescence of microcracks but also leads to more pronounced plastic deformation, which largely contributes to the toughening of composite materials. Promotion of interface debonding through microstructure design can effectively improve the fracture toughness of MMCs.
In this study, a rapid powder consolidation method combining powder compact hot pressing and extrusion was utilized to consolidate relatively cheap, high impurity blended powder mixture Ti–6Al–4V alloy. The purpose of this work was to investigate whether a suitable microstructure deriving from a particular heat treatment balance out or compensate for the presence of high interstitial impurity contents. From mechanical property data attained, it was clear that annealing in high α–β region gave a much better combination of mechanical properties: impact toughness (14 J), yield strength (878 MPa), ultimate tensile strength (1092 MPa), and ductility/plastic strain (6.2%) compared to as-extruded material despite the presence of 0.44 wt% oxygen. Therefore, it can be concluded that optimization of microstructures provides improvement to the fracture related properties and Ti–6Al–4V produced in this way is suitable for less demanding applications. For further enhancement in properties, utilization of low oxygen starting powders is vital.
Thermal cycling of planar solid oxide cell (SOC) stacks can lead to failure due to thermal stresses arising from differences in thermal expansion of the stack’s materials. The interfaces between the cell, interconnect, and sealing are particularly critical. Hence, understanding possible failure mechanisms at the interfaces and developing robust sealing concepts are important for stack reliability. In this work, the mechanical performance of interfaces in the sealing region of SOC stacks is studied. Joints comprising Crofer22APU (preoxidized or coated with MnCo2O4 or Al2O3) are sealed using V11 glass. The fracture energy of the joints is measured, and the fractured interfaces are analyzed using microscopy. The results show that choosing the right coating solution would increase the fracture energy of the sealing area by more than 70%. We demonstrate that the test methodology could also be used to test the adhesion of thin coatings on metallic substrates.
The wafering of thin silicon substrates is done by wire sawing technology. In this work a numerical model for the investigation of microstructural mechanisms like cracking and damage evolution during the sawing process is presented. A three-dimensional finite element model representing the phase transformation properties of silicon is validated by loading curves from nano-indentation experiments. By using cohesive zone finite elements, the crack lengths as well as crack initiation depths can be quantified and compared with the experimental results in terms of the maximum depth of subsurface damage.
Interfaces can influence the mechanical properties of metallic multilayers, even between different combinations of face-centered cubic (FCC)/body-centered cubic (BCC) constituents, as reported from many experiments. Recent literature has shown promise for fracture being delayed or even stopped at these interfaces. However, no studies have investigated the influence of their constituents on the subsequent mechanisms of fracture leading to failure. We performed in situ microfracture bending tests of the notched clamped beams made from physical vapor deposited Cu/Nb and Al/Nb multilayers. A catastrophic, linear elastic, brittle fracture was observed for the Cu/Nb beams, whereas a more delayed fracture with a gradual crack propagation was observed for the Al/Nb beams. These observations reveal differences in mechanisms because of the FCC element, interface/boundary blocking of dislocation motion, and effect of grain boundaries in the multilayers. Through this study, FCC/BCC metallic multilayers can be designed with enhanced fracture resistance and mechanical strength.
Biological materials such as bone, teeth, and nacre boast remarkable structures and toughening mechanisms, many of them unmatched by engineering materials. In these materials, fracture toughness is key to fulfill critical structural functions and achieve high strength, reliability, robustness, damage tolerance, and notch performance. In this article, we review and discuss some of the main toughening strategies found in hard biological materials. In particular, we underline a “universal” strategy where well-defined microarchitectures, stiff building blocks, and weak interfaces operate in synergy to resist crack propagation. These natural materials have been inspiring the development of a myriad of synthetic materials that duplicate some of these features at the nanoscale and at larger scales. While recent materials show impressive properties, duplication of the architectures and mechanisms seen in natural materials still presents formidable challenges.
In fracture mechanics, established methods exist to model the stability of a crack tip or the kinetics of crack growth on both the atomic and the macroscopic scale. However, approaches to bridge the two scales still face the challenge in terms of directly converting the atomic forces at which bonds break into meaningful continuum mechanical failure stresses. Here we use two atomistic methods to investigate cleavage fracture of brittle materials: (i) we analyze the forces in front of a sharp crack and (ii) we study the bond breaking process during rigid body separation of half crystals without elastic relaxation. The comparison demonstrates the ability of the latter scheme, which is often used in ab initio density functional theory calculations, to model the bonding situation at a crack tip. Furthermore, we confirm the applicability of linear elastic fracture mechanics in the nanometer range close to crack tips in brittle materials. Based on these observations, a fracture mechanics model is developed to scale the critical atomic forces for bond breaking into relevant continuum mechanical quantities in the form of an atomistically informed scale-sensitive traction separation law. Such failure criteria can then be applied to describe fracture processes on larger length scales, e.g., in cohesive zone models or extended finite element models.
The presence of children in English voluntary hospitals during the eighteenth century has only recently come under academic scrutiny. This research examines the surviving admission records of the London Hospital, which consistently record inpatient ages, to illuminate the hospital stays of infant and child patients and examine the morbidity of children during the long eighteenth century. Traumatic cases were the most common category of admission. The proportion of trauma cases admitted to the London Hospital was higher than in provincial English child patient cohorts, potentially reflecting the differential risks faced by rural and urban children. In most cases of traumatic injury the inpatients stayed in hospital long enough for significant fracture healing to have occurred. Understanding the conditions surrounding children’s admission to hospital, their length of stay, the result of their stay, and which medical issues drove their parents or guardians to seek medical attention for them are critical to illuminating the morbidity of children during the long eighteenth century.
Chip-package interaction (CPI) and the related thermomechanical stress in microchips increase the risk of failure in on-chip interconnect stacks, caused by delamination along Cu/dielectrics interfaces (adhesive failure) and fracture in dielectrics (cohesive failure). High-resolution transmission X-ray microscopy (TXM) is a unique technique to image crack propagation in on-chip interconnect stacks. The visualization of crack evolution in Cu/low-k Backend-of-Line (BEoL) structures is demonstrated using an experimental setup which combines high-resolution X-ray imaging with mechanical loading. The application of an indenter manipulator at the TXM beamline of the synchrotron radiation source BESSY II provides an unprecedented level of details on the fracture behavior of microchips. This in-situ experiment allows to identify the weakest layers and interfaces and to evaluate the robustness of the BEoL stack against CPI.
Interventions to reduce disability from acute orthopedic injuries require a primary assessment of knowledge and need. There are no previous studies to assess this need in the remote provincial islands of the Philippines, an area recurrently affected by natural disaster.
A preliminary assessment of orthopedic knowledge and need was performed to be expanded for regional or national implementation.
Two independent surveys were conducted of households and mid-level providers who represent the first contact of care. The goal of the survey was to describe the local health care system, to identify barriers to care, and to assess gaps in knowledge for acute traumatic orthopedic injuries. Both surveys were conducted in June of 2015.
Population proportional sampling assessed a total of 100 households from 25 local Barangay communities. Questions focused on existing knowledge of acute traumatic orthopedic injuries and barriers to care.
The mid-level provider survey focused on knowledge and barriers to care regarding acute traumatic orthopedic injuries. A total of 10 school nurses and Barangay midwives representing 25 local Barangay were surveyed.
In the household population survey, 84% of respondents reported cost was either always or sometimes a barrier to care; 73% cited transportation as a barrier to care. A total of 68% of respondents reported that they would seek care at the provincial hospital for a suspected broken bone; 28% percent of respondents did not believe broken bones making an arm or leg crooked could be corrected without surgery. Only 55% percent believed care should be sought within six hours of injury, and 37% stated that more than three days after an injury was an appropriate timeframe to seek care.
Of the mid-level providers surveyed, 90% reported that they would refer possible broken bones to a higher level of care. Aggregate ranking of barriers to care from greatest to least were: cost, transportation, knowledge of time sensitive nature of treatment, religious beliefs, and other (not specified). In all, 100% reported that an education initiative regarding acute orthopedic injuries would increase the number of patients seeking care within 12 hours.
The survey describes perceived barriers to care and gaps in knowledge for acute orthopedic injuries. With some modification, this survey tool could be expanded and utilized on a regional or national level to assess gaps in knowledge and barriers to acute orthopedic care.
CourtneyCS, KirschTD. Orthopedic Knowledge and Need in the Provincial Philippines: Pilot Study of a Population-Based Survey. Prehosp Disaster Med. 2018;33(3):293–298.
In this paper, we report a method of increasing fracture toughness (KIC) and strain energy release rate (GIC) of vinyl ester matrix by adopting a dual reinforcement strategy. Reinforcements were carbon nanotubes (CNT) and graphene nanoplatelets (GNP). Both categories of nanoparticles were functionalized with COOH. The idea was to enhance crack bridging and interface sliding with CNT inclusions, given their high aspect ratio. In addition, promote crack-tip blunting and cross-linking density with GNP inclusions, due to their platelet structures. Both KIC and GIC were measured using ASTM D5045-14. An exhaustive experimental study revealed an optimum loading of both nanoparticles to be 0.25 wt% CNT and 0.5 wt% GNP, based on the highest combination of KIC and GIC values. We observed that stress intensity factor, KIC, of neat vinyl ester increased by 43% from 1.14 to 1.62 MPa*(m½). Meanwhile, the improvement in GIC was even greater with an increase of 65%, i.e., from 370 to 610 J/(m2). Differential scanning calorimetry (DSC) studies showed a discernible shift in glass transition temperature (Tg) from 123 to 128°C. The slight temperature increase was similar in thermogravimetric analysis (TGA). We observed the maximum thermal decomposition temperature (Tp) increase from 410 to 414°C, as was evident in the derivative TGA (DTG) curves.