Catastrophic Crack Propagation Research Articles

Interfacial fatigue fracture of elastomer bilayers under cyclic large deformation

Applications of stretchable polymeric soft materials, such as elastomers and hydrogels, in fields of biomedicine and soft devices frequently require strong bonding between these materials or these materials and other substrates. Methods of achieving the strong interfacial bonding have been well developed, but the fatigue behaviors of these bonding interfaces under cyclic large deformation have been rarely explored in both experiments and theories. In this work, an experimental methodology is presented to study the interfacial fatigue fracture of firmly bonded elastomer bilayers enabled by topological entanglements under cyclic large deformation. The relationship between the interfacial fatigue crack propagation speed v and the energy release rate G is obtained. Three regions are identified in this relationship: crack initiation, stable crack propagation, and catastrophic crack propagation regions. The threshold of energy release rate G0 for the crack initiation under cyclic large deformation is about 35 times smaller than the critical energy release rate Gc for the catastrophic crack propagation. Both in the crack initiation and catastrophic crack propagation regions, the logarithmic v increases with increasing the logarithmic G nonlinearly. While in the stable crack propagation region, the experimental points of logarithmic v at various logarithmic G yield a linear relationship with a positive slope. It is demonstrated that by designing the structure of the bonding interface or the bonding edge, the interfacial fatigue fracture of elastomer bilayers can be alleviated. The experimental methodology presented in this work can be utilized to study the interfacial fatigue fracture between stretchable materials of other types. The findings in this work help to reveal the mechanism of interfacial fatigue fracture between stretchable materials and the development in anti-interfacial fatigue bonding methods.

Improving strength and impact resistance of 3D printed components with helicoidal printing direction

Inspired by the fibre layup in the exoskeleton of mantis-shrimp, we show that the out-of-plane strength and impact toughness of 3D printed plates can be significantly improved by simply controlling the tool path (printing direction) of each layer to replicate a helicoidal layup. Without any increase in weight or fabrication time, or effects on the final shape, 3D printed helicoidal plates are shown to outperform conventional printed plates by up to 360% and 128% in terms of out-of-plane strength and impact energy dissipation. Moreover, the 3D printed plates with helicoidal layup can sustain 20% higher out-of-plane load than compression moulded plates and possess comparable impact resistance. The damage mechanisms of 3D printed plates are different under static loading and dynamic impact. Helicoidal layup excels in both conditions; the well-distributed voids in helicoidal layups are effective at preventing large crack formations during static loading; while spiralling cracks in the helicoidal layup prevent catastrophic crack propagation during impact. Fibre reinforced laminates have been reported to possess superior mechanical properties when arranged in a helicoidal fashion. This study is performed using plain unreinforced PLA, showing the benefits of such bioinspired substructure are not exclusive to composites.

Investigating the fracture behaviors of bulk metallic glasses under different dynamic loading rates using phase-field model

Bulk metallic glasses (BMGs) attract significant attention because of their superior mechanical properties. They are potential dynamic structural materials that may be applied in military and aerospace fields as energy penetrators. Unfortunately, the structural reliability under dynamic loading cannot be guaranteed at room temperature because of the catastrophic crack initiation and propagation. Thus, it is vital to ascertain the dynamic fracture behaviors of BMGs and their corresponding mechanisms. In this work, the dynamic fracture behaviors of Zr-based BMGs are investigated through the analysis of the mode-I fracture evolution under different high strain rates using the phase-field method. The crack initiation was observed to change from several branches at the notch tip to one branch with the rate increase, while the evolution in metallic glass changed from one stable crack to fragmentation with multiple branches as the rate increased. To investigate crack evolution features, the relationship between crack extension displacement xs and time t was obtained from the simulation results, and the crack velocity was derived through fitting. The changes in fracture behavior with strain rate are attributed to the effects of the strain rate on the crack dynamics and energy dissipation for the crack initiation. Comparison of crack velocities in different fracture modes allowed to determine the critical velocity for the bifurcation. These results can help design new materials for dynamic structures.

In situ study on fracture behaviors of SiC/2A14Al composite joint： Co-construction of microstructure and mechanical properties via laser welding

SiC p /Al matrix composite (SiC p /AMCs) has been widely used in aerospace field of structural frames, but it is difficult to realize the lightweight connection of complex structural parts. Understanding the fracture mechanism from the source will provide a decisive insight into the preparation (e.g. additive manufacturing) and processing (e.g. welding) of new high strength lightweight aluminum matrix composite. In this work, the fracture characteristics and the correlative damage mechanisms near fracture crack tip in a notorious SiC/2A14Al joint was conducted by in-situ scanning electron microscopy tensile test and transmission electron microscopy . The sandwich eutectic microstructure Mg 2 Si–Al–AlCu of clean interface and micron scale heterogeneous heterostructure inhibited the crack propagation . The sandwich eutectic microstructure disperses the extra stress caused by deformation, and the dislocation multiplies and moves at the interface of Mg 2 Si–Al and Al–AlCu phase, which slows down the stress concentration at the interface and avoids catastrophic crack propagation. Based on the fracture crack initiation source characterization of the joint, a rapid solidification process strategy was designed to further improve the mechanical properties of the joint, and the joint with tensile strength of 255 MPa was successfully prepared. Under the premise of ensuring full penetration and the same heat input, it is easier to obtain satisfactory tensile strength with high laser power - high velocity welding parameters, which is due to the suppression of the Al 4 C 3 . This work can be regarded as a breakthrough in the research of high strength aluminum matrix composites guided by reverse failure analysis strategy.

The Balance Strategy between Structural Safety and Sensing Accuracy Inspired by Slit-Based Mechanical Sensilla

In engineering, cracks are typically regarded as defects due to enormous stress amplification at tip of the crack. Conversely, scorpion ingeniously utilizes the “risky” near-tip stress field of a crack-shaped slit to accurately detect weak vibration signal without causing catastrophic crack propagation from the slit tip. The present paper focuses on the balance strategy between structural safety and sensing accuracy of slit-based mechanical sensilla. We performed a detailed structural and mechanical property study of tissue around the slit wake utilizing a complementary combination of various experimental methods. The results indicate that there is a special thin surface membrane covering the slit wake and the elastic moduli of the membrane and exoskeleton are 0.562 GPa and 5.829 GPa, respectively. In addition, the ratio of bending stiffness between exoskeleton and membrane tissue is about 8 × 104. The theoretical and simulation analysis show that the surface membrane—with appropriate elastic modulus and bending stiffness—can achieve different forms of deformation with the change of slit width for protecting the mechanosensory structure without sacrificing the sensing accuracy. This finding offers a crucial theoretical basis for the further design of bionic mechanical sensors based on the near-tip stress field of artificial cracks.

Strain Rate-Dependent Viscoelasticity and Fracture Mechanics of Cellulose Nanofibril Composite Hydrogels.

In this work, the composite hydrogel toughening behaviors as manifested by strain rate-dependent viscoelastic properties and enhanced fracture mechanics, that is, suppressed catastrophic crack propagation with increased resistance, are systematically examined by using cellulose nanofibrils (CNFs) as fillers in the polyacrylamide (PAAm) matrix. The uniaxial deformation tests show that the tearing energy increases with crack velocity and becomes dominated by the viscoelastic energy dissipation in front of the crack tip. The creep dynamics of the composite hydrogels under a constant stress is examined, and the results indicate that the incorporation of the CNF pronouncedly suppresses the creep deformation. In addition, the microdeformation and failure mechanisms are analyzed through the observation of morphology of arrested crack tips and the damage zone by transmission electron microscopy and scanning electron microscopy. By aligning the CNF along the crack direction, it is possible to focus on the study of interfacial slip mechanics and identify the role of interfacial slip during the energy dissipation process. The results indicate that the CNFs are largely orientated parallel to the loading direction to maximize the energy dissipation, where the initiation of crack propagation is the primary fracture mechanism in composite hydrogels. The coarse feature on the composite fracture surface implies that the CNF initiates deflection of crack propagation fronts and thus increases the strain energy for continuation of the fracture. It is envisioned that with the incorporation of interdisciplinary strategies, one can rationally combine multiple approaches toward the creation of nanocomposite hydrogels with enhanced mechanical properties.

Mechanical response of notched marble beams under bending versus acoustic emissions and electric activity

An experimental protocol, including the combined application of both innovative and traditional sensing techniques, is described aiming to explore the mechanical response of marble and also to check the possibilities of detecting precursor phenomena designating upcoming catastrophic fracture. The protocol consisted of three-point bending tests with notched prismatic beams made of Dionysos marble, the material extensively used for restoration of the Acropolis of Athens monuments. The sensing system improvised included techniques relying on completely different physical foundations, which permit simultaneous detection and recording of the Pressure Stimulated Currents, Acoustic Emissions, three dimensional displacement fields and Notch Mouth Opening Displacements. Analysis of the results revealed interesting features of the mechanical response of Dionysos marble and indicated, also, that classical Continuum Fracture Mechanics fails to describe accurately the response of marble, at least in the presence of notches. In addition, strong correlations between the Pressure Stimulated Currents, the rate of acoustic hits and the rate of change of the opening of the pre-existing notch have been enlightened. Moreover, the onset of catastrophic crack propagation appears following distinguishable changes of the Pressure Stimulated Currents recorded. Therefore (and taking into account the very small size of the respective sensors as well as the simple complementary equipment needed), it is concluded that the specific technique could be considered as a simple and reliable tool for an alternative approach to the in-situ Structural Health Monitoring of classical stone monuments.

Tensile deformation behavior of tungsten fibre-reinforced tungsten composite specimens in as-fabricated state

To overcome the inherent brittleness of tungsten, which is a promising candidate for a plasma-facing material in a future fusion device, tungsten fibre-reinforced tungsten composites (Wf/W) have been developed. As a part of the materials characterisation program on Wf/W, we present the results of first tensile tests of as-fabricated Wf/W in this contribution. The results give insight on the ultimate tensile strength properties and reveal the active toughening mechanisms under tension load within the composite. Fibre bridging, fibre necking as well as fibre pull out were observed. This is leading to the typical pseudo ductile behavior of the composite which is characterized by a rising load bearing capability despite multiple matrix cracks accompanied by non catastrophic crack propagation (in the matrix). The description of the mechanical tests is supplemented by detailed microstructural investigations.

Hierarchical Supramolecular Cross-Linking of Polymers for Biomimetic Fracture Energy Dissipating Sacrificial Bonds and Defect Tolerance under Mechanical Loading.

Biological structural materials offer fascinating models how to synergistically increase the solid-state defect tolerance, toughness, and strength using nanocomposite structures by incorporating different levels of supramolecular sacrificial bonds to dissipate fracture energy. Inspired thereof, we show how to turn a commodity acrylate polymer, characteristically showing a brittle solid state fracture, to become defect tolerant manifesting noncatastrophic crack propagation by incorporation of different levels of fracture energy dissipating supramolecular interactions. Therein, poly(2-hydroxyethyl methacrylate) (pHEMA) is a feasible model polymer showing brittle solid state fracture in spite of a high maximum strain and clear yielding, where the weak hydroxyl group mediated hydrogen bonds do not suffice to dissipate fracture energy. We provide the next level stronger supramolecular interactions toward solid-state networks by postfunctionalizing a minor part of the HEMA repeat units using 2-ureido-4[1H]-pyrimidinone (UPy), capable of forming four strong parallel hydrogen bonds. Interestingly, such a polymer, denoted here as p(HEMA-co-UPyMA), shows toughening by suppressed catastrophic crack propagation, even if the strength and stiffness are synergistically increased. At the still higher hierarchical level, colloidal level cross-linking using oxidized carbon nanotubes with hydrogen bonding surface decorations, including UPy, COOH, and OH groups, leads to further increased stiffness and ultimate strength, still leading to suppressed catastrophic crack propagation. The findings suggest to incorporate a hierarchy of supramolecular groups of different interactions strengths upon pursuing toward biomimetic toughening.

Moisture Ingress, Behavior, and Prediction Inside Semiconductor Packaging: A Review

Reliability issues associated with moisture have become increasingly important as advanced electronic devices are nowhere more evident than in portable electronic products. The transition to the Pb-free solders, which require higher reflow temperature, makes the problem further exacerbated. Moisture absorbed into semiconductor packages can initiate many failure mechanisms, in particular interfacial delamination, degradation of adhesion strength, etc. The absorbed moisture can also result in catastrophic crack propagation during reflow process, the well-known phenomenon called popcorning. High vapor pressure inside pre-existing voids at material interfaces is known to be a dominant driving force of this phenomenon. This paper reviews various existing mechanisms of water accumulation inside voids. The procedures to obtain the critical hygroscopic properties are described. Advanced numerical modeling schemes to analyze the moisture diffusion phenomenon are followed with selected examples.

Recording the mechanical response and fracture of marble DENT specimens using modern sensing techniques

Abstract: The mechanical response and fracture of Double Edged Notched Tensile (DENT) specimens, made of Dionysos marble, is studied in an effort to enlighten the mechanisms activated before and during catastrophic crack initiation and propagation in pre-cracked structures. The innovation of the study is that four modern sensing techniques, both contact and non-contact, are used simultaneously to monitor the response of the specimens, in the direction of pumping real-time data from both the surface and the interior of them. The analysis of these data, permitted comparative assessment of the efficiency of the techniques used and indicated that they provide clear and in mutual agreement signs, which precede the onset of crack initiation and can be considered as pre-failure indicators.

Experimental analysis of bonding strength in shape rolling of Al–Cu bimetallic circular pipes into square tubes

In the present research, the intermetallic layer situation and bond strength of square tubes produced by shape rolling of Al–Cu bimetallic pipes has been investigated. The explosive welding process was used for the production of bimetallic circular pipes. The macro- and microscopic features of explosive-welded pipes and shape-rolled specimens at various stages were experimentally measured by using shear and hardness testing and optical metallography. Moreover, scanning electron microscopy was used for fractography of the fracture surfaces after the last stage of forming. The experimental results showed that the shear strength variation is dependent on the intermetallic layer thickness and its continuity. Also, continuity in Al–Cu interfaces that have a thick intermetallic layer intensifies the microcrack propagation, while due to discontinuity in the thin intermetallic layer, the retardation of catastrophic crack propagation was observed in the early passes of the shape-rolling process. However, the results of the present study showed that as the passes proceed in the rolling process, the crack propagation phenomenon, which is the cause of a decrease in shear strength, is expected.

Effect of Specimen Geometry and Press-Fit on the Stress Intensity Factor Solution for Scaled Wheelset Axles under Bending

One problem in wheelset axles are the failures due to the propagation of fatigue cracks. To avoid those failures with sometimes catastrophic consequences analytical crack propagation simulations are used. For these simulations the knowledge of the crack propagation path and the corresponding stress intensity factor (SIF) is required. To calculate the SIF a numerical simulation, such as finite element method (FEM), is generally needed. To this end numerical simulations for shouldered solid shafts with elliptical surface cracks are done. In this work different influencing factors of the stress intensity factor expressions have been identified. Therefore, two different specimen types with three different stress concentration factors each were designed. The results represent an overview for the interaction of stress concentration factor, crack depth, crack aspect ratio, bending and press-fit load in shouldered solid shafts. To represent the rotation, different characteristic angular positions for one selected characteristic crack geometry have been considered.

Fracture Characterization of Roller Compacted Concrete Mixtures with Blast Furnace Slag and Industrial Sand

The global energy crisis and growing environmental concerns have prompted manufacturers in general to intensify their efforts to reuse the by-products and wastes they generate. In metallurgy, steel production generates dross in the form of slag. In the present study, roller-compacted concrete (RCC) mixtures were prepared and the sand fraction was composed with industrial sand and granulated blast furnace slag. The influence of these different fine aggregates was analyzed based on the compaction parameters (moisture and dry unit weight), indirect tensile strength, flexural strength, and modulus of elasticity. Toughness and fracture tests were carried out and R-curve was determined for the mixtures. It was found that the RCC mixtures containing slag required larger amounts of water for compaction, presented drops in tensile strength and modulus of elasticity as well as increased the mean values of propagation energy in little amount. Both the reference RCC and the slag RCC presented a consistently upward R-curve and, in absolute values, showed a better R-curve and KR performance than conventional and high-strength concretes previously studied in the literature. This is very important for paving purposes since a material with such a behavior shows greater resistance to catastrophic crack propagation, thus extending the service life of pavement layers.

Flaw Tolerance of Nuclear Intermediate Filament Lamina under Extreme Mechanical Deformation

The nuclear lamina, composed of intermediate filaments, is a structural protein meshwork at the nuclear membrane that protects genetic material and regulates gene expression. Here we uncover the physical basis of the material design of nuclear lamina that enables it to withstand extreme mechanical deformation of >100% strain despite the presence of structural defects. Through a simple in silico model we demonstrate that this is due to nanoscale mechanisms including protein unfolding, alpha-to-beta transition, and sliding, resulting in a characteristic nonlinear force-extension curve. At the larger microscale this leads to an extreme delocalization of mechanical energy dissipation, preventing catastrophic crack propagation. Yet, when catastrophic failure occurs under extreme loading, individual protein filaments are sacrificed rather than the entire meshwork. This mechanism is theoretically explained by a characteristic change of the tangent stress-strain hardening exponent under increasing strain. Our results elucidate the large extensibility of the nuclear lamina within muscle or skin tissue and potentially many other protein materials that are exposed to extreme mechanical conditions, and provide a new paradigm toward the de novo design of protein materials by engineering the nonlinear stress-strain response to facilitate flaw-tolerant behavior.

Al2O3와 TiO2의 반응소결로 제조한 Al2TiO5-기계가공성 세라믹스

Aluminium titanate(Al₂TiO?) has extremely anisotropic thermal expansion properties in single crystals, and polycrystalline material spontaneously microcracks in the cooling step after sintering process. These fine intergranular cracks limit the strength of the material, but provide an effective mechanism for absorbing strain energy during thermal shock and preventing catastrophic crack propagation. Furthermore, since machinable BN-ceramics used as an insulating substrate in current micro-electronic industry are very expensive, the development of new low-cost machinable substrate ceramics are consistently required. Therefore, cheap Al₂TiO?-machinable ceramics was studied for the replacement of BN ceramics. Al₂O₃-Al₂TiO? ceramic composite was fabricated via in-situ reaction sintering. Al₂O₃ and TiO₂ powders were mixed with various mol-ratio and sintered at 1400 to 1600℃ for 1 h. Density, hardness and strength of sintered ceramics were systematically measured. Phase analysis and microstructures were observed by XRD and SEM, respectively. Machinability of each specimens was tested by micro-hole machining. The results of research showed that the Al₂TiO?-composites could be used for low-cost machinable ceramics.

Computation of the J‐integral for large strains

AbstractThe phenomenon of failure by catastrophic crack propagation in structural materials poses problems of design and analysis in many fields of engineering. Cracks are present to some degree in all structures. They may exist as basic defects in the constituent materials or they may be induced in construction or during service life.Using the finite element method, a lot of papers deal with the calculation of stress intensity factors for two‐ and three‐dimensional geometries containing cracks of different shapes under various loadings to elastic bodies. In order to increase the accuracy of the results, special elements have been used. They are described together with methods for calculating the stress intensity factors from the computed results.At the vicinity of a crack tip, the strains are not always small, but they may also be large. In this case, the J‐integral can also be applied to characterize the cracks in elastic or elastic–plastic bodies.This paper describes the computation of the two‐dimensional J‐integral for large strains to elastic and elastic–plastic bodies and represents some numerical examples. Copyright © 2007 John Wiley & Sons, Ltd.

Fracture mechanisms of the Strombus gigas conch shell: II-micromechanics analyses of multiple cracking and large-scale crack bridging

Micromechanics analyses of the dominant energy-dissipating mechanisms responsible for the resistance to catastrophic fracture of the aragonitic shell of the giant Queen conch, Strombus gigas, are presented. The crossed lamellar microstructure of the shell is associated with a work of fracture that is three orders of magnitude higher than that of non-biogenic aragonite [J. Mater. Sci. 6 (1996) 6583]. Previous energy-based models predict that multiple “tunnel” cracks in the weak layers of the shell account for a factor of 20 of this increase in fracture energy. We show that the additional factor of ⪞300 results from the synergy between the tunnel cracking and crack bridging mechanisms, analogous to multiple energy dissipating mechanisms observed in brittle matrix composites. The theoretical models demonstrate that the microstructure of the shell of S. gigas is such that potential cracks evolve towards the desirable non-catastrophic ACK (Aveston–Cooper–Kelly) [Properties of fiber composites, Conference Proceedings 15, National Physical Laboratory, IPC Science and Technology Press, 1971] limit, a situation in which all bridging ligaments remain intact along the crack wakes. Load–deflection experiments at temperatures ranging from −120 to 200 °C suggest that a glass transition occurs within the organic (proteinaceous) phase at ∼175 °C, and demonstrate the critical role that this organic “matrix” plays in the resistance of the shell to catastrophic crack propagation.

Surface Oxide Effects on Static Fatigue of Polysilicon MEMS

ABSTRACTStatic fatigue – crack growth causing delayed failure under constant stress and involving stress corrosion cracking – was investigated in polysilicon MEMS using two different surface-micromachined devices. One exploited residual tensile stresses to create stress concentrations at micromachined notches, and the other involved a single-edge notched beam specimen integrated with an electrostatic comb-drive microactuator. Tests with both devices revealed that polysilicon is not susceptible to static fatigue in humid environments. However, when a relatively thick (45 to 140 nm) surface oxide was thermally grown on the microactuator devices, the specimens demonstrated delayed fracture in a humid ambient, presumably due to static fatigue of the surface SiO2. The stress intensities at the resulting cracks in the SiO2were then sufficient to cause catastrophic crack propagation through the polysilicon specimens. The implications of our data on the issue of fatigue in polysilicon is discussed.

Fracture toughness of polysilicon MEMS devices

Polysilicon fracture mechanics specimens have been fabricated using standard microelectro-mechanical systems (MEMS) processing techniques, with characteristic dimensions comparable to typical MEMS devices. These specimens are fully integrated with simultaneously fabricated electrostatic actuators that are capable of providing sufficient force to ensure catastrophic crack propagation. Thus, the entire fracture experiment takes place on-chip, eliminating the difficulties associated with attaching the specimen to an external loading source. The specimens incorporate atomically sharp cracks created by indentation, and fracture is initiated using monotonic electrostatic loading. The fracture toughness values are determined using finite element analysis (FEA) of the experimental data, and show a median value of 1.1 MPa m 1/2.