Glass Fracture Research Articles

Scratch‐induced yielding and ductile fracture in silicate glasses probed by nanoindentation

AbstractLateral nanoindentation provides access to the scratch hardness of glass surfaces. The specific sensitivity of the scratching experiment to surface mechanical properties can be enhanced when the local load at the tip apex is reduced. Here, we report on ramp‐load scratch tests on a range of silicate glasses using a sphero‐conical tip shape. Similar as with regular scratching experiments using sharp indenters, such tests create a sequence of micro‐ductile, micro‐cracking, and micro‐abrasive regimes. Detailed investigation of the indenter displacement h and of the lateral force FL as recorded in situ, however, reveals pronounced deviations in comparison to Vickers or Berkovich scratching experiments. Most notably, this includes an abrupt increase in both h and FL at moderate normal load, marking the onset of ductile fracture, and a yield point at the transition from fully elastic deformation to the elastic‐plastic regime at low load. For the range of examined silicate glasses, we find that structural cohesion controls yielding, whereas scratch‐induced fracture and micro‐abrasion are dominated by the volume density of bond energy.

Interface fracture of micro-architectured glass: Inverse identification of interface properties and a novel analytical model

Recently, it is demonstrated that bio-inspired interlocking micro-architectures within a brittle material can increase its ductility and fracture toughness remarkably. Despite showing tremendous promise, there is a lack of computational and analytical models for these interlocked systems to simulate opening-mode fracture behavior. Besides, due to curved geometry of the interface its properties are difficult to obtain from experiments alone. In the present paper the effective thickness, cohesive and contact properties of the interface, are inversely identified from the experimental data by developing and using a finite element (FE) model as a forward solver. The identification is challenging due to the fact that the influences of these parameters on the mechanics are interdependent. Present FE model has revealed important insights about the stress fields and the interface fracture process. Further, a novel approximate analytical model is derived to simulate the complete pullout response of the interlocking teeth, which involves the mixed mode cohesive fracture and the contact mechanics at the interface in addition to elastic deformation of the teeth. The analytical model is derived independently without any phenomenological input from the experiments or the finite element simulations. However, the predictions by the analytical model match accurately with both experiments and the finite element simulations. It is expected that the proposed experimentally validated, predictive computational and analytical models of interlocking micro-architectured materials would generate insights into their complex failure process and enable their performance optimization for large-scale applications.

Enhancing cover glass fracture resistance for hand-held devices with PEALD-based Al2O3 coating

Abstract Chemically strengthened glass (also known as ion exchanged glass) is widely used for hand-held devices due to excellent mechanical strength. However, only top and bottom planar surfaces are sufficiently protected following a series of machining processes such as drilling, cutting and milling for manufacturing and assembly. Micro cracks that remain on the machined surface of the glass edges become very vulnerable to shock impact loading. In this study we suggest a novel PEALD technique to encapsulate surface defects with a 100 nm thickness layer of high-strength ceramics such as Al 2 O 3 . In order to optimize PEALD parameters, we have examined best practices for testing chemically strengthened cover glasses. By conducting both Charpy impact test and 3-point bending test, it was found that impact resistance and flexural toughness of the PEALD-coated cover glass were 35%–40% and 21% higher compared to non-coated samples. Through the analysis of the state and elements of the machined surface with Al 2 O 3 (500 nm) coating it was verified that the coated layer uniformly covers the entire surface of the glass and fully infiltrates into the micro cracks. This study confirms that Al 2 O 3 ceramic coated with PEALD improves not only overall glass strength, but also overcomes the shortcomings of chemically strengthened glass machining.

48‐4: Analysis of Fracture Mechanism in Sand Paper Drop Test for Cover Glass

Fracture mechanism of cover glasses on mobile devices in a drop event was investigated through high speed camera observation and conventional fracture surface analysis. Drop tests were performed for various chemically strengthened glasses with different mechanical properties and stress profiles. These were dropped on sandpapers with grit size of 180. Based on the observations, we confirmed that the fracture of cover glasses in the drop tests is caused by two consecutive steps. The first one is nucleation of a crack due to the contact with the protrusions of the sandpaper, which causes a strong tensile stress localized around the protrusions. The second one is the propagation of the crack due to a stress caused by bending of the smart phone during the rebound process after the first contact with the sandpaper. According to these investigations, we established a prediction formula for the “fracture drop height” for the sandpaper drop tests. It enables us to understand key parameters to improve the fracture drop height. For example, in addition to the stress induced by chemical strengthening, the mechanical properties of the cover glass and the body of the smartphone are key factors to improve the fracture drop height. We believe the knowledge in this study is important for improving the strength of the cover glasses in the future.

Evidential significance of multiple fracture patterns on the glass in forensic ballistics

BackgroundIn the current scenario, glass plays a vital contribution in our standard everyday life. Fractured and fragmented glasses are most commonly encountered at the crime scene. During the commission of the crime, glass breaks and shatters into fragments or into minute particles that serve as trace evidence and may provide a significant lead for the probe. The fracture of glass could be the consequence of an intentional act or unintentional act. For the forensic purpose, study of glass fractures, i.e., fractography has paramount importance as it can provide noteworthy data and encourages an assortment of assessments to investigate the case especially in the shooting cases where a firearm is used. Multiple fractures on a glass pane also have enough potential to establish an investigative link. Presently, air guns are being used in many of the cases due to their simple accessibility. There are a number of cases that have been reported that involved the use of air guns.ResultThis study was conducted to evaluate the pattern of multiple fractures on the glass by the air rifle. For this purpose, glass panes of various thicknesses were test fired perpendicularly at a fixed range with an air rifle having round nose pellet. Measurements and analysis were made on some fixed parameter. Data was analyzed graphically and statistically to check the consistency in the trends.ConclusionConsistency in the trends was observed, which is analyzed by the test for the goodness of fit. The findings of this work have provided some significant different perspectives that may effectively contribute to the criminal investigation.

Numerical Analysis of the Impact Fracture of Metallic Glass Based on Free Volume Model

The yield behavior of amorphous metals including the metallic glass shows intrinsic dependence on the hydrostatic stress, so that yield criterion models such as Mohr-Coulomb and Drucker-Prager are often used. Both the models can explain the asymmetry in the yield stress under uniaxial compression and tension conditions, while the asymmetry in the angle of fracture surface is not able to be determined based on any of those models. The free volume model is able to provide that foundation. Shibutani et al. proposed a new constitutive model for amorphous metals that was derived from some free volume models and the flow rule using the Drucker-Prager yield function as a plastic potential, and investigated the yield behavior and the formation of localized shear band under some temperature conditions using the implicit static FEM code. The formation of shear bands is an unstable phenomenon that is greatly affected by the initial imperfection. In this model, on the other hand, the temperature or the strain rate also affects the yield behavior considerably. In this study, the impact fracture of metallic glass was investigated by implementing the constitutive model proposed by Shibutani et al. into the explicit dynamic FEM code DYNA3D, with laying emphasis on reproducing asymmetry in the angle of fracture surface and the examination of effects of strain rate and temperature change.

Elucidating how correlated operation of shear transformation zones leads to shear localization and fracture in metallic glasses: Tensile tests on Cu[sbnd]Zr based metallic-glass microwires, molecular dynamics simulations, and modelling

The plastic deformation of metallic glasses (MG) is well-known to occur via shear transformation zones (STZs) on the scale of atomic clusters, yet fracture of MG takes place via shear bands of the micron scale. So far, understanding on how the operation of STZs leads to shear localization and fracture remains limited. In this work, tensile tests on Cu/Zr-based MG micro-wires show that both the first-yield and fracture stress exhibit the Weibull distribution, and fractography reveals that shear localization in the form of intense shear bands leads to shear fracture. Molecular dynamics (MD) simulations show that shear bands form via the correlated emergence and operation of discrete STZs close to one another. To describe how the stochastic yet correlated occurrence of the discrete STZs leads to shear localization, a model is constructed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, if nucleation density of the STZs grows along the strain path, as prior shear events triggers the emergence of new STZs, then successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization and shear banding. Furthermore, the MD results suggest an exponential growth law for the occurrence of the STZs along the strain path, which also gives predictions in good agreement with the experimental Weibull distributions of the first-yield stress of Cu/Zr-based MGs.

Critical Analysis of an FeP Empirical Potential Employed to Study the Fracture of Metallic Glasses.

An empirical potential that has been widely used to perform molecular dynamics studies on the fracture behavior of FeP metallic glasses is shown to exhibit spinodal decomposition in the composition range commonly studied. The phosphorous segregation induces a transition from ductility to brittleness. During brittle fracture the atomically sharp crack tip propagates along a percolating path with higher P concentration. This embrittlement is observed to occur over a wide range of chemical compositions, and toughness decreases linearly with the degree of compositional segregation over the entire regime studied. Stable glass forming alloys that can be quenched at low quench rates do not, as a rule, exhibit such thermodynamically unstable behavior near to or above their glass transition temperatures. Hence, the microstructures exhibited in these simulations are unlikely to reflect the actual microstructures or fracture behaviors of the glassy alloys they seek to elucidate.

Indentation deformation and cracking in oxide glass –toward understanding of crack nucleation

Abstract Cracking is one of the most serious and unfavorable phenomena in glass. To enhance the damage resistance of glass, methods for controlling factors related to cracking have been discussed for many years. In this brief review, various works on indentation-induced fracture in oxide glass are introduced and relevant issues regarding indentation techniques are discussed. In particular, various aspects of the indentation fracture (IF) method, which is a well-known empirical technique for evaluating the apparent fracture toughness of brittle materials, are discussed and it is emphasized that indentation techniques are powerful tools for comparing crack resistances between glass compositions. Finally, some novel approaches to analyzing indentation deformation and fracture in oxide glass are introduced to aid in understanding the brittle behaviors of glass and improving the mechanical properties of glass.

Improving plasticity of metallic glass by electropulsing-assisted surface severe plastic deformation

Using either electropulsing (EP) or surface severe plastic deformation (SSPD) to process metallic glasses can improve their plasticity, however, the moderate improvement in plasticity does not warrant a commercial usage. This work, for the first time, demonstrates the integration of electropulsing and surface severe plastic deformation is much more effective in improving the plasticity of metallic glasses than EP or SSPD treatment alone, opening a new avenue towards an unprecedented combination of strength and plasticity in metallic glasses. It is found that SSPD can generate microstructure heterogeneity featured by a mixture of matrix and plastically displaced regions with increased atomic volume. When applying EP and SSPD simultaneously, a synergistic effect occurs to produce a hybrid network with excess free volume and nanocrystals uniformly embedded in amorphous matrix. Molecular dynamics simulation and fracture surface analysis further reveal that the hybrid network is able to effectively reduce shear band localization, and therefore delay the fracture of metallic glasses.

An intrinsic cohesive zone approach for impact failure of windshield laminated glass subjected to a pedestrian headform

Though windshield laminated glass is a simple sandwiched composite structure, it is of vital importance to investigate its damage and energy absorption behavior for the purpose of pedestrian safety protection. This paper proposes an intrinsic cohesive zone approach to achieve this end. A finite element windshield glazing model is established by inserting cohesive elements into the common surfaces between glass elements, and the interfaces between glass and PVB prior to simulations. The appeal of this model is that both of the two main failure patterns and their energy absorptions, i.e. glass fracture and glass-PVB debonding, of the windshield can be captured by means of the intrinsic cohesive zone approach. The effectiveness of the proposed model is validated by comparing the simulation results under center and corner impact with the corresponding experimental outcomes. Finally, the effects of adhesion parameters, PVB material parameters, and impact velocities and angles on the impact fracture behavior and energy absorption performance of the windshield are thoroughly investigated.

Effect of High‐Pressure Torsion on the Thermal and Mechanical Properties of La62Cu12Ni12Al14 Bulk Metallic Glass

Even though bulk metallic glasses (BMGs) are generally brittle, they receive large plastic deformations of over 1000% shear strain without fracturing through the application of combined shear and hydrostatic stresses. In this study, the effects of the severe plastic deformation by high‐pressure torsion (HPT) on the thermal and mechanical properties of La62Cu12Ni12Al14 BMG are investigated. Crystallization is not detected during the HPT processes with 5, 15, and 30 revolutions. However, increase in glass transition temperature, crystallization temperature, fracture strength, and fracture strain, and decrease in the coefficient of thermal expansion and hardness due to the HPT process are found. These changes in the properties are attributable to the increased free volume that results from the rejuvenated structure in the HPT‐processed BMG. In addition, the fracture in the HPT‐processed La62Cu12Ni12Al14 BMG proceeds through a softening mechanism in the microscale region.

Nanoshaping of glass forming metallic liquids by stretching: evading lithography

Lithography-free nanomanufacturing by elongation and fracture of glass forming metallic liquid is presented. The viscous metallic liquid confined in a cavity is laterally downsized to nanoscale by stretching. The extent of size-reduction can be controlled by tuning the active volume of liquid and the viscous and capillary stresses. Very high aspect-ratio metal nanostructures can be fabricated without using lithography or expensive molds. A systematic study is performed using glass forming Pt-Cu-Ni-P alloy to understand the effects of viscosity, surface tension, pulling velocity, and cavity size on the evolution of cylindrical liquid column under tension. The results are quantitatively described using a phenomenological model based on lubrication theory and surface tension induced breakup of liquid filaments. A new manufacturing approach based on variable pulling velocity and/or spinning of metallic liquid is proposed for fabrication of complex geometries.

Shear band fracture in metallic glass: Hot or cold?

Usually, the fracture of metallic glass accompanies with huge heat release, resulting in the well-known localized melting and formation of vein patterns. In this work, using X-ray tomography and tin-coating methods, we found that the shear fracture of metallic glass under compression can be also “cold” if slowing down the fracture process. The results clarify that the melting phenomenon is not a necessary outcome of shear-band fracture, while the temperature-rise relies strongly on the energy release per unit shearing time, even in the fracture process.

Numerical analysis of crack development of timber-glass composite I-beams in the extended finite element method (XFEM)

This article discusses the comparison of numerical analysis and experimental investigation results for composite timber-glass I-beams, which are an interesting alternative beams of ceilings and roofs. The reasoning behind the use of timber-glass I-beams is the combination of the best features of both materials – this enables the creation of particularly safe beams with regard to structural stability. The article is focused on the glass modelling in FEM software as it is much more brittle than concrete. A models for cracks were shown on the beginning of the paper, than numerical models for composite timber-glass I-beams were performed in ABAQUS and the approach of discrete crack model for glass were shown. Models differ with the mesh size, glass fracture energy and the model of adhesive. For the better verification of the performed numerical models, an experimental investigation was referred and the results of the analysis were compared these tests results.

Shear band fracture in metallic glass: Sample size effect

Previous studies have revealed that the shear band propagation of metallic glasses can be strongly influenced by decreasing the sample size, leading to the dramatically enhanced plasticity. Here we show that the sample size also affects the shear band fracture behavior. When reducing the sample size, the compressive shear band fracture mode is changed from the typical instant fracture with vein patterns into a gradual fracture mode without the formation of vein pattern. This can be explained by the marked reduction of energy dissipation density during shear band sliding in small samples and thus the enhanced shear band stability against the instable fracture.

Design of statically indeterminate reinforced glass beams: accounting for system action

ABSTRACTTransparency has become a trend in architecture. In the near future, the scale of application of structural glass will grow from elements (beams, columns, etc.) to entire systems (floors, roofs, façades, frames, etc.). Especially beams, being one of the most important and common building components, will be combined into load‐carrying systems (e. g. a supporting grid for floors and roofs). However, this scale‐up cannot be realized without incorporating structural safety and robustness on an element and system level, as required by modern building codes. For glass beams, research resulted in the concept of hybrid glass beams, in which a glass web is combined with flanges or reinforced with another material that provides post‐fracture strength and ductility, hence providing structural element safety and robustness. Especially the stainless steel reinforced glass beam, based on the principle of reinforced concrete, proved satisfactory. To investigate the feasibility of applying this concept in innovative beam systems, experimental five‐point bending tests on 3.0 m beams and membrane action tests on 4.3 m beams were performed, resulting in satisfactory load‐carrying behaviour with significant stress redistribution due to plastic hinge formation and membrane action, both compressive and tensile. Both features/mechanisms resulted in significant increases in the post‐fracture capacity of the beam systems. For the latter's design, focusing only on glass fracture is insufficient, as the element and system safety requirements should also be satisfied. Therefore, post‐fracture capacity and ductility should be incorporated in design. The latter requires an amount of reinforcing material, which decreases the beam's overall transparency. Therefore it is important that design aims at achieving maximum capacity and ductility with as little reinforcing material as possible. This paper investigates the effect of stress redistribution and membrane action on the achievable capacity of a reinforced glass beam system. It is concluded that both features/mechanisms, and especially membrane action can make a significant contribution to the ultimate capacity and ductility of the beam system. This paper also considers the incorporation of these mechanisms in design, resulting in optimal (i.e. minimal) usage of materials. Compared to statically determinate beams, a decrease in reinforcing material is possible, resulting in ever more transparent structures.

The geometrical properties of random 2D Voronoi tesselations for the prediction of the tempered glass fracture pattern

ABSTRACTThis paper presents spatial statistical investigations of randomly generated 2D Voronoi tessellations of planar point distributions for the prediction of the fracture pattern of thermally tempered glasses. Statistical distributions of geometrical characteristics of two dimensional Voronoi cells for three types of point processing in the plane of homogeneous Poisson point process (HPP), Matérn Hard‐Core point process (MHCP) and Strauss point process (SP) are numerically obtained based on the computer experiments. In this paper, required parameters of the point processes are presented. Samples of Voronoi cells in two dimensional spaces are generated from the point distributions. The generated random Voronoi tessellations are examined for their uniformity in comparison to a regular honey comb with hexagonal cells. The statistical distributions of geometrical characteristics, the cell area, the cell perimeter and the number of edges per cell are presented for each of the three point processes.

Field of Rods: Part 2. Mockup and Performance Testing of TSSA

ABSTRACTEric Owen Moss Architects has designed a glass cylinder roof for the 8511 Warner Drive project in Culver City, CA. Curved laminated glass tubes suspended from a structural frame provide a striking structure unequalled in structural glass design. Their pursuit of discovering methods that bring projects to public prominence is evident in this design of hanging tubes. The engineering design and redundancy was presented at Engineered Transparency 2016 and this is a continuation of the mockup and validation. The project uses nearly 200 curved annealed laminated tubes (of various lengths) suspended from a structure where mechanical fastening of the glass tube by drilled holes and bolts is architecturally unappealing. The top of the tube is designed with conventional structural silicone and a “Yoke” inside the middle of the tube is attached with TSSA. The Yoke is bonded to curved steel 65 mm × 125 mm rectangular patches with TSSA. The curved fittings attached to the glass have to be machined to match the glass that has a 305 mm radius. The engineering challenge is to use the TSSA to resist the rotational stresses between the two halves of the cylinder during wind and seismic events. Mockup challenges with glass fracture due to heat capacity of large metal fittings were resolved with engineered steel fittings that were smaller in mass and dimensionally adjusted. Additional challenges include dimensional tolerances in glass and machined fittings that resulted in trapped air in the adhesive. The aesthetic trapped air issues were overcome by layering TSSA films upon each other to resolve the tolerances. A customized quality assurance process was implemented to validate performance of the cured adhesive. Results of quality assurance tests are presented. The design team is in the process of executing the design through mockup and validation for full scale production.

Probing structural changes during ductile fracture in metallic glasses via in situ straining inside a MeV transmission electron microscope

Structural changes in the vicinity of a crack during ductile fracture in various metallic glasses (MGs) with the propensity of forming shear bands were investigated by in situ straining inside a MeV transmission electron microscope (TEM). During tensile loading, a plastic deformation zone with a width of ∼100 nm and shear bands with a width of ∼15 ± 5 nm developed from crack tips of ductile fracture in MGs. In particular, crack propagation by nanovoid formation and coalescence was observed in Ti40Zr29Cu9Ni8Be14 MG with the highest compressive plasticity (∼6.7 ± 0.5%) among the tested MGs, whereas this process was hardly observable in other MGs. This difference may be attributed to atomic scale heterogeneities in Ti40Zr29Cu9Ni8Be14 MG induced by icosahedral short range orders (ISRO), which cause pronounced shear band branching and thus delay fracture process. Meanwhile, no crystallization of MGs even with ISRO or with lower Tx (crystallization onset temperature) was observed during in situ straining inside the MeV TEM and uniaxial tensile test with ribbon samples; a possible effect of normal stress and nanovoid formation during in situ straining is discussed to compare with crystallization during uniaxial compression test of the same MGs. Our study provides an insight into intrinsic toughening of MGs and thus can give useful guidelines on how to delay ductile fracture processes of MGs by tuning nanovoid formation, taking advantage of structural features through customized design of MGs.