Occurrence Of Brittle Fracture Research Articles

Seismic behaviour of a steel moment resisting frame structure featuring hourglass double split tee joints

The occurrence of brittle fractures in welds of full-strength joints of steel moment-resisting frames (MRFs) during earthquakes in Northridge (1994) and Kobe (1995) marked a turning point in seismic design philosophies for steel structures. This led to the development of new strategies focusing on enhancing structural resilience and energy dissipation. While Reduced Beam Sections (RBS) were traditionally favoured, recent approaches include partial-strength joints and replaceable dissipative fuses, in some cases also equipped with self-centring components. These systems often employ friction or yielding dampers, ensuring that the weakest joint component comprises well-designed dampers for optimal ductility and energy dissipation.In this context, the present study evaluates the performance of a large-scale structure featuring MRFs with partial-strength double-split tee (DST) joints and hourglass dampers. These joints are conceptually similar to ADAS dampers and the Simpson Strong Tie moment connection prequalified by AISC 358–16. The research involves conducting a pseudo-dynamic test campaign on a two-storey steel structure designed according to EC8 provisions. Findings from experiments and a numerical model developed using OpenSees are discussed. Incremental Dynamic Analyses (IDAs) compare structures with hourglass DST joints to those with full-strength joints, showing enhanced self-centring capacity in DST-equipped structures despite lacking dedicated self-centring components. Additionally, structures with hourglass DST joints demonstrate significant low-cycle fatigue life, potentially reducing the need to replace dissipative components after severe seismic events.

Fabrication and mechanical behavior of structurally graded material (ceramic-reinforced polylactic acid/polylactic acid) for integrated engineering application

The goal of this study is to provide first-hand knowledge to the industrial community on the mechanical properties of structurally graded materials (polylactic acid/ceramic-reinforced polylactic acid) wall fabricated through fused filament fabrication. Structurally graded materials are inhomogeneous materials that consist of two or more materials to produce a final structure with distinctive material properties. Because of their specific gradient, structurally graded materials are commonly used in various applications such as aerospace, medicine, optoelectronics, biotechnology, etc. These applications have certain requirements and significant properties such as thermo-mechanical resistance and chemical stability. Filaments of polylactic acid were deposited over ceramic-reinforced polylactic acid to fabricate structurally graded material walls. Coupon studies revealed that structurally graded materials exhibited mechanical properties (tensile strength: 19.1 ± 0.3 MPa, flexural strength: 100.1 ± 0.23 MPa, compressive strength: 64.6 ± 2.35 MPa, impact energy: 1.32 ± 0.038 J, and hardness: 81.5 Shore-D) that are better than polylactic acid. It is revealed that structurally graded materials possess better mechanical properties than polylactic acid and exhibit behavior similar to ceramic-reinforced polylactic acid. Fracture morphology indicates the occurrence of brittle fracture in tensile sample.

Failure mechanism in the pearlite structure with lamellar graphite due to temperature

Critical operating temperature can be determined for the machine components that form the key functional part of the device. This article describes a change of the failure mechanism in the material of an engine, where the material structure consists of fine pearlite layers with lamellar graphite equivalent to grey cast iron. The mechanical properties of this material influence not only the layers formed by the perlite, but also the lamellar graphite acting as a stress concentrator. To assess the failure mechanism in the material, a fractographic analysis of the fracture areas was performed after the impact test. The range of test temperatures at which the impact test was performed on the test specimens with notch ranged from -80°C to +180°C. The occurrence of brittle fracture was detected at the low temperatures by cleavage along and over the planar layers of the lamellar graphite. The brittle fracture initiation at the higher temperatures was observed by cleavage in the basic layer of the perlite. Depending on the failure mechanism of the test specimens fracture area, critical temperatures were determined, in which the measured value of the notch toughness decreased. Regression of temperature dependence of notch toughness allows very precise quantification of this decrease.

Comparison of International Brittle Fracture Design Code Provisions for Steel Bridges

Brittle fracture is a major concern to structural engineers as it has significant consequences for safety and cost. Although modern day occurrences of brittle fracture are rare, it is well known that they can occur without warning and may lead to the sudden closure of a bridge, loss of service, expensive repairs, and/or loss of property or life. In Canada, steel bridge fracture is a particularly significant concern because of the harsh climate. If the toughness properties are improperly specified, many steels could be on the lower shelf of the toughness-temperature curve. A comparison of brittle fracture design provisions around the world reveals that more sophisticated approaches have been developed for modeling and understanding brittle fracture in existing and new bridges than those currently in use in North America, including Canada and the U.S.A. This paper describes the European brittle fracture provisions and presents a comparison of the North American and European design provisions using the example of a typical steel-concrete composite highway bridge. On the basis of this comparison, situations where one set of design rules may be more or less conservative are identified, and opportunities for improvement and areas warranting further study are highlighted.

A New Methodology for Predicting Brittle Fracture of Plastically Deformable Materials: Application to a Cold Shell Nosing Process.

The traditional theory of ductile fracture has limitations for predicting crack generation during a cold shell nosing process. Various damage criteria are employed to explain fracture and failure in the nose part of a cold shell. In this study, differences in microstructure among fractured materials and analysis of their surfaces indicated the occurrence of brittle fractures. The degree of “plastic deformation-induced embrittlement” (PDIE) of plastically deformable materials affects the likelihood of brittle fractures; PDIE can also decrease the strength in tension due to the Bauschinger effect. Two indicators of brittle fracture are presented, i.e., the critical value of PDIE and the allowable tensile strength (which in turn depends on the degree of PDIE or embrittlement-effective strain). When the maximum principal stress is greater than the latter and the PDIE is greater than the former, our method determines the likelihood of brittle fracture. This approach was applied to an actual cold shell nosing process, and the predictions were in good quantitative agreement with the experimental results.

Tensile properties and deformation behavior of ferrite and austenite duplex stainless steel at cryogenic temperatures

The tensile properties and deformation and fracture behaviors of a commercial ferrite (body-centered cubic) and austenite (face-centered cubic) duplex stainless steel were investigated at cryogenic temperatures, and the mechanism underlying the improvement in the tensile properties of the steel at cryogenic temperatures was discussed. The strength of the steel increased continuously with lowering temperature, and the elongation hardly decreased with a decrease in temperature from 293 K to 77 K. At 40 K, the elongation of the steel decreased drastically, however, it increased significantly at 8 K. The serration of the steel was measured during its tensile test at 8 K. The volume fraction of the deformation induced martensite was high at 77 K and 8 K, while it was low at 40 K. The strain accumulated in the ferrite and austenite phases during the deformation process remained almost constant at all the test temperatures. The ductile fracture occurred 293 K, 200 K, 77 K, and 8 K, while brittle fracture occurred at 40 K. The low volume fraction of the deformation induced martensite and the occurrence of brittle fracture can explain the low elongation of the steel at 40 K. At 8 K, the occurrence of serration may increase the temperature in the deformation region, which led to an increase in the volume fraction of the deformation induced martensite, as well as the suppression of brittle fracture, resulting in high elongation. The occurrence of deformation induced martensitic transformation, simultaneous deformation of ferrite and austenite, ductile fracture contribute to improvement of tensile property of ferrite and austenite duplex stainless steel at cryogenic temperatures.

Effect of Multiaxial Tensile Deformation on the Mechanical Properties of Semiflexible Polymeric Samples.

Molecular dynamics simulation is used to investigate the mechanical properties of the semiflexible polymer during multiaxial tensile deformations. The multiaxial tensile deformations can be imposed in totally or partially constrained modes. These types of deformations may be observed during the sudden deformation of polymeric material in the areas of aerospace, automobile, defense applications, etc. It is found that the constrained multiaxial deformation leads to the formation of nanovoids into the polymer sample. The high Young's modulus and yield strength for the totally constrained modes of tensile deformation are due to the energy required to create voids. The variation in von Misses stress, void volume, and bond order parameter with strain indicates the occurrence of brittle fracture during totally constrained tensile deformations. The partially constrained tensile deformations lead to the improvement in bond order parameter and lesser creation of nanovoids within the system. The system shows the characteristic strain hardening before failures.

Numerical simulation of improved RHS column-to-I beam flange-plate bolted connections for avoiding tensile fracture

In an attempt to find possible solution for avoiding premature occurrences of brittle fracture in rectangular hollow section (RHS) column-to-I beam connections, an improved type of connection was t...

Local melting mechanism and its effects on mechanical properties of friction spot welded joint for Al-Zn-Mg-Cu alloy

Local melting and the eutectic film and liquation crack formation mechanisms during friction spot welding (FSpW) of Al-Zn-Mg-Cu alloy were studied by both experiment and finite element simulation. Their effects on mechanical properties of the joint were examined. When the welding heat input was high, the peak temperature in the stir zone was higher than the incipient melting temperature of the Al-Zn-Mg-Cu alloy. This resulted in local melting along the grain boundaries in this zone. In the retreating stage of the welding process, the formed liquid phase was driven by the flowing plastic material and redistributed as a “U-shaped” line in the stir zone. In the following cooling stage, this liquid phase transformed into eutectic films and liquation cracks. As a result, a new characteristic of “U” line that consisted of eutectic films and liquation cracks is formed in the FSpW join. This “U” line was located in the high stress region when the FSpW joint was loaded, thus it was adverse to the mechanical properties of the FSpW joint. During tensile shear tests, the “U” line became a preferred crack propagation path, resulting in the occurrence of brittle fracture.

Influence of High-Temperature Ageing on Mechanical Properties of Low Alloy Steel Clad Plate Welded Joints

In this paper, the mechanical properties of welded joints of Q345R/304 stainless steel clad plates after high temperature ageing were studied. E309Nb welding rod, ER310 welding wire, and ER309L welding wire were used as cladding filling materials, and ER50-6 welding wire was used as base-layer filling material. The mechanical properties of welded joints were characterized by micro-hardness measurement and mechanics performance testing respectively, after ageing treatment at 600°C for 100 hours. Test results showed that there was lower carbon diffusion in the ER309L joint-base layer and transition layer than those of other two joints after high temperature ageing, and the hardness value had little change around ER309L joint-fusion zone. The microstructure of ER309L joint-fusion zone was different from that of other two joints. The impact test after high temperature ageing indicateed the impact toughness of three welding joints decreased obviously, and the weld zone of ER309L joint presented a worst toughness, which leded to the occurrence of brittle fracture.

Damage behaviors of self-compacting concrete and prediction model under coupling effect of salt freeze-thaw and flexural load

In this paper, the workability, mechanical properties and damage behaviors of self-compacting concrete (SCC) under coupling effect of salt freeze-thaw and flexural load were experimentally investigated. The results showed that the workability of fresh SCC and mechanical properties could be influenced by fly ash, blast furnace slag and silica fume added in the mixture. Salt freeze-thaw had small influences on the surface erosion and relative dynamic elastic modulus of the SCC used in this study. Under coupling effect of salt freeze-thaw and flexural load, different levels of flexural load were applied on the SCC specimens. The results showed that with increasing salt freeze-thaw cycles, the weight loss increased and the relative dynamic elastic modulus declined. The coupling effect had a slight influence on the surface erosion, while it had a significant influence on the occurrence of brittle fracture. Higher stress levels always caused earlier and more occurrences of brittle fracture. A prediction model of the damage degree of SCC was proposed by incorporating the test results achieved in the paper. The comparison results showed that the prediction values had good consistencies with the experimental values. The model is expected to be used to predict the damage degree of SCC under coupling effect of salt freeze-thaw and flexural load.

Removal and Fracture Characteristics of Cemented Tungsten Carbide under Nanoindenting and Nanoscratching

Nanoindenting and nanoscratching were used to investigate removal and fracture characteristics of cemented tungsten carbide (cWC). Nanoindentation results indicated that the elastic modulus and hardness of WC grains were significantly greater than those measured in cobalt binder rich regions, respectively. Few evidences of cracking or fracture were observed on the indented surfaces using both in-situ atomic force microscopy and scanning electron microscopy. However, the pop-in events were observed from indenting load-displacement curves and the corresponding acoustic emissions were detected, indicating the occurrences of brittle fracture. Nanoscratch results demonstrated that similar removal characteristics existed, but cracking was observed in both surface and subsurface of the scratched samples.

Mechanical properties of low carbon steel hardened by the Fe2SiTi phase at high volume fraction

The addition of Si and Ti to low carbon steel allows to obtain a nanometric precipitation of an ordered intermetallic phase, Fe2SiTi, with a volume fraction of the order of 6%; which is much higher than usual for microalloyed steels and more comparable to the situation encountered in aluminum alloys. The resulting precipitation hardening leads to extremely high mechanical properties, depending on the optimization of metallurgical route, which makes these materials very promising for the development of innovative, weight saving automotive solutions. In this contribution the precipitation sequence and kinetics are studied in details by a combination of experimental techniques including small angle neutron scattering and scanning or transmission electron microscopy. Some properties (yield stress, strain hardening, strain to fracture) are discussed in view of the measured precipitate characteristics, notably their size and volume fraction. The fracture mechanism, and particularly the occurrence of brittle fracture, is discussed in view of the state of precipitation.

Low-velocity impact response of injection-moulded polypropylene plates – Part 2: Effects of moulding conditions, striker geometry, clamping, surface texture, weld line and paint

The low-velocity, low-energy impact response of a mineral and elastomer modified polypropylene was characterised by instrumented falling-weight impact testing of plates with annular clamping. Most of the impact tests were performed at −30 °C with incident impact velocities in the range 1.0–4.4 m/s, and with plate thicknesses in the range 2.0–3.9 mm. The following factors were investigated: moulding conditions (mould temperature, melt temperature, holding pressure), striker geometry, clamping, plate surface texture, melt flow weld lines and paint. The occurrence of brittle fracture was affected by all these factors, except the moulding conditions. Reducing the striker hemisphere diameter or changing to a flat striker induced brittle fracture. Removing the annular clamping led to a more brittle response. Plates with a weld line were more brittle than standard plates. The surface texture caused brittle fracture when the textured side was in tension under the striker. The paint induced brittle fracture at −30 °C, but no adverse effect of the paint was observed at 20 °C.

Mechanical properties and fracture characteristics of cemented tungsten carbide with fine microstructure studied by nanoindentation

Nanoindentation was used to investigate mechanical properties and fracture characteristics of cemented tungsten carbide with fine microstructure. The elastic modulus and hardness of tungsten carbide grains were found to be significantly greater than those measured in cobalt binder rich regions, respectively. From the indented surfaces, very few evidences of cracking or fragmentation were observed using in-situ atomic force microscopy. However, the pop-in events were observed from the indentation load-displacement relationships and the acoustic emissions were detected during indenting processes, indicating the occurrences of brittle fracture. The latter observations were explained using a hypothesis that the fracture of the material under nanoindentation was mainly via sharp corner breaking due to the crash of individual tungsten carbide grains.

Failure mechanisms and surface roughness statistics of fractured Fontainebleau sandstone

In an effort to investigate the link between failure mechanisms and the geometry of fractures of compacted grains materials, a detailed statistical analysis of the surfaces of fractured Fontainebleau sandstones has been achieved. The roughness of samples of different widths W is shown to be self-affine with an exponent zeta=0.46+/-0.05 over a range of length scales ranging from the grain size d up to an upper cutoff length xi approximately =0.15 W. This low zeta value is in agreement with measurements on other sandstones and on sintered materials. The probability distributions pi delta z(delta h) of the variations of height over different distances delta z>d can be collapsed onto a single Gaussian distribution with a suitable normalization and do not display multiscaling features. The roughness amplitude, as characterized by the height-height correlation over fixed distances delta z, does not depend on the sample width, implying that no anomalous scaling of the type reported for other materials is present. It is suggested, in agreement with recent theoretical work, to explain these results by the occurrence of brittle fracture (instead of damage failure in materials displaying a higher value of zeta approximately =0.8 ).

Prediction of fracture behavior of steel beam-to-column connections with weld defect using the SINTAP

This paper aims to determine accurate and quick defect assessment of welded steel beam-to-column connections and to examine the effect of the different connection types and the electrode types having different toughness on their fracture behavior using the Structural Integrity Assessment Procedure (SINTAP) for European countries. The SINTAP-Level II developed for confidence in welded structures with defects is used. The procedure described in this study allows assessing the maximum crack size permitted in the welded connection under seismic loading without occurrence of brittle fracture. The results obtained from the present study agree well with those of the other studies.

Thermo-Mechanical Aspects of SOFC Operations

Mechanical properties of SOFCs appear to affect drastically their durability. In this work, special attention has been paid to the mechanical loading of a cell upon heating from room temperature to operation temperature and during operation. In the latter case, both hydrogen and methane feeding have been considered. A thermo-mechanical model has been developed to determine both the temperature gradient and the stress field within an anode supported cell structure. Residual stresses due to manufacturing processes and thermal stresses induced by the mismatch in thermal expansion coefficients have been taken into account. Mechanical calculations have been performed considering low compressive residual stresses in the thin electrolyte at room temperature and neglecting any mechanical external loading of the cell. After a heating step, during H 2 operation, a high tensile stress is calculated in the thin electrolyte. Its value is nearly homogenous all over the layer and is found to exceed the limit strength of the yttria stabilised zirconia. Moreover, a singular stress field analysis has allowed determining a high tensile stress at the anode edge (on the circumferential free surface).,The occurrence of brittle fracture of the electrolyte layer upon one thermal cycle is analysed using a Weibull distribution. Depending on the cell material characteristics and on the cell geometry, a critical electrolyte thickness ranging around 50 μm is determined which shows the highest rupture probability. In addition internal steam reforming induces an inhomogeneous temperature distribution in the cell including a cooled area at the inlet of the cell due to the endothermic steam reforming reaction. However, in representative simulation conditions, the temperature gradient in the cell is found to be low (0.2°C/mm). Thus, it cannot be involved in any mechanical degradation process of the cell upon operation.

High resolution transmission electron microscopy of dislocation core dissociations in gold and iridium

Abstract The occurrence of brittle fracture in iridium has attracted significant attention in recent years and is thought to be related to the energetics of the dislocation core, in particular the extremely high unstable stacking energy. Although it is not experimentally possible to measure the unstable stacking energy, first-principles calculations have been used to predict both this and the stacking-fault energy. These calculations suggest that, despite large differences in stacking-fault energy and elastic constants, gold and iridium exhibit similar dissociation behaviours, with screw dislocations in both metals dissociated by approximately 1 nm. In the current study, high-resolution transmission electron microscopy (HRTEM) has been utilized to observe experimentally the arrangement of atomic columns surrounding dislocation cores. Deviations from perfect lattice sites have been measured, and experimental observations quantified through comparisons with image simulations. In the case of screw dislocatio...

Residual Stresses in Welded Moment Frames and Implications for Structural Performance

Welding-induced residual stresses are usually regarded as secondary stresses and therefore not accounted for in the structural analysis. This treatment, however, was brought into question with the welding research community by the discovery of a widespread structural damage in the 1994 Northridge earthquake. Brittle fracture was identified in many of the prequalified weld joints in steel moment frames. Welding-induced residual stresses are believed to be one of the factors contributing to the brittle fracture. In this paper, a detailed investigation on residual stresses in welded moment frame connections is presented. Both the actual connection configurations of the moment resistant frames and local weld details are considered by means of advanced residual stress analysis techniques. Then, the effects of triaxial residual stress state on plastic deformation capacity are discussed based on finite-element results on a wide-plate specimen loaded in tension. Finally, a series of fracture mechanics analyses were carried out to assess the effects of residual stresses on the fracture behavior. As a result, a great deal of insight has been obtained on the fracture behavior of the moment frame welds. Among other things, the results indicate that welding-induced residual stresses significantly increase the fracture driving force due to the presence of high tensile residual stresses. The high triaxial residual stress states identified in these joints can greatly reduce the plastic deformation capability, consequently promoting brittle fracture. Therefore, it seems reasonable to postulate that weld residual stresses could have played an important role in the occurrence of brittle fracture in the pre-Northridge weld joints.