Change In Failure Mechanism Research Articles

Microstructure simulation and ballistic behaviour of weld zones in friction stir welds in high strength aluminium 7xxx plate

The effect of friction stir welding (FSW) on the knockdown in properties and failure mechanisms of high strength aluminium plate (AA7010-T7651) under ballistic impact has been investigated by modelling and ballistic testing, simulated, and actual weld zone microstructures, in order to systematically study the link to weld microstructural variation. FSW of thick plate leads to an extremely wide heat affected zone (HAZ). The microstructures within the weld zones were successfully modelled and simulated. However, the maximum loss of hardness in the HAZ resulted in only a 20% reduction in the ballistic limit (V50). The maximum V50 was associated with the highest level of hardness reached prior to a change in failure mechanism, from ductile hole enlargement, using an armour piercing (AP) projectile, or plugging with a fragment simulation projectile (FSP) to excessive spalling and associated adiabatic shear banding at too high hardness levels. The behaviour of the nugget zone has also been examined. The trends in the ballistic data are discussed in the context of the weld zone microstructures and fracture mechanisms.

Micro/nanomechanical behavior of magnetron sputtered Si–C–N coatings through nanoindentation and scratch tests

Si–C–N/Si based MEMs are advantageous to conventional MEMS because of their ease of fabrication and high temperature sustainability. The failure mechanism of such systems has to be known for their efficient performance. Nanoindentation and scratch behavior were performed on Si–C–N coatings deposited on silicon substrates by RF magnetron sputtering. Crack growth and propagation were studied using optical and SEM views of the deformed region. Different failure mechanisms were observed and analyzed. The crack deflection was due to nanocrystalline phases in the Si–C–N nanocomposite film, as no such deflection was observed in the amorphous CNx film. A different failure mechanism in the form of tensile and conformal cracking was observed. The films' failure mechanism changes from cohesive failure at lower loads to adhesive failure at higher loads during nanoscratching.

Elastoplastic Seismic Response Analysis of Masonry Buildings with Variable Wall Thickness along Height

The multistory masonry buildings with variable wall thickness along the height have suffered different degrees of damage subjected to the Wenchuan earthquake. In order to study the failure mechanism of such masonry structure under the earthquake, three types of five-story structure of computational model are firstly introduced in this paper, including (1) the wall thickness of five stories is 240mm; (2) the wall thickness of the first floor is 370mm, and that of the upper four stories is 240mm; (3) the wall thickness of the first and second story is 370mm, and that of the other stories is 240mm. Then, the elastoplastic time-history dynamic analysis is carried on with the story shear model by the finite element method, and the ground motion of El Centro waves are adopted as earthquake input motion. The analysis results show that variation of wall thickness along height can easily cause stiffness mutation of the upper and lower floor, lead to local floor deformation concentration and soft floors, and the change of failure mechanism of the structure. Finally, it is suggested that some appropriate seismic resistance measures should be taken to meet the lateral stiffness ratio of the upper and lower floor in the later design of this structure, or this kind of structure should be avoided using as far as possible.

Size and temperature effects on the fracture mechanisms of silicon nanowires: Molecular dynamics simulations

We present molecular dynamics simulations of [1 1 0]-oriented Si nanowires (NWs) under a constant strain rate in tension until failure, using the modified embedded-atom-method (MEAM) potential. The fracture behavior of the NWs depends on both temperature and NW diameter. For NWs of diameter larger than 4 nm, cleavage fracture on the transverse (1 1 0) plane are predominantly observed at temperatures below 1000 K. At higher temperatures, the same NWs shear extensively on inclined {1 1 1} planes prior to fracture, analogous to the brittle-to-ductile transition (BDT) in bulk Si. Surprisingly, NWs with diameter less than 4 nm fail by shear regardless of temperature. Detailed analysis reveals that cleavage fracture is initiated by the nucleation of a crack, while shear failure is initiated by the nucleation of a dislocation, both from the surface. While dislocation mobility is believed to be the controlling factor of BDT in bulk Si, our analysis showed that the change of failure mechanism in Si NWs with decreasing diameters is nucleation controlled. Our results are compared with a recent in situ tensile experiment of Si NWs showing ductile failure at room temperature.

Revealing the bearing capacity mechanisms of a penetrating spudcan through sand overlying clay

The safe and economic use of mobile jack-up structures is still hindered by limited understanding of the installation of their large conical ‘spudcan’ footings in layered sand over clay sites. This paper addresses this by detailing experimentally observed bearing failure modes induced during the penetration of a spudcan through a layer of sand into underlying normally consolidated clay. Digital images were captured continuously by installing a half-spudcan against a transparent window, and analysed using a particle image velocimetry technique coupled with close-range photogrammetry correction. As the experiments were performed in a geotechnical centrifuge the observed mechanisms occurred in stress conditions of similar magnitude to the offshore case. The experimental evidence provides: (a) failure modes at different spudcan penetration depths; (b) the transitional failure mechanism at the event of peak bearing resistance; and (c) the changes in overall failure mechanism due to varying geometric and strength conditions of the layered soil. The results provide useful references for the development and validation of analytical or numerical solutions for this problem.

Simulation of material failure behavior under different loading rates using molecular dynamics

Material failure behavior is generally dependent on loading rate. Especially in brittle and quasi-brittle materials, rate dependent material behavior can be significant. Empirical formulations are often used to predict the rate dependency, but such methods depend on extensive experimental works and are limited by practical constraints of physical testing. Numerical simulation can be an effective means for extracting knowledge about rate dependent behavior and for complementing the results obtained by testing. In this paper, the failure behavior of a brittle material under different loading rates is simulated by molecular dynamics analysis. A notched specimen is modeled by sub-million particles with a normalization scheme. Lennard.Jones potential is used to describe the interparticle force. Numerical simulations are performed with six different loading rates in a direct tensile test, where the loading velocity is normalized to the ratio of the pseudo-sonic speed. As a consequence, dynamic features are achieved from the numerical experiments. Remarkable failure characteristics, such as crack surface interaction/crack arrest, branching, and void nucleation, vary in case of the six loading cases. These characteristics are interpreted by the energy concept approach. This study provides insight into the change in dynamic failure mechanism under different loading rates.

Improving strength and ductility of continuous composite plate girder bridges

The ultimate load carrying capacity of continuous composite plate girder bridges is usually limited by the local buckling failure of steel girders at interior supports. This paper presents a simple reinforcement method which changes the failure mechanism of the continuous girder from local buckling to formation of plastic hinges at the interior supports and mid-span. Such a change in failure mechanism greatly improves the strength and ductility of the superstructure. In this method the compressive portion of the web near the interior support is braced against local buckling by bolting pairs of stiff bracing elements on opposite sides of the web. The bracing elements prevent local buckling failure of the support section and create a section which can rotate inelastically at plastic moment allowing the second hinge to form at mid-span. The bracing elements may be plates or longitudinal stiffeners which should be designed to remain elastic while the section undergoes plastic deformation. The behavior of plate girders which are reinforced by such bracing elements is studied using nonlinear finite element analyses.

Enhancement of the Compressive Strength of Kevlar-29/Epoxy Resin Unidirectional Composites

This paper examines the compressive and tensile strength of Kevlar-29/Derakane composites using as-received Kevlar-29 and a thermally enhanced version of the fiber. Previous studies have shown that thermal enhancement results in a form of Kevlar-29 with significantly better compressive strength at a cost of 10—20% of its tensile strength. This study compared the mechanical properties of unidirectional composites made using as-received and thermally enhanced forms of Kevlar-29 fibers. The composites made with the thermally treated fiber showed a decrease in tensile strength roughly proportional to the difference in tensile strength of the fibers. Although the improvement in the compressive strength of the composites made with the thermally treated fibers was statistically significant, it was less than anticipated. A change in predominant failure mechanism also occurred with most as-received fiber-based composites failing from a debonding/pull-out mechanism, while composites made with the thermally enhanced fiber failed primarily from microbuckling with attendant splitting.

Synthesis, phase behaviors, and mechanical properties of biphenyl-type epoxy resins and composites

Diglycidyl ether of 3,3′,5,5′-tetramethyl-4,4′-biphenyl (TMBPDGE) which has been found great applications in semiconductor packaging was synthesized. The liquid crystalline phases of diglycidyl ether of 4,4′-dihydroxybiphenol (BPDGE) cured with phenol novolac (PN) were studied by wide angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). BPDGE was in situ copolymerized with TMBPDGE to improve its thermal and mechanical properties by means of the LC domains retained in the crosslinked networks. The results indicated that a nematic phase was formed and fixed with proper curing schedule when BPDGE was cured with PN that had no neighboring active hydrogens and the LC domains could also be efficiently embedded into the composite systems. Dynamic mechanical properties showed that epoxy networks containing LC domains displayed higher α-relaxation temperature and linear elastic modulus traces. The impact toughness and Tg were improved with the addition of BPDGE. Scanning electron microscope observation of the fracture surfaces showed that there was a change in failure mechanism in the composite systems. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Rate dependent strengths of some solder joints

The shear strengths of three lead-free solder joints have been measured over the range of loading rates 10−3 to ∼105 mm min−1. Binary (SnAg), ternary (SnAgCu) and quaternary (Castin: SnAgCuSb) alloys have been compared to a conventional binary SnPb solder alloy. Results show that at loading rates from 10−3 to 102 mm min−1, all four materials exhibit a linear relationship between the shear strength and the loading rate when the data are plotted on a log–log plot. At the highest loading rate of 105 mm min−1, the strengths of the binary alloys were in agreement with extrapolations made from the lower loading rate data. In contrast, the strengths of the higher order alloys were found to be significantly lower than those predicted by extrapolation. This is explained by a change in failure mechanism on the part of the higher order alloys. Similar behaviour was found in measurements of the tensile strengths of solder joints using a novel high-rate loading tensile test. Optical and electron microscopy were used to examine the microstructures of interest in conjunction with energy dispersive x-ray analysis for elemental identification. The effect of artificial aging and reflow of the solder joints is also reported.

Effect of hafnium incorporation on the crystallization and mechanical behavior of a Zr-based bulk amorphous alloy

The crystallization and mechanical behavior of a hafnium-containing bulk amorphous alloy (Hf 28.35Zr 28.35Cu 15.3Ni 12.5Nb 5.0Al 10.0Y 0.5) was investigated and compared with a Hf-free alloy (Zr 56.7Cu 15.3Ni 12.5Nb 5.0Al 10.0Y 0.5) to understand the role of hafnium. Although the overall behavior of both alloys has similarities both in the as-received and heat-treated states, details of the physical, thermal, and mechanical properties are influenced by the incorporation of hafnium. Differential scanning calorimetry (DSC) results for the hafnium-containing alloy showed a glass transition at 440 °C and three exothermic peaks at 511, 559, 664 °C which correspond to primary and secondary crystallization. The increase in glass transition and the first crystallization temperature, compared to the Hf-free alloy, indicates stabilization of the amorphous state. Samples were heat-treated at various temperatures to examine the crystallization of various intermetallic phases. The phases were identified using X-ray diffraction (XRD) and the sizes varied between 100 and 200 nm, as observed by field emission scanning electron microscopy (FESEM), for samples heat-treated at 700 and 800 °C. The Vickers microhardness increased with heat-treatment from 5.2 to 7.2 GPa. Quasi-static compression tests showed decrease in failure stresses from 1843 MPa (as-received) to 994 MPa (heat-treated at 535 °C). The failure stress increased to 1661 MPa for the sample heat-treated at 800 °C, similar to the Hf-free alloy. Fracture morphologies changed from typical vein-type in as-received samples to smooth facets in heat-treated samples indicating embrittlement. Failure mechanism changes from shear banding in the as-received amorphous samples to void formation and coalescence in the heat-treated samples.

Transverse mechanical behaviour and moisture absorption of waterlogged archaeological wood from the Vasa ship

Abstract Damage on the hull of the 17th century Swedish warship Vasa has been observed recently. Damage in the form of indentations in the wood is caused by high compressive loads from the support structure. In the process of developing an improved support structure, radial mechanical properties and the deformation mechanisms of Vasa oak are particularly important. Causes of differences in PEG content and oak degradation are also of interest. The radial modulus and compressive strength of Vasa oak are 50% lower than for recent oak. Furthermore, a significant change in failure mechanism is observed. More brittle separation fracture of the rays of Vasa oak is observed compared to the continuous folds of rays in recent oak. Tangential stiffness and strength are also 30% and 50% lower, respectively. Comparably small differences in moisture absorption between PEG-extracted Vasa oak and recent oak indicate a low extent of degradation of the Vasa oak.

Analysis of ESD failure mechanism in 65nm bulk CMOS ESD NMOSFETs with ESD implant

Electrical and SEM analysis of gate-silicided (GS) ESD NMOSFETs in a 65nm bulk CMOS technology show that the failure mechanism changes from source-to-drain filamentation to drain-to-substrate short when a p-type ESD implant (ED) is used. Simulations show that the reason for change in failure mode is the different current and temperature distribution when the device is operated in bipolar mode due to the presence of ED. The size of the drain silicide blocking can be reduced from 3 to 0.75 μm by the use of ED while keeeping the same ESD failure current with the corresponding area saving benefit. When the ED implant extends under the drain contacts, the on-resistance (Ron) of the device can be reduced by ∼50% with respect to a design where ED is not located under the contacts.

Fracture Behaviours of Nano-Silica Modified Epoxies at Elevated Temperatures

An experimental investigation was conducted to characterize the mechanical properties of nano-silica modified epoxies at room temperature and elevated temperatures. A commercial nano-silica dispersed epoxy product was used as modifiers to toughen a DGEBA epoxy resin. Fracture toughness and other material properties of the cured epoxies were measured using the standard compact tension (CT), tensile and flexural test methods to investigate the effects of the nano-silica on fracture behaviours of the cured epoxies at 23oC, 50oC and 70oC. It has been found that the fracture toughness of the nano-silica modified epoxies have been significantly increased at 23oC and 50oC, while the strength and modulus of the materials remain unchanged or slightly higher. The increases in KIC at room temperature and 50oC are up to 50% and 70% respectively, compared to the unmodified epoxies. At 70oC, the effect of nano-silica on the fracture behaviour diminished because of the change of failure mechanisms from the brittle fracture to the plastic deformation. It is also very encouraging that the nano-silica modified epoxies did not show reduction in glass transition temperature (Tg).

Effect of Cu Migration in a Field Induced Dielectric Failure

AbstractFor the study of dielectric failures by Cu migration, TDDB (time dependent dielectric breakdown) and 1-D FDM simulation was carried out. We tested TDDB using a simple MIS structure with no barrier Cu electrode. From our TDDB results, the TTF's in the acceleration condition and the characteristic parameter of TDDB were obtained. In the simulation parts, 1-D FDM simulation was accomplished considering space charge effect due to Cu ions.The objective of TDDB is to predict of TTF (time to failure) in the service condition form the results of an accelerating condition. The characteristic of TTF's follows E model in the accelerating condition, in the service condition, the deviation from E model was observed. This different characteristic of TTF can be explained by the mechanism of Cu migration enhanced by an applied E field. Our simulation and TDDB results reveal that the deviation from E model does not mean the change of failure mechanism, but it shows the characteristics of Cu migration.

Devulcanization of Natural Rubber Vulcanizates by Mechanochemical Process

Using a suitable disulfide-based devulcanizing agent, which cleaved the sulfur cross-links in vulcanized rubber at high temperature, devulcanization of gum natural rubber was carried out. High sulfur and medium sulfur, as well as low sulfur–containing rubber vulcanizates were used to study the cleavage of sulfidic bonds. The cure characteristics and mechanical properties of vulcanized natural rubber and revulcanized natural rubber were studied. Thermal properties of the rubber were analyzed by thermogravimetric analysis (TGA), which indicates that the onset degradation temperature further increased on revulcanization with higher amount of disulfide. The properties of the revulcanized natural rubber increased with increasing disulfide concentration, also the mechanical properties of the devulcanized natural rubber were increased by decreasing the sulfur content in the original rubber vulcanizate. From the rheometric study increases in optimum cure time were observed when ground rubber vulcanizates were treated with higher amounts of disulfide. The scanning electronic microscopy (SEM) study suggested the change in failure mechanism as influenced by the type of cross-linking present and the devulcanizing agent used. From infrared (IR) spectroscopy it was observed that the oxidation of the main polymeric chain did not occur at the time of high temperature milling.

Experimental determination of cohesive failure properties of a photodegradable copolymer

In this paper we present a methodology to measure the material traction-separation relation for a poly(ethylene carbon monoxide) copolymer, ECO. This material exhibits a ductile-to-brittle transition when subjected to ultraviolet irradiation and undergoes a change of failure mechanism, from shear yielding to crazing, in the process. Single edge notched tension specimens of ECO irradiated for 50 h were used to generate slow-speed stable crack growth, predominantly from material crazing. Full-field measurements of the in-plane deformation around the growing crack tip were performed using the optical technique of digital image correlation. A multicamera setup was used in which simultaneous measurement of both the far-field displacement and that directly surrounding the craze was performed. The far-field results were used to obtain a value of the energy release rate supplied to the crack tip region. The near-tip results were used to extract a material traction-separation law in the regime of steady-state crack growth. A softening traction-separation relation was measured. The area under the traction-separation curve was then compared with the simultaneous far-field measurements and the agreement was very good (within 6.5%) validating the experimental methodology used.

Epoxy+liquid crystalline epoxy coreacted networks: II. Mechanical properties

The effect of coreacting diglycidyl ether of bisphenol F (DGEBP-F) with diglycidyl ether of 4,4′-dihydroxybiphenol (DGE-DHBP) on mechanical properties is investigated. DGE-DHBP shows a liquid crystal (LC) transition upon curing. Tensile, impact and fracture toughness test results are evaluated. Dynamic mechanical analysis is conducted to determine the effect of the DGE-DHBP component. Scanning electron microscopy of fracture surfaces shows changes in failure mechanisms compared to the pure components. The results indicate that the mechanical properties of these blended samples are significantly improved at 10–20% by weight of DGE-DHBP. This is a consequence of the rigidity of the LC component which thus provides the reinforcement.

Influence of phase microstructure on the mechanical properties of ternary phase polypropylene composites

The mechanical properties and phase microstructure of ternary phase polypropylene composites have been investigated using combinations of rigid glass beads and ethylene/propylene rubber (EPR) modifiers. Particular consideration has been given to the relative interaction between these components and their dispersion within the composite. In this regard, EPR and untreated glass beads existed as independent phases, whereas EPR functionalized with maleic anhydride showed a tendency to encapsulate the inorganic phase. As a consequence, marked differences in mechanical properties, in particular toughness, were exhibited by these systems, manifested by changes in failure mechanism. Structure in these composites was explored by Fourier transform infrared spectroscopy, dynamic mechanical analysis, and scanning electron microscopy. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 587–597, 1998

Low temperature mixed mode fracture of a pressure vessel steel subject to prior loading

The influence of prior loading on the low temperature mixed mode I/II fracture behaviour of BS1501-224 Gr490B steel has been investigated. A prior mode I proof load at room temperature resulted in an increase in the mode I low temperature fracture toughness and load bearing capacity. This benefit reduced with increasing shear (mode II) until pure mode II loading where there was no change in toughness. A prior mode II proof load at room temperature resulted in a decrease in low temperature fracture toughness and load bearing capacity. With increasing shear this reduction was not so great. For pure mode II loading there was no change in toughness or load bearing capacity at low temperature. Fractographic studies revealed that there was a change in failure mechanism from cleavage to ductile tearing with increasing mode II loading.