Ductile Failure Mechanisms Research Articles

The tensile deformation and fracture behavior of a magnesium alloy nanocomposite reinforced with nickel

In this paper the intrinsic influence of micron-sized nickel particle reinforcements on microstructure, micro-hardness tensile properties and tensile fracture behavior of nano-alumina particle reinforced magnesium alloy AZ31 composite is presented and discussed. The unreinforced magnesium alloy (AZ31) and the reinforced nanocomposite counterpart (AZ31/1.5 vol.% <TEX>$Al_2O_3$</TEX>/1.5 vol.% Ni] were manufactured by solidification processing followed by hot extrusion. The elastic modulus and yield strength of the nickel particle-reinforced magnesium alloy nano-composite was higher than both the unreinforced magnesium alloy and the unreinforced magnesium alloy nanocomposite (AZ31/1.5 vol.% <TEX>$Al_2O_3$</TEX>). The ultimate tensile strength of the nickel particle reinforced composite was noticeably lower than both the unreinforced nano-composite and the monolithic alloy (AZ31). The ductility, quantified by elongation-to-failure, of the reinforced nanocomposite was noticeably higher than both the unreinforced nano-composite and the monolithic alloy. Tensile fracture behavior of this novel material was essentially normal to the far-field stress axis and revealed microscopic features reminiscent of the occurrence of locally ductile failure mechanisms at the fine microscopic level.

Mechanisms governing cyclic fracture behavior of two high-strength steels: role of composition and microstructure

In this article, the cyclic fatigue fracture behavior of two high-strength specialty steels denoted as Tenax 310 and 300 M, is presented and discussed. The two chosen specialty steels have noticeably improved properties to offer than most steels in this category and even other competing high-strength steels. The observed improvement can be ascribed to be because of the synergistic and mutually interactive influences of chemical composition and secondary processing technique used for the two steels. The two chosen high-strength steels, designated by the manufacturer as Tenax 310 and 300 M, were produced by initial melting using vacuum arc remelting and then casting to obtain ingots. The cast ingots were mechanically deformed by hot working to get the starting blocks. At the fine microscopic level, cyclic fatigue fracture of these two steels revealed features reminiscent of the occurrence of “locally” ductile and brittle failure mechanisms. Over the range of maximum stress and at the two different load ratios, the two steels were cyclically deformed. It was observed that the cyclic fatigue resistance of the candidate steels was noticeably higher than their high-strength steels counterparts for which data are available in the literature. The key mechanisms responsible for the observed fracture behavior of the two steels, cyclically deformed over a range of maximum stress, are presented and discussed.

Ductile Failure Prediction of Spot Welded Lap Joint

A finite element (FE) model incorporating a progressive material damage with Rice-Tracey damage initiation criterion is developed in this study. The relationship between local ductility reduction and stress triaxiality was established experimentally. The FE model was validated by comparisons of load-displacement response of the spot welded lap joint specimen at displacement rate of 5 mm/min and the observed ductile failure mechanism. Results show that Rice-Tracey damage initiation criterion used is sufficient to reproduce the observed ductile failure response of the specimen. Failure of the spot welded lap joint is initiated at the HAZ/fusion zone interface with localized necking.

Effect of Contact Conditions on Void Coalescence at Low Stress Triaxiality Shearing

Recent numerical cell-model studies have revealed the ductile failure mechanism in shear to be governed by the interaction between neighboring voids, which collapse to micro-cracks and continuously rotate and elongate until coalescence occurs. Modeling this failure mechanism is by no means trivial as contact comes into play during the void collapse. In the early studies of this shear failure mechanism, Tvergaard (2009, “Behaviour of Voids in a Shear Field,” Int. J. Fract., 158, pp. 41-49) suggested a pseudo-contact algorithm, using an internal pressure inside the void to resemble frictionless contact and to avoid unphysical material overlap of the void surface. This simplification is clearly an approximation, which is improved in the present study. The objective of this paper is threefold: (i) to analyze the effect of fully accounting for contact as voids collapse to micro-cracks during intense shear deformation, (ii) to quantify the accuracy of the pseudo-contact approach used in previous studies, and (iii) to analyze the effect of including friction at the void surface with the main focus on its effect on the critical strain at coalescence. When accounting for full contact at the void surface, the deformed voids develop into shapes that closely resemble micro-cracks. It is found that the predictions using the frictionless pseudo-contact approach are in rather good agreement with corresponding simulations that fully account for frictionless contact. In particular, good agreement is found at close to zero stress triaxiality. Furthermore, it is shown that accounting for friction at the void surface strongly postpones the onset of coalescence, hence, increasing the overall material ductility. The changes in overall material behavior are here presented for a wide range of initial material and loading conditions, such as various stress triaxialities, void sizes, and friction coefficients.

APPLICATION OF DAMAGE MODEL WITH MATERIAL FLOW EVALUATION TO HIGH TEMPERATURE COMPRESSION PROCESSES

Mechanisms of ductile failure in a compression process are studied, based on micro-structural considerations, with analytical and experimental investigations performed on a cylindrical specimen. Geometrical softening is able to occur by void if the shear band is generated inside of materials. The original Gurson model is used to analyze tensile deformation, and a modified Gurson model is proposed, which incorporates damage accumulation under shear and compression. This modified Gurson model is also applied to the void closure of sintered material under compression loading. However, the compressed materials must be considered in both the void nucleation and closure, because the void is nucleated and closed at a specific angle. The results of individual analyses of nucleation and closure do not agree with observations, but the analysis considered here, of both the nucleation and closure of a void, corresponds well with experimental results.

Influence of strain rate on the seismic response of RC structures

At the strain rate levels induced by seismic excitations, dynamic properties of concrete and reinforcing steel exhibit non-negligible increases if compared with the values exhibited under quasi-static properties. This is expected to change the mechanical behavior of structural systems, under seismic loading conditions. The work here presented aims to appreciate the influence of strain rate sensitivity of concrete and reinforcing steel on the global seismic response of reinforced concrete (RC) structures. In order to assess the influence of the strain-rate, first (a) the modified material properties are used to derive moment–curvature relationships of a generic RC cross section, then (b) the seismic fragility of a 2D RC frame structure is derived accounting for seismic strain rate modified material properties, and (c) the seismic assessment of a real 3D RC frame structure at the significant damage (SD) limit state is conducted in both cases of quasi-static material properties and strain rate modified material properties. The results shows that considering updated material properties, to account for earthquake induced strain rate, a strength reserve of the structural system is experienced when only ductile failure mechanisms are accounted for; on the other hand, a reduction of the structural capacity is obtained when brittle failure mechanisms are included.

Grain boundary characteristics and tensile properties of Ti14 alloy after semi-solid deformation

The microstructure and room-temperature tensile properties of Ti14, a new α+Ti2Cu alloy, were investigated after conventional forging at 950°C and semi-solid forging at 1000 and 1050°C, respectively. Results show that coarse grains and grain boundaries are obtained in the semi-solid alloys. The coarse grain boundaries are attributed to Ti2Cu phase precipitations occurred on the grain boundaries during the solidification. It is found that more Ti2Cu phase precipitates on the grain boundaries at a higher semi-solid forging temperature, which forms precipitated zones and coarsens the grain boundaries. Tensile tests exhibit high strength and low ductility for the semi-solid forged alloys, especially after forging at 1000°C. Fracture analysis reveals the evidence of ductile failure mechanisms for the conventional forged alloy and cleavage fracture mechanisms for the alloy after semi-solid forging at 1050°C.

General notes on ductility in timber structures

The paper discusses the implications of ductility in design of timber structures under static and dynamic loading including earthquakes. Timber is a material inherently brittle in bending and in tension, unless reinforced adequately. However connections between timber members can exhibit significant ductility, if designed and detailed properly to avoid splitting. Hence it is possible to construct statically indeterminate systems made of brittle timber members connected with ductile connections that behave in a ductile fashion. The brittle members, however, must be designed for the overstrength related to the strength of the ductile connections to ensure the ductile failure mechanism will take place before the failure of the brittle members. The overstrength ratio, defined as the ratio between the 95th percentile of the connection strength distribution and the analytical prediction of the characteristic connection strength, was calculated for multiple doweled connections loaded parallel to the grain based on the results of an extensive experimental programme carried out on timber splice connections with 10.65 and 11.75 mm diameter steel dowels grade 4.6. In this particular case the overstrength ratio was found to range from 1.2 to 2.1, and a value of 1.6 is recommended for ductile design. The paper illustrates the use of the elastic–perfectly plastic analysis with ductility control for a simple statically indeterminate structure and compares this approach to the fully non-linear analysis and with the more traditional linear elastic analysis. It is highlighted that plastic design should not be used for timber bridges since fatigue may lead to significant damage accumulation in the connections if plastic deformations have developed. The paper also shows that the current relative definitions of ductility, as a ratio between an ultimate deformation/displacement and the corresponding yield quantity, should be replaced by absolute definitions of ductility, for example the ultimate deformation/displacement, as the latter ones better represent the ductile structural behavior.

Elastic and ductile design of multi-storey crosslam massive wooden buildings under seismic actions

The paper discusses the seismic design of multi-storey buildings made from cross-laminated timber panels (‘crosslam’). The use of seismic analysis methods such as the modal response spectrum and the non-linear static (push-over) analysis is discussed at length, including issues such as the modelling of crosslam walls and connections, the evaluation of the connection stiffness, and the schematization of floor panels. It was found that it is crucial to account for the flexibility of the connections (hold-downs and angle brackets) between upper and lower walls, since otherwise the vibration periods of the building would be underestimated. The basics of capacity design to ensure the attainment of ductile mechanisms in crosslam timber structures under seismic actions are presented. The ductile failure mechanism is characterized by plasticization of connectors (hold-downs, angle brackets and screws) between adjacent wall panels and between panels and foundations. The crosslam panels and the connections between adjacent floor panels must be designed for the overstrength of the connectors to ensure that they remain elastic during the earthquake and the ductile failure mechanism is attained. Based on the results of preliminary quasi-static cyclic tests, a value of 1.3 was found for the overstrength factors of hold-downs and angle brackets. A case study multi-storey crosslam massive wooden building was then analysed using the non-linear push-over analysis as implemented in the N2 method recommended by the Eurocode 8. The building was modelled using shell elements and non-linear links to schematize the hold-downs and angle brackets. The building ductility, calculated from the bilinear curve equivalent to the actual non-linear push-over curve, was then investigated. Such a quantity, defined as the ratio of the displacement at the near collapse state and the maximum elastic displacement of the top floor, was found to rise from 1.7 to 2.5 when ductile instead of brittle hold-downs and angle brackets are used. Furthermore, the maximum peak ground acceleration the building can resist raised from 0.2 g to 0.4 g , demonstrating the importance of using ductile connectors in seismic design.

횡방향철근이 감소된 중공사각단면 교각의 내진거동 특성

The seismic design concept of RC bridges is to attain the proper ductility of piers, yielding a ductile failure mechanism. Therefore, seismic design force for moment is determined by introducing a response modification factor (R), and lateral reinforcements to confine core concrete are specified in the current design code. However, these design provisions have irrationality, which results in excessive amounts of lateral reinforcements for columns in Korea, which are generally designed with large sections. To improve on these provisions, a new design method based on seismic performance has been proposed. To apply this to hollow sectional columns, however, further investigations and improvements must be performed, due to the different seismic behaviors and confinement effects. In this study, hollow sectional columns with different lap-splice of longitudinal bars and lateral reinforcements have been tested. Seismic characteristics and performance were investigated quantitatively. These research results can be used to derive a performance-based design for hollow sectional columns.

Internal ductile failure mechanisms in steel cold heading process

The occurrence of internal ductile failure in cold-headed products presents a major obstacle in the fast expanding cold heading (CH) industry. This internal failure may lead to catastrophic brittle fracture under tensile loads despite the ductile nature of the material. Comprehensive testing and investigation methodologies were used to this work to reveal the complicated interplay of process and material parameters contributing in the initiation and propagation of internal ductile failure in six CH quality AISI steel grades. The metallurgical and microscopic investigations showed that internal ductile failure occurs progressively by void nucleation and growth mechanisms with increasing plastic strain inside the highly localized adiabatic shear bands (ASBs). The void nucleation occurs by decohesion at second-phase particles, inclusion–matrix interfaces, grain boundaries and by particle or inclusion cracking. Therefore, the number and morphology of any inclusions and second-phase particles are key factors in material formability. The metallurgical investigations showed that under compressive loading conditions, the nature of the metal flow pattern promotes different rates of material flow around the inclusions and stringers which supports decohesion and void nucleation since the early stages of deformation. At advanced stages of deformation, the metal flow pattern contributes to the ASB localization in supporting void growth and coalescence along the band leading to narrow void sheets. All tested materials in this work experienced ductile failure by void nucleation and coalescence, forming cracks along the ASBs. The ductile failure of each material was the result of the contribution of all the mechanisms of void nucleation at the inclusion–matrix interface, second phase–matrix interface and at the grain boundaries. However, the level of contribution of each mechanism in the final ductile failure varied depending on material properties and their microstructure.

The interactions between shear bands and rigid lamellar inclusions in a ductile metal matrix

A ductile metal matrix (modelled as a nonlinear elastic material) containing a dilute suspension of an iso-oriented lamellar stiff phase (modelled as stiffeners, i.e. zero thickness, rigid inclusions) is subject to a simple shear of finite amount, parallel to the inclusion orientation, and subsequently perturbed through an incremental Mode I loading, uniform at infinity. Solution to this problem permits analytical investigations of the emergence of shear bands and their interaction with a rigid inclusion (involving a stress square-root singularity at its tip) and discloses the mechanisms of ductile failure in reinforced materials (explaining for instance the experimental evidence that shear bands tend to nucleate and grow parallel to thin hard inclusions). Finally, investigated beyond the elliptic range, the obtained solution becomes non-unique and reveals non-decay and singularity of the fields, facts that provide analytical justification for the difficulties associated with numerical treatment of shear bands.

The quasi-static deformation and final fracture behavior of aluminum alloy 2219

In this paper is presented the results of an experimental study aimed at understanding the tensile deformation and fracture behavior of aluminum alloy 2219. Uniaxial tensile tests results reveal the alloy to have acceptable strength and ductility (elongation-to-failure) for both the anodized and non-anodized conditions. The ductility, quantified in terms of reduction in area, of the anodized sheet is marginally superior to the ductility of the non-anodized sheet. No drastic change in tensile fracture mode was evident as a function of anodized condition of the sheet. On a microscopic scale, tensile fracture surfaces of the alloy revealed features reminiscent of locally ductile and brittle failure mechanisms. The fracture behavior of the alloy is discussed in light of intrinsic microstructural features; deformation characteristics of the alloy, local stress state and grain boundary failure.

Micromechanical Modeling of the Static Loading of an Al 359-SiC MMC

The objective of this work was to use micromechanical finite element models to simulate the static mechanical behavior of a metal matrix composite: a cast Al 359 alloy reinforced with 20% SiC particles, at two different temperatures: room temperature and 150°C. In the simulations, periodic unit cell models incorporating the explicit representation of the matrix, reinforcing particles and precipitated primary silicon crystals in both 2D and 3D were used. Micromechanical models with both idealized and realistic reinforcing particle geometries and distributions were generated. The realistic particle geometries and distributions were inferred from experimental SEM micrographs. The pattern and intensity of the plastic deformation within the matrix was studied and the macroscale behavior of the composite was inferred from average stress and strain values. In order to include the effects of residual stresses due to the processing of the material, a quenching simulation was performed, prior to mechanical loading, and its effects on the macroscopic and microscopic behavior of the MMC was assessed. The effects of introducing the damage mechanisms of ductile void growth and brittle failure of the reinforcing particles was also investigated. The results of the simulations were compared with experimental results for the MMC in terms of macroscopic tensile stress–strain curves and conclusions were drawn.

Microstructural evolution of AS21X HPDC alloy during thermal treatment

The microstructure of a High Pressure Die Cast Magnesium (HPDC) AS21X alloy was investigated after various heat treatments. The material, supplied in the as cast state, consisted of Mg-α grains separated by intermetallic particles such as Mg17Al12, Mg2Si and Al–Mn. The alloy was subjected to solution treatment at 415° C for times ranging from 0.5 to 48 h and to aging to assess grain growth stability and precipitation hardening. Light microscopy showed that Mg-α grains increase slightly in size whereas intermetallic particles do not disappear but assume a more rounded shape. Static precipitation and/or dissolution were followed by electrical conductivity, hardness measurements and X-ray diffractometry. Tensile properties at room temperature were evaluated on both the as cast and solution treated samples. Density was used as an indicator of porosity to explain the scatter in elongation to fracture data. Study of the fracture surfaces revealed the morphology of porosity and the otherwise ductile fracture failure mechanism.

Factors influencing the extent of hydrogen-enhanced brittle cracking in a Cu-strengthened HSLA steel during monotonic loading

The extent of brittle crack as a function of hydrogen charging conditions was studied for a HSLA steel using circumferentially notched cylindrical tensile samples. Two different notch depths were used. The effect of hydrogen could be well represented by an effective hydrogen potential which was defined using a representative hydrogen concentration and a diffusive time parameter, for relatively faster strain rates. The high triaxiality in deep-notched samples led to the initiation of ductile failure mechanisms overwhelming the brittle cracking process.

일반도로교의 내진해석모델 개발

일반설계에서 요구하는 구조물의 안전성은 탄성영역에서 일정수준의 안전계수를 확보하여 만족된다. 그러나 내진설계에서의 안전성은 소성영역에서 구조물의 형상에 따라 특정한 연성파괴메카니즘을 유도하여 확보하도록 요구하고 있다. 그러므로 이러한 안전성은 구조물의 기본설계단계에서 여러 개의 대안을 가지고 비교, 검토를 수행하여 확보되어야 하며 실시설계단계에서 이를 확인하는 작업이 이루어져야 한다. 이 연구에서는 일반도로교량을 대상으로 하여 기본설계와 실시설계에 사용하는 모델을 설정하였으며 양 모델의 동적거동특성인 주기와 모드형상을 비교하고 다중모드스펙트럼해석을 적용하여 파괴메카니즘을 규명하였다. 기본설계와 실시설계에 사용하는 모델로 각각 확인한 파괴메카니즘을 비교하여 기본설계모델의 타당성을 입증하고 실무에 적용할 수 있는 내진해석모델로 제시하였다. The structural safety required in general design is to be proved with safety factors provided for structural members in elastic range. But, for the safety requirement in the earthquake resistant design, a specific ductile failure mechanism in plastic range should be verified according to the structural configuration. Therefore such verifications should be done in the preliminary design stage by comparing various design alternatives. In the main design stage only a confirmation of the ductile failure mechanism is required. In this study typical roadway bridges are selected and analysis models are presented for the preliminary and main design. For the two models, vibration periods and mode shapes are compared and the multi-mode spectrum method is applied to determine failure mechanisms. The failure mechanisms obtained with the two models are compared to check the properness of the model used for the preliminary design, which may well be used as an earthquake resistant analysis model in practice.

Crack growth predictions by cohesive zone model for ductile fracture

For crack growth by a ductile failure mechanism, the fracture process zone is represented in terms of special interface elements in the crack plane ahead of the crack. A porous ductile material model is used in the interface elements to represent the growth of voids to coalescence. In relation to the finite element mesh, the initial width of the interface is taken to be zero, but the traction separation relations represented by the interface are based on assuming an interface width of the order of the void spacing. The tangential stresses inside the interface are determined by the requirement of compatibility with the surrounding material in the tangential direction. Some comparison with predictions based on related crack growth models is used to discuss the performance of the special cohesive zone model presented here.

Development of Ductility Design Guidelines for RC Beams with FRP Reinforcing Bars

As corrosion is a common problem in steel reinforced concrete (RC), fibre reinforced plastic (FRP) reinforcement, that is corrosion resistant, has gained the attention of structural engineers. When overloaded, structural components must behave in a ductile fashion to provide adequate warning of imminent failure but due to the brittle nature of FRP materials, it has been difficult to incorporate ductility into FRP RC beams. However, previous research has revealed that placing FRP reinforcement in both the compression and the tensile zones of a beam section can result in a ductile failure mechanism, as the compressive FRP affects the ductile plateau of the moment-curvature sectional response. This paper quantifies minimum ductility requirements for the redistribution of bending moment in continuous FRP RC beams. Validation of ductility models by experimental testing allowed for the development of design rules to ensure that a beam incorporating FRP reinforcement will behave in a ductile fashion.

The influence of tensile stress states on the failure of HY-100 steel

The failure of an HY-100 steel plate has been examined as a function of stress state using notched and un-notched axisymmetric tensile specimens. The results show that increasing stress triaxiality leads to a rapid decrease in failure strains in a manner that is exponentially dependent on the degree of triaxiality. Two ductile failure mechanisms are identified: a void coalescence process, in which relatively equiaxed voids grow to impingement, and a void-sheet process, which links by a shear instability process large, elongated inclusion-initiated voids. The void-sheet mechanism intervenes and limits ductility at high-stress triaxialities in transversely oriented HY steel plate material, whereas the former process controls failure in longitudinally oriented material. These orientation effects are related to the morphology and alignment of the nonmetallic inclusion stringers that act as the primary void nucleation sites. Calcium treatments for inclusion-shape control improve ductility, especially at intermediate-stress triaxialities, primarily by suppressing the local conditions which give rise to the void-sheet instability process.