Pull-out Mode Research Articles

Weibull Statistics of Tensile-Shear Strength of 5083 Aluminum Alloy after Friction Stir Spot Welding

Friction stir spot welding has been adopted to replace resistance spot welding for Al alloys. This study investigates the effects of plunge depth, rotation speed, and welding duration on the microstructural characteristics of the weld, bonding strength and reliability of 5083 Al alloy. Bonding strength is evaluated using a tensile-shear test and the reliability is analyzed using a Weibull model. According to the experimental results, the major difference in the microstructure was the range of the stirring zone (SZ). The range of the SZ and the tensile-shear failure loading (TSFL) were enhanced when the welding duration or plunge depth was increased. The optimal rotation speed changed with plunge depth. Two fracture modes (tearing mode and cup pull-out mode) appeared for all parameter combinations. When the range of the SZ was deep and broad, the cup pull-out mode was the main fracture mechanism. The TSFL of the cup pull-out mode was higher than that of the tearing mode. Furthermore, the Weibull parameters of the cup pull-out mode were higher and the distribution curve shapes were better. Higher plunge depth, longer welding duration, and a suitable rotation speed made the fracture mode of the tensile-shear test be the cup pull-out mode and led to high bonding strength and good reliability. [doi:10.2320/matertrans.M2014281]

Joining of Dissimilar alloy Sheets (Al 6063&AISI 304) during Resistance Spot Welding Process: A Feasibility Study for Automotive industry

Present design trends in automotive manufacture have shifted emphasis to alternative lightweight materials in order to achieve higher fuel efficiency and to bring down vehicle emission. Although some other joining techniques are more and more being used, spot welding still remains the primary joining method in automobile manufacturing so far. Spot welds for automotive applications should have a sufficiently large diameter, so that nugget pullout mode is the dominant failure mode. Interfacial mode is unacceptable due to its low load carrying and energy absorption capability. Strength tests with different static loading were performed in, to reveal the failure mechanisms for the lap-shear geometry and the cross-tension geometry. Based on the literature survey performed, venture into this work was amply motivated by the fact that a little research work has been conducted to joining of dissimilar materials like non ferrous to ferrous. Most of the research works concentrated on joining of different materials like steel to steel or aluminium alloy to aluminium alloy by resistance spot welding. In this work, an experimental study on the resistance spot weldability of aluminium alloy (Al 6063) and austenitic stainless steel (AISI304) sheets, which are lap joined by using a pedestal type resistance spot welding machine. Welding was conducted using a 45-deg truncated cone copper electrode with 10-mm face diameter. The weld nugget diameter, force estimation under lap shear test and T – peel test were investigated using digital type tensometer attached with capacitive displacement transducer (Mikrotech, Bangalore, Model: METM2000ER1). The results shows that joining of Al 6063 and AISI 304 thin sheets by RSW method are feasible for automotive structural joints where the loads are below 1000N act on them, it is observed that by increasing the spots per unit length, then the joint with standing strength to oppose failure is also increased linearly incase of interfacial failure mode and non linear for pullout failure mode.

Bioinspired design and assembly of layered double hydroxide/poly(vinyl alcohol) film with high mechanical performance.

Inspired by the hierarchical structure and excellent mechanical performance of nacre, LDH nanosheets with an appropriate aspect ratio to withstand significant loads and at the same time allow for rupture under the pull-out mode were synthesized as artificial building blocks for the fabrication of nacre-like films. Multilayered PVA/LDH films with a high tensile strength and ductility were prepared for the first time by bottom-up layer-by-layer assembly of pretreated LDH nanosheets and spin-coating of PVA. The weight fraction of inorganic LDH platelets in the hybrid PVA/LDH films (wp) was controlled by changing the concentration of PVA solution applied in the spin-coating process. The resulting films revealed that the PVA/LDH hybrid films were piled close together to form a well-defined stratified structure resembling the brick-and-mortar structure of natural nacre. In the hybrid films, the content of inorganic LDH platelets was comparable to the value in nacre, up to 96.9 wt %. It could be clearly seen that the mechanical performance of the as-prepared PVA/LDH films was greatly improved by increasing the rigid building-block LDHs. The tensile strength of the 2 wt % PVA/LDH hybrid film reached a value of 169.36 MPa, thus exceeding the strength of natural nacre and reaching 4 times that of a pure PVA film. Meanwhile, its elastic modulus was comparable to that of lamellar bone.

Bond Performance of Basalt Fiber-Reinforced Polymer Bars to Concrete

This paper presents the test results of a study on the bond behavior of basalt fiber-reinforced polymer (BFRP) bars to concrete. Thirty six concrete cylinders reinforced with BFRP bars and twelve cylinders reinforced with glass fiber-reinforced polymer (GFRP) bars were tested in direct pullout conditions. Test parameters included the FRP material (basalt and glass), the bar diameter, and the bar embedment length in concrete. Bond-slip curves of BFRP and GFRP bars revealed similar trends. BFRP bars developed average bond strength 75% of that of GFRP bars. All BFRP specimens failed in a pullout mode of failure along the interfacial surface between the outer layer of the bar and the subsequent core layers. The influence of various parameters on the overall bond performance of BFRP bars is analyzed and discussed. The well-known BPE and modified-BPE analytical models were calibrated to describe the bond-slip relationships of the bars. Test results demonstrate the promise of using the BFRP bars as an alternative to the GFRP bars in reinforcing concrete elements.

Similar and dissimilar resistance spot welding of advanced high strength steels: welding and heat treatment procedures, structure and mechanical properties

Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel and a 980 MPa transformation induced plasticity (TRIP) steel were resistance spot welded under different welding and heat treatment parameters. The microstructure and mechanical properties of spot welds were evaluated using metallographic technique, microhardness and tensile shear tests. The results showed that the tendency to fail in the pullout mode increased in the order of DP/DP, TRIP/TRIP and DP/TRIP welds, which was caused by the different hardness distribution, carbon equivalent and susceptibility to shrinkage void formation of spot welds for different combinations. In the study of the effects of heat treatments on the DP/TRIP welds, the pre-heating procedure improved the splash of welding to some extent. When the cooling time was larger than or equal to 1000 ms, the post-heating procedure improved the mechanical properties of spot welds owing to the temper of spot weld microstructure.

Deformation behavior of high performance fiber reinforced cementitious composite prepared with asphalt emulsion

A novel engineered cementitious composite (ECC) was prepared with the complex binder of Portland cement and asphalt emulsion. By adjusting the amount of asphalt emulsion, different mixture proportions were adopted in experiments, including four-point bending test, compressive test, and scanning electric microscopy (SEM). The SEM observation was conducted to evaluate the contribution of polyvinyl alcohol (PVA) fiber and asphalt emulsion to the composite toughening mechanism. The tests results show that the most remarkable deflection-hardening behavior and saturated multiple cracking are achieved when the content of asphalt emulsion is 10%. However, excessive content of asphalt emulsion causes severe damages on the deformation behavior as well as loss in compressive strength of the mixture. SEM observation indicates that the influence of asphalt emulsion on the fiber/matrix interfacial property changes the dominant fiber failure type from rupture into pull-out mode, and thus causes beneficial effects for strain-hardening behavior.

Dissimilar ultrasonic spot welding of Mg-Al and Mg-high strength low alloy steel

Sound dissimilar lap joints were achieved via ultrasonic spot welding (USW), which is a solid-state joining technique. The addition of Sn interlayer during USW effectively blocked the formation of brittle al12Mg17 intermetallic compound in the Mg-Al dissimilar joints without interlayer, and led to the presence of a distinctive composite-like Sn and Mg2Sn eutectic structure in both Mg-Al and Mg-high strength low alloy (HSLA) steel joints. The lap shear strength of both types of dissimilar joints with a Sn interlayer was significantly higher than that of the corresponding dissimilar joints without interlayer. Failure during the tensile lap shear tests occurred mainly in the mode of cohesive failure in the Mg-Al dissimilar joints and in the mode of partial cohesive failure and partial nugget pull-out in the Mg-HSLA steel dissimilar joints.

Characterization of ultrasonic spot welded joints of Mg-to-galvanized and ungalvanized steel with a tin interlayer

Ultrasonic spot welded (USWed) Mg-to-bare steel, Mg-to-galvanized steel and Mg-to-bare steel with Sn interlayer (placed in-between Mg and bare steel) were studied. Weak joining occurred in the USWed Mg-to-bare steel, since Mg and Fe do not react with each other. The intermetallic compounds (IMCs) of Mg7Zn3 and Mg2Zn11, which led to the failure of the joint, were largely present in the USWed Mg-to-galvanized steel joint. The introduction of a Sn interlayer in the USWed Mg-to-bare steel actively worked as an intermediate medium to join Mg to Fe, and led to the presence of a distinctive composite-like Sn and Mg2Sn eutectic structure. The lap shear strength of Mg-to-bare steel with Sn interlayer joint was significantly higher than that of the Mg-to-bare steel and Mg-to-galvanized steel joints. Failure during the tensile lap shear tests occurred mainly in the partial nugget pull-out mode in the dissimilar joints of Mg-to-bare steel with Sn interlayer. All the joints of Mg-to-galvanized steel failed from the interface (cohesive failure). The addition of Sn interlayer resulted in energy saving since the welding energy required to achieve the maximum strength decreased from 1750 to 1500J in the Mg-to-steel joints.

On the Failure Behavior of Highly Cold Worked Low Carbon Steel Resistance Spot Welds

Highly cold worked (HCW) low carbon steel sheets with cellular structure in the range of 200 to 300 nm are fabricated via constrained groove pressing process. Joining of the sheets is carried out by resistance spot welding process at different welding currents and times. Thereafter, failure behavior of these welded samples during tensile-shear test is investigated. Considered concepts include; failure load, fusion zone size, failure mode, ultimate shear stress, failure absorbed energy, and fracture surface. The results show that HCW low carbon steel spot welds have higher failure peak load with respect to the as-received one at different welding currents and times. Also, current limits for failure mode transition from interfacial to pullout or from pullout to tearing are reduced for HCW samples. Fusion zone size is the main geometrical factor which affects the failure load variations. Ultimate shear stress of spot welds is increased with decreasing the heat input and for HCW samples at a specific welding current and time, it is lower than that of the as-received ones. Before pullout mode, failure absorbed energy (FAE) for HCW low carbon steel spot welds is higher than that of the as-received one, but after failure mode transition, trend would be vice versa and FAE of the as-received spot welds is extremely higher (about 3 times). In addition, spot welds fracture surface (in interfacial failure mode) for HCW sample is more dimpled which indicates higher energy absorption than that of the as-received one.

Effect of boron content and welding current on the mechanical properties of electrical resistance spot welds in complex-phase steels

When complex phase steel where tensile strength is more than 1GPa grade is joined by resistance spot welding (RSW) optimum boron (B) content should be chosen to satisfy weldability and mechanical properties. Therefore, in this study, the effect of the B content (0–40ppm) on the tensile-shear strength of the RSW were investigated. As the resistivity of the base metal was independent on the B content it did not affect to nugget diameter. Regardless of the B content the specimens under 5t1/2 (t=sheet thickness) were fractured at interfacial failure mode. In the low welding current condition (lower than 6.4kA), measured nugget diameters were smaller than calculated critical nugget diameter regardless of the amount of B addition so that fracture mode was interfacial failure. Pull out failure occurred at the softened zone which was boundary between the base metal and the heat affected zone. Tensile-shear load of the specimen failure at the pull-out mode was increased as the fractured diameter and hardness of the softened zone were increased. Shear load was only dependent on the fractured diameter. The equations to calculate the shear and tensile-shear load were suggested for the specimens fractured at interfacial and pull-out failure modes respectively. Correlation coefficients between measured and calculated values of shear and tensile-shear load were 0.98 and 0.97, respectively. Therefore, shear and tensile-shear load of advanced high strength steel joined by RSW could be predicted successfully using the suggested equation.

A finite–discrete element framework for the 3D modeling of geogrid–soil interaction under pullout loading conditions

The behavior of a geogrid–soil system under pullout mode is known to depend on the properties of the geogrid material, the backfill soil and the interface condition. Modeling the geogrid–soil interaction taking into account the true geogrid geometry is a challenging numerical problem that requires the consideration of the discontinuous nature of the soil and the different modes of resistance that contribute to the pullout capacity of the geogrid layer. In this study, a coupled finite–discrete framework has been developed to investigate the behavior of a biaxial geogrid sheet embedded in granular material and subjected to pullout loading. Validation is performed by comparing experimental data and numerically calculated results using the proposed model. The detailed behavior of the geogrid and the surrounding soil is then investigated. The numerical results indicated the suitability of the coupled model to solve this class of problems.

Metallurgy and mechanical performance of AZ31 magnesium alloy friction spot welds

Microstructural features were studied along the cross-section of AZ31 magnesium alloy friction spot welded joints made using different combinations of welding parameters. Static lap shear testing was performed to evaluate the mechanical properties of the welded joints, and the resulting fracture mechanisms and crack propagation paths were fully examined. Failure load is optimized when the welding procedure is performed with the combination of parameters that maximizes the material mixing, the size of fully metallurgical bonding and simultaneously minimizes the vertical displacement of hook region. The welds demonstrate three failure modes during lap shear testing: through the weld and non-circumferential pull-out modes, in which crack propagation crosses the recrystallized zone, and circumferential pull-out mode, around this zone.

Bond and splitting behaviour of reinforced concrete confined by steel jackets without grouting

This study conducts a bond test of reinforced concrete confined by a new steel jacketing method without grouting, which was suggested previously. The steel jacket does not show composite behaviour with concrete because it has no grouting. The aims of this study are to determine the performance of the steel jacketing method in improving bond behaviour and to assess splitting stress developed during bond tests. For the purpose, this study uses concrete cylinders with the dimension of 100 mm × 200 mm and 150 mm × 300 mm (D × L), and D22 reinforcing bars (nominal diameter of 22·2 mm) are embedded in concrete. The specimens of 100 mm × 200 mm are designed to show splitting failure under unconfined conditions. On the contrary, the specimens of 150 mm × 300 mm are expected to produce pull-out failure under unconfined conditions. This study analyses the relation of circumferential strain to bond stress and suggests a model of splitting stress–strain for the confined concrete. The new steel jacketing method transfers the bond failure from splitting to pull-out mode and increases bond strength and toughness satisfactorily. This study suggests the ratio of bond stress to splitting stress as a function of slip that can be used to develop an analytical model of bond behaviour. In general, the bond strength increases with the confinement of the steel jackets; however, too heavy a confinement is not effective in continuously increasing the bond strength. In contrast to the bond strength, more confinement effectively decreases circumferential strain. The bond stress–circumferential strain curves generally show hook-shaped behaviour for pull-out failure mode for unconfined or confined specimens.

An approach in prediction of failure in resistance spot welded aluminum 6061-T6 under quasi-static tensile test

The aim of this article is to predict the failure load in resistance spot welded aluminum 6061-T6 sheets with 2 mm thickness under quasi-static tensile test. Various welding parameters, e.g. welding current, welding time and electrode force are selected to produce welded joints with different quality. The results show that for all the samples in this study only interfacial failure mode was observed in tensile-shear test and no pull-out mode was observed. According to the failure mode, an empirical equation was used for the prediction of failure load based on nugget size and hardness of failure line. Microstructure study has been carried out to investigate microstructural changes in the welded joints. For determination of the minimum hardness, microhardness tests have been carried out to find hardness profiles. The minimum hardness value was observed for a thin layer around the nugget with large and coarse grains. The results show that by using the presented empirical equation, the failure can be predicted with a good agreement only by measuring nugget size.

Dissimilar resistance spot welding of DP600 dual phase and AISI 1008 low carbon steels: correlation between weld microstructure and mechanical properties

The aim of this work is to investigate and analyse the microstructure and mechanical properties of dissimilar low carbon steel/dual phase steel (DP600) resistance spot welds. The failure modes of spot welds during the tensile–shear test were detailed by examination of the weld fracture surfaces. Relationships between the fracture path and the mechanical properties (peak load and energy absorption) were developed using the observed microstructures in the fusion and heat affected zones. It was found that the failure of DP600/low carbon steel is initiated from the stronger side (i.e. DP600 side). This was explained in terms of hardness profile, difference in tensile strength and workhardening behaviour of the base metals. A transition in the failure mode from interfacial failure mode to pullout failure mode was observed with increasing the fusion zone size caused by increasing the welding current. However, when expulsion occurred, the spot welds failed in the partial thickness–partial pullout mode, with reduced energy absorption and peak load, compared to those spot welds with the same or smaller nugget size, which failed in the pullout mode. This can be related to the low ductility of the location of the failure initiation (i.e. heat affected zone of DP600 side) in this mode.

Reliability Based Earthquake Resistant Design for Internal Stability of Reinforced Soil Structures

This paper presents a method to evaluate reliability for internal stability of reinforced soil structures using reliability based design optimization. Using limit equilibrium method and assuming the failure surface to be logarithmic spiral, analysis is conducted to maintain internal stability against both tensile and pullout failure of the reinforcements. Properties of backfill soil and strength of the geosynthetic reinforcement are considered as random variables. For the seismic conditions, reliability indices of all the geosynthetic layers in relation to tension and pullout failure modes are determined for different magnitudes of seismic accelerations both in the horizontal and vertical directions, surcharge load and design strength of the reinforcement. The efforts have been made to obtain the number of layers, pullout length and total length of the reinforcement at each level for the desired target reliability index values against tension and pullout modes of failure. The influence of horizontal and vertical earthquake acceleration, surcharge load, design strength of the reinforcement, coefficient of variation of soil friction angle and design strength of the reinforcement on number of layers, pullout length and total length of the reinforcement needed for the stability at each level is discussed.

Impact Tension Damage Mechanism Analyses of Co-Woven-Knitted Composite from Hilbert–Huang Transform

The impact tensile behaviors of co-woven-knitted (CWK) composites were tested with split Hopkinson tension bar apparatus along 0°, 45°, and 90° directions under the strain rates from 1589 to 2586 s−1 and compared with that in quasi-static strain rate of 0.001 s−1. The impact tension damage mechanisms were analyzed in frequency domain with the Hibert–Huang transform (HHT) method. Specifically, the stress time histories curves of the CWK composite were transformed to find the frequency distributions for the different tension failure modes. It was found that thefailure mode of fiber breakages, fiber pull-out, resin cracks, and resin shear failurewere located at a specific frequency band. And also, the different failure modes under high strain rate loading are all located at the high frequency range. With such kind of HHT analyses, the damage mechanisms could be explored more precisely when combined with the observation results of the fractographs of the CWK composite under various strain rates.

Monotonic and cyclic bond behavior of confined concrete using NiTiNb SMAwires

This study conducts bond tests of reinforced concrete confined by shape memoryalloy (SMA) wires which provide active and passive confinement of concrete.This study uses NiTiNb SMA which usually shows wide temperature hysteresis;this is a good advantage for the application of shape memory effects. The aimsof this study are to investigate the behavior of SMA wire under residual stressand the performance of SMA wire jackets in improving bond behavior throughmonotonic-loading tests. This study also conducts cyclic bond tests and analyzescyclic bond behavior. The use of SMA wire jackets transfers the bond failurefrom splitting to pull-out mode and satisfactorily increases bond strength andductile behavior. The active confinement provided by the SMA plays a major rolein providing external pressure on the concrete because the developed passiveconfinement is much smaller than the active confinement. For cyclic behavior,slip and circumferential strain are recovered more with larger bond stress. Thisrecovery of slip and circumferential strain are mainly due to the external pressureof the SMA wires since cracked concrete cannot provide any elastic recovery.

Structure-Property Relationship of Burn Collagen Reinforcing Musculo-Skeletal Tissues

An investigation has been conducted on burn ligaments, addressing the specific conditions in burn arising from dehydration and heating, and how these affect the structure-property relationship of collagen for reinforcing the tissue. Collagen fibres were isolated from a sheep’s anterior cruciate ligament, i.e. our model for this study, and divided into six groups. The first group was designated as control; the second (D) group was dehydrated without exposure to elevated temperature. The remaining (DH) groups were dehydrated followed by heating at 120oC for 30 minutes, 2, 4 and 24 hours, respectively. Tensile test to rupture was carried out to derive the fibre modulus of elasticity (E), maximum stress (σ), strain at maximum stress (ε) and strain energy density to maximum stress (u). Electron micrographs of the ruptured ends reveal a mixed mode of fibril pull-out and rupture: fibril pull-out dominates in the control group but fibril rupture dominates increasingly in the other groups, i.e. with increasing exposure time to elevated temperature. Apart from ε, there is significant increase in E, σ and u in the D and DH groups with respect to the control group but there is no evidence of variation among the D and DH groups. The results of this study implicate (1) the removal of water in the hydrated proteoglycan-rich matrix, leading to shrinkages at micrometer length-scale during dehydration, and (2) the alteration of the collagen organisation arising from the underlying changes in the crystallinity and denaturation during heating, on the mechanical properties of burn collagen fibres.

Mechanical characterization of friction stir spot microwelds

Microstructures and failure mechanisms of friction stir spot microwelds in 300-μm thin sheet of aluminum 1050 alloy were investigated. As an alternative to conventional soldering and welding in joining thin metals for electronic, medical and microdevices, friction stir welding may be utilized in order to limit the excessive heat damage. Transmission electron microscopy micrographs of the cross sections of friction stir spot microwelds in lap-shear specimens were examined. These microwelds showed the failure mode of nugget pullout under lap-shear loading conditions. The experimental observations suggested that under lap-shear loading conditions, the failure was initiated near the possible original notch tip in the stir zone and the failure propagated along the circumference of the nugget to final fracture. Microindentation hardness data of base metal, heat affected zone, thermal-mechanical affected zone and stir zone were obtained. The interface between the heat affected zone and the thermal-mechanical affected zone was the softest region, where the cracks of friction stir spot microwelds in the lap-shear specimens under the loadings initiated and leaded to fracture of the specimens.