Types Of Fracture Modes Research Articles

Prediction of Residual Life of In-Service P91 Steel Joints Based on Fracture Behavior.

P91 steel and P91 steel joints experience performance degradation after serving for 30,000 h in working conditions. To clarify the damage and failure behavior and remaining life of the joints during subsequent service, further creep testing was conducted on the welded joints of P91 steel that had been in service for 30,000 h at three temperatures: 550 °C, 575 °C, and 600 °C. The fracture surface and the cross-section damage behavior were characterized by SEM and EBSD methods. The results show that there are two types of fracture modes in the joints at different temperatures: ductile cracking occurring at the BM, and type IV cracking occurring in the FGHAZ. The threshold stress for fracture mode transition decreases with an increase in working temperature. Type IV cracking near the HAZ is the main reason for the premature failure of joints during service. And based on the fracture mode, the dual-constant L-M method was proposed to predict the strength of in-service joint materials. The testing data are discussed and classified based on the fracture mode in this method, which has high accuracy and can prevent the premature failure of joints.

Study on precise weld diameter validations by comparing destructive testing methods in resistance spot welding

The torsion test is rarely used for resistance spot-welded joints since they are not subjected to torsion in applications. Normal, shear, and/or peel loads are usually the main stresses. Extensive scientific investigations in the context of Kunsmann’s dissertation date back more than 50 years. These investigations are still the basis of ISO 17653 and the German guideline DVS 2916-1. Recent scientific investigations only use torsion tests, but do not describe the reason for its use. A decisive advantage of the torsion test over the other standardized destructive testing methods lies in the types of fracture modes that occur and the properties of the fracture surfaces. Torsional loading results in either interfacial or button-pulled fracture modes. No material residues occur on the fracture surfaces for ductile and advanced high-strength steels. Hence, the measurement of weld diameter is achievable with minimal constraints, resulting in reduced variability and facilitating objective assessments of spot welds. This article delineates these attributes through a comparative analysis of various destructive testing methods employing statistical approaches. Additionally, the article expounds on the design concept of the developed rig for conducting torsion tests on spot welds.

The effect of RFSSW parameters on load capacity of EN AW-6082-T6 aluminum alloy and AlCu bimetallic joints

This paper presents research into the creation of refill friction stir spot welding (RFSSW) single-lap joints of 6082-T6 aluminum alloy and roll-bonded AlCu bimetallic sheet. The latter sheet comprises aluminum EN AW-1050A and electrolytic copper M1E in Z6 temper condition (after rolling) and with a 1:1 ratio of the Cu:Al layers. The welding parameters involved a variable plunge depth and welding time. The mechanical properties of the joints were determined in tensile/shear tests. The highest average value of the load capacity of the joint was observed in joints made with a tool plunge depth dt = 2.6 mm and total welding time tw = 4 s. The load capacity for this variant was an average of 5986.75 N with standard deviation SD = 166.15 N. A plug-type fracture on the aluminum layer of the bimetallic sheet was observed in RFSSW joints produced with a plunge depth of 2.6 mm. Increasing the tool rotational speed and thus generating heat resulted in a plug-type fracture on the upper sheet (aluminum alloy). In the case of joints produced with a plunge depth of 2 mm equal to the thickness of the upper sheet, the type of fracture mode with both welding times was found to be nugget debonding.

Relationship between cross tension strength and carbon content of lower sheet in friction element welded steel joints

High-strength steels (HSSs) have been increasingly used in car bodies to simultaneously achieve the weight reduction and high collision safety of vehicles. In resistance spot welding, which is widely used for joining car bodies, low cross tension strength (CTS) of joints using HSSs is a problem. In this study, we focused on friction element welding (FEW) to enhance the CTS. To investigate the joint strength of steel sheets jointed by FEW, a pre-hole was provided in the upper sheets, and the element was passed through the pre-hole to joint with a lower sheet, and then cross tension tests of the joints were conducted. In addition, the microstructures of the joints were observed to investigate the fracture positions. In the cross tension test, three types of fracture modes were observed. When the carbon content of the lower sheet was as low as 0.07–0.15 mass%, the joint broke at the head of the element. However, when the carbon content was increased to 0.20 mass%, the joints were fractured at the softened area of heat affected zone in the thickness direction of lower sheets. While the carbon content was further increased to 0.30 mass% (C30), the cracks propagated inside the area quenched from the two-phase temperature region (inter-critically quenched area) of the lower sheets and then fractured in the lower sheet thickness direction. Consequently, the CTS was decreased. To clarify the mechanism of the decrease in CTS, the fracture surface was observed together with the hardness difference measurement between ferrite and martensite by the nanoindentation method. As a result, it was clarified that the hardness difference between the two phases was significantly large, leading to the ductile fracture. Based on these findings, the low CTS of C30 was inferred to be caused by the poor local ductility in the inter-critically quenched area.

Plane bending fatigue behavior of interstitial-free steel at room temperature

Abstract Interstitial-free steels are widely used as thin sheet for automobile external panels. For this type of application, essential properties of the steel are deep-drawability and stretch-formability. However, low-strength interstitial-free steel is nearly pure iron and work on fatigue fracture behavior of this steel might be adopted as a basic study to understand the behavior of high-strength steels used for structural components. In this study, the fatigue fracture behavior of low-strength interstitial-free steel has been studied at room temperature in air and in an argon atmosphere. Fractographic observations on fatigue fracture surfaces reveal a mixed type of fracture mode (intergranular and transgranular cracking) for all specimens. The major portion of intergranular fracture has been found at the regions of low and medium crack lengths. Further increase in the crack length causes a decrease in the proportion of intergranular cracking and, finally, the fracture mode becomes completely transgranular. At the same test temperature, the proportion of intergranular fracture drastically decreased when the test was carried out in the argon atmosphere.

(Student Battery Slam Best Presentation Award Winner) Sodiation Induced Chemo-Mechanics in Sn Electrodes

Binder free self-supporting tin (Sn) foil with the promise of high theoretical capacity and outstanding volumetric capacity is an outstanding anode candidate for sodium-ion batteries. However, it suffers from an undesirable initial coulombic efficiency (ICE), indicating higher irreversible losses associated with particle isolation and solid electrolyte interphase (SEI) formation. In the present work, we have studied the electrochemical behavior (Cycling performance, Electrochemical Impedance Spectroscopy) and damage mechanisms of Sn foil in the context of thickness and temperature. Bulk Sn foil showed mechanical failure caused by pulverization and microscopic images revealed different types of fracture modes at different temperatures and different thicknesses of the foil. The crystallographic changes during sodiation/de-sodiation and ex-situ XRD patterns were analyzed to understand the phase transformation. The pristine Sn foil has a polished surface and post-mortem observations show random cracks formed on the Sn surface after cycling. In lower temperature, the thinner Sn foil cracked into pieces, where the thicker foil showed deformed Sn structures. Different types of physical, electrical, and/or electrochemical degradation of the electrode led to continuous cell capacity loss with cycling. This study focuses on understanding different types of electrode damage behavior on thick and thin Sn foil during the sodiation/de-sodiation process, and thus open new avenues for dense Sn foil-based anode to be used in full cell batteries.

Simulation on the mechanical performance and fracture behavior of unidirectional carbon fiber-reinforced composites

Unidirectional carbon fiber reinforced polyamide 6 (CF/PA6) laminates after tensile tests exhibited two different types of fracture modes, namely, splitting and step-like. In this study, the mechanical properties and fracture behavior of the unidirectional CF/PA6 laminates were simulated and analyzed by using finite element simulation of a tensile test. The simulations considered the actual deformation and damage mechanism (matrix plastic deformation, stochastic fiber fracture, and interfacial debonding between the fiber and the matrix) experimentally observed in previous research. Results of the numerical simulations were in good with the experimental stress–strain curves. Moreover, the above-mentioned two types of fracture mode were well simulated by changing the interfacial properties. The simulated results showed that the unidirectional laminates with weak interface properties exhibit the splitting fracture mode. Meanwhile, the unidirectional laminates with strong interface properties show the step-like fracture behavior.

Microstructure and mechanical properties of friction stir spot welded AA5052-H112 aluminum alloy

The influences of the pin geometry and rotational speed on the microstructure evolution and mechanical properties of the AA 5052-H112 FSSW joint were investigated. Results showed that hook height and fully bonded region (FBR) width were significantly dependent by both pin geometry and rotational speeds. Both pin geometry and tool rotational speed had an apparent influence on the strength of the weld. At 900 and 1400 rpm, the strength of welds prepared using a cylindrical pin was higher compared to that of a step pin. For the step pin, the cross-tension strength of the welds increased as rotational speed increased. For cylindrical pin, the tensile/shear and cross-tension loads achieved the maximum values of 3589 and 3419 N at 1400 rpm, respectively. Under tensile/shear loading, shear and tensile/shear fractures were observed. On the other hand, under cross-tension loading, two types of fracture modes, namely nugget debonding and nugget pull-out were detected.

The impact of macro/microstructure features on the mechanical properties of refill friction stir spot–welded joints of AA2219 alloy with a large thickness ratio

AA2219 aluminium alloy plates of large thickness ratios were successfully produced by refill friction stir spot welding (RFSSW) method. In addition to common defects, such as holes and hook, insufficient refill and weak bonding also appeared in the joints with large thickness ratios. Besides, the stir zone of the typical RFSSWed joint exhibited refined and equiaxed grains because of the dynamic recrystallization, while the elongated grains with a large proportion of the low angle grain boundaries were the main microstructure features in the thermomechanically affected zone due to insufficient heat input and deformation. Moreover, in all cases of large thickness ratio, the micro-hardness distribution of the joints presented an M-shaped profile. The maximum tensile-shear strength reached 7.2 kN, corresponding to the rotation speed of 2000 rpm, welding time of 3.6 s and plunge depth of 2.2 mm, which displayed a close correlation with the defect morphology, such as the weak bonding length and hook height. Also, two types of fracture modes were observed in the tensile-shear specimens, plug fracture mode and mixed fracture mode.

Elucidation of fatigue characteristics and fracture mechanism of friction stir spot‐welded tension–shear joint steels

AbstractThe fatigue life and fracture mechanism of friction stir spot welded tension–shear joints using 590‐MPa class steel as a base material under constant‐amplitude conditions were investigated with focus on welding dimension variations caused by tool wear. The fatigue limit of the friction stir spot welding (FSSW) joint used for this study is significantly low compared with the static tensile strength of the joint itself. It was clarified that the FSSW joint in this study exhibited two different failure morphologies regardless of the applied load level: base metal fracture and weld area fracture. Although the welding state changes due to the tool wear phenomenon that produce two types of fracture modes in relation to the welding rip diameter, they have no effect on the fatigue strength, regardless of the applied load.

Effect of Sleeve Plunge Depth on Interface/Mechanical Characteristics in Refill Friction Stir Spot Welded Joint

Refill friction stir spot welding was employed to produce 6061-T6 aluminum alloy joints with different sleeve plunge depths. The interface characteristics of joint-line remnant and hook are investigated by optical and scanning electron microscopy. The joint-line remnant consists of primary bonding region and secondary bonding region, and two types of hook can be identified as downward hook and upward hook. Tensile shear results demonstrate that joint-line remnant and hook make interaction effects on tensile shear properties. The optimal joint is achieved when sleeve plunge depth was 2.0 mm with the corresponding failure load of 8673.4 N. Three different types of fracture mode are exhibited in joints produced at different sleeve plunge depths, which are closely related with the morphology of interface characteristics.

Fracability Evaluation of Shale of the Wufeng‐Longmaxi Formation in the Changning Area, Sichuan Basin

The fracturing technology for shale gas reservoir is the key to the development of shale gas industrialization. It makes much sense to study the mechanical properties and deformation characteristics of shale, due to its close relationship with the fracability of shale gas reservoir. This paper took marine shale in the Changning area, southern Sichuan Basin of China as the research object. Based on field profile and hand specimen observation, we analyzed the development of natural fractures and collected samples from Wufeng Formation and Longmaxi Formation. Combining with the indoor experiment, we investigated the macroscopic and microscopic structural features and the remarkable heterogeneity of shale samples. Then we illustrated the mechanics and deformation characteristics of shale, through uniaxial compression test and direct shear test. The shale has two types of fracture modes, which depend on the angular relation between loading direction and the bedding plane. Besides, the Wufeng shale has a higher value of brittleness index than the Longmaxi shale, which was calculated using two methods, mechanical parameters and mineral composition. Given the above results, we proposed a fracability evaluation model for shale gas reservoir using the analytic hierarchy process. Four influence factors, brittleness index, fracture toughness, natural fractures and cohesive force, are considered. Finally, under the control of normalized value and weight coefficient of each influence factor, the calculations results indicate that the fracability index of the Wufeng Formation is higher than that of the Longmaxi Formation in Changning area, southern Sichuan Basin.

Microstructures of longitudinal/transverse welds and back-end defects and their influences on the corrosion resistance and mechanical properties of aluminum alloy extrusion profiles

Aluminum alloy profiles manufactured by semi-continuous porthole die extrusion have three kinds of intrinsic features: longitudinal welds, transverse welds and back-end defects, which affect the corrosion resistance and mechanical properties of the profiles. In this work, the microstructure, corrosion resistance and mechanical properties of these intrinsic features were studied by microstructural characterization, chemical immersion and in-situ test, and the relationship between the material flow behavior and the evolution of the back-end defect and transverse weld was revealed through numerical simulation. It is found that, in terms of corrosion resistance and mechanical properties, both the transverse weld and back-end defect are poor, while the longitudinal weld is relatively good. Impurities and oxide particles are densely distributed in the aluminum alloy matrix in the back-end defect area. In the transverse weld area, the bonding interface has impurities and oxides, and grains are separated by the bonding interface; while in the longitudinal weld area, the grains across the bonding interfaces are formed. The bonding interface of the ring-shaped transverse weld which is far from the longitudinal weld has a low bonding degree. When the transverse weld becomes no longer ring-shaped and very close to the longitudinal weld, its bonding degree is greatly enhanced. The bonding degree of all the transverse welds is lower than that of all the longitudinal welds. During the tensile tests of the extruded profiles, strain concentration phenomena occur at the positions of the longitudinal welds, transverse welds and back-end defects, and there are two types of fracture modes: “uniform plastic deformation → strain concentration → fracture” and “uniform plastic deformation → strain concentration → debonding → fracture”.

Microstructure and size effect of interfacial intermetallic on fracture toughness of Sn3.0Ag0.5Cu solder interconnects

The effects of solid state thermal aging and liquid state reflow on the interfacial fracture toughness of Sn3.0Ag0.5Cu solder interconnects are experimentally investigated. Solid state thermal aging for compact tension specimens was treated at 210 °C up to 48 h, the liquid state reflow was treated at 230 °C up to 48 h as well. Three types of fracture modes are observed including fracture in solder, intermetallic/solder interface and intermetallic layer. In general, the fracture toughness of intermetallic decreases with increasing of treating time under both liquid and solid states. After solid state thermal aging, the fraction of intermetallic fracture decreases while the proportion of solder fracture and interfacial fracture increases. After liquid state reflow, the fracture keeps being dominated by the intermetallic fracture. The morphology evolution and grain size of intermetallic layer are analyzed. In solid state thermal aging, the intermetallic layer tends to transit from the initial scallop-type to lamellar-type. While in liquid state reflow, the intermetallic layer tends to keep scallop-type. Both intermetallic morphology and grain size are found to be essential factors that affect the fracture type and toughness. The effect of intermetallic grain size on fracture toughness is investigated based on the size effect theory. The predicted fracture toughness of solder interconnects after 230 °C reflow agree well with the experimental results.

Three Dimensional Finite Element Analysis of Rolling Contact between Wheel and Rail

The fatigue performance of the rails is affected by many factors, including service conditions, loading, mechanical properties, environment factors, and manufacturing processes. In this paper, the investigation on wheel-rail to identify the initial damages caused by Rolling Contact Fatigue (RCF) cracks and the location that experienced damages is presented. UIC 54kg rail (grade 900A) was used as the model in three dimensional (3D) finite element contact analysis. The fatigue crack growth on wheel-rail was carried out by considering the Hertz contact pressure. The finite element analysis results show that maximum stress concentration zone was between the wheel-rail surface (rail inside curve gauge corner) and it is above the yield stress limit for wheel-rail steel. Fatigue crack propagation within a depth affected stress concentration region was predicted. The stress intensity factors (SIF) for mode I, mode II and mode III fracture were plotted from ANSYS simulation. Three types of fracture modes were affected the UIC54kg rail Steel to fail or develop initial failure when the crack propagation exceeds 5 mm.

Mechanical Characterization of Austempered Ductile Iron Obtained by Two Step Austempering Process

Austempered ductile iron (ADI) is known to have a good combination of mechanical properties due its unique ausferrite microstructure. The strength of ADI is mainly a function of the austempering temperature and the stability of ausferrite matrix. To increase the stability of the ausferritic matrix, two stage austempering processes was developed. During this investigation, in the Ist step, ductile iron specimens were austenitized at 900 °C for 60 min followed by quenching to 250 °C in salt bath. In the IInd step, after quenching at 250 °C, the salt bath was gradually heated to 350 °C, 400 °C and 450 °C respectively where specimen were soaked for 120 min. The tensile strength and impact strength were evaluated according to ASTM standards. The results were compared with that obtained by conventional austempering process by quenching directly into salt bath at 400 °C for 120 min. Both tensile and impact strength were found to have improved by two step austempering process. During Ist stage of austempering, martensite was observed while during IInd stage of austempering microstructures revealed acicular ferrite and carbon stabilized austenite. The fractographic examination revealed mixed type of fracture mode and intergranular fracture was seen under SEM. It was further observed that the tensile strength decreased whereas the impact strength increased with IInd stage of austempering temperature.

T型片側すみ肉溶接継手の異なる破壊モードに対する統一的な疲労設計マップの構築

T型片側すみ肉溶接継手の異なる破壊モードに対する統一的な疲労設計マップの構築

Fatigue behaviour of aluminium alloy/steel joints by spot friction stirring

Fatigue tests of Al–Mg alloy and low-carbon steel joints, by means of the spot friction stir welding, were carried out to investigate the basic fatigue properties. The influences of joining conditions on the fatigue behaviour were also studied by using the joints which were joined under the different plunge depth conditions. From the experimental results, it was found that the fatigue life and strength were slightly different with difference in the plunge depth in case of tests under the higher applied load levels. There were three types of fatigue fracture modes which depend on both of plunge depth and applied cyclic load level. They were shearing type fracture due to detaching of the joining interface, fatigue fracture type in the aluminium alloy due to fatigue crack growth in perpendicular to the loading direction and the plug type fracture due to crack growth along the spot FSW region. Under the applied load which was the border level between two types of fracture mode, both fracture processes propagated concurrently and the final fracture occurred when either process attained to the critical stage.

Anisotropy of high cycle fatigue behavior of a Ni-base single crystal superalloy

The rotary bending high cycle fatigue properties of a single crystal superalloy in three orientations were tested at 700°C in ambient atmosphere. The [011] orientation shows the highest fatigue strength. Moreover, the fatigue strength with 〈111〉 orientation is obviously higher than that with 〈001〉 orientation. There are two types of fracture modes, one is cleavage fracture for the [001] orientation, the other is the mixed fracture of cleavage and tearing for the [011] orientation. The 〈011〉 orientation shows more pronounced crystal lattice rotation than the 〈111〉 orientation. Nevertheless, lattice rotation does not occur in the crystals with [001] orientation. Fatigue steps of the [001] and [111] orientated crystals propagate along different {111} glide planes, which means that the slip in 〈111〉 orientation moves more easily than in 〈011〉 orientation. The typical deformation mode of [001], [011] or [111] orientation is respectively slip bands running through γ and γ′ phases, dislocation pairs shearing γ′ particles, or superlattice stacking faults besides high-density dislocation shearing γ′ particles.

Flexural strength and failure modes of layered ceramic structures

Objective To evaluate the effect of the specimen design on the flexural strength ( σ f ) and failure mode of ceramic structures, testing the hypothesis that the ceramic material under tension controls the mechanical performance of the structure. Methods Three ceramics used as framework materials for fixed partial dentures (YZ – Vita In-Ceram YZ; IZ – Vita In-Ceram Zirconia; AL – Vita In-Ceram AL) and two veneering porcelains (VM7 and VM9) were studied. Bar-shaped specimens were produced in three different designs ( n = 10): monolithic, two layers (porcelain-framework) and three layers (TRI) (porcelain-framework-porcelain). Specimens were tested for three-point flexural strength at 1 MPa/s in 37 °C artificial saliva. For bi-layered design, the specimens were tested in both conditions: with porcelain (PT) or framework ceramic (FT) layer under tension. Fracture surfaces were analyzed using stereomicroscope and scanning electron microscopy (SEM). Young's modulus ( E) and Poisson's ratio ( ν) were determined using ultrasonic pulse-echo method. Results were statistically analyzed by Kruskal–Wallis and Student–Newman–Keuls tests. Results Except for VM7 and VM9, significant differences were observed for E values among the materials. YZ showed the highest ν value followed by IZ and AL. YZ presented the highest σ f . There was no statistical difference in the σ f value between IZ and IZ-FT and between AL and AL-FT. σ f values for YZ-PT, IZ-PT, IZ-TRI, AL-PT, AL-TRI were similar to the results obtained for VM7 and VM9. Two types of fracture mode were identified: total and partial failure. Significance The mechanical performance of the specimens was determined by the material under tension during testing, confirming the study hypothesis.