Fracture Mode Of Joints Research Articles

Investigation of ultrasonic welding of CF/PA66 using nylon mesh energy directors

AbstractCarbon fiber reinforced polyamide 66 (CF/PA66) sheets are ultrasonically welded by applying nylon meshes as energy directors (EDs) in this paper. The influence of mesh dimensions is revealed by observing the joint formation process, joint mechanical performance, joint cross‐sectional morphology and joint failure mode. Results show that the joint formation process can be divided into wire flattening stage, ED spread stage, tight bonding stage, and mixing stage. The nylon mesh ED with moderate wire spacing, small wire diameter and small mesh area promotes better weld quality. Welding energy will be dispersed rather than concentrated if the mesh area is too large, resulting in reduced failure load. The joints exhibit two fracture modes: interfacial fracture (IF) and workpiece fracture (WF), and the fracture mode of the joints with nylon mesh EDs transits from IF to WF as the welding energy increases.Highlights Five types of nylon mesh are used as energy directors to join CF/PA66 sheets by ultrasonic welding. The influence of nylon mesh dimensions on the mechanical property and fracture mode of joints is investigated. The formation process of ultrasonically welded joints can be separated into wires flattening stage, ED spread stage, tight bonding stage, and mixing stage.

An improved J-integral based adhesive joint fatigue life estimation method for automotive structural durability analysis

ABSTRACT To estimate the fatigue life of lightweight automotive structures in compliance with top-down design principles, there is a need for an accurate adhesive joint fatigue life prediction method that is compatible with industrial finite element modeling practices. This paper proposes a method to split the analytical J-integral solution based on the fracture mode of joints. The split mode I and II J-integrals correspond to the opening mode and sliding mode of the bonded joint, respectively. By splitting the J-integral and introducing the concept of the mixed mode ratio, an improved approach for estimating adhesive joint fatigue life is presented, considering the influence of loading modes. The proposed fatigue life prediction method is validated through fatigue tests on a sub-component level bonded structure. The results of the validation demonstrate that the split J-integral analytical solution method, considering the mixed mode ratio, provides better predictions compared to its predecessor approach.

Microstructure and mechanical properties of refill friction stir spot welded joints: Effects of tool size and welding parameters

A novel refill friction stir spot welding (RFSSW) technique employing large-sized tools is proposed. The microstructure and mechanical properties of joints produced with a large-sized welding tool and a conventional tool are compared. The results show that the exit line resulting from the sleeve becomes longer with increasing the plunge depth, and the diameter of nugget increases with higher rotational speed for the joints produced by both conventional and novel tools. The plunge depth increases from 2.0 mm to 2.2 mm, then to 2.4 mm, which affects the hook defect to bend upwards, almost parallel to the lap interface, to bend downwards, respectively. The joints produced with the novel tool have a flat hook compared to the conventional tool. The microstructure evolution of the conventional and novel joints is similar. The tensile-shear and tearing forces measured of the novel joints are higher than those of the conventional joints for the same welding parameters. For conventional joints, the maximum tensile-shear and tearing forces are 8.6 ± 0.1 kN and 4.4 ± 0.2 kN, respectively. The maximum tensile-shear and tearing forces for novel joints are 10.9 ± 0.1 kN and 5.6 ± 0.1 kN, respectively. After the tensile-shear test, there are three modes of fracture, the upper-mixed, the lower-mixed, and the shear fracture one. The plunge depth has a pronounced effect on the fracture mode of joints.

Hybrid joining mechanism of rivet plug oscillating laser welding for dual-phase steel and magnesium alloy

A novel hybrid joining method of rivet plug oscillating laser welding (RPOLW) was explored for joining DP590 dual-phase steel to AZ31B magnesium alloy. Microstructure, mechanical properties, and fracture mode of joints were analyzed, and force field, temperature, and flow field of molten pool were simulated. The results showed that the RPOLW joint presented the characteristic of asymmetrical double molten pool. The load-bearing areas of the joint were categorized into the metallurgical joining area and the riveting joining area. Meanwhile, the metallurgical joining area was the core load-bearing area of the joint, which was composed of a mixed-phase zone and an Al-rich zone. The tensile-shear load of RPOLW joint reached 2.2 kN at the optimal parameter, which was 36% higher than that of riveting joint. The heat accumulation in left molten pool resulted in a greater penetration depth of this side, the mixed-phase zone and Al-rich zone of the left molten pool were more evenly distributed. However, Al-rich zone was embedded in mixed-phase zone in the right molten pool, which was caused by the migration of the flow-field center. Consequently, the fracture position of joint was transferred to the mixed-phase zone in left molten pool.

Advanced high-temperature resistant (RT-1000 °C) aluminum phosphate-based adhesive for titanium superalloys in extreme environments

To facilitate the repairing and connecting processes for titanium superalloys, an advanced high-temperature resistant adhesive that can be converted to a composite of intermetallics and ceramics was prepared by modifying aluminum phosphate-based adhesive with various additives. The composition evolution of the adhesive, the structure changes in the bonding layer, the reaction process at interfaces and the fracture mode of joints are comprehensively studied to explore the bonding mechanism. The results showed that the chemical bonding based on the formation of TiSi and Ti5Si4 started to work at 600 °C, and became the crucial bonding mechanism at higher temperatures. A diffusion reaction layer with size up to 6 μm effectively alleviated the difference of composition and performance between alloy substrate and adhesive. From RT to 900 °C, the coefficient of thermal expansion (CTE) of adhesive matched well with that of alloy substrates, and the difference of their CTE was always lower than 2 × 10−6 k−1. The bonding strength reached ~16 MPa after pre-treatment at 900 °C without pressure, and remained over 13 MPa within the common operating temperature range (400–600 °C) of TC4 alloys. Without any pre-treatment, the adhesive could still provide above 5 MPa in range of RT-600 °C.

Classification and variation of fracture modes in laser shock hole-clinching

Laser shock hole-clinching is a high–strain rate mechanical joining process in which the metal foils are joined together based on plastic deformation generated by a laser-induced shock wave. In the process, fracture is a typical defect and seriously influences the clinching quality and production efficiency of joints. However, the classification and variation of fracture modes in laser shock hole-clinching are of less concern. In this study, the fracture mode of Cu-Fe joints in laser shock hole-clinching was experimentally investigated. Both optical microscopy and scanning electron microscopy were adopted to analyze the fracture surface morphology of clinched joints. The influence of laser power density, laser spot diameter, the initial grain size and thickness of metal foil, and spacer height on the fracture mode of joints was evaluated. It is revealed that the temperature increase caused by high–strain rate plastic deformation has no impact on the fracture behavior of the joining partners. Fracture always occurs on joining partner I, and it can be divided into four modes, including bottom surface fracture, bottom corner fracture, neck fracture, and mixed fracture. It is found that fracture on the bottom surface is seldom seen but mixed fracture accounts for most of the cracked specimens. Neck tensile fracture rarely appears alone, and it usually exists accompanied by fracture on the bottom corner. The fracture mode varies from a tensile fracture mode on the bottom corner to a mixed fracture mode and then to a shear fracture mode on the neck with the enhancement of laser power density. In addition, the initial grain size of joining partner I has a significant impact on the fracture mode of clinched joints. The fracture mode varies from bottom corner fracture to mixed fracture in relation to the change of mechanical properties of metal foil with an enlarged grain size. However, the mixed fracture mode always appears with the enlargement of both laser spot diameter and spacer height.

Microstructure and Mechanical Characteristics of Dissimilar TIG Welded 9% Cr Heat-Resistant Steels Joints

Dissimilar 9% Cr heat-resistant steels (G115 and CB2) with good creep properties for ultra-supercritical steam turbines were butt-joined by tungsten inert gas welding. The microstructure of welded metal (WM) was quenched martensite without carbide precipitates and lath packets existed inside prior austenite grains (PAGs), which leaded to higher hardness of WM. Partially melted zone at G115 side was composed of untempered martensite within equiaxed PAGs. The lowest hardness occurred in both G115 and CB2 steels which was attributed to tempered martensite with many M23C6 precipitates. The heat-affected zone consisted of three sub-grains and their microstructure was detailly analyzed in current work. As current increased from 130 to 150 A, both the tensile strength at room temperature and 650 ℃ increased while strength had no obvious change with further increasing current. The values of 673 MPa and 309 MPa corresponded to the tensile stress with 150 A at room temperature and 650 ℃, respectively. The fracture mode of joints at room temperature was cleavage and ductile failure at 130 and 150 A, respectively. The high-temperature fracture surface at 150 A was composed of deep and fine dimples.

IMC microstructure modification and mechanical reinforcement of Sn–Ag–Cu/Cu microelectronic joints through an advanced surface finish technique

Intermetallic compound(s) (IMC) that nucleates at the interface between solder and Cu trace during a soldering reaction, is one of the most crucial factors for microelectronic packaging reliability. This study was conducted to modify the IMC microstructure and to reinforce the mechanical strength of Sn–Ag–Cu/Cu microelectronic joints through various surface finish coatings, including organic solderability preservative (OSP), immersion Ag (ImAg), immersion Sn (ImSn), Au/Pd (electroless palladium/immersion gold, EPIG), and Au/Pd/Au (IGEPIG) layer(s). We confirmed that the type of surface finish dominated the IMC growth morphology and mechanical characteristics of the Sn–Ag–Cu/Cu solder joints, even though these surface finishes were eliminated in a few seconds of the soldering process. A dense Cu6Sn5 layer with a scallop-like appearance was obtained for the traditional OSP case, while a prismatic, loose Cu6Sn5 microstructure was produced for the alternative cases (metal films). This loose Cu6Sn5 microstructure offered numerous molten solder channels for the in-diffusion of Sn to Cu, retarding undesired Cu3Sn growth at the Cu6Sn5/Cu interface; consequently, a brittle-to-ductile transition in the joint fracture mode with a high shear strength and fracture energy was obtained in the high-speed ball shear (HSBS) test. A significant mechanical reinforcement of the Sn–Ag–Cu/Cu microelectronic joints can be achieved with the replacement of the traditional OSP coating by the newly developed IGEPIG trilayer surface finish.

Effect of welding parameters on resistance thermocompression microwelded joint of insulated copper wire

Insulated copper wire has been widely used in many industries, but the insulating coating must be removed before welding because it will hinder the formation of joints. Therefore, in this work, the resistance thermocompression microwelding process for insulated wire without removing the coating in advance was proposed. Through detailed mechanical testing and metallurgical examination, the effects of welding voltage on the joint appearance, joint macro-/microstructure, joint breaking force, and joint fracture mode were investigated. The results showed that joints were achieved by solid-state bonding. As the welding voltage was further increased, the joint breaking force increased first and then decreased. Corresponding to the change of the welding voltage, the fracture mode of the joints changed from interfacial fracture to partial pullout fracture. The fracture position also moved backwards gradually, which was caused by the expansion of the bonding area, the improvement of the interfacial strength, and the amount of heat input. Finally, an orthogonal experiment was conducted to investigate the significance level of three parameters. Under the optimal for joint breaking force parameters (2.1 V, 24 ms, and 12 N), the average achievable force was 1.1723 N, which was about 83.7% of the breaking force of the as-received wire.

Bonding performance and mechanism of a heat-resistant composite precursor adhesive (RT-1000∘C) for TC4 titanium alloy

To facilitate the repairing and connecting processes for the non-main bearing TC4 alloy, a high-temperature (up to 1000[Formula: see text]C) resistant adhesive that is converted to the composite of intermetallics and ceramics is prepared. The composition evolution of the adhesive, the structure changes in the bonding layer, the reaction process at interfaces and the fracture mode of joints are comprehensively studied to explore its bonding mechanism. The results show that chemical bonding mechanism based on the formation of Ti5Si3 plays a critical role at 600[Formula: see text]C, and acts as the crucial one at elevated temperatures. As the reaction interlayer (2–5[Formula: see text][Formula: see text]m) is far thinner than the entire bonding layer (60–70[Formula: see text][Formula: see text]m), mechanical properties of the adhesive dominate the bonding performance, which is tied up with the composition and structure evolution. The differ of coefficient of thermal expansion (CTE) between the adhesive and the substrate remains lower than [Formula: see text][Formula: see text]K[Formula: see text] in range of 500–1000[Formula: see text]C. Specifically, the formation of composites from intermetallics and ceramics improves the mechanical properties and heat-resistant of the adhesive. The bonding strength reaches [Formula: see text]40[Formula: see text]MPa after pre-treatment at 1000[Formula: see text]C without pressure, and remains over 30[Formula: see text]MPa within the normal operating temperature range of 500–700[Formula: see text]C.