Steel bridges fabricated with Q345qD steels face critical challenges when operating in cold regions with a low ambient temperature. This study aims to investigate, via an experimental program, the low-temperature fatigue crack propagation behavior of Q345qD bridge steel base material and its butt welds. The testing program comprises a series of Charpy impact tests and fatigue crack propagation tests at the room temperature, −20°C and −60°C. The experimental results demonstrate a reduced crack propagation rate in the base material, but an increasing crack propagation rate in the butt welds, with a decreasing ambient temperature. The base material also shows enhanced fatigue crack propagation thresholds with the decreasing temperature. The ductile-to-brittle transition temperature for fatigue is lower than that for fracture in the base material while the weld metal exhibits an opposite trend. Generally, the butt welds present higher resistance against fatigue crack propagation and larger Charpy toughness values than do the base material at all tested temperatures. The Paris-law parameters measured at the room temperature for the base material leads to a conservative assessment of the crack propagation life for a welded joint under a low ambient temperature.

Fatigue crack propagation for Q345qD bridge steel and its butt welds at low temperatures

The joining of various parts in engineering applications such as automotive components, aircrafts, medical implants, sensor devices, interconnections etc., are a great concern nowadays. Various types of low melting point fillers, braze and alloys have been also developed but problem of crack generation and failures are not satisfactorily treated. This paper reviews some of the developments in traditional and modern joining techniques in automotive joining and welding. Currently, joining of composites is attracting researchers for weight reduction and economy. There are many practical techniques for joining metals and ceramics to themselves and to dissimilar materials. An overview of various processes of joining metal-metal, ceramic-metal and composites, factors affecting joining and approaches used to improve the brazed joint structures are presented here. In addition, new modifications in welding and brazing processes are outlined for further directions.

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Advances in Joining and Welding Technologies for Automotive and Electronic Applications

Evaluation of the forming limit curve of medium steel plate based on non-constant through-thickness normal stress

Lamellar tearing and crack-induced brittle failures along steel plates in the through-thickness direction seriously threaten the safety and reliability of steel thick plate structures in construction and service, especially at low ambient temperatures. Three kinds of tests, including uniaxial tensile tests, Charpy V-Notch impact tests, and three-point bending (TPB) tests were performed at normal and low temperatures to investigate the through-thickness mechanical properties, impact and fracture toughness of Q345B structural steel plates with thicknesses from 60 to 165 mm. The test specimens were mainly sampled along the through-thickness direction of the plate, but transverse specimens along the rolling direction were also involved. The ductility index (percentage reduction of area), impact toughness index (Charpy impact energy), and fracture toughness index (critical crack tip opening displacement (CTOD) values) all decrease as the temperature declines. All the mechanical properties and the impact and fracture toughness along the through-thickness direction are worse than those along the rolling direction. The results also offer experimental support for the determination of an evaluation indicator for structural steel thick plates with through-thickness characteristics.

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Experimental study on the through-thickness properties of structural steel thick plate and its heat-affected zone at low temperatures

Oxygen-free copper (OFC) is widely used in high energy physics detectors and accelerators. The mechanical properties of OFC, such as stress–strain relation and impact toughness, are of great importance when designing OFC-based load-bearing shielding tanks or structural components. In order to study the mechanical properties of OFC, tensile and impact tests were separately carried out in the present work. The relations between the type and rolling direction of the material and its mechanical properties were intensively studied. Subsequently, a modified two-stage Ramberg–Osgood (R–O) model was used to fit the tensile test data. The results show that the nonlinearity of the stress–strain curve of OFC was pronounced. Although the yielding stage of OFC was not obvious, this material showed a ductile behavior. The rolling direction had a significant effect on the tensile properties of the OFC sheet, but had an insignificant effect on the impact toughness. OFC tubes possessed similar strength levels, but inferior ductility compared to the OFC sheets in the longitudinal rolling direction. The modified R–O model is suitable to describe the constitutive relation of OFC.

Experimental study on the mechanical properties of oxygen-free copper used in high energy physics detectors and accelerators

The thick plate induces the variation of mechanical properties and fracture toughness, especially in cold regions. At the low temperature, the brittle behaviour of steel becomes worse. A series of tests (such as uniaxial tensile test and three-point bending test) were carried out at low temperature to investigate the mechanical properties and fracture toughness of structural steel plates of Q345B with thickness of 60 to 150 mm, as well as the fracture toughness of 150 mm thick butt welded plate. The test specimens are all manufactured from plates along thickness with small size, and the tensile test specimens included through-thickness specimens additionally. The ductility index (percentage reduction of area) and the fracture toughness index (critical CTOD values) all decrease with the temperature decreases and the distance from plate surface increases. The results obtained in this paper provide technical basis for preventing brittle fracture of thick plate steel structures in cold regions.

Experimental study on mechanical properties and fracture toughness of structural thick plate and its butt weld along thickness and at low temperatures

At a low temperature, the mechanical properties of thermit welding joints of rail steels get worse and brittle failures happen more easily. This threatens the railway transportation seriously. It is necessary to find out the mechanical properties of thermit welding joints of rail steels at low temperature because there are more and more railway constructions at low temperature such as Qinghai-Tibet railway. This paper studied the mechanical properties of tow common types of thermit welding joints in China, whose mother metal are U71Mn and U75V of 60kg/m, and got the Tensile strength, Proof strength (nonproportional extension), Percentage elongation after fracture and Percentage reduction of area of the tow types of thermit welding joints at a series of low temperatures. Temperature sensitivity coefficients of the two types of thermit welding joints were calculated and then the tensile strengths and proof strengths at low temperature could be forecasted. At last, which type of rail steel should be chosen at low temperature was proposed through the comparison of U71Mn and U75V thermit welding joints.

Zongwen Hu

Papers

Experimental study on mechanical properties and fracture toughness of structural thick plate and its butt weld along thickness and at low temperatures

Experimental Study On Mechanical Properties of Rail Thermit Welding Joints At Low Temperature