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To investigate the cracking reason in welding structure with transition changing faces at cylinder block on boom of medium excavator during fatigue test, the finite element model of boom was established by using ANSYS finite element software. The stress distribution of the boom was obtained under fatigue loading spectrum of medium excavator. In addition, the fine model of welding structure with solid element length 2 mm was set up with the help of sub-model technology in ANSYS software to get the accurate hot spot stresses around welding toes. Finally, the fatigue life of this welding structure was calculated according to IIW fatigue class. The results show that the stress gradient along with the thickness direction can be acquired by fine three-dimensional finite element sub-model of welding structure. Therefore, the weakest point in thick plate was located, which was in accordance with the experiment result.

In this work, weld-repaired Bisplate80 joints without or with a buffer layer (BL) were subjected to a post-weld heat treatment (PWHT) at 930 °C for 1 h followed by air cooling. The mechanical properties and fracture mechanism of the PWHT specimens were compared to those from corresponding as-welded specimens to study the influence of the PWHT and to compare them with as-received Bisplate80 specimens to inspect the welding quality of the joints. Fatigue crack growth curves and micro-hardness profiles across the welded joints for the as-weld or PWHT conditions were measured together with relevant scanning electron microscope observations along the crack growth path. Special attention was paid to the areas at and around the welded interfaces. The results showed that the incorporation of a BL with an appropriate thickness significantly improved the fatigue crack growth behavior of the welded joint for the PWHT condition, as the da/dN over the entire area of the welded joint was much lower than for both the as-welded and as-received Bisplate80 specimens, showing that the incorporated BL greatly improved the fracture mechanisms of the fatigue specimens.

A simple method of applying and distributing multi-walled carbon nanotubes (MWCNTs) onto grit-blasted steel substrates has been investigated in this study to overcome the difficulty of mixing MWCNT...

By means of finite element modeling (FEM) and fatigue experiments, we study the fatigue performance of the rounded welding region between the diaphragm plate and closed rib of orthotropic steel bridge deck in this work. A local sub-model of the rounded welding region from the orthotropic steel bridge deck was developed to analyze the stress distributions. Based on the analysis results we designed the fatigue specimen for the fatigue test of this detailed structure. The fatigue experimental results revealed that the crack initiates from the weld toe of the rounded welding region and the stress concentration at the rounded welding region is the main mechanism of fatigue crack initiation. In addition, we propose three improvements to reduce the stress concentration of the rounded welding region, and the local structure optimization scheme of the diaphragm&ndash, rib weld can effectively improve the fatigue resistance of the detailed weld structure.

The wettability of grit blasted steel substrates is improved by a simple surface treatment method proposed in this study. The shear strength of epoxy adhesive joints has been improved by over 50% after optimizing this “resin pre-coating” surface treatment method. The shear strength is further enhanced by incorporating multi-walled carbon nanotubes (MWCNTs) and short Kevlar fibres into the adhesive joints, with over 70% improvement for the combined surface-treated and MWCNTs toughened adhesive joints. The reinforcing mechanisms along the “interfacial zone” defined by the surface topography and the toughening mechanisms within the “composite adhesive joints” because of MWCNTs and short Kevlar fibres are discussed together with detailed Scanning Electron Microscopy (SEM) observations and experimental results.

The cutout hole locating at the place of rib-to-diaphragm welded connection is adopted to minimize the restraint, which is caused by the floor-beam web to rib rotation at the support due to the unsymmetrical loads in orthotropic deck. In practice, an inevitable problem is that there is a large number of welding joint's cracks formed at the edge of cutout hole. In this study, a comparative experiment is carried out with two types of cutout hole, the circular arc transition and the vertical transition. The fatigue life estimation of specimens is investigated with the application of the structural hot spot stress approach by finite element analyses. The results are compared with the ones of the fatigue tests which are carried out on these full-scale specimens. Factors affecting the stress range are also studied.

a b s t r a c t Welding residual stresses, Vickers hardness and microstructure of welded high-strength low-allow steel, known for high strength and low carbon content, were studied under the following conditions: as- received high-strength low-alloy steel, welded high-strength low-alloy steel without a buffering layer, and welded high-strength low-allow steel with various thickness buffering layer. A soft buffering layer with a modest thickness between the parent metal and the weld metal could be incorporated to welded high-strength low-alloy steel to effectively reduce the widths of tensile residual stress zone and welding softening zone, to change the residual stress (in y-direction) nature at the weld root from tensile to com- pressive, and to refine the grains of the welded high-strength low-alloy steel. The width of the tensile residual stress (in x-direction) zone was approximately equivalently to that of the welding softening zone for those welded high-strength low-alloy steel with and without buffering layers. © 2013 Elsevier B.V. All rights reserved.

The effects of tensile over-load (OL) on fatigue crack growth behavior of a weld-repaired high-strength low-alloy (HSLA) steel were studied by measuring both the fatigue crack growth rate and sample-thickness variation along the fatigue crack growth path. The thickness variation, indicating the degree of plastic deformation (PD), provided an indirect measurement of associated residual compressive stresses at the crack-tip. The applied tensile OL with one-hour holding period in each test generated a damage zone at the crack tip. Microscopic details of the crack-tip damage zone were characterized by scanning electron microscopy. Three groups of expanded compact-tension (E-CT) samples, 10 mm in thickness, were tested: weld-repaired HSLA without soft buffer layer (BL), and weld-repaired HSLA with 4 mm or 10 mm thick BL. The experimental results showed that the OL-induced PD, closely linked to the crack-tip residual compressive stresses, reduced the subsequent fatigue crack growth rate, and that the HSLA with a 10 mm BL had the lowest growth rate, indicating a soft BL with an adequate thickness could further improve the fatigue resistance.

Binhua Wang, Pengmin Lu, Yuhong Shao Key Laboratory for Highway Construction Technology and Equipment of Ministry of Education, Chang’an University, Xi’an 710064, Shaanxi, China Email: wangbh@chd.edu.cn Abstract In order to minimize the restraint provided by the floor-beam web to rib rotation at support due to unsymmetrical loading in orthotropic decks, the structure of cutout holes are adopted at the place of rib-to-diaphragm welded connection. However, there is a large number of welding joint’s cracks formed at the edge of cutout holes. In this study, the comparative specimens with two types of cutout holes are designed, i.e. the circular arc transition structure and the vertical transition structure. And the fatigue life estimation of specimens is investigated with the application of the structural hot spot stress approach by use of finite element analyses. The results of the finite element calculations are compared with the results of the fatigue tests which are carried out on these full-scale specimens.

Residual stresses (RS) in a bulk weld-repaired steel structure and thin samples sliced from the welded-section are different due to inevitable variations in the stiffness and boundary conditions. In this study, thin extended compact tension (E-CT) samples for fatigue crack growth measurements were sliced from three extensively weld-repaired high-strength low-alloy (HSLA) blocks with three different welding conditions, i.e. welding with a 4 or 10 mm thick soft buffer layer (BL) and welding without the soft BL. Deformation in the thin E-CT samples with notches due to the partial release of welding-induced RS) was measured, and then the samples were used to measure the fatigue crack growth. In this study, we reported detailed measurements of residual stress-induced (RS-induced) deformation in thin test samples and corresponding fatigue crack growth behavior of an extensively weld-repaired HSLA with or without a thin BL. Three groups of E-CT samples were prepared: weld-repaired HSLA without BL, and weld-repaired HSLA with a 10 mm or 4 mm BL. The RS-induced deformation and corresponding fatigue crack growth were measured, and studied together with the influence of BL. The results showed that the incorporation of a 4 mm BL had a profound influence on reduction of the RS-induced deformation, and the incorporation of a 10 mm BL had a significant influence on the fatigue crack growth behavior in the parent metal, heat-affected-zone and weld metal. Detailed SEM observations show that fatigue characteristics of the weld-repaired HSLA were also influenced by the buffer layer and welding-induced RS.

It is a contradiction to homogeneous material fatigue behavior characterized by widely used linear Paris law, welded-joint fatigue issues need to be reassessed because fatigue crack growth behavior going through heterogeneous region will be different. For a welded joint, log(da/dN) is no longer linearly related to log(ΔK) in heterogeneous region because of the change in fatigue properties resulting from the welding process. Theoretical model of the fatigue crack growth rate without artificial adjustable parameters was proposed by considering the effects of heterogeneity in a welded joint and load-amplitude variation on fatigue crack growth curve. In this fatigue heterogeneous region, the relationship between log(da/dN) and log(ΔK) is similar to a concave-down parabola. Predicted results from the proposed model agreed better with the experimental data obtained from fatigue tests conducted in this study and open published literatures for welded joints in comparison to the widely used Paris model.

A soft buffer layer (BL) between the weld metal (WM) and the parent metal (PM) was introduced in this study. To study the effect of a BL on mechanical properties, two types of joints made from welded high-strength low-alloy (HSLA) specimens with or without a BL were prepared. To investigate the influence of the notch location, the welded HSLA specimens machined with a U-notch in the WM or the PM. These conditions were examined to determine the effects of the BL and the notch location on the fatigue characterizations and the microscopic features of the fatigue-fractured surface. Extended-compact tension (E-CT) specimens were used for all tests. Paris fatigue curves and hardness distributions of the welded HSLA specimens with or without a BL were measured, and relevant scanning electron microscope (SEM) images were taken. The results showed that the presence of a BL significantly improved the fatigue crack growth behavior when a U-notch was in the WM. The notch location greatly influenced the fatigue behavior and the surface features of the fatigue-fractured specimens.

The microstructure variation and its subsequent influence on the fatigue behavior and Vickers hardness of an extensively weld-repaired high strength low alloy (HSLA) were investigated in this study. Microstructures of the as-received HSLA, or parent metal (PM), welded HSLA without buffer layer (BL), and welded HSLA with 4 or 10 mm BL between the PM and weld metal (WM) were examined and compared with the corresponding fatigue behavior, hardness measurement, and fatigue-fractured surface. It was found that the fatigue resistance and hardness in part of the WM and in the melted parent metal (MPM) during welding of the welded HSLA without BL were lower than those of PM. However, the welded HSLA with a 4-mm-BL produced a marked increase in fatigue resistance and hardness, especially within the WM, within the mixed area of BL and MPM (BL + MPM). This was because the WM and MPM of welded HSLA without BL had block ferrite and coarse columnar grain structures. Furthermore, no obvious differences were found in fatigue property, hardness and microstructure between the MPM of welded HSLA without BL and the BL + MPM of welded HSLA with 10-mm-BL. In addition, optical microscopy and SEM examinations of the corresponding regions revealed that block ferrite and non-metallic particles were the main reason of poor fatigue resistance and low hardness.

Heat-affected zones (HAZs) in extensively weld-repaired Bisplate80, or the parent metal (PM), have been studied and examined using hardness measurements in conjunction with fatigue crack growth measurements and detailed scanning electron microscopy (SEM) observations. Three different groups of specimens have been prepared, i.e. as-received PM, weld-repaired PM with a thin 4 mm BL between PM and weld metal (WM), and weld-repaired PM without BL. The extended compact tension (E-CT) specimens for fatigue measurements are prepared according to the ASTM specifications, which are also used for hardness measurements. Hardness and fatigue crack growth variations across the boundaries between WM, BL and PM have been measured together with detailed SEM observations. It has been found that the welding process without BL reduced both hardness and fatigue resistance, especially around the weld interface and within the HAZ. Incorporation of a thin soft BL between WM and PM has increased both hardness and fatigue resistance around the weld interface and within the HAZ.