To solve the intractable problems of optimal rank truncation threshold and dominant modes selection strategy of the standard dynamic mode decomposition (DMD), an improved DMD algorithm is introduced in this paper. Distinct from the conventional methods, a convex optimization framework is introduced by applying a parameterized non-convex penalty function to obtain the optimal rank truncation number. This method is inspirited by the performance that it is more perfectible than other rank truncation methods in inhibiting noise disturbance. A hierarchical and multiresolution application similar to the process of wavelet packet decomposition in modes selection is presented so as to improve the algorithm’s performance. With the modes selection strategy, the frequency spectrum of the reconstruction signal is more readable and interference-free. The improved DMD algorithm successfully extracts the fault characteristics of rolling bearing fault signals when it is utilized for mechanical signal feature extraction. Results demonstrated that the proposed method has good application prospects in denoising and fault feature extraction for mechanical signals.

Metal structures of ladle cranes are typical complex welded structures, where fatigue cracks are prone to initiate in some specific locations. In this paper, on-site tests and theoretical analysis were performed for a in-service ladle crane to investigate the crack damage mechanism at some local areas beneath the wheel loads. The on-site tests included crack observation of the entire bridge structure, geometry inspection of the main girder and stress test of the key locations. It was found that fatigue cracks had densely occurred in some spots with similar geometry details. The stress test results showed that obvious rail gnawing phenomenon happened during the ladle crane operating process. By finite element analysis (FEA) with a multi-layer model and DLOAD subroutine, stress time histories of the damaged spots under moving wheel loads were simulated. Combined with the tested stresses, influences of rail gnawing force on the stress cycling characteristics at the cracked areas were analyzed. Based on the in-service stress spectrum, the fatigue lives of the cracked areas were predicted, revealing that the random rail gnawing force is the dominant factor causing large amount of cracks at the damaged areas.

Aiming at the base metal (BM) zone, heat affected zone (HAZ) and fusion zone (FZ) of the weld, the tip plastic zone and initiation behavior of microscopic short cracks (MSCs) in the toe of cross-welded joints are studied. Based on the theory of crack tip plastic zone and the fatigue index parameter (FIP), the shape and boundary size of the plastic zone at the tip of MSCs as well as the initiation life were calculated. The results show that the plastic zone was bone-like under plane stress and butterfly-like under plane stress; when the crack length increased from 2 μm to 30 μm, the boundary size of the plastic zone at the MSC tip increased by 15 μm and 2 μm under the action of plane stress and plane strain in the X-axis direction, respectively. As the grain gradient decreased from 0.5 to 0.1, the boundary size of the plastic zone at the tip of the MSC increased about 50%. MSC initiation occurs first in the BM zone, and life of initiation was about several hundred thousand cycles, which was consistent with the experimental result. The MSC expansion rate in BM zone was the fastest, with the average expansion rate about 2.5 × 10⁻³ μm/cycle; the MSC expansion rate in the FZ and HAZ was about 3.1 × 10⁻⁴ μm/cycle.

Through the comparison of fatigue properties of components made of composite materials and high-strength structural steel materials, this study proves that composite materials can replace traditional steel materials used in the production of mechanical structural components. The focus of this study was a low-speed, high-torque radial piston motor mounted on a roadheader. According to different theories, the motor block was designed using a composite material made of carbon fiber, a classic high-strength structural steel, and an aluminum alloy. The thickness of the motor cylinder obtained by theoretical calculation was verified by finite-element numerical simulation technology, and the fatigue phenomenon caused by the time change of the piston cylinder pressure was considered. The results showed that the stress results of the numerical simulation verify the rationality of the theoretical calculation of the cylinder size. In terms of safety factors, the motor cylinder made of composite materials was close to the motor cylinder made of high-strength structural steel, and the difference between the static safety factor and fatigue safety factor was only 0.8 and 0.86. The weight of the motor cylinder made of composite material was reduced from 32 N to 7 N compared with steel material, which was about 78% lighter. This is of great significance for improving the use efficiency of equipment and reducing fuel costs.

Based on linear elastic fracture mechanics (LEFM), the fatigue crack evolution process and behavior of corrugated web girders were studied. The global finite element analysis (FEA) model of corrugated web girders was first developed and the equivalent structural stress method was used to reveal the dangerous locations along the weld under the bending load. The weld toe between the tension flange and the web weld, which is near the intersection of the inclined fold and the parallel fold, was determined as the fatigue crack easy-initiating location. Then a small region containing the crack-prone site was extracted as the sub-model for a crack propagating simulation. A semi-circle initial crack with 0.1 mm radius was inserted at the crack easy-initiating location. The stress intensity factors (SIFs; K_I, K_II, and K_III) of some discrete points along the crack front were calculated by the M-integral method. Based on Nasgro law, the geometry of the new crack front with a given extension length was obtained. Finally, the complete evolution process of the crack propagation was simulated. Results showed that the dominant crack propagating mode is open type (Mode I) and K_I is the most important propagating driving force. According to the crack front shape evolution, the whole propagating process was divided into 6 stages. An obvious kink of the crack was found in stage 1, which covered only a very short time. The stages 3, 4 and 5 accounted for the majority of life, among which the stage 3 accounted for as high as 81% of the total life. Therefore, the cycles of the weld toe crack propagating from 0.1 mm to the thickness of the flange can be defined as the prediction life of this kind of structures.

The real weld toe geometry is generally not mathematically perfect, resulting in obvious stress concentration effects, both on the weld section and along the longitudinal direction of the weld toe. The true stress-strain state at the local weld toe directly affects the fatigue performance and behavior of the welded structure. Therefore, a Fiber Bragg Grating (FBG) sensor based method for testing the cyclic strain at the weld toe was proposed. Cruciform welded joints were fabricated as specimens on which FBG sensors were arranged at several characteristic points along the weld toe curve. Strains at all the characteristic points under cyclic tensile load were measured and recorded, which showed the proposed measuring method could accurately obtain the complete local strain time histories along the weld toe. The strain time histories clearly reflected the cyclic hardening phenomenon in the early stage and the plastic yielding phenomenon in the final stage. Furthermore, based on the cyclic stress-strain constitutive model of the weld material, the stress-strain response curves of all the characteristic points were drawn. Combined with the fatigue fracture morphology, the mechanism of the unsynchronized initiation of the multiple cracks in the weld toe was investigated.