Steel platform trolleys, a common load-carrying vehicle in logistics, experience stress distribution patterns influenced by load magnitude, location, and structural form. When unloaded or lightly loaded, stress concentration in the frame primarily occurs at the axle-frame connection. These areas experience abrupt changes in stress lines due to geometrical changes, resulting in localized stress peaks. During this period, the frame is in an elastic deformation phase. After unloading, the structure fully recovers, and stress distribution is relatively uniform. However, residual stress at welded joints, combined with external loads, can significantly increase local stress levels.
As load increases, the stress distribution pattern in steel platform trolleys changes significantly. The lack of support on the platform surface generates bending stresses, creating new stress concentration areas at the intersection of the frame's crossbeams and longitudinal beams. At this point, the material gradually enters an elastic-plastic deformation phase, increasing deflection in the center of the platform, leading to significantly higher stress levels in this region compared to the edges. If the load is offset, the frame undergoes a more complex stress state due to a combination of torsional and bending deformations. Stress increases on one longitudinal beam, while compressive stress zones appear on the other. This unevenness worsens with increasing eccentricity.
When the load approaches the rated value, a steel platform trolley enters a critical load-bearing state, exhibiting three typical stress distribution characteristics: Constraints at the four corners of the frame create biaxial tensile stress fields, reaching values that can reach a high percentage of the material's yield strength; triaxial stresses at the axle-frame connection cause localized plastic hardening of the material; and bending deformation on the platform surface produces a wavy stress distribution, with significant stress differences between adjacent crests and troughs. This complex stress state causes irreversible changes in the material's microstructure, creating a potential for fatigue crack initiation.
Changes in the load application location significantly influence the stress distribution in a steel platform trolley. Under central load, the frame exhibits symmetrical bending deformation, with stresses distributed parabolically along the longitudinal beams. Eccentric loads induce a combination of torsion and bending, significantly increasing stress on one longitudinal beam and creating a compressive stress zone on the other. When the load application point approaches the edge of the frame, local stresses can reach several times the design value, inducing significant plastic deformation. This non-uniformity can accelerate structural fatigue damage under frequent eccentric loading conditions.
The structural geometric parameters of a steel platform trolley have a crucial influence on stress distribution. Increasing the spacing between crossbeams reduces the platform's bending stiffness, increasing mid-section deflection and significantly increasing bending stress. Reducing the cross-sectional height of longitudinal beams reduces bending stiffness, forcing more load to be transmitted through shear forces, leading to shear stress concentration. Furthermore, transition areas with excessively small fillet radii can experience severe stress concentrations, with stress concentration factors reaching high levels, creating high-risk areas for structural failure.
The impact of material properties on the long-term stress response of a steel platform trolley cannot be ignored. Cold-rolled steel sheets exhibit significantly lower fatigue life than annealed steel due to residual tensile stresses. Surface sandblasting creates a compressive stress layer that can extend the crack initiation life. Under alternating loads, the dislocation density in the material microstructure continuously increases, leading to localized increases in hardness and decreases in plasticity reserve. This conflicting characteristic exacerbates the risk of brittle fracture in stress-concentrated areas.
From a design optimization perspective, adding hem flanges to the steel platform trolley improves bending stiffness and reduces stress levels in the center. Adopting a variable-section longitudinal beam design improves material utilization while keeping maximum stress within a reasonable range. Numerical simulations show that a rational rib layout significantly improves the stress uniformity coefficient. These improvements, based on a deep understanding of stress distribution patterns, demonstrate the critical role of mechanical principles in engineering practice.
