Dynamic Impact Loading Conditions Research Articles

Brittle fracture development through sets of preexisting small-scale cracks under quasi-static and dynamic impact loading

Fracture development and its possible relation with wave motion in a brittle rectangular solid material with sets of preexisting small-scale parallel cracks are investigated. Experimental observations using high-speed cameras under external quasi-static tensile or dynamic impact loading conditions indicate that depending on the inclination angle, length and distribution pattern of cracks, fractures evolve considerably differently. Quasi-static load generally produces dynamic or quasi-static primary fracture with ensuing dynamic secondary retrograde fractures that are initiated at distant positions. Fractures can be arrested and jump easily, often resulting in clusters. Impact-related waves can induce primary fracture moving in both prograde and retrograde directions.

Fatigue life modelling of railway prestressed concrete sleepers

Railway sleepers, which transfer wheel loads to the formation, are an essential component of railway track systems. Prestressed concrete is the most commonly used type of railway sleepers around world. Crushing is a common damage (under resultant compressive stress) in concrete sleepers, which is attributable to excessive flexural, shear, pre-stress and bond actions. Dynamic impact loading conditions are often the cause of failure in the concrete sleepers. However, accumulated damage due to cyclic loads can also cause concrete crushing. Most previous research has investigated the impact load characteristics and the ultimate limit states of prestressed concrete railway sleepers. There is a knowledge gap with respect to serviceability limit states and fatigue failure of prestressed concrete sleepers. This study presents new results of extensive numerical, analytical, and experimental investigations aimed at predicting fatigue lives under cyclic loads. A numerical study validated by 30 full-scale experimental tests is executed and extended to assess fatigue limit states, while theoretical fatigue analysis methods based on S-N curve and Miner linear cumulative damage are additionally introduced for benchmarking. This paper is the world's first to demonstrate a remaining fatigue life assessment for prestressed concrete sleepers and benchmark with the theoretical methods. Parametric studies are conducted to unprecedentedly illustrate influences of support conditions, dynamic load distribution, and track stiffnesses on the remaining fatigue lives of prestressed concrete sleepers. This paper highlights the rationales associated with the development of fatigue limit states. The insights will provide design flexibility and improve railway sleeper maintenance and inspection criteria.

Energy Absorbing Properties Analysis of Layers Structure of Titanium Alloy Ti6Al4V during Dynamic Impact Loading Tests.

This paper presents the testing methodology of specimens made of layers of titanium alloy Ti6Al4V in dynamic impact loading conditions. Tests were carried out using a drop-weight impact tower. The test methodology allowed us to record parameters as displacement or force. Based on recorded data, force and absorbed energy curves during plastic deformation and sheet perforation were created. The characteristics of the fractures were also analyzed. The impact test simulation was carried out in the ABAQUS/Explicit environment. Results for one, two, and three layers of titanium alloy were compared. The increase in force required to initialize the damage and the absorbed energy during plastic deformation can be observed with an increase in the number of layers. The increase in absorbed energy is close to linear. In the simulation process, parameters such as Huber–Mises–Hencky stress value, equivalent plastic strain, temperature increase, and stress triaxiality were analyzed.

Structural response and performance of hexagonal thin-walled grooved tubes under dynamic impact loading conditions

This paper numerically investigates the structural response and crashworthiness performance of a hexagonal thin-walled grooved tube subjected to axial and oblique impact loading conditions. First, an analytical formulation that estimates the mean crushing force and total energy absorption are obtained for both hexagonal and circular tubes subjected to high dynamic impact loading conditions. The solutions of the analytical model are used to verify and compare the approximate solutions of the finite element model. The inclusion of grooves to the conventional hexagonal thin-walled tubes shows the great potential for improving the crashworthiness performance such as the energy absorption capacity, crushing force efficiency and specific energy absorption. In further, parametric studies are performed, deformation modes of different models are obtained and their crushing force-displacement characteristics are described. The improved crashworthiness performance of the hexagonal thin-walled grooved tubes make them good candidate used as energy absorbers.

The role of force chains in granular materials: from statics to dynamics

This paper investigates the role of force chains in granular materials under both quasi-static biaxial loading and dynamic impact loading conditions. This study is mainly interested in investigating the role of force chains on the global mechanical responses of samples as well as on the microstructure of granular materials. For this purpose, discrete element methods are adopted to perform different loading tests. Granular materials are modelled as aggregations of poly-disperse spherical particles. Results first show that force chains are responsible for the strength of the sample: in 2D biaxial test, the establishment and buckling of force chains are responsible for the evolution in terms of deviatoric stress; in 3D dynamic impact test, the development and the buckling of force chains are responsible for the evolution of impact force on the impacting projectile. Second, force chains are found to play an important role in the microstructure of granular materials: in statics, the buckling of force chains mainly locates inside the shear band, and is related to the increase in local void around the buckling grain as well as the increase in kinetic energy inside the sample; in dynamic test, the number of buckled force chains is correlated with the energy dissipation inside the sample.

Lightweight Hybrid Electrical Vehicle Structural Topology Optimisation Investigation Focusing on Crashworthiness

As focus on the world climate rises, so does the demand for ever more environmentally friendly technologies. The response from the automotive industry includes vehicles whose primary propulsion systems are not based upon fossil fuels. On this basis a Low Carbon Vehicle Technology Project, partly funded by the European Regional Development Fund, is currently under way; part of this project involves designing a lightweight Body In White (BIW). This has been specifically tailored to suit the drive train and general packaging requirements associated with a Hybrid Electric Vehicle (HEV). The future opportunities for new lightweight vehicle architecture have been investigated using a technique entitled topology optimisation, which extracts the idealised load paths for a given loading. The topology optimisation includes equivalent NCAP dynamic impact loading conditions, as well as torsional rigidity performance. Initially a total of 7 loading scenarios are applied on a structure comprising of various battery and range extender layouts. Two different optimisation modelling techniques have been undertaken comparing conventional boundary conditions against inertia relief, as well as studying the sensitivity of the BIW topology against the influence of load case direction and battery box stiffness. Optimal locations for the two components having the highest mass, i.e. a single battery pack and a combined range extender and fuel tank have been studied focusing upon the effects of the location of their Centre of Mass. It has been assumed that advances in battery technology will reduce the external dimensions of the battery package, thereby enabling an increased number of possible locations within the BIW.

Bicycle helmet retention system testing and evaluation

Bicycle helmet testing is conducted under laboratory conditions, which do not always reflect real-world conditions. This paper reports on a survey of chin strap use among a test group of bicycle riders and the results of three tests: (1) a retention harness system static test to determine elongation properties; (2) a loop and position device test to determine the effects of the combination of the webbing positioner and the elongation properties of the webbing; and (3) a dynamic test to demonstrate the compatibility of the helmet retention system under real-world type dynamic impact loading conditions. The survey showed that bicyclists wear their helmets with an average of 4.76 cm of strap clearance when tested with a 500 gram load. Tests of chin strap shows that substantial motion and elongation can occur under nominal loading conditions. Comparative dynamic testing resulted in a bicycle helmet rotating forward significantly and exposing the back of the dummy head, while an equestrian helmet with a different retention system design rotated only slightly forward and remained on the dummy head. These tests demonstrate that effective helmet retention testing must take into account consumers’ normal use of helmets, and how all the elements in the helmet and its retention system work together.