Experimental determination and numerical prediction of the dynamic forming limits of a press hardened steel

Material characteristics such as yield strength, failure strain, strain hardening and strain rate sensitivity parameter are affected by loading speed. Therefore, the strain rate dependency of materials for plasticity and failure behavior is taken into account in crash simulations. Moreover, a possibility for consideration of instability at multi-axial dynamic loadings in crash simulations is the use of dynamic forming limit curves (FLC). In this study, the dynamic FLC of the press hardened automotive steel Usibor 1500 (AlSi coated 22MnB5) is investigated. The experimental results are obtained from a unique high-speed Nakajima setup. Two models are used for the numerical prediction. One is the numerical algorithm CRACH as part of the modular material and failure model MF GenYld+CrachFEM 4.2. Furthermore, the extended modified maximum force criterion considering the strain rate effect is also used to predict the dynamic FLC. The comparison of the experimental and numerical results are presented and discussed.

Optimal control of a rocket projectile according to the "minimum force" criterion

The article solves the problem of optimal control of a rocket projectile by its delivery from a given initial position to a given final position, taking into account the air resistance force. The motion of the projectile is described by the vector differential equation of I.V. Meshchersky. The control quality criterion is taken in the form of "minimum force", the minimization of which ensures minimal overloads for the projectile. Three types of the norm of the control force vector are considered. For each of them, an optimal control is obtained that solves the task. The analysis of the results of the numerical experiment is carried out, confirming the general theoretical provisions.

Crack Resistance Increase of Bushings while Assembling Joints with Tension

In order to restore worn parts during repair work, it is often used to press the repair bushings. While assembling joints, cracks may form in the spanning part (bushing), as it may have surface and structure defects. Therefore, an urgent task is to increase the crack resistance of joints with tension during its assembly. The paper examines the influence of the geometry of the spanning part on the process of cracking. To assess the effect of defects on the crack formation, the method of modeling defects with a surface crack of a semi elliptical shape was used. The crack resistance is estimated using the force criterion of fracture mechanics – the stress intensity coefficient. As a result, it was found that the ratio between the wall thickness of the enclosing sleeve and its outer diameter has a significant effect on the crack resistance of the joint with tension, which allows reducing the origin probability and crack development during its pressing by varying these parameters. While assembling joints, the following methods are often used: mechanical-using a press and thermal with heating of the covering part. When the method of joint with cooling of the covered part, the greatest strength of the joint is achieved, but the probability of cracking increases. This is because when the temperature of the enclosing sleeve decreases in contact with the cooled shaft, the critical stress intensity coefficient (fracture toughness), which is a mechanical characteristic of crack resistance, decreases. To reduce the effect of cooling the sleeve, you can use a combined method of assembling the joint, in which the covered part is cooled and the covering part is heated. It is shown that to assess the fracture resistance at low temperatures, it is optimal to conduct full-scale tests, where the cooling and heating temperatures of the press joint parts are the experimental factors.

Cobot with Prismatic Compliant Joint Intended for Doppler Sonography

This paper deals with a collaborative robot, i.e., cobot, coupled with a new prismatic compliant joint (PCJ) at its end-effector. The proposed collaborative solution is intended for Doppler sonography to prevent musculoskeletal disorders issues. On one hand, the Doppler sonographer’s postures are investigated based on motion capture use during the arteries examination. This study highlighted that configurations adopted by angiologists lead to the musculoskeletal disorder. On the other hand, the proposed PCJ with variable stiffness gives an intrinsic compliance to the cobot handling the probe. This feature allows preserving the human safety when both human and cobot share a common workspace. The effectiveness of the proposed solution is experimentally validated through a 7-DoF Franka Emika robot virtually coupled with the PCJ, during the execution of a trajectory performed during a Doppler ultrasound exam. The impact force criterion is considered as a safety performance.

A Graphical Method to Estimate Forming Limit Curve of Sheet Metals

Since its foundation, the concept of forming limit diagram has been widely accepted in sheet metal forming community as a powerful tool for studying formability. There are pyramid models that were developed to estimate the forming limit curve theoretically, for example, Swift's diffuse necking criterion, Hill's localized necking criterion, Marciniak and Kuczynski model, Modified Maximum Force Criterion, etc.. Implement of these models, however, is a laborious task. To simply the task, this study presents a graphical method to estimate forming limit curve of sheet metal. Some new insights into the Modified Maximum Force Criterion, the Hora method, are discussed. The insights pertain to the use of a graphic tool to estimate limit strains at three critical forming modes in sheet metal forming that are the uniaxial tension, plane strain, and equi-biaxial tension. Connecting three points by linear lines yields to a simple graph of forming limit curve. Method validation is supported by comparing the estimated forming limit curve with experimentally measured data for several automotive sheet metals.

Additional Injury Prevention Criteria for Impact Attenuation Surfacing Within Children's Playgrounds

More than four decades have passed since the introduction of safety standards for impact attenuation surfaces (IAS) used in playgrounds. Falls in children's playground are a major source of injuries and IAS is one of the best methods of preventing severe head injuries. However, the ability of IAS in prevention of other types of injuries, such as upper limb fractures, is unclear. Accordingly, in this paper, ten synthetic playground surfaces were tested to examine their performance beyond the collected head injury criterion (HIC) and maximum G-force (Gmax) outputs recommended by ASTM F1292. The aim of this work was to investigate any limitations with current safety criteria and proposing additional criteria to filter hazardous IAS that technically comply with the current 1000 HIC and 200 Gmax thresholds. The proposed new criterion is called the impulse force criterion (If). If combines two important injury predictor characteristics, namely: HIC duration that is time duration of the most severe impact; and the change in momentum that addresses the IAS properties associated with bounce. Additionally, the maximum jerk (Jmax), the bounce, and the IAS absorbed work are presented. HIC, Gmax, If, and Jmax followed similar trends regarding material thickness and drop height. Moreover, the bounce and work done by the IAS on the falling missile at increasing drop heights was similar for all surfaces apart from one viscoelastic foam sample. The results presented in this paper demonstrate the limitations of current safety criteria and should, therefore, assist future research to reduce long-bone injuries in playgrounds.

Evaluation of the dynamic loads impact on the conditions of drill string failure during tripping operations

The conditions of drill string failure during tripping operations with external or internal transverse annular cracks were defined. At the same time, considering the experimental evaluation of the force criterion for failure of metal of backup and operated drill string pipes, the interrelationship between the depth of the critical external or internal transverse annular cracks in drill string pipes with the weight of the drill string was established, considering the effect of dynamic loads during tripping operations. It was determined that dynamic loads during run-in-hole operations are more dangerous at depths of more than 3.7 km, while at lower depths, the dynamic loads occurring during pulling out of string are considered more dangerous. It was demonstrated that, when pulling out operated drill strings, internal transverse annular cracks at depths exceeding 1.1 km, are more dangerous than external ones, while at depths up to 1.1 km the external transverse cracks are more dangerous.

Head, chest and femur injury in teenage pedestrian–SUV crash; mass influence on the speeds

This work studies the teenage pedestrian–sport utility vehicle (SUV) crash; injury to the vital parts of the body, such as the head and chest, and to the femur is evaluated. More advanced injury criteria are applied, as provided in the rules. The multibody technique is applied by making use of SimWise software and of the teenager anthropomorphic model, the use of which is now consolidated. Head injury criterion (HIC) is used for the head, thoracic trauma index (TTI) criterion for the thorax in the case of side impact and 3 ms criterion in the case of frontal impact, while the force criterion is used for the femur. Both the TTI and femur load evaluation require non-substantial modifications of the dummy, by insertion of sensors for the measurement of the acceleration of the 4th rib and the 12th vertebra and two very thin plates at the knees for the correct individuation of the contact point with the vehicle bumper. Particular attention is paid to the front shape of the vehicle, concluding that the SUV examined in this paper is less dangerous than the sedan studied in a previous work, since its frontal dimensions (bonnet angle, bumper height and bonnet height) are more advantageous. However the teenage pedestrian in a lateral position is less prone to injuries in the head and chest, with respect to the frontal position; the pedestrian’s position has little influence on femur damage. Furthermore, the braking of the vehicle reduces the possibility of crash fatality. In conclusion, a theoretical approach is shown, to highlight the influence of the vehicle mass on the pedestrian speed after the impact.