Sandia Fracture Challenge: blind prediction and full calibration to enhance fracture predictability

The Impact and Crashworthiness Lab at Massachusetts Institute of Technology participated in the Sandia Fracture Challenge and predicted the crack initiation and propagation path during a tensile test of a compact tension specimen with three holes (B, C, and D), using a very limited number of material properties, including uniaxial tensile tests of a dog-bone specimen. The maximum shear stress and modified Mohr–Coulomb fracture models were used. The predicted crack path of A–C–E coincided with two out of thirteen experiments performed by Sandia National Laboratories, and the maximum load, as well as the load level at the first and second crack initiation, was accurately captured. However, the crack-tip opening displacements (CODs) corresponding to the initiation of the two cracks were overestimated by 12 and 24 %, respectively. After the challenge ended, we received the leftover material from Sandia and did full plasticity and fracture calibration by conducting extra fracture tests, including tensile tests, on a specimen with two symmetric round notches, a specimen with a central hole, and a butterfly specimen with double curvature. In addition, pure shear tests were carried out on a butterfly specimen. Newly identified fracture parameters again predicted the A–C–E crack path, but the force–COD response could be reproduced almost perfectly. Detailed calibration procedures and validation are discussed. Furthermore, in order to investigate the influence of the machining quality on the results, a pre-damage value was introduced to the first layer of finite elements around the starter notch, A, and the three holes, B, C, and D. This accelerated shear localization between holes A and D (and between D and C as well) and changed the crack path to A–D–C–E. Parametric study on the pre-damage value showed that there exist two competing crack paths, and the corresponding force–COD curve is influenced by the pre-damage value. The effect of mesh size and boundary conditions are also discussed.     

Principles and droplet size distributions of various spraying methods: a review

Spraying refers to the process of forming small droplets of a liquid and dispersing them in air or on the surface of an object. Spraying is ubiquitous because it enables a liquid widely and uniformly dispersed. The existing spraying methods can be broadly categorized as ultrasonic, liquid compression, and two-phase nozzle-based. These different methods utilize different principles, with advantages and disadvantages. Thus, it is necessary to understand how different methods are suited to the applications of interest. In particular, the droplet size plays a significant role. Many studies have been conducted to characterize different spraying methods, but little systematic organization and summarization with respect to the droplet size has been done. Herein, we introduce the most widely used spraying methods and explain their spraying principles, fields of use, and appropriate operating environments. We focus on the droplet size aspect. This review is likely to be useful in the design and development of spraying devices.