This work was sponsored by the US Army’s Natick Soldier Systems Center to investigate additively manufactured lattice structures for improved blunt impact protection for helmets. The idea is simple enough, modern helmets are designed to deflect or mitigate the impact forces due to bullets (high velocity) but not so much for blunt force impacts (lower velocity). In military operations, blunt force impacts are common, albeit sometimes accidently, due to falls or in the rush to enter-exit buildings and vehicles. In combat, flying debris also present challenges to helmet designers where the impacts can be both high- and low-velocity. Our work was to set the foundation for the exploration of polymeric 3D lattice structures to create the next generation of energy-absorbing helmet liners for military applications. Current foam liners, whether multi-layer or sculptured, all exhibit more-or-less the same energy-absorbing response which is fine for high-energy impacts but lacks the sensitivity for low-energy impacts. If one can move away from the use of foam and toward that of a 3D polymeric lattice structures, then it should be possible to engineer a helmet liner to have a more variable or tailored energy-absorbing response. To create such structures, the additive manufacturing process was used.
The applications for fiber reinforced plastic (FRP) composite structures have increased tremendously in the automotive and aerospace industries due to their lightweight nature. However, because of their high manufacturing cost, composite structures are typically only used for high-end parts. The reason behind this is the relatively low mass production rate of composite structures. Among the various composite manufacturing methods available, pultrusion is a continuous production process, meaning that the potential for mass production is there, if the process can be made fast enough. The process of pultrusion is defined as extrusion with pulling, in contrast with the conventional ‘extrusion process’, which is used for manufacturing uniform cross section structures such as circular bars, hollow tubes, I section beams etc. [1] [2]. Currently, pultrusion has a wide range of applications in the “architecture, transportation, construction, agriculture, chemical engineering, aircraft, and aerospace industry” [2]. On the basis of the polymer used in the manufacturing process, pultrusion can be divided into two types namely: thermoset and thermoplastic pultrusion. Many studies in the past have been conducted on thermoset pultrusion whose main advantage over thermoplastic pultrusion is the fiber impregnation, or ‘wetting out of the fiber reinforcement’ due to the resin’s low viscosity [3]. On the other hand, thermoplastic pultrusion can create parts which are recyclable, post formable, weldable, have excellent environmental stability and good mechanical properties such as high “fracture toughness, higher damage resistance” [1]. Due to such economic, environmental and mechanical advantages, many researchers have contributed to the development of thermoplastic pultrusion mainly in the field of fiber impregnation with thermoplastic resins [1] [4] [5]. With the advancement in thermoplastic prepreg technologies, pultrusion experiments with pre-consolidated tapes (PCT), powder coated tow-pregs and commingled yarns were performed and studied [1]. Moreover, in the early 1990s, thermoplastic pultrusion models were developed by researchers in order to understand the workings of the process [3] [6]. Most of the current research is focused mainly on investigating the effects of process and material parameters on the mechanical performance of the pultruded part. However, the interrelationship between the materials, process, and product is still not fully understood or has been incorporated into a complete CAE processing chain. The development of analytic, computational, and experimental approaches continues and the need of a fully developed simulation model, which can be used to optimize process parameters, avoid a trial and error approach and to improve productivity still exists.
Taking into account the strain and thickness distributions of cold formed parts is well established in LS-DYNA. Additionally, *MAT_TAILORED_PROPERTIES offers the possibility to use a tabulated set of flow curves dependent on a history variable. The physical quantity represented by the chosen history variable can be defined by the user. Getting the distribution of this history variable may be a difficult task. For press-hardening simulation exclusively, LS-DYNA offers *MAT_244/248 to calculate the distribution of mechanical properties based on the distribution of metallurgical phases. The phase distribution is a result of the thermo-mechanically coupled simulation of the production process. To overcome the limitations of these two material models, *MAT_GENERALIZED_PHASECHANGE was implemented. This material has been used successfully for the simulation of press-hardening, welding and 3D-Printing. The current work presents a new field of application for *MAT_GENERALIZED_PHASECHANGE, simulating the “bake-hardening”-effect of specific aluminium alloys. The local final strength of hardenable aluminum alloys for automotive applications depends on the local pre-strain from the forming process and the local time-temperature-profile during paint bake. An initial approach to model this behavior is given. Implemented extensions to *MAT_GENERALIZED_PHASECHANGE, which enable are more precise description of the underlining mechanisms, will be shown.
In the recent past, a lot of effort has been made towards the closing of the simulation process chain for all different kinds of materials. Besides the regular transfer of resulting stress, strain, and history data, main focus from a material’s perspective has been on the transfer of fiber orientations from process simulations for continuous fiber reinforced composites [1] together with various homogenization approaches for short fiber reinforced plastic materials [2].
The ITEA VMAP project aims to gain a common understanding of and interoperable definitions for virtual material modelling in Computer Aided Engineering (CAE). Using industrial use cases from major material domains and representative manufacturing processes, new concepts will be created for a universal material exchange interface for use in virtual engineering workflows. [1] In the VMAP consortium with nearly 30 partners, 4a is focusing on injection molding of plastics. Two sub use cases – namely impact behavior of fiber reinforced thermoplastics and structural behavior of foamed parts - are presented in this contribution.
Impact Analysis of Polymeric Additive Manufactured Lattice Structures