Tomasz Jankowiak

Taylor’s Test Technique for Dynamic Characterization of Materials: Application to Brass

The article reports experiments using the Taylor test to define the dynamic behavior of brass. This material does not exhibit strain rate dependency therefore it allows the validation of an analytical description of flow stress as a function of strain with strain hardening for the data obtained from the Taylor test. The experiment is used to study the process of damage and fracture with fragmentation which is dependent on the impact velocity. Using the experimental data a numerical model of the Taylor test is developed to determine the strain level and the strain gradient along the specimen.

Verification of a Thermoviscoplastic Constitutive Relation for Brass Material Using Taylor's Test

This paper presents a methodology to define and verify the dynamic behavior of materials based on Taylors test. A brass alloy with a microstructure composed mainly of two pure metals that have two different crystal structures, copper (face-centered cubic (fcc)) and zinc (hexagonal closed-packed (hcp)), is used in this study. A combined approach of different principal mechanisms controlled by the emergence and evolution of mobile dislocations as well as the long-range intersections between forest dislocations is, therefore, adopted to develop accurate definition for its flow stress. The constitutive relation is verified against experimental results conducted at low and high strain rates and temperatures using compression screw machine and split Hopkinson pressure bar (SHPB), respectively. The present model predicted results that compare well with experiments and was capable of simulating the low strain rate sensitivity that was observed during the several static and dynamic tests. The verified constitutive relations are further integrated and implemented in a commercial finite element (FE) code for three-dimensional (3D) Taylors test simulations. A Taylors test enables the definition of only one point on the stress–strain curve for a given strain rate using the initial and final geometry of the specimen after impact into a rigid surface. Thus, it is necessary to perform several tests with different geometries to define the complete material behavior under dynamic loadings. The advantage of using strain rate independent brass in this study is the possibility to rebuild the complete process of strain hardening during Taylors tests by using the same specimen geometry. Experimental results using the Taylor test technique at a range of velocity impacts between 70 m/s and 200 m/s are utilized in this study to validate the constitutive model of predicting the dynamic behavior of brass at extreme conditions.

A Study on Reduction of Friction in Impact Compressive Test Based on the Split Hopkinson Pressure Bar Method by Using a Hollow Specimen

The split Hopkinson pressure bar (SHPB) technique is widely-used to describe the impact compressive behavior of different materials including metals. During the impact test, the specimen deforms in a wide range of impact strain rate from 102 to 104 s-1. It is a reason why the method is studied for many years even though the structure of the apparatus based on the SHPB is simple. Actually, the cylindrical specimens are widely used for a compressive test and it is clearly seen that stress measured by the test includes the increment of stress (an error) derived by friction effect between a specimen and pressure bars. Therefore, it is important that the measured stress should indicate similar value as the proper stress of the material by reducing friction effect during not only quasi-static but also the impact test. Various attempts to reduce a friction effect in past have been conducted. A method to reduce friction effect is in general a use of lubricants. However, it is ineffective because it can be considered that this method contributes to an attenuation of the stress wave for obtaining the stress-strain curve under impact loading. Thus, rise time of waves obtained by the experiment becomes longer compared with a case not to use lubricants. Recently, a study can be found using a ring specimen, however, the determined thickness of the specimen is quite thin and it can be considered that a buckling effect cannot be vanished. In this study, a use of hollow specimen is suggested to solve the problem related to reduce the friction effect by decreasing a contact area between a specimen and pressure bars instead of a cylindrical specimen. The compressive experiments at various strain rates are conducted by using a hollow specimen.

Safety of Concrete and Masonry Structures under Unusual Loadings

In the paper the behavior of selected brittle materials and structures (concrete and masonry) subjected to explosive loadings is discussed. For concrete the accepted Cumulative Fracture Criterion (CFC) is exposed. It describes the degradation of the material under fast dynamic processes accompanied by the strong waves propagation phenomenon and large strain rates of deformation. To overcome the computational difficulty in the analyses of such complex problems, the sub-modeling technique as well as splitting of the calculations into two separate parts: analysis of acoustic wave in the air and the propagation of stresses in structures, were used. Some instructive numerical examples of concrete and masonry walls are in focus of the presentation. The numerical tools and computer simulations allow for proper estimation of the structures safety and for taking the design decisions on how to ensure their expected strength.

Dynamic Behavior of Materials. Constitutive Relations and Applications

In this chapter a particular attention has been directed on the dynamic behavior of materials and structures subjected to dynamic loading. Based on experimental observations, it is clear that the homogeneous material behavior of metals has several non linearities related for example to the strain rate and temperature sensitivity. Therefore, the material description and more precisely the constitutive relation used during numerical simulations for example must include all macroscopic observations. It has to be noticed that constitutive relations described in this chapter are defined in a macroscopic scale. Considering some examples, it is clear that the constitutive relation that was used is the key point to simulate a global problem allowing to avoid as frequently used some numerical tricks to obtain an agreement between experiments and numerical predictions. Following the concept described in this part, the reader will be available to propose new models to fit precisely their own materials for specific applications.