Measurement of deformation history during a high-speed mechanical test plays an important role in establishing the dynamic behavior of materials. Traditional strain measuring techniques such as extensometers and strain gauges have limitations such as frequency response and range of strain. Kevlar 49 fabrics were tested in tension within a strain-rate range of 25 to 170 s-1 using a high-speed servohydraulic testing system. Results show that the dynamic material properties in terms of Young’s modulus, tensile strength, maximum strain, and toughness increase with increasing strain rate. The woven nature of Kevlar 49 fabric results in large displacements and shape changes during tests. Noncontacting strain measuring technique is therefore highly preferred. A technique was developed using image analysis to obtain the deformation of Kevlar 49 fabrics at the tested strain rates. Using image analysis results, the stress-strain curves of Kevlar 49 fabrics at different strain rates were constructed and compared wi...

Dynamic tensile tests are conducted on aluminum alloy (AA) 6061-T6 using a high-speed servohydraulic machine at intermediate strain rates to validate the testing technique and to investigate the strain-rate effect on the material’s stress-strain behavior and failure mode. We present the experimental procedures and results discussing the constitutive response of the alloy at strain rates up to approximately 200 s-1. The predominant frequencies of the high-speed testing machine were characterized by modal analysis, and we analyzed the effect from vibration of the system and loading rate on flow stress by using a single degree-of-freedom (SDOF) spring-mass-damper model. We tested two different specimen sizes at a wide range of actuator velocities to achieve the desired strain rates. Results show that the yield strength, ultimate strength, and failure strain were dependent on strain rate. We fitted the data to the Johnson-Cook (JC) constitutive model, and the resulting parameters are comparable to published ...

The football helmet is used to help mitigate the occurrence of impact-related traumatic (TBI) and minor traumatic brain injuries (mTBI) in the game of American football. While the current helmet design methodology may be adequate for reducing linear acceleration of the head and minimizing TBI, it however has had less effect in minimizing mTBI. The objectives of this study are (a) to develop and validate a coupled finite element (FE) model of a football helmet and the human body, and (b) to assess responses of different regions of the brain to two different impact conditions – frontal oblique and crown impact conditions. The FE helmet model was validated using experimental results of drop tests. Subsequently, the integrated helmet–human body FE model was used to assess the responses of different regions of the brain to impact loads. Strain-rate, strain, and stress measures in the corpus callosum, midbrain, and brain stem were assessed. Results show that maximum strain-rates of 27 and 19 s⁻¹ are observed in the brain-stem and mid-brain, respectively. This could potentially lead to axonal injuries and neuronal cell death during crown impact conditions. The developed experimental-numerical framework can be used in the study of other helmet-related impact conditions.

The need for accurate material models to simulate the deformation, damage, and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. There are a variety of material models currently available within commercial transient dynamic finite-element codes to analyze the response of composite materials under impact conditions. However, there are several features that are lacking in the currently available models that could improve the predictive capability of the impact simulations. To address these needs, a combined elasto-plastic model with damage suitable for implementation within transient dynamic finite-element codes has been developed. A key feature of the improved material model is the use of tabulated stress-strain data in a variety of coordinate directions to fully define the stress-strain response of the material. Currently, the model development efforts have focused on creating...

Test procedures for characterizing the orthotropic behavior of a unidirectional composite at room temperature and quasi-static loading conditions are developed and discussed. The resulting data consisting of 12 stress–strain curves and associated material parameters are used in a newly developed material model—an orthotropic elasto-plastic constitutive model that is driven by tabulated stress–strain curves and other material properties that allow for the elastic and inelastic deformation model to be combined with damage and failure models. A unidirectional composite—T800/F3900, commonly used in the aerospace industry, is used to illustrate how the experimental procedures are developed and used. The generated data are then used to model a dynamic impact test. Results show that the developed framework implemented into a special version of LS-DYNA yields reasonably accurate predictions of the structural behavior.