Brian T. Werner

A Constitutive Model for Fiber-reinforced Polymer Composites

A constitutive model for fibrous polymer composites was established based on an elastic—plastic approach. The proposed potential function is a linear combination of functions related to deviatoric and dilatational deformations. A unidirectional carbon/epoxy composite, AS4/3501-6, and a woven-glass/vinylester composite were fabricated and tested under quasi-static off-axis tension and compression and used in developing and verifying the model. Model parameters were determined from the off-axis test results. Stress—strain curves predicted by the model were in good agreement with experimental results. The constitutive model established can describe the nonlinear orthotropic mechanical behavior of the composites, and it accounts as well for their tension—compression behavior.

Characterization and Constitutive Modeling of Composite Materials under Static and Dynamic Loading

Composite materials were characterized under quasi-static and dynamic loading and a constitutive model was adapted to describe the nonlinear multi-axial behavior of the materials under varying strain rate. The materials investigated were unidirectional glass fiber/vinylester, and carbon fiber/epoxy composites. Multiaxial static and dynamic experiments were conducted using off-axis specimens to produce stress states combining transverse normal and in-plane shear stresses. Stress-strain curves were obtained for various loading orientations with respect to the fiber direction at three strain rates, quasi-static, intermediate and high strain rate. A nonlinear constitutive model is proposed to describe the rate-dependent behavior under states of stress including tensile and compressive loading. Experimental results were in good agreement with predictions of the proposed constitutive model.

Deformation and Failure of Angle-Ply Composite Laminates

Angle-ply carbon/epoxy composite laminates were tested under uniaxial compression at two strain rates (10−4 and 1 s−1). In this study, failure is defined as maximum ply damage. This is determined by noting where the stress–strain curve of the laminate reaches a characteristic damage state and the stress–strain behavior reaches a terminal modulus. The characteristic damage state stress is compared to predictions from the recently introduced Northwestern (NU) theory at each strain rate. Residual and interlaminar stresses were considered in the analysis and experiment design. Most classical theories fail to predict the exact failure behavior of the laminates. The NU theory is in good agreement in most cases with the exception of cases where the state of stress includes a non negligible stress component in the fiber direction. However, for determining lamina failure due to matrix dominated failure modes, the NU theory is in excellent agreement with experimental results.

Strain Rate Effects on Failure of a Toughened Matrix Composite

The objective of this study was to characterize the quasi-static and dynamic behavior of a toughened matrix composite (IM7/8552) and apply the Northwestern (NU) failure theory to describe its strain-rate dependent failure under multi-axial states of stress. Unidirectional and off-axis experiments were conducted at two strain rates, quasi-static (10−4 s−1) and intermediate (~1s−1) using a servo-hydraulic testing machine. Stress–strain curves were obtained and the nonlinear response and failure were measured and evaluated based on classical failure criteria and the NU theory. Predicted failure envelopes were compared with experimental results. The NU theory was shown to be in excellent agreement with experimental data.

Characterization and Modeling of Polymeric Matrix Under Static and Dynamic Loading

A polymeric matrix (3501-6) used in composite materials was characterized under multi-axial loading at strain rates varying from quasi-static to dynamic ones. Tests were conducted under uniaxial compression, tension, pure shear and combinations of compression and shear. Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted using a split Hopkinson Pressure Bar (Kolsky bar) system built for the purpose. This SHPB system was made of a glass/epoxy composite (Garolite) bars having an impedance that is closer to that of the test polymer than metals. The typical stress-strain behavior of the polymeric matrix exhibits a linear elastic region up to a yield point, a nonlinear elastoplastic region up to an initial peak or “critical stress,” followed by a strain softening region up to a local minimum and finally, a strain hardening region up to ultimate failure. A general three-dimensional elasto-viscoplastic model was developed incorporating strain rate effects, and including the large deformation region. The model was formulated in strain space unlike most models in the literature. The stress-strain curves obtained were used to develop and validate the new elasto-viscoplastic constitutive model.

Effect of Ply Dispersion on Failure Characteristics of Multidirectional Laminates

The influence of layer thickness and ply interdispersion was investigated on the failure characteristics of multidirectional laminates. The composite material investigated was a high strength carbon fiber composite with a toughened epoxy matrix (IM7/8552). The basic unidirectional material was fully characterized in a previous study under static and dynamic loading conditions. In this study the investigation was extended to angle-ply laminates with varying layer thickness while including the effects of residual stresses. Intralaminar and interlaminar failure mechanisms were observed and found to be strongly related to the layer thickness for the same layup. For thin layer thicknesses failure modes included fiber breaks resulting in higher ultimate strengths. For thicker layers consisting of multiple stacked parallel plies, more matrix dominated intralaminar and interlaminar failures were observed resulting in lower ultimate strengths. This trend reaches a lower limiting plateau as the layer thickness increases. Failure modes and ultimate strengths were further investigated as a function of strain rate. All results were evaluated by the recently developed Northwestern (NU) failure theory.

Compression Testing of Aged Low Density Flexible Polyurethane Foam

Flexible open celled foams are commonly used for energy absorption in packaging. Over time polymers can suffer from aging by becoming stiffer and more brittle. This change in stiffness can affect the foam’s performance in a low velocity impact event. In this study, the compressive properties of new open-cell flexible polyurethane foam were compared to those obtained from aged open-cell polyurethane foam that had been in service for approximately 25 years. The foams tested had densities of 10 and 15 pcf. These low density foams provided a significant challenge to machine cylindrical compression specimens that were 1 “in height and 1” in diameter. Details of the machining process are discussed. The compressive properties obtained for both aged and new foams included testing at various strain rates (0.05. 0.10, 5 s−1) and temperatures (−54, RT, 74 °C). Results show that aging of flexible polyurethane foam does not have much of an effect on its compressive properties.

Effect of Threaded Joint Preparation on Impact Energy Dissipation Using Frequency-Based Kolsky Bar Analysis

Threaded joints are used in a wide range of industries and are relied upon in maintaining component assembly and structural integrity of mechanical systems. The threads may undergo specific preparation before assembly in applications. In order to ensure a tight seal the threads may be wrapped with PTFE tape or to prevent loosening over time an adhesive (thread locker) may be used. When a threaded joint is subjected to impact loading, the energy is transmitted through the joint to its neighbors while part of it is dissipated within the joint. In order to study the effect of the surface preparation to the threads, steel and aluminum joints were tested with no surface preparation, application of PTFE tape, and with the use of a thread locker (Loctite 262). The tests were conducted using a Kolsky tension bar and a frequency based analysis was used to characterize the energy dissipation of the various thread preparations on both steel/steel and steel/aluminum threaded joints.

Mechanical and failure behavior of composite materials under static and dynamic loading

Composite materials were characterized under quasi-static and dynamic loading and failure theories were developed/expanded to describe static and dynamic failure under multi-axial states of stress. The materials investigated were unidirectional glass fiber/vinylester, and carbon fiber/epoxy composites. Multi-axial static and dynamic experiments were conducted using off-axis specimens to produce stress states combining transverse normal and in-plane shear stresses. A Hopkinson bar apparatus was used for multi-axial characterization of the above materials at high strain rates. Quasi-static and dynamic failure envelopes were obtained by the various available failure theories including the recently introduced Northwestern (NU) theory and compared with experimental results. The NU theory was extended to the dynamic loading regime and was shown to be in excellent agreement with experimental results.

Calibration of a Simple Rate Dependent Elastic-Plastic Constitutive Model for a Toughened Carbon Epoxy Composite System

The concept of progressive failure modeling is an ongoing concern within the composite community. A common approach is to employ a building block approach where constitutive material properties lead to lamina level predictions which then lead to laminate predictions and then up to structural predictions. There are advantages to such an approach, developments can be made within each step and the whole workflow can be updated. However, advancements made at higher length scales can be hampered by insufficient modeling at lower length scales. This can make industry wide evaluations of methodologies more complicated. For instance, significant advances have been made in recent years to strain rate independent failure theories on the lamina level. However, since the Northwestern Theory is stress dependent, for adequate use in a progressive damage model, a similarly robust constitutive model must also be employed to calculate these lamina level stresses. An improper constitutive model could easily cause a valid failure model to produce incorrect results. Also, any global strain rate applied to a multi-directional laminate will produce a spectrum of local lamina level strain rates so it is important for the constitutive law to account for strain rate dependent deformation.