Elastomeric polyurea coating has proven to be an excellent candidate for enhancing the energy absorption and dynamic performance of concrete and metal structures. Some of the interesting attributes of this material is:
- Inexpensive retrofit
- A glue with short curing time
- Well-established methods for large-scale application through spray-casting
- Fire resistant
- Excellent abrasion can chemical resistance
In addition to these unique chemical and physical features, polyurea has very interesting mechanical properties. At UCSD/CEAM we have focused on quantifying the mechanical response of polyurea under various conditions, seeking to develop a physics-based model that can reproduce the wide range of test results recently reported by us and others. Our efforts can be divided into two main categories:
(a) Experimental characterization and development of a material constitutive model
(b) Dynamic tests for evaluation of the failure performance of steel-polyurea bilayers
Experimental characterization and development of a material constitutive mode
Dynamic Hopkinson bar experiments: In these experiments a thin sample is placed between two long steel bars (see figure). The incidence bar is struck by a striker bar. An axial compressive wave (incident pulse) travels along this bar and loads the sample. The reflected pulse in the incidence bar and the transmitted pulse in the transmission bar as well as the initial incident pulse are measured using strain gages. Based on these measurements, the strain rate at which the sample is loaded, and the stress on the sample at each instant can be calculated. Finally the stress-strain curve of the sample can be drawn. The initial temperature of the sample and the average strain rate applied to the sample can be effectively controlled. In the figure two different setups are shown. In one setup, the sample is not restrained from radial deformation; therefore its state during the test is closest to uni-axial stress. In the second setup, the sample is constrained radially; therefore it is in the state of uni-axial deformation. The transverse stress can be estimated using a strain gage on the outside of the constraining tube. Examples of the stress-strain curve in various effective strain rates and room temperature are shown below.
Quasistatic Tests: In quasi-static tests, the rate of deformation is much lower than dynamicHopkinson bar experiments. However, similar to dynamic tests, the state of stress (uni-axial stress or strain), temperature, and strain rate can be controlled. Some examples are shown in the figure.
DMA characterization: Using a TA-2820 Dynamic Mechanical Analyzer, the relaxation curve of the polyurea is measured at various temperatures in uniaxial stress state; see figure. As explained below, the low temperature relaxation data can be used for prediction of the room temperature response of polyurea at high strain-rates.
DSC characterization: Using Modulated Differential Scanning Calorimetery, the heat capacity and reversible and irreversible thermal transitions including the glass transition temperature can be obtained.
The model developed at CEAM is coded as a material constitutive subroutine that can be used independently or along with LS-DYNA, as an explicit finite element program. The components of this nonlinear temperature- and pressure-dependent viscoelastic model are summarized here. For more detail and verification of this model as well as specific material parameters please see our paper: Amirkhizi, A. V., Isaacs, J., McGee, J., and Nemat-Nasser, S. (2006) An experimentally-based viscoelastic constitutive model for polyurea, including pressure and temperature effects, Philosophical Magazine, Vol. 86, 5847-5866.
Dynamic tests for evaluation of the failure performance of steel-polyurea bilayers
To characterize and study the effect of polyurea coating on the dynamic performance and fracture resistance of steel plates a comprehensive experimental and numerical work is performed at CEAM. The experiments are performed on monolithic steel samples and steel-polyurea bilayer samples (Fig 1.).
Fig. 1. Three different plate configuration: monolithic steel plate (right), bilayer plate
Two sets of experiments are performed at CEAM and paralleled by full-scale finite-element models:
- Reverse Ballistic
- Direct Ballistic
Reverse Ballistic Experiments and Models
A series of experiments performed to assess the dynamic response of circular monolithic steel and steel-polyurea bilayer plates to impulsive loads. A convenient technique to enhance the energy absorption capability of steel plates and to improve their resistance to fracturing in dynamic events, is to spray-cast a layer of polyurea onto the plates. Since polyurea readily adheres to metallic surfaces and has a short curing time, the technique may be used to retrofit existing metallic structures to improve their blast resistance. We have examined the effectiveness of this approach, focusing on the question of the significance of the relative position of the polyurea layer with respect to the loading direction; i.e., we have explored whether the polyurea layer cast on the front face (the impulse-receiving face) or on the back face of the steel plate would provide a more effective blast mitigating composite.
The experimental results suggest that the polyurea layer can have a significant effect on the response of the steel plate to dynamic impulsive loads, both in terms of failure mitigation and energy absorption, if it is deposited on the back face of the plate. And, remarkably, when polyurea is placed on the front face of the plate, it may actually enhance the destructive effect of the blast, promoting (rather than mitigating) the failure of the steel plate, depending on the interface bonding strength between the polyurea and steel layers. These experimental results are supported by our computational simulations of the entire experiment, employing realistic physics-based constitutive models for the steel (DH-36, in the present work) and polyurea.
Direct Ballistic Experiments and Models
The response of monolithic steel plates and steel-polyurea bilayer plates to impulsive blast loads produced in direct ballistic experiments, focusing on the deformation and failure modes of the plates. Using high-speed photography, the deformation and fracturing of some of the plates are also captured. In addition, the total force acting on the steel plate is measured as a function of time in a few cases.
The experimental results suggest that the presence of polyurea on the back face (opposite to the load receiving side) of the steel plates can enhance the energy absorption of the plates and help to mitigate their failure. On the other hand, when polyurea is placed on the front face (load receiving side), it will magnify the initial shock effect and promote failure. These experimental results are paralleled by our numerical simulations of the entire experiment, employing physics-based models for the DH-36 steel and polyurea.
Fig. 2. Schematic view of the direct Ballistic experimental setup
Fig. 3. Finite-element model of direct ballistic experiment, showing: the aluminum projectile at velocity V0, impacting the loading target (polyurethane/water) that rests on a Hopkinson bar (top: angled view; bottom: side view).
Fig. 4.Selected set of photos showing the sequence of the deformation and failure of the monolithic steel plate, S-64. The frame number is marked on the top left side of each frame. The frames are at 40μs time intervals.
Multiscale Analysis of Deformation and Fracture of Composite Plates
The deformation and fracture of the monolithic and bilayer plates tested in the direct ballistic experiments are modeled using commercially available finite-element code, LS-DYNA. A comprehensive metalography and fractography of the tested samples are performed using SEM and Optical microscope:
Fig. 5. The process of preparing the samples for metalography and final micro-structure
Fig. 6. SEM Fractography of tested sample. Dimpled fracture surface is typical of ductile fracture.
Based on the micro-scale observations using SEM and Optical microscope, a fine-scale finite-element model is introduced.
Fig. 7. Microscopic view of the necked region and the finite-element model
The fine scale model is capable of predicting the necking and fracturing pattern of the deformed samples.
Fig. 8. Microscopic view of a failed sample and the finite-element model prediction