Durability and Damage Tolerance of High Temperature Polymeric Composites

Modern durability and damage tolerance predictions for composite material systems rely on accurate estimates of the local stress and material states for each of the constituents, as well as the manner in which the constituents interact. In this work, an number of approaches to estimating the stress states and interactions are developed. First, an elasticity solution is presented for the problem of a penny-shaped crack in an N-phase composite material system opened by a prescribed normal pressure. The stress state around such a crack is then used to estimate the stress concentrations due to adjacent fiber fractures in composite materials. The resulting stress concentrations are then used to estimate the tensile strength of the composite. The predicted results are compared with experimental values. In addition, a cumulative damage model for fatigue is presented. Modifications to the model are made to include the effects of variable amplitude loading. These modifications are based upon the use of remaining strength as a damage metric and the definition of an equivalent generalized time. The model is initially validated using results from the literature. Also, experimental data from APC-2 laminates and IM7/K3B laminates are used in the model. The use of such data for notched laminates requires the use of an effective hole size, which is calculated based upon strain distribution measurements. Measured remaining strengths after fatigue loading are compared with the predicted values for specimens fatigued at room temperature and 350 F (177 C)

Vibrothermography and Ultrasonic Pulse-Echo Methods Applied to the Detection of Damage in Composite Lamintates

It has recently been shown in our laboratories that quasi-isotropic, graphite-epoxy, composite laminates develop a typical damage state that eventually leads to final failure. This damage state cannot be represented by a single through crack that propagates in a self-similar manner in the fashion ordained by fracture mechanics. To the contrary, the damage state is a complex one which begins by transverse cracking in the weakest lamina, continues by an increase in transverse crack density until a stable equilibrium spacing is achieved, proceeds by growth into the adjacent laminae.and ends by final, catastrophic failure. In certain stacking sequences, the damage state is further complicated by delamination. Several NDE methods are being developed in our laboratories specifically to identify and quantitatively describe this damage state. The vibrothermography technique uses low amplitude vibrations as a steady state energy source in the composite laminate. The mechanical energy is preferentially absorbed in the region of damage and converted to heat, which can then be detected by thermography. This technique is especially applicable to detecting delamination. An ultrasonic pulse-echo method utilizing a straightforward diffraction analysis is. being developed to detect the transverse cracks which, as they approach and attain an equilibrium spacing, present the appearance of a changing diffraction grating to the ultrasonic beam

Fatigue Damage in Notched Composite Laminates under Tension-Tension Cyclic Loads

Composite materials are established as reliable and efficient materials for a large number of structural applications. Although composites have gained widespread use, we do not, as of yet, have a precise and complete understanding of the mechanismsm of damage development in composite materials. Recent research results have pointed out the need to treat damage as a collective condition; i.e., a damage state, rather than as an assembly of discrete and independent damage modes. The process of the development of the damage state and the subsequent response of the composite laminate throughout the loading history can then be related. This report presents the results of an investigation to determine the damage states which develop in graphite epoxy laminates with center holes due to tensio-tension cyclic loads, to determine the influence of stacking sequence on the initiation and interaction of damage modes and the process of damage development, and to establish the relationships between the damage states and the strength, stiffness, and life of the laminates.Two quasi-isotropic laminates were selected to give different distributions of interlaminar stresses around the hole. The laminates were tested under cyclic loads (R=0.1, 10 Hz) at maximum stresses ranging between 60 95 percent of the notched tensile strength. Damage was monitored nondestructively throughout the loading history using stereo X-ray radiography, acoustic emission recording, and stiffness change. Some specimens were deplied after specific numbers of cycles to determine the nature and distribution of damage in each ply araound the hole and to confirm the components and size of the damage state observed nondestructively. Fatigue life and residual strength tests were also performed. Fatigue damage in the two laminates included matrix cracks in all plies followed by delaminations. The density of matrix cracks and the distribution of the damage zone (matrix cracks plus delaminations) in laminates cycled at the same percent of notched tensile strength were strongly dependent on the local constraint and distribution of interlaminar stresses as governed by the stacking sequence. The distinctly different damage states which developed in the two initially quasi-sitropic laminates due to similar load histories produced stiffness changes of 15-20 percent, different rates of residual strength degradation, and a factor of four difference in fatigue life. The results of this study are interpreted to establish relationships between the loading history, the progressive development of the damage state, and the response of the notched laminates

Evaluation of coated metallic bipolar plates for polymer electrolyte membrane fuel cells,”

Abstract Metallic bipolar plates for polymer electrolyte membrane (PEM) fuel cells typically require coatings for corrosion protection. Other requirements for the corrosion protective coatings include low electrical contact resistance, good mechanical robustness, low material and fabrication cost. The authors have evaluated a number of protective coatings deposited on stainless steel substrates by electroplating and physical vapor deposition (PVD) methods. The coatings are screened with an electrochemical polarization test for corrosion resistance; then the contact resistance test was performed on selected coatings. The coating investigated include Gold with various thicknesses (2 nm, 10 nm, and 1 m), Titanium, Zirconium, Zirconium Nitride (ZrN), Zirconium Niobium (ZrNb), and Zirconium Nitride with a Gold top layer (ZrNAu). The substrates include three types of stainless steel: 304, 310, and 316. The results show that Zr-coated samples satisfy the DOE target for corrosion resistance at both anode and cathode sides in typical PEM fuel cell environments in the short-term, but they do not meet the DOE contact resistance goal. Very thin gold coating (2 nm) can significantly decrease the electrical contact resistance, however a relatively thick gold coating (>10 nm) with our deposition method is necessary for adequate corrosion resistance, particularly for the cathode side of the bipolar plate

Evaluation of Coated Metallic Bipolar Plates for Polymer Electrolyte Membrane Fuel Cells

Metallic bipolar plates for polymer electrolyte membrane (PEM) fuel cells typically require coatings for corrosion protection. Other requirements for the corrosion protective coatings include low electrical contact resistance, good mechanical robustness, low material and fabrication cost. The authors have evaluated a number of protective coatings deposited on stainless steel substrates by electroplating and physical vapor deposition (PVD) methods. The coatings are screened with an electrochemical polarization test for corrosion resistance; then the contact resistance test was performed on selected coatings. The coating investigated include Gold with various thicknesses (2 nm, 10 nm, and 1 μm), Titanium, Zirconium, Zirconium Nitride (ZrN), Zirconium Niobium (ZrNb), and Zirconium Nitride with a Gold top layer (ZrNAu). The substrates include three types of stainless steel: 304, 310, and 316. The results show that Zr-coated samples satisfy the DOE target for corrosion resistance at both anode and cathode sides in typical PEM fuel cell environments in the short-term, but they do not meet the DOE contact resistance goal. Very thin gold coating (2 nm) can significantly decrease the electrical contact resistance, however a relatively thick gold coating (\u3e10 nm) with our deposition method is necessary for adequate corrosion resistance, particularly for the cathode side of the bipolar plate

Prediction Of Polymer Composites

With the increasing use of composite materials for diverse applications ranging from civil infrastructure to offshore oil exploration, the durability of these materials is an important issue. Practical and accurate models for lifetime will enable engineers to push the boundaries of design and make the most efficient use of composite materials, while at the same time maintaining the utmost standards of safety. The work described in this dissertation is an effort to predict the strength and rupture lifetime of a unidirectional carbon fiber/polymer matrix composite using micromechanical techniques. Sources of material variability are incorporated into these models to predict probabilistic distributions for strength and lifetime. This approach is best suited to calculate material reliability for a desired lifetime under a given set of external conditions