Various damages, which develop in structural composites, cause a softening behavior which can cause significant load redistribution in non-critical as well as critical areas of various structural elements. The strain energy dissipation (SED) concept provides a set of consistent nonlinear constitutive relations for stress analyses and the in-plane loader (IPL) provides a way for material characterization. Damage surfaces for moderate values of dissipated energy densities (DED) were estimated to match the response of conventional (+/-theta) sub ns coupon tests, which are influenced by matrix mode damages. Softening behaviors for high values of DED which can result from fiber breakage/pull out were estimated using engineering judgement. Nonlinear finite element analyses were performed for two classes of structural elements; namely. (i) a (+/- theta) sub ns cylindrical pressure vessel, where the nonlinearities in load- deformation responses are noticeable before fiber failure. and (ii) open hole tension of two quasi-isotropic layups, where local failure and softening in the fiber direction become critical to cause catastrophic failure. The results show the usefulness of the approach for design purposes and for explaining the complex problem of the so called "hole size" effect. Extension of the SED concept to the 3-D problem is suggested and a new approach to the problem of certification is discussed. Usefulness of the data generated by the IPL could not be assessed since they were not available for use. However, it appears that such data would be useful if they can provide full information up to high DED levels required for complete softening due to fiber breakage and pull out. Additional studies are suggested for the following phases for utilizing the full potential of the approach. Use of the test devices (IPL) and/or other tests by other testing organizations are also recommended to determine their acceptability and versatility

Fabric reinforced polymer matrix composites are an integral structural material used in wind turbine blades. Wind turbines are expected to experience growth both in physical size and utilization as the focus of power generation shifts towards utilizing renewable sources more efficiently. Even current generation blades are experiencing reliability concerns. These factors are now driving improvements in design and manufacture of wind turbine blades. With this, progress in characterizing the mechanical behavior of materials is necessary. Composite materials possess unique damage mechanisms due to their constituent materials and these require further study. Composite materials were manufactured into four layups from four fabrics and an epoxy matrix. Acoustic emission sensors were applied in a linear locating arrangement to capture elastic waves from damage mechanisms during a tensile test. Data critical to this work that was extracted from the elastic waveforms include peak frequency and absolute energy. A static loading scenario was used to characterize the materials and their damage progression while an LUR loading scenario was used to correlate absolute energy to dissipated strain energy. Results of the material characterization found that a greater range of frequencies, thus damage mechanisms, were observed for increasingly complex fabric architectures. The observed frequency and energy data provided valuable information on the interaction of the various constituent materials. Attempts at obtaining an accurate and consistent correlation value were not successful with the LUR tests. However, total accumulated energy emerged as a consistent metric of equal value between static and LUR tests that shows promise of being an indicator of coupon damage state. Acoustic emission was found to provide a unique analysis that can identify and characterize damage and fabric composition in composite materials. The frequency content provides a consistent method of identifying damage mechanisms between varying materials. Correlating acoustic energy to strain energy dissipated appears to be more complex than is developed here but the concept does hold merit and requires further study. An applicable database of AE characteristics of the materials was created that will be of great use for future more complex sub-structure component testing.