Douglas S. Cairns

Transient response of graphite/epoxy and Kevlar/epoxy laminates subjected to impact

The influence of different parameters on the impact behavior of laminated composite plates is considered analytically. A Rayleigh-Ritz energy method was used to spatially discretize the time-varying boundary value problem and a set of coupled, ordinary differential equations in time were obtained based on the discretized system Lagrangian. The effects of shearing deformation, bending-twisting coupling, and nonlinear contact behavior were included in the model. The resulting equations were integrated using the implicit Newmark beta method without the effects of rotary inertia. The results indicate that the effective mass of the plate is often an important effect in the response to impact events. In general, the influence of the constitutive behavior dominates for very low velocity impact, whereas the target mass properties become more important as the impactor velocity increases. This importance of mass clearly shows that impactor kinetic energy is not sufficient to characterize the impactor as the impactor mass is shown to have a large influence on the resulting dynamic behavior. In addition to these parameters, the effects of preload and material properties are considered and discussed.

Fatigue of Composite Materials and Substructures for Wind Turbine Blades

This report presents the major findings of the Montana State University Composite Materials Fatigue Program from 1997 to 2001, and is intended to be used in conjunction with the DOE/MSU Composite Materials Fatigue Database. Additions of greatest interest to the database in this time period include environmental and time under load effects for various resin systems; large tow carbon fiber laminates and glass/carbon hybrids; new reinforcement architectures varying from large strands to prepreg with well-dispersed fibers; spectrum loading and cumulative damage laws; giga-cycle testing of strands; tough resins for improved structural integrity; static and fatigue data for interply delamination; and design knockdown factors due to flaws and structural details as well as time under load and environmental conditions. The origins of a transition to increased tensile fatigue sensitivity with increasing fiber content are explored in detail for typical stranded reinforcing fabrics. The second focus of the report is on structural details which are prone to delamination failure, including ply terminations, skin-stiffener intersections, and sandwich panel terminations. Finite element based methodologies for predicting delamination initiation and growth in structural details are developed and validated, and simplified design recommendations are presented.

A Consistent Engineering Methodology for the Treatment of Impact in Composite Materials

An approach is formulated to treat the behavior of composites subjected to impact as the sequential effect of a number of events. The sequence for the analysis of im pact presented here is as follows. First, a global analysis of impact of the structure is con ducted. In this study, a plate is analyzed in a time marching scheme to properly account for nonlinearities during the impact event. This provides loads introduced into the struc ture during impact. These loads are then introduced into a local deformation and strain re sponse model just below the impactor to refine the analysis in the local region where damage occurs. Damage is predicted in the laminate on a ply-by-ply basis. Excellent agreement between the models and test data taken during the impact event is found. This is denoted as the Damage Resistance portion of the problem. Given the pre-existing damage, an analysis of the damaged structure is conducted. This entails using an average strain criterion for fiber-dominated fracture around the damage and a sublaminate buck ling analysis for compression loading. Good agreement is found between the analysis and experimental data. This is denoted as the Damage Tolerance portion of the problem. Finally, all of the models are combined to provide combined Damage Resistance/Damage Tolerance curves for useful design data.

Modeling of resin transfer molding of composite materials with oriented unidirectional plies

Abstract One of the limiting factors in the application of composites to primary structural applications has been the relatively high cost of converting basic material forms to structural configurations. One potential manufacturing method is Resin Transfer Molding (RTM). In RTM, resin is injected into a fibrous preform. In this study, the processing parameters are defined and investigated analytically and experimentally. A basic model is proposed that incorporates Darcys law in fibrous bundle regions and channel flow equations between bundles. A model description is provided along with experimental procedures. The model results are compared to experimental data for unidirectional, stitched preforms, and multi-layer configurations with good experimental agreement. It was found that incorporating channel flow is an important feature for properly modeling the RTM process. Pressure profiles, resin velocities, and resin flow fronts are predicted accurately, and are available for manufacturing process development. The same materials were compared to fully stitched preforms. It was found that although the shapes for resin flow are similar between experimental and analytical results, the stitching affects the permeability such that unidirectional ply data do not accurately capture the times for resin flow.

Analysis of a Composite Blade Design for the AOC 15/50 Wind Turbine using a Finite Element Model

A fiberglass blade was designed for the Atlantic Orient Corporation (AOC) H/50 wind turbine through the use of finite element (FE) modeling techniques. In this initial design phase, the goals were: 1) make the blade as stiff as the previously designed laminated wood blade, 2) minimize resonant operating conditions, 3) design the blade to withstand extreme wind conditions, and 4) make the blade compatible with reasonable manufacturing techniques. The modeling assumptions used are discussed and the final results, for this initial design phase, are presented. Based on the J?E model, the designed blade will be able to withstand extreme wind conditions through elastic deformation, and resonant operating conditions will be minimized. This document is an overview of the design and manufacturing synthesis data of composite wind turbine blades for applications to the Sandia National Laboratories’ NuMAD wind turbine blade design tool.

Compression fatigue response for carbon fiber with conventional and toughened epoxy matrices with damage

This paper compares the open hole compression, compression after impact, compression fatigue of open hole specimens, and compression fatigue after impact response of quasi-isotropic laminates with IM7 carbon fiber and 3501-6 and 8551-7 epoxy matrices. These matrices can be considered to be a relatively brittle and a high-toughness resin, respectively. The objective was to establish whether the improved compression after impact response associated with high toughness matrices also held after fatigue loading. The results of impact and compression fatigue tests show that residual strengths of the toughened epoxy matrix system were approximately twice that of the brittle matrix system, and that fatigue resistance after impact and of open hole specimens was generally improved.

Static and Dynamic Mode II Strain Energy Release Rates in Toughened Thermosetting Composite Laminates

In this work, the static and dynamic Mode II fracture properties of several thermosetting resin-based composite laminates are presented. Two classes of materials are explored. These are homogeneous thermosetting resins and toughened, multiphase, thermosetting resin systems. A new dynamic Mode II test is presented for composite laminates based on an end-notched flexure (ENF) specimen geometry. The specimen is impacted in three-point bending with an instrumented impact tower. Fracture initiation and propagation energies under static and dynamic conditions are determined analytically and experimentally. The test results show that the toughened systems provide superior fracture initiation and higher resistance to propagation under dynamic conditions. The results indicate that static fracture testing is inadequate for determining the fracture performance of composite laminates subjected to conditions such as low velocity impact. A good correlation between the basic Mode II dynamic fracture properties and the performance in a combined material/structural compression after impact (CAI) test is found. The results underscore the importance of examining rate-dependent behavior for determining the longevity of structures manufactured from composite materials.

Moisture and Thermal Expansion Properties of Unidirectional Composite Materials and the Epoxy Matrix

Experimental techniques are presented for determining the moisture- and thermal- expansion coefficients of polymers and polymer-matrix composite materials. Materials tested included Hercules 3501-6 neat epoxy resin, Hercules AS/3501-6 graphite/epoxy unidirectional composite, and Owens-Corning S2 glass fibers in the same Hercules 3501-6 epoxy matrix. Correlations of the experimentally determined moisture- and thermal-expansion properties with a nonlinear, finite element, micromechanics analysis are presented. Thermal expansion results for all three materials, both dry and moisture-conditioned, were obtained. Moisture diffusivity constants were also ex perimentally determined. It is demonstrated that the moisture and thermal expansion of composite materials can be determined experimentally and predicted numerically, with generally good results.

Characterization of Manufacturing Defects Common to Composite Wind Turbine Blades: Flaw Characterization

The preliminary results from a survey of wind turbine blade manufactures, repair companies, wind farm operators and third party investigators has directed the focus of this investigation on types of flaws commonly found in wind turbine blades; waviness, and porosity/voids. A variety of flaw geometries as defined by in-field collection of production scale blade data has been investigated and complied. Basic statistical analysis has shown that the data generally follows standard distributions. The preliminary results from this effort and coupon level testing have established a protocol by which a defect in a blade can be characterized quantifiably. With this data and other parameters it is possible to develop criticality models that can be used in the field to evaluate the risk of leaving an as manufactured flawed structure in service. The basic metrics for this model have been developed and are described herein.

Biaxial testing of composite tubes

The development of a tubular test specimen for the characterization of advanced composite-material lamina and laminate mechanical properties is presented. The specimen design and supporting analyses are given for a practical tubular specimen that minimizes end effects. Test results for simplified loading conditions are shown to be in good agreement with values obtained in other tests. Failure strains in laminates are higher than those obtained in comparable flat-coupon tests. The results obtained indicate that the specimen and apparatus are useful for obtaining response properties and failure values under general in-plane loading.