Laminated materials

Piezoelectric sensors are one of the primary devices used in smart structures because of their capability to act as both, sensors and actuators. A finite element model has been developed to predict elastic behavior and electrical response of laminate composites with embedded piezoelectric sensors. Correlations with experimental results indicate that the model is capable of forecasting the elastic and electrical response of the structure with good accuracy. The important issue of debonding of any of the faces of the sensors is also studied in the current work. Finite element results indicate significant changes in the elastic response caused by debonding, as well as unreliable electrical outputs.

This thesis presents an experimental and analytical investigation of concrete structural members strengthened with externally bonded composite laminates with varying configurations. Parameters, such as size, type of laminate, debond, etc., are evaluated from the viewpoint of stress patterns and their influence on interfacial debonds. Stress patterns in the structure and stress intensity factors around crack tips are determined using a finite element model developed for this purpose. The study also includes a precise description of cracking and the failure function of each parameter investigated. Besides the development of an innovative finite element program, which enables the study of interfacial cracks in structures with highly nonlinear behavior and multiple irregular cracking patterns, the significant contributions include the effect of laminate geometry, the inefficiency of laminate prestressing, the negative effect of end debond, and the insignificant effect of midspan debond on the cracking and the strength of a laminated concrete structural member.

Delamination growth has been investigated as a potential failure mechanism for filament-wound composite cylinders used for offshore and underwater structures. Analysis and experiments on DCB, ENF, and MMB beam fracture specimens machined from angle-ply laminate panels and filament-wound composite cylinders are presented. Bending analysis of beam fracture specimens machined from flat panels and composite cylinders was derived from first order shear deformation theory and one-dimensional expressions obtained from laminated plate and shell theories. For the DCB specimens, elastic foundation effects were modeled. Experiments on flat, glass/polyester laminate beam specimens considered [0]6, [+/-30]5 and [+/-45] 5 lay-ups with mid-plane delaminations. Experiments on beam specimens machined from composite cylinders were conducted on [+/- q ]6 and [+/- q ]12 lay-ups with mid-surface delaminations where q = 30 degrees, 55 degrees and 85 degrees. For all lay-ups and specimen configurations, beam model predictions of compliance were in good agreement with experimental data over the range of laminate thicknesses, ply angles, and crack lengths examined. Fracture toughness for delamination propagation was examined for flat glass/polyester panels and glass/epoxy cylinders. The initiation value of mode II fracture toughness, GIIc, was much larger than the initiation value of mode I fracture toughness GIc. The initiation value of mixed mode fracture toughness, Gc, increased with decreased ratio GI/GII and increased ply angle q . Debonding of transversely oriented fiber bundles was observed as a major crack arrest and fracture resistance mechanism for the flat, glass/polyester angle-ply laminates. Bridging by interlaced fiber bundles and crack jumping to another interface contributed to crack arrest and limited the growth in the curved, glass/epoxy angle-ply laminates. For all lay-ups, the crack propagated in a non-uniform manner across the width of the specimen as explained by elastic coupling effects in the laminate beams of the cracked region.

This study focused on the fracture and fatigue crack growth behavior in polyvinylchloride (PVC) and polyethersulfone (PES) foams. A new sandwich double cantilever beam (DCB) test specimen was implemented. Elastic foundation and finite element analysis and experimental testing confirmed that the DCB specimen is appropriate for static and cyclic crack propagation testing of soft polymer foams. A comprehensive experimental mechanical analysis was conducted on PVC foams of densities ranging from 45 to 100 kg/m3 and PES foams of densities ranging from 60 to 130 kg/m3. An in-situ scanning electron microscope study on miniature foam fracture specimens showed that crack propagation in the PVC foam was inter-cellular and in the PES foam, failure occurred predominately by extensional failure of vertical cell edges. Sandwich DCB specimens were loaded cyclically as well. For the PVC foams, the crack growth rates were substantially influenced by the density. For the PES foams, there was no clear indication about the influence of foam density on the crack growth rate.

In an effort to obtain an improved mode III fracture toughness test suitable for a testing standard, mechanics analysis, experimental testing, and finite element analysis (FEA) have been conducted. Of particular concern are the merits of one-point and two-point edge crack torsion (ECT) test methods, the influence of specimen geometry that overhangs beyond load/support points, and the influence of crack length on the compliance and energy release rate. Shear stress distributions at the crack front are determined to examine the uniformity of mode III loading and mode II influence. The shear stress distributions in the one-point and two-point tests are virtually identical, indicating that either of the two tests could be used interchangeably. Based on the uniformity of the mode III shear stress distribution along the crack front, it was found that the ECT specimen should have minimum overhang. Longer crack lengths tend to produce nonuniform shear stress distributions. A modified two-point ECT test fixture was developed to allow testing of specimens with a range of dimensions. This development enabled experimental verification of the results from the FEA overhang series. The specimens with a minimum overhang produced consistant mode III toughness data. The most reliable way to reduce data is through the original compliance calibration method. A modified ECT specimen was developed with a staggered crack front to produce uniform mode III crack growth. Finite element analysis of the modified ECT specimen shows a uniform mode III stress distribution along the crack front with little mode II interaction.