Carlsson, Leif A.

Debond failures in structural sandwich may lead to severe reductions in load-bearing capability of the structure because of impartial transfer of shear and tensile forces between facing and core due to the lack of interfacial bonding. Analysis of interfacial bonding in sandwich specimens subjected to transverse tensile and shear forces is presented. Stress intensity factors computed based on the near-tip displacement field are related to experimental crack growth observation on the sandwich beams with aluminum skins on a wide range of PVC foam cores. Experimentally it was found that the crack tends to grow at the interface between the bondline and core as opposed to skin/bondline interface. In shear dominated fields, a pre-existing flow tended to deflect into the core rather than grow along the interface. The tendency for kinking and the direction of the kink is examined experimentally and analyzed using the finite element method.

Recently developed electronic packages called Overmolded Pad Array Chip Carrier (OMPAC) IC packages frequently fail at the interface between the overmold compound and the substrate. In this study, this generic type of structure was evaluated by a combination of experimental and analytical methods. Model specimens representative of OMPAC structures were designed, manufactured and tested to failure. Detailed finite element models of the specimens were developed and analyses conducted to calculate debond stresses. Analytical methods were refined to include the effect of stress singularities. Stress results were averaged over a distance of.010 in. around the stress singularities to capture the intensity of the stress. These results were used in a combined stress failure criterion to calculate interfacial strengths based on macroscopic failure loads. The interfacial strengths were found to approach, but not exceed, those of the bulk overmold compound.

Mode I interlaminar fracture toughness, G IC, of interleaved graphite/epoxy has been investigated with DCB specimens, beam theory, and finite element analysis. Finite element modeling aimed to investigate the influence of interleaf thickness on compliance and energy release rate and possible mixed mode loading in the case of asymmetric interfacial crack. Another objective was to compute crack tip yield zone dimensions as a function of thickness and elastic properties of the interleaf material. The analysis is correlated with experiments. Thermoplastic interleaves enhanced G IC to a great extent. The toughness increased sharply with film thickness to a maximum at 16 mu m and decreased for the thicker interleaves. On the other hand, inadequate adhesion preempted the toughness potential of thermoset interleaves.

Interlaminar mode II fracture toughness, GIIC, of thermoset and thermoplastic interleaved (TSI and TPI) composites were investigated over a wide range of interleaf thickness. TPI specimens had four to about seven times larger GIIC than those without an interleaf. Poor adhesion observed for some TPI specimens were likely to be due to contaminated film materials. Thermoset interleaves were less effective in enhancing the mode II fracture toughness. However, even 0.043 mm thermoset interleaves gave three times larger G$\sb{\rm IIC}$ than those without an interleaf. Estimates of the volume of the yielded material around the crack tip based on a quasi-elastic finite element approach and Irwin's model showed that the yield zone height reaches a peak value for increasing interleaf thickness for both TSI and TPI specimens. Furthermore, fracture toughness data correlates well with yield zone heights.

The objective of this thesis is to report on an experimental study on the compressive behavior of foam cored sandwich composite specimens with and without face/core debond. A test fixture was designed which enables a precisely machined sandwich specimen instrumented with back-to-back strain gages to be loaded in edgewise compression. Tests were conducted on specimens without implanted face/core interface debonds over a range of core densities and gage lengths. The experimentally determined compression strengths and failure modes were compared to closed-form predictions and finite element analysis. Specimens with an implanted through-the-width face/core debond were also tested and mechanism of failure was analyzed using finite element analysis. Good agreement between collapse loads predicted using geometrically nonlinear analysis and experimentally measured strengths was observed.

Fracture toughness of a large range of PVC foam cores was determined using the single edge notch bend (SENB) specimen. Debond fracture toughness for sandwich specimens with the same foam cores was characterized using the TSD specimen. Examination of the crack propagation path in specimens with adequate face/core adhesion revealed that the debonding process occurred by crack propagation in the core, near the face/core interface. It was observed that the debond toughness and core toughness are of similar magnitude although the debond toughness exceeds the core toughness, especially for the higher density cores. Possible reasons for the elevation of the debond toughness over the core toughness such as plastic zone enlargement, mode mixity, core thickness, and gradient of properties of the core are examined. It was found that the plastic zone enlargement is a major factor for increase in debond toughness over the other factors examined.

Effects of face/core debonding on the structural integrity of curved sandwich beams subjected to opening bending moments has been examined experimentally. Curved beams of glass/polyester faces and PVC H30 foam core were manufactured. Various sizes of debonds were created using thin Teflon sheets inserted at the outer face/core interface during processing. A fixture for testing curved beams in flexure was designed, manufactured and evaluated. Surface strains at the middle of the curve were recorded. Buckling of the debonded face sheet occurred followed by face/core propagation of the debond. Strength reduction of the beams under opening moments due to face/core debonding was substantial.

A computational approach for characterization of curl of paper under humidity changes is presented. Asymmetric papers with nonuniform through-thickness fiber orientation distribution are considered. Testing of the constituent layers of the papers considered was conducted at various constant relative humidities to obtain the mechanical properties, moisture content, moisture expansion coefficients and stress relaxation curves. Experiments were performed on asymmetric two-ply laboratory made papers to determine the curl response under moisture loading. The influence of viscoelastic stress relaxation on the curl response was first investigated. Geometrically nonlinear finite element analysis was conducted. It was found that the curvatures relax at an increasing rate with increasing humidities because of moisture enhanced viscoelastic dominance. Computed time-dependent curvatures were compared to experimental measurements which verified the mode shape and time-dependent relaxation response. Geometrically nonlinear finite element analysis revealed that initial deflections may strongly influence the subsequent curl behavior. A sheet with initial curvatures may undergo a bifurcation transition (buckling curl response) if the curvatures strongly interact. After the bifurcation transition, the sheet may or may not assume an unexpected shape. Experiments showed sensitivity of the response to the directions of the initial curvatures, and there are indications of a bifurcation as a result of curvature interaction. A two-ply laminate model was used to analyze curvatures of various asymmetric papers. Differences in fiber orientation distribution and principal fiber orientation angle between the two plies were considered. The analysis showed that the sheet typically bifurcated into a cylindrical and/or twisted shape. A sheet with known through-thickness fiber orientation demonstrated a complex curl response that could be simulated using the approach presented, given that the initial curl shape is known.

A new test specimen, named tilted sandwich debond specimen (TSD), has been Introduced to promote face/core debonding over crack kinking and enable characterization of an important failure mode of sandwich structures. An experimental compliance calibration procedure was developed for evaluation of debond fracture toughness in a straight-forward manner. The specimen has been evaluated through kinematics analysis, elastic foundation model, finite element analysis and a comprehensive experimental investigation. An elastic foundation model of the TSD specimen was developed to obtain analytical expressions for specimen compliance and strain energy release rate. A design equation for the maximum tolerable crack length was derived. Finite element analysis of various configurations of the TSD specimen was conducted to obtain the mixed mode stress intensity factors, crack kinking angle, specimen compliance and strain energy release rate. The results revealed that the bimaterial character of the TSD specimen influences the mode mixity for the specimen and that crack kinking was more likely for thick and low density cores. The presence of the interphase layer only slightly influenced the mode mixity and kinking angle. The debonding characteristics of several sandwiches consisting of glass/vinylester face sheets and PVC foam cores of various densities were examined using the TSD specimen. Crack propagation from the beelcore precrack involved "micro-kinking" or kinking deeply in the core for all specimens at the first crack propagation increment(s). Crack kinking in the intermediate density core could be suppressed by selecting a certain range of tilt angles. After kinking, crack returned to a path parallel and close to the interface in agreement with the analysis of sub-interface cracks. Cracks propagated in a stick/slip manner. Measurements of the debond fracture toughness, Gc, using the TSD specimen revealed that Gc is fairly independent of crack length and increases with increasing core density. The debond toughness was of similar order as the mode I toughness of the core.

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.