Numerical Simulations of Transverse Compression and Densification in Wood

Numerical modeling, such as finite element analysis (FEA), of complex structures and complex materials is a useful tool for stress analysis and for failure modeling. Although FEA of wood as an anisotropic continuum is used, numerical modeling of realistic wood structures, including details of wood anatomy and variations in structure within specimens, has been beyond the capabilities of FEA and other methods. In contrast, the recently derived material point method (MPM) has features that make it amenable to analysis of realistic wood structures. To demonstrate the capabilities of MPM, simulations were done for wood in transverse compression. Some advantages of MPM are that it is easy to discretize micrographs of wood specimens into a numerical model, it can handle large deformations, it can model elastic-plastic cell-wall properties, and it automatically accounts for contact between cell walls. MPM simulations were run for softwood and hardwood loaded in either radial or tangential compression. The simulations reproduced many features of wood compression, gave insight into effects of wood anatomy on compression, and may be the first numerical calculations of realistic wood structures extended through to full densification without numerical difficulties

Microcracking, microcrack-induced delamination, and longitudinal splitting of advanced composite structures

A combined analytical and experimental study was conducted to analyze microcracking, microcrack-induced delamination, and longitudinal splitting in polymer matrix composites. Strain energy release rates, calculated by a variational analysis, were used in a failure criterion to predict microcracking. Predictions and test results were compared for static, fatigue, and cyclic thermal loading. The longitudinal splitting analysis accounted for the effects of fiber bridging. Test data are analyzed and compared for longitudinal splitting and delamination under mixed-mode loading. This study emphasizes the importance of using fracture mechanics analyses to understand the complex failure processes that govern composite strength and life

Studies of fiber-matrix adhesion on compression strength

A study was initiated on the effect of the matrix polymer and the fiber matrix bond strength of carbon fiber polymer matrix composites. The work includes tests with micro-composites, single ply composites, laminates, and multi-axial loaded cylinders. The results obtained thus far indicate that weak fiber-matrix adhesion dramatically reduces 0 degree compression strength. Evidence is also presented that the flaws in the carbon fiber that govern compression strength differ from those that determine fiber tensile strength. Examination of post-failure damage in the single ply tests indicates kink banding at the crack tip

The initiation, propagation, and effect of matrix microcracks in cross-ply and related laminates

Recently, a variational mechanics approach was used to determine the thermoelastic stress state in cracked laminates. Described here is a generalization of the variational mechanics techniques to handle other cross-ply laminates, related laminates, and to account for delaminations emanating from microcrack tips. Microcracking experiments on Hercules 3501-6/AS4 carbon fiber/epoxy laminates show a staggered cracking pattern. These results can be explained by the variational mechanics analysis. The analysis of delaminations emanating from microcrack tips has resulted in predictions about the structural and material variables controlling competition between microcracking and delamination failure modes

Fracture Toughness of Wood and Wood Composites during Crack Propagation

Mode I fracture toughness as a function of crack length of medium-density fiberboard (MDF), particleboard (PB), and Douglas-fir (DF) was measured using a new energy-based method. PB and MDF are examples of composites that develop fiber bridging during crack propagation, which causes their toughness to increase with crack length. Longitudinal cracks in DF also displayed fiber-bridging behavior, but only when the crack plane was normal to the tangential direction. MDF and PB experiments were performed for both in-plane and out-of-plane cracks. The toughness of the former was much higher than the latter. The in-plane crack toughness of MDF was higher than PB, but its out-of-plane toughness was lower. PB made using a new soy-based resin had an in-plane toughness similar to commercial PB but an out-of-plane toughness three times higher. Out-of-plane crack propagation is suggested as an improved method for measuring internal bond (IB) properties. When the fracture method was compared with conventional IB tests, both methods showed that the soy PB was better but the fracture method provided a clearer distinction

Numerical Simulation of Orthogonal Cutting Using the Material Point Method

A material point method simulation of orthogonal cutting that can simulate cutting into steady-state chip curling is described. The modeling used ductile fracture mechanics using cohesive zone in the cutting path. Robust simulations required a new mechanism to damp kinetic energy artifacts associated with dynamic crack propagation. The simulations displayed two regimes — crack-tip touching, where the tool reaches the crack tip, and plastic bending, where the tool is separated from the crack tip by a gap. The simulations were compared to analytical models that were revised to account for rubbing forces and hardening laws.Keywords: Material Point Method, Cutting, Cohesive Zones, Computational Mechanic

Generalized crack closure analysis for elements with arbitrarily-placed side nodes and consistent nodal forces

A new approach was developed for the evaluation of energy release rate by the virtual crack closure technique in quadratic and linear elements. The generalized method allows arbitrary placement of the side nodes for quadratic elements and thus includes both standard elements, with mid-side nodes, and singularity elements, with quarter-point nodes, as special cases of one general equation. It also accounts for traction-loaded cracks. The new derivation revealed that the proper nodal forces needed for crack closure calculations should be the newly-defined “nodal edge forces,” rather than the global or element forces from standard finite element analysis results. A method is derived for calculating nodal edge forces from global forces. These new forces affect energy release rate calculations for singularity elements and for problems with traction-loaded cracks. Several sample calculations show that the new approach gives improved accuracy.Keywords: Crack closure, Finite elements analysis, FractureKeywords: Crack closure, Finite elements analysis, Fractur

Master plot analysis of microcracking in graphite/epoxy and graphite/PEEK laminates

We used a variational stress analysis and an energy release rate failure criterion to construct a master plot analysis of matrix microcracking. In the master plot, the results for all laminates of a single material are predicted to fall on a single line whose slope gives the microcracking toughness of the material. Experimental results from 18 different layups of AS4/3501-6 laminates show that the master plot analysis can explain all observations. In particular, it can explain the differences between microcracking of central 90 deg plies and of free-surface 90 deg plies. Experimental results from two different AS4/PEEK laminates tested at different temperatures can be explained by a modified master plot that accounts for changes in the residual thermal stresses. Finally, we constructed similar master plot analyses for previous literature microcracking models. All microcracking theories that ignore the thickness dependence of the stresses gave poor results