Timothy Breitzman

Bond breaking in stretched molecules: multi-reference methods versus density functional theory

Several quantum chemistry methods were compared for modeling the breaking of bonds in small molecules subjected to extreme strain. This provides a rigorous test of quantum mechanical methods because a high degree of dynamical and non-dynamical correlation is required to accurately model bond breaking in a strained molecule. The methods tested included multi-reference methods, unrestricted Kohn–Sham density functional theory (DFT) using several functionals, and unrestricted coupled-cluster singles and doubles. It is challenging to employ the multi-reference method in a balanced way for the molecules considered due to the computational cost. While the DFT methods are less costly and provide balanced correlation, they do not have enough static correlation to properly model the bond-breaking curve to dissociation. Despite this, for the N12 DFT method the artifacts due to spin contamination of the unrestricted Kohn–Sham method were the least severe and tolerable. Given this, and the low computational cost, the N12 method was chosen for subsequent dynamical simulations for modeling fracture inception in polymers under extreme strain. The physical characteristics of the bond-breaking process are discussed as well as the influence of secondary conjugation on the process.

Local Field Assessment inside Multiscale Composite Architectures

We introduce asymptotic expansions for recovering the local field behavior inside multiscale composite architectures in the presence of residual stress. The theory applies to zones containing abrup...

Simulation of Mode I Fracture at the Micro-Level in Polymer Matrix Composite Laminate Plies

A technique for simulating evolving matrix damage through a polymer matrix composite ply was developed. This method simulates disbonding between fiber and matrix and cracking within the matrix at the micro-scale. The aim of the study is to develop a methodology whereby the mode I traction-separation law (cohesive zone) for a given lamina could be obtained by simulation only using the fiber and matrix constituent properties as inputs. The results obtained with randomly spaced fibers representing approximately 1/3 of a ply thickness are encouraging.

Prediction of Incipient Nano-Scale Rupture for Thermosets in Plane Stress

There is limited experimental evidence that fracture nucleation in polymers includes a small number of covalent bond scissions followed by rapid void growth by chemo-mechanical processes. Generalized criteria for predicting such bond scission, then, would help anticipate fracture in polymer matrix composites. Strain states at incipient bond scission for thermoset resins in plane stress are here predicted by atomistic simulation. Several cured epoxy systems were examined, each having a different chain length. For biaxial extension and a portion of the shearing regime, scission occurs at a critical value of the larger principal strain. This value increases with increasing chain length. The corresponding dilatation is largest for biaxial extension and decreases to nearly zero for pure shear. Results are compared with strain invariants at fracture measured from experiments in which polymer matrix composites having various ply stacking sequences were loaded to rupture.

Detailed Morphology Modeling and Residual Stress Evaluation in Tri-axial Braided Composites

Local strain fields in tri-axial braided composites arising in the vicinity of a saw cut due to the release of thermal processing stresses were experimentally measured by using moire interferometry technique and compared to those obtained by 3D stress analysis. The independent mesh method (IMM) and digital chain methods were used to perform the 3D stress analysis and the composite tow morphology modeling respectively. A significant degree of morphological detail was required to achieve good comparison with experimental data. Three degrees of refinement were produced in direction of matching the actual morphology of the tri-axial braded composite which was tested experimentally. These levels of refinement included (i) correct tow path angle and curvature variation based on braid parameters (ii) addition of the effect of compaction process during the cure stage and (iii) addition of the surface sanding affects during moire test preparation stage. The results obtained by using IMM were able to capture sharp variations of the strain components observed by using the moire interferometric technique both in terms of spatial distribution and magnitude and provide accurate evaluation of the residual strain levels in the triaxial braided composites.

A Hi Fidelity Asymptotic Theory For Local Field Recovery Inside Pre‐stressed Composite Media

We introduce a new mathematically rigorous high fidelity asymptotic theory for recovering the local field behavior inside complex composite architectures. The theory applies to zones containing strong spatial variance of local material properties. The method is used to recover the local field across ply interfaces for a pre‐stressed multi‐ply fiber reinforced composite. The results are shown to be in good agreement with direct numerical simulations for realistic fiber sizes and fiber‐matrix elastic properties.

Optimal Design of a Composite Scarf Repair Under Uni-axial Tension Loading

Benchmark un-notched strength testing was used to characterize material properties for IM6/3501-6 composite material and to establish parameters for critical failure volume (CFV) (see [8]) analysis tools. Critical failure volume was used to predict the strength of scarfed composites, as well as composites having a scarf repair patch. Baseline repairs were created both without and with over-plies. Simplex optimization was performed on the analytical models to determine the repair stacking sequence that would result in the largest tensile strength for the repairs. The repair was optimized in the linear elastic regime, but strength predictions took into account both geometric nonlinearities of the respective materials and the material nonlinearities of the adhesive. Predicted strengths were in good agreement with experimental results, and the resultant optimal designs increased the strength of the repair under uni-axial tensile load by 10–20%.

Composite Scarf Repair Patch Optimization Under Uniaxial Loading Conditions

Simplex optimization algorithm was applied to predict the fiber orientations of the scarf repair patch for a given quasi-isotropic panel to maximize strength retention of the repair. The optimal stacking sequence of the repair patch avoids 0 degree plies in the direction of the load. Such a stacking sequence prolongs the life of the adhesive and results in a predicted 13% strength increase as compared to the traditional ply-by-ply replacement. The second optimization problem solved was one of finding the least favorable stacking sequence of the repair patch. Such stacking sequence inserts stiff plies into the patch and leads to the failure of the repair patch as early as 20% below the reference failure load of the repair patch with traditional ply-by-ply replacement. The strength prediction model consisted of nonlinear constitutive modeling of adhesive behavior and fiber failure prediction loads in the adherents based on critical failure volume (CFV) (see [8]) strength prediction method. Benchmark analysis was performed on the virgin, scarfed, and repaired (ply-by-ply replacement) panels and was in good agreement with experimental data.