Yibin Xue

Micromechanics Study of Fatigue Damage Incubation Following an Initial Overstrain

Understanding and quantifying the effects of overloads/overstrains on the cyclic damage accumulation at a microscale discontinuity is essential for the development of a multi-stage fatigue model under variable amplitude loading. Micromechanical simulations are conducted on a 7075-T651 Al alloy to quantify the cyclic microplasticity in the matrix adjacent to intact or cracked, life-limiting intermetallic particles. An initial overstrain followed by constant amplitude cyclic straining is simulated considering minimum to maximum strain ratios of 0 and —1. The nonlocal equivalent plastic strain at the cracked intermetallic particles reveals overload effects manifested in two forms: (1) the cyclic plastic shear strain range is greater in the cycles following an initial tensile overstrain than without the overstrain and (2) the initial overstrain causes the nonlocal cumulative equivalent plastic strain to double in subsequent tensile-going half cycles and triple in subsequent compressive-going half cycles, as compared with cases without an initial tensile overstrain. The cyclic plastic zone at the microdiscontinuity corresponds to that of the maximum strain during the initial overstrain and the nonlocal cyclic plastic shear strain range in the matrix near the intact or cracked inclusion is substantially increased for the same remote strain amplitude relative to the case without initial overstrain. These results differ completely from the effects of initial tensile overload on the response at a macroscopic notch root or at the tip of a long fatigue crack in which the driving forces for crack formation or growth, respectively, are reduced. The micromechanical simulation results support the incorporation of enhanced cyclic microplasticity and driving force to form fatigue cracks at cracked inclusions following an initial tensile overstrain in a fatigue incubation model.

Kenaf Bast Fiber Bundle–Reinforced Unsaturated Polyester Composites. IV: Effects of Fiber Loadings and Aspect Ratios on Composite Tensile Properties

Mechanically retted short kenaf bast fiber bundle (KBFB)–reinforced unsaturated polyester (UPE) composites were fabricated. The effects of fiber loadings and aspect ratios on composite tensile properties were evaluated experimentally and theoretically. Tensile properties of KBFBs and the neat cured UPE were determined, and kenaf-UPE shear bonding strengths were measured. These measured properties were used to predict the tensile properties of the short KBFB-reinforced UPE composites using classical models in micromechanics. Theoretical tensile moduli predicted by Halpin–Tsai, Mori–Tanaka, and Self-Consistent models were in good agreement with experimental results. Theoretical tensile strengths predicted by the Kelly–Tyson model correlated well with experimental results at high fiber aspect ratios. Both composite tensile moduli and strengths increased consistently with increasing fiber loadings up to 75 percent (vol/vol).

Kenaf bast fiber bundle-reinforced unsaturated polyester composites. II: Water resistance and composite mechanical properties improvement.

Water resistance properties of kenaf bast fiber bundle (KBFB)–reinforced composites with either unsaturated polyester (UPE) or vinyl ester (VE) matrices, developed in previous research, were studied. The effects of styrene content in UPE resin on the neat UPE resin tensile properties and molding pressure on composite flexural properties were evaluated. The effect of laser and plasma radiation on fiber-matrix interfacial shear strengths was studied. The composites exhibited high water uptake during short- and long-term water immersion. Encapsulation by surface coating and edge sealing improved composite water resistance. Statistical analyses indicated that molding pressures of 5 to 7 MPa were preferable to achieve the maximum composite mechanical properties, and 37.6 percent (wt/wt) styrene in the UPE resin gave the highest UPE matrix tensile properties in the studied range. Laser and plasma radiation of the KBFBs significantly improved fiber-matrix interfacial bonding.

Kenaf Bast Fiber Bundle-Reinforced Unsaturated Polyester Composites. III: Statistical Strength Characteristics and Cost-Performance Analyses

The tensile, flexural, and impact strength distribution and the cost-effectiveness of kenaf bast fiber bundle (KBFB)- reinforced unsaturated polyester composites were studied. Probability models including normal, two-parameter Weibull, gamma, lognormal, exponential, Burr, Pareto, and inverse Gaussian models were fitted against measured composite strengths. Taking the 5th percentile values as the composites strength design values, the two-parameter Weibull model provided the most conservative composite strength design values. A cost-effectiveness analysis showed these composites were more cost-effective than glass fiber-reinforced sheet molding compounds (SMCs) for carrying tensile and flexural loads when their fiber loadings reached 51.2 and 56.3 percent (wt/wt), respectively. The KBFB-reinforced unsaturated polyester composites were less cost-effective than glass fiber-reinforced SMCs for carrying impact loads. This work suggests that natural fiber-reinforced composites have the potential to be viable replacement materials in applications where impact resistance is not critical.

Kenaf bast fiber bundle-reinforced unsaturated polyester composites. I: Processing techniques for high kenaf fiber loading.

The fabrication of kenaf bast fiber bundle/unsaturated polyester composites with high (60% to 67, wt/wt) fiber contents was explored in this study. Mechanically ground kenaf bast fiber bundles were...

Modification of Wood Flour Surfaces by Esterification with Acid Chlorides: Use in HDPE/Wood Flour Composites

Modification of wood fiber/flour (WF) surfaces can improve their compatibility with hydrophobic plastic matrices and reduce composite water uptake. WF was esterified with octanoyl chloride and palmitoyl chloride. Modified WF was analyzed by FT-IR. More extensive esterification occurred in highly polar dimethylformamide (DMF) than in much less polar CHCl3 or methyl tert-butyl ether (MTBE). DMF penetrates into the fiber far more than CHCl3 or MTBE, making more –OH groups available for esterification. Increasing the acid chloride chain length from C8 to C16 decreased the mole fraction of esterification. Longer chains cover surface –OH groups, retarding reactions with nearby hydroxyls after esterification. Longer chain acid chlorides also have lower reactivity and penetrate into the hydrophilic wood fiber more slowly. Modified wood flour surfaces were covered by a hydrophobic layer of ester groups (SEM). Modified wood flour surfaces and WF/HDPE composite fracture surfaces were studied by SEM. C8-modified wood flour (60 wt%)/HDPE composites exhibited far less water absorption after 24 h and 216 h immersions compared with unmodified WF (60 wt%)/HDPE composites. Water absorption continues over the 216 h period. Esterified WF/HDPE composites exhibited lower flexural strengths and moduli. In contrast to C8-esterification, the addition of maleated polypropylene (MAPP) to WF/HDPE composites improved composite mechanical performance and gave similar water absorption properties to C8-esterified WF composites.

Micromechanical Simulation of Cyclic Plasticity at Inclusion Particles with Pre-Over-Straining

Understanding and quantifying correctly the effects of overload on the cyclic damage accumulation at a microscale discontinuity is essential for the development of a multstage fatigue model under variable loading. Micromechanical simulations were conducted on a 7075-T651 Al alloy to quantify the cyclic microplasticity induced by constant amplitude cyclic loads following a pre -overload. The initial overstraining amplitudes were selected in the region of limited macroscopic plastic deformation to account for both macroscopic and microscopi c plastic overloading effects. The nonlocal equivalent plastic strain at the micrometer-scale discontinuity showed the overload effects primarily in two forms: 1) the cyclic plastic dissipation is greater in the cycles following a pre -overstraining than that without a pre -overstraining; 2) the overtraining causes the nonlocal equivalent plastic strain to increase two times in a tensile loading step and three times in the compression loading steps, as compared to those without a pre -overstraining. The cyclic plastic zone at the microdiscontinuity corresponds to the maximum load in o verstraining. The micromechanical simulation results support a cyclic damage accumulation rule that captures the cyclic microplasticity accumulation induced by an overstraining for a high fidelity fatigue incubation model under variable amplitude loading.

Quantitative Uncertainty Analysis for a Mechanistic Multistage Fatigue Model

A quantitative approach is pre sented to evaluate the uncertainty of a microstructurebased multistage fatigue (MSF) model for fatigue life predictions and fatigue damage estimations. The microstructurally-based MSF model manifests the varying mechanisms of fatigue damage incubation, microstructurally and physically small crack growth, and long crack growth. The MSF model also incorporates the effects of microstructur al features in fatigue damage progression: 1) the size and morphology of discontinuities, such as intermetallic particles and casting pores, 2) primary and secondary dendrites in cast alloys, and 3) grain size and texture in wrought alloys. The MSF model lays a foundation to evaluate the stochastic symbiosis of fatigue cracks with microstructural features that induce large scatter in the fatigue life. The quantitative uncertainty analysis demonstrates analytically the sensitivity of the MSF model parameters, random microstructural features to the fatigue life and uncertainty propagation of each parameter with respect to the fatigue life. The uncertainty magnification factor and uncertainty percentage contribution of each model parameter are quantified to provide understanding of each parameter’s contribution to the overall fatigue behavior. This paper demonstrates that a mechanistic multistage fatigue model, with multiple parameters related directly to microstructur al features , induces less uncertainty in the results. The MSF model is a true high fidelity model that possesses great potential in structural prognosis in various applications.