Fatigue crack propagation in microcapsule toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered doublecantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 micron diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.published or submitted for publicationis peer reviewe

Who Chooses Defined Contribution Plans?

This paper provides new evidence on what types of individuals are most likely to choose a defined contribution (DC) plan over a defined benefit (DB) plan. Making use of administrative data from the State Universities Retirement System (SURS) of Illinois, we study the decisions of nearly 50,000 new employees who make a one-time, irrevocable choice between a traditional DB plan, a portable DB plan, and an entirely self-managed DC plan. Because the SURS-covered earnings of these employees are not covered under the Social Security system, their choices provides insight into the DB vs. DC preferences of individuals with regard to a primary source of their retirement income. We find that a majority of participants fail to make an active decision and are thus defaulted into the traditional DB plan after 6 months. We also find that those individuals who are most likely to be financially sophisticated are most likely to choose the self-managed DC plan, despite the fact that, given plan parameters, the DC plan is inferior to the portable DB plan under reasonable assumptions about future financial market returns. We discuss both rational and behavioral reasons that might explain this finding.

Is a Bird in Hand Worth More than a Bird in the Bush? Intergenerational Transfers and Savings Behavior

This paper provides new evidence on the decomposition of aggregate household wealth into life-cycle and transfer wealth. Using the 1998 Survey of Consumer Finances, it finds that transfer wealth accounts for approximately one-fifth to one-quarter of aggregate wealth, suggesting a larger role for life-cycle savings than some previous estimates. Despite the smaller aggregate size of transfer wealth, its concentration among a small number of households suggests that it can still have an important effect on the savings decisions of recipients. Estimates suggest that past receipts of transfer wealth reduce life-cycle savings by as much as dollar-for-dollar, while expected future transfers do not produce such a crowd-out effect.

Executive Financial Incentives and Payout Policy: Firm Responses to the 2003 Dividend Tax Cut

We test whether executive stock ownership affects firm payouts using the 2003 dividend tax cut to identify an exogenous change in the after-tax value of dividends. We find that executives with higher stock ownership were more likely to increase dividends after the tax cut in 2003, whereas no relation is found in previous periods when the dividend tax rate was higher. Relative to previous years, firms that initiated dividends in 2003 were more likely to reduce repurchases. The stock price reaction to the tax cut suggests that the substitution of dividends for repurchases may have been anticipated, consistent with agency conflicts.

Individual Account Investment Options and Portfolio Choice: Behavioral Lessons from 401(k) Plans

This paper examines how the menu of investment options made available to workers in defined contribution plans influences portfolio choice. Using unique panel data of 401(k) plans in the U.S., we present three principle findings. First, we show that the share of investment options in a particular asset class (i.e., company stock, equities, fixed income, and balanced funds) has a significant effect on aggregate participant portfolio allocations across these asset classes. Second, we document that the vast majority of the new funds added to 401(k) plans are high-cost actively managed equity funds, as opposed to lower-cost equity index funds. Third, because the average share of assets invested in low-cost equity index funds declines with an increase in the number of options, average portfolio expenses increase and average portfolio performance is thus depressed. All of these findings are obtained from a panel data set, enabling us to control for heterogeneity in the investment preferences of workers across firms and across time.

In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene

Microencapsulated healing agents that possess adequate strength, long shelf-life, and excellent bonding to the host material are required for self-healing materials. Ureaformaldehyde microcapsules containing dicyclopentadiene were prepared by in situ polymerization in an oil-in-water emulsion that meet these requirements for self-healing epoxy. Microcapsules of 10-1000 ??m in diameter were produced by appropriate selection of agitation rate in the range of 200-2000 rpm. A linear relation exists between log(mean diameter) and log(agitation rate). Surface morphology and shell wall thickness were investigated by optical and electron microscopy. Microcapsules are composed of a smooth 160-220 nm inner membrane and a rough, porous outer surface of agglomerated urea-formaldehyde nanoparticles. Surface morphology is influenced by pH of the reacting emulsion and interfacial surface area at the core-water interface. High yields (80-90%) of a free flowing powder of spherical microcapsules were produced with a fill content of 83-92 wt% as determined by CHN analysis.published or submitted for publicationis peer reviewe

401(k) Matching Contributions in Company Stock: Costs and Benefits for Firms and Workers

This paper examines why some employers provide matching contributions to 401(k) plans in company stock and explores the implications of match policy for employee retirement wealth. Unlike stock option grants to non-executives, a firm's decision to match in company stock does not appear to be strongly correlated with cash flow or with measures of the benefits of aligning incentives of employees and employers. Rather, we find evidence that firms are more likely to provide the match in company stock if firm risk is low (i.e. lower stock price volatility and lower bankruptcy risk) and employees are also covered by a defined benefit plan. These findings suggest that firms consider the retirement security of their workers in making the match decision, either because firms want to minimize the risk of violating their fiduciary responsibility or because employees more fully value company stock at companies with lower firm-specific risk. Evidence also indicates that firms may want to match in company stock to boost employee ownership, perhaps to help deter takeovers, or because of the tax advantages for dividends on the company stock match. Simulation results suggest that sufficiently risk-tolerant individuals actually prefer a 401(k) plan at a company with a company stock match to a plan at a company with an unrestricted match, unless the equity premium is reduced substantially.

The Effect of Inheritance Receipt on Retirement

This paper uses the receipt of an inheritance to measure the effect of wealth shocks on retirement. Using the Health and Retirement Study (HRS), we first document that inheritance receipt is common among older workers %u2013 one in five households receives an inheritance over an eight-year period, with a median value of about $30,000. We find that inheritance receipt is associated with a significant increase in the probability of retirement. In particular, we find that receiving an inheritance increases the probability of retiring earlier than expected by 4.4 percentage points, or 12 percent relative to the baseline retirement rate, over an eight-year period. Importantly, this effect is stronger when the inheritance is unexpected and thus more likely to represent an exogenous shock to wealth.

The Nature of Hypervelocity Stars and the Time between Their Formation and Ejection

We obtain Keck HIRES spectroscopy of HVS5, one of the fastest unbound stars in the Milky Way halo. We show that HVS5 is a 3.62 ± 0.11 M_☉ main-sequence B star at a distance of 50 ± 5 kpc. The difference between its age and its flight time from the Galactic center is 105 ± 18 (stat) ±30 (sys) Myr; flight times from locations elsewhere in the Galactic disk are similar. This 10^8 yr "arrival time" between formation and ejection is difficult to reconcile with any ejection scenario involving massive stars that live for only 10^7 yr. For comparison, we derive arrival times of 10^7 yr for two unbound runaway B stars, consistent with their disk origin where ejection results from a supernova in a binary system or dynamical interactions between massive stars in a dense star cluster. For HVS5, ejection during the first 10^7 yr of its lifetime is ruled out at the 3σ level. Together with the 10^8 yr arrival times inferred for three other well-studied hypervelocity stars (HVSs), these results are consistent with a Galactic center origin for the HVSs. If the HVSs were indeed ejected by the central black hole, then the Galactic center was forming stars ≃200 Myr ago, and the progenitors of the HVSs took ≃100 Myr to enter the black hole's loss cone