Simo-Pekka Hannula

State Variable Theories Based on Hart’s Formulation

Constitutive equations based on state variables are very attractive in stress analysis because of their ability to characterize the consequences of mechanical history by current values of state variables. By virtue of this a state variable theory can organize and rationalize a wealth of accumulated data, and the ensuing economy in thought will automatically lead to economy in experiment.1 Moreover, with the aid of state equations, material behavior under difficult or even unmanageable conditions may be predicted by more readily performable tests; for example, long-term creep behavior at high temperatures might be estimated based on combined short-term tensile and relaxation testing at a lower temperature.

Influence of nitrogen alloying on hydrogen embrittlement in AISI 304-type stainless steels

Hydrogen embrittlement of AISI 304-type austenitic stainless steels has been studied with special emphasis on the effects of the nitrogen content of the steels. Hydrogen charging was found to degrade the mechanical properties of all the steels studied, as measured by a tensile test. The fracture surfaces of hydrogen charged specimens were brittle cleavage-like whereas the uncharged specimens showed ductile, dimpled fracture. In sensitized materials transgranular cleavage mode of fracture was replaced by an intergranular mode of fracture and the losses of mechanical properties were higher. Nitrogen alloying decreased the hydrogen-induced losses of mechanical properties by increasing the stability of austenite. In sensitized steels the stability of austenite and nitrogen content were found to have only a minor effect on hydrogen embrittlement, except when sensitization had causedα′-martensite transformation at the grain boundaries.

Repeated load relaxations of type 316 austenitic stainless steel

Several experiments have shown that the shape of the load relaxation curve after reloading in a logarithmic stress vs. logarithmic strain rate plot may differ from that after the initial loading. In a recent study Korhonen and Li showed that the apparent kink in a log sigma versus log epsilon plot of the stress relaxation data, after mainly elastic and anelastic loading, is due to change of the deformation mode from an anelasticity dominated one to a plasticity dominated one. According to the state variable model by Hart, the relaxation curve in reloading should overlap with the original one after this transition in the absence of structural changes. Therefore, the crossing of stress relaxation curves after an initial plastic loading and subsequent elastic and anelastic reloadings in commercially pure aluminum at room temperature was accounted for by thermally induced effects. In the same study, consecutive stress relaxation runs were conducted on 316 SS, and no cross-over behavior was observed, which was associated with the lack of thermal effects in 316 SS at room temperature. The results demonstrate that strain aging has an effect on relaxation behavior even at room temperature, the effect being more pronounced at high strains. The phenomenamorexa0» can be accounted for according to a state variable model by modifying the rate constant, which is affected by the amount of mobile dislocations as well as dislocation mobility.«xa0less

Evaluation of Mechanical Properties of Thin Wires for Electrical Interconnections

A miniaturized universal testing machine was used to measure the yield stress, elastic and anelastic modulus, load relaxation, and low cycle fatigue properties of bonding wires. These material properties were found to be sensitive to the mierostructure of the material produced by a variety of thermal and mechanical treatments. The relations established between structure and properties can potentially be used as a basis for optimizing the bonding wire perform- ance.

Workhardening correlations based on state variables in some FCC metals in monotonic loading

This paper reports an investigation of the workhardening properties of 316 stainless steel, commercial purity aluminum, and OFHC copper at room temperature by using a state-variable approach. Based on the results of experiments, it is proposed that a generalized workhardening correlation can be formulated by using two state variables only, the structural state parameter and the inelastic strain rate. Particularly, an absolute workhardening coefficient is proposed to be a product of two parts. The athermal part, which is structure dependent only, will be approached at low homologous temperatures and/or high strain rates, while the strain rate sensitive part describes a dynamic recovery contribution at high homologous temperatures and/or low strain rates. Correspondingly, based on this approach a new way of separating thermal and athermal contributions to workhardening rate are suggested. Moreover, it is shown that a single master curve can be constructed to display the dependence of workhardening coefficient on structure and strain rate.

Hardening of graphite surface by ion beam amorphization

Ion implantation has been shown to possess potential as a practical method for improving the mechanical properties of surfaces. So far a number of tests have been performed. In these studies, metals - most often steels or other hard materials - have been used. However, the improvement of the mechanical properties at the surface is not only important for substances that already have considerable hardness, but also for materials with other unique properties. Graphite is useful in many applications. Mechanically, it is easily machined and has good dimensional stability. It has self lubricity and under certain conditions has excellent sliding properties. Chemically, it is generally impermeable, resistant to corrosion, and compatable with mating materials. It is a good conductor of both heat and electrical current. The ion implantation of graphite has been studied elsewhere but without reference to the mechanical properties of the surface. It is expected, however, that heavy ion bombardment will cause structural changes at the graphite surface resulting in a modification of the mechanical properties. In this investigation the authors focus on these properties. The main objectives have been to study how the surface mechanical properties are changed by an ion bombardment and to find the relationshipmorexa0» between mechanical properties and microstructure.«xa0less