D. Canadinc

On the fatigue behavior of ultrafine-grained interstitial-free steel

Abstract The present paper reports on the cyclic stress–strain response of body-centered cubic ultrafine-grained (UFG) interstitial-free (IF) steel severely plastically deformed at room temperature utilizing equal channel angular extrusion (ECAE). Low-cycle fatigue tests were conducted with various strain amplitudes and strain rates on samples obtained through different ECAE routes and number of ECAE passes in order to determine the optimum processing route(s) for improved fatigue response of this material. UFG IF steel is superior to its coarse grained counterpart under both monotonic and cyclic loading in terms of properties, such as stress ranges tolerated, strength levels attained, and the corresponding fatigue behavior. All UFG steels subjected to more than 4 ECAE passes exhibit stable cyclic stress–strain response. Moreover, it was shown that dynamic grain coarsening, which usually leads to cyclic softening in UFG materials, is not prevalent in the ECAE processed UFG IF steel. For representing the fatigue life of UFG IF steel, the parameter after Smith, Watson and Topper, which is an indication of energy dissipation per cycle, proved adequate while comparing materials obtained through different ECAE routes.

Estimation of fracture toughness of liver tissue: Experiments and validation

The mechanical interaction between the surgical tools and the target soft tissue is mainly dictated by the fracture toughness of the tissue in several medical procedures, such as catheter insertion, robotic-guided needle placement, suturing, cutting or tearing, and biopsy. Despite the numerous experimental works on the fracture toughness of hard biomaterials, such as bone and dentin, only a very limited number of studies have focused on soft tissues, where the results do not show any consistency mainly due to the negligence of the puncturing/cutting tool geometry. In order to address this issue, we performed needle insertion experiments on 3 bovine livers with 4 custom-made needles having different diameters. A unique value for fracture toughness (J=164±6 J/m(2)) was obtained for the bovine liver by fitting a line to the toughness values estimated from the set of insertion experiments. In order to validate the experimental results, a finite element model of the bovine liver was developed and its hyper-viscoelastic material properties were estimated through an inverse solution based on static indentation and ramp-and-hold experiments. Then, needle insertion into the model was simulated utilizing an energy-based fracture mechanics approach. The insertion forces estimated from the FE simulations show an excellent agreement with those acquired from the physical experiments for all needle geometries.

Stress-strain-temperature behaviour of [001] single crystals of Co49Ni21Ga30 ferromagnetic shape memory alloy under compression

The conventional shape memory effect (SME) and pseudoelasticity (PE) in as-grown [100] single crystals of Co49Ni21Ga30 alloy under compression are reported. The parent single crystals exhibit about 5% transformation strain at compressive stress levels as low as 4 MPa, and a pseudoelastic strain of 4.5%. Complete PE was observed in the temperature range from 35 to 285°C, along with increasing stress hysteresis with temperature. The latter is attributed to increasing number of variants and the corresponding variant–variant interactions. We demonstrate that the current material can be utilized in applications that demand high strength at elevated temperatures. Moreover, the current results also indicate the potential of this material to exhibit magnetic shape memory effect, which could broaden the scope of utility of this material upon further research.

Evaluation of passive oxide layer formation-biocompatibility relationship in NiTi shape memory alloys: geometry and body location dependency.

A systematic set of ex-situ experiments were carried out on Nickel-Titanium (NiTi) shape memory alloy (SMA) in order to identify the dependence of its biocompatibility on sample geometry and body location. NiTi samples with three different geometries were immersed into three different fluids simulating different body parts. The changes observed in alloy surface and chemical content of fluids upon immersion experiments designed for four different time periods were analyzed in terms of ion release, oxide layer formation, and chemical composition of the surface layer. The results indicate that both sample geometry and immersion fluid significantly affect the alloy biocompatibility, as evidenced by the passive oxide layer formation on the alloy surface and ion release from the samples. Upon a 30 day immersion period, all three types of NiTi samples exhibited lower ion release than the critical value for clinic applications. However; a significant amount of ion release was detected in the case of gastric fluid, warranting a thorough investigation prior to utility of NiTi in gastrointestinal treatments involving long-time contact with tissue. Furthermore, certain geometries appear to be safer than the others for each fluid, providing a new set of guidelines to follow while designing implants making use of NiTi SMAs to be employed in treatments targeting specific body parts.

Modeling the role of external stresses on the austenite-to-bainite phase transformation in 51CrV4 steel

A new model is proposed to successfully predict the initiation and evolution of the austenite-to-bainite phase transformation, capturing specifically the time-dependent transformation kinetics. In particular, the isothermal bainitic transformation in 51CrV4 steel is experimentally observed for various constant stress conditions, and significant improvement is obtained in comparison with the existing models. Specifically, both the transformation kinetics and the resultant transformation strains can be simultaneously predicted using the same variant growth approach. Simulation results are in good agreement with the experiments, evidencing the success of the proposed model in describing the transformation phenomena in terms of kinetics and transformation plasticity. Furthermore, the proposed formulation provides a basis for incorporating variant–variant interactions and cementite formation in the residual austenite matrix.

Early detection of crack initiation sites in TiAl alloys during low-cycle fatigue at high temperatures utilizing digital image correlation

Abstract Fatigue-induced damage accumulation was investigated in a third generation titanium aluminide alloy both at room temperature and at a temperature of 700 °C promoting oxidation. The digital image correlation technique was utilized for monitoring the evolution of local strain fields with cyclic deformation at both temperatures. With the aid of a newly adopted surface patterning technique, digital image correlation successfully detected the crack initiation sites prior to the actual formation of the cracks. Despite the oxidation at elevated temperatures, digital image correlation could detect the crack initiation sites at the early stages of the cyclic deformation, laying out the potential of this technique for monitoring the damage evolution in various metallic materials under severe service conditions.

On the role of the cooling rate and crystallographic orientation on the shape memory properties of CoNiAl single crystals under compression

Crystallographic orientation and cooling rate dependences of the shape memory effect and pseudoelasticity are reported for CoNiAl alloy single crystals oriented along the [001], [110] and [123] directions. Phase transformation response, pseudoelasticity and two-way shape memory effect were investigated through thermo-mechanical experiments. The experimentally determined transformation strains exceeding the theoretical values in the [110] orientation single crystals are attributed to the additional contribution from the detwinning strain. High cooling rates (oil and water quenching) were found to lead to a lower γ-phase volume fraction that resulted in higher shape memory strains under low external stresses. Moreover, the [001] oriented single crystals demonstrated perfect pseudoelasticity (up to 4.3% strain), a large pseudoelastic window (>140 °C), high resistance to dislocation slip and small thermal hysteresis. The existence of detwinning, easy reorientation of favorable variants under low external stresses and significant fully recoverable strains have important implications regarding the magnetic field-induced shape changes in this class of alloys.

On the Cyclic Stability and Fatigue Performance of Ultrafine-Grained Interstitial-Free Steel under Mean Stress

This paper reports on the fatigue performance of an ultrafine-grained (UFG) interstitialfree (IF) steel deformed at various mean stress levels. The UFG microstructure was achieved using equal channel angular extrusion processing at room temperature (RT) and along an “efficient” route, giving way to the formation of high angle grain boundaries (HAGBs) with a high volume fraction. The current study not only confirms the previous finding that a high volume fraction of HAGBs promotes cyclic stability, but also inquires into the role of mean stress level on the cyclic stability. It is shown that the UFG IF steel exhibits a stable cyclic deformation response in the lowcycle fatigue regime within the medium applied mean stress range of -75 to 75 MPa. The corresponding fatigue lives can still be predicted with the Smith-Watson-Topper approach within this range. Furthermore, the present study demonstrates that the evolution of mean strains with cyclic deformation can be linked to the evolution of mean stresses in strain-controlled loading.

Twinning activity in high-manganese austenitic steels under high velocity loading

Deformation temperature and manganese content dependencies of twinning activity in two types of high Mn austenitic steels were investigated upon high velocity tensile loading. It was observed that nanotwin formation within previously formed twins dominates at subzero temperatures and significantly contributes to work hardening.

Evaluation of the biocompatibility of NiTi dental wires: a comparison of laboratory experiments and clinical conditions.

Effects of intraoral environment on the surface degradation of nickel-titanium (NiTi) shape memory alloy orthodontic wires was simulated through ex situ static immersion experiments in artificial saliva. The tested wires were compared to companion wires retrieved from patients in terms of chemical changes and formation of new structures on the surface. Results of the ex situ experiments revealed that the acidic erosion effective at the earlier stages of immersion led to the formation of new structures as the immersion period approached 30 days. Moreover, comparison of these results with the analysis of wires utilized in clinical treatment evidenced that ex situ experiments are reliable in terms predicting C-rich structure formation on the wire surfaces. However, the formation of C pileups at the contact sites of arch wires and brackets could not be simulated with the aid of static immersion experiments, warranting the simulation of the intraoral environment in terms of both chemical and physical conditions, including mechanical loading, when evaluating the biocompatibility of NiTi orthodontic arch wires.