David A. Miller

Thermomechanical fatigue of shape memory alloys

As shape memory alloys (SMAs) gain popularity as high energy density actuators, one characteristic that becomes particularly important is the thermomechanical transformation fatigue life, in addition to maximum transformation strain and stability of actuation cycles. In this paper, a novel test frame design and testing protocol are discussed, for investigating the thermally activated transformation fatigue characteristics of SMAs under various applied loads for both complete and partial phase transformation. A Ni50Ti40Cu10 (at.%) SMA was chosen for this investigation and the effects of various heat treatments on the transformation temperatures and the transformation fatigue lives of actuators were studied. For selected heat treatments, the evolution of recoverable and irrecoverable strains up to failure under different applied stress levels was studied in detail. The influence of complete and partial transformation on the fatigue life is also presented. The irrecoverable strain accumulation as a function of the number of cycles to failure for different stress levels is presented by a relationship similar to the Manson–Coffin law for both partial and complete transformations.

Hydrodynamic Deformation and Removal of Staphylococcus epidermidis Biofilms Treated With Urea, Chlorhexidine, Iron Chloride, or DispersinB

The force‐deflection and removal characteristics of bacterial biofilm were measured by two different techniques before and after chemical, or enzymatic, treatment. The first technique involved time lapse imaging of a biofilm grown in a capillary flow cell and subjected to a brief shear stress challenge imparted through increased fluid flow. Biofilm removal was determined by calculating the reduction in biofilm area from quantitative analysis of transmission images. The second technique was based on micro‐indentation using an atomic force microscope. In both cases, biofilms formed by Staphylococcus epidermidis were exposed to buffer (untreated control), urea, chlorhexidine, iron chloride, or DispersinB. In control experiments, the biofilm exhibited force‐deflection responses that were similar before and after the same treatment. The biofilm structure was stable during the post‐treatment shear challenge (1% loss). Biofilms treated with chlorhexidine became less deformable after treatment and no increase in biomass removal was seen during the post‐treatment shear challenge (2% loss). In contrast, biofilms treated with urea or DispersinB became more deformable and exhibited significant biofilm loss during the post‐treatment flow challenge (71% and 40%, respectively). During the treatment soak phase, biofilms exposed to urea swelled. Biofilms exposed to iron chloride showed little difference from the control other than slight contraction during the treatment soak. These observations suggest the following interpretations: (1) chemical or enzymatic treatments, including those that are not frankly antimicrobial, can alter the cohesion of bacterial biofilm; (2) biocidal treatments (e.g., chlorhexidine) do not necessarily weaken the biofilm; and (3) biofilm removal following treatment with agents that make the biofilm more deformable (e.g., urea, DispersinB) depend on interaction between the moving fluid and the biofilm structure. Measurements such as those reported here open the door to development of new technologies for controlling detrimental biofilms by targeting biofilm cohesion rather than killing microorganisms. Biotechnol. Bioeng. 2011;108: 2968–2977.

The SNL/MSU/DOE Fatigue of Composite Materials Database: Recent Trends

Trends of recent test data in three areas are described for wind blade materials in the SNL/MSU/DOE fatigue of composite materials database 1 . First, a complete 3-D set of static elastic constants and strength properties is given for a thick infused glass fabric/epoxy laminate. Second, results are presented which explore the effects of fabric structure and resin type on the tensile fatigue resistance. Using aligned strand structure as a baseline, the efficiency of stitched fabric reinforcement is quantified for static and fatigue properties, and the origins of poor fatigue performance with some resins are identified. Third, an overall comparison is given of the tensile fatigue sensitivity of various blade materials including laminate in-plane and interlaminar failure, epoxy based blade adhesives and core materials. Comparisons of fiber dominated and resin dominated failure modes show clear trends in the fatigue exponent, depending on the resin system.

Thermomechanical training and characterization of Ni?Ti?Hf and Ni?Ti?Hf?Cu high temperature shape memory alloys

Nickel–titanium (NiTi) is the most commonly used shape memory alloy (SMA) for actuator applications, though its usefulness is limited to temperature ranges below 100 °C. High temperature SMAs are formed by adding ternary elements to NiTi, but their usefulness as actuators is still in question. The purpose of this research was to characterize and train two high temperature SMAs, NiTi29.7Hf20 and NiCu5Ti29.7Hf20, to determine their effectiveness as linear actuators. Low temperature martensitic phase and high temperature austenitic phase stress–strain tests were performed to characterize the materials’ behavior followed by temperature cycling under constant stress. Temperature cycling under constant stress is known as thermomechanical training and resulted in small amounts of plastic strain growth and the development of two-way shape memory (TWSM). The results from these tests support the conclusion that hafnium distorts slip planes within the martensitic material phase, and that (Ti,Hf)2Ni and (Ti,Hf)3Ni4 particulates form during aging and annealing. The distorted slip planes cause slip and martensite reorientation to occur simultaneously, which develops a strong internal stress field during training within the first few cycles. The internal stress field develops TWSM, but limits further plastic growth. The particulate formation also embrittles the material. The transformation temperatures of both alloys were below creep and annealing temperatures making them ideally suited for high temperature actuators.

Performance of Composite Materials Subjected to Salt Water Environments

This paper presents the recent trends in the mechanical characterization of composite systems under consideration for Marine Hydro Kinetic (MHK) applications exposed to salt water environments. First, a testing protocol for environmental effects has been developed for resin infused in-house fabricated laminates. Unidirectional ([0] and [90]) mechanical test samples were submerged in synthetic sea water at 40°C and 50°C, with the weight recorded at time intervals over the entire period. Additional witness coupons were submerged to monitor effects of fiber orientation and cure temperature. Next, after conditioning to both full saturation and partial saturation, static compressive and tensile strength properties at temperatures of 0°C, 20°C and 40°C were collected. Tensile fatigue resistance was also measured at room temperature for the 0° samples. These results show trends of reduced tensile and compressive strength with increasing moisture and temperature in the 0° (longitudinal) direction. In the 90° (transverse) direction, compression strength decreases but tensile strength is little affected as temperature and moisture increase. The fatigue resistance of environmentally conditioned samples is reduced at high stress levels, but matches the un-conditioned samples at low stress levels. Finally, both mechanical and chemical analysis results are presented for samples conditioned in a Salt Fog Chamber for unidirectional, [0] and [90], samples.

Creep/Fatigue Behavior of Resin Infused Biaxial Glass Fabric Laminates

This study has explored creep/fatigue effects for infused glass/epoxy biaxial (±45 o ) fabric laminates typical of shell/web areas of wind blades. Loading conditions were cyclic fatigue at R-values 0.1, -1 and 10, as well as tensile and compressive creep. Cyclic stress-strain loops for load and strain control are compared. The results are discussed in terms of the relative creep or cyclic dominance and appropriate fatigue design criteria. Laminates with a low content of 0 o unidirectional plies added to the biaxial plies were also briefly studied in terms of the interaction of creeping, softening effects of the biaxial plies on the performance of the 0 o plies.

7 – Effects of resin and reinforcement variations on fatigue resistance of wind turbine blades

: This chapter explores the influence of resin and reinforcing fabric variations on the fatigue sensitivity for a wide range of typical blade laminates reported recently in the SNL/MSU/DOE database. Test results are presented for static and fatigue property variations with resin type, reinforcing fabric construction and weight, fiber content and laminate construction. Critical resin/fabric interactions and damage mechanisms are identified. The effects of resin and fiber type are also explored for material transitions at ply drops, where ply delamination is the dominant damage.

Analysis of SNL/MSU/DOE Fatigue Database Trends for Wind Turbine Blade Materials 2010-2015.

Wind turbine blades are designed to several major structural conditions, including tip deflection, strength and buckling during severe loading, as well as very high numbers of fatigue cycles and various service environments. The MSU Database Program has, since 1989, addressed the broad range of properties needed for current and potential blade materials through static and fatigue testing and test development in cooperation with Sandia National Laboratories and wind industry and supplier partners. This report is the latest in a series, giving test results and analysis for the period 2010-2015. Program data are compiled in a public database [1] and other reports and publications given in the cited references. The report begins with an executive summary and introductory material including background discussion of previous related studies. Section 3 describes experimental methods including processing, test methods, instrumentation and test development. Section 4 provides static tension, compression and shear stress-strain properties in three directions using coupons sectioned from a thick infused unidirectional glass/epoxy laminate. The nonlinear, shear dominated static properties were characterized with loading-unloading-reloading (LUR) tests in tension and compression to increasing load levels, for ±45O laminates. Section 5 explores the origins of tensile fatigue sensitivity in glass fiber dominated laminates with variations in fabric architecture including specially prepared fabrics and aligned strand laminates. Several types of resins are considered, with variations in resin toughness and bonding to fibers, as well as cure cycle variations for an epoxy. Conclusions are drawn as to the limits of tensile fatigue resistance

Influence of Fabric Architecture on Damage Progression Evidenced by Acoustic Emission Measurements

Previous experimental results have studied the effect of fabric architecture on the static and fatigue response of glass reinforced polymer composites. Additional studies have analyzed the acoustic emission (AE) signatures of glass/epoxy composites which are monotonically loaded to failure [1-3]. These studies have identified systematic testing methodologies, and general relationships between measured acoustic properties and specific internal damage modes. The primary acoustic emission property utilized within this work is the peak frequency of each acoustic event. A plot of this peak frequency as a function of the applied strain show distinct levels, or bands, often attributed to specific damage modes, e.g., matrix cracking, interphase failure etc. Common fiberglass fabrics utilized within wind turbine blades have heavy unidirectional fibers held in place with stitching, a layer of random mat, or a combination of both. Results presented within this paper include the acoustic emission results for unidirectional composites monotonically loaded in both longitudinal and transverse directions, and biaxial composites from the same fabric. These results are compared with acoustic emission frequencies from glass/epoxy pre-preg materials with no stitching or additional backing material. It is shown that the addition of plies at different orientations and the removal of stitching effect the number of acoustic events and the strain to failure.

Fatigue Resistance of Wind Blade Laminates Containing In-Plane Waviness Flaws

Summary Preceding studies 1,2 have explored the effects of waviness flaws on the static strength of blade laminates, showing severe strength loss for many flaw geometries. Blades containing undetected/unrepaired waviness are subject to spectrum fatigue loading in service, where the flaws may extend sufficiently to produce failure at low service loads. This paper will report the results of fatigue loading of laminates containing flaw geometries selected from the preceding studies. Preliminary results show progression of the waviness flaws to produce failure at relatively low applied fatigue loads, consistent with knockdowns found in static tests. The effects of waviness flaws on statistical aspects of strength and lifetime will be included in the paper. Modeling of the waviness flaws has shown that damage progression under axial loading is primarily driven by local shear stresses in the wave area 1 . Separate research over the past several years 3,4 has addressed creep/fatigue interactions for similar infused glass/epoxy laminates without artificial flaws, under shear loading. This work is being extended to explore the dominant creep/fatigue mechanisms responsible for damage progression in waviness flaws; in particular, whether criteria which control damage growth are cyclic or cumulative time based. Various loading conditions and flaw geometries will be included in the paper, although preliminary fatigue results are currently only available for a single case.