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Seven laboratories participated in an interlaboratory study to assess the applicability of ASTM E606, Test Method for Strain-Controlled Fatigue Testing, to strain-controlled fatigue of a tungsten fiber/copper matrix composite (Battelle Columbus, University of Iowa, IITRI, MCL, MarTest, Rockwell International Science Center and University of Twente). Because of the material and specimen fabrication methods, the number of fibers in the 9, 25, and 36 % fiber volume fraction specimens is approximately the same and the strain-life data are nearly identical for each of the volume fractions. Although the specimen aspect ratio is within the nominal range, the specimen diameter is less than the nominal value specified in ASTM E606. Laboratories reporting data noted that a small specimen diameter and a large gage section aspect ratio made specimen alignment difficult. Thus, specimen buckling was problematic at high strain ranges. The study data are reported and analyzed with nonparametric and semiparametric statistical methods to assess the effect of study covariates on fatigue life. The median fatigue life, median absolute deviation from the median, and interquartile range are reported as measures of central tendency and variability. ASTM E606 may be used as a guide to evaluate the fatigue response of this type of composite material if the length-to-diameter aspect ratio, specimen alignment in the fatigue machine, and specimen surface finish are closely monitored to ensure valid data. [ABSTRACT FROM AUTHOR]

ASTM E2948, Standard Test Method for Conducting Rotating Bending Fatigue Tests of Solid Round Fine Wire, is a procedure for rotating bending fatigue tests of fine wire to obtain the fatigue strength for metallic materials at a specified life in the fatigue regime where the strains are predominately and nominally linear elastic. This method is limited to the fatigue testing of small diameter solid round wire subjected to constant amplitude, completely reversed, periodic strain. The methodology can be useful in assessing the effects of inclusions, melt technique, and cold work processing. Data obtained in accordance with ASTM E2948 is subject to variability because of many factors. An inter-laboratory study (ILS) was performed to assess variability of the data and to develop a precision statement for ASTM E2948. Ten laboratories participated in the ILS. The samples consisted of two materials, Nitinol and 35N LT. Each material was supplied from a single lot from a single manufacturer. Samples were tested at three different strain amplitudes with ten replicates at each strain level. Variability of the resulting data was quantified as reproducibility limits, between laboratory variability, and repeatability limits, within laboratory variability. A precision statement for ASTM E2948 was formulated based on the reproducibility and repeatability limits determined from the inter-laboratory study. The viability of the limits is assessed with Monte Carlo simulations. Alternative methods of analysis using nonparametric statistics and bootstrap methods are discussed. [ABSTRACT FROM AUTHOR]

ABSTRACT Studies in fatigue can be summarized into two stages, fatigue crack initiation and crack propagation. Fatigue damage may increase the risk of failure under cyclic load. Energy dissipation, termed damping, occurs in engineering metals and is a function of the cyclic loading history. Damping behaviour of materials has been estimated using many different experimental techniques, and parameters i.e. the loss factor vs. strain amplitude, frequency range, etc. However, micro-structural changes in the form of fatigue damage are also contributors to damping in engineering materials. In order to measure energy dissipation, a damping monitoring method has been used. Under a constant cyclic load up to the point of fatigue crack initiation, the effects of fatigue on damping factor were studied for 6061-T6511 aluminium alloy. In the experiments, the stress levels were below yield point, 50% and 70% of ultimate strength. Experimental results showed that the damping factor changes with the number of fatigue cycles. Percentage increase in damping energy was calculated using experimental data. [ABSTRACT FROM AUTHOR]

Breast implant strength and durability is presently an important topic in biomaterials science. Research studies are being conducted to determine the mechanisms and rates of failure in order to assess the in vivo performance of breast implants. Fatigue life is a measure of breast implant durability since fatigue failure is a potential in vivo failure mechanism. This study describes the characterization of the fracture surface morphology of breast implant shell regions that have failed due to cyclic fatigue. Saline breast implants were fatigue tested to failure using a laboratory apparatus in which flat plates cyclically compressed the implants. The implants were unimplanted control devices of both textured and smooth saline implants. The failure surfaces of the fatigued shells were examined using scanning electron microscopy (SEM). The morphological features of the failure surfaces are described for implants with short and long fatigue lifetimes. The details of both the inside and outside surfaces of the shell at the failure location are described. Two different modes of failure were observed in both the textured and smooth shells. These modes depend on the magnitude of the cyclic load and corresponding number of fatigue cycles at failure. The first mode is a tear in the shell of about 18 mm in length, and the second mode is a pinhole approximately 1 mm in diameter. Details of the surface morphology for these two types of failure modes and shell thickness data are presented herein. There was no significant change in the crosslink density of the shell as a result of fatigue.

Several generations of silicone gel breast implants have been produced by implant manufacturers. The primary material usually viewed as the base material in the manufacture of implants is polydimethylsiloxane. Polymeric reactions are notorious for their variability and nonuniformity. The elastomer used in different types of implants can have vastly different properties. Furthermore, the material properties associated with a particular type of implant can vary considerably from one lot to the next. Considering the various designs, styles, and manufacturing techniques associated with silicone gel implants, knowledge of the original properties of the implants before implantation is important in determining the effects of aging in vivo. This study was conducted to investigate differences in key mechanical and chemical properties of silicone gel breast implant materials. The two types of implants chosen for analysis were Silastic I and Silastic II control implants. Material property data were determined for both types of controls and significant differences were found in their values. Lot-to-lot variability was also investigated and found to be significant.

In this article, mechanisms of breast-implant failure caused by surgical instruments commonly used to perform implantation, breast biopsies, needle localization procedures, cyst aspirations, and explantation are described. Failure was artificially induced in breast-implant shells using various types of surgical instruments, including scalpels, suture needles, hypodermic needles, hemostats, and Adson forceps. Field-emission scanning electron microscopy (SEM) was used to document the morphology of the failure sites produced by these instruments. Micrographs were used to categorize failure according to a specific type of surgical instrument. SEM micrographs were also obtained on explants that failed in situ, and the morphology of the corresponding failure sites was examined. The study was designed to document a range of failure mechanisms associated with gel-filled, saline-filled, double-lumen (saline-gel), and soybean oil-filled implants. The results of the study also demonstrate that SEM can often be used to determine the cause of breast-implant failure.

The transport of octamethylcyclotetrasiloxane (D4), one of the major constituents of silicone fluids and rubbers, and low viscosity polydimethylsiloxane oil into a silica filled cross-linked silicone elastomeric rubber was measured as a function of temperature, cross-link density of the rubber, and concentration of the D4 in methanol solution. A small amount of material, approximately 3 wt%, is extracted from the rubber with hexane. The extraction process has a large effect upon D4 solubility in the rubber, increasing from approximately 160 to 180 wt% after extraction. The heats of solution for both penetrants into the rubber are essentially zero and the activation energies for diffusion are small, approximately 8 and 15 kJ molt(-1) for D4 and PDMS, respectively. The diffusion process is Fickian and the diffusion coefficient of D4 into silicone/silica rubbers is essentially independent of concentration over the concentration investigated, i.e. from 1 to 100 vol% D4 in methanol. The permeability, i.e. the product of the diffusion coefficient and the solubility, decreases rapidly for D4 concentrations less than 50 vol% (0.1 mol fraction). This suggests that the permeation of D4 out of any encapsulation device, such as a silicone breast implant, is linearly dependent upon the concentration of D4 in the prosthesis. Swelling is isotropic and was measured by dimensional changes in rectangular samples and correlates well with the volume of D4 sorbed.

The reasons for the failure of silicone gel breast implants are unclear. One potential failure mechanism is the weakening of the implant shell during its insertion into the breast. Such local weakening could eventually lead to implant failure. We recently reported on the effect of implant surgery on the overall mechanical properties of SILASTIC II gel-filled implants. In the earlier study, the mechanical properties of 34 Dow Corning SILASTIC II gel-filled breast implants from the same manufacturing lot were measured. Twenty of the thirty four implants were not implanted but were evaluated to establish a baseline of control data. The other fourteen lot-matched implants were inserted into a subglandular pocket through an inframammary incision in a cadaver breast and then removed. The experimental augmentation scenario was designed to represent actual breast implantation as closely as possible. The mechanical properties of the anterior and posterior sides of the control implants (not implanted) and explants (implanted in a cadaver) were measured and compared to determine whether differences existed between the explant and control groups. We found that the implantation surgery process did slightly reduce the average tensile strength. Although not as statistically significant, other mechanical properties such as breaking energy and moduli were less for the explants than the controls. The reduction was a relatively small percentage in the context of overall shell properties. Elongation and tear resistance were unaffected. Our findings suggested that the surgical act of implanting a breast implant has a small but detectable weakening effect on the average tensile strength, breaking energy and moduli of the elastomeric shell of the device. The present study is an extension of the previous investigation. Here we have analyzed the explant shell region where the surgeon's fingers forced the implant through the incision. Our results indicate that the implant shell can be locally damaged due to the implantation process.