Some applications of thermo-mechanical rheology for testing of plastics

Short-time and long-term loading stress capacity are the fundamental mechanical engineering properties of plastics. The proper mechanical measurement of these properties is complicated and expensive, since it involves longterm testing. the aim of this paper is to present the possibilities of the DTA (Differential Thermal Analysis)- method to basic calculation of determination these loading capacity of plastics.     

A laboratory investigation of the flow in the left ventricle of a human heart with prosthetic, tilting-disk valves

The understanding of the phenomena involved in ventricular flow is becoming more and more important because of two main reasons: the continuous improvements in the field of diagnostic techniques and the increasing popularity of prosthetic devices. On one hand, more accurate investigation techniques gives the chance to better diagnose diseases before they become dangerous to the health of the patient. On the other hand, the diffusion of prosthetic devices requires very detailed assessment of the modifications that they introduce in the functioning of the heart. The present work is focussed on the experimental investigation of the flow in the left ventricle of the human heart with the presence of a tilting-disk valve in the mitral position, as this kind of valve is known to change deeply the structure of such a flow. A laboratory model has been built up, which consists of a cavity able to change its volume, representing the ventricle, on which two prosthetic valves are mounted. The facility is designed to be able to reproduce any arbitrarily assigned law of variation of the ventricular volume with time. In the present experiment, a physiologically shaped curve has been used. Velocity was measured using a feature-tracking (FT) algorithm; as a consequence, the particle trajectories are known. The flow has been studied by changing both the beat rate and the stroke volume. The flow was studied both kinematically, examining velocity and vorticity fields, and dynamically, evaluating turbulent and viscous shear stresses, and inertial forces exerted on fluid elements. The analysis of the results allows the identification of the main features of the ventricular flow, generated by a mitral, tilting-disk valve, during the whole cardiac cycle and its dependence on the frequency and the stroke volume.     

Dual-camera DPIV for flow studies past artificial heart valves

A video-based digital-particle-image-velocimetry (DPIV) system with a time-delayed dual-camera recording was built for investigation of the unsteady flow structure in a pulsating flow behind artificial heart valves. The delay between the master and slave image is arbitrary by adjusting a phase-shift in the phase synchronization of the consumer video-cameras which represents a cheap and easy way to perform video-based cross-correlation DPIV at high velocities like that in heart valve flows. A calibration procedure was necessary to correct for misalignment of the image acquisition system with regard to translation and tilt of both image planes. With a framing rate of 50 Hz and continuous recording, a large number of PIV recordings at a rate of 50 Hz can be captured which enables to resolve the flow evolution in detail as demonstrated by a sequence showing the generation and sweep down of starting vortices behind the leaflets for the Bjork—Shiley—Monostrut (BSM) and Sorin—Bicarbon (SB) heart valves during the opening phase. Long-term recording with the video system over several hundreds of beating cycles offers statistical investigation of cycle-resolved fluctuations and evaluation of shear stresses. In general, this system is very suitable for video-based DPIV at higher velocities when a pulsed and synchronized illumination is not available or difficult to implement.     

Development of an apparatus for fatigue testing in vacuum

The development and use of a small, light-weight, remotely controlled fatigue apparatus are described. The particular application was for fatigue tests of 7075-T6 aluminum alloy in vacuums of 8.6×10^?8 to 4×10^?9 Torr. The fatigue life in vacuum was 1.8 times greater than the fatigue life in air based on the upper 95 percent confidence limit for tests in air and the lower 95 percent confidence limit for tests in vacuum.     

Resonant fatigue testing of riveted joints

A special test facility was constructed for evaluating the fatigue strength of turbine-blade components under steady pullout and vibratory bending loadings. The use of mechanical resonance for these applications was particularly attractive in that failure in inaccessible areas could be detected by the change in natural frequency of the specimen. The sensitivity of the method allowed determination of a fatigue crack as early in life as possible. The application of this facility to the determination of the fatigue strength of a type of riveted connection employed on certain types of blades is described in this paper. A steady load representing the centrifugal force was applied hydraulically through a long flexible pullrod which in combination with the test specimen was excited laterally at its resonant frequency by an electromagnetic shaker. An accelerometer provided closed-loop control on both vibration amplitude and resonant frequency through special circuitry. Fatigue-strength reduction factors at 10⁷ cycles were determined for three types of riveted test specimens in air at room temperature. These values exceeded the elastic stress-concentration factors estimated from earlier photoelastic tests for all three cases.     

Growth of part-through and through-thickness fatigue cracks in sheet glassy plastics

Linear-elastic fracture mechanics is used to interpret observations of through-thickness fatigue crack growth in sheet specimens of polymethylmethacrylate, polycarbonate and an unfilled epoxy, and to correlate measurements of the growth of part-through and through-thickness fatigue cracks in sheet polymethylmethacrylate. It is shown that at least one of these materials may be useful for model studies of the growth of part-through thickness fatigue cracks in metallic components.     

A methodology for thermomechanical fatigue crack growth testing in MMCs

This paper describes the experimental techniques used in an investigation of the crack growth characteristics of a four-ply, unidirectional, silicon carbide fiber reinforced, titanium matrix composite (SCS-6/Ti–6Al–2Sn–4Zr–2Mo) subjected to thermomechanical fatigue. A mechanical test system was assembled which is capable of conducting fully automated, computer-controlled thermomechanical fatigue crack growth tests. The system is able to simultaneously impose operator-defined arbitrary mechanical and thermal histories on the specimen. Crack lengths in single-edge tension [SE(T)] or middle tension [M(T)] specimens are measured by the direct-current electric potential method and optically using a unique telemicroscope system. A series of isothermal, in-phase and out-of-phase crack growth tests was conducted to obtain baseline data for material modeling purposes. The test temperatures ranged from 150°C to 538°C, and the highest thermal frequency was 0.0083 Hz.     

Study of the closing mechanism of natural heart valves

At present, there are still some controversial considerations on the closing mechanismof natural heart valves. In this area. a lot of phenomena still remain mysterious.particularlyfor the mechanism of earlier partial closure of the valve white the blood ejection is still in itsacceleration phase. It is the purpose of this study to focus on the problem of earlier closuremechanism of heart valves by using both theoretical analysis and experimentalverifications. A certain two-dimensional mathematical model of closure with allcontributing factors and the whole operating process being considered is investigated with anew analytical method Several new conclusions were found. The main points are asfollows:I. During the opening phase, the main factor of motion of the valve is the acceleratingflow, and the decelerating flow plays a main role during most the of time of the decelerationphase. But during the quasi-steady phase of blood ejection. it is the vortex in the sinus thatcontrols the valve closure.2. For Ih     

Isothermal low-cycle fatigue testing of microscale solder interconnections

A series of isothermal low-cycle fatigue studies of small to extremely small-volume solder joints has been conducted. These solder microjoints were designed and fabricated using processing which duplicates the microelectronics interconnection structures that might be used in high density, highly integrated flip-chip packaging and all fatigue tests were conducted in fully reversed simple shear both with and without dwells at maximum strain. Results of low-cycle fatigue tests of both single and double-bump 95/5 Pb/Sn solder microjoints in the form of Manson-Coffin (plastic strain amplitude versus fatigue life) plots and post-test failure mode analysis (FMA) carried out using scanning electron microscope (SEM) fractography are presented and evaluated. Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–12.     

Strain and fatigue testing of large expansion bellows

An experimental study was performed to investigate the strain and fatigue behavior of two 635-mm-diam 316 H stainless-steel convolute bellows. Bellows No. 1 was initially subjected to an axial deflection test and a pressure test, both at ambient temperature. The primary objective of these tests was to acquire strain data utilizing strain-gaging techniques. These strain tests were followed by an elevated-temperature fatigue test. Bellows No. 2 experienced the fatigue test.     

Multi-scale modelling of textile reinforced artificial tubular aortic heart valves

Tissue engineered heart valves equivalent to the native aortic heart valves are in development as an alternative to available prostheses. To achieve sufficient mechanical stiffness for application in tissue engineered valves exposed to the systemic circulation, the tissue is reinforced by a textile scaffold. Mechanical testing of structurally different textiles used as reinforcement in tissue engineered heart valves is expensive and time-consuming. The current study seeks to predict the behaviour of textile reinforced artificial heart valves using a multi-scale modelling approach. The complex textile structure was divided into simplified models at different scales. Virtual experiments were conducted on each of these models and their response was fitted by appropriate isotropic and anisotropic hyperelastic material models. The textile response was then used in a macro heart valve model, which was subjected to dynamic cardiac loading. It was shown that the current modelling approach is in good agreement with the real valve behaviour.     

The in-situ fatigue testing of a cast aluminum-silicon alloy

The study reported in this paper deals with the experimental determination of the effect of the two-phase microstructure of an aluminum-silicon alloy on the propagation of fatigue-induced fracture. The work involved the use of a computerized apparatus which applied four-point bending loads to test specimens inside the vacuum chamber of a scanning electron microscope (SEM). The use of the SEM allowed for the in-situ monitoring of the progression of the fatigue crack with respect to the microstructure of the material.A constant load amplitude fatigue test was established for the experimental system and an analytic model is proposed to predict the growth of the crack in the specimen using compliance measurements. In-situ tests were performed on a variety of test aluminum-silicon alloy specimens. Preliminary results show that a relationship exists between the silicon phase in the aluminum matrix and propagating fatigue crack. Data from these tests were used to evaluate both the model mentioned above and an analytic relation for the stress intensity factor for a beam containing a crack subjected to four-point bending.     

Changes in modal parameters of a bridge during fatigue testing

The effects of high-cycle fatigue on the dynamic modal properties of a full-scale bridge test section are examined in this study. A span 30.5 m (100 ft) in length was salvaged from an interstate bridge for experimental testing to establish the fatigue performance of the retrofit procedures which could be used for future field repairs of the fatigue cracks. Horizontal cracks had formed in the webs of the twin bridge girders in the vicinity of the web-flange weld at or near the connections of the girders to the floor beams. An electrohydraulic vibration shaker was used to excite the test span in the torsional and bending modes to promote fatigue crack growth. In addition to the experimental evaluation of the retrofit procedures, the dynamic response characteristics of the test span during the high-cycle fatigue test were monitored. Experimental data have been used to establish mode shapes and modal damping ratios during the four-million-cycle fatigue test. Damping ratios ranged from 1.09 percent to 0.53 percent. Modal stiffnesses were calculated based on the experimental mode shapes. There is a modest correlation between the changes in stiffness and the observed cracking of the steel girders and the subsequent repairs. Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Porland, OR on June 5–10.     

Review and evaluation of the double-torsion technique for fracture toughness and fatigue testing of brittle materials

This paper reviews the double-torsion (DT) test as an experimental technique for the measurement of fracture toughness and slow-crack-growth behavior in brittle materials based on the authors' experiences and an evaluation of current literature. The DT technique has numerous advantages due primarily to the fact that the stress intensity is independent of crack length, at least for the central half of the specimen. Although the technique was first proposed about 20 years ago, and has been used extensively since then, there are a number of important unresolved questions concerning the methodology. To date there has been no standardization of test procedure or specimen geometry. A review of specimen geometries in use indicates that the proportions that are most commonly employed (based on the literature and experience) are width:W, length: 3W, thickness:W/6–W/15. Grooves on both the top and the bottom surfaces have been used to guide the crack, however it has been found that ungrooved, but very accurately aligned, specimens give the best results. Theoretical thickness-correction factors which account for both relatively thick specimens (with respect to width) and the effect of the size of the loading points, have been considered. The effect of crack-front profile on measured values of crack velocity and stress intensity is contentious. Althogh the stress intensity,K, varies along the crack front, the front merely translates axially. For the presentation of crackvelocity stress-intensity (V-K) data, the consensus seems to be that the only rational velocity to use is that based on the crack's intersection with the tensile surface. Despite some of the shortcomings mentioned above, the DT technique is widely accepted and gaining in popularity. It is particularly useful under cyclic fatigue conditions for investigating the effect of a change in a single parameter on crack-growth rate, using the very effective and elegant ‘changeover’ method.     

Spectral estimation and analysis of LDA data in pulsatile flow through heart valves

The Laser Doppler Anemometry (LDA) measurement technique presents an inherent difficulty when spectral analysis is applied to it. The random nature of the LDA signal prohibits sampling at regular, equi-spaced, time instants. Irregular sampling presents additional variability of the spectral estimator. In order to reduce this variability, spectral analysis of LDA data is performed according to the method of direct Fourier transform of short blocks of data, as suggested by Gaster and Roberts. The LDA data is measured in a flow field distal to prosthetic heart valves with varying degrees of stenosis. The spectral estimates of velocity data sampled during the rapid closure stage of the valve are achieved with excellent frequency resolution. Important and useful information about dominant frequency peaks and preferred modes which exist in the flow, otherwise smeared or concealed in the spectral contents, are then derived from the spectral information. These modes are quantitatively analysed in light of vortex formation and related flow mechanisms. Comparative studies of normal and stenosed valves show that the preferred modes are governed by the valve geometry and dynamic behavior and are correlated to the severity of the stenosis.     

determination of dynamic errors generated during fatigue testing with use of Krylov functions

Institute of Mechanics, Academy of Sciences of the Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 27, No. 12, pp. 78–85, December, 1991.