Decrease In Fatigue Resistance Research Articles

Fatigue behaviour of a box‐welded joint under biaxial cyclic loading: effects of biaxial load range ratio and cyclic compressive loads in the lateral direction

ABSTRACT The biaxial fatigue of a steel plate (JIS SM400B) having a box‐welded (wrap‐around) joint was experimentally studied. Special concerns were focused on the effects of the biaxial load range ratio and compressive cyclic loading in the lateral direction. The direction of fatigue crack propagation under biaxial cyclic tensile loading, which has a phase difference of π, changed according to the biaxial load range ratio, Rxy = ΔPx/ΔPy. When Rxy was less than 0.56, fatigue cracks propagated along the toe of the weld in the x‐direction because the principal tensile stress range Δσy at that location exceeded the orthogonal value Δσx at the box‐weld toe. The fatigue lives in biaxial tests related well to the data from uniaxial tests when invoking the Δσ5 criterion. However, the location and direction of Δσ5 should be chosen according to the Rxy value and the failure crack direction. An increase in Δσ5, as induced by the Poisson's ratio effect from either the out‐of‐phase tensile loading or the in‐phase compressive loading in the y‐direction, leads to an increase in fatigue damage (decrease in fatigue resistance or specifically a faster crack propagation rate), and this effect can be successfully estimated from uniaxial fatigue test data.

Effects of boronising on fatigue life of plain carbon and low alloy Cr–Mo steels

In the present paper the hardness profiles, stress versus number of cycles to failure (S–N curves), and fatigue threshold stresses of thermochemically boronised plain carbon and low alloy Cr–Mo steels have been investigated. A comparison has been made with the fatigue behaviour of the steel without any surface treatment. In addition, X-ray diffraction has been employed to determine the boride phases formed. Optical and scanning electron microscopy have been used to examine the microstructure and fracture surfaces of all boronised and fatigue tested specimens. The results obtained revealed a boronised layer with a thickness of about 100 μm, for both types of steel. In comparison with untreated specimens, a 50% increase and a 20% decrease in fatigue resistance for boronised plain carbon and Cr–Mo steel specimens, respectively, was observed. In addition, it was found that hardening and tempering the boronising specimens of both grades of steel decreased the fatigue strength, however, the fatigue strength of the boronised plain carbon steel was still greater than that of the as received plain carbon steel.

Fracture and fatigue properties of acrylic bone cement: The effects of mixing method, sterilization treatment, and molecular weight

The purpose of this study was to characterize the relative and combined effects of sterilization, molecular weight, and mixing method on the fracture and fatigue performance of acrylic bone cement. Palacos® R brand bone cement powder was sterilized using ethylene oxide gas (EtO) or gamma irradiation. Nonsterile material was used as a control. Molecular weights of the bone-cement powders and cured cements were measured using gel permeation chromatography. Hand and vacuum mixing were employed to mold single edge-notched bend specimens for fracture toughness testing. Molded dog-bone specimens were used for fatigue tests. Electron microscopy was used to study fracture mechanisms. Analysis of variance and Student t-tests were used to compare fracture and fatigue performance between sterilization and mixing groups. Our results indicate that vacuum mixing improved significantly the fracture and fatigue resistance (P <.05, P <.07) over hand mixing in radiation-sterilized and EtO-sterilized groups. In vacuum-mixed cement, the degradation in molecular weight resulting from gamma irradiation decreased fracture resistance significantly when compared with EtO sterilization and control (P <.05). A corresponding decrease in fatigue resistance was observed in the cement that was degraded severely by a radiation dose of 10 MRad (P <.05). In contrast, EtO sterilization did not result in a significantly different fracture resistance when compared with unsterilized controls for vacuum-mixed cement (P >.1). For hand-mixed cement, fracture and fatigue resistance appeared to be independent of sterilization method. This independence is believed to be the result of higher porosity that compromised the mechanical properties and obscures any effect of sterilization. Our results indicate that a combination of nonionizing sterilization and vacuum mixing resulted in the best mechanical performance and is most likely to contribute to enhanced longevity in vivo.

Compression and tension fatigue resistance of medical grade ultra high molecular weight polyethylene: the effect of morphology, sterilization, aging and temperature

It is shown in this study that polymer morphology, sterilization with or without accelerated aging, and temperature affect the fatigue crack propagation behavior of ultra high molecular weight polyethylene (UHMWPE). Changes in UHMWPE orthopedic implants due to sterilization and shelf aging are of great clinical interest and importance. In recent years, accelerated aging has been used to simulate shelf aging and post-sterilization oxidation in UHMWPE orthopedic components. In order to understand the role of sterilization and post-sterilization aging, two commercially available polymers (GUR4150HP and Hylamer®-M) with distinct morphologies were subjected to one of four sterilization techniques (gamma radiation in air, gamma radiation in vacuum, ethylene oxide gas, and gas plasma). Non-sterile samples were used as controls for all studies. Half the groups were tested in the unaged condition. The remaining samples were aged under accelerated conditions via elevated temperature (70°C) and oxygen pressure (5atm) for 14 days to simulate five years of shelf aging. Subsequent to sterilization and aging treatments, specimens were subjected to cyclic compression loading followed by cyclic tensile loading in order to characterize the fatigue resistance. Scanning electron microscopy was used to provide an understanding of fatigue fracture mechanisms. Fourier transform infrared spectroscopy and density gradient column methods were used to determine the structural changes brought about by sterilization and aging. It is found in this work that accelerated aging causes a decrease in fatigue resistance regardless of sterilization method. Fatigue degradation is most severe for gamma radiation in air coupled with accelerated aging conditions. The results of this study have important implications for the design of fatigue resistant polymers used in medical applications.

An analysis of the reasons for the reduction in reliability of machinery structures under low temperature climate conditions

The influence of low temperature on the reliability of machine parts and on the characteristics of strength of steels and their weld joints is analyzed. It is established that a drop in temperature in impact cyclic loading causes a decrease in fatigue resistance of weld joints of low-alloy steels and a corresponding increase in relative frequency of crack formation in them. A comparison of experimental and service data shows that the increase in service relative frequency of crack formation in welded machine parts at reduced temperature is an indication of their fatigue origin. It was concluded that the increase in the frequency of failures of welded machine parts with a reduction in temperature observed under service conditions is caused by a reduction in their fatigue resistance and an increase in the tendency of their materials toward brittle fracture.

Responses of contractile properties in rat soleus to high-energy phosphates and/or unloading.

Responses of contractile properties of soleus to unloading and/or changes in high-energy phosphate contents were studied in rats. Reduction of high-energy phosphates, especially phosphocreatine, in ankle extensors was induced by feeding beta-guanidinopropionic acid (beta-GPA). The major finding in the study was that the fatigability and speed-related contractile properties responded to unloading and creatine supplementation in a similar manner. The high-energy phosphate contents tended to be elevated after 10-d supplementation of creatine and hindlimb suspension. The shift toward slow-type, mainly due to an increased one-half relaxation time, was seen in rats fed beta-GPA. Such a shift was reversed by feeding creatine or by hindlimb suspension; however, the suspension-induced shift of contractile properties toward fast-type was not prevented completely by beta-GPA feeding. Although the muscle fatigue resistance did not change by beta-GPA feeding alone, the decrease in fatigue resistance following suspension and creatine supply was less in the beta-GPA group. It is suggested that the levels of high-energy phosphates and tension production play important roles in the regulation of contractile properties of the soleus muscle.

Beta-receptor properties in soleus muscles from spontaneously hypertensive rats.

We have compared the properties of beta-adrenergic receptors in slow-twitch, oxidative skeletal muscles (soleus) from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at three different ages. The investigation was based on the hypothesis that the increase in Na+ content and decrease in fatigue resistance observed previously in the soleus of SHR might be the result of a down regulation of muscle beta-receptors. Activation of beta-adrenergic receptors in skeletal muscle stimulates sarcolemmal sodium-potassium adenosine triphosphatase, which produces an efflux of Na+ and an influx of K+. Receptor down-regulation would be expected to reduce hormonal stimulation of Na+ pump activity, particularly during exercise. The results of receptor binding studies, however, and an investigation of cyclic adenosine monophosphate (cAMP) production in response to applied epinephrine indicated that there were no significant differences in receptor properties in the soleus muscles of SHR and WKY rats. Receptor number and affinity were the same in the two strains, and the rate, magnitude, and duration of the increase in cAMP in response to 10(-6) M epinephrine were also similar. beta-Adrenergic receptor down-regulation does not appear to be a generalized phenomenon in tissues of SHR, despite the appearance of other physiological changes in the tissue.

Effect of neonatal denervation-reinnervation on the functional capacity of a 129ReJ [formula omitted] murine dystrophic muscle

The sciatic nerves of 14-day-old 129 ReJ normal (++) and dystrophic ( dy dy ) mice were transected in the midthigh region. The cut ends of the nerves were approximated to facilitate regeneration. One hundred days after denervation, contractile properties of denervated-reinnervated, normal and dystrophic extensor digitorum longus (EDL) muscles were compared to agematched normal and dystrophic muscles. In dystrophic muscle, in vitro twitch and tetanic tensions were reduced, compared to those of normal muscle. The denervation-reinnervation procedure resulted in an increase in these parameters as compared to unoperated dy muscle. These data correlated with increases in total myofiber cross-sectional areas. Twitch contraction time was not significantly affected by the dystrophic condition or by the denervation-reinnervation protocol. Whereas dystrophic muscle had a longer half-relaxation time than normal muscle, denervation-reinnervation of the dystrophic EDL resulted in a significantly faster half-relaxation time. While fatigue resistance was greater in dystrophic muscles than in normal muscle, there was a significant decrease in fatigue resistance in the denervated-reinnervated dystrophic muscle. Transient neonatal denervation results in modification of both the morphological and physiological characteristics of murine dystrophy.

Sustained force development: specializations and variation among the vertebrates.

The kinds of muscle fibre that are recruited for sustained force production by different vertebrates are described. Although aerobic metabolism always accounts for a significant proportion of their ATP turnover, no single characteristic such as colour, number and form of motor endplates, membrane properties, myosin isotype or contraction speed is diagnostic of such muscles. As mechanical power output increases, there is a tendency for a decrease in fatigue resistance with repetitive usage and an increase in both aerobic capacity and the fraction of energy requirements derived from glycolysis.

The effect of heat treatment on the fatigue and corrosion fatigue behavior of a CuNiCr alloy

Fatigue experiments were conducted on a CuNiCr alloy (IN838) in air and in 0.5 N NaCl solutions under conditions of free corrosion and of applied anodic currents. The alloy was heat treated to produce a solutionized structure and also to produce a precipitation hardened structure. The fatigue behavior of the solutionized alloy was unaffected by free corrosion although increased corrosion rates resulted in a decrease in fatigue resistance for small applied anodic currents. The age hardened alloy showed a decrease in fatigue resistance under free corrosion conditions and a further decrease in resistance with small applied anodic currents. In both heat treatments fatigue in air resulted in mixed transgranular-intergranular crack initiation and propagation while corrosion increased the relative amount of intergranular cracking. These results can be explained by a consideration of previously developed fatigue and corrosion fatigue models of pure copper and copper aluminum alloys.

Sonic Fatigue of Simulated Riveted Joints

The fatigue of a structure clamped to a support with a row of simulated rivets was investigated. The structure consisted of a 2024-T3 aluminum sheet (2 in.×3.5 in.×0.032 in.) cantilevered in a closely fitting slot in a siren test duct. Three aluminum bolts were used to simulate a clamping row of countersunk rivets. This design was chosen because it eliminated membrane stress at the rivet line and provided for control of clamping tension. The cantilever was excited at its fundamental resonance by plane waves of sound propagating at grazing incidence in the duct. The nominal strain at the rivet line was set by controlling the amplitude of motion at the end of the cantilever. Three different rivet diameters were used in fatigue life measurements: 0.125, 0.250, and 0.375 in. Each rivet size was tested at three strain levels. The fatigue lives ranged from 0.4×105 to 4×105 cycles. The fatigue-strain-concentration factors measured in this experiment varied from 1.5 for the 0.125-in.-diam rivet to 1.7 for the 0.375-in.-diam rivet. The concentration factor for the 0.125-in. diam rivet was found to be close to that measured in a subsequent experiment on reversed bending of a cantilever beam with a 0.125-in.-diam hole. An important decrease in fatigue resistance occurred in going from a rivet diameter of 0.125-in. to a diameter of 0.250 in.; a relatively small change was observed from 0.250 in. to 0.375 in. It appears that small rivets will increase sonic fatigue resistance of riveted joints. (Research sponsored by Aeronautical Systems Division, Air Force Systems Command, U. S. Air Force.)

Investigation of the Role of Intermolecular Forces in the Mechanism of High-Elastic Deformation. VII. The Effect of Molecular Interaction on the Fatigue Resistance of High Polymers Having Highly Developed Spatial Structures

Abstract It is shown experimentally that an irregular change of the fatigue resistance of loaded natural-rubber vulcanizates with increase of the degree of swelling in paraffin oil and in dibutyl phthalate is caused by the superposition of two processes: the increase of fatigue resistance, as a result of the reduction of mechanical losses, and the decrease of fatigue resistance due to the decrease of tensile strength proportional to the higher degree of swelling.

Changes of Fatigue Resistance of Vulcanized Natural Rubbers during Swelling

Abstract A method of fatiguing swollen vulcanizates of natural rubber while maintaining a minimum deforming load, independent of the change of modulus of elasticity of the experimental specimens, was devised. The effect of swelling vulcanized natural rubber in petrolatum and in dibutyl phthalate on fatigue resistance was studied. With increase of swelling of a vulcanizate in dibutyl phthalate and in petrolatum, an initial increase of fatigue resistance was observed up to 1.8 times the fatigue resistance of the unswollen vulcanizate. Following this, a decrease of fatigue resistance was observed. The equation obtained for the change of fatigue resistance as a function of the degree of swelling is explained by two simultaneous processes: (1) increase of fatigue resistance because of decrease of energy loss in the destructionof intermolecular bonds, and (2) initiation of chemical processes leading to destruction, and a smaller difference between the deformation and the relative elongation at rupture. The effect of the character of the solvent used for swelling on the nature of the fatigue resistance-swelling relation is shown.