A comparison of results of axial-tension,rotating-beam and pressurized-cylinder fatigue tests

In a previous paper by Davidson, Eisenstadt and Reiner, it was noted that, as the diameter ratio of an open-end, thick-walled cylinder approached unity, the stress state due to internal pressure approaches that of uniaxial tension. It was, therefore, proposed that the fatigue life of a cyclically pressurized thick-walled cylinder might be predicted from the results of axialtension fatigue tests. In this paper, the results of the thick-walled-cylinder fatigue tests, reported in Ref. 2, extrapolated to a diameter ratio of unity, are compared with the results of axial-tension fatigue tests on the same material. The effect of oil in contact with the surface of the axial fatigue specimens and that of varying the cyclic speed from 1800 cpm to 200 cpm are investigated. Rotating beam fatigue test results for the same material are also reported. The results of the axial-tension fatigue tests do not agree with the extrapolated thick-walled cylinder results in the range of fatigue lives from 10⁴ to 10⁶ cycles with the cylinder results showing the shorter lives. For less than 10⁴ cycles, the results converge. No effect of cyclic speed or of oil in contact with the surface was found. The results of the rotating-beam tests generally lie between the axial-tension and extrapolated cylinder results.     

Random-load fatigue tests on a fail-safe structural model

Experiments performed on a ten-member redundant “fail-safe” structural model subjected to randomized load sequences confirm predictions of fatigue life and reliability based on a probabilistic approach. The statistical variation in ultimate strength of 2024-T4 aluminum alloy combined with an exponentially distributed, Markovian, bending load-amplitude sequence with a constant-amplitude S-N relation for single specimens, is utilized in the analysis. Experimental results are presented for the statistical distribution of ultimate bending strength of 2024-T4 aluminum alloy. Constant load-amplitude flat-bending fatigue tests on single specimens and on multimember structures, and variable-amplitude flat-bending tests on fail-safe structures are reported. Life to failure of the weakest member, as well as the remaining “fail-safe” life in the parallel structure, are examined.     

Torsional fatigue tests of anodized aluminum rods

Fatigue tests in reversed torsion were run on 2024-T351 aluminum-alloy specimens in (1) uncoated state and (2) anodized with 0.3 and 0.05-mil (0.0076- and 0.0013-mm) coatings. Tests on both groups were run at low (20–25 percent), medium (55–60 percent), and high (86–91 percent) relative humidity. One lot of uncoated specimens was tested that had been polished after machining, and one lot of anodized specimens was tested that had been polished before coating. Tests on uncoated and on anodized specimens were run at stress levels of approximately 13,000, 17,000 and 20,000 psi (89.62, 117.2 and 137.9 MPa, respectively). Results of tests on both coated and uncoated specimens showed longer fatigue life at low humidity than at high humidity. Little difference was found between tests at medium and at high humidity. No difference was found between polished and unpolished specimens. With coated specimens, the thickness of coating showed no effect on fatigue life. Comparing the results on coated and uncoated specimens at low humidity, coated specimens had a longer fatigue life. However, at high humidity, there was no significant difference.     

Algorithms for controlling fatigue tests of airplanes

The efficiency of algorithms for automatic control of forces prescribed in the test program is analyzed by an example of a large-aspect-ratio wing considered as a beam loaded by aerodynamic and inertial forces.     

汽车动力总成悬置系统振动解耦计算方法研究

目前,汽车发动机动力总成悬置系统设计的主要任务是选择悬置元件的刚度、位置和角度,使悬置系统自由振动模态频率避开发动机怠速激励力频率与车身自振频率,并尽量提高各模态振型的解耦程度,从而提高悬置系统隔振效果。悬置系统按预定频率严格解耦设计是使设计出的悬置系统模态频率完全等于按汽车设计频率规划预定的频率,并使各模态的振型严格解耦,即各向振动能量的解耦度等于1。本文从悬置系统的自由振动方程出发给出了对悬置系统按预定频率严格解耦设计的方程组,通过方程组构造函数进而求出该方程组的解,从而提供比当前的悬置系统模态优化设计更为简便高效的优化设计方法。相应的算例验证了本文提出的按预定频率严格解耦设计方程和求解方法的正确性。     

Evolution of fatigue damage using the fatigue damage map method

Using only readily available material properties and the concept of the fatigue damage map the work attends to develop a methodology able to predict crack tip propagation rates characterising each of the fatigue stages, namely crack arrest, microstructurally and physically short crack (Stage I), long crack growth (Stage II) and Stage III growth. Crack tip propagation rates are predicted through the combination of an Elasto-Plastic crack tip opening displacement model and the crack blunting theory. The methodology provides a theoretical framework for a 3D version of the fatigue damage map for damage tolerance design. In addition, the work attends to explain a number of fundamental problems starting from crack tip propagation discrepancies, found in the short cracking stage, as well as to provide an insight towards the effect of grain size on the fatigue limit/threshold stress intensity factor.     

Continuous measurement of material damping during fatigue tests

An experimental procedure for continuously measuring strain level, temperature, and energy-dissipation rate during resonant fatigue tests is described. The technique is based on a previous method for measuring loss factor (Ref. 3) using base-excited cantilever-beam specimens vibrating at resonance. The amplitude and frequency dependence of loss factor is therefore included directly in the measurement. For beams vibrating in vacuum, energy-dissipation rate and temperature measurements provide a basis for irreversible thermodynamic analysis of fatigue. This procedure provides a continuous measurement of energy-dissipation rate during fatigue-crack nucleation, and is the basis for experimental study of the hypothesis that the entropy gained during fatigue failure is a material constant.     

Service-simulation tests to determine the fatigue life of outside-diameter-notched thick-wall cylinders

Simulation specimens were used to model the fatigue behavior of outside-diameter-notched internally pressurized cylinders of alloy steel. Results from continuum mechanics and finite-element analyses are described for use in selection of simulation-test conditions. The effects of notch depth and residual stress on fatigue life are determined from the simulation tests.     

Effect of low cycle fatigue pre-damage on very high cycle fatigue

Carbon-manganese steel is often applied in components of pipes in nuclear plant. Ultrasonic fatigue tests following low cycle fatigue (LCF) cycles damaged are used to study the strength of very high cycle fatigure (VHCF). The comparison of test results of simple VHCF and cumulative fatigue (LCF plus VHCF) shows that LCF load influences the following VHCF strength. Continuum damage mechanics model is extended to VHCF region. The effect of LCF load on VHCF is studied by an improved cumulative damage model.     

Effect of plastic zone on the fatigue crack propagation behavior between two fatigue cracks

In this study, the fatigue crack propagation behavior in the stress interaction field between two different fatigue cracks is studied by experiment and finite element analysis. In the experiment, the offset distance between two cracks and the applied stress are varied to create different stress interaction fields. The size of the plastic zone area is used to examine the crack propagation path and rate. Three types of crack propagation in the interaction field were found by experiment, and the crack propagation behavior of two cracks was significantly changed as different stresses were applied. The size of the plastic zone obtained by finite element analysis can be used to explain crack propagation behavior qualitatively.     

Effect of the secondary slip system on early fatigue damage

A face-centered cubic polycrystal under cyclic loading is considered. It is shown that slip in the primary slip system causes the resolved shear stress in a second slip system to increase to the critical value and then this second slip system slides. Slip in this second slip system greatly increases the rate of the local plastic strain build-up in the primary slip system. Hence the occurrence of the second active slip system may greatly increase the rate of extrusion and intrusion commonly observed in face-centered cubic metals.     

Fundamental study on rupture by low-cycle fatigue of polymers applying the fine-grid method

Few studies have been made on the fracture mechanics of polymers, their resistance to plastic failure, fatique rupture, and the adverse effects of environmental conditions, in contrast to the numerous studies conducted on metallic materials. Since fatigue is characterized by very local and cyclic fractures, in the present study a real-time fine-grid method was applied to study the fatigue rupture of polymers: to examine changes in local strain at the root of the notch during the process of crack initiation, the local strain at the tip of the crack during crack propagation and the relation between the plastic zone formed in front of the crack tip and the rate of crack propagation. As a result, strong correlation between three proposed parameters of the local crack-tip strain, the crack initiation and the propagation rate was obtained, and the mechanism of low-cycle fatigue rupture of polymers could be discussed.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Comparison of low-cycle fatigue results with axial and diametral extensometers

Low-cycle fatigue tests at room temperature have been carried out on a stainless steel and an aluminum alloy utilizing an axial extensometer with cylindrical specimens and a diametral extensometer with hour-glass specimens. In all cases, the axial strain was the controlled parameter. The results obtained with both extensometers are compared. For the materials studied, it is found that the data obtained with a diametral extensometer correspond to a somewhat longer life (by a factor of the order of 1.6 for the parameters used). Furthermore, two tests have been carried out on cylindrical specimens with both extensometers where the axial-strain computed through the diametral strain was controlled throughout the material life and the measured axial strain was simultaneously recorded. Results indicate that geometry of specimen is the predominant factor influencing fatigue life.     

On the correctness of the energy criterion in fracture mechanics

S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 31, No. 10, pp. 28–34, October, 1995.     

Effect of stress biaxiality on the high-strain fatigue behavior of an aluminum-copper alloy

Although there is now a considerable volume of high-strain (<10⁵ cycles) fatigue data for uniaxial tension-compression and simple-bending conditions, relatively little information is available regarding the effects of stress and strain biaxiality. A method which has been used to study the effects of biaxiality on longlife fatigue strength is to subject thin-walled tubes to repeated internal pressure and an end load which is in phase with, and a linear function of, the pressure. The object of the present research was to use this method to study the influence of stress biaxiality on the high-strain fatigue behavior of a high strength, aluminum-4% copper alloy at room temperature. From a continuum-mechanics point of view, this material is completely elastic after the first few load cycles. Cylinder results for hoop to axial stress ratios of 2:1, 1:1, 1:2 and 2: ?1 suggest that fatigue failure of this material in the life range 10³ to 10⁵ cycles is primarily dependent on the maximum range of tensile stress. This conclusion and a study of fracture surfaces led to the use of linear-elastic fracture mechanics to interpret the fatigue and brittle fracture behavior of these cylinders.