Laboratory Fatigue Tests Research Articles

An On-Line Correction Technique of Random Loading with a Real-Time Signal Processor for a Laboratory Fatigue Test

Abstract The authors present an application of the Solartron 1200 Real Time Signal Processor to correcting the on-line random loading in a fatigue test system. By inputting a power spectrum described by a data file to a computer, its corresponding random process may be simulated. Since the processor is connected with a computer, results of signal processing done by the processor are sent to the computer periodically during the test. The computer performs pattern matching using the correlation coefficient value (CCV) between a loading spectrum and its predetermined target. If a CCV exceeds its threshold, the old data file will be replaced with a new one and the random loading exerted on a specimen will be updated. When the new data file is obtained based on optimal control strategy, the updated loading spectrum will approach its target. Therefore, an electrohydraulic fatigue test system equipped with a real-time signal processor can realize an on-line correction of a random loading based on the target spectrum desired. Since this updating does not change the original software package and hardware of the system, the performance of the system and testing quality are safely improved.

Fatigue Crack Growth in Pavements

A common laboratory fatigue test for life estimation of pavements uses a repeated load flexural device with nonlayered beam specimens. As a result of such factors as traffic wander, and also due to different crack growth rates between such beam specimens and layered media, highway pavements have been found to sustain from 10 to 100 times the number of load applications estimated by this type of laboratory test. This study focuses on identifying the differences in crack propagation between beam specimens and layered pavements. A 2D finite-element program with specialized fracture mechanics capabilities is used in the analysis. An isotropic linear elastic material is assumed. Noncohesive cracks are introduced in the mesh as geometrical discontinuities. Cracks are propagated through the material under cyclic loading. Stress intensity factors for mixed mode are calculated for each crack length. Two different configurations are analyzed: laboratory beam specimens and layered pavements. It will be shown that predicted crack growth rates in pavements for the same maximum longitudinal strain are much lower than in beam specimens, which is consistent with laboratory and field observations. Two different crack growth rates are identified in pavements. The first is associated with tensile stresses at the bottom of the surface course. The second one is caused by tensile stresses at the top of the surface course, when the load is ahead of or behind the crack. The growth rate for surface cracks (growing downward) is much smaller than the internal crack growth rate (growing upward).

Fatigue damage evaluation of a power plant component from analysis of the dislocation structures

The paper presents a method for estimating the degree of fatigue damage in an engineering ferritic–bainitic steel. A dislocation microstructure chart is first established from laboratory fatigue tests. Misorientation measurements are carried out after fatigue failure between dislocation cells produced by cyclic loading in ferrite grains and between dislocation cells in bainite originated by the heat treatment. There is shown to be a clear dependence of these values on the strain amplitude. On this basis, the analysis of the microstructure of a steel taken from a component which failed in service allowed the determination of the strain amplitude value to which the component had been subjected.

Performance of Superpave Mixtures Under Accelerated Load Testing

The performances of two Superpave® test sections, constructed with 12.5 mm (0.5 in.) nominal maximum size Superpave mixture (SM-2A) with varying percentages of river sand at the Kansas Accelerated Testing Laboratory (K-ATL), are described. A 150-kN (34-kip) tandem axle with dual wheels and 620-kPa (90-psi) tire pressure was used for loading. After 10,000 repetitions, the sections were loaded by 160-kN (36-kip), 150-kN (34-kip), and 144-kN (32.5-kip) tandem axles and 98-kN (22-kip), 90-kN (20-kip), and 80-kN (18-kip) single axles, for estimating relative pavement damage due to different axle loads and configurations. Critical pavement responses were measured on each test section with the transverse strain gauges and soil pressure cells under both K-ATL tandem axle and falling weight deflectomter (FWD) loads. The pavement responses were also estimated from a multilayer elastic analysis program, ELSYM5. Laboratory fatigue tests were conducted on the SM-2A beams. Fatigue and rutting damage analyses were then conducted. The results indicated very little fatigue damage on these sections. The Superpave mixtures appeared to be less susceptible to fatigue damage, presumably due to better aggregate structure and higher binder content than the traditional asphalt mixtures. Rutting on the section with 15 percent river sand was mainly due to consolidation of the SM-2A and/or other layers since no significant flow of the Superpave mixture was evident. The AASHTO load equivalency factors were found to be much higher than those calculated in this study.

Evaluation of Fatigue Cracking

The continuous growth of damage from fatigue cracking is described as a two-stage process of crack initiation and crack propagation. The crack-initiation stage can be characterized by conventional laboratory fatigue tests. The crack-propagation stage is described using the Paris-Erdogan phenomenological law. Problems encountered in implementing the theory are presented and discussed. For the sake of simplicity, only Mode II (shearing mode of the crack tip) is used. Computations of the stress-intensity factors and of the number of load repetitions for propagating the crack are presented. The applicability of the above fracture approach is evaluated using a pavement design method that includes an additional term to represent the crack-propagation phase.

A STUDY ON REHABILITATION AVAILABILITY AND USABILITY OF PRESTRESSED CONCRETE PAVEMENT WITH SURFACE CUTTINGS

The prestressed concrete pavement planned in Tokyo International Airport Offshore Expansion Project has cuttings for taxiway lighting wiring. Aiming at learning whether there is a problem in using pavement with such cuttings as apron and whether “Prestressed Concrete Lift-Up Method” needs to be modified, the authors carried out a laboratory fatigue test, flexural test, two-dimensional analysis using an elastic beam model and three-dimensional analysis of a plate on prestressed concrete. Proposed as a result are revised specifications for lift-up mechanism of prestressed concrete pavement with surface cuttings.

Designing Oceanographic Surface Moorings to Withstand Fatigue

Abstract A method is presented for predicting the fatigue resistance of a single-point oceanographic mooring. The method determines the fraction of life that the components of a mooring lose during a deployment by combining results of dynamic analysis, data from laboratory fatigue tests of mooring components, and observations of sea-state occurrence. Calculations based on the 1989 and 1991 moorings used in the Marine Light-Mixed Layers experiment confirm the failure of a pear-shaped sling link on the 1989 mooring and the success of the redesigned 1991 mooring. Dynamic analysis shows that the large oscillatory tensions that cause the fatigue in oceanographic moorings are due primarily to the mass and damping of the instruments that are attached to the upper, wire-rope portion of the mooring line. Developing new instruments that are smaller and lighter would greatly increase fatigue resistance of oceanographic moorings and allow for longer deployments.

Growth of multiple cracks and their linkup in a fuselage lap joint

An issue of concern in aging aircraft is the growth of multiple cracks emanating from a row of fastener holes, typically in a pressurized aircraft fuselage lap splice. This multisite damage (MSB), or widespread fatigue damage, if allowed to progress, can suddenly become catastrophic. The understanding of the failure behavior dictates the level of compromise between safety and economy. The complexity of the structure due to various stiffening elements makes it unamenable to a simple direct analysis. A two-step elastic finite element fatigue analysis combining a conventional finite element method and the Schwartz-Neumann alternating method with analytical solutions is developed to understand fatigue growth of multiple cracks and to obtain a first estimate of the residual life of a stiffened fuselage shell structure with MSD in the riveted lap joint. The analysis procedure is validated by simulating a laboratory fatigue test on a lap joint in a flat coupon. Both the coupon and the shell panel are found to have fatigue lives only up to the first linkup of neighboring crack tips.

Investigation of Laboratory Fatigue Testing Procedures for Asphalt Aggregate Mixtures

The primary purpose of this paper is to evaluate laboratory test procedures for measuring the fatigue response of asphalt paving mixtures. Available test methods are conveniently classified into the following categories: simple flexure, supported flexure, direct axial, diametral, triaxial, fracture mechanics, and wheel‐track testing. Criteria used to evaluate each method for its potential use as a laboratory standard included: (1) Ability to simulate field conditions; (2) applicability of test results for use in modeling pavement performance; and (3) simplicity. The three most promising methods are considered to be simple flexure, diametral fatigue, and tests based on fracture mechanics principles. Although not a fatigue test in itself, direct tension testing offers considerable potential as a simple surrogate for more complex fatigue tests. Finally, in most prior work, the maximum principal tensile strain has been used as the cause or determinant of fatigue damage. Other damage determinants, such as work...

Automated fatigue testing stand

Design principles, structure, and operating features of a stand with extended functional possibilities for laboratory fatigue tests are examined. With a highly reliable protection system, the stand facilitates detection of the instant at which a damage originates in the tested object and recording of the process of its development from start to complete failure.

High-cycle, long-life fatigue behavior of welded steel details

Abstract Results from laboratory fatigue tests of large-scale welded attachments under random variable-amplitude loading have indicated that the existence of a fatigue limit below which no fatigue cracks develop is assured only if none of the stress cycles exceeds the constant-amplitude fatigue limit. In the extreme life region of fatigue behavior, the contribution of the lower cycles of a stress range spectrum to crack propagation is a function of the larger stress cycles. However, the value of the constant-amplitude fatigue limit is not precisely known and in some cases will differ from the assumed value. As a consequence of the variability in the fatigue limit, the lower bound variable-amplitude fatigue resistance on large-scale welded steel details is defined by the straight-line extension of the S-N design curves below the constant-amplitude fatigue limit. A summary of the test data is given and the effect of the variability on the fatigue behavior of welded steel details in the high-cycle, long-life regime is discussed.

Contact Loading of a High Rail in Curves Physical Simulation Method to Investigate Shelling

SUMMARY According to present knowledge, the contact fatigue failures of rails, referred to as shelling, most frequently appear on the inner side of high rail in curves of moderate radii. In order to improve the understanding of the failure mechanism the studies on the several related research subjects are necessary, among them the laboratory fatigue tests. This paper presents two methods for physical simulation of rail contact loading in curves of moderate radii using a contact fatigue machine. The description of methods is followed by the estimation of the available range of parameters influencing contact forces on the machine, a theory of contact forces occurring on the machine and by comparison between forces generated by a rail vehicle on curved track and the loading on the machine for one and two-point contact between wheel and rail on curve.

Pile Fatigue Failures. IV: Fatigue Description

Laboratory fatigue tests were performed on girth butt weld specimens taken from both vertical piles and batter piles salvaged from the terminal. Specimens were selected to be representative of welds found to be acceptable by irt‐situ radiographic inspection. All batter pile tests and all but two vertical pile tests were terminated when the number of applied stress cycles exceeded design requirements current in 1974–75. These cycles were in addition to those that had been applied earlier during the construction period. Results gave assurance that piles containing welds found to be normal quality by on‐site inspection would function adequately with little expectation of fatigue failure. Available data from the tests are used to develop a probabilistic approach to fatigue life estimation, including the effects of mismatch in girth butt welds.

FATIGUE SUBSTANTIATION OF TITANIUM LUGS

The purpose of this paper is to calculate the fatigue life of titanium lugs from full scale fatigue substantiation tests using the stress--endurance curve together with a knowledge of the variation of fatigue strength with the mean stress. The tests were carried out using lugs with bushes whereas a helicopter components all have bushed holes. Since, the Goodman diagram for titanium lugs has never been defined, the tests provide sufficient data to draw a Goodman diagram with confidence. A computer program for curve fitting has been designed to determine the different S-N curves from the laboratory fatigue test data. The flight fatigue damage and the safe life for the sections of the titanium lugs as used in helicopter components are calculated on the basis of the flight load levels.

A Bayesian method for establishing fatigue design curves

From a reliability viewpoint, the simple conventional procedure for establishing design S− N curves from laboratory fatigue test data suffers from two important limitations. Firstly, the calculated fatigue reliability on the design curve only reflects the observed “physical” uncertainty associated with the fatigue process itself. “Statistical” uncertainties, connected with estimating the parameters of the fatigue model, are not considered. Secondly, it is not possible to account for fatigue run-outs (non-failures) in a rational manner. A new procedure, based on Bayesian statistical inference, is presented which is capable of handling both the above problems. Its use is illustrated via the analysis of some typical fatigue data sets and a number of general points arising from the results are discussed.

Logical analysis of assumptions in fatigue design

Abstract Whatever method is used to estimate the safety of a structure under fatigue loading, there is inevitably a mismatch between the predictions of the theoretical model used and the performance of the actual structure when built. It is the purpose of this paper to present an analysis of this mismatch and to suggest a method of fuzzy logical analysis which may be used to assess it. Three types of parameters are identified. The first group are those which can be precisely defined, measured and controlled in a laboratory experiment; the second are those which are difficult to define and measure even in the laboratory and the third are those unknown parameters which account for the scatter in laboratory fatigue tests. A fuzzy logic model of the mismatch between the behaviour of the completed structure and the laboratory models and between the service loading and those loadings used in the laboratory is presented. The probability of a particular class of weld detail in a bridge structure is calculated usi...

Testing methods for fatigue properties of rubber materials and vibration isolators

The main purpose of this investigation was to find out if laboratory fatigue tests in tension give results which can be useful in predicting the service life of a vibration isolator. Four natural rubber vulcanizates with different curing systems were used in the tests. The fatigue life in cyclic loading was determined in tension on standard tensile specimens (dumb-bells) and in shear on two types of vibration isolators. The results indicate that the information achieved in tension is relevant for estimating the service life of an isolator. Both relaxing and non-relaxing tests in tension were made and the relaxing tests seem to give the best correlation between dumb-bells and isolators. As the relaxing tests are also the least time-consuming they are to be preferred for material testing.

Long Time Observation of a Fatigue Damaged Bridge

Fatigue cracking was first observed at the Yellow Mill Pond Bridge on the Connecticut Turnpike (I-95) in 1970. Fatigue crack growth resulted in complete fracture of a tension flange in one of the girders. Smaller cracks were discovered in several other beams. Fatigue cracking started at the end weld of cover plates welded on the rolled section, which form the longitudinal girders of the bridge structure. Several inspections were made between 1970 and 1979. The stress range due to the traffic was recorded in 1971, 1973 and 1976. Maximum stress ranges as high as 10.5 ksi (72.4 MPa) were measured at the end weld of the cover plates. The results from the bridge observations are compared with laboratory fatigue test data on small size and full size cover plated beams. The laboratory tests and analytical predictions of crack growth agree well with the observations made on the Yellow Mill Pond Bridge.

Fatigue Crack Growth under Random Loading (4th Report)

Fatigue crack growth under random loading has been extensively studied since nearly a decade in relation with the design of offshore structures, ship hulls, pressure vessels and so on.For the sake of simplicity, random loads in service conditions are often simulated by block programmed loads in laboratory fatigue tests. However, very few studies have been conducted on the difference of fatigue crack growth behaviour between random loads and block programmed loads.The authors previously presented an experimental study on fatigue crack growth under block programmed loads, and discussed on the effect of block size on fatigue crack growth life. A proposed model gave a good prediction of fatigue crack growth life in block programmed loading tests. In the present paper, fatigue crack growth tests were performed under three different types of block or random loads. Experimental results show that fatigue crack growth behaviour under random loads is very similar to that under block programmed loads with short periods. Miner's rule gave a non-conservative estimation for crack growth life under both random and short block loads. Life estimation by the model previously proposed agreed well to the experimental results.

Improved Service Life of Automotive Suspension Bushings through the Use of Semi-Efficient Vulcanization Systems

Abstract The relative merits of antiozonants, efficient vulcanization systems, and semi-efficient vulcanization systems for natural rubber in dynamic applications have been previously evaluated in a number of laboratory studies. This report covers an evaluation of the effects of these three factors on the laboratory fatigue testing of automotive suspension bushings. The use of semi-efficient vulcanization systems along with a good soluble antioxidant will provide the optimum resistance to the combined effects of oxidation and mechanical fatigue experienced in this application. Efficient vulcanization systems produce products with very poor service life. The use of substituted p-phenylenediamine (ppd) antiozonants can be an advantage or disadvantage in compression fit bushings, depending upon the type of service. Excessive amounts of ppd at the rubber-to-metal interface can cause slippage and early failure; therefore, the use of ppd's must be approached with caution in this application.