Sidney A. Guralnick

Piezomagnetism and fatigue: II

The cumulation of damage in test specimens of AISI 1018 steel, subjected to repeated cycles of tension and compression leading to fatigue failure at Nf cycles, is correlated with the evolution of stress–strain (σ − e) hysteresis and piezomagnetic (B–e) hysteresis. Specifically, the σ − e hysteresis loop areas, when plotted as a function of the number of loading cycles N, show systematic variations that can be identified with the three principal stages of fatigue: initial accommodation (i.e. strain softening or hardening), N < N2; accretion of damage, N2 < N < N3, and terminal failure (crack coalescence and growth); N3 < N < Nf. Data from 49 fatigue trials, spanning the range 1219 ≤ Nf ≤ 250 200, show that the transitional cycles N2 and N3 have an approximately invariant relation to final fatigue failure at Nf: i.e. N2/Nf ≈ 12% and N3/Nf ≈ 90%. Piezomagnetic hysteresis develops in parallel with stress–strain hysteresis and also exhibits transitions at N2PM and N3PM corresponding to N2 and N3. Detailed analyses of eight fatigue trials yield the approximately invariant ratios N2PM/Nf ≈ 12% and N3PM/Nf ≈ 93% where 3561 ≤ Nf ≤ 189 629.

Real-time inspection of pavement by moire patterns

The necessary basis for any cost-effective highway and bridge deck pavement management program is reliable complete and up-to-date information concerning the state of pavement surface. An imaging system using the Shadow Moire method for the quantitative inspection of highway pavement surface in real-time has been developed which includes image analysis software for performing image enhancement and the analysis of Moire fringe patterns. Several enhancement schemes and thinning algorithms have been implemented and tested. The prototype system possesses a high degree of automation and gives enhanced pictures and accurate results. Also needed surface geometry information may be obtained efficiently and effectively.

Correlation between the piezo-Barkhausen effect and the fatigue limit of steel

Many types of ferrous metals can sustain an indefinite number of repeated loading cycles (N ≥ 107 cycles) provided that the maximum imposed stresses do not exceed certain critical values usually referred to as fatigue or endurance limits. In current practice, these limits are primarily inferred from statistical analyses of numerous fatigue experiments that relate the number of cycles to failure, Nf, to the loading programs. Numerous attempts have been made to bypass these time consuming tests by the direct observation of changes in material microstructures utilizing a variety of physical effects ranging from neutron diffraction, x-ray radiography, acoustic emission and even positron radiation patterns; but none of these approaches has yielded any unambiguous indices of damage. Recently, it has been found that the evolution of piezomagnetic hysteresis, due to magnetization changes induced in ferromagnetic steels by tension and compression, is a reliable indicator of the development of fatigue damage and can lead to practical predictions of service life. Further detailed information concerning processes at the microstructural level can be obtained from measurements of flux jumps associated with the piezomagnetic fields. Sequences of flux variations of the order of 10−3 Mx or 10−11 Wb, comparable to those observed in conventional Barkhausen experiments, appear when ferromagnetic steels are subjected to tension or compression. The amplitude distribution of these piezo-Barkhausen pulses increases markedly in the vicinity of the endurance limit and appears to provide a rapid means for distinguishing between stable, i.e. safe, loading regimes and those terminating in fatigue failure.

The Origin and Inception of Fatigue in Steel — A Probabilistic Model

This paper presents a simple model to simulate the fatigue behavior of materials. The model consists of a system of parallel springs, each with a non-linear stress-deformation behavior. The applied stress is modeled as a random variable. Using a probabilistic analysis, the system is subject to a series of stress applications. Upon each stress application, a certain number of springs fail. When a sufficiently large number of springs fail, the entire system is considered to have failed. The number of stress cycles corresponding to this condition is considered to represent the fatigue life. This system’s behavior is similar to fatigue damage accumulation in materials and fatigue failure. The description of the model is presented in the paper along with a numerical illustration to demonstrate its applicability. The paper also discusses the areas where the model can be further developed to obtain a broader understanding of the fatigue behavior of materials.

DEVELOPMENT OF AUTOMATED ROAD INSPECTION VEHICLE FOR NONDESTRUCTIVE EVALUATION OF ROAD SURFACE CONDITION

An ever-growing need exists for a fast, objective, and relatively inexpensive automated road inspection system. Unlike traditional methods of road inspection, the proposed automated road inspection system relies on video images and distance tracking systems housed within a survey vehicle. The road inspection vehicle is capable of acquiring full field road surface distress information at speeds in excess of 85 km/hr. Only two individuals are needed to drive the towing vehicle and trailer along the appropriate roadways where a surface analysis is required and to operate the acquisition equipment. This type of road inspection process eliminates the need to class a road for inspection and greatly increases safety levels when compared with the traditional road inspection methods. Acquisition of roadway surface distress information is accomplished using the shadow moire method. The shadow moire method has been proven to be a fast and accurate means of measuring out-of-plane surface deformation. This method is relatively inexpensive and requires minimal maintenance while providing accurate results. The objective of this research is to refine or improve, or both, the various subsystems that form the integrated prototype system.

Piezo-Barkhausen Emission as an Indicator of the Fatigue Limit of Steel

The fatigue properties of two variants of AISI 1018 steel samples were measured in a series of 33 experiments using new kinds of magnetic diagnostics. An MTS-810 servohydraulic test machine applied sinusoidal fully reversed (R = −1) loads under strain (Є) control in the range of 0.0008 ≤ (Є)≤ 0.0020. In 28 experiments, the number of cycles to fatigue failure Nf varied between 36,000 < Nf < 3,661,000. By contrast, in five runs extending over 107 cycles, the specimens showed no detectable signs of weakening or damage. The corresponding “S-N” or classical Wohler plots indicated that the transitions from fatigue failure to nominally infinite life (i.e., the fatigue limit) occurred at strains of about Є = 0.0009 and Є = 0.0010, respectively, for the two types of steel. Every loading cycle of each test was instrumented to record continual values of stress and strain. Flux gate magnetometers measured the variations of the piezomagnetic fields near the specimens. A 1000-turn coil surrounding the test pieces detected the piezo-Barkhausen pulses generated by abrupt rearrangements of their internal ferromagnetic domain structures. Analyses of the magnetic data yielded four independent indices each of which located the fatigue limits in complete agreement with the values derived from the Wohler curves.

A probabilistic mechanical model for simulating fatigue failure in metals – modelling the damage accumulation process

It is demonstrated that analysis of a probabilistic model for simulating the fatigue process in metals previously reported leads to the well-known Palmgren-Miner Law for fatigue damage accumulation. In a paper published in the Journal of Damage Mechanics, a mechanical model was presented that was used to simulate the S-N relations in metal fatigue problems. The simulation is accomplished by a model which is a system containing a number of sliding blocks connected by springs which rupture sequentially during cycles of applied stress. When a sufficiently large number of springs have ruptured, the entire system is presumed to have failed in fatigue. Further work on the model was completed by the authors on the capabilities of the model. The behaviour of the system, when subjected to a number of stress cycles, was investigated by analysing the cycle by cycle accumulation of stress-strain hysteresis. Through simulations, it has been shown that the model can closely portray the linear damage accumulation process as predicted by the Palmgren-Miner Law.

A Probabilistic Mechanical Model for Simulating the Fatigue Failure Process in Metals

A model for simulating fatigue damage accumulation and the fatigue failure process in metals is presented. The simulation is achieved by modeling material behavior with a series of nonlinear mechanical springs with randomized behavior. With each applied stress, a certain number of springs rupture. The damage accumulation process is modeled by the number of springs that have ruptured during the entire stress application cycle. When a sufficiently large number of springs rupture, the entire system is considered to have failed. This constitutes fatigue failure. This article follows two previous publications by the first two authors and extends the model further by incorporating additional random variables, investigating the significance of uncertainty in the spring behavior and simulation of the hysteresis behavior of metals during the fatigue damage accumulation process. Results similar to (1) the Wöhler S—N curve and (2) the hysteresis loss versus the number of stress cycle relationship, observed in laboratory testing of metal specimens, are presented.

Fast Fourier transform method for shadow moire fringe pattern analysis

An image system using the shadow moire method for the quantitative inspection of highway pavement surface has been developed. Studies have shown that a single moire image is ordinarily not sufficient to yield complete information about the gradient of a set of contour lines pertaining to a particular area of surface. The pavement surface reconstruction algorithm that has been developed to date uses two moire interferograms of any point on the surface to obtain overlapping information to correctly measure elevation. In this paper, a fast Fourier transform (FFT) method is described which generates accurate surface topographical information from a single interferogram. The fast Fourier transform used in conjunction with the shadow moire technique provides the basis for an automated pavement survey system now under development. This new approach is expected to significantly decrease system hardware and processing time requirements for an automated pavement survey system.