Fatigue refers to the irreversible changes in a material or component that is exposed to cyclic loading. These changes are microstructural changes before the onset of macroscopic crack growth. Changes in microstructure are closely related to changes in the density and structure of dislocations. The fatigue process is commonly divided into three stages: accumulation of cyclic plastic deformation which is closely related to bulk material properties, formation of slip bands and nucleation and growth of microcracks in the surface, and finally growth of a macroscopic crack to failure.

How to measure fatigue
The material changes during a fatigue process cause reduced permeability and increase of electrical resistance in the surface. These changes in the outer surface and beneath are monitored by the FEMMs transient voltage response. This response changes according to the degree of changes in these parameters, e.g. when micro-crack density increases and grows to more continuous cracks, the response signal changes accordingly. By analyzing the response signal, changes of permeability and resistance are estimated, and based on these parameters the degree of material degradation can be characterized in due time before any crack is visible.

Stress measurements in ferromagnetic steel are relative to a previous measurement, preferably taken with known stress level, e.g. no stress. Calibration curves can be established and quantification of actual stress level is thus possible.

Plasticity, permanent changes in the material caused by stress exceeding yield, is detected. This is e.g. observed when monitoring areas with residual stress. Changes of residual stresses related to welds are measured, e.g. when monitoring fatigue in welds.

The steel ″remembers″ previous external loads, i.e. the load has changed the magnetization of the steel. This is measured by FEMM and is called remanent stress, which shows the highest stress since the last measurement. This magnetization is wiped out by the measurement, and the permanent change can then be measured.

Differentiation between these changes are related to the analyses of changes of permeability and resistance and the depth of these changes. This has been verified by several high cycle fatigue tests.

Accumulation of cyclic plastic deformation due to cyclic loads causes formation of slip bands and nucleation and growth of microcracks which finally lead to formation and growth of a macroscopic crack.

How are cracks measured
The pin matrix is designed to give highest possible sensitivity for crack detection and crack growth monitoring, whether the crack starts in the outside surface or inner surface of a pipe. The matrix sensing direction is oriented perpendicular to the expected crack direction. If this direction is not known, a matrix sensing in both directions can be installed. When the monitoring records a significant crack, the maximum depth can be estimated. Thereafter the crack growth is monitored and reported.

Corrosion internal in pipelines is more or less uneven and the degree of corrosion attack can vary e.g. to be more severe in the bottom section. This makes it advantageous to monitor a continuous area of the pipe to be able to get a representative picture of the distribution and degree of attacks to quantify the deepest pit in the area.

How is corrosion measured
The location expected to be most exposed to corrosion is selected for installation of the FEMM sensors. Based on the expected type and location of corrosion, e.g. localized attacks in the bottom section, a sensing matrix is designed and installed. The whole area covered by the sensing matrix is monitored for any internal metal loss. Initially, a measurement is taken and stored and used as a reference for the following measurements, which give the change of corrosion attacks. When a significant metal loss is detected, the actual depth of e.g. a pit can be estimated using the patented FEMM algorithm based on the transient signal.

Erosion internal in pipes most frequently occurs in bends and tends to be most severe in the outer radius of the bends. The shape of the erosion varies related to the bend geometry and flow. Both carbon steel and duplex steel can be monitored.

How is erosion measured
The sensing pin matrix is designed for optimized sensitivity based on expected shape of erosion, e.g. if erosion is wide the matrix is distributed along the outer radius. If the erosion is expected to be a narrow groove along the outer radius the matrix can be along the bend’s circumference which will significantly improve sensitivity. Also a combination of these matrices can be applied in case the shape of the erosion is difficult to predict. Sufficient area is covered to be sure to pick up the most severe attacks. When significant metal loss is detected, the actual depth of the deepest attack can be estimated based on the patented FEMM algorithm.