Particle Counting

Introduction

An Optical Particle Counter gives the count of particles and their size in the solution being considered. There are 2 main techniques for particle counting – light extinction and light scattering (Moubray, 1997).
The particle counter consists of an optical system illuminated by a light source. This system detects the contamination in fluid by collecting the light scattered from each particle with a detector. The electronic device provides amplification of the low level signals received from the photo-detector and these scattered light pulses are converted to a corresponding size category (proportional the height of pulse), which is then collected in a logger. Every scattered pulse corresponds to a particle count, and this is incremented in the appropriate size category to obtain particle concentration in a given size interval (Barth, 1984).

Light Scattering Particle Counter

Light Extinction Particle Counter

Particle counting based on the principle of light obscuration is known as light extinction. The LEPC consists of a bright light source, an object cell and photo detector. The subject fluid moves through the object cell under specific volume conditions and controlled flow. When the opaque particles present in the fluid come across the light beam it blocks an amount of light which is proportional to the size of the particle. The number and size of the particles in the sample fluid determines how much light is blocked or reflected, and passes through to the photo diode. The resultant change in the electrical signal at the photo diode is analysed against a calibration standard to calculate the number of particles in predetermined size ranges and displays the count.

Advantages and Disadvantages of Particle Counters

The LEPC is faster than visual graded filtration. It lacks the Intensity and Consistency of light scattering PC and fails to overcome reaction of many different wavelengths of light. The Accuracy of counter is dependent on fluid opacity, the number of translucent particles, air bubbles and water contamination. Resolution is limited to 5 microns particle range. Also the counter does not provide information on the chemical composition of the contaminants.
The LSPC performs well in settings where conditions are controlled. It has higher accuracy than the LEPC. It can measure particles of size as small as 2 microns. With LSPC it is possible to monitor continuously. Along with the points that are stated for light extinction process in this counter it is often required to dilute fluid to refrain from having high particle concentration so as to avoid coincidence error where several particles bunch together and appear as one large particle (Friend, 1997).

Development of Life Usage Monitoring System

The Life Usage Monitoring System has consistently been upgraded with the development of new technologies. In the 60s, the tracking unit for life of sensitive parts was in terms of the flying hours. The parts were tested in multiple flights and damage parameters were maintained in a database. This database provided a conversion factor between the flying hours and Low Cycle Fatigue damage for each tracked part. Few years later, aircrafts were equipped with flight tape recorders to record for each engine, the engine parameters (Time, Low Pressure Rotational, Speed etc) were measured every second. The magnetic tape from the recorders was retrieved and downloaded in a microcomputer to calculate life consumption of the part.
Actually the life consumed during an engine run or during a flight is based on stresses and temperatures at the critical areas of the subject components. The temperature changes results to cyclic stress which cause premature fracturing of components also termed as Thermal Fatigue (Mattingly, 2002).
We determine the thermal and mechanical boundary conditions for the engine components based on the measured time histories of engine operating parameters. On the basis of these boundaries, the transient temperature development within the components is calculated. Also, we calculate the stresses at critical fracture areas. Resultant of these stresses along with the corresponding temperature histories form the basis of prediction of the damage.
The fatigue is calculated as a resultant of HCF and LCF. These cause premature failure of the engine i.e. before it completes its life cycle.
Low cycle fatigue is generally caused by repeated loading cycles caused by operation, i.e. starting or takeoff conditions. If an engine blade goes through more than the threshold cycle takeoff max power setting it may fail due to fatigue. High cycle fatigue is similar, but is generally based on cycles with a higher frequency due to vibration. That same engine blade may see more than threshold cycles during a single takeoff if the blade is vibrating badly. All these factors result in failure of the component.
The engine is run multiple times and the resultant damage for each run is recorded. For most of the critical areas, we predict the number of reference cycles which the critical area can undergo until a fatigue crack is developed to the predicted depth of 0.4 mm. To describe the relationship between applied stress range and the threshold number of cycles required for crack initiation, material SN curves can be used. We examine the effect of loading frequency on the development of inelastic strains under concurrent thermal and mechanical loading. Thus, aircraft will have a greater accumulation of creep strains and, consequently, a greater possibility of material damage in its engine components over the same total flying time. This causes a growth in the crack (Morgan, 1996).

Conclusion

Therefore, by calculating the number of cycle necessary to propagate the crack up to the malfunctioning criterion, we can derive the secure crack propagation life. So, we can predict the length of service of the ruptured part by summing up the crack propagation time-period and the crack initiation time.