Preliminary Requirements

Procedure

1. SUBTASK 70-22-03-860-001 Introduction

   1. This procedure has been written for the measurement by replication of small surface features whose depth is greater than 0.0005 inch (approximately 0.0125 mm). The procedure specifies the method for making a replica, the assessment of the condition of a replica and the method for taking a measurement from a replica. The procedure is intended for use in the assessment of damage to components for Technical Variance purposes. The surface replication material permits the examination of surfaces with a resolution better than 0.1 micron; the maximum depth that can be measured depends upon the size of field of view of the optical microscope.

   2. This replication procedure is intended only for the measurement of surface feature depth but it does in effect use a focused inspection of the damage with a binocular microscope. While it must not replace any other inspection requirement (for example, penetrant, or visual, or binocular), the replica can show cracks in a surface. Cracks and microcracks will be seen on the surface of the replica as fine lines (ridges). These ridges usually vary in height and frequently have a fringe of replica material along their length. They can usually be easily distinguished from scores, which are straight, well defined, and of equal height.

   3. Surface replicating materials can be used to examine surfaces for cracks, microcracks, impact damage, and pits. Surface replicating material can also replicate the grain structure at the surface of a polished and etched metal.

2. SUBTASK 70-22-03-860-002 Health and Safety

   

   WARNING

   IT IS THE RESPONSIBILITY OF THE OPERATOR TO OBTAIN AND OBSERVE THE MANUFACTURER'S MATERIAL SAFETY DATA SHEETS FOR CONSUMABLE MATERIALS. THESE CONTAIN INFORMATION SUCH AS, HAZARDOUS INGREDIENTS, PHYSICAL/CHEMICAL CHARACTERISTICS, FIRE, EXPLOSION, REACTIVITY, HEALTH HAZARD DATA, PRECAUTIONS FOR SAFE HANDLING, USE AND CONTROL MEASURES AND ALSO TO TAKE LOCAL REGULATIONS IN TO CONSIDERATION.

3. SUBTASK 70-22-03-860-003 Consumable Materials

   CoMat 01-031 ACETONE (CH3)2CO		Acetone
   CoMat 01-124 ISOPROPYL ALCOHOL		Isopropyl alcohol
   CoMat 01-338 CLEANING SOLVENT		Cleaner solvent
   CoMat 01-437 CLEANING SOLVENT		Cleaner solvent
   CoMat 01-491 CLEANER SOLVENT		Cleaner solvent
   CoMat 01-506 SOLVENT, HANDWIPING		Solvent handwiping
   CoMat 02-181 MODELLING CLAY		Modeling clay
   CoMat 08-133 SURFACE REPLICATING MATERIAL		Surface replicating material
   CoMat 08-165 SURFACE REPLICATING MATERIAL,QUICK SET THIXOTROPIC		Surface replicating material, quick set thixotropic
   CoMat 08-166 SURFACE REPLICATING MATERIAL,QUICK SET		Surface replicating material, quick set
   CoMat 08-167 SURFACE REPLICATING FOIL		Surface replicating foil and liquid
   CoMat 12-026 P.V.C. SELF ADHESIVE TAPE		PVC self-adhesive tape
   CoMat 12-026 P.V.C. SELF ADHESIVE TAPE		PVC self-adhesive tape
   CoMat 08-133 SURFACE REPLICATING MATERIAL is available as fluid or thixotropic grades, with normal or rapid cure rates.

4. SUBTASK 70-22-03-280-001 Definition of Measurement Requirements

   1. Before an assessment of the depth of a surface feature on a component is made, it is important to consider what is to be replicated and measured. Features to consider are:

      1. Single Impact/Indentation Damage.

         1. The feature is the only area of consideration and is automatically identified as the area to be replicated, measured, and reported.

      2. Multiple Impact/Indentation Damage.

         1. If there are several surface features, they should all be replicated, measured, and reported. All surface features in all different identifiable areas of the component must be examined separately. For example, if there is impact damage on the rim and diaphragm of the disc, then both areas must be examined, not just the deepest of the two.

      3. Many Areas of Impact/Indentation Damage.

         1. Sometimes a very large number of impact marks can be made on a component, for example, by repeated contact with a piece of debris. In this instance, it is not practicable to replicate and measure the depth of every impact mark. A representative sample of what is considered the worst impact marks must be replicated and measured. All the measurements must be reported.

      4. Corrosion/Erosion Pitting.

         1. For this type of damage there are usually many surface features. Visually inspect the component and from each separate area make replicas of the worst (deepest) features. The replicas can then be compared systematically with a binocular microscope. Section the deepest pits, measured the depth, and reported the depth.

5. SUBTASK 70-22-03-860-004 Description of Replicating Material Compounds

   NOTE

   Some replica compounds are available in fluid and thixotropic grade.

   1. There are two basic grades of replica material compounds: fluid and thixotropic.

   2. Fluid replica materials (for example, CoMat 08-133 SURFACE REPLICATING MATERIAL, or CoMat 08-166 SURFACE REPLICATING MATERIAL,QUICK SET).

      1. Fluid replica materials are best for the replication of:

         1. Fracture surfaces.

         2. Holes.

         3. Cavities.

         4. Rough surfaces.

      2. The relatively low viscosity of this material lets it:

         NOTE

         Because of the low viscosity of this material, make a well with CoMat 12-026 P.V.C. SELF ADHESIVE TAPE to contain the correct amount of fluid above the feature to be replicated.

         1. Flow to give good coverage of a surface.

            Easily pump through a tube to a remote location, if necessary.

            NOTE

            A remote location is, for example, the inner surface of a shaft.

   3. Thixotropic replica materials (for example, CoMat 08-133 SURFACE REPLICATING MATERIAL, or CoMat 08-165 SURFACE REPLICATING MATERIAL,QUICK SET THIXOTROPIC, or CoMat 08-166 SURFACE REPLICATING MATERIAL,QUICK SET).

      1. This grade of replica compound has a higher viscosity that the fluid type. It is best for the replication of features on:

         1. Vertical surfaces.

         2. Overhead surfaces.

      2. Thixotropic grades of replicating compound are not the best to use on areas of tight geometry because it is difficult to apply to the surface to be replicated (for example, within very tight holes).

      3. When cured, CoMat 08-133 SURFACE REPLICATING MATERIAL has the flexibility of rubber that makes removal from the surface easier. This permits it to be used in areas that are difficult to access, or where the geometry does not permit other "hard-setting" replicas to be extracted.

      4. When cured, higher viscosity replica materials frequently contain air bubbles. You can minimize air bubbles if you carefully apply the material.

   4. Each basic grade of replicating material compound is available as "Normal Cure" and "Rapid Cure".

      1. Normal Curing replicating materials (for example, CoMat 08-133 SURFACE REPLICATING MATERIAL) have a cure time of approximately 30 minutes and a working life of 4 minutes at 77 deg F (25 deg C). The working life is the time that the replica can be in the applicator tube before it begins to set (cure).

      2. Rapid Curing replicating materials (for example, CoMat 08-133 SURFACE REPLICATING MATERIAL, or CoMat 08-165 SURFACE REPLICATING MATERIAL,QUICK SET THIXOTROPIC, or CoMat 08-166 SURFACE REPLICATING MATERIAL,QUICK SET) have a cure time of approximately 5 minutes and a working life of 30 seconds at 77 deg F (25 deg C). The advantage of the rapid cure time needs to be offset against the need to get the mixed replicating compound onto the surface in a very short time. If the replica were to be applied through a long tube to an otherwise inaccessible site, then this type would be unsuitable. It is the usual practice of IAE to use the normal cure variant for the assessment of damage.

6. SUBTASK 70-22-03-280-002 Application of Replica Material Compound

   1. Choose the applicable replicating compound for the job.

   2. Prepare the Surfaces to be Replicated.

      1. The surface to be replicated must be clean. It must not be contaminated with greases, dirt, or debris. Replicating compounds only replicate the surfaces they touch. Grease, or dirt, or debris will all prevent contact with the surface. This will produce artifacts (errors) on the replica. To measure the depth of corrosion pits on a surface, it is necessary to remove all debris from the pit. If debris stays in the corrosion pit, the correct depth may not be measured.

         NOTE

         To make sure the surface is dry and without contamination from, for example, fibers remaining from cleaning cloths, blast the surface with air from a compressed air can or a "blow-brush".

      2. Clean the features to be replicated with CoMat 01-031 ACETONE (CH3)2CO or CoMat 01-124 ISOPROPYL ALCOHOL, or CoMat 01-338 CLEANING SOLVENT or CoMat 01-437 CLEANING SOLVENT or CoMat 01-491 CLEANER SOLVENT or CoMat 01-506 SOLVENT, HANDWIPING. Let the solvent evaporate before you apply the replicating compound to the surface.

      3. Mask the surface around the feature to be replicated and make a well deep enough to produce a replica thickness of 0.1 to 0.2 inch (3 to 5 mm). If the thickness of the replica is less than this, the replica could be too delicate to cut and/or handle. CoMat 02-181 MODELLING CLAY can be used to make a well on flat surfaces (for example diaphragms) to contain the replica fluid. You can use CoMat 12-026 P.V.C. SELF ADHESIVE TAPE effectively for other geometries (for example, firtree posts) to make a well to contain the replica fluid.

   3. Prepare the Applicator Gun.

      1. Assemble the applicator gun and cartridge and attach a new mixing nozzle. Pump the trigger of the gun to prime the replicating fluid within the nozzle.

      2. Discard the first quantity of replica fluid that comes from the nozzle because sometimes this initial amount of replicating material will not contain the full amount of hardener and so will not cure (solidify/set) fully. The quantity to discard must be decided with reference to the size of the mixing nozzle.

   4. Apply the Replica Material to the Surface.

      1. Apply the replica fluid with a smooth pouring action. Keep the nozzle as steady as possible because this will help prevent air bubbles becoming caught in the replica.

      2. Fill each area to be replicated individually. Complete each area before you move to the next. It is important to remember that the mixed replica fluid has a working life. If the replica material stays in the nozzle for a period of time approaching the working life, it will begin to cure and will not make a satisfactory replica of the surface.

      3. After the replica material has been applied, let it cure for the time specified by the manufacturer. The replica material must be allowed to cure fully before it is removed from the surface or handled, or touched. Removal of the replica before it is cured fully can cause a distorted (twisted) replica.

   5. Remove the Replica from the Surface.

      1. Peel the replica away from the surface with a smooth action. The replica must not tear.

      2. Examine each replica for quality of reproduction, refer to Step. Poor reproductions, caused by contamination from grease, dirt, fibers, or air bubbles, must not be used for measurement.

      3. Put the replica in a clean, clear, plastic bag to protect it from contamination.

7. SUBTASK 70-22-03-280-003 Description and Application of Surface Replicating Foil

   NOTE

   The replicating foil and solvent liquid is usually available a "replicating kit".

   1. Replicating foil is a metal-backed polymeric film used with a solvent to make a surface replica. It is particularly useful to make a replica of an etched section.

      1. Make the polymeric film moist with the solvent and apply it to the surface to be replicated.

      2. Press the replicating foil onto the surface to be replicated.

      3. Let the solvent evaporate before removal of the replica from the surface.

      4. Usually the replica is then attached to a holder (for example, a glass microscope slide) for safe handling and storage.

   2. For specific instructions to make a foil replica, refer to the manufacturer's instructions.

8. SUBTASK 70-22-03-280-004 Examination of the Surfaces of the Replica

   1. Make sure the replica is clean and free from dust. The replica can quickly become coated with dust and fibers during inspection. Contamination of the replica can have the effect of masking small features. If the replica becomes difficult to examine because of dust or fibers then it is permitted to wash the replica in a soap solution and to flush with water. Use a cool-air blower to dry the replica.

   2. Visually examine the quality of the replica. Use a binocular microscope with a minimum magnification of 20X.

      1. The feature whose depth is to be measured should be clearly defined on the replica and must not include air-bubbles or fibers within the body of the replica at that location. These artifacts can cause local distortions and make measurements difficult.

      2. If the surface of the replica contains debris it has removed from the surface within the feature to be measured, then the true depth of the feature will not have been replicated.

      3. The examination of corrosion pits in nickel based alloys has shown that the replica will reveal the outline of the grain structure because the grain boundaries are attacked preferentially during corrosion. This can be seen with a binocular microscope. The visibility of an outline of a granular structure on the surface of the feature confirms the cleanliness of a corrosion pit.

   3. For impact damage marks it is usual for there to be plastic deformation in the material immediately around the impact point. This can be seen as a shallow ridge around the impact mark on the metal surface. This ridge is reproduced on the replica as a shallow trough around the impact mark. Evidence of this area of plastic deformation must be reported along with the depth measurement.

9. SUBTASK 70-22-03-280-005 Measurement of Features Using a Replica

   1. To measure the depth of a feature (for example, a pit, or impact damage mark), cut the replica through the position of maximum feature depth. You can then view the section with an optical microscope and the depth of the feature can be measured.

   2. Section the Replica.

      1. Examine the replica must be examined with a binocular microscope and determine the position on the replica that relates to the maximum depth of the feature.

      2. With the replica placed on a hard flat surface, cut a section through the feature at the position that relates to the maximum depth with a clean, sharp scalpel. The scalpel cut must be as perpendicular to the surface as possible. Use a magnifier to view the replica as you make the cut to let you see the replica more clearly.

      3. View the position of the cut with the aid of a binocular microscope. It is necessary to view both pieces.

         1. If the cut did not pass exactly through the deepest part of the feature, then the piece with the deeper part of the feature must be selected for measurement.

         2. If the deepest part of the feature is close to the position of the cut, it will still be visible in profile and suitable for measurement.

      4. Examine the section to make sure that there are no air bubbles caught close to the feature to be measured.

         1. Trapped gas bubbles in this area can cause slight local distortions in the replica and will have an effect on the accuracy of the measurement.

         2. If there are such gas bubbles, discard the replica and make a new one.

      5. A further cut must be made in the replica parallel to the cut that passes through the feature to make a thin slice. This is to make it easier to position the cut surface horizontal when the measurements are made.

   3. Measurement of Feature Depth on the Sectioned Replica.

      1. Measurement of the feature depth must be done on a calibrated optical microscope. To do this, use of an optical microscope fitted with either an eyepiece graticule or a calibrated digital imaging system.

         1. It is necessary that the depth of the feature relative to the level of the component surface is measured. To do this, the replica must be correctly aligned to give an accurate measurement:

            1. Put a ball of CoMat 02-181 MODELLING CLAY between two glass microscope slides.

            2. Then put the replica on the uppermost slide. Tilt the upper most slide to correctly align the replicated component surface.

         2. The method of correct alignment will depend upon the apparatus used to view the replica.

            1. If a binocular microscope (variable objective lens) is used, the replica can be tilted to make the replicated surface of the component perpendicular to the field of view. The large field of view provided by binocular microscopes makes this possible. As the replica is tilted, the replicated surface of the component will diminish in the field of view until it is just no longer visible - only the surface of the scalpel cut through the feature will then be visible. The depth of the feature relative to the component surface can then be measured.

            2. If an optical microscope (fixed objective lenses) is used, the method outlined for binocular microscopes may not be possible because of the very limited depth of field of view of fixed objective lenses. For optical microscopes, it is preferable to view the surface made by the scalpel slice and tilt the replica until as much of the surface is in focus as possible. This of course then relies upon the scalpel cut being as close as possible to perpendicular to the replicated surface of the component.

         3. Alignment of the replica must be made at low magnifications. Once the replica is aligned, the magnification must be increased so that the replicated feature fills as much of the field of view as possible. It should be noted however that the depth measurement is to be made relative to the replicated surface of the component. Consequently, the field of view must contain an amount of surface sufficient to make the measurement.

         4. The depth of the feature must be measured relative to the replicated component surface. The depth of the feature can be measured with an eye-piece graticule or by a digital imaging system.

            1. An eye-piece graticule is a scale marked onto the eye piece of the microscope. The scale must be calibrated for each objective lens setting. Suitable calibrated scales are commercially available for this purpose. These consist of a calibrated scale etched onto a piece of metal, mounted on a glass slide, which can be viewed through the microscope. These provide a convenient method for testing the calibration for every measurement made. This can also be used for computer-based digital imaging systems.

            2. If a digital imaging system is used, it is important that the system is correctly calibrated (just the same as for measurements with an eye piece graticule). For systems connected to binocular microscopes with continuously variable objective lenses, each selection of objective setting must be calibrated.

         5. The error introduced by measurement along a line that is not perpendicular to the replicated component surface is not significant for small angles. For any measurement to be within +/- 10 percent of the true value, the measurement must be made along a line that is at an angle less than 25 degrees to the line perpendicular to the replicated surface. This should be easily achievable with the techniques specified in this procedure.

   4. Any unusual feature on a replica, which is not an artifact of the replication process (for example, trapped fiber, gas bubble, or dirt) must be reported along with the depth measurements. If it is suspected that an unusual feature is an artifact, then the damage shall be recleaned, replicated, and examined again for the unusual feature.