1.0 Introduction (Back to Contents)
This booklet is essential reading for anybody who is considering purchasing Flawed Specimens or Mock-ups. It contains useful information on what should be considered when buying Specimens, how Specimens differ from real components and how substantial financial savings can be made. This booklet also gives guidelines for going out to bid and how to compare bids in order to maximise value for money.
This Buyers Guide is based on many years experience of designing and manufacturing this unique product. It is not written with any particular certification or performance in mind. It is equally applicable for Specimens to be used for general certification, through to full blown performance demonstration and validation programmes where equipment, procedure and technicians are subject to test.
1.2 What is a Flawed Specimen/Mock-up?
Flawed Specimens/Mock-ups are NDE products designed (with flaws) to be used exclusively by the NDE industry. As they will not form any part of a working component they can be built with different considerations. This is the case for both performance demonstration and practice Specimens, yet there is more scope for cost savings on practice/training Specimens. Flawed Specimens should be designed with their specific use in mind and never be confused with the real component they are simulating.
1.3 What are Flaws?
In most cases it is impossible, and certainly not economical or practical, to produce naturally occurring flaws. Some flaws will have taken many years to propagate and their sizes are impossible to measure accurately. It is normal practice to create real flaws using artificial methods. It is not important to replicate how a natural flaw occurred; only its resulting characteristics i.e. planar, volumetric, rough, smooth, etc. The image from most NDE methods will not fully identify a flaw type; it will only provide a certain amount of evidence. The actual flaw type is often decided from a combination of NDE characteristics, material, location and environment. Real flaws cannot be substituted by artificial slots and notches because they are not characteristic.
2.0 Definitions/Standards (Back to Contents)
There are no National or International specifications for flawed Specimens. Historically, it has been left to individuals to write their own specification and some countries, organisations and certification bodies have developed specifications which can be used or referred to in order to get some commonality/consistency.
Dilemma: Manufacturing specifications do not allow cracks yet they are often required in a flawed Specimen! Most specifications allow small flaws yet in flawed Specimens these may lead to confusion with the intended flaws. It is important to work to the ‘spirit’ of existing relevant specifications.
3.0 Going out to bid (Back to Contents)
In each case, refer to the relevant section below or page. As a minimum, you must specify the following as the basis for obtaining a price:
• Specimen Type(s)/Quantity Required - Plate, Pipe, Tee, Nozzle, Node etc.
• Flaws (Ref. Section 5) The number, type, size, location and shape of the flaws in each specimen. In all cases a tolerance for flaw size and location should be stipulated.
• Materials (Ref. Section 6) Specify the material used in the real component. Money can sometimes be saved by using alternative materials which have almost identical NDE characteristics.
• Design (Ref. Section 7) The dimensions of the Specimen i.e. pipe diameters, thickness, any geometric condition required, i.e. counterbores etc. A drawing should be supplied whenever possible. State whether the Specimens are to be used for Training, Practice or for Examination/Certification. Wherever possible stipulate the name of the Examination/Certification scheme.
• Welding Standard (Ref. Section 8) The Specimen must replicate the real component but it does not necessarily have to be welded/constructed to the same standard. A high standard of welding must be achieved to include intended flaws but minimise unintentional flaws.
• NDE Standard (Ref. Section 9) The Specimen’s final use is for training and qualifying NDE technicians, it is therefore critical that the Specimen manufacturer is fully conversant with each applicable NDE method. suitably trained, qualified and experienced personnel must be involved at each stage of manufacture.
• Documentation (See section 10)
The Specimen must be accompanied by conclusive evidence of the flaw(s) existence, size, shape and location. These items will often be governed by your plant type, how many people you plan to qualify and how much training you intend, as well as the size of your budget. Remember that most Specimen manufacturers will supply whatever you ask for. It may mean a bigger order for them! Do you really need everything you are asking for? The following text will help you to be sure!!!
4.0 Comparing Bids (Back to Contents)
Remember, you are buying flaws, not just Specimens. Do not make your choice of supplier based only on the SpecimenÕs overall dimension/size. The following are the most important considerations:
- Quality and Quantity of Flaws
- Fitness for Purpose/Usefulness of the Specimen
- Quality of Welding and Inspection
- Quality of Relevant Manufacturing Experience
- Quality and Meaningfulness of Documentation (not volume)
- Price & Delivery
5.0 Flaws (Back to Contents)
The major cost to produce a Specimen is the number, type, size/shape, location and tolerance of the flaws. When comparing prices, it is important to be able to compare the overall area of flaws. This is not easy to calculate if there are a number of different flaw types, but if there are only cracks then the overall cracked area can be easily calculated.
It is essential to compare the number of flaws, their length and through-wall dimensions, as these sizes have a major effect on the cost of the Specimen. Ask the proposed manufacturer to confirm the total number and total length/area of flaw so that you can compare competitive quotations.
The price to produce an unflawed Specimen is constant. As a general rule the larger the number of flaws, the greater the overall cost, but the price per flaw decreases as the number increases. A large population of flaws can spoil the value of a Specimen. It is permissible for practice Specimens to contain more flaws than performance demonstration Specimens.
Flaw types fall into the following categories:
- Volumetric flaws include slag inclusions, group porosity, single pores, incomplete penetration.
- Planar flaws include cracks, lack of fusion, laminations. These are often split into two categories,
i.e. rough and smooth.
- Crack types include mechanical fatigue, thermal fatigue, intergranular stress corrosion, solidification.
- Artificial flaws are used for reference and equipment calibration, or as an inexpensive alternative to real
flaws for personnel training and practice.
There are two tolerance which you should consider:
i) Working tolerance - The Specimen manufacturer should be allowed as much tolerance as possible in placing the flaw, i.e. distance from datum, depth, etc. If, for example, the flaw is wrongly located and will not affect its final use, then it should be accepted. The greater you make this tolerance, the more you reduce the risk to the manufacturer and therefore reduce the cost.
ii) Final tolerance - The final tolerance must be agreed by the purchaser and manufacturer during the bid stage. This tolerance is for the final position and size.
The tolerance of the intended flaws will have a major effect on the price. Bidders should be made aware of your desired tolerances.
5.5 Size/Shape and Location
Flaw size, location and, to a lesser extent, the shape are major factors affecting the price of a Specimen and should be clearly identified.
It is important also that the flaw sizes are NOT measured using any one NDE method, as this gives little advantage over those under examination.The flaw size truth should be measured by some physical means. It is commonly accepted that NDE measurements be used as a back-up and ensure no propagation has occurred.
5.6 NDE Characteristics
The NDE characteristics of the flaws are critically important and will drastically affect the use of the Specimen. It is important to ensure the proposed Specimen manufacturer understands the use of the product. This includes, but is not limited to, the flaw’s image, NDE response and signal amplitudes.
Once the Specimen is complete it is impossible to re-check the flaw size, so it is important to ensure that the Specimen manufacturer has the relevant experience in conducting this type of work and has completed trials to prove their ability.
Evidence of the flaw size and shape can be collected during stages of manufacture, i.e. tracings, photographs, etc. Hold points can be organised to allow the purchaser to monitor inspection. It should be noted that the evidence does not give the final size of the flaw and that allowances/adjustments have to be made in most cases.
For surface breaking flaws, a crack is the most expensive flaw to produce and the cost increases with the size. It is therefore critical when comparing bid prices to calculate the total flaw size (area) for each bid.
Other flaw types are much less expensive and can be compared by number and length.
Buried or sub-surface cracks are also expensive to produce, but the depth from the surface is another important consideration. When comparing bids the flaws should be the same, or of similar size and depth.
5.9 Overall Area of Flaws
Multiply the length (circumferential) by the through-wall height for each flaw and add all the flaw sizes together.
5.10 Unintended Flaws
When purchasing Specimens ensure that each bidder is aware of the size and number of permitted unintended flaws. This can be as specified by a particular training/certification programme, or to your individual specification. Do not insist on a particular welding standard, as this may allow flaws which can interfere with the use of the Specimen.
5.11 Geometric Conditions
The geometric condition of the Specimen can greatly affect its use and cost. You should specify the geometry, including details such as counterbore, mismatch, root, excess-penetration, undercut and weld crown contour.
The geometry will affect the price. You should highlight the total length of the conditions you require and ensure that all bidders quote to the same specification.
6.0 Materials (Back to Contents)
6.1 Type and Grade
The Ultrasonic characteristics of the material used in flawed Specimen manufacture should match as close as possible those of the actual component. It is especially important to use the same form of material, i.e. forged or cast. The exact chemical composition, grade of steel is not important.
The material should not contain any flaws which will affect the use of the Specimen. In most cases the required quality of the raw material is higher than that specified in any recognised standard. Choice of raw material, therefore, depends on the use of the Specimen and should be considered for each individual specimen.
In most cases it is possible and cost effective to use small sections and/or other similar material forms to produce test Specimens. For example, large diameter pipe can be simulated by 120û/rolled plate.The type/grade and quality of material is less important for practice/training Specimens, as any discrepancies can be notified to the technician and ignored (there is no possibility of dispute). Greater use of alternative materials and section is also possible.
6.4 Free Issue Materials
It is always worth considering issuing material for two main reasons:
i) To reduce cost
ii) To ensure that the material is representative
The down side is the handling/shipping costs and time scale.
7.0 Specimen Design (Back to Contents)
For economic reasons, performance demonstration Specimens and practice/training Specimens can be treated separately. For example, performance demonstration Specimens must conform to the specification.
Equally, for training and practice Specimens there are many ‘cost cutting’ ways that the Specimens can be manufactured without affecting their usefulness.
7.1 Performance Demonstration/ Certification Specimens
Most performance demonstration specifications stipulate that flawed Specimens should have the following as a minimum:
- Contain a number of intentional flawed and unflawed areas. Often a 1:2 ratio, and that each area should have a minimum length, i.e. 75mm.
- Contain flaws for both detection and sizing; often treated separately.
- Concealed flaw areas to allow blind tests to be conducted.
- Adequate documented evidence to show the flaw size truth.
- Integrity of the Specimen manufacturer and confidentiality of flaw location report(s).
- There is less scope to reduce the cost for these Specimens.
7.2 Practice/Training Specimens
There is no specification for practice/training Specimens but they must be typical of the performance demonstration Specimens. There is scope therefore to reduce the cost. This can be done by:
- Combining detection and sizing Specimens. They can have a number of accurate flaws for detection and sizing, and other less accurate flaws for detection only. (Accurate sizing flaws are most expensive to manufacture.)
- Reducing the number of unflawed areas.It is most important that the practice Specimens replicate the performance demonstration Specimens in the following ways:
- Geometric conditions, i.e. weld root and crown, counterbore, mismatch, etc.
- Flaw sizes, shapes and characteristics, etc.
- The Specimens can be supplied with less evidence of flaw size as there is no reason for the technician to dispute sizes.
- Specimens do not have to be secure.
8.0 Welding (Back to Contents)
The volume of welding will also greatly affect the cost of manufacture. It is important to consider this when designing the Specimen.
Reduced welding also reduces the risk of unintended flaws. As a general rule for implanting flaws, large planar flaws should be closest to the surface and volumetric flaws the deepest.
The following items address welding considerations. Deviation from these may affect the cost of the Specimen.
Specimens are NDE products. The strength and condition of any weld is not important; cleanliness is.
Welds have only to be fit for their purpose.
8.2 Weld Quality
For Specimens where welding is part of the Specimen, i.e. pipe welds, the weld should be consistent with that used in the real weld, yet not contain confusing unintended flaws. Where implanting is carried out, the weld should be ÔinvisibleÕ to ultrasound or other NDE methods.
IMPORTANT NOTE: The high quality of welding will increase the cost of the Specimen, it is important that the bidders are instructed to achieve the same standard.
8.3 Welders Qualification
The usual welder qualifications are not relevant for the manufacture of Specimens. Because the Specimens contain intentional flaws, it is very difficult for any meaningful welder qualification test to be applied.
In general, most welder qualification/certification standards allow welders to leave small ‘acceptable’ flaws which are unacceptable in test Specimens.
Considerable welding experience, a good knowledge of Specimens and their uses and a knowledge of NDE is however essential. Qualifying welders to each specific weld type would be very expensive and therefore add to the manufacturing costs.
IMPORTANT NOTE: It is not important to ask for specific welder qualification; only evidence that the required standard can be consistently achieved.
8.4 Weld Procedures
General welding procedures are important to ensure consistency of welding quality in a given welding process. They are unnecessary for ensuring mechanical properties. Producing specific welding procedures is prohibitively expensive and would increase the manufacturing cost.
IMPORTANT NOTE: It is not important to ask for a specific welding procedure, only general procedures to ensure consistency of welding. For stainless steel the welding position is important and should be specified to the bidder as this will affect the price.
Repairs are carried out if any unintended flaws are found. It is not necessary to keep records of such repairs as they will have no effect on the use of the specimen.
8.6 Weld Stress Relief
In general, stress relieving of Specimens has no positive advantage and, although it is often carried out to make the end product the ‘same’ as the real component, it is not necessary. It does add to the cost. NDE has to be carried out twice; and stress relieving can cause the propagation of the intended flaws, which can only be remeasured using NDE.
9.0 Non Destructive Evaluation (Back to Contents)
NDE of the end product and at various stages of manufacture is critical. This ensures that the product is fit for its purpose.
9.2 NDE Technicians
It is important to employ qualified NDE technicians. However, like welders, the technician should have a good knowledge of the product and its uses. Experience in testing flawed Specimens is essential. The technician’s qualifications cannot be specific. It must, however, be an internationally recognised standard and be generally applicable.
9.3 NDE Procedures
General NDE procedures are important to consistency in the level of inspection. However, specific procedures are not necessary as the Specimen is used to develop procedures.As a rule, all NDE will be carried out manually to ensure that the flaws are visible and their characteristics typical, that the flaw has not propagated and that no other unintended flaws exist or interfere with the use of the Specimen.
9.4 Stage NDE
It is not necessary to report each stage of inspection. If the inspection is acceptable then this is OK. If not, a repair is carried out and the area is re-tested.
It is not always appropriate to ask for certification for the technician who tests the Specimens. These may not be at all relevant to the particular Specimen type, or to the technician’s ability to detect or size the flaws (i.e. pass the test). The technician will have had full knowledge of the flaw’s location. The manufacturer should be asked to supply evidence of the technician’s ability/experience.
The equipment used to test the Specimen does not have to resemble in any way the equipment on which the Specimen is being used. It is not the intention to prove the ability of this equipment.
10.0 Documentation (Back to Contents)
Documentation should be limited to relevant information applicable to the use of the Specimen. It is strongly recommended that the following is requested as a minimum for performance demonstration Specimens.
- As-built drawing showing flaw details.
- Flaw Size Statement for each flaw.
- A tracing or photograph of each flaw, where appropriate.
- Material and consumable certification.