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CTOD Testing

The concept of fracture toughness was introduced in an earlier Connect article, which discussed the Charpy-V test, a simple qualitative test that gives only an indication of the toughness of a metal.
The next few articles will look at the tests that enable fracture toughness to be accurately measured in a quantitative manner by using a full size specimen containing a crack with loading that is representative of service conditions.
This allows a fitness-for-purpose analysis to be carried out which enables a critical defect size to be calculated. Thus, prior to fabrication, realistic acceptance standards can be set and decisions on appropriate NDE techniques and detection sensitivities can be made.
For equipment already in service, it is possible to justify the continued use of cracked or otherwise flawed components until such time as repair or replacement can be effected. Such engineering critical assessments can save an operator large amounts of time and money, running into perhaps hundreds of millions of pounds in the case of an oil rig for example. Whilst the Crack Tip Opening Displacement (CTOD) test was developed for the characterisation of metals it has also been used to determine the toughness of non-metallics such as weldable plastics.
The CTOD test is one such fracture toughness test that is used when some plastic deformation can occur prior to failure - this allows the tip of a crack to stretch and open, hence 'tip opening displacement'.
Unlike the inexpensive 10mm by 10mm square Charpy-V test piece with a blunt machined notch, the CTOD specimen may be the full thickness of the material, will contain a genuine crack and will be loaded at a rate more representative of service conditions. Conventionally three tests are carried out at the relevant temperature to ensure consistency of results.
The test piece itself is 'proportional' - the length, depth and thickness of each specimen are inter-related so that, irrespective of material thickness, each specimen has the same proportions.
There are two basic forms - a square or a rectangular cross section specimen. If the specimen thickness is defined as 'B', the depth (W) will be either B or 2B with a standard length of 4.6W. A notch is machined at the centre and then extended by generating a fatigue crack so that the total 'defect' length is half the depth of the test piece- see Fig.1. A test on a 100mm thick weld will therefore require a specimen measuring 100mm thick, 200mm wide and 920mm long - an expensive operation, the validity of which can only be determined once the test has been completed.

Fig.1. Proportional rectangular cross section CTOD specimen

The test is performed by placing the specimen into three point bending and measuring the amount of crack opening. This is done by means of a strain gauge attached to a clip placed between two accurately positioned knife edges at the mouth of the machined notch (Fig.2)

Fig.2. Typical test arrangement. The specimen can be easily immersed in a cooling bath

As bending proceeds, the crack tip plastically deforms until a critical point is reached when the crack has opened sufficiently to initiate a cleavage crack. This may lead to either partial or complete failure of the specimen. The test may be performed at some minimum temperature eg the minimum design temperature or, more rarely, at a range of temperatures.
The values that are required for the calculation of toughness are firstly the load at which fracture occurs and secondly the amount by which the crack has opened at the point of crack propagation (Fig.3).

Fig.3. Position of CTOD specimen immediately prior to crack propagation

Since the length of the crack and the opening at the mouth of the notch are known it is a simple matter to calculate the crack tip opening by simple geometry. Whilst the test is in progress the results are recorded automatically on a load/displacement chart (Fig. 4). This illustrates the various shapes of curve that may be produced - (a) is a test where the test piece has fractured in a brittle manner with little or no plastic deformation. (b) exhibits a 'pop-in' where the brittle crack initiates but only propagates a short distance before it is arrested in tougher material - this may occur several times giving the curve a saw tooth appearance or after this one pop-in deformation may continue in a ductile manner as in (c) which shows completely plastic behaviour.

Fig.4. Load vs crack opening displacement curves showing three types of fracture behaviour

The location of the notch in the weld HAZ or parent metal is important as an incorrectly positioned fatigue crack will not sample the required area, making the test invalid. To be certain that the crack tip is in the correct region, polishing and etching followed by a metallurgical examination are often carried out prior to machining the notch and fatigue cracking. This enables the notch to be positioned very accurately. Examination may also be carried out after testing as further confirmation of the validity of the test results.
Once the sample is broken open the crack surface is examined to ensure that the fatigue crack has a reasonably straight front. The residual stresses present in a welded joint may cause the fatigue crack front to be irregular - if this is excessive the test may be invalid. To overcome this problem the test piece may be locally compressed at the machined notch tip to redistribute the residual stress.
Two depressions each side of the sample can often be seen where this compression has been carried out. The fatigue cracking itself should be carried out using a low stress range. The use of high stresses to speed up the fatigue cracking process can result in a large plastically deformed area ahead of the fatigue crack and this will invalidate the results of the test.

Other causes of test failure can unfortunately only be determined once the test has been completed and the crack surface examined. The precise length of the fatigue crack is measured - this is required for the analysis - but if the length of the crack is not within the limits required by the specification the test is invalid. If the fatigue crack is not in a single plane, if the crack is at an angle to the machined notch or if the crack is not in the correct region the test may need to be repeated.

Related specifications

BS 7448	Parts 1- 4 Fracture Mechanics Toughness Tests
BS 7910	Guide on Methods for Assessing the Acceptability of Flaws in Metallic Structures.
ASTM E1820	Standard Test Method for Measurement of Fracture Toughness.
BS EN ISO	15653 Metallic Materials - Method of test for the determination of quasistatic fracture toughness of welds

This article was written by Gene Mathers

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A comparison of ISO 15614 Part 1 and ASME IX

Job knowledge

The question is sometimes asked ‘Can I use our existing welding procedure qualifications?’ where the qualification specification required by the contract is one that has not previously been used by the organisation. This is particularly relevant when substantial costs and/or delays will be incurred if re-qualification of the welding procedures is necessary. The two most frequently encountered specifications are ISO 15614 Part 1 and ASME IX and whilst these are written with the same purpose (that of giving assurance that a welding procedure will provide the desired joint properties)  there are major differences between the two specifications that mean that they are not equivalent. It will not be possible in this short article to cover every welding variable and its range of approval in the two specifications. Where compliance is required then reference MUST be made to the appropriate specification.
With respect to ASME IX the specification requirements can be applied in two ways; ASME intent and ASME stamp. If the welded item is to be ASME stamped this can only be done by a manufacturer who has a quality system accredited by ASME and who holds an appropriate stamp, N stamp for nuclear components, U for unfired pressure vessels, S for power boilers etc. All the requirements of the ASME specifications MUST be complied with, even to the extent of dimensions of the mechanical test pieces and the calibration of testing equipment.
ASME intent is used where the item is not to be code stamped but is perhaps only designed to the relevant ASME code and some flexibility is possible with respect to the manufacturing aspects of specification compliance. Such flexibility may allow the manufacturer to submit to the client or inspecting authority procedure qualification records (PQR) to ISO 15614 Part 1 for approval that can be shown to be technically equivalent to an ASME PQR.
ASME IX covers the qualification of welders and welding operators, welding procedures, brazing operatives and brazing procedures for the complete range of ferrous and non-ferrous engineering metals (steels, copper, nickel, aluminium, titanium and zirconium alloys) and oxy-gas, arc, power beam, resistance and solid phase welding processes. ISO 15614 Pt1 covers the welding procedure qualification of arc and gas welds in steel and nickel alloys only. Other alloys and joining processes are covered by additional specifications within the ISO 15614 series.
Both specifications identify essential variable (although ISO 15614 Pt1 does not describe them as such) to each of which is assigned a range of approval. A change to an essential variable outside of its range of approval requires the welding procedure to be re-qualified. ASME IX in addition identifies supplementary and non-essential variables. Supplementary variables are only invoked when toughness requirements are specified by the application code, eg ASME VIII or ASME B31.3. Non-essential variables, as the name suggests, are those variables that are not regarded as affecting the quality or mechanical properties of the welded joint and comprise such variables as the weld preparation, shield gas flow rate, method of back gouging, shield gas nozzle size etc. Although these variables are non-essential it is a requirement that they should be referenced on the welding procedure. It is therefore NOT acceptable to use a butt welding procedure to specify how a fillet weld should be made.
ISO 15614 Pt1 does not identify any variables as non-essential;  where a variable is not regarded as significant it is simply not referenced in the specification. There are several variables in both specifications where there is no range of approval;  the manufacturer, the welding process and the application or deletion of post weld heat treatment (PWHT) for example.
In order to reduce the amount of qualification testing, both specifications group alloys of similar characteristics together. Qualifying the welding of one alloy within the group allows the other alloys within the group to be welded. ASME IX assigns the groups  numbers with steels being numbered P1 to P15F. Any alloy that does not have a P number is regarded as unassigned; a procedure qualification carried out using an unassigned alloy qualifies only that specific designation of alloy. Until recently only alloys that complied with the ASME and/or ASTM material specifications and/or had a UNS number were assigned P numbers. However, a limited number of EN, Canadian, Chinese and Japanese alloys have now been introduced into the list of assigned alloys.
ISO 15614 Pt. 1 also groups steel and nickel alloys into families with similar properties but is somewhat less prescriptive than the ASME code in that, provided alloys have similar chemical compositions and mechanical properties, the material specification is not relevant – for example a plain carbon steel with less than 0.25%C and a minimum specified yield strength less than 460MPa  falls into Group 1 irrespective of whether or not it is a pressure vessel or structural steel or supplied in accordance with EN or ASTM material specifications. To determine into which group the alloy falls reference should be made to ISO/TR 15608, the specification that lists both ferrous and non-ferrous alloys and assigns them a group number.
Other significant differences between the two specifications with respect to the arc welding processes are :-

• ASME IX requires only tensile and bend tests to qualify a butt weld. ISO 15614 Pt1 requires a far more extensive test programme of visual inspection, radiography or ultrasonic examination, surface crack detection, tensile and bend tests and macro-examination. In certain circumstances Charpy-V impact tests and hardness surveys are also required. 
• ASME IX specifies that the tensile strength of the cross joint tensile specimen shall be at least that of the minimum specified for the parent metal and that bend test coupons should have no discontinuity greater than 3mm. ISO 15614 Pt1 has identical requirements for these mechanical tests but in addition specifies an acceptance standard for the non-destructive testing; impact test results, when required, that match the parent material toughness and hardness limits when hardness testing is required.
• ISO 15614 Pt 1 requires Charpy-V impact testing for steels over 12mm thick when the material specification requires it. ASME requires impact testing only when specified in the application standard. This requirement makes heat input a supplementary essential variable in ASME IX but an essential variable in ISO 15614 Pt1.
• Hardness testing is required by ISO 15614 Pt1 for all ferritic steels with a specified minimum yield strength greater than 275MPa. A maximum hardness for joints in either the as-welded of PWHT’d condition is specified. ASME IX does not require hardness testing.  
• ASME IX allows a reduction in preheat of 55^OC before requalification is required. ISO 15614 Pt1 does not permit any reduction in preheat from that used in the qualification test.
• ASME allows the maximum interpass temperature to be 55^OC above that measured in the qualification test. ISO 15614 Pt 1 permits no such increase.
• ASME IX requires pressure containing fillet welds to be qualified by a butt weld procedure qualification test. Non-pressure retaining fillet welds may be qualified by a fillet weld test only. ISO 15614 Pt1 requires a fillet weld to be qualified by a butt weld when mechanical properties “.... are relevant to the application...” i.e when it is a load carrying fillet weld. In addition, whilst a butt weld will qualify a fillet weld “....fillet weld tests shall be required where this is the predominant form of production welding...” i.e. an ISO compliant welding procedure where the majority of the welding is of load carrying fillet welds must reference both a butt weld and a fillet weld procedure qualification.
• Weld metal transfer mode, where relevant, is an essential variable in both ISO 15614 Pt1 and ASME IX but the current type is an essential variable in ISO 15614 Pt1 and a supplementary essential variable in ASME IX.
• A change from manual to automatic welding is an essential variable in ISO 15614 Pt1 but a non-essential variable in ASME IX.

Whilst there are several other variables in the two specifications that have substantially different ranges of approval there are many that have ranges that are very similar – material thickness being but one example.

This article has highlighted some of the significant differences but to ensure that the welding procedure and its supporting procedure qualification record are compliant the specifications must be referred to. The answer to the question posed at the start of this article is therefore – it depends upon what you can persuade the client and inspecting authority to accept!
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Fatigue testing

Fatigue as a specific failure mechanism has been recognised since the early part of the nineteenth century but it was the development of rail travel that resulted in a major increase of interest in this type of fracture.
The premature failure of wagon axles led to Wohler in Germany investigating fatigue failure under rotating loading. This led to the design of the first standardised test - a reversing stress rotating specimen, illustrated in Fig.1.

Fig.1. Wohler rotating fatigue test

There are many mechanisms that can lead to failure but fatigue is perhaps one of the most insidious since it can lead to a catastrophic failure with little or no warning - one well known example being the failure of the Comet aircraft in the 1950s.
Failure can occur at a fluctuating load well below the yield point of the metal and below the allowable static design stress. The number of cycles at which failure occurs may vary from a couple of hundreds to millions. There will be little or no deformation at failure and the fracture has a characteristic surface, as shown in Fig.2.

Fig.2. Typical fatigue crack fracture surface

The surface is smooth and shows concentric rings, known as beach marks, that radiate from the origin; these beach marks becoming coarser as the crack propagation rate increases. Viewing the surface on a scanning electron microscope at high magnification shows each cycle of stress causes a single ripple. The component finally fails by a ductile or brittle overload.
Fatigue cracks generally start at changes in section or notches where the stress is raised locally and, as a general rule, the sharper the notch the shorter the fatigue life - one reason why cracks are so damaging.
There are two stages in the process of fatigue cracking - a period of time during which a fatigue crack is nucleated and a second stage where the crack grows incrementally leaving the ripples described above. In an unwelded component the bulk of the life is spent in initiating a fatigue crack with a shorter period spent in crack propagation.
An unwelded ferritic steel component exhibits an endurance limit - a stress below which fatigue cracking will not initiate and failure will therefore not occur. This is not the case with most non-ferrous metals or with welded joints - these have no clearly defined endurance limit.
The reason for this is that in arc welded joints there is an 'intrusion' - a small defect at the toe of the weld, perhaps only some 0.1mm deep. Provided that the applied stress is sufficiently large a crack will begin to propagate within an extremely short period of time. The endurance limit for a welded joint is therefore dependent on the intrusion size that does not result in crack propagation at the applied stress range. In the case of a welded joint, therefore, a fatigue limit - a 'safe life' is specified, often the stress to cause failure at 2x10⁶ or 10⁷ cycles.
During fatigue the stress may alternate about zero, may vary from zero to a maximum or may vary about some value above - or below - zero.
To quantify the effect of these varying stresses fatigue testing is carried out by applying a particular stress range and this is continued until the test piece fails. The number of cycles to failure is recorded and the test then repeated at a variety of different stress ranges.
This enables an S/N curve, a graph of the applied stress range, S, against N, the number of cycles to failure, to be plotted as illustrated in Fig.3. This graph shows the results of testing a plain specimen and a welded component. The endurance limit of the plain specimen is shown as the horizontal line - if the stress is below this line the test piece will last for an infinite number of cycles. The curve for the welded sample, however, continues to trend down to a point where the stress range is insufficient to cause a crack to propagate from the intrusion.

Fig.3. S/N curves for welded and unwelded specimens

By testing a series of identical specimens it is possible to develop S/N curves. In service however, there will be variations in stress range and frequency. The direction of the load may vary, the environment and the shape of the component will all affect the fatigue life, as explained later in this article.
When designing a test to determine service performance it is therefore necessary to simulate as closely as possible these conditions if an accurate life is to be determined. In order to enable the fatigue life to be calculated when the stress range varies in this random manner, the Palmgren-Miners cumulative damage rule is used.
This rule states that, if the life at a given stress is N and the number of cycles that the component has experienced is a smaller number, n, then the fatigue life that has been used up is n/N.
If the number of cycles at the various stress ranges are then added together - n₁/N₁ + n₂/N₂ + n₃/N₃ + n₄/N ₄etc - the fatigue life is used up when the sum is of all these ratios is 1. Although this does not give a precise estimate of fatigue life, Miners rule was generally regarded as being safe. This method, however, has now been superseded with the far more accurate approach detailed in the British Standard BS 7608.
The design of a welded joint has a dominant effect on fatigue life. It is therefore necessary to ensure that a structure that will experience fatigue loading in the individual joints has adequate strength. The commonest method for determining fatigue life is to refer to S/N curves that have been produced for the relevant weld designs.
The design rules for this range of joint designs were first developed by TWI and incorporated with the bridge code BS 5400 in 1980 and then into the industry design rules for offshore structures. Further refinements and improvements finally resulted in the publication of BS 7608 Code of practice for fatigue design and assessment of steel structures. This standard will be looked at in more detail in a future article.
This article was written by Gene Mathers.
Part 2
Part 3
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Heat Treatment of welded joints Part 4 - Precipitation or age hardening

There are several methods that may be used to increase the strength of a metal; alloying, quenching of steel, work hardening, and one very specific form of heat treatment, that of precipitation or age hardening (the two terms are synonymous). Many ferrous and non-ferrous alloys are capable of being age hardened and, as the name suggests, this method of increasing the strength relies upon the formation of precipitates. To achieve the optimum combination of mechanical properties the heat treatment cycles must be very closely controlled.
Unfortunately, to understand how the precipitates affect the mechanical properties it is necessary to introduce some fundamental metallurgy.
The precipitation hardening mechanism requires the solubility of the alloying element, the solute, in the metal, the solvent, to increase as the temperature increases as shown in the phase diagram in Figure 1 where the solvus line shows decreasing solubility of alloying element B in the solvent A as the temperature falls.
An analogy is that of salt in water; as the temperature increases more salt can be dissolved but the converse happens as the solution is allowed to cool when salt crystals begin to form or precipitate.
The same process occurs in suitably alloyed metals except that the processes of dissolving and precipitating take place in the solid and are hence much slower as atoms find it more difficult to move in a solid than a liquid solution.
A consequence of this is that once the precipitates have been dissolved by taking the metal alloy to a sufficiently high temperature, ie above the solvus line, they can be prevented from re-forming by rapid cooling or quenching.
This heat treatment is known as solution heat treatment and is carried out to form an unstable super-cooled solid solution which, if reheated to a lower ageing or precipitation hardening temperature, will begin to re-form the precipitates, these growing in size as the heat treatment proceeds. A schematic of such a heat treatment cycle is also given in Figure 1 for alloy N.

Figure 1: Phase diagram showing decreasing solubility of B in A and heat treatment cycle

In the solution heat treated metal the atoms of the alloying element, the solute, are randomly distributed throughout the matrix but once the temperature is raised the precipitates begin to form by a nucleation and growth process. At relatively low temperatures and in a short timescale the solute atoms begin to cluster together to form extremely small and very finely dispersed precipitates known as Guinier-Preston (GP) zones, named after the two metallurgists who first identified them. The GP zones are so small that they are not visible using normal optical microscopes but can be seen using electron microscopy at magnifications of around x100,000.
The GP zones are described as coherent, in other words they have the same crystal structure as the solvent metal. However, they distort the crystal lattice, the framework on which the atoms are positioned. This makes it more difficult for dislocations to move through the lattice and it is dislocation movement that enables metal to deform; tensile strength and hardness therefore increase but ductility and toughness decrease. As the ageing treatment continues or the temperature is raised the tensile strength also continues to increase as the precipitates grow and coarsen whilst still remaining coherent. At some point, however, the precipitates begin to lose their coherency; they become incoherent, forming separate particles within the metal with a different crystal structure from the solvent and at this stage they become visible using an optical microscope.
Just before this point is reached is when the alloy has the very highest tensile strength. As these incoherent particles form and grow in size the tensile strength progressively decreases. The alloy then is said to be overaged although the precipitates still contribute towards the tensile strength of the alloy. The high strength low alloy (HSLA) steels are a good example of this where incoherent, overaged precipitates are used to give a substantial increase in the tensile strength.
In order to achieve the best combination of properties the precipitates need to be evenly distributed throughout the grains of the alloy and of an optimum size. The ageing temperature and/or time can obviously be changed to tailor the distribution and size of the precipitates; longer times and/or higher temperatures generally result in a reduction in strength but an increase in ductility, an overaged structure giving the lowest tensile strength but the highest ductility.
Typical heat treatment times and temperatures of a range of different alloys are given in Table 1. The ferritic and nickel based alloys are generally used in the overaged condition in order to ensure a reasonable degree of ductility. It can be seen that with some alloys, eg 17/4PH stainless steel, the precipitation mechanism is sufficiently sluggish that the component can be cooled in still air or, as with the A286 stainless steel, long ageing times are required.
On the other hand the aluminium-copper alloy 2219 is capable of ageing at room temperature if left for a couple of days. Some of the 6000(Al-Si-Mg) and series 7000 (Al-Zn-Mg) alloys will similarly age at ambient temperature. This is known as natural ageing; – aging at an elevated temperature is known as artificial ageing.

Table 1: Typical ageing heat treatments and properties of a range of age hardening alloys

The close control of heat treatment times, temperatures and cooling rates is therefore essential if the required properties are to be obtained. For the solution treatment of aluminium alloys a salt bath is frequently used, artificial ageing taking place in a forced air circulation furnace. Illustrated in Figure 2 is the effect of varying the time and temperature on the ultimate tensile strength of an Al-4%Cu alloy such as alloy 2025 where it can be seen that a difference as small as 40OC in the ageing temperature can have a major effect on the strength. The higher temperature needed by the nickel and ferrous alloys generally requires the use of gas fired or electrical furnaces with sufficient thermocouples to ensure the correct temperatures are consistently achieved throughout the component.

Figure 2: Effect of varying ageing times and temperatures on tensile strength

This article was written by Gene Mathers.

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Radiography Part 3

The previous two articles dealt with what may be termed conventional radiographic techniques employing either gamma- or X-ray sources and photographic film. The development of electronics, in particular the increase in computing power over the last 20 or 30 years, has enabled what were laboratory based radiographic methods - real time radiography and computerised tomography (the “CT” scan that is perhaps more familiar in a medical context) -to be implemented both on the shop floor and on site.

The fundamental difference between film radiography and real time radiography is the way in which the radiographic image is handled - the image in real time radiography being produced electronically rather than on film. This means that the image can be viewed, as the name suggests, in real time – an instant result rather than the significant time delay that occurs when radiographic film is carried to a darkroom, developed and dried before viewing can take place. The real time image is viewed on a monitor screen and the results can be stored and transmitted electronically. An additional difference from film radiography is that the image is a positive rather than the negative image of film radiography, denser materials transmitting less radiation and therefore appearing darker and hence voids, slag entrapment, porosity etc show as light areas in a welded joint.

The early real time radiographic equipments used a fluorescent screen which interacted with the x-ray radiation to produce an image that was then passed through an image intensifier. This enabled a video film to be produced and the image to be displayed on a monitor screen in real time. Developments in computing power now enable the image to be digitised and enhanced and then analysed. Comparison with a set of pre-programmed parameters e.g. an acceptance standard, enable the process of inspection and acceptance/rejection to be automated.

Fluorescent screens to a great extent have been replaced by flat or curved screen photodiode arrays based on Si sensors with other alloys e.g. PbI and HgI, being researched in order to improve further the sensitivity of the examination method. Photo-conductors such as selenium or cadmium telluride are also used in what is termed “direct conversion” to give a sharper image than that from the photo-diode panels.

The X-ray source has been progressively reduced in size such that the focal spot is now as small as 2 or 3 microns and with micro-focus units to as small as 0.1mm, again resulting in improved image quality. These features also enable the image to be enlarged with magnifications of over 1000 times being available with little or no loss in image quality.

Real time radiography is employed in applications where a rapid in-production inspection is required. It has found extensive use in the electronics industry for the in-line examination of circuit boards, in the aerospace and automotive industries for the examination of castings and, often using gamma ray sources, in the process industry for the detection of corrosion. By moving the component between the X-ray source and the detector screen seam welded tube and tube butt welds can be inspected.

Robust, portable manipulating equipment is commercially available for the on-site examination of process pipework and cross country pipelines. Orbiting heads carrying both an X-ray source and, diametrically opposed, the detection screen, are capable of radiographing pipe butt welds with a substantial reduction in time compared with a conventional film radiograph, particularly where multiple exposures are required to give full coverage of the weld. The head rotates around the pipe to give a DWSI (double wall single image) with very high resolution and good contrast. Internal crawlers equipped with a rotating X-ray head and an external detector can be used to provide a SWSI (single wall single image) radiograph. The examination results are instantly available for display on a laptop screen, the images being stored on disc or memory stick. No film processing or darkroom are required resulting in a further reduction in cost. A typical commercially available unit is illustrated in Fig 1.

Fig 1. Portable Real Time Radiographic unit for pipeline inspection. Courtesy Shaw Pipeline Services.

Computed tomography (CT) is a process that uses the techniques of real time radiography to produce three dimensional, rather than two dimensional, images of components. Both the external surfaces and the internal structures of an item can be imaged. All materials – metals, plastics, composites, rocks, fossils, the human body – can be scanned, making CT scanning a very powerful investigatory and diagnostic tool. Industrial CT scanning is used in many applications for the internal inspection of components, metrology, and for reverse engineering.

The earliest CT units utilised an X-ray beam collimated to create a fan shaped beam of radiation which is scanned across the item, the beam being collected at the detector screen. The signal is then manipulated by a powerful software programme, enabling a 3D image to be created. The technique was further improved when the fan shaped beam was replaced by a cone shaped beam. Rotating the part beneath the beam enables a sequence of 2D images to be obtained, generally between 360 (one image per degree) to 3,600 images, depending on the desired resolution, with scanning of the object being completed in as little as a few seconds. The images are subsequently combined using complex software to provide a 3D image of the item, the image containing all of the information relating to both the external and internal surfaces of the component. A typical commercially available 225Kv microfocus unit is shown in Fig. 2.

Fig. 2 CT scanning equipment showing control console, viewing screen and radiation proofed cabinet containing the X-ray source, turntable and detector screen. Courtesy Nikon Metrology UK Ltd.

The 3D image can be manipulated to provide slices through the object and these slices can be rotated so that the precise size and position of internal features can be accurately determined. Assembly or machining errors can be identified. Similarly, flaws within a welded joint or casting can be categorised and the size and position of inclusions, lack of fusion, cracks etc accurately measured, thus allowing an accurate Engineering Critical Assessment (ECA) to be carried out. The method can also be used to examine non-metallics – a couple of examples being the inspection of composite wind turbine blades for delaminations and the positioning of the cores in lost wax casting. The technique can also be used for accurate measurement of the internal and external surfaces of a component, the image being capable of being overlaid on a CAD drawing of the object. This is particularly useful where parts are provided by a number of suppliers to a manufacturer for subsequent assembly. The items can be checked against a CAD drawing before despatch and, if required, the manufacturer instantly provided with the results, thus reducing the risk of being supplied with non-fitting parts. 

Unfortunately, unlike real time radiography that has moved from being a non-portable method to construction site use, CT scanning requires the use of a protective enclosure containing the X-ray source, the detector screen and a precision turntable. The method is currently therefore confined to within a factory or laboratory environment. 

The final radiographic method is neutron radiography. Neutrons, like X- and gamma-radiation, will pass through solid materials and can be used to produce an image on photographic film or to react with a detector screen. Neutrons, however, unlike X- and gamma radiation, are strongly affected and absorbed by certain elements such as hydrogen, carbon and boron but to a far lesser extent by iron, nickel and aluminium. Neutron radiography is therefore useful in detecting the presence of material containing hydrogen or hydrocarbons – this includes corrosion products, water, explosives, oil and plastics. Composite items e.g. a combination of metal and plastic, can be non-destructively examined for assembly faults or manufacturing defects; explosive ordnance can be examined for complete filling and fluid flow analysis can be carried out in real time – oil flow in vehicle engines, fluid flow in piping systems.

Neutron radiography can be used with conventional film with real time techniques and for CT scanning. The neutrons are produced by particle accelerators or, with poorer image definition, from certain isotopes, primarily californium²⁵². The method is currently not portable but development work is in progress to address this limitation.

This article was written by Gene Mathers.

RADIOGRAPHY TESTING PART 2

The previous article dealt mostly with the basic principles of radiography – this part will cover the methods of ensuring that a radiograph is of an acceptable quality and capable of showing relevant imperfections. As mentioned in the previous article the quality of a radiograph is assessed using three factors: density, contrast and definition or sharpness of the image. Density and contrast have already been covered but there also has to be some method by which the sensitivity (the ability to reveal imperfections) can be measured. To do this devices known as image quality indicators (IQI’s), formerly called, are used. These can be of several forms as illustrated in Fig.1

Fig. 1. Image Quality Indicators

The wire type is the most frequently used IQI in radiography-by-film. The design of the IQI is given in EN ISO 19232 Part 1 or ASTM E747. Both specifications list a series of IQIs containing six or seven wires of increasing diameter, from 1-8mm, from 10-50mm in length and in a range of metals – iron, nickel, aluminium, magnesium, copper and titanium. The wires are mounted side by side in a flexible plastic sheath which also carries appropriate identification, generally lead letters that will be clearly seen on the radiograph. The IQI is selected with respect to the metal type and the component thickness; the thicker the component the thicker the IQI’s wires. The IQI carries an identification and serial number so that it can be confirmed at a later stage that the correct IQI has been used.

Ideally the IQI is placed on the source side of the component and, in the case of a weld, transversely across the joint although this is not always possible when radiographing pipe and tube butt welds. The sensitivity is taken as the smallest diameter wire that can be seen divided by the component thickness, expressed as a percentage. Most application codes specify a sensitivity of between 2-4% ; this is a maximum, the smaller the figure the greater the sensitivity of the radiograph. Alternatively an actual wire diameter that must be visible is specified.

The step hole IQI is used less frequently. It is a stepped wedge with a hole drilled in each step, the hole diameter matching the thickness of the step. As with the wire IQI, the material and dimensions of the step wedge are selected to match the application. The diameter of the smallest hole visible on the radiograph determines the sensitivity, this being calculated as hole diameter divided by component thickness expressed as a percentage. The sensitivity measured by the use of a wire IQI is not the same as the sensitivity using a step wedge IQI.

As with any film, the method of processing will affect the quality of the image. Care must be taken to ensure that there is no light contamination, the processing chemicals are at the correct concentrations and temperatures and that drying the film does not leave marks and stains that would leave spurious indications and would make accurate interpretation difficult.

Interpretation of the radiographs must be undertaken by trained and experienced radiographers. In addition to being fully conversant with radiographic techniques such individuals should also have a comprehensive knowledge of welding processes, joint design and the various imperfections that may occur.  Many application specifications require such individuals to be independently certified to a suitable certification scheme such as PCN, administered by the British Institute for NDT or CSWIP, administered by TWI Certification Ltd. Viewing should be carried out in a darkened room, allowing a period of time for the viewer’s eyes to be accustomed to the conditions. The luminance of the viewing screen will need to vary with the density of the radiograph – there is generally a rheostat control (a dimmer switch) on the viewer to enable the luminance to be varied. The light itself should be white and diffuse and there should be as little light as possible leaking around the edges of the radiograph. 

Radiography of flat plates and cylinders large enough to permit entry for placement of the film is a relatively simple operation, as shown in Fig. 1 of the previous article. Lead numbers are placed at fixed intervals along the plate adjacent to the weld or around the circumference of a pipe to enable the position of any imperfections to be accurately located. The weld also carries a unique identification number reproduced on the radiograph by the use of lead figures placed adjacent to the weld.

Radiography of pipes, however, where access to the bore to place the film is not possible presents some problems and terms such as SWSI and DWSI are used as shorthand to identify the various techniques that may be used.

Single wall, single image (SWSI) is a technique whereby the radiographic source is placed inside the pipe by some suitable method, the film wrapped around the outside of the pipe and the exposure made as shown in Fig. 2. This may also be known as a panoramic exposure. The IQI is placed on the outside of the pipe immediately beneath the film. Both X- and gamma-radiography can be used, the source being placed in position by the use of a pre-placed spider or by means of a crawler unit. This method is most commonly used for the inspection of pipelines where the weld can be radiographed in one exposure, making the technique rapid and cost effective.

Fig. 2 Single wall, single image (SWSI) or panoramic radiographic technique

Where access to the bore is not possible or the pipe diameter is too small to permit the use of an internal source then the double wall, single image (DWSI) technique is used. Here the film is placed on the outside of the pipe on the farthest side from the radiographic source, as shown in Fig. 3. The source may be offset slightly to avoid an image of the upper part of the weld to be projected onto the film or directly in line. The source may be close to or a substantial distance from the pipe, the location being a compromise between a less sharp image but short exposure time for a small stand-off and sharper image but longer exposure time for a large stand-off. The need to penetrate two wall thicknesses means that the sensitivity will be poorer than with the single wall single image technique. The technique also requires multiple exposures to enable the complete circumference of the pipe to be examined – specification or contract requirements frequently specify the minimum numbers of exposures to ensure complete coverage and images of an acceptable quality. The technique is generally used on pipes over 80mm in diameter.

Fig. 3 Double wall, single image

The last technique is double wall, double image (DWDI), generally used only on pipes less than 75-80mm in diameter. By offsetting the source from the weld centre line and using a long source to film distance it is possible to project an image onto the film of both the upper and the lower parts of the weld as shown in Fig. 4. As with the DWSI technique multiple exposures are required to achieve complete coverage.

Fig. 4. Double wall double image radiograph of a pipe butt weld. Note the IQI, identification numbers and position markers.

Part 3 will look at some of the more sophisticated radiographic techniques and the advantages and disadvantages of radiography.

This article was written by Gene Mathers.

Radiography Testing PART 1

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Radiography

Fig. 1. Principles of radiography

The previous two Connect articles dealt with the defect detection techniques of MPI, LPI and eddy current testing. These methods are capable of detecting surface or very near surface imperfections and some process is therefore required to enable buried imperfections to be reliably detected – a so-called volumetric detection method. The first such method to be used in manufacturing to determine the quality of fabricated components is radiography. X-rays generated from a cathode ray tube were discovered in the late 1890’s, soon followed by the discovery of gamma radiation from radioactive isotopes.

The ability of this radiation to penetrate both living and inert objects with the amount of transmitted radiation depending on the density, thickness, and atomic number of the object was eventually understood.  The transmitted radiation was found to produce images on photographic film and these findings resulted in the development of industrial radiography as illustrated in Fig. 1.

Fig. 2 Schematic of an X-ray tube

X-radiation is produced from a vacuum tube - this contains a cathode which produces a beam of electrons when an electric current is passed, the higher the voltage the more intense is the stream of electrons and the deeper the penetration – see Table 1. Industrial X-ray tubes use voltages between 20kV and some 450kV but up to 30MV in linear accelerator and betatron equipment. The stream of electrons impacts an anode, some 1% of the energy being emitted as a beam of X-rays, the remainder being released as heat, requiring the anode to be cooled, often with water or oil. A schematic of an X-ray tube is shown as Fig. 2. The anode can be designed to provide a beam as illustrated in Fig. 2 or a 360O panoramic beam, typically used for pipe butt weld inspection with the X-ray source inside the pipe.

kV	Maximum Thickness (mm)
	Steel	Aluminium
50	1	12
100	20	60
150	30	80
200	50	100
300	80	150
400	100	200
2000(2MV)	200	500
25000(25MV)	500	1400

Table 1 Applied kV and maximum thickness

Gamma radiation is naturally occurring and is produced by the decay of a radioactive isotope which, when it decays, emits three types of radiation, alpha, beta and gamma. Alpha and beta radiation are short range and very easily absorbed – gamma radiation, however, is very energetic, typically over 100keV, and can easily pass through a metal, the thickness that can be penetrated depending on the type and size of isotope. The isotopes are stored in shielded containers from which they can be wound to expose the isotope, as shown in Fig 3.

Fig 3. Typical arrangement of gamma ray NDE equipment

Each isotope has a “half life”- a length of time by which half (50%) of the radioactive isotope has decayed into a stable element – two half lives means the source has only 25% of its original strength, three half lives 12.5%. As the source decays and becomes less energetic the length of the exposure time must be increased to achieve the same density of image on the radiographic film. There is thus a point at which use of the gamma ray source is discontinued and replaced with a fresh isotope. In addition, because the gamma radiation emitted by an isotope cannot be varied in quality there is a range of material thickness for which each source will give acceptable results. The commonest isotope in regular use is iridium192 with cobalt60  being used for very thick components.

Isotope	Half Life	Typical Thickness Range – steel  (mm)
Iridium 192	74 days	10 - 50
Cobalt 60	5.26 years	25 - 200
Ytterbium 169	32 days	< 10
Thulium 170	128 days	< 10
Selenium 75	119 days	5 - 20
Caesium 137	30 years	20 - 80

Table 2 Half life and typical thickness range of industrial isotopes.

The film is a fine grained photographic film with a coating of a light sensitive emulsion on both sides which is loaded in a darkroom into a flexible cassette. The cassette contains an intensifying screen, a card mounted thin lead foil, 0.05 mm to 0.5mm thick, held in close contact with the film. This screen absorbs stray radiation and emits electrons that enable the exposure time to be substantially reduced without affecting the radiographic quality.

The quality of a radiograph is assessed using three factors – density, contrast and definition or sharpness of the image. Most specifications require film densities in the range 1.8 to 2.5  (film density = 1, 1/10th of light is transmitted: film density = 2, 1/100th of light is transmitted). The density of the film can be measured using a densitometer; films outside of the specified range of density would be rejected. The density is affected by the exposure time and metal thickness – this can make the radiography of components with dissimilar thicknesses problematic – thin sections being over-exposed, thick sections under-exposed.

Fig 4. Formation of a penumbra

The contrast is determined by the differences in absorption between the metal and the defect and by the type of film used. The speed of the film in particular affects the  contrast of the image. Most  weld defects are less absorbing than the surrounding metal – slag, porosity, lack of fusion or penetration etc therefore appear dark against a lighter background. The only weld defects that are more absorbingr than the parent metal are tungsten inclusions that appear as bright white specks on the film.

Sharpness of the image is a function of a number of factors, a radiograph with poor sharpness being somewhat similar to an out-of-focus photograph with fine details being blurred or not visible. Film with a very fine grain size is preferred for high quality radiography, being capable of resolving fine details. There is a geometric unsharpness caused by the size of the radiation source, known as the “focal spot”. A large diameter source will cause a penumbra which means that the edges of an image become blurred as shown in Fig. 4. The further away from the item being radiographed than the less obvious is this effect – unfortunately the further away the source is from the object then the longer is the exposure time – twice the distance quadruples the time. To minimise the penumbra and increase sharpness the source should be the smallest diameter possible; the film should be as close to the back of the object as possible; the source should be as far from the object as possible, bearing in mind the lengthening of exposure time; fine grained film should be used.

Additional unsharpness is caused by the release of electrons within the film emulsion that darken the adjacent area.

To ensure that radiographs are of an acceptable quality with respect to image sharpness, contrast etc it is a requirement that an image quality indicator (IQI) is used as can be seen in Fig. 1. This topic will be covered in the next article, as will the techniques for radiography of a variety of joint types and component configurations.