Nondestructive-testing.docx

Group 5: Gapuz, Mark Millan Gorospe, Mary Rose N. Munar, Rhoy Orodio, Tristan Paul G. Ordinario, Lanz Pitaca, Zyrell

NONDESTRUCTIVE TESTING (NDT)

Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. Non destructive testing (NDT) covers a wide group of techniques used to evaluate the properties of a material, part, product, weld, or system without materially affecting the integrity of the unit being inspected or investigated under the test procedure.

Objectives of NDT        

To ensure product integrity, and in turn reliability To detect internal or surface flaws To determine the materials’ structure To measure the dimensions of materials To ensure customer satisfaction and maintain the manufacturer's reputation To avoid failures, prevent accidents and save human life To maintain uniform quality level To control manufacturing processes

 

To aid in better product design To ensure operational readiness.

Major Types of NDT

Test method names often refer to the type of penetrating medium or the equipment used to perform that test.

Detection of surface flaws   

Visual Inspection Magnetic Particle Testing (MT) Liquid Penetrant Testing (PT)

Detection Of Internal flaws  

Radiographic Testing (RT) Ultrasonic Testing (UT)

Why NDT?    

Test piece too precious to be destroyed Test piece to be reuse after inspection For quality control purpose Test piece is in service

Visual Inspection Most basic and common inspection method. Tools include fiberscopes, borescopes, magnifying glasses and mirrors.

Fiberscope

Boroscope

Magnifying Glass

Mirrors

Robotic crawlers

Magnetic Particle Inspection (MPI)

Nondestructive testing method used for defect detection. Fast and relatively easy to apply and part surface preparation is not as critical as for some other NDT methods. – MPI one of the most widely utilized nondestructive testing methods. MPI uses magnetic fields and small magnetic particles, such as iron fillings to detect flaws in components. The only requirement from an inspectability standpoint is that the

component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. 

Electromagnetic Yoke - A magnet that induces a magnetic field in the area of a part that lies between the poles. The same shape as the permanent magnet yokes, can also be used. These are U-shaped cores of soft iron, usually laminated, with a coil wound around the base of the U. When we pass an electric current through the coil the result is a north and south pole at the ends of the core. This makes it look like a horseshoe magnet. Direct current yokes have some advantages over the permanent magnet yokes. It is possible to vary field strength using varying current and the EM yoke is also easy to place onto and remove from the part as no field exists until current is applied.

Advantages and Disadvantages of MPI Advantages: 

AC or DC fields



Useful in confined spaces



Low voltages



No poles to attract particles



Control of amperage

Disadvantages: 

Danger of arcing



Danger of overheating



Heavy transformer required



Possible to switch on without creating field



Possible contamination of the testpiece by the electrode



Must have good electrical contact



Usually two-man operation

Liquid Penetrant Testing

This method employs a penetrating liquid, which is applied over the surface of the component and enters the discontinuity or crack. This method is utilized for the detection of surface flaws in welds that may or may not be visible. An NDT method that is used to reveal surface breaking flaws through bleed out of a colored or fluorescent dye from the flaw. Can be used on metals and nonmetals, including glass, rubber, plastics, ceramics, etc.

Liquid Penetrant Testing provides a means of:



Obtaining a visual image of a discontinuity on the surface of the specimen under



examination Disclosing the nature of the discontinuity without impairing the material (presence of



cracks, porosity, etc.) Separating acceptable and unacceptable parts in accordance with predetermined standards (the standards are included as “acceptance criteria” within a test procedure)

Steps for Performing a PT

1) Surface Preparation  

Surface must be free of paint, oil, grease, water, or other contaminants May require etching

2) Penetrant Application

 

Can be done by spraying, brushing, or immersion in a penetrant bath (dip) Penetrant must be allowed to “dwell” for a minimum time period  Dwell time gives penetrant time to be drawn into a discontinuity  Time specified by penetrant manufacturer or procedure

3) Excess Penetrant Removal 

Penetrant to be removed from the surface of the part without removing penetrant from discontinuities

4) Developer Application  Developer acts as a blotter to draw the penetrant back to the surface of the part so it can be seen

 

Either a dry powder, dip, or spray Also given time to process (usually a minimum of 10 minutes)

5) Inspection 

Part is visually inspected under appropriate lighting to detect indications of flaws

6) Final Surface Cleaning  Required to remove developer and penetrant from the part Advantages of PT as an NDT Method:       

High sensitivity to small surface discontinuities Few material limitations Large areas and large volumes of parts can be inspected rapidly and at low cost Can inspect parts with complex geometric shapes Indications produced directly on surface of part for visual representation of flaw Portable method Relatively inexpensive

Disadvantages of PT as an NDT Method     

Can only detect discontinuities that are open to the surface Can only inspect parts with nonporous surfaces Must pre-clean/post-clean parts Inspector must have direct access to the part surface Time-consuming

Safety Precautions         

Flammability Use exhaust fans to disperse vapors Ignition sources must be avoided Skin Irritation Wear gloves to protect hands Wear safety glasses to protect eyes from splashing UV Light Lamps get hot – be cautious! Report missing or cracked filter on lamps  UV rays can cause sunburn and eye damage if filters not used or not functional

Ultrasonic Testing

Ultrasonic methods of weld testing use beams of sound waves (vibrations) of short wavelength and high frequency, transmitted from a probe and detected by the same or other probes. This method is utilized for detection of internal flaws in welds such as slag inclusions, internal porosity, or internal cracks. The following list includes the steps, and the accompanying hardware and software pieces, required to get one pulse and the subsequent echoes:



Application software - The user interacts with the application software to set up



the test and presentation parameters. Motion control - The ultrasonic transducer is moved over the appropriate area over the UUT.



Communication - The pulser/receiver operation parameters, such as pulse energy, pulse damping, and bandpass filtering, are set. The communication path



is typically RS232 or USB. Pulser/receiver - This device generates the high-voltage pulse that is required by



the ultrasonic transducer. Ultrasonic transducer - The transducer is pulsed, sending out an ultrasonic wave. The subsequent echoes generate a voltage in the transducer, which is sent back



to the pulser/receiver. Pulser/receiver - The analog signal from the ultrasonic transducer is amplified



and filtered before it is sent back to the digitizer within the PC. Digitizer - The waveform sent from the pulser/receiver is converted from voltage



to bits using an analog-to-digital converter (ADC). Application software - Data from the digitizer is processed, analyzed, and presented according to the user-defined parameters.

Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements. • Ultrasonic examinations can be conducted on a wide variety of material forms including castings, forgings, welds, and composites. • A considerable amount of information about the part being examined can be collected, such as the presence of discontinuities, part or coating thickness. Basic Principles of Sound Sound is produced by a vibrating body and travels in the form of a wave. • Sound waves travel through materials by vibrating the particles that make up the material. • The pitch of the sound is determined by the frequency of the wave (vibrations or cycles completed in a certain period of time). • Ultrasound is sound with a pitch too high to be detected by the human ear. The measurement of sound waves from crest to crest determines its wavelength (λ). • The sound wavelength is inversely proportional to its frequency. (λ = 1/f) • Several wave modes of vibration are used in ultrasonic inspection. The most common are longitudinal, shear, and Rayleigh (surface) waves.

Ultrasound Generation The transducer is capable of both transmitting and receiving sound energy.

Ultrasound is generated with a transducer. A piezoelectric element in the transducer converts electrical energy into mechanical vibrations (sound), and vice versa.

Test Techniques • Ultrasonic inspection techniques are commonly divided into three primary classifications. 

Pulse-echo and Through Transmission (Relates to whether reflected or transmitted



energy is used) Normal Beam and Angle Beam (Relates to the angle that the sound energy enters the test article)



Contact and Immersion (Relates to the method of coupling the transducer to the test article)

Inspection Applications Some of the applications for which ultrasonic testing may be employed include: • Flaw detection (cracks, inclusions, porosity, etc.) • Erosion & corrosion thickness gauging • Assessment of bond integrity in adhesively joined and brazed components • Estimation of void content in composites and plastics • Measurement of case hardening depth in steels Data Presentation • Information from ultrasonic testing can be presented in a number of differing formats. • Three of the more common formats include: – A-scan – B-scan – C-scan A-scan presentation displays the amount of received ultrasonic energy as a function of time.

• Relative discontinuity size can be estimated by comparing the signal amplitude to that from a known reflector. B-scan presentations display a profile view (cross-sectional) of a test specimen.

• Only the reflector depth in the cross-section and the linear dimensions can be determined. C-scan presentation displays a plan type view of the test specimen and discontinuities.

• C-scan presentations are produced with an automated data acquisition system, such as in immersion scanning. Advantage of Ultrasonic Testing • Sensitive to both surface and subsurface discontinuities. • Depth of penetration for flaw detection or measurement is superior to other methods. • Only single-sided access is needed when pulse-echo technique is used. • High accuracy in determining reflector position and estimating size and shape. • Minimal part preparation required. • Electronic equipment provides instantaneous results. • Detailed images can be produced with automated systems. • Has other uses such as thickness measurements, in addition to flaw detection. Limitations of Ultrasonic Testing

• Surface must be accessible to transmit ultrasound. • Skill and training is more extensive than with some other methods. • Normally requires a coupling medium to promote transfer of sound energy into test specimen. • Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect. • Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise. • Linear defects oriented parallel to the sound beam may go undetected. • Reference standards are required for both equipment calibration, and characterization of flaws.

Radiographic Testing

Radiography is used in a very wide range of applications including medicine, engineering, forensics, security, etc. Radiographic testing (RT) offers a number of advantages over other NDT methods, however, one of its major disadvantages is the health risk associated with the radiation. In general, RT is method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials.

After more than a century, radiographic inspection is still the nondestructive method of choice for most projects, whether in the laboratory or in the field. Examples of application of radiographic inspection are:         

Pipeline welded joint inspection Pressure vessel fabrication quality control Welder qualification testing Structural steel fabrication Detect Discontinuities in weld Determine the extent of corrosion (internal and external) Locate reinforcement bar and conduit Locate post-tension cables Detect Discontinuities in weld

Advantages: Radiographic Testing       

Both surface and internal discontinuities can be detected. Significant variations in composition can be detected. It has a very few material limitations. Can be used for inspecting hidden areas (direct access to surface is not required) Very minimal or no part preparation is required. Permanent test record is obtained. Good portability especially for gamma-ray sources.

Limitations: Radiographic Testing       

Hazardous to operators and other nearby personnel. High degree of skill and experience is required for exposure and interpretation. The equipment is relatively expensive (especially for x-ray sources). The process is generally slow. Highly directional (sensitive to flaw orientation). Depth of discontinuity is not indicated. It requires a two-sided access to the component.

Types of Radiographic Testing 

Film Or Paper Radiography

      

Computed Radiography Real Time Radiography Neutron Radiography Stereo Radiography X-ray Radiation Gamma Rays Radiation Neutron Radiation

Flaws Detection : Radiographic Testing     

Sand inclusions Blow holes Shrinkage Cracks Inclusions

Merits: Radiographic Testing    

No need of washing and developing films. Low cost. Image viewed immediately on screen. Time consumption is less.

Demerits: Radiographic Testing   

Poor resolution Low image contrast Electronic image intensifier required for increasing the contrast