Design and Metallurgy of Weld Joints (MEM-510)
Testing of Weld Joints I
Dr. Chaitanya Sharma 1-1
Destructive Testing of Weld Joints Lesson Objectives
In this chapter we shall discuss the following: 1. Need for testing weld joints. 2. Different methods of weld testing. 3. Destructing testing of welds 4. Different methods of destructive testing.
Learning Activities 1. Look up Keywords 2. View Slides; 3. Read Notes, 4. Listen to lecture
Keywords:
Need of weld testing, Destructive testing, Non destructing testing, Tensile testing, Bend test, Hardness testing, Impact testing, Fatigue testing, Fracture testing etc.
Why Testing of Weld Is Needed? Testing of weld joints is imperative due to following reasons: Testing helps to 1. Assess the suitability of the weld joint for a particular application 2. Take decision on whether to go ahead (with further processing or accept/reject the same) at any stage of welding 3. Quantify the performance parameters related with soundness and performance of weld joints. •
Thus weld testing identify hidden defects, ensure right
quality, integrity and soundness , prevent of cartographic failure and ensure safe functioning of weld joints and structures.
Methods of Weld Testing • In general weld joints are tested by one or more techniques of following methods: 1. Destructive testing 2. Nondestructive evaluation • Destructive testing methods damage the test piece to more or less extent, thus making it unusable. • Non-destructive testing methods do not damage test specimens so they can be used for intended purpose in anyways. • Each technique has capabilities ,limitations and sensitivity reliability and requirement for special equipment and operator skill.
Visual Inspection • Visual inspection reflects the quality of external features of a weld joint (such as weld bead profile) and external defects (such as craters, cracks, distortion etc.) only. • Most widely used welding inspection method. • Human inspector visually examines for: – Conformance to dimensions, – Warpage – Cracks, cavities, incomplete fusion, and other surface defects • Limitations: – Only surface defects are detectable – Welding inspector must also decide if additional tests are warranted
Non Destructive Testing • NDT means Inspect or measure without doing harm. • After NDT welds or specimens can be put into operation. Common NDT techniques include: • Visual inspection • Ultrasonic testing - high frequency sound waves through specimen to detect cracks and inclusions • Radiographic testing - x-rays or gamma radiation provide photograph of internal flaws • Dye-penetrant and fluorescent-penetrant tests - to detect small cracks and cavities at part surface • Magnetic particle testing – iron filings sprinkled on surface reveal subsurface defects by distorting magnetic field in part
Destructive Testing • Tests in which weld is destroyed either during testing or to prepare test specimen are called destructive testing. • Common Destructive Testing includes: • Mechanical tests - purpose is similar to conventional testing methods such as tensile tests, shear tests, etc • Metallurgical tests - preparation of metallurgical specimens (e.g., photomicrographs) of weldment to examine metallic structure, defects, extent and condition of heat affected zone, and similar phenomena .
Why Destructive Testing Is Needed? • Weld joints are generally subjected to destructive tests
such as hardness, toughness, bend and tensile test for: 1. Developing welding procedure specification and assessing the suitability of weld joint for a particular application.
2. Welding procedure qualification and welder performance qualification testing, 3. Sampling inspection of production welds, 4. Research inspection, and failure analysis work. 5. Determining weld integrity or performance.
• Typically they involve sectioning and/or breaking the welded component and evaluating various mechanical and/or physical characteristics.
Destructive Tests For Welds Mechanical Tests:
Metallurgical Tests:
1. 2. 3. 4. 5. 6. 7. 8.
• OM • SAX • SEM • EPMA • EDAX • AES (Spectrometry) • XRD • TEM All needs preparation of metallurgical specimens (e.g., photomicrographs) of weldment to examine metallic structure, defects, extent and condition of heat affected zone, and similar phenomena .
Tensile test Hardness test Bend test Impact test Fatigue test Fracture Toughness test Corrosion and creep tests Testing of spot welds
Tension-Shear test Cross-Tension test. Twist test Peel test
Tensile Test (ASTM E8M-09) • Tensile test is most widely used to determine behavior of weld joints. • Tensile and shear strength of weld joints are tested. • It gives tensile properties of the weld joints namely yield and ultimate strength and ductility. • Test may be conducted in ambient condition or in special environment (low temperature, high temperature, corrosion etc.) depending upon the requirement of the application. • Tensile test is usually conducted at constant strain rate (ranging from 0.0001 to 10000 mm/min). • Tensile specimen may be from longitudinal or transverse direction of weld. i.e. all weld metal or all zone specimen. • Engineering stress and strain diagram are obtained.
Tensile Test: Specimen, • Tensile test specimen may be: – Round or rectangular in cross-section – Transverse or all weld
• The region between the grips usually being of reduced cross–section. • Flat/rectangular specimens are easy to machine than dumbbell/ round shaped specimens and save lot of machining time. Further, Marking of gauge length and identification of failure location is easier. Flat
Round
Tensile Test:σ-ε Curve • During test, specimen subjects to an axial elongation
and the resultant load on the specimen is measured. • Stress-Strain/load-extension curve is plot to obtain proof stress, yield strength, ultimate tensile strength, toughness, ductility (% elongation/reduction in CS area). BM SA
450
AW ST
NA STA
AA
Engineering Stress (MPa)
400 350 300 250 200 150 100 50 0 0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 Engineering Strain (%)
What Is Needed for Tensile Test Results? • Tests results must includes information on following point about test conditions: – Type of sample (transverse weld, all weld specimen) – Strain rate (mm/min) – Temperature or any other environment in which test was conducted if any – Topography, morphology, texture of the fracture surface indicating the mode of fracture and respective stress state
Tensile Test: Fractography Tensile fracture may be: – Ductile fracture (Cup-Cone, Slant fibrous fractured edges) – Brittle fracture (Flat featureless fracture surfaces) – Mixed mode fracture
• Usually weld joints fracture from low strength zones. • Fractograps yields information fracture mode, effect of welding process, process parameter etc.
Dimples with secondary crack
Feature less flat surface
Hardness Test ASTM E384-11e1 • Hardness tests are essentially simple and rapid to carry out and are virtually non-destructive, so well-suited for quality control. a b • The hardness of materials is mostly measured as: – Resistance to indentation and – Height of rebound of a ball or hammer. • Indentation is the penetration of a pointed object (harder) into other object (softer) under external MgZn2 precipitates 20 µm • MgZn precipitates 2 20 µm load. • Resistance to penetration of indenter depends on c d hardness of material • All methods of hardness testing are based on the principle of applying the standard load through the indenter (a pointed object) and measuring the penetration in terms of diameter/diagonal/depth of 20 µm 20 µm indentation. • Higher the penetration softer is the material with low hardness.
Different Hardness Testing Techniques •
• •
Hardness testing methods can be compared on the basis of following: 1) Indenter type, 2) magnitude of load & 3) measurement of indentation. Brinell and Vickers measures the size of the impression left by an indenter whereas the Rockwell measures the depth of penetration of an indenter. Diamond pyramid test gives best results as the indenter depth is very small.
Applied load by indenter, kg Hardness = Area of indenter, mm2
Hardness Measurement • Weld joints are inherently heterogeneous. • Hardness measurement along the transverse plane quickly and
easily illustrate impact of process, process parameters, PWHT etc. • Hardness test confirms the findings of tensile testing e.g. failure location must
Retreating side
BM
have lowest hardness than other zone.
HAZ
• Hardness depends on the morphology of, population
of
strengthening
precipitates as well as extent of work hardening. • Hardness
measurement
information microstructure.
about
160
easily changes
yields in
HAZ
AW SA
NA ST
AA STA
RS
150
Microhardness (Hv)
size,
Advancing sid WNZ
TMAZ
AS
WNZ
140 130 120 110 100 90
HAZ
80 -15
-10
-5 0 5 10 Distance from weld centre (mm)
15
Bend Test (ASTM E290-09) • Bend test determines ductility and strength of welded joints and reveals linear fusion defects (porosity, inclusion). • The specimen is bend to a specified bend radius/angle. • Bending can be free or guided and performed using simple compressive/bending load and die of standard size for free and guided bending respectively. • For bend test, the load increased until cracks start to appear on face or root of the weld and angle of bend at this stage is used as a measured of ductility of weld joints. • Higher is bend angle (needed for crack initiation) greater is ductility of the weld. • Fracture surface of the joint from the face/root side due to bending reveals the presence of internal weld discontinuities if any.
Bend Test Methods
Free Bend Test
Various methods of bend tests used to evaluate the ductility and soundness of welded joints are as follows: – Face, Root and Side bend test – Three point bend test – Four point bend test – Free bend test – Guided bend test Three point test – Wrap around bend test
Wrap bend test
Guided bend test
Four point test
Different Bend Test • Free bending can be face or root bending while guided bending is performed by placing the weld joint over the die as needs for better control of bending condition. • Face bend tests are made with the weld face in tension, and root bend tests are made with the weld root in tension. • The root side bending shows the lack of penetration and fusion if any at the root. • When bend testing thick plates, side bend test specimens are usually cut from welded joint and bent with the weld cross section in tension. • Guided bend tests are usually taken transverse to weld axis & may be bent in plunger type test machines or in wrap-around bend test jigs. • The guided bend test is most commonly used in welding procedure and welder performance qualification tests.
Impact Test (ASTM E23 ) • Charpy, Izod Impact tests are high strain rate test, here a bar of material is broken by a swinging pendulum and energy lost by pendulum in breaking the sample is obtained from height of swing after the sample is broken. • The energy lost/absorbed is the measure of toughness of the material and the yield strength. • Initial potential energy = WH = WR (1 – cos α ) • Potential energy after rupture = Wh = WR (1 – cosβ ) • Energy required to rupture the specimen = WH – Wh = W (H – h) = WR (cos β – cos α ) = WR cos β, when α = 90°
Disadvantage: Reproducibility of the experimental conditions is difficult.
Charpy & Izod Impact tests Charpy test • Specimen, a bar of 10 mm x 10 mm x 55 mm length. • It has a V-notch 2 mm deep of 45° included angle and a root radius 0.25 mm. • Specimen is placed on the supports as a simply supported beam.
Izod test • Cantilever test piece, of 10 mm x 10 mm section and 75 mm length . • It has a V-notch 2 mm deep and
the angle of the notch is 45°.
• Test piece is placed vertically &
hammer strikes near its free end.
Energy Absorbed (Jules)
Impact Test: Fractography Why zones of welds absorbed energy in varying amounts?
Charpy test
6 5 4 3
2 1 0
Nugget
TMAZ
HAZ
Base Material
• Fracture occurs because of a sudden loading and fracture surface shows the river bank pattern. • These river bank patterns occur when the material breaks by a high amount of impact load and the dimples elongated in the direction of loading.
Fatigue Test (ASTM E466– E468) • • • • • • •
• •
Fatigue is a common type of catastrophic and insidious failure wherein the applied stress level fluctuates with time. Due dynamic and fluctuating stresses, fatigue failure occur at a stress level considerably lower than tensile or yield strength. Almost 90% of all metallic failures; occurs due to fatigue. Fatigue failure is brittle like in nature and failure occur suddenly and without warning, with very little gross plastic deformation. Fatigue failure occurs by initiation and propagation of cracks, and ordinarily fracture surface is normal to direction of applied load. Test data are plotted as stress Vs log of no of cycles to failure. For many metals and alloys, stress diminishes continuously with increasing number of cycles at failure. And for other metals/alloys, at some point, stress ceases to decrease with time, and becomes independent of, the number of cycles; fatigue behavior of these materials is expressed in terms of fatigue limit (107-108) . Fatigue strength and fatigue life are the parameters used to characterize the fatigue behavior of these materials.
Fatigue Testing Machines • Fatigue testing is usually performed on computerized, servo hydraulic universal testing machine (UTM). However dedicated fatigue testing machine like rotating bending are also available. • These machines apply fluctuating loads in tension, compression, torsion, bending, or combinations of such loads, and the number of cycles to specimen failure is recorded. • Smooth, unnotched testpieces are prepared, carefully avoiding sharp changes in cross-section that may give rise to stress concentrations.
1 - 25
Fatigue Test: Specimen Fatigue test specimen may: Flat or round and with and with out notch. Fatigue life is affected by surface conditions (presence of stress raiser i.e. scratch, notch etc.) so prior to testing samples must be polished to reduce stress concentration, making crack initiation difficult.
1 - 26
a
S-N Curve
b).Thus, fatigue will ultimately occur regardless of the magnitude of the stress.
For these materials, the fatigue response is specified as fatigue strength, which is defined as the stress level at which failure will occur for some specified number of cycles
a
Nominal stress range (MPa)
Fatigue life of a material is the number of cycles to cause failure and is taken from the S–N curve . For some ferrous (iron base) and titanium alloys, the S–N curve (Fig a) becomes horizontal at higher N values; called the fatigue limit (or endurance limit), below which fatigue failure will not occur. Most nonferrous alloys (e.g., Al, Cu, Mg) do not have a fatigue limit, S–N curve slope in downward at increasingly greater N values (Fig
b
190 180 170 160 150 140 130 120 110 100 90 80 70 60 50
b
0
500000
O Base metal O FSW Joint
1000000 1500000 2000000
Number of cycles to fatigue failure (N)
Reasons For Fatigue Failure Reasons for for fatigue failures are: – Surface imperfections like machining marks and surface irregularities. – Stress concentrations like notches, keyways, screw threads and matching under-cuts. – At low temperature the fatigue strength is high and decreases gradually with rise in temperature.
Fatigue Life May be Extended by: – Reducing the mean stress level; – Eliminating sharp surface discontinuities; – Improving the surface finish by polishing; – Imposing surface residual compressive stresses by shot peening; and – Case hardening by using a carburizing or nitriding process.
Fatigue Fracture • Fatigue fracture results from the presence of fatigue cracks. • These cracks normally nucleate on the surface of a component and initiated by cyclic stresses, at some point of stress concentration. • Although the initial stress concentration associated with these cracks are too low to cause brittle fracture, however they cause slow growth of the cracks into the interior. • In brittle materials the crack grows to a critical size from which it propagates right through the structure rapidly, whereas with ductile materials the crack keeps growing until the area left cannot support the load and an almost ductile fracture suddenly occurs. • Characteristic fatigue surface features are beachmarks & striations. • Beachmarks form on components that experience applied stress interruptions; they normally may be observed with the naked eye. • Fatigue striations are of microscopic dimensions, and each is thought to represent crack tip advance distance over a single load cycle.
Fatigue Test: Fractography
1 - 30
Fatigue Failure Comparison with Tensile Failure •
1. 2.
3. 4. • • • •
One can recognize fatigue failures by the appearance of fracture which has many specific features such as:
It occurs at lower stresses than the failure at static loads.
Failure starts on the surface (or near it) locally, in places of stress (strain) concentration.
Failure occurs in a number of stages: A failure usually has the initial zone of destruction (the zone of nucleation
of micro cracks), the fatigue zone, and the final failure zone. The initial zone of failure is usually near the surface and has small size and smooth surface. The fatigue zone is the zone where a fatigue crack gradually develops & propagate . It has typical concentric ripple lines which are an evidence of jumpwise propagations of fatigue cracks. The fatigue zone develops until the increasing stresses in the gradually diminishing actual section attain a level at which instantaneous destruction takes place and forms the zone of final failure.
Fracture Toughness Test • The significant discrepancy between actual and theoretical fracture strengths of brittle materials is explained by the existence of small flaws that are capable of amplifying an applied tensile stress in their vicinity, leading ultimately to crack formation. • Fracture ensues when the theoretical cohesive strength is exceeded at the tip of one of these flaws. • The fracture toughness of a material is indicative of its resistance to brittle fracture • when a crack is present. • It depends on specimen thickness, and, for relatively thick specimens (i.e., conditions of plane strain), is termed the plane strain fracture toughness. • This parameter is the one normally cited for design purposes; its value is relatively large for ductile materials (and small for brittle ones), and is a function of microstructure, strain rate, and temperature.With regard to designing against the • possibility of fracture, consideration must be given to material (its fracture toughness), the stress level, and the flaw size detection limit.
Fracture Toughness: Specimen
Creep test • Creep is the time-dependent and permanent deformation of materials when subjected to a constant load or stress. • It is observed in all materials but becomes important only for temperatures greater than about ( absolute melting temperature). • This requires the measurement of four variables: stress, strain, temperature and time.
Creep test • • • • • • • • •
• •
Creep test determine the continuing change in the deformation of materials at elevated temperatures when stresses below the yield point. At one end of the specimen a platinum wire is spot welded and to the other end a platinum tube is spot welded. The platinum wire-slides inside the tube and the reference marks are put on the both, wire and tube, which we may observe through the telescope at the middle. There is a scale inside the telescope and elongation can be measured. The specimen is loaded in the furnace along the pullies. With the help of wire, specimen is made to fix on the ground of thefurnace Thermocouples are fixed along both the ends of specimen to measure temp.. As soon as the load is applied, the specimen come under the tension. First, a load is applied and initial or instantaneous elongation is measured with the help of telescope. For a constant load, all that is needed is a dead weight and a system of levers to multiply it to the required load. The temperature of the furnace is allowed to increase upto a specified temperature. With the help of the
Fillet Break Test • Fillet weld break test involves breaking a single fillet weld, usually in press. • The sample has load applied to its unwelded side, transverse to weld & directed to its unwelded side . • The load is increased until the weld has failed. • The failed sample is then inspected to establish the presence and extent of any welding discontinuities. • This test will provide a good indication as to the extent of discontinuities within the entire length of weld tested. • This type of weld inspection can detect such items as lack of fusion, internal porosity and slag inclusions.
Tensile Testing of Spot Welds
Fig : (a) Tension-shear test for spot welds. (b) Cross-Tension test. (c) Twist test. (d) Peel test
Macro Etch Testing • Macro etching involves the removal, polishing and etching of small samples of the welded joint. • Samples are polished across their cross-section and then etched using some type of mild acid mixture. • The acid etch provides a clear visual appearance of the internal structure of the weld e.g. fusion line. • Depth of penetration, lack of fusion, inadequate root penetration, internal porosity, cracking and inclusions can be detected during inspection of the etched sample. • This is obviously a snapshot of overall weld length quality when used for sampling inspection of production welds. • This is extremely used to pinpoint welding problems such as crack initiation, when used for failure analyses.