Paper presented at Institute of Mechanical Engineers Seminar on Plastics & Polymers, September 2002
This paper describes various mechanical tests that are currently used in industry to determine the quality of welds in plastics. These tests include short-term tests, such as bend tests, tensile tests, lap shear tests and impact tests, and long-term tests such as the tensile creep rupture test. The advantages and disadvantages of each test will be summarised. In addition, a number of novel tests developed for specific applications will be described.
Welded joints in thermoplastics materials can sometimes act as a failure initiation point. This may be a result of factors such as:
- Poor welding technique.
- Presence of defects in the weld.
- Poor joint design.
- Introduced stress concentrations.
In order to qualify thermoplastics welds there are a number of mechanical tests that can be used. Since the mechanical behaviour of plastics can be strongly influenced by factors such as strain rate, temperature and the surrounding environment, it is important that the mechanical test chosen for any particular welded component should be as representative as possible of the conditions that would be expected in service.
Short-term tests have the advantage that they are relatively inexpensive to perform and provide results within a short time. They are normally mainly used for QA purposes, to ensure that the welds conform to previously established acceptance criteria, or for optimising welding conditions. However, for design purposes, it is normally necessary to define the long-term properties of the weld. This necessitates the use of long-term tests, which are normally far more expensive and, by their nature, take a long-time to provide results.
In order to reduce testing costs it is desirable to assess the quality of a weld using small-scale coupons rather than the component itself. Coupon tests can be divided into short-term tests and long-term tests.
There are a number of short-term coupon tests for assessing the quality of welded plastic joints. However, there is very little data to correlate the results of these tests with the in-service performance of a component. The most commonly used short-term small-scale tests are outlined below.
The tensile test is probably the most common method used to determine weld quality. There are several variations on the tensile test that may be applied, depending on factors such as the quality of the weld and the form of the material to be tested.
Tests on flat samples
This test is described in the European Standard EN 12814-2: 2000.  It employs either a parallel strip or a dumb-bell type specimen. Both welded and unwelded reference specimens, ideally from the some component, are tested. A short-term tensile welding factor, f s, defined as:
is then determined.
Typical acceptance values for f s range from 0.7 to 0.9, depending on the material and welding process. These are given in the European Standard EN 12814-8. 
For some materials and welding processes, e.g. hot plate welds in polyethylene (PE), a short-term tensile welding factor of 1 can be obtained, i.e. fracture occurs outside the weld. If this happens, in order to, for example, optimise welding conditions, fracture can be induced at the weld by drilling a hole in the specimen at the centre of the joint. This procedure is described in the German Welding Society (Deutscher Verband Für Schweisstechnik) guideline DVS 2203-2. 
Another tensile test that induces fracture at the weld is the low temperature tensile test. This test was developed in Switzerland, specifically for butt welds in polyvinylidene fluoride (PVDF) and polypropylene (PP), and is described in the European Standard EN 12814-6:2000.  It uses a waisted test specimen geometry and is conducted at a temperature of -40°C in order to induce brittle fracture at the weld.
Tests on curved samples
For curved samples, such as pipes, it is often difficult to grip the specimens described above. For this reason specimens incorporating holes for locating loading pins can be used. Such specimens, which are normally also waisted to induce fracture at the weld, are described in various standards and specifications. [5-7] In WIS 4-32-08  only the failure mode is noted, and described as one of the following:
- Ductile - large-scale drawing out of material from both fracture surfaces.
- Brittle - flat fracture surfaces.
- Mixed - intermediate characteristics.
In ISO 13953(5) both the failure mode and tensile strength are determined. EN 12814-7  specifies a tensile energy welding factor, f e, defined as the energy to break of the welded test specimen divided by the energy to break of an unwelded specimen.
Tests for ultrasonic welds
The German Electrical Manufacturers Association (ZVEI) has developed a tensile test based upon practical rather than scientific requirements for the assessment of ultrasonic welds. It uses a cylindrical test specimen, which can be ultrasonically welded either as a shear joint or projection joint. It can also be spin welded and hot plate welded, which makes it useful for comparing different welding techniques.
The American Welding Society (AWS) has also developed a tensile test for assessing ultrasonic welds, which employs an I-beam specimen geometry. 
A bend test for the assessment of butt welds in plastics is described in the European Standard EN 12814-1:2000.  Rectangular section specimens, with the weld in the middle, are loaded in three-point bend and the angle (or ram displacement) at which either fracture occurs or a crack initiates is measured. The acceptance values for the bend test are given in EN 12814-8 as graphs of bend angle and ram displacement against wall thickness for various thermoplastics and welding techniques.
The quality of a weld may be assessed by the force required to peel apart the materials being joined. This is of particular interest for materials that are joined using some form of overlap joint, such as films, sheet and pipe socket joints. Various peel tests are defined in the European Standard EN 12814-4. 
A T-peel test is used for welded sheets. The two unwelded ends of the specimen are pulled at constant speed in opposite directions until complete failure. The peel resistance of the welded joint is expressed as the maximum force divided by the specimen width. A similar test to that described in EN 12814-4 is given in ASTM D6392. 
This test, which is also specified in WIS 4-32-08, is used to assess the quality of electrofusion joints in plastics pipes. A rectangular specimen is cut from the pipe/coupler joint and pulled at constant speed perpendicular to the joint interface. Based on the sample geometry and maximum load during the test, a 'fracture toughness' of the joint can be calculated. Analysis of the fracture surfaces also provides qualitative information about the joint integrity. Ductile yielding, with signs of stress whitening and drawn material between the wires indicates a good quality weld.
A variation of this test is defined in WIS 4-32-08 and also in the International Standard ISO 13954,  where the joint interface is parallel to the load.
This is a test for assessing the quality of socket joints in small diameter ( ≤ 90mm) plastic pipes. The welded pipe/fitting assembly is cut in half lengthways. The pipe portion is then squeezed in a vice until the inner surfaces meet and held in this position for 10 minutes. If the weld is of good quality, there should be no evidence of cracking at the weld interface.
In a number of situations, resistance to impact loading is one of the primary properties required by design engineers.
The simplest impact test is to hit a section of welded plastic with a hammer and examine the appearance of the fracture surface. This can indicate whether or not the joint is brittle. A more rigorous approach is to use a standardised pendulum impact testing machine, which measures the energy absorbed by the specimen during the test. As in other tests, a welding factor can be determined, in this case from the ratio of the impact energy for the welded specimen to that of the unwelded reference specimen.
The two main types of impact test used for assessing plastics welds are the Charpy test and the tensile impact test.
This test is based on the equivalent test for parent materials.  A notched rectangular bar is supported at both ends in such a position that the pendulum, when released, strikes the specimen at the weld and directly behind the notch. When applying this test to plastics welds the position and sharpness of the notch become far more critical than when testing parent material since, in order to determine the properties of the weld, the notch tip must be positioned in the weld region. When the weld bead is small and symmetrical, it is possible to use the centre of the bead as an accurate locator of the weld zone. However, when the weld bead is larger or unsymmetrical this approach may not be valid.
Tensile impact test
This test is intended primarily for samples that are too thin to test using the Charpy test (<4mm). This test is described in the German guideline DVS 2203-3  and uses a dumb-bell specimen, one end of which is held in a rigidly mounted base and the other in a movable head. On release, the pendulum is able to pass the rigid base and impacts the movable head. The specimen consequently fractures in tension.
The only standardised test method for determining the long-term performance of welded coupons is the tensile creep test. 
This is basically a stress rupture test in which dumb-bell specimens are subjected to a constant stress at elevated temperature and the time to failure is recorded. Tests are performed over a range of stresses in order to generate creep rupture curves for both welded and unwelded specimens. The long-term welding factor, f l
, is determined from the ratio of the two stress values at which equal lifetime of the welded joint and parent was obtained. In order to shorten the duration of the test a surface active medium and/or higher test temperatures can be used.
Typical acceptance values for f l range from 0.4 to 0.8, depending on the material and welding process. These are given in the European Standard EN 12814-8. 
Whole component tests
A welded plastic component may well behave differently to a coupon cut from the component. This can be due to the existence of effects such as residual stresses and the constraint imposed by neighbouring material. Hence, full-scale tests are preferred to assess the performance of a welded structure, even though they are significantly more expensive than coupon tests.
Many of the test methods and research into the performance of welded plastics structures have been developed by the gas and water industries, based on pipe assemblies. Consequently, many of the large-scale tests reported below are related to those industries.
A general lack of correlation between test data from coupon impact tests and failure of whole components through impact loading has led to the development of many qualitative assessment methods for specific welded products. These involve dropping a missile of known mass from a predetermined height on to the component and examining the impact failure damage. Alternatively, the welded component may be dropped from a known height on to a hard surface. Such tests are used for products such as mobile phones, storage drums, window frames and pipe tee joints.
Hydrostatic pressure tests
Short-term hydrostatic burst tests can provide a quick indication of weld quality. These tests typically involve pressurising the welded component to a pre-determined level at a fixed rate. The sample is then held at that pressure for a specified amount of time, followed by a further pressure increase up to the point of burst.
Long-term hydrostatic pressure tests are used to determine the design stresses of plastics pipes. Data are obtained at elevated temperature and for a range of internal pressures, giving test durations over two or more logarithmic decades. These are then analysed using regression methods to determine a 50-year design stress.
It should be noted that this test is mainly intended for parent pipe materials, but can be applied to welded pipes. However, since the hoop (circumferential) stress in this test is twice the axial stress, failure will normally occur in the parent pipe, away from the weld. This may not be the case for in-service welded pipes, which may fail at the weld due to a combination of internal pressure and external loading. Consequently, this test method is not recommended to determine the long-term performance of welded joints in plastics pipes.
Tensile creep rupture test
In order to generate long-term failures at the weld in plastics pipes TWI has developed a tensile creep rupture test rig for welded whole pipe lengths. This test subjects whole pipe sections to a constant axial load at elevated temperature. Results have shown that failure is indeed induced at the welded joint. This unique equipment therefore enables experimental regression curves of axial stress versus time to joint failure to be generated.
The number of test methods available for welded plastics can make the selection of the appropriate test confusing. The test method (or methods) employed should be selected by considering the following:
- The geometry of the weld in the test specimen should be as close as possible to the geometry in the welded component. If possible, the specimens should be extracted from the component. Ideally, a whole component (or sub-component)test should be considered.
- The mode of loading of the test specimen/sample should reflect the loading of the joint in service.
- The test rate should reflect the intended application of the component. It is often convenient to qualify a welded joint or weld procedure using a short-term test. However, for a component held under constant stress in service, along-term test is more appropriate. Alternatively, a weld in a structure prone to impact damage should be assessed using an impact test.
Finally, it should be noted that the results of the different tests should not be compared to each other, due to differences in specimen geometry, strain rates, temperature, etc.
- EN 12814-2:2000 'Testing of welded joints of thermoplastics semi-finished products - Part 2: Tensile test'.
- EN 12814-8:2001 'Testing of welded joints of thermoplastics semi-finished products - Part 8: Requirements'.
- DVS 2203-2 'Testing of welded joints of thermoplastics materials: Tensile test', 1985.
- EN 12814-6:2000 'Testing of welded joints of thermoplastics semi-finished products - Part 6: Low temperature tensile test'.
- ISO 13953:2001 'Polyethylene (PE) pipes and fittings. Determination of the tensile strength and failure mode of test pieces from a butt-fused joint'.
- Water Industry Specification WIS 4-32-08 'Specification for the fusion jointing of polyethylene pressure pipeline systems using PE80 and PE100 materials', 2002.
- EN 12814-7:2002 'Testing of welded joints of thermoplastics semi-finished products - Part 7: Tensile test with waisted test specimens'.
- AWS G1.2M/G1.2:1999 'Specification for standardized ultrasonic welding test specimen for thermoplastics'.
- EN 12814-1:2000 'Testing of welded joints of thermoplastics semi-finished products - Part 1: Bend test'.
- EN 12814-4:2001 'Testing of welded joints of thermoplastics semi-finished products - Part 4: Peel test'.
- ASTM D6392-99 'Standard test method for determining the integrity of nonreinforced geomembrane seams produced using thermo-fusion methods'.
- ISO 13954:1997 'Plastics pipes and fittings - Peel decohesion test for polyethylene (PE) electrofusion assemblies of nominal outside diameter greater than or equal to 90mm'.
- EN ISO 179-1:2001 'Plastics. Determination of Charpy impact properties. Non-instrumented impact test'.
- DVS 2203-3 'Testing of welded joints of thermoplastics; tensile impact test', 1985.
- EN 12814-3:2000 'Testing of welded joints of thermoplastics semi-finished products - Part 3: Tensile creep test'.