One of the main requirements to prevent failures is to assess whether a particular polymer /environment combination is susceptible to ESC. Preferably ESC susceptibility tests should be conducted prior to service examining all the chemicals the polymer is likely to come into contact with. However, due to the vast range of different chemicals, this is extremely difficult and as a consequence, it is found that most ESC failures are due to unintended exposure to secondary fluids such as cleaning agents or lubricants. ESC susceptibility tests are therefore vital to the monitoring of ESC and subsequent failure analysis. The evaluation of ESC in thermoplastics is covered by a number of national and international standards. The resistance of polymeric materials to ESC is known as ESCR. ESCR can be quantified by critical strain, critical stress, or stress time to failure.
Keywords: Polymer cracking, ESC, ESCREnvironmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present. According to ASTM D883, stress cracking is defined as “an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength”. This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces. Environmental stress cracking may account for around 15-30 % of all plastic component failures in service. This behavior is especially prevalent in glassy, amorphous thermoplastics. Amorphous polymers exhibit ESC because of their loose structure which makes it easier for the fluid to permeate into the polymer. Amorphous polymers are more prone to ESC at temperatures higher than their glass transition temperature (Tg) due to the increased free volume. When Tg is approached, more fluid can permeate into the polymer chains. Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazing process, initiating crazes at stresses that are much lower than the stress-causing crazing in the air (Figure 1).
The action of either a tensile stress or a corrosive liquid alone would not be enough to cause failure, but in ESC the initiation and growth of a crack is caused by the combined action of the stress and a corrosive environmental liquid. These test methods can be divided roughly into two groups those that are based on an applied deformation and those based on applied load. The main international for testing ESC resistance are:
• Bent strip
• Bent strip test for flexible materials
• Ball and pin impression
• Constant tensile deformation
• Slow strain rate testing
• Constant tensile stress
• C-ring tests
The following section gives a brief overview of the ESC test methods that have been standardized. It should be noted, that there are no standards for testing ESC resistance under either cyclic or biaxial stresses. However, recent work indicates that ESC resistance can be significantly poorer when biaxial stresses are used rather than uniaxial stress.
The bent strip test (ISO 4599) involves clamping the test specimen to a semicircular former to apply a known strain to the specimen. The radius of curvature of the former can be varied to induce different levels of stress in the specimen. This strain may be calculated using the following equation:
where d is the thickness of the specimen and r is the radius of the former. Once the specimen has been strained it is brought quickly in contact with the chemical environment. After an agreed time, the specimen is removed from the apparatus and either visually inspected for crazing or mechanically tested to assess its residual strength. This test is most commonly used for assessing the ESC susceptibility of amorphous polymers. It is not suitable for semi-crystalline polymers, which are susceptible to rapid stress relaxation, as the stress applied to the specimen will decrease during the test.
This test was developed by Bell laboratories in the USA and has since been standardized as ASTM D1693. The technique is suitable for flexible polymers such as polyethylene but should only really be used for quality control purposes. An illustration of the type of apparatus used in this test method is shown in Figure 2. The specimens used in this test are notched rectangular strips (38 × 13 × 3 mm) that are clamped in a jig so that the sample folds over on itself at an angle of 180° to produce stress within the specimen. Once loaded into the jig the specimens are immediately exposed to the chemical environment at the required test conditions. The specimens are then inspected either visually or using an automatic inspection technique at agreed time intervals and the time required for 50% of the specimens to fail is noted.
The ball and pin impression test is used primarily for complex finished components. The method involves drilling a series of holes of a specific diameter into the polymer. A series of oversized balls or pins are inserted into the holes to induce a range of different stresses. One hour after the pins have been inserted the specimens are immersed in the environment for 20 hours. The specimens are then dried and visually examined for crazes. The smallest ball to cause visible crazing is used to determine the ESC resistance of the polymer.
The constant tensile deformation test is a relatively new test that is currently being developed as an ISO standard as ISO DIS 22088 part 5. The test method involves applying a constant deformation to the specimen and monitoring the stress relaxation that occurs while it is immersed in the chemical environment. The test is repeated using progressively smaller levels of deformation until the stress relaxation curves of consecutive tests superimpose on one another (Figure 3). The applied stress required to produce this level of deformation is defined as the critical stress. The ESC resistance of the material is determined by comparing the critical stress obtained in the environment to that obtained in air.
The slow strain rate method has been used only comparatively recently for characterizing the performance of plastics although it is a well-established for metals and it is now currently being developed into a standard as ISO DIS 22088 part 6. The test method involves subjecting a specimen to a gradually increasing strain at a constant displacement rate whilst it is exposed to the chemical environment. The tests are conducted under uniaxial tension at low strain rates to enhance the influence of the Figure 3 Stress relaxation curves obtained using progressively smaller levels of deformation (1>5) until consecutive curves superimpose on one another (3 and 4). Note: S0 is the initial stress and S is the stress at time t. Load and displacement are monitored continuously to enable stress-strain curves to be produced. The development of crazes within the specimen causes the strain to be taken up locally at the crazes such that the stress required to deform the specimen is reduced compared to that in an inert environment. The onset of craze initiation can therefore be detected by the departure of the stress-strain curve in the chemical environment from that in the air Figure 4. The main advantages of the slow stain rate test are that it is relatively rapid, requires few specimens, and can be automated.
The distinctive feature of this test is that a constant load is applied to the specimens, thereby avoiding the problem of stress relaxation that is found in the constant strain test methods. An illustration of the type of apparatus used in this test method is shown in Figure 5. The technique involves subjecting the specimen under investigation to a constant tensile stress at a stress below the tensile yield stress of the polymer. This is usually achieved using a dead weight that is suspended from one end of the specimen. The specimen is then immersed in the stress-cracking agent and inspected at regular intervals to establish the onset of crazing. The time required for crazes/cracks to appear after the specimen has been exposed, or the threshold stress below which no crazes appear in a specific time period (typically 100 hours) can be used as a measure of the ESC resistance.
C-ring specimens are often used for the testing of tubing and pipes and have been standardized for the testing of polyethylene pipes in ASTM F-1248. Typical apparatus for testing C-ring specimens is shown in Figure 6. Circumferential stress is of principal interest and this stress varies around the circumference of the C-ring from zero at each bolthole to a maximum at the outer surface of the middle of the arc opposite the stressing bolt. C-rings can also be stressed in the reverse direction by spreading the ring and creating tensile stress on the inside surface. An almost constant load can be developed on the C-ring specimen by placing a calibrated spring on the loading bolt. This enables the C-ring to be self-loading particularly important for monitoring degradation in inaccessible places as they can be used as in situ test coupons.
Self-loading tensile tests are not common in the testing of plastics and have not been standardized. However, they are extremely useful for the monitoring of plastics as they can be placed in the same environment as the material under investigation, for example in a polymer pipeline and then removed at regular intervals for testing. There are various means by which specimens can be self-loaded, the most common of which is to produce a constant stress in the specimen by using a compressed spring to apply a known load. An example of this type of jig is shown in Figure 7. The specimen is held within the tube and the stress is applied by turning the bolt at the end of the tube to compress the spring. The advantage of this arrangement over the C-ring tests is that it produces a simple, uniform stress pattern within the specimen.