Читать книгу Handbook of Plastics Testing and Failure Analysis - Vishu Shah - Страница 24

Automation, speed, and instrumentation are the key new developments in testing technology since the new millennium. An increasing number of test equipment manufacturers are engaging in developing instruments that require less human intervention and operate at faster speeds. Automation is accomplished by the use of robotics. This type of automated system has been proven to be cost‐effective for performing tensile, flexure, and impact tests on large quantities of samples, while freeing up personnel for more demanding tasks. In combination with a six‐axis industrial robot and a bar code scanner, a system can be configured to feed multiple testing machines with a single robot. A system developed by a leading manufacturer (6) uses state‐of‐the‐art web technologies (webcam, e‐mail, SMS), ensuring a constant process control and remote diagnostic of the system. The data generated by the instrument are analyzed and seamlessly integrated with other office automation programs. Figure 1‐1 shows one such highly automated test setup. Automation has also led to highly accurate and reproducible specimen preparations. A microprocessor‐controlled notching instrument for impact tests can notch up to 50 specimens all in one operation. The notcher uses a linear cutting motion that reduces friction and induced stresses generated by traditional rotary notchers. Figure 1‐2 shows an automated notcher.

More and more companies demand a fast turnaround of test results. Equipment manufacturers have met this challenge by developing faster instruments, such as a twin‐bore capillary rheometer, as shown in Figure 1‐3. Twin barrels allow the Bagley correction in one run and they also double the output effectively combining two rheometers into one unit. This means that testing time is considerably reduced; more pressure–velocity increments can be applied to one change of material and the test can run automatically while the operator performs other tasks. The use of the Internet to deliver data to customers is increasing. The customer receives the data within minutes after the tests are completed. Such web‐based material data management systems (7) have the ability to securely store, display, and output diverse material properties for a variety of materials in complex formats, including CAE material model parameters. Material suppliers now have the ability to store and publish or selectively distribute their data instantly across the globe.

The use of instrumented testing to study the impact properties of plastic materials is also on the rise, and industry has witnessed a significant expansion in the application of instrumented testing. Instrumented testers are capable of generating and providing much more meaningful data as compared to conventional instruments. Instrumented instruments are better suited for research and development, as well as simulating real‐life conditions. Whereas the non‐instrumented tests generally measure the energy necessary to break the specimen, instrumented impact tests provide curves of high‐speed stress–strain data that distinguish ductile from brittle failure and crack‐initiation from crack‐propagation energy. The latter gives a more nuanced picture of the “toughness” of a specimen (8). With instrumented impact, the falling dart’s tip or the pendulum’s hammer is fitted with a load cell. The force–time data during the actual impact are stored by a high‐speed data‐acquisition system. These data can be used to generate curves showing force, energy, velocity, and deformation versus time. By analyzing these curves, one can learn (a) the force, energy, and deformation necessary to initiate a crack and then to cause total failure, (b) the rate sensitivity of a material to impact loading, and (c) the temperature of a material’s transition from ductile to brittle failure mode. The advent of piezoelectric sensors for instrumented impact testers is said to provide greatly increased sensitivity, allowing for testing of very light films, foams, and most other materials used in packaging. Developments in fracture mechanics analysis applied to polymers have now been incorporated into ISO 17281, so it is now possible to study the intrinsic impact properties of plastic materials. Two properties, the critical stress intensity factor (K1C) and the energy release rate (G1C), can be isolated as inherent fracture characteristics independent of the actual geometry and dimensions of the finished product; as such, they can be utilized in CAD tools to determine fracture performance. One such instrumented impact tester is shown in Figure 1‐4.

Figure 1‐1. Automated test setup.

(Source: Courtesy of Instron.)

Figure 1‐2. Automated notcher.

(Source: Courtesy of Instron.)

Figure 1‐3. Twin‐bore capillary rheometer.

(Source: Courtesy of Instron.)

Figure 1‐4. Instrumented impact tester.

(Source: Courtesy of Instron.)

While budget cutbacks and reduced staffing levels have affected nearly every segment of the plastics industry, large research centers operated by major manufacturers have been particularly under scrutiny. As a result, the outsourcing of analytical and testing services to independent laboratories has been increasing. According to the latest estimate, nearly $10 billion in analytical services is currently being outsourced, and growth of more than 20 percent per year is anticipated. By using outside laboratory services, a company can reduce large investment in underutilized lab instrumentation, reduce expensive overheads, and keep in‐house laboratory staff focused on higher‐value development and problem‐solving work.

Figure 1‐5. Outdoor weathering box.

(Source: Courtesy of Atlas Material Testing Technology LLC.)

The use of plastics in increasingly demanding applications has created a need for developing tests that simulate real‐life applications. The traditional method of comparing properties of plastics and rating one material against another is no longer adequate since the results do not reflect the behavior of the product in real‐life situations. Laboratories are focusing on testing products under special circumstances and conditions. New regulations have spurred auto manufacturers and suppliers to test and characterize the energy‐absorbing properties of the materials used in instrument panels and other components that drivers and passengers may come in contact with in the event of an accident. High‐rate impact testers simulate the dynamic nature of a car crash much better than traditional impact tests. To simulate the conditions found in the interior of an automobile, an outdoor weathering box has been developed by a leading material testing company. This under‐glass exposure cabinet was primarily designed to accommodate nonstandard specimen sizes such as complete automotive assemblies as well as standard ten × 15‐mm samples. Various types of windshield or side window glass can be installed for evaluating the effects of different types of glass on automotive interior components. Color and gloss measurements and visual inspections are performed at intervals according to applicable test standards. Figure 1‐5a illustrates one such box capable of housing a variety of components, including headphones, remote control devices, CD covers, and window tinting films. Figure 1‐5b shows an instrument panel exposed in a box designed to simulate an automobile interior.

A new type of test methodology for characterizing engineering plastics has been developed. These techniques simulate the extrusion and molding process to show what a polymer would undergo in terms of shear, temperature, pressure, and residence‐time deformation. The behavior of a compound can be accurately predicted before processing. An online‐type rheometer continuously measures the viscosity of the polymer from the die on a real‐time basis, and the data is used to make a screw speed adjustment to keep the viscosity consistent (9).