Process Control Applications for Materials Testing and Characterization
Along with the expansion of materials science has come the need for better, more accurate testing capabilities. Indeed, testing is a necessity, not only for the comparison of new materials to old, but also to ensure that the properties of existing materials are being maintained at prescribed levels.
Choosing the proper testing system from among a wide selection of products and systems available need not be a difficult task. Without considering the cost of equipment, one needs only to determine the information they wish to monitor, the detail at which they wish to report that information, and the types of products available to meet their requirements. One factor entering into this equation more and more is the availability of automated equipment to save time and operating costs by simplifying the process.
In the thermal processing industry, customer-specified design tolerances and mechanical properties are generally the two key areas that are considered when quality inspections are done. Mechanical properties are listed typically as a "minimum" tensile strength, yield strength, elongation, and hardness, while design or machining tolerances are listed generally as a certain value with a "maximum" range of allowance. Computer controlled test systems not only provide increased accuracy for these measurements, but also higher efficiency through increased speed of analysis.
Common Quality Tests
The most common mechanical tests performed for quality assurance purposes are hardness testing and tensile testing. Both of these tests are considered static or monotonic (single factor) tests. Other less common tests include compression, friction, fatigue, fracture toughness, creep, and stress rupture testing.
The Rockwell hardness test is the most widely used method for determining hardness, primarily because the test is simple to perform and does not require highly skilled operators. By use of different loads (forces) and indenters, Rockwell hardness testing can determine the hardness of most metals and alloys. Readings can be taken in a matter of seconds with conventional manual operation and in even less time with automated setups. All readings are direct; optical measurements are not required eliminating most operator errors.
The tensile test is the most common form of static test. Generally, the point of failure is of much interest and is characterized by: 1) the "ultimate tensile strength" or maximum load a material can support; and 2) the strain to failure. Other critical measurements are the "0.2% offset yield strength" or the load a material can support at 0.2% strain (European standards typically call for a 0.1% offset yield strength), and the "total elongation" or plastic deformation a material can exhibit without failing or separating (Fig. 1). Each of these values is a calculated value based on the cross-sectional area or starting length of the sample being tested.
Because of the high volume of testing required in the metals industry, automation of test calculations is highly desirable to improve productivity and facilitate record keeping. Today's digital control systems enable test conditions to be accurately established and measured.
AUTOMATED SYSTEMAs example of the capabilities built into automated mechanical testing systems, a suite of materials testing software programs have been developed by Instron Corporation, Canton, MA, for materials test set-up, control, data collection, result generation, and report preparation for the company's 5500 testing systems. The software (MerlinT) features a graphical user interface fully implemented in WindowsR. It provides up to four real-time numerical displays (digital and/or analog) of test data as well as graphs and tables of processed test results.
The software applications packages are versatile testing modules that are tailored to meet the specific needs of a testing program. The packages include test control, results calculations, units of measurement, specimen information, etc., for the type of testing to be performed. The packages are applicable to tension testing, compression testing, flexure testing, peel, tear, and friction testing and cyclic applications.
Test controls for tension testing (Fig. 2) include pre-cycling, pre-loading, automatic test speed change, creep and relaxation control, pre-tension grip control and test end/break detection control. Test results and data handling functions include modulus, yield, break, unlimited pre-set point detection, peak values, slack correction, creep/relaxation results (total or delta), replay facility, and data import/export. Complete control of the testing procedure (except specimen loading and unloading) can be obtained through the software control programs. Test controls for compression testing are similar.
Flexure test controls include pre-loading, position control and optional load, stress, and strain control, creep and relaxation controls, test end/break detection for rate of load fall, load threshold, and any channel value. Test results and data handling include modulus, outer fiber stress and strain, yield, break, unlimited pre-set point detection, peak values, a replay facility and data import/ export.
Peel, Tear and Friction
Test controls available for these tests include pre-loading, position control, pneumatic grip control (option), and automatic test end detection. There are tests for 90 peel, 180 peel, T-type peel, tear, friction, etc. Peel/tear test results include average peel/ tear load, load/displacement at first peak, energy at first peak, energy between limits, and actual peel length. Friction test results include coefficients of static and dynamic friction. Other functions are also available
Benefits of Automated Testing
The primary benefits of automated testing systems are:
- removal of operator error associated with measurements;
- feedback control of the testing sequence;
- automatic calculations based on input information and actual test data;
- ease of machine control; and
- ease of report generation.
Each of these factors can help save time in materials or product evaluation and increase throughput for a quality laboratory. On the other hand, in order to preserve accuracy, repeatability, and consistent operation, automated systems require the same attention to the calibration of sensors and equipment as do non-automated systems.
SUMMARYThe use of automated testing equipment has increased in recent years as a result of improvements in computer/ software technology and the need to improve production rates. Quality assurance programs must adjust to these demands to ensure products are meeting or exceeding customer expectations.
Not only is accuracy increased through the use of automated systems, but also repeatability is improved by eliminating much of the operator error that sometimes plagues the reporting of test results and data. The application of computer software programs and automatic control systems to mechanical testing can help quality labs to continue to meet the demands of the 21st century manufacturer. IH