Hardness testing is a fundamental characteristic in analyzing component properties. As an important materials-testing tool, this critical parameter can be performed and measured by a multitude of methods and techniques.
|Fig. 3. High-quality image of Vickers indentation on glass sample, previously not possible with older techniques|
While some hardness-test types such as the Rockwell test will yield fast, single-process results based on indentation depth, many of the commonly used test types such as Vickers, Knoop and Brinell require a secondary process to determine the size of the indentation (in microns or millimeter) surface area. These secondary processes can be time consuming, inefficient and prone to subjective errors. One means of improving productivity while providing consistency to the process is through automatic indentation and impression reading utilizing image analysis.
Significant improvements in recent years in hardness-testing instrumentation as well as computer hardware, electronics, imaging algorithms and software capabilities have opened the door to extremely precise and reliable testing processes that provide results more quickly than ever before. These components and techniques have proven to be beneficial in raising efficiency, speed and accuracy to levels never before achieved.
|Fig. 1. Fully automated traversing Knoop/Vickers and Rockwell hardness-testing system testing multiple samples|
Why Automatic Hardness Testing?
Two common hardness tests are Knoop and Vickers, which are used in micro and/or macro testing to determine material hardness based on measuring the size of a diamond-shaped impression left from an application of a specified force.
The Knoop diamond produces an elongated rhombic-based, diamond-shaped indent with a ratio between long and short diagonals of about 7 to 1. Knoop tests are typically performed at test forces from 10-1,000 g; are often referred to as microhardness or microindentation tests; and are best used in small test areas or on brittle materials as minimal material deformation occurs on the short diagonal area.
The Vickers diamond produces a square-based pyramidal shape with a depth of indentation about 1/7th of the diagonal length. The Vickers test has two distinct force ranges – micro (10-1,000 g) and macro (1-100 kg) – to cover a multitude of testing requirements. Vickers tests are typically referred to as macroindentation tests and are used on a wider variety of materials including case-hardened and steel components. Vickers indents are also less sensitive to surface conditions than the Knoop test. The nature of these test types typically dictates a force consistent with the material being tested, usually resulting in extremely small impressions that must be measured at the micron level.
Traditional techniques, still widely practiced today, involve microscopes with objectives of varying resolution integral to the hardness tester. The objectives are used to manually measure the impression through an eyepiece, based on human interpretation. Predictably, this is time-consuming, inefficient and, in today’s fast-paced, extreme environment, it is increasingly unacceptable. It is not uncommon for a technician to produce and measure by eye many hundreds of indentations during a day, with fatigue likely compromising the measurement process as the indents increase in quantity. Add to this the need to produce a full analysis of a hardness traverse often consisting of more than 15 indents each (many times on a single sample) as well as the importance of accurate, well-reported results, and the need for more advanced, automated techniques becomes evident (Fig. 1).
|Fig. 2. Fully automated traversing and image-reading Knoop/Vickers system testing four mounted samples at a time|
Automatic Hardness-Testing Techniques
Over the past several years and no doubt increasingly in the future, these manual processes have and will continue to rapidly give way to automation in every aspect of the testing process. New, extremely efficient techniques in material preparation and handling, mount fixturing, stage movement, results interpretation and analysis, and even reporting have been introduced to the hardness-testing industry. An important and productive technology being integrated into many hardness systems around the world is automatic stage traversing and image analysis of Knoop, Vickers and Brinell indentations.
An automatic hardness system typically consists of a fully controllable tester, including an auto-rotating or revolving turret as well as actuation in the Z axis either from the head/indenter housing or from a spindle-driven system used for both applying the indent at a predetermined force as well as for automatically focusing the specimen. Add to this a standard computer with dedicated hardness software, an automatic XY-traversing motorized stage and a USB video camera, and the result is a powerful, fully automatic hardness-testing system. After initial setup with samples and an applicable traverse and parameter program, the system can be left alone to automatically create, measure and report on an almost unlimited number of indentation traverses (Fig. 2).
This newer technology eliminates much of the hardware that in the past caused operational challenges and cluttered workspaces. For example, current-design stages are moved by a virtual joystick, and stage controllers are fully integrated into the stage assembly on some systems. Advances in stage-movement algorithms and mechanical design have made XY accuracy and repeatability better than ever, which is paramount in precision traverse requirements such as case-depth analysis.
Image analysis is not new, but the technology driving it continues to advance. The indent measurement process is considerably improved from a more limited form with inadequacies measuring smaller indents and samples with fewer surface finishes. These high requirements in regard to surface preparation along with process restrictions meant previous systems were lacking in effectiveness as a complete solution.
Now, for example, camera technology has evolved from frame grabber to IEEE Firewire to USB formats, eliminating additional hardware while at the same time increasing camera resolution and field-of-view possibilities. The capabilities of current and developing cameras, coupled with the processing capacity of today’s PCs and continually improving software packages, have significantly improved the accuracy, repeatability and dependability of automatic indentation reading. All digital cameras have pixel arrays. Each pixel can be either on or off. If a black and white image is projected on the pixel array, the pixels in the dark areas will be off and the light ones will be on. By counting the number off, the size of the dark spot on the image can be determined and subsequently the image area. The size of any indent is used in combination with the indenter and applied force to determine hardness value.
It is now possible to accurately and repeatedly read smaller-than-ever indents and locate and analyze indents on surfaces and materials previously not possible (e.g., glass). In addition, new developments in microscope objectives and digital zooming technology are allowing for wider magnification ranges than ever before (Fig. 3).
|Fig. 4. Fully automated traversing Rockwell hardness-testing system testing multiple samples|
Expanding productivity even further is the ability to utilize larger-size automatic-traversing XY stages capable of holding two, four or even six samples at a time in an array of fixturing types. Pre-programmed and saved traverses are opened, samples are aligned in holders and, with a single click, the indentation, reading and reporting of a multitude of traverses on each sample is initiated. Autofocus mitigates any compromise of indent clarity due to small parallelism position variation. Newer software even allows different scales, forces and microscope objectives within and between traverses, creating new possibilities in multi-sample and case-depth analysis. This fully frees the operator from manually moving the sample from test to test for both the indentation as well as the measurement process and quickly provides an ROI and benefit that is readily evident and clearly increases the ability to evaluate a variety of materials.
|Fig. 5. Automated Rockwell system testing multiple samples with nine indents on each sample|
Automated testing is also increasingly beneficial for Rockwell hardness testing, particularly in repetitive-pattern requirements such as Jominy testing, where a number of bars can be fully tested and reported, unmanned after one click of the mouse. The use of an automated stage and software integrated with a Rockwell tester capable of automatic actuation allows for multiple sample testing. In some cases manufacturers are automatically testing more than 15 parts on a stage with multiple indents on each part (Figs. 4 & 5).
As in Knoop and Vickers testing, Brinell testing is by nature a labor-intensive and manual process that, in its conventional state, requires constant human intervention and processing. Since the traditional Brinell test consists of a single, controlled test force made with a specified diameter tungsten-carbide ball, the resulting impression must be optically measured (diameter in mm) to determine the material hardness. This is typically performed using a low-power, hand-held microscope, a process that is both laborious and subjective. As in Knoop and Vickers, fatigue-induced errors from performing measurements repeatedly are common, and the process itself can be inefficient and time-consuming.
With many processes requiring 100% inspection and productivity that depends on quick turnaround, it is no surprise that a means to both accelerate the process and mitigate the possible manually induced errors is in demand. The method that may be most applicable is dependent on a variety of factors, including test-time requirement, specimen geometry, loading and unloading technique, material properties, ASTM standards requirements and adherence, and of course budgetary alignment.
The production Brinell test is one unique method of automatically and accurately determining Brinell hardness in a production environment. Through the use of the Rockwell test principle of measuring depth of penetration to determine hardness, the production Brinell test eliminates the costly and time-consuming procedures associated with conventional Brinell testing. First, the part is pre-clamped with sufficient pressure to prevent it from moving during the test process. Next, the test is performed applying a pre and full test force for a specified dwell time. The part is unclamped upon dwell completion. The test result is obtained by measuring the difference between the reference depth and the final depth after recovery has taken place.
An ASTM standard, ASTM E103 – Standard Test Method for Rapid Indentation Hardness Testing of Metallic Materials exists for this test type. This test method covers the procedure for rapid-indentation hardness testing of metallic materials as an alternative to ASTM Test Method E10 on standard Brinell hardness and includes methods for the verification of rapid indentation hardness-testing machines. Production Brinell systems can be integrated to production automation lines or stand alone to perform quick and consistent Brinell testing. They are available in a variety of formats and frame configurations and can be customized to meet an abundance of application requirements. If the test must adhere to the more common, optical Brinell standard, ASTM E-10, then other means of productively performing optical Brinell measurements are available.
When using a conventional Brinell floor- or bench-model tester that performs the indent portion of the test only, an alternative to the handheld manual process involves utilizing a handheld digital camera that can accurately and efficiently measure the diameter of the impression, automatically using image analysis techniques as described above. As a result, it has become relatively easy to measure Brinell indents through a camera. If a handheld imaging system, which requires manual intervention, is lacking in the desired production level, a fully automatic, optical Brinell system can provide adherence to ASTM E-10 while allowing for fully automated optical testing.
A fully integrated automatic optical Brinell testing system can quickly and accurately perform the entire Brinell test process in accordance with ASTM E-10. This includes accurate indentation application and providing an image-analysis system to autofocus on, identify and record indent size and hardness measurement. All of this, coupled with flexible user-friendly software, gives the operator extensive capabilities in generating tests, completing the analysis and generating reports. An operator only has to locate the sample in the tester and press the start button. Indentation is automatic as is the rotation of a revolver or turret-style system that turns the measuring objective/microscope into position. The automatic focus and imaging process is then initiated, and results are returned quickly in both indent diameter and Brinell hardness.
Past limitations in regard to surface finish, lighting, preset calibration and pixel sizing have been mitigated and are continually undergoing improvement. The result is increased ability and dependence on “letting the instrument do the work,” contributing to substantial increases in throughput and consistency while freeing up the operator for other responsibilities. With a fully integrated system, the labor-intensive, subjective and error-prone process is virtually eliminated and instead replaced with a significantly more accurate and productive process. IH
For more information: Contact Bill O’Neill, sales manager, Wilson Instruments, 825 University Ave., Norwood, MA 02062; tel: 781-5757-5873; fax: 781-575-5770; e-mail: firstname.lastname@example.org. For automated hardness-systems information please visit www.wilson-hardness.com.