Advanced Techniques for Hardness Testing | Buehler

Materials testing, including hardness testing, is a fundamental but useful element in analyzing component properties, and can be accomplished by a multitude of methods and techniques. Determining the hardness value can provide valuable insight to the performance, durability, strength, flexibility, and capabilities of a variety of material types from raw materials to prepared specimens and finished goods. In today's extremely competitive global market, with high expectations on accuracy and productivity, there are heavy consequences for quality and/or productivity errors. Manufacturing, research, and quality control now more than ever must depend heavily on new and evolving techniques to revolutionize, streamline, and bolster their existing, more traditional processes if they want to maintain a competitive pace. Significant advances, developing at exponential rates, in hardware, electronics, algorithm's and software has led to much more sophisticated material and hardness testing equipment that can quickly, reliably, and with extreme precision, provide useful, material critical information.

Automatic Traversing and Image Analysis

Two of the most common hardness tests are Knoop and Vickers, used in micro and 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 nature of the test process typically dictates a relatively light force application, resulting in extremely small impressions that must be measured at the micron level. Traditional techniques, still widely employed today, involve the use of microscopes with objectives of varying resolution integral to the hardness tester, to manually measure through an eyepiece. Predictably, this is a time consuming, subjective, and potentially error filled process. It's 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 impressions quantities in excess of 15 indents each, usually several times on a single sample, and the desire to instill current advanced technological techniques becomes evident. During the past several years, and increasingly in the future, these manual processes are and will continue to rapidly give way to implementation of automation into every aspect of the process. New techniques are being realized in material preparation, stage movement, results analysis, and even reporting. One such technology being implemented into many labs around the world is automatic stage traversing and image analysis reading of Knoop and Vickers indentations.

An automatic Knoop or Vickers system typically consists of a fully controllable tester including auto-rotating turret, as well as actuation in the Z axis for both applying the indentation and as a means of automatic focusing on the specimen. Add to this a typical powered PC loaded with dedicated software, an automatic XY traversing motorized stage, and a USB video camera, and you have a fully-automated hardness testing system that, once initially setup with samples and a stored program, can be left alone to automatically make, measure, and report on an almost limitless amount of indentation traverses.

Newer technology eliminates much of the hardware that in the past posed operational challenges and cluttered workspace. Movement of the stage is through virtual joystick and on some systems, stage controllers are integrated into the stage housing. Advances in stage movement algorithms and mechanical design have made XY accuracy and repeatability better than ever, paramount in precision traverse requirements such as case depth analysis. While image analyzing and automated stage movement is not new to hardness testing, developments are ongoing and continue to factor into dramatically improving the process. Camera technology has evolved from frame grabber, to IEEE Firewire to USB, 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. Past limitations in regards to surface finish, lighting, pre-setting calibration, threshold, and pixel sizing has all but been eliminated while continually undergoing improvement. The result is increased ability and dependencies on "letting the instrument do the work", contributing to substantial increase in throughput, consistency, and importantly, freeing up the operator to perform other responsibilities. Expanding on productively even further is the ability to utilize larger sized XY stages capable of holding 2, 4, or even 6 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 issues of indent clarity due to Z position variation. Newer software's even allow different scales, forces, and microscope objectives, within and between traverses. Automation testing is also increasingly beneficial on Rockwell hardness testing applications, 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.