Tensile Testing - There's a Fine Line Between Gripping and Slipping
October 9, 2007
With the complex materials science and engineering and physics that goes into the tensile testing of metals, it is hard to imagine the accuracy of the results and the safety of the operation coming down to something as seemingly simple as the right sample grips. However, aren’t the right tools the secret to the successful outcome of just about every endeavor?
ASM International devotes an extensive section of its tensile testing primer to test setup with an emphasis on grips selection and use, stating that “only two rules apply: the grips must properly fit the specimens (or vice versa), and they must have sufficient force capacity so that they are not damaged during testing.” Examples abound about the kinds of bad outcomes that can result from sample-grip mismatches. Threaded grips help to ensure a tighter hold on specimens, but sample sizing, placement and alignment are critical to avoid damage to the grips or injury to the operator when high force is applied. Misalignment causes off-center loading and bending loads. Also, repetitive testing virtually guarantees eventual wear of even the hardest grip material. Test operators need to be on the lookout for differences in the teeth marks, a sure sign of worn grips.
Grip DevelopmentAt Tinius Olsen, emphasis on gripping systems is best summarized by their philosophy that “if you can hold it, you can test it.” The company has built a tremendous knowledge base on grip development over its 127-year history. It has engineered an exceptional array of both common and highly specialized grips, taking special pride in never having been seriously challenged by a material or component that could not be held and supported for a successful test. Its dedicated grip-design department can create customized solutions to specific needs. They will take a testing standard and customer application input, interpret requirements and build a grip precisely matched to the job.
Grip ExamplesIn addition to familiar styles of tensile grips, such as wedge grips and split-collar grips, Tinius Olsen has developed many application-specific models. For testing something as flexible as foil, a light scissor grip uses two knurled rollers. It is self-tightening and self-aligning. To grip tension springs, zipper components and items like canned-drink ring pulls, tension hooks are often employed in conjunction with other gripping devices. Rigid dumbbells, cut from a section of gas or water pipe, can be tested by gripping the curved shoulder of the dumbbell between two bollards.
Sample ChallengesSometimes a very limited amount of material is available for producing a specimen – such as a special-alloy automobile wheel. To address this need, Tinius Olsen developed hardened split collect chucks to hold round-turned dumbbell test pieces. A precise shoulder radius prevents premature test-piece breakage within the grip.
Sheet-metal specimens in flat, rigid dumbbell form can be tested by having an 8-mm-diameter hole drilled in each end. The test piece is located between two serrated dies, and the dies are clamped on it using a high-tensile bolt.
To test threaded and headed high-strength metal specimens in accordance with ASTM E8, Tinius Olsen offers a very specific set of holders used with two spherically seated plates and nuts that have matching studs. These are typically made for testing 0.505-inch-diameter, threaded specimens. Adaptors are available for accommodating 0.357-inch or 0.252-inch-diameter test bars or even cast-iron bars of 0.5-inch, 0.75-inch or 1.25-inch diameter. Additionally, the company makes split adaptors for headed specimens in various sizes and a metric version for both headed and threaded holders (Fig. 1).
For high-strength metals testing, stud-mounted grips are offered that have a unique spherically seated design to ensure positive and automatic alignment. The spherical nut moves freely within a matching spherical countersink in a mounting plate. A helical spring absorbs the shock and grip recoil when the sample fails. Another gripping option is designed especially for rapid testing of standard headed specimens in accordance with ASTM E8, A370 and B557. The sample is inserted into two pairs of split sockets and an outer clamping ring slides down over them to lock the sample into place (Fig. 2).
Wire, cable and other high-strength, flexible metal materials need very special tensile grips. Tinius Olsen developed a system in which the sample is wrapped around 4.5-inch- (114-mm)-diameter grooved spools with each end securely held between split spools by action of integral clamping bolts. This arrangement is ideal for testing wire and cable up to 3/8 inch (9.5 mm) in diameter.
ConclusionDevising the right system of upper and lower grips to securely hold the material sample under test and accurately transmit the forces being applied is an ongoing challenge. This is especially true as new materials are being developed or new component configurations are required. Many of these challenges have already been met, however, so it is worth a careful exploration to find the right existing grip design or one that can be adapted easily to minimize the expense of a highly customized solution. IH
For more information, contact: John Kraus, regional sales, Tinius Olsen, 1065 Easton Road, Horsham, PA 19044 USA, (215) 675-7100, Fax (215) 441-0899, www.TiniusOlsen.com.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: tensile testing, bending load, extensometer, double shear
SIDEBAR: Tough Testing Standards Require Flexible Testing SystemsImagine you’re a company that prides itself on staying ahead of the technology curve. Imagine also that measurements that are “good enough” just won’t do. Now imagine that you want to combine accuracy with innovation in order to deliver cutting-edge results to your clients.
That was the dilemma for Wallace-Kuhl & Associates, Inc. (WKA), a geotechnical and materials-testing organization headquartered in West Sacramento, Calif. For years the company had been using Tinius Olsen’s 120 Super L static hydraulic testing machine, which still functioned well with upgrades. Yet when they looked to purchase new technology for the main office, they wanted the most precise measuring technology available. In fact, they wanted a lot more.
Josh Nitchman, senior lab technician for WKA, had plenty of “must-haves” for his next stabilometer and testing system, so he shopped. After looking at numerous other companies without finding anyone who offered everything integrated on one machine, he headed to Tinius Olsen with his list of items that a new machine must have. Wallace-Kuhl tests materials such as concrete, soils and steel in pre-construction and construction applications. Field testing of bolts and other materials is also performed, and it was necessary to improve the speed and efficiency of this process.
Upon consideration of all testing needs, a proposal was offered for one of the most loaded and customized testing systems designed in their long history. The result was a 400,000 pound-force Super L with adjustable extra height; hydraulically actuated grips; dedicated grips to test rebar, seven-strand cable, bolts and concrete cylinders; and a host of other customizations.
Also included was a special software routine for referencing a variety of field-use bolt testers. WKA uses hand-pumped or electrically pumped hydraulic rams to test the strength of high-tensile bolts in their final application. This test was being performed in the same way as other materials-testing groups – each calibration taking up to an hour to complete.
Not any more. Bolt testing in the field now takes seconds. Tinius Olsen’s Test Navigator software, which works in tandem with the 400 Super L, gives the operator a means to reference pressure on the mobile testers to fixed load values on the test pressure. Even the video extensometer, which is so cutting-edge that no program runs with it, now has its own program and automatic readings.
Nitchman and WKA are satisfied customers in more than one respect. Nitchman says the savings to his company is a bonus. “We’ve been able to reduce some of our costs because we don’t need as many people, and we don’t have to subcontract any of our work. That savings goes back to our clients, which gives them both a cheaper price and direct access to the test results.”
Wallace-Kuhl & Associates, Inc. – founded in 1984 – employs a staff of 200, including environmental scientists, geologists, engineers, technicians and support personnel. They have three offices in the greater California Central Valley, located in Yolo County, Placer County and San Joaquin County. www.wallace-kuhl.com
SIDEBAR: Perryman Co. Envisions it - Tinius Olsen Makes it HappenSpecialty metal requirements are tricky enough to address, but especially when a company is collaborating with a vendor. Such was the case recently for Perryman Company, a Houston, Pa.-based global specialty titanium-products provider. They worked closely with Tinius Olsen – the Horsham, Pa.-based supplier of materials-testing machinery and systems – on some specialized testing challenges. Perryman’s products are used in the aerospace, automotive, medical and recreational industries.
Perryman uses a Tinius Olsen electro-mechanical tensile-test machine equipped with ball screws to perform two critical tests – tensile and double shear. Because the nature of these tests varies significantly, however, employees lose time modifying the machine in between. It is a very time-consuming process to convert the machine, which requires removal and reattachment of the grips.
A few years ago, the crew at Perryman saw the possibility of avoiding the cumbersome setup between each test. They approached Tinius Olsen about a single machine for both tensile testing and shear testing without major set-up time. “In the past, we would have to schedule shear tests during one half of the day and tensile tests the other half. It wasn’t the best way to work,” indicated Sy Sweet, chief metallurgist for Perryman.
In response, a special tool was designed that fits around the bottom of the grip so that it bolts to the bottom cross member. Affixing the grips in the proper position allows the operator to change quickly between tests. On Perryman’s test machine, the operator uses grips. When pulling a tensile test, the grips are fixed from the center bar to the top of the cage. The modified materials-testing machine is working – Sweet reports a 30% increase in productivity.
Headquartered in Houston (near Pittsburgh), Pa., Perryman is ISO 9001-2000 and AS-9100 approved. Its focused manufacturing facilities produce the following titanium products: precision-finish coil, centerless-ground bar, drawn net shapes, premium fine wire and hot-rolled products. www.perrymanco.com.