eXpert 1000 Servo Hydraulic Testing Machine

Summary of Test

ASTM A370 is performed by steel manufacturers and commercial and government laboratories serving the construction industry to measure the strength of rebar. Values of ultimate tensile strength, yield and elongation are required for the most common sizes.

Why is it done?

Steel manufacturers are required to test the products they sell to ensure that the specifiction cited is met. Significant liabilities can result from providing noncompliant product. Tensile testing to failure is the most common method employed. These often include the use of extensometers for the measurement of Yield.

In the United States, the size designations of these mild steel bars used to reinforce concrete are set by ASTM International. Rebar is generally supplied in 20- and 60-foot lengths.

Most bars are “deformed,” that is, a pattern is rolled onto them which helps the concrete get a grip on the bar. The exact patterns are not specified, but the spacing, number and height of the bumps are. ASTM A615 defines these markings.

Three grades are defined:

The size designations up through size 8 are the number of eighths of an inch in the diameter of a plain round bar having the same weight per foot as the deformed bar. So, for example, a number 5 bar would have the same mass per foot as a plain bar 5/8 inch in diameter.

Users of rebar are required to test the rebar they receive using independent sources prior to acceptance. There have been issues with substitution of grades and materials not meeting the intended specification. Testing helps to minimize the possible installation of out of spec rebar.

Why is it important?

The strength of rebar is a fundamental requirement in the design and construction of buildings, dams, bridges, roads and airports. The failure of a single rebar may result in significant loss of strength in the structure under construction with the possibility of catastrophic failure.

What ADMET equipment is needed?

Many steel manufacturers and independent testing laboratories own tensile testing machines of considerable force capacity. These older machines constitute a considerable investment so automating an existing system via upgrade/retrofit is a prudent decision. The upgrade/retrofit is performed at the customer’s site including training.

If new equipment is desired, ADMET ExPress servohydraulic testing machine of capacities 20,000 to 300,000 LBF depending upon rebar size and specification are available.

Rebar fixtures including holders and long gauge length extensometer are among the tools ADMET offers for the testing of rebar.

Changes in the force rate being applied to the rebar can affect the yield strength, tensile strength and elongation values which are highly strain rate sensitive. In general the yield strength and tensile strength will increase with increasing strain rate. Elongation values generally decrease as the strain rate increases.

Rebar testing using manually operated machines places a burden on the technician performing the test. The rate of testing is deliberately slow and thus the length of time to test can be long. All the while the operator is expected to be monitoring and adjusting the controls of the machine to ensure there is consistent force application at the specified rate. This is often not the case and inconsistent force application from test to test is the inadvertent result. Undesirable scatter in the test data can occur and sometimes good product gets rejected while at other times out of spec product can be deemed acceptable. Automatic control minimizes this scatter and provides inherent documentation of the testing force application.

Table 1. Rebar size versus yield strength in pounds force for 3 grades

Note: Not all sizes are available in all strengths. Ultimate tensile strength is typically between 1.33 (larger sizes) to 1.75 (smaller sizes) the yield strength.

Test method ASTM F2634 is used to determine the quality of polyethylene (PE) butt fusion joints. This test is desinged for pipes with a diameter greater than 2.37″ and a wall thickness greater than 0.25″.

Test Summary:

In this test a PE “dogbone” shaped specimen is pulled in tension at a rate of 4-6 inches per second. The specimen has holes on each end which allows them to be pinned to the upper and lower crosshead. The speed of this test is quite fast and the fixturing provides clearance to allow the machine to quickly gain enough speed before the impact takes place. After the test, the following are typically reported:

• Yield Energy
• Average Rate of Speed
• Recorded force / vs time chart
• Max force
• Rupture energy
• Yield stress
• Type of rupture

Equipment required:

1. Testing machine with a controlled rate of crosshead movement (constant rate of extension)
   1. Speed of 6 inches per second (360 inches per minute)
   2. Data aquizittion rate of 1000 HZ (1000 samples per second)
2. Load cell
3. Fixturing to provide impact clearance and hold samples
4. Position indicator (LVDT, encoder, extensomer, etc.

Equipment available:

McElroy, a company that designs and manufactures a complete line of fusion equipment, makes a machine specifically for this test called the McSnapper. It looks nice but we don’t know anything about it.

At ADMET we studied ASTM F2623 and determined that our ExPress line of hydraulic universal testing machines offer the best solution for this test. A standard ExPress system with a modified pumping system to increase the maximum speed would satisfy all requirements of the specification.

This is our recommended system:

1. Express hydraulic universal testing machine. For example, the Express 50kN would have the following specifications:
   1. Capacity: 50kN (11,250 LB)
   2. Power Stroke: 4 inches (Other Stroke Lengths Are Optional)
   3. Tension Opening: 38.5 inches without load cell.
   4. Crosshead Thickness: 4 inches
   5. Speed (max): 165 mm/sec (6.5 inches/sec)
   6. Column Separation: 508 mm (19 inches)
   7. Column Diameter: 63.5 mm (2.5 inches)
   8. Dimensions:
      1. Height: 2,362 mm (93 inches)
      2. Width: 838 mm (33 inches)
      3. Depth: 813 mm (32 inches)
   9. Power: 220/440 VAC Three Phase
2. MTESTQuattro testing software
   
3. ASTM F2634 pre loaded test method and software analysis module
4. ASTM F2634 fixturing

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Niacc-Avitech Technologies is an aircraft component overhaul and manufacturing facility in Fresno, CA. A subsidiary of HEICO Aerospace, the company repairs and overhauls starter/generators, fuel assemblies, cable harnesses and electronics, brake and hydraulic components, and other aircraft appliances and components.

Niacc recently received a customer request to add overhaul capabilities for landing gear shock absorbers to its portfolio of services. The technical specifications required load vs. position and cyclic properties certification of shock absorbers according to OEM specifications that would require new test equipment.

“The load issue was one of the real challenges,” Explained Rayan Kabeer, Niacc head of Engineering and Product Development. “For this type of shock absorber we were looking into 49,000 pounds. Everything other companies had was too big or too small. They could have custom made it, but it would have taken a tremendous amount of time.”

“We talked with ADMET about load and pressure and what the requirements were,” he said. “We gave them all the parameters that we needed and there was a lot of back and forth. They pretty much said, ‘we can do it.’”

ADMET specified an ExPress 300 kN (60,000 lbf) dual-column servo-hydraulic materials testing machine with a top mounted actuator (as specified in the CMM). ADMET also recommended the MTESTQuattro Materials Testing System, a Windows-based software controller with advanced analysis and reporting capabilities.

The unit arrived within six weeks. “We just had to hook [the machine] up with our computer and then load the software. We installed the load cell ourselves and it was pretty much plug and play,” Kabeer said. “The best thing about ADMET is that, with their software, we can control [the test] pretty much hands-off.”

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Researchers at the University of Connecticut, in cooperation with the Connecticut Department of Transportation, tested and developed a low-cost passive wireless crack sensor using commercial RFID tags (single tag and multi-tag) for crack detection of field metal structures. 

Tensile testing was completed with an ADMET eXpert 1655 Hydraulic Universal Test System with 250kN load cell. The tensile grips were clamped flush with the interior edge of the anti-buckling plates. The test setup included a digital image correlation (DIC) system to observe the crack propagation area. Pre-cracks were formed by applying a tensile load at low strain rate, typically 0.0025 in/min until a maximum load was reached followed by a visible crack extending from the machined notch, approximately 0.0875 inches for small samples and 0.1875 inches for large samples. The sample was monitored at roughly 10x magnification during this operation and crack length was measured to 0.05-mm precision. To enable communication between the RFID tags and the reader antenna, a substrate material between the tag and the metal surface was found. 

The first major experimental study was on the crack detection capability and sensitivity to damage of both sensor configurations: single-tag and multiple-tag and the damage sensitivity of multiple sensors were studied for crack detection. The results showed that the developed sensors have great potential to effectively detect the existence, location, and the degree of cracks. Therefore, the complete damage sensitivity of the RFID-based crack sensors were successfully validated.

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