The article will focus on the significance of the ASTM-A-931 test involving the tensile testing of wire ropes and strands, the way the test is done and in which industries this test applies and also highlight issues faced by users.
The types of extensometry currently used for performing the test will be discussed, concluding with how to use Imetrum’s UVX non-contact video extensometer to more accurately and efficiently carry out the test.
ASTM A-931 Test and Methodology
ASTM A-931 encompasses the tensile testing of steel wire strands and ropes at room temperature. The test purpose is to determine the yield strength, breaking force, modulus of elasticity and elongation.
The test is done till the load drops below the maximum and at this point strands can be heard breaking. The test speed must be below 0.032mm/min for each mm of specimen length (i.e. 910mm specimen should be tested at a max speed of 28mm/min).
According to the standard, determining the modulus requires specialized equipment and it is not considered as a requirement. However, as several persons want to know the modulus, they measure it as part of the test.
Industries Requiring ASTM A-931 Testing
The industries using wire rope include:
- Oil and gas
- Structural (suspension bridges, cable stayed bridges, tied arch bridges, stayed masts and towers, roof structures)
- Cranes and lifting equipment (container cranes, tower cranes, mobile cranes, dockside cranes, overhead cranes)
Rope testing is done by almost all rope manufacturers in their research and development departments and also for quality control and quality assurance processes. Testing may also be done by certain end users and academic institutes.
Current ways of testing to ASTM A-931
Presently available methods of ASTM A-931 testing are:
- Crosshead displacement in combination with a ruler to establish the initial gauge length.
- Contacting extensometers
- Non-contact laser extensometers are used sometimes if the specimens would not support the weight of a clip-on or when a high-energy failure is expected.
Cross head displacement
For steel rope, cross head displacement is used to measure the extension. However, there is normally a movement within the gripping method such as bedding in of the poured socket and potential slack within the load train. This causes an inaccurate measurement.
In order to avoid damage to the extensometer it must be removed before specimen failure. The specimen will be subjected to a load so this poses a significant risk to the operator as there are chances that the specimen will break while removing the extensometer.
In case the extensometer is still attached, it may be thrown off. This may destroy the instrument and it also poses a hazard to any persons in the area.
Laser system resolution may not suffice for modulus measurement. The working range of laser extensometers is limited so they are not suited for long rope lengths. They are mostly mounted close to the sample hence are vulnerable to damage by failing specimens.
Rope twisting or rotation may pose to be an issue with attached reflective targets going out of sight. Many-a-time, the markers are too large to fit on narrow ropes. The laser may find it difficult to track the reflective markets for long gauge lengths.
Issues Faced by Customer
During rope testing, high-energy specimen failures may be experienced. In case of using a contacting measurement device, it will be damaged and turned into a projectile if it was not removed before failure. The preferred technique is thus a non-contact solution.
Benefits of the UVX
The video gauge offers the same accuracy as of a contacting extensometer but without any of the drawbacks:
- It works well with all specimen types and G.L.s including large diameter (>125mm) and long samples (>6m)
- It is possible to position the camera at a safe distance so there is no risk of damage
- There is no need for the operator to remove the device before failure thus avoiding the risk of personal injury and enabling strain and extension to be measured all the way through to failure
- It does not have any contact points and no stress concentrations. This may reveal the rope to be stronger/stiffer than before.
- Since the test can be post-processed, further measurements can be made even after the rope has been tested to failure. This enables focused measurements to be made on specific areas such as the area of failure or sections where the rope is spliced.
Test Methodology using the UVX
The methodology followed is:
- In case the rope to be measured is arranged vertically, the 5M pixel camera is placed on its side as seen in Figure 1. If there is a protective screen, the camera must be arranged behind the test machine in order to get a clear view.
- In case a Perspex screen is being used for protection, it is important that the camera is positioned at an angle in order to avoid reflection of light and camera in the image.
- For maximizing the measurement range, with longer ropes, it is advisable to ensure that the fixed end of the rope is close to the edge of the image.
- It is essential to know the appropriate lens and approximate Field of View (FOV) for the application.
- Spreadsheets are available, but in order to use them, the Gauge Length, total elongation and rope length must be known.
- One must choose a GP lens that will achieve the required FOV
Table 1 shows typical fields of view
Table 1. Approximate fields of view for various gauge lengths (assumes the gauge length is half the distance between grips and max strain is <10%).
|Gauge Length (mm)||Field of View (mm)|
- Light must be positioned at the same height as camera and slightly to the side
- In case the rope is vertical, mount the light on its side
- Set far enough back that it evenly illuminates the entire length of a fully extended specimen.
- For long samples/GLs one light per target is required
Several light and dark bands round the rope create a suitable target that will work well even if the rope twists during the test. This is possible by printing a set of light & dark stripes on a sticky label and then wrapping this around the rope to create the bands.
Figure 2. Light and dark bands around the rope
A ruler is the used to apply the marks at the required gauge length. The rope testing spread sheet provides an indication of the needed target length and diameter.
The rope testing spread sheet will output an indication of the required target length diameter. Some typical values are shown in Table 2 below. The standard is five dark bands and four light bands on the target.
|Gauge Length (mm)||Target length (mm)||Min Target diameter (mm)|
Video Gauge configuration
The configuration is done by:
- Positioning target cross-hairs on centre of target
- Sizing the target as per photo.
Figure 3. Target cross hairs positioned at target centre
- Apply a slight pre-load so that the specimen is pulled straight before test.
- Begin the Video Gauge
- Begin tensile test machine
About Imetrum - Non-Contact Precision Measurement
Imetrum, a spin out from Bristol University, invented and continues to develop Video Gauge, a unique, non-contact, video-based measurement system that allows the user to test a variety of over 200 important parameters (strain, rotation, displacement etc.) in real time, in a single test. The Video based measurement system is ideal for use in exacting market areas such as aerospace, automotive, defence, oil & gas, energy, R&D and high technology motorsport, where highly accurate measurements directly traceable to national standards are critical for their materials, component or sub-assembly development. In materials it is the world’s first video extensometer capable of being calibrated to BS EN ISO9513 Class 0.2, which also equates to the equivalent American standard ASTM E83 Class B-1. As the Imetrum system is video based, a record of the test can be saved so engineers can either post process the test or complete further analysis without the need for repeating the test, saving both time and money.
منبع : تست جرثقیل،لیفتراک و آسانسور |تست كشش سيم بكسل بر مبناي استاندارد ASTM
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