Source: Ice City + 2021/11/05
In the early morning, the calm sea surface of the test port was slowly set with a layer of gold edge, and the sunlight reflected on the test equipment also brought the news of victory, and as the loading force under the last millimeter loading step dropped, the eye sockets of the 24-hour unseen researchers were moist, and they ushered in the sunrise and also ushered in the success of the experiment, for this moment, someone has been chasing for 17 years. In 2004, Li Chenfeng, who studied under Professor Ren Huilong of Harbin Engineering, began scientific research in the field of ultimate strength of hull structure, and now that he has become a teacher, he has not forgotten his original intention, and has always taken the narrowing of the technological gap with the international leading level as his goal. Finally, at this moment, the test results confirmed that the technology he led the team to develop achieved cornering overtaking, and the overall test technology of extreme strength reached the international cutting-edge level.
An old photo from 8 years ago He found a breakthrough in cracking the "card neck" technology
"The model is assembled, the hydraulic loading system is installed and positioned, and the model attitude adjustment is completed... All channels check for good. With the sound of a password, the extreme strength test of the damaged cabin section under the action of the bending and torsional combination of the Xinghai Academic Team of Harbin Engineering entered the key link of the system joint commissioning.
Ships in the design of the finalization of the need for strength tests, the ultimate strength is an important indicator of the vitality of the ship, the ship in the waves navigation by different directions of force, will form "bend" and "twist" two forms, and the previous model test can only measure the simple "bend" or "twist" form. If the forms of "bend" and "twist" are simulated at the same time to act together on the hull (bending and torsional coupling synchronous loading), it can be greatly close to the real force state of the ultimate strength of the ship structure.
However, the state of simulating "bending and twisting coupling" is a technical problem that has plagued the community of ship structural mechanics for many years, and there have been no successful and effective cases in related fields at home and abroad. A year ago, after the Xinghai academic team of Harbin Engineering University's "Environmental Load and Structural Strength" received a national key basic research task, Associate Professor Li Chenfeng, the head of the project, led the team to start a journey to tackle tough problems.
One by one, the loading scheme was proposed with great anticipation, and it was denied again and again by the theoretical analysis and simulation results, and the team fell into a confusion. Li Chenfeng kept looking for the reason, and when he looked through the information, he found a breakthrough in an old photo from 8 years ago. "This is the photo of our bending moment measurement limit strength test that year, I found that in order to simulate the damage to the hull side of the ship, the model was opened, and the asymmetry of the structure led to the proximity of the limit state, and the loading points evenly arranged horizontally at both ends of the model had inconsistent loading forces, which made the model in a torsion state, which is the bending and torsional coupling load that the entire industry is struggling to pursue!"
After the idea was clear, the team began to design and validate the new loading scheme in detail. After several rounds of optimization, the "four-point eccentric" torsional coupling loading technology was finally innovatively proposed, compared with the bending and torsional combined loading method proposed by other institutions at home and abroad, the new loading technology not only avoids the longitudinal change of the cantilever loading bending moment along the model, but also avoids the mutual interference of the step-by-step loading and the load is not synchronized, ensuring the effective input of the torsional load.
In order to achieve the most comprehensive and accurate test results, the team manually sprayed "leopard spots" on the hull
After the realization of the loading technology, the next difficulty is the measurement of the limit strength, and the "point" measurement data used in the traditional way is not comprehensive enough, so the team began to tackle the high-precision measurement technology of the limit strength.
After extensive study and research, the team selected two new technologies, 3D laser scanning and digital image correlation (DIC) techniques that have never been formally adopted in extreme intensity tests. Both have strict requirements for lighting, model surface chromatic aberration, measurement angle, etc., how to reduce the interference between related test equipment and improve the measurement accuracy according to the characteristics of the hull ultimate strength test, is the team needs to solve the problem.
"When measuring the accuracy of dic, we need to spray the hull with dense 'leopard' points first, and under the resolution of high pixels, we can see the deformation of each leopard point to determine the magnitude of its force." Li Chenfeng introduced. Speckle quality is an important factor to ensure the accuracy of dic measurement, for common planes and surfaces generally use standard speckle tools to make speckles.
Due to the complex structure of the hull, the "leopard point" made by the scattering tool could not be effective, the team decided to manually put on the "leopard dot" load for the hull, the team members continued to try and practice the "strength", "angle", "distance" and "movement speed" of the spraying, when the scattered mass quality evaluation interface appeared purple symbolizing the best quality, the "leopard dot" was successfully made.
The ultimate strength test level of China's hull structure has achieved curve overtaking
"The system is zeroed out, the experiment officially begins, and the loading step 1." At 6 o'clock in the morning of October 13, the test officially began, the loading system was normal, the test system was normal, the data was normal, and the success of the system joint commissioning enhanced everyone's confidence. Halfway through the load, it was nearly noon, the sun was shining through the window high in the laboratory, and suddenly an abnormal data broke the calm, "Scattered spots are not normal, dic is wrong." Graduate student Cai Qinggang shouted.
"Has anyone touched the camera?" Has anyone touched a light source? Has anyone moved the parameter settings? After receiving all the negative answers, the team members checked the mass of the plaque and found that a large purple area was missing, replaced by a gray, indicating that the mass quality was extremely poor.
The experiment was forced to pause, the team members analyzed one by one, one by one, the time passed minute by minute, but the cause of the problem was still not found, and everyone fell into a long silence. "Let's see, the last 2 loading steps of the photo is obviously brighter than the previous one, it is lighting, the sunlight coming in from the window interferes with the dic device." Cai Qinggang made a major discovery after comparing the photos taken before and after dic.
Through testing, it was proved that the quality of the scattered spots was indeed reduced due to direct sunlight, and the previous standard specimen dic suitability test and system joint commissioning were in closed or no direct sunlight conditions, so the problem has not been discovered.
In order to ensure the accuracy of the DIC device measurement, the team decided to re-start the test after dark after the DIC was recalibrated. After nightfall, the extreme strength test of the damaged cabin section under the action of the bending and twisting combination officially began. As the loading level increases, the plastic deformation of the structure gradually appears, the three-dimensional laser scanner obtains the accurate morphology of the model under each loading step, and the dic effectively measures the deformation field and strain field of the deck plate frame. The "High-Precision Measurement Technology for Structural Response Field" based on non-contact measurement has been successfully applied and verified.
At 4:30 a.m., the team members are still insisting, and dare not relax for a moment, because the model limit state of the theoretical forecast is coming, which is a big test of the "four-point eccentric" bending and twisting coupling loading technology, and it is also an assessment of the team's limit strength theoretical forecast level. "Load step 75, add another 1 mm." The data was rising in extremely slow fluctuations, 1 minute, 2 minutes, 3 minutes, and suddenly the data began to fall. This means that the limit state has arrived! "Quickly record, analyze and analyze errors with theoretical values." After 2 minutes, Dr. Zhu Zhiyao, who was in charge of the model design, shouted after repeated calculations and confirmations: "The test value is 209.4, the theoretical forecast value is 216.1, and the error is 3.2%." ”
At the test site, experts from 701, 702, Shanghai Jiao Tong University and Wuhan University of Technology reviewed the test plan: the limit strength test of the damaged hatch under the combination of bending and torsion was a complete success, the large load bending torsional coupling synchronous loading technology based on the "four-point eccentricity" load was effectively verified, and the high-precision measurement technology of structural response field based on advanced non-contact measurement was successfully applied.
Since then, the Xinghai academic team of "Environmental Load and Structural Strength" of Harbin Engineering University has successfully overcome the problems of model design, combined load loading and structural response full-field measurement in the limit strength test, and the overall technical level has reached the international frontier level, so that the ultimate strength test level of China's hull structure has achieved curve overtaking. Li Chenfeng said excitedly, "Since 2004, when I studied graduate school with Teacher Ren, I had the dream of catching up with the international advanced, and he has always encouraged and supported me, and finally realized my dream at this moment." ”
Author: Zhu Hong Reporter Wang Yue Editor: Xu Hui