Cause Analysis and Process Optimization of Non uniformity of FRP Anti current Vibration Carrier Shell

. For improving the test system, it needs to build measuring carrier and optimize shell design. To obtain valid and reliable test data, it needs the measurement uncertainty of test system with 3dB.But the test results show the measurement of results between the inside and outside the shell are quite different. This has great impact on the measurement results. This paper carried out cause analysis from five links of people, machine, material, method, environment for sound transmission performance of FRP housing. It found out the main reason of poor sound transmission of FRP housing, Single side vacuum adsorption molding method have been taken, good results and expected purpose have been achieved.


Introduction
In the underwater radiated noise test system, in order to reduce the interference of the ocean background to the test system, improve the measurement capability and the validity and reliability of the measurement data, It is necessary to build a measuring carrier with anti current and vibration, Optimize the design of hydrophone shell carrier, improve the anti current and vibration performance of the test system. Related research at home and abroad found that FRP materials have light weight, high strength, chemical corrosion resistance, good electrical insulation and sound transmission etc. It can be used as the shell material of anti current and vibration damping carrier. It is found that the thickness of anti current vibration carrier is uneven, to ensure the measurement uncertainty of the test system is within 3dB, Improve the Measurement Reliability of FRP Anti current Vibration Absorbing Carrier, In this paper, the influence of the thickness and non-uniformity of FRP on the sound transmission performance is studied, Optimize the structure design and process flow, improve the yield of structural carrier, and then improve the sound transmission performance of anti current vibration carrier.

Anti current vibration carrier
Anti current vibration carrier shall meet the following requirements: (1) It has good resistance to current and vibration, and it can effectively reduce the influence of self noise of hydrophone.(2) It has good sound transmission performance to ensure the undistorted transmission of acoustic signals to the maximum extent;(3) It can fix the position of hydrophone; (4) It can prevent the hydrophone from being damaged by marine organisms. The schematic diagram of underwater acoustic signal transmission through the anti current damping vibration is as follows.

Analysis of the influence of thickness on sound transmission performance of FRP
To study the influence of thickness on sound transmission performance of FRP, FRP test pieces of the same material are selected for this sample, it selects 3mm, 4mm, 5mm and 6mm thick materials as samples, tested by professional metrology institution, the comparison of sound transmission coefficients of FRP shells with different thicknesses is shown in Figure 2.
The following conclusions are drawn from the investigation of the influence of the thickness on the sound transmission performance of the FRP shell: The thickness of each part of FRP housing is uneven, thickness varies from 3mm-7mm. For broadband noise, the smaller the thickness of FRP is, the higher the sound transmission coefficient is. The sound transmission coefficient value bit of 6mm thick is 0.89, the sound transmission coefficient value bit of 3mm thick is 0.98.  For the non-uniformity of shell thickness, the causes of poor sound transmission of FRP are analyzed from five aspects: man, machine, material, method and environment, it draws the cause tree diagram of the thickness and uniformity of the FRP shell, It analyzes the causes of 7 end causes respectively, the main reasons for uneven shell thickness are obtained, as shown in Table 1.

Causal analysis
Hand pasting method is adopted for initial production of FRP shell, the process flow is shown in Figure 4. When the glass fiber reinforced plastic shell is made by hand pasting method, it uses scraper, brush, scissors and other simple and rough tools to apply resin, it is difficult to apply the resin evenly. At the same time, the thickness and evenness of each part are also unstable, which is easily affected by the skill level of operators, so the stability of product quality is poor. Due to the complexity of the shell, there are many curved surfaces and radians, the groove corners are not fully impregnated with resin by hand pasting process, resulting in dislocation, displacement and uneven distribution of glass fibers, even resulting in uneven thickness of each part. For the shell processed by hand pasting process, the thinnest part is 3mm, the thickest part is 7mm, and the thicker part is mostly found at the corners with radians. After many tests, the shell thickness has been improved to 5mm, but it is still difficult to reach the target thickness of 3mm.

Process optimization
The hand paste process is greatly affected by human factors, it cannot reach the target thickness. It learns from advanced process experience at home and abroad, and combines with the structural characteristics of the anti current and vibration carrier, three optimization processes are proposed, including molding scheme, molding vacuum forming negative pressure method, and singlesided vacuum adsorption molding method, the scheme is analyzed and verified to determine the optimal one.

Molding scheme
The molding method in the molding process is adopted .the metal molding is used to constrain the molding material between the molding surfaces of the two halves of the mold. The molding material is solidified under a certain temperature and pressure.
The molding method adopted in this scheme makes the surface roughness of the test housing reach the ideal level, However, as the filling material and staple fiber are clamped by the mold during molding, the internal situation is invisible, if the problem cannot be solved in time, the void ratio and thickness in this scheme cannot meet the test requirements, and the scheme needs to be further improved.

Method of forming negative pressure by mould pressing and vacuum pumping
Lay glass fiber, mold release agent, filling materials, etc. on the cured gel coat, and seal the edges around the cavity tightly. Vacuum the cavity and inject resin into the cavity. Under vacuum ,the resin flows along the resin pipeline and the fiber surface flows to impregnate the fiber bundle, so as to achieve overall pressure, and the product is solidified and shaped under room temperature or heating conditions. Scheme features: It has improved mechanical properties, The strength, stiffness and other physical properties of the components formed by vacuum induction process can be increased by 1.5 times; It has good repeatability, relatively constant resin ratio of components, low porosity, generally ≤ 1%, which is greatly improved compared with hand pasting process; It is light in weight, with fiber content up to 75-80%, and requires no additional materials to connect the core materials; Environmentally friendly, the vacuum induction process is almost a closed mold forming process, and volatile organic compounds and toxic air pollutants are limited in the vacuum bag. This scheme has outstanding advantages and is very suitable for the manufacturing of test shells. However, due to the lack of manual intervention, some indicators still need to be improved. This method requires upgrading of existing equipment, professional training of operators, and improvement of materials and manufacturing methods. Due to high cost of time and capital, it cannot be completed in a short period of time, so it still needs to be improved on the basis of this scheme.

Single side vacuum adsorption molding method
The method of forming negative pressure by mold pressing and vacuum pumping is improved, it's a liquid pressure method, the process advantages of scheme II are retained, and the production process of carbon fiber is used for reference, the outer surface of the shell is vacuumized on one side, and the inner surface uses a vacuum bag to adsorb the material onto the shell, at the same time, manual intervention shall be carried out during the curing process. Once problems are found, timely intervention shall be carried out to finally cure and shape. Scheme features: The time cost and capital cost are greatly reduced, and the shell required for the test can be manufactured.
Through group discussion and comparison, the first method is too rough, and the sound transmission performance of the FRP shell produced by processing is not improved greatly, which still cannot meet our test requirements; The improvement effect of Scheme II is the best. However, due to the need to purchase expensive equipment, the high capital cost and time cost of investment, and the lack of human intervention in the processing of glass fiber reinforced plastic shell, it is easy to produce defective products, so it is not considered. Finally, the team members agreed to adopt Scheme IIIsingle-sided vacuum adsorption molding method, which is easy to operate and has low investment cost. The thickness of the FRP shell can be controlled within 3mm, and the shell surface is also very uniform.

Effect verification
Single side vacuum adsorption molding method is adopted for construction, the effect of samples is checked, the measuring points at typical positions on the shell surface are selected for measurement, the thickness of each position of the improved process is shown in the figure below, After implementing the new scheme, the gap disappears, the thickness is between 2.8mm-3mm, the deformation is small, the roughness is about 3.2 µ m, the sound transmission coefficient is 0.98-0.99, the sound transmission performance is excellent, which meets the technical requirements, and can be used as the construction process of anti current and vibration reduction carrier.

conclusion
In view of the problem that the initial construction process is greatly affected by human factors, the single-sided vacuum adsorption molding method is adopted by drawing on the advanced achievements at home and abroad, the thickness of the fabricated FRP shell is uniform within 3mm, the sound transmission coefficient reaches 0.98-0.99, High sound transmission and comprehensive index requirements for anti current and vibration damping performance are realized.