| Skin parts are external parts of aircraft, whose surface smoothness and shape accuracydirectly affect the flight performance of the aircraft. So, attention should be paid onmanufacturing quality of skin parts. Stretch forming (SF) machine plays a key role inproducing skin parts. Unfortunately, SF machines used at home are mainly imported fromoverseas, which have the following disadvantages such as outdated and high cost, as well ashave bad conformability during the forming process and low material usage. Moreover, one ormore steps of heat treatments are required when forming large or complicated curvature parts,which would lead to a low forming efficiency and result in a poor forming quality. Therefore,to develop SF machine with independent intellectual property right also guarantee highforming precision has a profound significance on industrial application and national defence.High flexible stretch forming (HFSF) is a kind of technological innovation of modernsheet metal forming process, aims to reduce equipment development cost, improve workpieceforming quality and save manufacturing cost. HFSF machine differs from the traditionalstretch forming (TSF) machine by the fact that straight jaws in traditional SF machine aresubstituted by a pair of discrete clamping mechanisms, and the special cylinders aresubstituted by general cylinders. Through these improvements, HFSF could realize adaptiveadjustment between sheet metal deformation and motion of loading mechanisms, and lead to apart with more homogeneous stress and strain distributions. It should be also pointed out thatHFSF could obtain a better conformability of sheet metal, as well as increase material usage,reduce manufacturing cost and improve forming quality.With the rapid progress of continuous improvement on computer hardware and numericalsimulation technology in the past few decades, numerical simulation has become an importantway in exploring new products for some enterprises and research institutions. Stress and strainvariations during the forming process can be addressed from all aspects by applying numericalsimulation in HFSF. Furthermore, numerical simulation could be also utilized in predicting forming defects and optimizing processing parameters. In this paper, numerical simulation wasadopted to investigate the influences of transition length and loading sequence of hydrauliccylinders on forming results. In addition, thickness and layer of elastic pad, as well as frictioncoefficients on forming results in HFSF were explored. Meanwhile, incremental high flexiblestretch forming (IHFSF) was proposed aimed at eliminating wrinkling in HFSF, and relevantinvestigations on IHFSF have been conducted through numerical simulation.The main contents and conclusions are summarized as follows:1. Research on forming principle, mechanical and adaptive adjustment processes ofHFSFForming principle and technology characteristics of TSF were analyzed, which pointedout that the bad sheet metal conformability together with low material usage were twoimportant factors limiting the application range of TSF. The forming principle, equipmentstructure and characteristics, together with application range and prospect of HFSF wereexplored detailtedly. Mechanical process of HFSF and the evolution processes of stress andstrain along the thickness section during the forming process were analyzed. Self-adjustmentprocess of HFSF was analyzed, and the results pointed out the elastic connection elementwould not only effectively improve the flow state of the sheet contact with the clampingmechanisms (decrease material failure), but also drive the clamping mechanisms to realizelateral rotation (drive edge portion of sheet metal to conform to the shape of forming die).2. Establishment of the finite element model of HFSFImplementation procedures for dynamic explicit and static implicit algorithms inABAQUS were introduced. Springback model of HFSF and its required boundary condtionswere explored. FE models of HFSF were built and the related material model, elements,contact and boundary conditions were considered. In addition, friction coefficients at theinterfaces of different contact pairs (polyurethane rubber-polyurethane rubber, polyurethanerubber-45#steel,45#steel-45#steel) under different lubricant conditions were measured usingUniversal friction tester, providing data support for the following numerical simulations.3. Research on the forming process of HFSF and transition lengthComparative analyses on TSF and HFSF processes were conducted through numericalsimulation. The results showed HFSF could remarkably improve sheet metal conformabilitycompared to TSF. Influences of transition length on forming force, stress and strain, thicknessand forming precision of formed parts were investigated. And the results revealed that theforming force decreases with the transition length decreased, and the uniformities of stress andstrain increase with the transition length decreased. Besides that the results also indicated that uniformity of strain distribution was an important factor in affecting the forming precision. Inthe end, related experiments were carried on self-developed apparatus, which showed a goodcorrelation with the simulation results.4. Research on friction coefficients and loading sequences of hydraulic cylinders inHFSFInfluences of horizontal-vertical (HV) and horizontal-tilting-vertical (HTV) loading pathson forming results were studied, as well as the influences of friction coefficients on formingresults in both loading paths. Investigations of pre-stretch force on strain and formingprecision in HV loading path were studied, and the results revealed that the vertical forcesexerted by vertical cylinders are almost at the same level when the pre-stretch force varied in acertain range. Furthermore, the results also indicated the maximum strain in the forming zoneof the simulated part increases with the pre-stretch force increased, as well as a pre-stretchforce made the sheet blank into the initial yield point would give a higher forming precision.Analyses of wrapping force on strain and forming precision in HTV loading path werenumerically investigated. The results indicated that the maximum strain in the forming zoneincreases with the tilting force decreased. It is also found the wrapping force has a significantsignificance on forming precision. Friction coefficients on strain, thickness and formingprecision of simulated part were investigated. The results indicated that the maximum strain inthe forming zone decreases with the friction coefficient increased, while the strain uniformityincreases with the friction coefficient increased. It was also found that the friction coefficientin HTV loading path has a weak influence on forming precision compared to that in HVloading path.5. Research on wrinkling mechanism of HFSF and wrinkle depression methodsWrinkling mechanism of HFSF together with wrinkle depression methods was explored.Incremental high flexible stretch forming (IHFSF) was proposed, and its forming process andwrinkling mechanism were studied. In addition, target shape construction of multi-point die(MPD) in IHFSF was explored. The forming process of IHFSF was studied by taking thehyperbolic paraboloid surface part as the research object through numerical simulation. Theresults indicated that the wrinkling defect could effectively eliminated by IHFSF. Aimed at thespringback in IHFSF, springback compensation based displacement adjustment method (DAM)was investigated to show their influences on forming precision, as well as the springbackcompensation factor on forming results. The numerical results indicated that the formingprecision of simulated part can be significantly improved by using DAM just once. It was alsofound that the compensation factor equals to1.1would give a higher forming precision. 6. Influences of elastic pad on forming results in HFSF based on multi-point dieTwo kinds of evaluation methods of sheet metal conformability were explored, and theadvantages and disadvantages for both methods were summarized. Influences of elastic padhardness were studied by taking spherical surface part as research object. The results indicatedthat a lower hardness would cause a larger shape distortion of formed part. However, a higherhardness would cause a larger strain in the final product. Influences of elastic pad layer onforming results were investigated under the same total thickness of elastic pad. The resultsshowed that the instability of elastic pad decreases with the layer increased. However, usingjust one layer of elastic pad would cause difficulties in making sheet metal to conform to theshape of the forming die. Influences of friction coefficient at the interfaces of sheet blank–MPD, elastic pad–elastic pad, elastic pad–elastic pad on forming results were numericalinvestigated by using three-factor and three-level orthogonal test. The results revealed thatfriction coefficient between elastic pad and MPD was the most significant factor in affectingforming precision, followed by the coefficient between sheet blank and elastic pad, and thenthe coefficient between elastic pad and elastic pad. |
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