The metallic sandwich plate is composed of a relatively low-density core placed between two face sheets. Due to their lightweight characteristics and load-bearing properties with multi-functionality, metallic sandwich plates can be applied to diverse areas. They can be used with energy absorbers, thermal isolators, heat exchangers, sound dampers, catalyst supports, and current collectors. Despite many benefits, expanding their practically applicable areas remains a challenge. Sandwich plates cannot be deformed efficiently by commercial forming processes due to their low formability. The development of a deformable sandwich plate could not only expand the range of application, but also improve the bending characteristics due to the ability to prevent the local failures that weaken load-bearing resistance. In order to design a bendable sandwich plate, the axiomatic design theory is employed as a design methodology. The functional requirements were defined, and then the design parameters were established by the conceptualization process, numerical forming windows, analytic design maps, U-bending analysis, and 3-point bending analysis.
A sandwich plate with a pyramidal truss core was determined as the object for a basic parametric study. An efficient method that employs the virtual equivalent projected model (VEPM) is introduced and formulated to simulate local failures in a sandwich plate. This approach is based on two-dimensional FE-analysis that uses the projected shapes of 3D pyramidal truss cores and utilizes virtual properties from the homogenized equivalent elastic-plastic properties of the original material and geometries. The results of simulations have shown that the load-displacement curves and deformed shapes with local failures of the VEPM are analogous to 3D pyramidal truss cores irrespective of the materials and core geometries. In addition, the computational time was greatly reduced.
Numerical design procedures using the VEPM were carried out to design bendable sandwich plates. U-bending processes were simulated with respect to various ranges of geometric parameters. Optimal geometric parameters and safe design zones for DP590 sandwich plates with the total thickness of 3 mm and a face thickness of 0.5 mm were determined in terms of die radius in the forming windows. However, it is difficult to fabricate the designed pyramidal truss cores, when the crimping process is used to create pyramidal truss cores from the expanded metal. From the viewpoint of the axiomatic design, a pyramidal truss core has a disadvantage, because coupled design matrix was constructed.
Bendable sandwich plates with a sheared dimple core were newly introduced and designed. The core shear strength increased as the gap between the attachment points decreased. This characteristic produces benefits, such as preventing face buckling and core shear failure. A multi-point resistance welding process was employed as a bonding mechanism for deformable sandwich plates with sheared dimple cores to improve productivity. During U-bending, a de-bonding failure occurred due to the shear deformation of the core. An analytic study that investigated the bending mechanism was carried out to determine the control parameter of core shear stress. The deformed shapes of a sandwich plate were described as a combination of straight lines and arcs. Homogenized properties of core shear stress- shear strain curves calculated by using pure shear FE-analyses were used in the analysis. For numerical verification of the proposed analysis, the U-bending simulations were carried out for sandwich plates with various inner structures including a pyramidal truss core, a honeycomb core, corrugated cores, and a sheared dimple core. The analytic analysis enables the acceptable prediction of the load curves and deformed shapes during U-bending. The analytic estimation revealed that the shear stress of the core could be reduced drastically by increasing the clearance between the upper die and the lower die. In addition, from the U-bending analysis, the de-bonding behavior with respect to clearance could be effectively predicted. The fabricated sandwich plates could be bent without failure when the clearance was larger than the thickness of the sandwich plate by three times.
The bending responses of metallic sandwich plates with a sheared dimple core were investigated through 3-point bending tests and dynamic plane bending tests. Analysis for large deformation of a sandwich plate subject to 3-point bending was carried out, where friction work was considered and a face sheet was treated as a rigid-plastic material with Hollomon’s flow rule. Load-deflection curves and the deformed shapes of the simulation and the experiment were efficiently predicted by the proposed analysis. The effect of friction work on the peak load and energy absorption increases as the bending angle increases. The dynamic bending responses of the sandwich plates, such as the effect of a deformed shape, failure mode, energy absorption, and absorption ratio were compared with those of a SPFC780 high-strength steel sheet. It was demonstrated that a bendable sandwich plate was stronger construction than a high-strength steel sheet under the tested conditions, despite the weight reduction and weaker properties.
In the axiomatic design, decoupled design matrix was established by employing sheared dimple cores. It was experimentally shown that all of the functional requirements related with productivity, weight reduction, bending performance, and bendability were satisfied by the defined design parameters.
In order to demonstrate the industrial effectiveness of the sandwich plates, a front bumper back beam with the designed sandwich plate is fabricated by roll forming and stretch bending. The impact test for the measurement of intrusion and deflection was carried out. The criteria of the domestic company were satisfied, while the weight of the front bumper back beam was reduced by approximately 12%. It was shown through the fabrication of a front bumper back beam that the practicality of the welded sandwich plates can be assured through the use of lightweight, load-bearing and multifunctional materials.
금속샌드위치판재는 초경량, 고강도, 고강성, 고기능성 판재로서 연구되고 있으며 다양한 요구조건을 충족할 수 있도록 공정변수와 설계변수의 변경이 용이하고 대량생산이 가능하다는 장점이 있다. 그러나, 많은 장점에도 불구하고 일반박판에 비해서 나쁜 성형성으로 인하여 그 적용 및 응용 범위가 제한되고 있다. 본 연구에서는 성형 가능한 금속샌드위치판재를 개발하여 샌드위치판재의 적용 및 응용범위를 넓히고 자 한다.
기초연구대상으로 피라미드형 트러스 코어를 분석하여 성형이 가능한 설계조건을 도출하였다. 유한요소 해석은 3차원 구조체를 2차원 투영형상과 가상물성을 이용하여 효율적으로 해석하였으며 해석기법은 해석적으로 비교 및 검증하였다. 제안된 가상등가 투영형상을 이용한 2차원 해석기법은 3차원의 변형형상, 하중선도, 결함모드를 거의 동일하게 예측하면서 해석시간은 최대 $\frasl{1}{100}$ 수준으로 단축시킬 수 있는 효율적 해석기법임을 보였다. 성형지도(Forming Window)를 이용하여 트러스 코어의 설계조건을 정량화하였다. DP590 0.5t 표면판재를 가지는 두께 3mm 샌드위치 판재를 성형하기 위해서는 상대공간밀도 26.4% 이상 되는 DP590 트러스 코어가 접점간 틈새간격 1.5mm 로 배치되어야 하며 이때 임계곡률반경은 9mm 이다. 트러스 코어는 점점간 틈새간격이 좁아짐에 따라 코어의 상대공간밀도가 증가하고 단위무게당 전단강도가 감소하는 경향이 있어 성형성 측면에서 불리한 구조이며 설계된 샌드위치판재는 절곡공정을 이용하여 제작하기에 어려운 치수이다.
따라서, 설계조건을 다시 균질화하여 일반적을 내부구조가 가지는 등가물성으로 설계조건을 변경한 후, 설계조건을 만족하는 전단형(Sheared Dimple) 내부구조의 형상 및 치수를 설계하였다. 접점간이 좁아지더라도 코어의 상대공간 밀도가 크게 변하지 않는 전단형(Sheared dimple) 내부구조를 개발하였고 접점간 틈새간격 1.48mm, 등가전단강도 62.2Mpa 보다 크게 설계하였다. 이론적 디자인맵(Design Map)을 이용하여 설계조건을 검증하였으며 유한요소 해석결과 설계된 샌드위치판재가 표면판재좌굴, 표면판재파단, 내부구조전단결함의 구조적결함이 발생하지 않는 것을 확인할 수 있었다.
설계사양에 맞게 샌드위치 판재를 제작하여 U-벤딩 실험을 통하여 설계 검증한 결과 곡률 반경 10mm 로 굽힘성형실험에서 접합부 파단이 발생하였다. 공정변수를 제어하여 접합부 파단 및 표면판재 파단, 내부구조 전단결함이 발생하지 않는 조건을 도출하기 위해서 샌드위치판재의 굽힘성형을 이론적으로 분석하였다. 샌드위치판재의 변형형상을 호와 직선의 조합으로 가정하였으며 내부소산일율이 최소가 되도록 하는 변형형상을 수치적으로 계산하였다. 표면판재의 굽힘변형모드와 내부구조조의 전단변형모드 그리고, 마찰에 의한 일율을 고려하였으며 미세증분에 대하여 점진적으로 변형 스템을 증가시키며 해를 구하였다. 이론식은 유한요소 해석와 실험과 비교 분석한 결과, 제안된 이론식이 샌드위치판재의 U-벤딩공정에서의 하중선도와 변형형상을 비교적 잘 예측할 수 있음을 확인하였다. 변수민감도분석을 통하여 틈새간격이 좁아짐에 따라 샌드위치판재의 전체두께편차에 따라 내부구조가 받는 최대전단력에 대한 민감도가 크게 증가하는 것을 확인하였다. 틈새간격을 조절하여 접합부파단을 방지할 수 있다는 결론을 얻었고 틈새간격을 조절하여 실험한 결과, 틈새간격이 9mm 에서 접합부 파단없이 곡률반경 10mm로 성형 가능하였다.
제안된 전단형(Sheared dimple) 내부구조의 샌드위치판재를 범퍼벡빔에 적용하기 위해서 기존범퍼강이 SPFC780 1.8t 소재와 정적/동적 굽힘성능을 비교 분석하였다. 그 결과 샌드위치판재가 최대하중, 에너지 흡수량, 처짐량 측면에서 더 우수한 성능을 보임을 확일할 수 있었다. 단순지지 굽힘조건에서 샌드위치판재의 굽힘거동을 이론적으로 해석하였으며 제안된 이론식이 유한요소해석결과와 실험결과를 잘 예측하였다. 특히, 최대하중과 변형에너지를 예측하는데 효과적임을 입증하였다. 그리고, 25단 롤포밍 금형으로 샌드위치 범퍼빔을 제작하여 충격 실험을 실험을 실시한 결과, 국내회사의 처짐량과 밀림량에 관한 제품규격을 만족하면서 약 12% 경량화를 달성하였다.
따라서 이 구조는 굽힘 변형으로 제작될 수 있는 구조물에 바로 적용될 수 있으므로 샌드위치 판재의 적용 및 응용 범위를 크게 확대시킬 수 있는 계기가 될 것으로 기대 된다.
