HYDRAULIC BULGE TESTING TO COMPARE FORMABILITY OF CONTINUOUS AND STRETCH BROKEN CARBON FIBER PREPREG LAMINATES

The use of carbon fiber reinforced polymer composites has increased with the increased need for high-strength, low-density materials, particularly in the aviation industry. Stretch broken carbon fiber (SBCF) is a form of carbon fiber created by the randomized breaking of aligned fibers in a tow at inherent flaw points, resulting in a material constituted of collimated fiber fragments longer than chopped fibers. While continuous carbon fibers possess desirable material properties, the limited formability prevents their wider adoption. SBCF composites exhibit pseudo-plastic deformation that can potentially enable the use of traditional metal forming techniques like stamping and press forming well established in mass production applications. To investigate the formability of SBCF composites prepared with either continuous or stretch broken Hexcel IM-7 12K fiber, impregnated with Huntsman RDM 2019-053 resin, hydraulic bulge testing was performed to explore the strain behavior under biaxial stress conditions at elevated temperature under atmospheric pressure. Initial results show better formability of SBCF compared to continuous fiber, characterized by the axisymmetric response to the applied stress.

Inverse Identification for the Balanced Biaxial Yield Stress of AA5182-O Alloy Sheet

Advanced yield functions, such as Yld2004, could describe the elastic boundary of materials better than the traditional. However the balanced biaxial yield stress σb which is essential to determine the parameters of advanced yield functions is hard to measure using frequently used test equipment. This work presented an inverse method to calibrate σb of AA5182-O alloy sheet based on the Erichsen test. The maximum punch force (MPF) measured from this test was used for the inverse identification. A modification coefficient was used to drop down the simulation MPF from shell element, as the application of shell element result in higher simulation punch force. Then the relationship between σb and MPF was established based on the plane stress Yld2004. With this relationship and the real measured MPF, σb could be inversely identified. Additionally, a hydraulic bulge test was performed to verify the accuracy of this inversely obtained σb.

Influence of the Laser Heat Treatment on the AA5754-H32 strain path during hydraulic bulge tests

The hydraulic bulge test represents an effective experimental method to characterise sheet metals since the equivalent strains before failure are much larger than those measured during tensile testing and there is nearly no frictional effect on the results. Recently this test has been proposed not only for extracting data concerning the equi-biaxial strain condition, but to determine the forming limit diagram (FLD) in the range of positive minor strains. In the proposed methodology, different strain paths can be obtained by merely using a test blank having two holes with a suitable geometry and position to be tested, without the need of dies with elliptical apertures. However, a carrier sheet is necessary, thus implying results may be affected by friction effects. This paper proposes a new methodology for the determination of the right side of the Forming Limit Curve (FLC), based on the adoption of local heat treatments aimed at determining different strain paths on the blank to be tested while using the classical circular die for bulge tests. In particular, the formability of the alloy AA5754-H32 was investigated; 3D Finite Element simulations were conducted setting different laser strategies and monitoring the resulting strain path. Results revealed that the proposed methodology supports obtaining many additional points in the right side of the FLC, thus being effective and friction free.

The split Hopkinson bar bulge setup: a novel dynamic biaxial test method

In sheet metal forming, very often, large plastic deformations are imposed to a thin plate. An accurate description of the material’s elastoplastic response is therefore of paramount importance to perform finite element (FE) simulations of an actual forming operation. Reliable stressstrain data till significantly larger strains compared to tensile tests can be identified by means of bulge test. In this work, a dynamic hydraulic bulge test is proposed. The novel split Hopkinson bar bulge setup, combines features of classical split Hopkinson pressure bar (SHPB) and hydraulic bulge tests. The special configuration of the Hopkinson bars leaves the sample surface fully accessible. As such, high-speed optical measurements can be performed on the sample surface allowing the application of, for instance, digital image correlation (DIC) for full-field displacement strain mapping. The potential of the facility is explored by performing experiments on 0.8mm thick Al2024-T3 sheet.

Formability prediction of tailor-welded blanks in hydraulic bulging using flow curves from biaxial tensile tests

In this work, the formability of laser-welded tailored blanks of low carbon steel of two different thickness combinations in hydraulic bulging has been studied by numerical simulation. For material modeling, flow curves of the parent sheets were obtained in biaxial stress condition by conducting hydraulic bulge tests. These curves were used to extrapolate the uniaxial tensile curves up to large strains using the work equivalence principle. The limiting dome height in conventional forming and hydraulic bulging of tailor-welded blanks has been predicted in finite element simulations using the flow curves obtained directly from the hydraulic bulge tests and the extrapolated uniaxial tensile curves. Hydraulic bulging and conventional forming experiments on tailor-welded blanks have also been conducted to validate the predicted results. It has been found out that the predicted limiting dome height of the tailor-welded blanks in conventional forming and hydraulic bulging using extrapolated uniaxial tensile curves is closer to the experimental values when compared to the results obtained by using stress–strain curves obtained from hydraulic bulge tests. It has also been found that using an extrapolated uniaxial tensile curve it is also possible to predict strain distribution and percentage thinning more accurately. It has been observed that with an increase in thickness ratio, the peak pressure increased but the predicted values of peak pressure using flow curves obtained directly from hydraulic bulge tests are closer to the experimental values.