Tube Hydroforming: System Analysis and Process Design

 PART I 
The THF System 
 
 
 
 
 
 Part I, Chapter 1 
Introduction 
1.1. A brief history of the hydroforming technology 
Increasing use of hydroforming in automotive applications requires intensive research and development on all 
aspects of this relatively new technology to satisfy an ever-increasing demand by the industry. 
Tube Hydroforming (THF) has been called with many other names depending on the time and country it was 
used and investigated. Bulge forming of tubes (BFTs) and liquid bulge forming (LBF) were two earlier terms, for 
instance. 
Hydraulic (or hydrostatic) pressure forming (HPF) was another form of name used for a while by some 
investigators. Internal high pressure forming (IHPF) has been mostly used within German manufacturers and 
researchers. Throughout this paper, the acronym THF will be used to describe the metal forming process 
whereby tubes are formed into complex shapes with a die cavity using internal pressure (which is usually obtained 
by pressurizing water trough an intensifier) and axial compressive forces simultaneously.  
Even though THF process has been in practical industrial use only more than a decade, development of the 
techniques and establishment of the theoretical background goes back to 1940s [Grey et al., ‘39]. 
In 1960s, experimental and theoretical investigations on instability of thin-walled cylinders were performed by 
many researchers at different countries. Fundamental investigations on thin- and thick-walled cylinders helped 
theoretical improvements in LBF operations [Mellor, 1960]. Use of hydrostatic pressure in metal forming 
processes, in particular, for bulging of tubular parts was first reported in the late 1960s [Fuchs, 1966], [Ogura et 
al., 1968]. 
In 1970s, research on different aspects of bulge forming continued both experimentally and theoretically by 
various authors. New shapes, materials, different tooling configurations and new machine concepts were 
introduced, whereas the fundamentals remained the same. For instance, instead of polyurethane, rubber and 
elastomer were used to provide internal pressure [Al-Qureshi, 1970]. 
Starting from 1980s, researchers in Japan concentrated on determining the material properties and their effects on 
tube bulging operations [Manabe, 1983], while several theoretical models for the study of the process and 
appeared [Hashmi et al., 1985]. 
In the late eighties, the process started to spread industrially, especially in Germany, and a lot of work was 
conducted, based on the previous theoretical studies, along with real and new industrial applications of this 
technology [Dohmann et al., 1991].  
In the early 1990s, researchers started utilizing the capabilities of continuously developing FEA and computer 
controls in their experimental and analytical works [Bohm, 1993]. 
In the late 1990s most of the research work has been addressed towards the selection and control of the process 
parameters and the investigation of several possible part types, with a heavy use of FEA [Altan, Koc et al., 1999]. 
Use of FEA for THF process simulations is now a standard development tool. Application of current commercial 
FEA software, such as LS-DYNA, PAM-STAMP, ABAQUS, MARC, AUTO-FORM, DEFORM, etc., for 
stamping and forging processes into THF was performed and presented successfully. Consequent and seamless 
simulation of bending, pre-forming and hydroforming, and sometimes annealing, results in accurate predictions in 
terms of producibility, formability and thinning of the desired part as well as points out necessary changes in tool 
M. Strano, Tube HydroForming: System Analysis and Process Design 
11 
design. In order to shorten the development time and efforts for THF process, supplemental codes and 
techniques are being developed. Adaptive simulation technique, for instance, iterates between appropriate internal 
pressure and axial feeding inputs to ensure a part without any fracture and wrinkles. These techniques are still 
under development and Part II of the present dissertation is focused on them. 
1.2. The system approach 
A system approach is chosen in the present work for the analysis of THF technology. Indeed, both in research 
activities and in design and development of a new Tube HydroForming operation, attention must be paid to 
several aspects and issues of the technology and a system approach to the resolution of problems is highly 
recommended. In other words, when designing a new process, problems and improvements in each area of the 
THF technology and their interaction should be considered. The main components and key issues of a complete 
THF system can be listed as in Figure I- 2. 
For each and every one of the mentioned components of the THF system, several issues should be studied and 
considered for process analysis and design. However, the most critical points for each of the mentioned 
components, from an industrial point of view, are: 
A. determination of quality and material properties of incoming tubes (see Chapter I-2); 
B. design of preform shapes, optimization of pre- (bending, crushing) and post- forming (punching, trimming) 
operations (see Chapter I-3); 
C. determination of guidelines for rapid design of dies and tools (see Chapter I-4); 
D. models for evaluation and prediction of die-workpiece interface conditions (wear, friction and lubrication) 
(see Chapter I-5); 
E. numerical and analytical methods for analysis of deformation mechanics (see Chapter I-6); 
F. design of low cost equipment and press (see Chapter I-7); 
G. evaluation of performance of the hydroformed part (mechanical resistance, springback) (see Chapter I-8); 
The most recent industrial and research trends and activities for each of the listed points will be described in the 
following chapters. Most of the issues presented in Chapters from I-2 to I-4 derive from original ideas and 
research work, which are presented in an appropriate reference scientific framework. Chapter from I-5 to I-8 
result from a deep and prolonged bibliographic study, although the matter is presented with an emphasis on the 
activities carried out at the ERC/NSM and the approach is strongly influenced by the research strategy and 
approach used at the mentioned institute. 
 
 
A. incoming tubes; 
B. preforming and post-
forming systems and 
methods; 
C. dies and tools; 
D. die-workpiece interface; 
E. deformation Mechanics; 
F. equipment and press; 
G. hydroformed part . 
C
Tools /
Dies
A
Incoming
Tube
D
Tool-Workpiece
Interface
E
Deformation
Mechanics
F
Equipment /
Environment / Press
G
Hydroformed
part
B
Bending /
Preforming
 
Figure I- 2: The Tube HydroForming System 
 Part I, Chapter 2 
Properties and Quality of Incoming Tubes 
The quality of the incoming tube is very critical for the success of any hydroforming process. The basic material 
properties (i.e., elasticity modulus, ultimate tensile strength, chemical composition, weld type) and dimensions 
(tube diameter and thickness) of the tube should be determined based on the final part requirements. However, 
for process simulation and development, more information is needed on the mechanical behavior of the material 
and, more precisely: 
• plastic anisotropy, 
• fragility and non-uniformity induced by the weld seam, 
• true stress – true strain diagram in the plastic field, 
• forming limits, 
The plastic anisotropy of sheet metals used to manufacture the tubes can be very important for a successful 
operation, either as a beneficial or as a detrimental factor, depending on the die geometry and the nature of the 
anisotropy.  
Similarly, the quality of welds can strongly influence the performance of the process. The tubes used in THF 
usually do not fail in correspondence of the welding seam, but in other regions, unless the quality of the welds is 
very poor or unless the weld is located in a critical area of tube expansion. Nevertheless, the presence of the weld 
itself inevitably causes a non-uniform distribution of mechanical properties along the circumference. 
The non-uniformity of tubular materials induced by anisotropy and by the welding lines is obviously to be 
considered when designing a THF process. However when using FEA or other design tools for planning a THF 
process, issues concerning the true stress – true strain diagram and the forming limits are far more important. For 
this reason, the following sections of this chapter are focused on these last two points. 
In the current industrial practice of tube hydroforming (THF) operations, very often the mechanical properties 
and the formability of tubes are derived from the tensile test data of the flat sheets used to manufacture the tubes. 
Alternatively, the material data are determined by running a tensile test directly on the tubes, rather than on the 
sheets. 
In both cases, these practices present some drawbacks, as also stated in previous works (see as an example 
[Fuchizawa and Narazaki, 1993]). One disadvantage is that the maximum effective strain value achievable with an 
ordinary tensile test before localized necking occurs is remarkably lower than the effective strain values usually 
reached during the hydroforming process. Furthermore, when using material data obtained by tensile tests of 
sheets, they should at least be corrected to consider the straining due to the bending process used to form the 
tubes. 
For the reasons stated above, several alternative testing procedures and tooling have been proposed so far, like 
the sheet bulge test (extensively described in the literature), the tube bulge test or more complex combined tests 
[Hora et al., 2000]. The hydraulic bulge test for tubes is gaining always more and more attention from the 
hydroforming industry Hydraulic bulge test equipment has been developed by several research institutes, 
hydroforming press manufacturers and tube suppliers. 
M. Strano, Tube HydroForming: System Analysis and Process Design 
13 
2.1. The tube bulge test 
In order to obtain reliable data on material properties of the tube, a test procedure should be used, that is as close 
as possible to the hydroforming process. Although the results of the tensile test can provide information about 
the stress-strain relationship, they can hardly be used to evaluate formability of tubes for hydroforming, since the 
tensile test induces a uniaxial state of stress, while the THF process is mainly biaxial. In other words, a test 
generating a biaxial tensile stress state in the sample (such as a bulging test) would be closer to the real process 
conditions and this would insure a much more effective evaluation of formability1. 
The principle of the bulge test is very simple: a metal tubular specimen is loaded with internal pressure (usually 
hydraulic) and expands, undergoing plastic deformation until bursting occurs. By measuring the internal pressure 
and the tube deformation at the crown of the tube, much information on its mechanical properties can be 
attained. 
Starting from 1980s, researchers in Japan concentrated on determining the material properties and their effects on 
tube bulging operations. [Manabe and Nishimura, 1983] investigated the influence of the strain-hardening 
exponent and anisotropy on forming of tubes in hydraulic bulging and nosing processes. They briefly presented 
the maximum internal pressure as a function of tube radius, thickness, strain hardening exponent, and strength 
coefficient assuming that there was no axial loading. 
[Manabe et al., 1984] published their work on examination of deformation behavior and limits of forming for 
aluminum tubes under both internal pressure and axial force. Axial cylinders and internal pressure were controlled 
by a computer-control-system to obtain pre-defined stress ratio during their experiments. They utilized 
fundamental analysis of thin-walled cylinders in their pre-dictions for internal pressure and axial force. 
Also Fuchizawa [Fuchizawa, 1984], [Fuchizawa, 1990], analyzed bulge forming of finite length, thin-walled 
cylinders under internal pressure using incremental plasticity theory. He presented the influence of strain-
hardening exponent on limits of bulge height. Later, he extended his studies to explore the influence of plastic 
anisotropy on deformation behavior of thin-walled tubes under only internal pressure. He based his analysis on 
deformation theory and Hill's theory of plastic anisotropy. Longitudinal anisotropy was found to be effective on 
the critical expansion limit while anisotropy in hoop direction was affecting the maximum internal pressure 
required. With increasing anisotropy in longitudinal axis, thinning is reduced while obtainable expansion gets 
larger with less internal pressure requirement. Experimental results were eventually compared with theoretical 
findings. Different materials including aluminum, brass and copper were tested in their tooling, which only 
utilized internal pressure in a closed cavity. Assuming that the tube materials obey power law of strain hardening, 
experimental and calculated results were found to be in good agreement. Studies of Manabe and Fuchizawa on 
anisotropy effects were mostly found useful in THF applications involving aluminum products. 
Hydraulic bulging of tubes was also used in determining the stress-strain characteristics of tubular materials by 
[Fuchizawa et al., 1993]. Annealed aluminum, copper, brass and titanium tubes were tested under only internal 
pressure. With the instrumentation and control systems available, tube thickness, radius of curvature in both 
longitudinal and hoop directions, and internal pressure measured and recorded during formation of the bulge. 
Using analytical methods by membrane and plasticity theories, stress-strain relations were derived. These findings 
were also compared with those obtained from tensile tests. Stress-strain relations for aluminum, copper and brass 
were found to be similar by two tests, whereas that for titanium were different. Since they did not use axial 
compressive load during bulging, stress-strain relation obtained was limited to low strain values up to 0.7. 
At the ERC/NSM, tube properties are currently determined by a hydraulic bulge test. A sketch of this test is 
shown in Figure I-4: the tube is locked on both ends and stretched freely using hydraulic internal pressure. 
During each experiment, the internal hydraulic pressure and the maximum bulge diameter are measured 
continuously. These data are used to calculate the flow stress (s ) of the tube material as a function of effective 
strain (e ) in the form of the equation nK )( 0 ees += , under the assumption of isotropic behavior [Altan et al., 
1999], [Aue-u-lan et al., 2000]. 
If a circle grid is etched on the tubular samples before bulging, the test can also be used to determine the 
experimental Forming Limit Diagrams and Forming Limit Stress Diagrams [Strano et al., 2000a] directly from the 
tubes, rather than from the original sheet material roll formed to manufacture the tube. 
                                                 
1
 This concept is well established in the scientific literature. See, as an example [Jevons, 1942].