Conical Punch Research Articles

Comparison of hole expansion ratios in a dual phase steel for holes prepared by conventional punching and fine piercing

Dual phase steels generally exhibit poorer stretch flangeability compared to lower strength steels. Stretch flangeability is normally evaluated using hole expansion testing, wherein a specimen with a 10 mm central hole is expanded using a conical punch till onset of cracking at the edge of the hole. The percentage increase in hole diameter is referred to as the hole expansion ratio (HER). In this study, hole expansion ratios of a DP 600 steel were evaluated. Holes were prepared on a standard 90 mm x 90 mm specimen using two different hole preparation conditions: i) conventional punching where the hole edge exhibits both fracture and shear regions and ii) fine piercing where the hole edge exhibits almost completely smooth shear. Hole expansion testing was carried out on these specimens as per standard ISO 16630. The specimens with holes produced by fine piercing exhibited lower values of HER in spite of having a hole edge with a smooth surface. The specimens with holes prepared by conventional punching exhibited higher HER values. The influence of strain hardening of the cut edge during hole preparation and the surface roughness of the cut edge on the HER are discussed.

Effect of meshing technique and time discretization size on thickness strain localization during hole-flanging simulation of DP980 sheet at high strain level

In the simulation of the hole-flanging process, many had reported that the effect of mesh sensitivity on strain localizations is insignificant. In this study, mesh layouts including the number of elements along the hole circumference (CD), element sizes in radial directions (R1) and the time discretization (or increment) sizes (T) are varied in the 3D FE simulation of 10-mm hole flanging process of DP980 thin sheets using a conical punch. Its effect on wall thickness distributions and forming load profiles are investigated. Also, the effect of punch surface meshing techniques i.e., analytical and discrete rigid on the distributions is investigated. The simulation results showed that the largest % wall thinning range around the expanded hole edges for punch displacement, S = 15 mm and 20 mm are 25.8–29.2% and 33.3–64.2%, respectively. The load-displacement curves during the continuous hole expansion are not linear for R1 ≥ 0.5 mm. For CD70 & R1 = 0.1 mm, the increase in T values from 0.1 s to 1.0 s has significantly increased the total simulation time by 25% due to the convergence problems. The automatic time increment mode in each step at T = 1.0 s has failed to reduce the total simulation time. The discrete rigid punch surface produces more concentrated wall thickness distributions than the analytical one, leading to higher amount of wall thinning, particularly for high S values. Since the model stiffness reduces with decreasing mesh size. It is recommended to match the simulated peak load with the experimental peak under the instability condition with a smooth hole e.g., a laser cut hole to determine the appropriate meshing method for the mesh sensitivity study. With that, the stiffness of the simulation model is calibrated with the experiment.

Numerical and experimental investigation of hole flangeability of AA6061 alloy

Hole flangeability is a measure of edge formability or local formability, and it is important for forming processes like stretch flanging. Hole flangeability of sheet metals is evaluated by determination of hole expansion ratio (HER) using a standard hole expansion test (HET). In the present work, numerical and experimental investigation of hole flangeability has been carried out on 1.6mm thick AA6061 alloy sheets. The material model plays an important role in the accuracy of predicted results in numerical simulations. The simulations have been carried out using two different work hardening laws (Swift and Voce). The failure has been predicted using the experimentally determined forming limit curve (FLC) of AA6061 alloy sheets. HER and thickness have been predicted along the edge of the expanded hole by simulation. The predicted HER and thinning from FE simulation have been validated experimentally by performing hole expansion tests according to ISO 16630-2009 using a conical punch. The predicted results have been found to be in good agreement with the experimental results.

Hot indentation of a FGM-coated thermoelastic half-space by a conical punch: Approximated analytical solution of the contact problem

Hot indentation of a thermoelastic coated half-space by a rigid punch is considered. The coating is assumed to be functionally-graded with all group of properties arbitrary varying with depth. The deformation of the half-space caused by the simultaneous mechanical and thermal loading is considered. The punch is subjected to a normal centrally applied force and preliminary heated to a uniformly distributed temperature. The contact problem is reduced to the solution of a system of dual integral equations over two unknown functions: Hankel transform of normal stresses and heat flux in the contact area. Heat flux is obtained using the bilateral asymptotic method. This method was sufficiently generalized to take into account additional function describing the displacements of the surface caused by thermal heating and obtain contact stresses. The solution is obtained in an analytical form which clearly demonstrates the dependence of the contact characteristics from the initial physical parameters of the problem. Numerical analysis is provided to illustrate the effect of the presence of the coating and its thickness on the results. Differences between the homogeneous and functionally-graded coatings is illustrated.

The Effect of Adhesion on Indentation Behavior of Various Smart Materials

The nanoindentation technique plays a significant role in characterizing the mechanical properties of materials at nanoscale, where the adhesion effect becomes very prominent due to the high surface-to-volume ratio. For this paper, the classical adhesion theories were generalized to study the contact behaviors of various piezoelectric materials indented by conical punches with different electric properties. With the use of the Hankel integral transform, dual integral equations, and superposing principle, the closed-form solutions of the physical fields for the Johnson-Kendall-Roberts (JKR) and Maugis-Dugdale (M-D) models were obtained, respectively. The contribution of the electrical energy to the energy release rate under the conducting punch was taken into consideration. The relationships between the contact radius, the indentation load, and the indentation depth were set up using the total energy method for the JKR model and the Griffith energy balance for the M-D model, respectively. Numerical results indicate that increasing the half cone angle of the conical punch enhances the adhesion effect, which can significantly affect the accuracy of the results of characterization in nanoindentation tests. It was found that the effect of electric potential on adhesion behaviors is sensitive to different material properties, which are not revealed in the existing studies of axisymmetric adhesive contact of piezoelectric materials and multiferroic composite materials. The load-displacement curves under conical punches with different half cone angles have very different slopes. These results indicate that the half cone angle has a prominent effect on the characterization of mechanical properties of piezoelectric solids in nanoindentation tests.

Experimental investigation of hole expansion ratio for automotive-grade steels

Hole expansion ratio (HER) is widely used to quantify the stretch-flangeability of sheet metal. HER is determined from the maximum limit of successful expansion of a central hole by a conical punch. The central hole edge is prepared by the punching process. Around 45° cracks are noticed at the central hole edge after successfully completing the hole expansion test. HER of punched hole correlates with uniaxial tensile properties like yield strength, ultimate tensile strength, total elongation, post-uniform elongation, and coefficient of normal anisotropy. Comparisons of strength (yield strength, ultimate tensile strength), anisotropy (coefficient of normal anisotropy) and deformation (total elongation, post-uniform elongation) parameters among steel grades are essential to relate HER among steel grades. Interstitial free steel is the highest, and SPFH steel is the lowest HER among the four steel grades. HER correlates nicely with the notch mouth opening displacement at peak load.

Effect of Different Conical Punch Angle Geometries and the Initial Hole Diameters on the Hole Expansion Ratio of DP steels

In the recent decade, advanced high strength steels (AHSS) have gained a great popularity in the automotive manufacturing industries due to their high strength to weight ratio, which significantly improves the safety of the manufactured automobiles while reducing the weight and thus, enabling to improve the fuel efficiency. However, it is known that some types of AHSSs, especially DP steels, are highly susceptible to edge cracking behaviour during the forming operations. Edge cracking behaviour is generally investigated with a 600 conical punch as suggested by the ISO 16630 standard. However, in this study, to observe the behaviour of edge cracking ability of DP steels under different conical punch angles for different initial hole diameters, hole expansion tests have been performed with conical punches with three different angles (300 ,600, 900) for three different initial hole diameters (14, 16, 18 mm). The results have shown that the hole expansion ratio (HER) does not differ considerably with the variation of the conical punch angle and the initial hole diameter due to low fracture strain of DP steels observed after hole expansion tests. The major factor for the edge stretching ability of DP steels have been observed to be microstructure rather than geometrical factors such as conical punch angles.

Theoretical studies of reverse extrusion of steel pipe by conical punch

The reverse extrusion operation of steel pipe by a conical punch for manufacture of semi-finished parts of the "body" type with a difference in wall thickness along the height is considered. Computer simulation and mathematical modelling of this process is carried out. Rational modes of deformation during reverse extrusion of pipe blanks are established.

Tube Expansion by Single Point Incremental Forming: An Experimental and Numerical Investigation

In this paper, we revisit the formability of tube expansion by single point incremental forming to account for the material strain hardening and the non-proportional loading paths that were not taken into consideration in a previously published analytical model of the process built upon a rigid perfectly plastic material. The objective is to provide a new insight on the reason why the critical strains at failure of tube expansion by single point incremental forming are far superior to those of conventional tube expansion by rigid tapered conical punches. For this purpose, we replaced the stress triaxiality ratio that is responsible for the accumulation of damage and cracking by tension in monotonic, proportional loading paths, by integral forms of the stress triaxiality ratio that are more adequate for the non-proportional paths resulting from the loading and unloading cycles of incremental tube expansion. Experimental and numerical simulation results plotted in the effective strain vs. stress triaxiality space confirm the validity of the new damage accumulation approach for handling the non-proportional loading paths that oscillate cyclically from shearing to biaxial stretching, as the single point hemispherical tool approaches, contacts and moves away from a specific location of the incrementally expanded tube surface.

Expansion of Additive‐Manufactured Tubes: Deformation and Metallurgical Analysis

Herein, the mechanical and metallurgical feasibility of integrating tube end‐forming operations with additive manufacturing is investigated. The work makes use of wire‐arc additively manufactured AISI 316L stainless steel preforms that are subsequently machined into tubes and expanded with a tapered conical punch. Experimental measurements of force, surface strains, thickness, and microhardness combined with microstructure observations and fractography of the fractured surfaces are utilized to characterize plastic deformation and formability limits of the additively manufactured tubes and understand the main differences against the results obtained from commercial wrought tubes of the same material. Results show that the material deformation characteristics, namely, the evolution of microhardness along the expanded tube length, and the formability limits by necking and fracture, are strongly influenced by the columnar microstructure originated by a noncyclic dendritic growth aligned with the building direction. Still, results demonstrate that the additively manufactured AISI 316L tubes are ductile enough to be successfully included in hybrid additive manufacturing routes.

The effect of strain hardening on the limit forming ratio on the single-step incremental hole flanging process on aluminium plates

Hole-flanging is the process of forming a plate to produce a collar around the hole. This formed collar has several important functions to increase the bonding surface area, as a supporting medium for other components, as a guide, increase construction rigidity, etc. This collar is generally formed through conventional hole flanging which involves making the initial hole in the plate, then punch with a certain profile and a larger diameter pressing the plate in one step of the forming process. This forming process will cause the deformation of the plate so that a collar is formed around the hole. In recent studies, it can be found that the collar can be formed not only with one stage of the conventional hole flanging process, but can also be achieved by a multi-stage incremental hole flanging process using a punch with a multi-purpose general profile. In this research, an experimental study was carried out on the effect of ‘strain hardening’ on the limit forming ratio (LFR) in the single-step incremental hole flanging process on aluminium plates using conical punch. The research hypothesis shows the influence of ‘strain hardening’ on the limit forming ratio (LFR) in the single-step incremental hole flanging process.

Determination of Young’s modulus of samples of arbitrary thickness from force distance curves: numerical investigations and simple approximate formulae

We present simple expressions for load required to indent a layer of arbitrary thickness with a conical, paraboloidal or cylindrical punch. A rigid substrate underneath the sample leads to an increase of load required for indentation. This effect has to be corrected for to prevent overestimation of Young’s modulus from indentation measurements, such as force - distance curves recorded with the Atomic Force Microscope (AFM). The problems of the frictionless contact of an axisymmetric punch and an isotropic, linear-elastic layer are reducible to Fredholm integral equations of the second kind. We solved them numerically and used the Remez algorithm to obtain piecewise polynomial approximations of the load - indentation relation for samples that are either in frictionless contact with the rigid substrate or bonded to it. Their relative error due to approximation is negligible and uniformly spread. Combining the numerical approximations with asymptotic solutions for very thin layers, we obtained equations appropriate for samples of arbitrary thickness. They were implemented in a new version of AtomicJ, our free, open source application for analysis of AFM recordings.

Упрощенное аналитическое решение контактной задачи о вдавливании конического штампа в полупространство с покрытием

The present paper is devoted to the construction of the approximate solution of the contact problem on a conical punch indentation into an elastic isotropic half-space with coating. The solution is suitable for both homogeneous (when elastic moduli are constant) and multilayered or functionally-graded coatings (when elastic moduli change with depth). The solution was obtained using the bilateral asymptotic method in the analytical form. The kernel trans-form approximation was obtained as the ratio of two quadratic functions and contains only one parameter. Thus, the scheme of the approximate analytical solution was constructed in a significantly simplified manner in comparison with the general case in which the product of fractional-quadratic functions is used. Such an approach allowed us to obtain explicit analytical expressions for the distribution of contact stresses and the force-displacement dependences in a simplified form, convenient for engineering calculations. The influence of the parameter characterizing the relative Young's modulus of the coating on the characteristics of contact interaction was analyzed. The accuracy of the obtained solution was studied depending on the ratio of the elastic moduli of the coating and the substrate using a series of homogeneous coatings as an example. Particular attention was paid to the investigation of the indentation stiffness - the most important characteristic used in experimental researches. The results of the work can be used to describe an experiment on nanoindentation of materials with coatings using either a conical or a Berkovich indenter.

Simplified Analytical Solution of the Contact Problem on Indentation of a Coated Half-Space by a Conical Punch

The contact problem on indentation of an elastic coated half-space by a conical punch is considered. To obtain an explicit analytical solution suitable for applications, the bilateral asymptotic method is used in a simplified form. For that purpose, kernel transform of the integral equation is approximated by a ratio of two quadratic functions containing only one parameter. Such an approach allows us to obtain explicit analytical expressions for the distribution of contact stresses and relations between the indentation force, depth, stiffness and contact radius. The obtained solution is suitable both for homogeneous and functionally graded coatings. The dependence of the characteristics of contact interaction on a relative Young’s modulus of the coating and relative coating thickness is analyzed and illustrated by the numerical examples. Ranges of values of elastic and geometrical parameters are obtained, for which the presence of a coating sufficiently changes the contact characteristics. The accuracy of the obtained simplified expressions is studied in detail. Results of the paper sufficiently simplify engineering calculations and are suitable for inverse analysis, e.g., analysis of indentation experiments of coated materials using either a conical or a pyramidal (Berkovich) indenter.

Comparison of experimental methods to evaluate seam welds quality in extruded profiles

Abstract The wedge and bulge expansion tests were compared in the assessment of the seam welds strength in a tubular profile extruded at two ram speeds. In the wedge test, the expansion was determined by moving a conical punch into the tube until the specimen fracture. In the bulge test, a hydrostatic tensile stress state was applied by expanding the specimen with an internal rubber plug. The two methodologies were compared in terms of load and hoop strain at fracture and by detecting the fracture morphology and location. Then, the effect of a number of design parameters was investigated in order to evaluate the robustness of the standard testing conditions. For both tests, ductile fractures appeared in the seam welds location, but the bulge test was more robust and conservative with respect to the wedge test, showing less scattered data. Thus, the performances of a second die for the tube profile, designed to optimize the seam welds quality, have been successfully assessed by the bulge test and results compared to those achieved by a novel numerical quality index, coming to a final good matching.

Effect of punch profile on deformation behaviour of AA5052 sheet in stretch flanging process

Stretch flanging is a type of bending process widely used in automobile and aerospace industries. Forming of the stretch flange is mainly affected by three important parameters: materials of the sheet, the geometry of tools and different process parameters. This work focuses on the effect of punch profile on deformation behavior of AA5052 alloy sheet to form the stretch flange. Six punches of different geometry i.e. cylindrical, two stepped, three stepped, six stepped, conical and hemispherical are used. Results are presented in the form of edge crack in the sheet at edge corner and its propagation towards center, forming load comparison for different punch profile and distribution of radial and circumferential strain in the sheet. It is observed that the punch profile has a considerable effect on the deformation behavior of the sheet. Circumferential strain, radial strain and load requirement to form the flange are found to be minimum in hemispherical punch profile as compared to other punch profiles. Experiments are performed to validate the FE simulation results and results are found in very good agreement in terms of edge crack length. Fractography study shows uniform and large number of small size dimples at the fractured surface for hemispherical and conical punch profile.

Effect of pre-strain on stretch flange deformation limit of steel sheets

The effect of pre-strain on the stretch flange deformation limit of steel sheets was investigated by hole expansion tests under various pre-strain conditions. Pre-strain was given by stretch forming with a cylindrical punch. The strain ratio and amount of pre-strain were controlled by changing the blank shape and forming height. The hole expansion tests were carried out with a conical punch and cylindrical punch in order to clarify the effect of the strain gradient. The effect of the strain ratio of pre-strain on the stretch flange deformation limit can be expressed by equivalent strain. In the case of higher strain gradients, the stretch flange deformation limit increased with increasing pre-strain. However, at lower strain gradients, the stretch flange deformation limit was almost constant. These results suggest that the formability of the stretch flange deformation area given by pre-strain should be determined from not only the total equivalent strain but also the strain gradient and the amount of pre-strain expressed by equivalent strain.

Effect of punch geometry on hole expansion ratio

Stretch-flangeability of sheet metal is normally quantified by hole expansion ratio. Researchers have determined hole expansion ratio with various punch geometries such as conical, flat-bottom and hemispherical, and reported different hole expansion ratio values for identical test condition. Finite element investigation confirms that alteration of deformation path with punch geometries consequence different hole expansion ratio values. Necking and failure take place slightly away from the central hole edge for flat-bottom and hemispherical punches, while at the central hole edge for conical punch. Approximately plane strain tensile deformation prevails at the failure location for flat-bottom and hemispherical punches, while pure uniaxial tensile deformation prevails for conical punch.

Analysis and experimental evaluation of contact interaction of simple punch shapes with concrete

The article presents the results of the study of contact interaction of cylindrical, conical and roller-shaped punches with concrete. The stress-strain state of the material is assessed during penetration. The paper discusses how the angle of tapering of a simple punch shape affects the contact strength of the material. There are some hypotheses in the paper that describe the nature of the interaction of simple punch shapes with the material, there is a high convergence of the results with the main provisions of the mentioned hypotheses. In particular, it is found that the contact stresses in the area of contact of a punch with the material in areas significantly exceeding the strength limit of concrete can be characterized by the ratio of the strained volume of the material to the volume of the pressure bulb of the material. Its size is determined by the properties of the material.

Half‐space contact problem considering strain gradient and surface effects: An analytical approach

AbstractIn this article, an analytical approach is proposed for the problem of an elastic half‐space under symmetrically distributed normal force with arbitrary profile including uniform pressure, Boussinesq flat‐ended punch, conical punch and Hertz's spherical punch. The formulation of the work is on the basis of Mindlin's first strain gradient theory with five material length scale parameters. Also, the surface effects are taken into account using the Gurtin‐Murdoch surface elasticity theory. The developed mathematical formulation is general so that it can be simply reduced to different strain gradient‐based theories such as the modified versions of strain gradient and modified couple stress theories (MSGT and MCST), the simplified strain gradient theory (A‐SGT), as well as the classical theory (CT). In the solution procedure, an analytical method is applied using the Fourier and Hankel transforms. In the numerical results, the effects of material length scale parameters and kind of punch on the surface displacements and stresses of the half‐space are analyzed. The surface influences are also comprehensively studied. Moreover, comparisons are made between the predictions of MSGT, MCST, A‐SGT and CT. This work shows that the material length scale parameters of Mindlin's first SGT play important roles in the response of half‐space. Also, the presented results provide a comparison between the intensity of effects of strain gradient parameters and surface parameters.