Flat-ended Punch Research Articles

The influence of the piercing punch profile on the stress distribution on its cutting edge

Modifying the piercing punch profile may have a positive effect on the belt perforation process. Using the proper shape of the tool may reduce the perforation force and improve the quality of the holes. However, complex geometry of the punch can also cause an adverse stress distribution, which leads to a faster tool wear. In the presented paper, several different piercing punch profiles were tested using FEM analyses in ABAQUS and the obtained stress distributions along its cutting edges were analyzed. For the selected group of the piercing punches, the influence of variable geometrical features (a radius of rounding, an angle of chamfering or a depth of the bowl) on the stress distribution were also shown. Based on the results, it is possible to predict how modifying the punch profile will affect the shortening of the tool life, compared to the basic flat-end cylindrical piercing punch. The following research can be useful in the design process of the punching tools used for vacuum conveyor belts perforation.

Ferroelastic behavior of LaCoO3: A comparison of impression and compression techniques

Ferroelastic mechanical behavior of LaCoO3 was investigated by the impression test and compared with the conventional compression test using digital image correlation technique for deformation measurement. The results show that the coercive stress for ferroelastic deformation can be detected by impression test, which is less than critical stress for such deformation in the compression test. Room temperature ferroelastic impression creep characterization showed different equilibrium penetration of flat-end punch at different constant stresses, which can be characterized by Prony series equation.

An analytical solution of eccentric transverse deflection in rigid-plastic solids

For a rigid-plastic solid obeying Tresca's yield criterion which deforms by the mechanism of extended slip, it conforms to the principle of rotation-rate continuity if the yield shear stress remains constant. It has been proved on this basis that the divergence of the strain-rate tensor is zero and the velocity vector field obeys the Laplace equation in the deformation region. This property can lead to two general solutions, one is derived into three-dimensional slip-line field theory; the other is applied to solve the problem of transverse deflection of a clamped plate deflected by a flat-ended rigid punch. For concentric loading conditions explicit analytical solutions can be obtained directly, while for eccentric loading conditions the method of fundamental solutions (MFS) is applied in the deformation region which is seen as a doubly-connected domain obeying the harmonic equation. The punching force needed for the deflection and strain components are then calculated. It has been obtained that the contour lines and the lines of steepest descent are two principle lines of any point in the deformation region, so the method of steepest descent is used to depict the trajectories of the principle lines. Numerical calculations have been made using commercial finite element software ANSYS-LSDYNA. It turns out that the results obtained from numerical calculations fit quite well with the analytical results based on the principle of rotation-rate continuity and MFS. Finally, NURBS curves and Coons surface are used to construct the configurations of slip-planes in this problem, where the normal of slip-planes and the planes on which the two families of slip-lines located are both curved.

Indentation of a Sensitive Clay by a Flat-Ended Axisymmetrical Punch

This paper presents a description and the results of instrumented macro-scale laboratory indentation testing of Champlain Sea Clay with a stainless steel, flat-tipped, cylindrical indenter. The results of the tests were analyzed using published analytical solutions for the indentation testing of elastic–plastic materials. The relationship between the depth of indentation, h, and the load on the indenter, P, the hardness, the undrained shear strength and the Young’s modulus of the clay were determined from these methods. The undrained shear strength and Young’s modulus were compared to values from conventional geotechnical testing. Indentation testing was simulated numerically using the finite element method. The clay was modelled using the conventional Mohr–Coulomb elastic–plastic model. The distribution of total vertical stress below the indenter from the simulations compared well with an analytical solution based on elastic theory. The simulations using material properties derived from the conventional geotechnical tests (Set 1) and derived from the indentation testing (Set 2) with a Poisson’s ratio of 0.49 (undrained loading) did not produce load-penetration (P–h) curves in good agreement with the curves from indentation testing. However, simulations with the material properties derived from indentation testing and a Poisson’s ratio of 0.2 (drained loading), produced load-penetration curves in good agreement with the indentation testing results.

Onset of detachment in adhesive contact of an elastic half-space and flat-ended punches with non-circular shape: analytic estimates and comparison with numeric analysis

A recent paper by Popov, Pohrt and Li (PPL) in Friction investigated adhesive contacts of flat indenters in unusual shapes using numerical, analytical and experimental methods. Based on that paper, we analyze some special cases for which analytical solutions are known. As in the PPL paper, we consider adhesive contact in the Johnson–Kendall–Roberts approximation. Depending on the energy balance, different upper and lower estimates are obtained in terms of certain integral characteristics of the contact area. The special cases of an elliptical punch as well as a system of two circular punches are considered. Theoretical estimations for the first critical force (force at which the detachment process begins) are confirmed by numerical simulations using the adhesive boundary element method. It is shown that simpler approximations for the pull-off force, based both on the Holm radius of contact and the contact area, substantially overestimate the maximum adhesive force.

Stiffness of frictional contact of dissimilar elastic solids

The classic Sneddon relationship between the normal contact stiffness and the contact size is valid for axisymmetric, frictionless contact, in which the two contacting solids are approximated by elastic half-spaces. Deviation from this result critically affects the accuracy of the load and displacement sensing nanoindentation techniques. This paper gives a thorough numerical and analytical investigation of corrections needed to the Sneddon solution when finite Coulomb friction exists between an elastic half-space and a flat-ended rigid punch with circular or noncircular shape. Because of linearity of the Coulomb friction, the correction factor is found to be a function of the friction coefficient, Poisson's ratio, and the contact shape, but independent of the contact size. Two issues are of primary concern in the finite element simulations – adequacy of the mesh near the contact edge and the friction implementation methodology. Although the stick or slip zone sizes are quite different from the penalty or Lagrangian methods, the calculated contact stiffnesses are almost the same and may be considerably larger than those in Sneddon's solution. For circular punch contact, the numerical solutions agree remarkably well with a previous analytical solution. For non-circular punch contact, the results can be represented using the equivalence between the contact problem and bi-material fracture mechanics. The correction factor is found to be a product of that for the circular contact and a multiplicative factor that depends only on the shape of the punch but not on the friction coefficient or Poisson's ratio.

Periodic Contact Problems in Plane Elasticity: The Fracture Mechanics Approach

In this study, the concept of the fracture mechanics is used to solve the: (i) frictionless purely normal contact and (ii) the similar material contact under the mutual actions of the normal and tangential load. Considering the contact region is simply connected, the out-of-contact regions can be treated as periodic collinear cracks. Through evaluating the stress intensity factor (SIF), we are able to obtain the size and location of the contact/out-of-contact region. Then, the normal traction, shear traction and interfacial gap can be directly determined by the Green's function of the periodic collinear crack. In the case of frictionless purely normal contact, the new approach is applied to two classic problems, namely, the Westergaard problem (sinusoidal waviness punch) and the periodic flat-end punch problem. Then, the sinusoidal waviness contact pair in the full stick and the partial slip conditions under the mutual actions of the normal and tangential loads are solved by the newly developed approach.

Use of loading-unloading compression curves in medical device design

The paper presents a method and experimental results regarding mechanical testing of soft materials. In order to characterize the mechanical behaviour of technological materials used in prosthesis, a large number of material constants are required, as well as the comparison to the original. The present paper proposes as methodology the comparison between compression loading-unloading curves corresponding to a soft biological tissue and to a synthetic material. To this purpose, a device was designed based on the principle of the dynamic harness test. A moving load is considered and the force upon the indenter is controlled for loading-unloading phases. The load and specimen deformation are simultaneously recorded. A significant contribution of this paper is the interpolation of experimental data by power law functions, a difficult task because of the instability of the system of equations to be optimized. Finding the interpolation function was simplified, from solving a system of transcendental equations to solving a unique equation. The characteristic parameters of the experimentally curves must be compared to the ones corresponding to actual tissue. The tests were performed for two cases: first, using a spherical punch, and second, for a flat-ended cylindrical punch.

Hybridization effects on quasi-static penetration resistance in fiber reinforced hybrid composite laminates

Abstract Quasi static penetration tests (QSPT) were conducted on fiber reinforced hybrid composite laminates with a circular and cylindrical punch. Woven Carbon, Kevlar and S-glass fibers were used as reinforcing fibers for production of hybrid and non-hybrid composite laminates. Punch shear test procedures were employed until the perforation and complete plugging shear out of the laminate. During QSPT experiments, a flat end punch with diameter of 12.7 mm has been used for two different support spans (25.4 and 63.5 mm) to obtain two different support span (D s ) to punch diameter (D p ) ratios (SPR = D s /D p = 2 and 5). After the experiments, the extent of compression-shear and tension-shear based failure modes were characterized at constant punch displacements of 8 and 10 mm for SPR = 2 and 5, respectively. In addition to visualize of front and back side surfaces, composite samples were half sectioned at damage region to observe the amount of delamination and damage through the thickness of the laminate. It was found that hybridization effects were distinctly observed with dominated tension-shear damage mode at SPR = 5.

Torsional Tribological Behavior and Torsional Friction Model of Polytetrafluoroethylene against 1045 Steel.

In this work, the plane-on-plane torsional fretting tribological behavior of polytetrafluoroethylene (PTFE) was studied. A model of a rigid, flat-ended punch acting on an elastic half-space was built according to the experimental conditions. The results indicate that the shape of T–θ curves was influenced by both the torsional angle and the normal load. The torsion friction torque and wear rate of PTFE exponentially decreased when the torsion angle rose. The torsional torque increased from 0.025 N·m under a normal load of 43 N to 0.082 N·m under a normal load of 123 N. With sequentially increasing normal load, the value of torque was maintained. With rising normal load, the wear mass loss of PTFE disks was increased and the wear rate was decreased. Good agreement was found with the calculated torque according to the model and the experimental torque except for that under a normal load of 163 N. The difference under a normal load of 163 N was caused by the coefficient of friction. Usually the coefficient of friction of a polymer decreases with increasing normal load, whereas a constant coefficient of friction was applied in the model.

Response of thin-skinned sandwich panels to contact loading with flat-ended cylindrical punches: Experiments, numerical simulations and neutron diffraction measurements

The response of aluminium foam-cored sandwich panels to localised contact loading was investigated experimentally and numerically using flat-ended cylindrical punch of four varying sizes. ALPORAS and ALULIGHT closed-cell foams of 15 mm thickness with 0.3 mm thick aluminium face sheets (of 236 MPa yield strength) were used to manufacture the sandwich panels. Face sheet fracturing at the perimeter of the indenter, in addition to foam cells collapse beneath the indenter and tearing of the cell walls at the perimeter of the indenter were the major failure mechanisms of the sandwich panels, irrespective of the strength and density of the underlying foam core. The authors employed a 3D model in ABAQUS/Explicit to evaluate the indentation event, the skin failure of the face sheets and carry out a sensitivity study of the panel's response. Using the foam model of Deshpande and Fleck combined with the forming limit diagram (FLD) of the aluminium face sheet, good quantitative and qualitative correlations between experiments and simulations were achieved. The higher plastic compliance of the ALPORAS led to increased bending of the sheet metal and delayed the onset of sheet necking and failure. ALULIGHT-cored panels exhibited higher load bearing and energy absorption capacity, compared with ALPORAS cores, due to their higher foam and cell densities and higher yield strength of the cell walls. Additionally, they exhibited greater propensity for strain hardening as evidenced by mechanical testing and the neutron diffraction measurements, which demonstrated the development of macroscopically measurable stresses at higher strains. At these conditions the ALULIGHT response upon compaction becomes akin to the response of bulk material with measurable elastic modulus and evident Poisson effect.

Determination of Elastic Modulus of Gelatin Gels by Indentation Experiments

Mechanical characterization of hydrogels is a challenging task because they are much softer than metals, ceramics or polymers. The elastic modulus of hydrogels is within 100-102kPa range. Because they easily break and slump under their own weight, tensile and bending tests are not suitable configurations to assess elastic modulus. This work reports on the determination of elastic modulus of a gelatin gel by indentation experiments. Indentation is very simple configuration, it is of technological importance and it can be applied at different length scales with high accuracy. The gelatin hydrogel behavior is first calibrated by uniaxial compression and low strain rheological measurements. It behaves as a hyperelastic solid with strain hardening capability at large strains and shows no dependence with frequency in the linear viscoelastic range. It can be properly characterized by the First order Ogden material model. Indentation experiments are carried out at macro and nanoscales using spherical and flat-ended cylindrical punches. Elastic contact solutions and inverse analysis accounting for hyperelasticity are used to extract the elastic modulus from experimental force-depth curves. Adhesion between punch and hydrogel influences the indentation response and affects the accuracy of elastic modulus determination in a larger extent than the assumption of linear elasticity. Adhesion leads to overestimation of elastic modulus values. The influence of adhesive forces increases with decreasing the length scale. A markedly decay of elastic modulus with increasing maximum load is observed at nanoscale. A hybrid model based on Hertz elastic contact solution and Johnson-Kendal-Roberts model for adhesion is used to determine elastic modulus. This model yields an elastic modulus in good agreement with that obtained from uniaxial compression test.

Indentation on two-dimensional hexagonal quasicrystals

This paper is concerned with contact problem for a half-space of two-dimensional hexagonal quasi-crystal punched by three common indenters (cylindrical flat-ended, conical and spherical punches). Based on the Green’s functions for the half-space subjected to an external phonon source exerted on the surface, the superposition principle is applied to constructing the boundary integral equation. Relations between the indentation force and the penetration depth, and the indentation stiffness constants are explicitly obtained for these three indenters. Complete and exact fields in the half-space are given in terms of elementary functions. The present theoretical solutions can not only serve as benchmark for computational contact mechanics, but also apply to guiding future indentation experiments.

The JKR-type adhesive contact problems for transversely isotropic elastic solids

The JKR (Johnson, Kendall, and Roberts) and Boussinesq–Kendall models describe adhesive frictionless contact between two isotropic elastic spheres or between a flat end punch and an elastic isotropic half-space. Here adhesive contact is studied for transversely isotropic materials in the framework of the JKR theory. The theory is extended to much more general shapes of contacting axisymmetric solids, namely the distance between the solids is described by a monomial (power-law) function of an arbitrary degree d⩾1. The classic JKR and Boussinesq–Kendall models can be considered as two particular cases of these problems, when the degree of the punch d is equal to two or it goes to infinity, respectively. It is shown that the formulae for extended JKR contact model for transversely isotropic materials have the same mathematical form as the corresponding formulae for isotropic materials; however the effective elastic contact moduli have different expression for different materials. The dimensionless relations between the actual force, displacements and contact radius are given in explicit form. Connections of the problems to nanoindentation of transversely isotropic materials are discussed.

Failure mode maps for circular composites sandwich plates under bending

Abstract Competing failure modes are investigated for circular sandwich plates comprising quasi-isotropic E-glass/epoxy composite faceplates (with [−60/0/60] ns configuration) and polyvinyl chloride (PVC) foam core under bending. Clamped sandwich plates are loaded using flat-ended punch at the center of the plate. Three competing failure modes, viz., core indentation, core shear and face failure/microbuckling, are considered. Analytical estimates for elastic response (stiffness) and initial failure load are proposed, and these are verified by experimental measurements and finite element (FE) predictions. Good agreement (with in ±20%) is observed among analytical estimates, experimental measurements and FE predictions. Analytical estimates for the failure modes are used to plot the failure mode map in non-dimensional plate radius versus faceplate thickness plane for a given material system. The failure mode map thus constructed is assessed by considering a few sandwich plate geometries. Normalized sandwich mass and failure load contours are superimposed onto the failure mode map to identify the locus of minimum weight design by numerical search. Effect of geometrical parameters of the sandwich plate on failure load and mode is also investigated.

The JKR-type adhesive contact problems for power-law shaped axisymmetric punches

The JKR (Johnson, Kendall, and Roberts) and Boussinesq–Kendall models describe adhesive frictionless contact between two isotropic elastic spheres, and between a flat-ended axisymmetric punch and an elastic half-space respectively. However, the shapes of contacting solids may be more general than spherical or flat ones. In addition, the derivation of the main formulae of these models is based on the assumption that the material points within the contact region can move along the punch surface without any friction. However, it is more natural to assume that a material point that came to contact with the punch sticks to its surface, i.e. to assume that the non-slipping boundary conditions are valid. It is shown that the frictionless JKR model may be generalized to arbitrary convex, blunt axisymmetric body, in particular to the case of the punch shape being described by monomial (power-law) punches of an arbitrary degree d≥1. The JKR and Boussinesq–Kendall models are particular cases of the problems for monomial punches, when the degree of the punch d is equal to two or it goes to infinity respectively. The generalized problems for monomial punches are studied under both frictionless and non-slipping (or no-slip) boundary conditions. It is shown that regardless of the boundary conditions, the solution to the problems is reduced to the same dimensionless relations among the actual force, displacements and contact radius. The explicit expressions are derived for the values of the pull-off force and for the corresponding critical contact radius. Connections of the results obtained for problems of nanoindentation in the case of the indenter shape near the tip has some deviation from its nominal shape and the shape function can be approximated by a monomial function of radius, are discussed.

Numerical study of contacts between a flat-ended punch and a half-space embedded with inhomogeneities

This paper presented a numerical approach to solving the problem of a flat-ended punch in contact with a half-space matrix embedded with multiple three dimensional arbitrary-shaped inhomogeneities. Based on the semi-analytical method (SAM) and the equivalent inclusion method, numerical procedures were developed and the effects of inclusion shape and distribution were analyzed. Fast Fourier transform technique was implemented to accelerate the calculation of surface deformation and subsurface stress. Interactions of inter-inclusions and inclusion-matrix were taken into account. Numerical results showed the presence of inhomogeneities (i.e., microstructures in solids) indeed had a great effect on local contact pressure and a strong disturbance to the subsurface stress field in the vicinity of inclusions. The effects were dependent on the shape and distribution of inclusions and inter-inclusion interactions. The physical significance of this study is to provide an insight into the relation between the material microstructure and its response to the external load, and the solution approach and procedures may find useful applications, for example, the analysis of fatigue and crack propagation for composite materials, prediction of stress field in solids containing material defects, and study of the mechanism of chemical-mechanical polish (CMP) for inhomogeneous materials, etc.

Effect of intermediate principal stress on flat-ended punch problems

The intermediate principal stress has certain effects on the yield strength of metallic materials under complex stress states. The flat-ended punch problem is a classical and fundamental problem in plasticity theory and mechanical engineering in which the metal beneath a flat-ended punch is under complex stress states. Using the finite difference codes, fast Lagrangian analysis of continua and Unified Strength Theory, the effect of the intermediate principal stress on the flat-ended punch problem is analyzed in this paper. First, the limit pressures of strip and circular punches pressed into an elastoplastic and homogeneous metallic medium are calculated by the two-dimensional finite difference method. The problems of square and rectangular punches are analyzed by the three-dimensional finite difference method. Finally, the effect of the intermediate principal stress on flat-ended punch problems with different punch geometries is analyzed.

Assessment of anisotropic tensile strength using a modified shear punch test for Cu–Cr–Zr alloy processed by severe plastic deformation

The effect of severe plastic deformation on the anisotropic strength change was investigated by equal-channel angular pressing (ECAP) experiments with various pass numbers. To overcome the size limitation for the tensile test specimen, this study proposes a shear punch test modified by using a flat-end rectangular punch to evaluate uniaxial tensile strength in multiple directions in the longitudinal and transverse sections of the ECAP-processed Cu–Cr–Zr alloy. The validity of the modified method was verified by the linear relationship between the shear punch test data and tensile test data in the longitudinal direction. Specimens were prepared at three angles (0°, 45° and 90°) with respect to the extrusion direction in the longitudinal sections and the horizontal direction in the transverse sections of the ECAP bar. The results showed that tensile strength deceased when the angle increased from 0° to 90° in both the longitudinal and transverse sections. The longitudinal sections showed a more pronounced anisotropy than the transverse sections. As the amount of plastic deformation by ECAP increased, the degree of the anisotropy decreased in the longitudinal section but increased in the transverse section. Based on the results of this study, the strength of an ECAP material must be assessed in multiple directions in both the longitudinal and transverse sections.

Indentation Behavior of a Closed-Cell Aluminum Foam at Elevated Temperatures

A series of deep indentation and uniaxial compression tests of closed-cell aluminum foams at room temperature as well as elevated temperatures were conducted. A flat-ended punch (FEP) was used in the indentation tests. Cross-sectional views of the specimens after tests show that the deformation is roughly confined to the material directly underneath the indenter with slightly lateral spread. It is found that plastic collapse strength and energy absorption of the specimens are temperature dependent in both loading conditions. Tear energy of the foam in FEP indentation depends on the indentation depth and temperature.