Punch Penetration Research Articles

Application of Digital Image Correlation to Small Punch Test for Determination of Stress–Strain Properties

The analysis of the small punch test is based on the force on the moving punch and the deflection data acquired at a single point of the specimen bottom. However, the spatial distribution of stress and strain at any given instant is non-uniform and its variations with increase in punch penetration are quite complex. In this work, the digital image correlation (DIC) technique is integrated with small punch test for in-situ full field strain measurement in the bottom surface of the specimen. The DIC results reveal that with the progression of deformation, the peak equivalent plastic strain at the bottom surface shifts from the center of the specimen to a characteristic radial location, where the strain rapidly builds up and concentrates leading to instability and cracking. Combining DIC-based strain results with finite element model-based stress estimation at the characteristic radial location, a methodology for determining the stress–strain curve from small punch test is formulated and the outcomes are compared with tensile test results for four different metallic alloys.

Finite element simulation of the complete sheet metal blanking cycle: Effect of blanking clearance on force curve and cut edge quality

Sheet metal blanking process is a manufacturing process that is widely used in sheet metal forming and particularly in the electronics and automotive industries. The outcome and quality of the blanked parts are affected by various parameters such as the blanking clearance, the sheet metal mechanical properties and the wear of tools. However, experimental campaigns to assess the effects of these parameters on the process outcome can be prohibitively expensive for the industry. As a result, numerical simulation of the blanking process is becoming increasingly important to gain insight into the process and study these effects.The present paper provides a novel finite element (FE) simulation procedure to predict the complete blanking cycle including sheet metal cutting, sheet metal springback, punch penetration in die, and stripping phases. A blanking force curve showing the punch force at all phases can thus be obtained. The commercial finite element code ABAQUS® has been chosen to develop this procedure using ABAQUS/Explicit® code for the cutting phase and ABAQUS/Standard® code for the remaining phases. Based on this procedure, the influence of the blanking clearance on the punch force-displacement curve and on the cut edge quality has been studied. The obtained results have been compared to previous literature results and experimental findings.

Dieless blanking using noncylindrical micropunches

Dieless blanking/punching are useful for piercing microholes because removing the die makes it possible to overcome the difficulty in piercing a small-sized die hole and assembling a die set. However, only cylindrical punches had been employed in previous studies on dieless blanking/punching. Piercing holes using noncylindrical micropunches was therefore attempted. The workpiece was backed up with adhesive tape instead of the die. Stainless-steel sheets were blanked using cemented tungsten carbide micropunches fabricated by electrical discharge machining. Blanking was successfully carried out in a 5-μm-thick sheet using micropunches with a rectangular cross section of 18 μm on the long side and 16 μm on the short side and a semicircular cross section of 20 μm in diameter. Corners of the pierced holes were finely processed, especially at the hole entrances. This result demonstrates that noncircular microholes can be pierced as well as circular ones by dieless blanking. An investigation into the relationships between blanking conditions and blanking characteristics showed that the punch stroke must be at least 30 μm for micropunches to penetrate a 5-μm-thick sheet at a punch feed speed of 50 μm/s. Furthermore, the maximum punch load increased with increasing punch feed speed for semicylindrical micropunches and was almost the same for quadrangular-prism micropunches. Rotating micropunches were also used and circular microholes were successfully pierced, indicating that noncylindrical micropunches can be employed for piercing both circular and noncircular holes. In addition, the punch load decreased with increasing punch rotation speed, likely because the cutting action by the peripheral edges of micropunches facilitated punch penetration.

Effect of small corner radius in cold punching of 22MnB5 die-quenched boron steel sheets

The effect of small corner radius in punching process of ultra-high strength category of steel i.e. 22MnB5 die-quenched boron steel was investigated. Although the die-quenched steel sheet can reach superior strength of above 1500 MPa, the ductility of the steel sheet is low, making punching process totally challenging. Punching of hole at 22MnB5 boron steel commonly carried out in hot condition prior to die-quenching and usually needed for complex tools for preheating, punching, transferring, and cooling the part. Inaccurate of hole dimension after quenching is inevitable due to the change of grain structure during cooling. In this study, a pre-hardened 22MnB5 boron steel sheet is punched under a small clearance in a normal cold condition. The small corner radius disperses the stress concentration at the punch edge and thus increases the material flow during initial stage of the punch penetration. The action of small clearance between punch and die reduce the tensile stress and therefore, the initiation of crack is delayed. The quality of the sheared edge surface was improved with the high percentage of area cover with the burnished surface.

Pin Extrusion for Mechanical Joining from Orbital Formed Tailored Blanks with Local Material Pre-Distribution

Pin extrusion is a common process to realise pin structures in different geometrical dimensions for a subsequent joining operation. Nevertheless, the process of pin extrusion offers process limits regarding sheet thinning as a consequence of the punch penetration depth into the sheet. Thereby, cracks at the residual sheet thickness can occur during strength tests, resulting in a failure of the complete joint due to severe thinning. Therefore, measures have to be taken into account to reduce the thinning. One possibility is the application of orbital formed tailored blanks with a local material pre-distribution, which allows a higher sheet thickness in the desired area. Within this contribution, the novel approach of a process combination of orbital forming and pin extrusion is investigated. To reveal the potential of a local material pre-distribution, conventional specimens are compared with previously orbital formed components. Relevant parameters such as the residual sheet thickness, the pin height as well as the average hardness values are compared. The results show a significant positive influence of a local material pre-distribution on the residual sheet thickness as well as the resulting pin height. Furthermore, the strain hardening during orbital forming can be seen as an additional advantage. To conclude the results, the process limits of conventional pin extrusion can be expanded significantly by the application of specimens with a local material pre-distribution.

Dieless punching of ultrasmall-diameter holes

The dieless punching of ultrasmall-diameter holes was attempted because it is difficult to punch such holes by conventional punching methods. Removing the die made it possible to solve the problem of difficulty in piercing an ultrasmall-diameter die hole and assembling a die set. The workpiece was backed up with adhesive tape instead of the die. Stainless-steel sheets were punched using cemented tungsten carbide small-diameter punches fabricated by electrical discharge machining. As a result, an 8.5-μm-diameter hole was successfully punched in a 5-μm-thick sheet using a 10-μm-diameter punch. A hole with a diameter smaller than 6 μm was also punched in a 3-μm-thick sheet using a 6-μm-diameter punch. Furthermore, the punch load evolution was measured using a load cell, and the punch load was shown to reach a positive peak when the punch penetrated the workpiece. An investigation into the relationships between punching conditions and punching characteristics showed that the punch stroke must be at least 30 μm for a 10-μm-diameter punch to penetrate a 5-μm-thick workpiece at a punch feed speed of 50 μm/s. In addition, the maximum punch load decreased with increasing punch feed speed. When the punch was rotated, the hole diameter increased, likely because of the cutting action of the punch peripheral surface. The cutting action facilitated punch penetration, resulting in a reduced punch load. Ultrasonic oscillation was also employed by oscillating the workpiece in the punch axial direction. The punch load was considerably reduced, demonstrating that ultrasonic oscillation is effective even when the workpiece is oscillated instead of the punch.

Effect of Die Edge Roundness on Crack Initiation near Punch Edge in Blanking

In this study, the effect of die edge radius on crack initiation was discussed in finish blanking. Experiments were carried out using die with rounded cutting edge. As a result, it was confirmed that crack initiation near the punch edge occurred earlier than that near the die edge. It was also confirmed that the burnished surface increased with an increase in the die edge radius. Finite element method was carried out from the initial stage of shearing and until the punch penetration depth was achieved just before the occurrence of the cracks to investigate the distribution of the mean stress. As a result, it was confirmed that the punch stroke, where the mean stress began to increase rapidly to a positive value, increased with an increase in the die edge radius, and the punch stroke at which the mean stress increased rapidly also increased.

A new approach on punch penetration test for prediction of boreability of jointed rock mass at northern section of Kerman water conveyance tunnel

A new approach on punch penetration test for prediction of boreability of jointed rock mass at northern section of Kerman water conveyance tunnel

Comparison of strength-based rock brittleness indices with the brittleness index measured via Yagiz’s approaches

Rock brittleness is one of the most significant properties of rock having a major impact not only on the failure process of intact rock but also on the response of rock mass to rock excavation. In fact, the brittleness is a combination of rock properties including not only the uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) but also density and porosity of rocks. Due to that, the brittleness should be examined very carefully for any excavation projects, i.e., mechanized excavation, drilling and blasting. The aim of this paper is to compare the strength-based brittleness indices with both the rock brittleness index (BIo), directly obtained via punch penetration test (PPT) and also estimated via Yagiz’s approach (BI1) as a function of strengths and density of rocks. For the aim, database including more than 45 tunnel cases are used to compute common rock brittleness indices (BI2, BI3, BI4), different combination of UCS and BTS. Further, these indices are compared with both BIo and BI1 as well as each other. It is found that the BIo and BI1 have a significant relations (ranging of determination coefficients (r2) from 0.69 to 0.88 with strength-based brittleness indices commonly used in practice. Also, based on findings, several rock brittleness classifications are also revised herein.

Analysis of bicycle helmet damage visibility for concussion-threshold impacts

ABSTRACT Any helmet involved in an accident should be replaced, regardless of appearance after impact. However, consumer compliance and interpretation of this recommendation is unclear, for which there is additional ambiguity for lesser impacts. This study aims to investigate the relation between helmet damage visibility and lesser impacts in line with concussion. As a preliminary model, a commercially available road-style helmet was chosen. Twelve helmets underwent impact attenuation testing; four were dropped from the standard testing height of 2 m, and eight from lower drop heights (0.34 and 0.42 m) associated with the production of linear accelerations (90 and 100 g, respectively) consistent with the production of concussion. Expanded polystyrene damage was assessed via flat punch penetration testing. American adults were then polled on helmet damage visibility based upon before and after photos. All helmets demonstrated damage to the expanded polystyrene liner in the form of altered material properties. Helmets dropped from 2 m displayed significant changes in elastic buckling (p < .01) and densification behavior (p < .01) as compared with lower drop height results. Adverse change in elastic buckling behavior was found to increase linearly with drop height (p < .001). Damage visibility was significant for helmets dropped from a 2-meter height, however, such a relation among the helmets impacted at the threshold for concussion was lacking. These findings suggest that for the chosen helmet model, consumers may be unable to distinguish between new helmets and helmets with diminished protective abilities.

Investigation of the penetration behavior of silicon dioxide epoxy hybrid nanocomposites

The use of nanomaterials is gradually increasing with the progress of nanotechnology. In particular, the production of nanocomposites incorporating nanoparticles is one of the most significant areas in which nanomaterials are being used increasingly. The first objective of this research was to detect the punch shear or penetration resistance behavior and damage mechanisms of hybrid nanocomposites obtained by using silica (SiO2) nanoparticles. For that purpose, six different SiO2 hybrid nanocomposites with different laminations three layer (3La), 5La, 7La, 11La, 15La and 21La and different thicknesses (HC) of 0.95∼4.98 mm, were made by using vacuum assisted transfer molding (VARTM). During this research, quasi-static punch-shear (QS-PS) tests at span punch ratios (SPRs) of 1.16, 1.33, 1.67, 2.00, 2.33, 2.67, and more were conducted to determine quasi-static penetration mechanics and penetration resistance behavior. Moreover, deflection, energy dissipation, damage area, stiffness, and peak force values were investigated through experimental results and scanning electronic microscope (SEM) images.

A new rock brittleness index on the basis of punch penetration test data

Brittleness is one of the most important properties of rock which has a major impact not only on the failure process of intact rock but also on the response of rock mass to tunneling and mining projects. Due to the lack of a universally accepted definition of rock brittleness, a wide range of methods, including direct and indirect methods, have been developed for its measurement. Measuring rock brittleness by direct methods requires special equipment which may lead to financial inconveniences and is usually unavailable in most of rock mechanic laboratories. Accordingly, this study aimed to develop a new strength-based index for predicting rock brittleness based on the obtained base form. To this end, an innovative algorithm was developed in Matlab environment. The utilized algorithm finds the optimal index based on the open access dataset including the results of punch penetration test (PPT), uniaxial compressive and Brazilian tensile strength. Validation of proposed index was checked by the coefficient of determination (R2), the root mean square error (RMSE), and also the variance for account (VAF). The results indicated that among the different brittleness indices, the suggested equation is the most accurate one, since it has the optimal R2, RMSE and VAF as 0.912, 3.47 and 89.8%, respectively. It could finally be concluded that, using the proposed brittleness index, rock brittleness can be reliably predicted with a high level of accuracy.

Numerical Modeling of a Punch Penetration Test Using the Discrete Element Method

Abstract Understanding the brittleness of rock has a crucial importance in rock engineering applications such as the mechanical excavation of rock. In this study, numerical modeling of a punch penetration test is performed using the Discrete Element Method (DEM). The Peak Strength Index (PSI) as a function of the brittleness index was calculated using the axial load and a penetration graph obtained from numerical models. In the first step, the numerical model was verified by experimental results. The results obtained from the numerical modeling showed a good agreement with those obtained from the experimental tests. The propagation path was also simulated using Voronoi meshing. The fracture was created under the indenter in the first step, and then radial fractures were propagated. The effects of confining pressure and strength parameters on the PSI were subsequently investigated. The numerical results showed that the PSI increases with enhancing the confining pressure and the strength parameter of the rock, including cohesion and the friction angle. A new relationship between the strength parameters and PSI was also introduced based on two variable regressions of the numerical results.

Justification of instrument parameters for the construction soil strength determination

A portable instrument has been introduced in the construction industry of the Russian Federation for the determination of soil strength grades, which is based on the free weight falling principle. Until the present the theory of such instrument hasn’t yet been completely developed and needs to be improved. In this study on the basis of the theorem on normal stresses on surfaces of a cone to be penetrated into the soil, a logically justified theory of the instrument basic parameter selection has been developed. Stresses on horizontal areas of construction soils are equal to twice the stresses on inclined areas of the cone. At the end of the working section of a rod to be penetrated into the soil there is a round flat punch, which area is A=1 cm2. During the punch penetration into the soil by an impact method, a soil compacted cone body appears before the flat punch, which is affected by normal pressure forces and tangential friction forces. For the first time the equation of normal stresses σ1 has been obtained on inclined areas of the soil cone. The mathematical model of the instrument doesn’t contain soil parameters, that allows to recommend it for any soil and media. The obtained analytical dependences allow to design similar instruments in order to measure the soil strength using any value of the falling weight mass. In this paper the instrument parameter justification has been made with the impact weight mass m 1 = 2.5 kg.

Numerical modeling of the tool-rock penetration process using FEM coupled with SPH technique

The numerical simulation of penetration into rock is an important tool to gain insights into rock drilling mechanisms, since it can be exploited as an alternative to the expensive field testing. This research aims to present an innovative computer simulation of rock penetration process on the basis of the finite element method (FEM) coupled with smoothed particle hydrodynamics (SPH). An advanced material model, namely the Karagozian and Case Concrete (KCC) model, was employed for this purpose. The Punch Penetration test (PPT) was carried out on a medium strength sandstone for validating the numerical method. The comparison of the numerical and experimental results obtained concluded that the FEM coupled with SPH method in conjunction with the fully calibrated KCC material model is a reliable method for the study of rock penetration due to its ability to deal with large deformations and its realistic constitutive modeling. The modeling approach was finally applied to estimate the required force to penetrate an offshore reservoir rock block under the in-situ confining pressure with a double conical tool up to 5 mm depth. The effective stresses in sedimentary basins of Agosta and Dosso Campus at a depth of 3000 m below the seabed are considered as the confining pressures of this study.

TBM performance prediction model with a linear base function and adjustment factors obtained from rock cutting and indentation tests

A new TBM performance prediction model consisting of a basic penetration function and adjustment factors for different boundary conditions is presented. The basic penetration function is implemented as a linear function with an offset from zero based on Frenzel et al. (2012). Adjustment factors are used to refine the model which reflects the model architecture used by Gehring (1995). The well-established CSM model (Rostami, 1997) was used as a reference to account for tool geometry and spacing. In addition, a new approach to account for rock brittleness is presented based on the rock mechanical interpretation of Punch penetration tests (PPT). The proposed new model was developed using full scale linear rock cutting tests from 19 igneous, metamorphic, sedimentary and artificial rock types. It was validated with 36 TBM penetration tests from tunnels in the Swiss Alps (Villeneuve, 2017), Koralm tunnel and Røssåga tunnel (Wilfing, 2016). It has a strong predictive capability for a wide range of rock and rock mass parameters. The main strengths of the model are high accuracy, simplicity of the basic penetration function and the highly transparent development process that enables a clear understanding of its strengths and limitations.

Mechanical behaviour of aerogels and composite aerogels submitted to specific penetration tests

Silica aerogels are potentially ideal candidates to capture fast moving particles in their ultra-porous compliant structure and aerogel capture cells have been deployed in low earth orbit on a number of occasions. It is consequently interesting to better characterize the mechanical properties of these ultra-porous materials under very large deformation and collapse. Some special flat punch indentation experiments are used in this study to test these properties. The large flat punch penetration induces the creation of a track in the sample with a series of pilled Hertzian cone cracks and samples can resist without breakage for penetration depth of several millimeters. The influences of the aerogel density and of the penetration rate are checked in both standard and fibers reinforced aerogels. The lower the density, the more compliant the aerogel is to large penetration.

Development of a new rock drillability index for oil and gas reservoir rocks using punch penetration test

Punch penetration test has shown a good relationship with the rate of penetration (ROP) of tunnel boring machine (TBM) into hard rock. In addition, various indices have been determined from the analysis of the force-penetration curve of the penetration test to present rock drillability, boreability, and brittleness. Therefore, in the present paper, punch penetration test will be used to determine rock drillability index (RDI). In this research, 9 samples were taken from the same formation in two wells drilled at southwestern Iran. Considering the limitations in taking more samples, and also to enhance reliability of the results, each of the samples was divided into smaller specimens, so as to conduct multiple tests at each sampling point. Evaluation of the force-penetration curves of samples indicated that most samples of well 1 have little changes of forces early in the test, but they increase as the test proceeds. Because the samples from the Well 1 were taken from shallow depths, The change in the slope of the curve determined on the basis of the samples taken from Well 1 can be explained by the pores existing in these samples. Different indices were extracted from the analysis of the force-penetration curve of each specimen, including first peak slope index (BI1), maximum peak slope index (BI2), area under the curve (BI3), and RDI. Initial examination of relationships between each of indices and the measured ROP follow an either exponential or power-law model. Evaluation of these models between each of these indices and actual ROP indicated that, an exponential curve gives the best fit to the obtained values. A comparison between estimated ROPs from the fitted relationships against measured values showed that the estimated ROP using the proposed index is highly accurate and reliable and can present a good measure of the reservoir rock drillability.

EMI database analysis focusing on relationship between density and mechanical properties of sedimentary rocks

The Earth Mechanics Institute (EMI) was established at the Colorado School of Mines (CSM) in 1974 to develop innovations in rock mechanics research and education. During the last four decades, extensive rock mechanics research has been conducted at the EMI. Results from uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), point load index (PLI), punch penetration (PP), and many other types of tests have been recorded in a database that has been unexamined for research purposes. The EMI database includes over 20,000 tests from over 1,000 different projects including mining and underground construction, and analysis of this database to identify relationships has been started with preliminary results reported here. Overall, statistically significant correlations are identified between bulk density and mechanical strength properties through UCS, BTS, PLI, and PP testing of sedimentary, igneous, and metamorphic rocks. In this paper, bulk density is considered as a surrogate metric that reflects both mineralogy and porosity. From this analysis, sedimentary rocks show the strongest correlation between the UCS and bulk density, whereas metamorphic rocks exhibit the strongest correlation between UCS and PP. Data trends in the EMI database also reveal a linear relationship between UCS and BTS tests. For the singular case of rock coral, the database permits correlations between bulk density of the core versus the deposition depth and porosity. The EMI database will continue under analysis, and will provide additional insightful and comprehensive understanding of the variation and predictability of rock mechanical strength properties and density. This knowledge will contribute significantly toward the increasingly safe and cost-effective geostructures and construction.

УПРОЩЕННОЕ АНАЛИТИЧЕСКОЕ РЕШЕНИЕ ЗАДАЧИ О ВНЕДРЕНИИ СФЕРИЧЕСКОГО ИНДЕНТОРА В ПОЛУПРОСТРАНСТВО С ПОКРЫТИЕМ

The contact problem of a rigid spherical punch penetration into a linearly elastic isotropic half-space with a functionally graded or homogeneous coating is considered. The problem is reduced to solving the dual integral equation. Using a one-parameter approximation of the kernel transform of an integral equation in the form of a ratio of two quadratic functions containing only one parameter, an approximate asymptotically exact solution of the problem was obtained in a simplified analytical form. The solution allows one to take into account both the elastic properties of the coating and the contribution of the substrate. In this case, the scheme for constructing an approximate analytical solution is significantly simplified in comparison to the general case. Simplified analytical expressions describing contact pressure, force, depth, and indentation rigidity are obtained. The accuracy of the solution obtained is investigated depending on the ratio of the elastic moduli of the coating and the substrate, the nature of the change in the elastic modulus, and the relative thickness of the coating. It is shown that the proposed formulas allow to achieve a sufficiently high accuracy for describing of the solution of the problem for thin and thick coatings, as well as for coatings of arbitrary thickness in the case of a relatively small difference in the elastic moduli of the coating and substrate. Using a simple one-parameter approximation makes it possible to obtain a solution to the problem in an explicit analytical form, convenient in practice for engineering calculations.