Punching Load Research Articles

Punching shear resistance of strengthened reinforced concrete interior slab–column connections using ultra-high-performance strain-hardening cementitious composite material

The main target of this article is to find out the most applicable strengthening technique to change the brittle punching shear failure of slab–column connection to a ductile one. The considered strengthening technique uses the combined effect of the internal single-leg stirrups and a thin layer of ultra-high-performance strain hardening cementitious composite material provided in either tension or compression side. Thus, 12 slab specimens divided into four groups were prepared and configured. It was found that the slab provided with properly anchored single-leg stirrups with a cross-shaped thin layer of ultra-high-performance strain hardening cementitious composite material at the compression side proved the most efficient technique among all the considered configurations. This technique enabled the slab to sustain a punching load of about 2.38 times that of the control unstrengthened slab. In addition, it exhibited ductile failure showing strain hardening and softening plateaus. Finally, the experimental shear resistances of all the strengthened slabs were compared against the design resistances stipulated by different design standards such as the ECP 203-2007, ACI 318-14, DIN 1045-1, and EC2-2004 codes. It can be concluded that the failure characteristics stipulated by the DIN 1045-1 code showed the most rational results compared with those of the experimental findings.

A Study on the Punch Shape for Improving Tool Life in Shearing AHSS

The adoption of hot rolled AHSS(Advanced High-Strength Steel) has been increased in accordance with the weight reduction of the chassis. The problem of tool life in punching the hole for high-strength steels is more important. Using the high-grade die material is one of the method to improve tool life. Another method for increasing the tool life is reducing the maximum punching load. We investigated the effect of tool shape on reducing the punching load. Conventional, shear-angled and humped tool geometry are considered for punching the flat sheets with 780MPa strength. In flat sheet, the maximum punching load is compared for each case of puch types. Maximum punching load is reduced by 40% of conventional punching process. For investigating the effect of punch shape in a real part, the punching process for the vent hole of the conventional wheel disc is modeled with FEM. The maximum load is efficeintly reduced by using the shear-angled punch.

Study of Machining Accuracy of Micro Punching Mold Using Micro-EDM

As one of the micro-forming technologies, micro punching process produces workpieces in high efficiency and low cost. It has attracted the attention of researchers from all over the world in recent years. The current micro punching mold is prepared on-line by micro electrical discharge machining (EDM) with simple cross-sectional shape. During punching, the punching load is small due to the soft material and small circumference of workpiece. To form micro part with complex cross sectional shape using micro punching technology, three-dimensional micro-EDM milling and reversed micro EDM are used to generate micro die and punch on-line to provide a solution to alignment of die and punch on machine. In order to obtain high dimensional accuracy of micro punching mold, in this paper, dimensional errors including electrode, reversed electrode, micro die and micro punch caused during machining process are identified. The correlation between dimensional errors and the clearance of micro die and micro punch are analyzed. Based on preliminary experimental results and dimensional error control strategy, the final punching clearance of micro punch molds was machined within the required range.

Experimental investigation into the electropulsing assisted punching process of 2024T4 aluminum alloy sheet

During the punching process of high strength lightweight materials such as 2024T4 aluminum alloy, the severe die wear and edge fracture of the punched part are one of the main concerns of manufacturing industries and the scientific community. Researches have found that the electric current can reduce the metals’ flow stress, improve their ductility, and promote the dislocation annihilation below the annealing temperature. Therefore, the electric current was proposed to help improve the punching process of 2024T4 aluminum alloy sheet in this paper. The experiments of electropulsing assisted punching (EAP) were carried out by applying different values of electric current on the specimens at different punching phases. For convenience to compare the punching results, the punching load, the surface quality of the punched profile and edge strain hardening (ESH) were analyzed under different electropulsing ways. In addition, the uniaxial tensile tests of punched parts were carried out to evaluate the influence of electropulsing on the ultimate tensile load of punched specimen with a hole. The results show that the electropulsing has a significant effect on reducing the punching load and improving the surface quality of the punched profile. Moreover, the anisotropy of material and the ESH of the punched edge, which are characterized by the level or distribution of microhardness, are eliminated partially due to the electropulsing induced annealing effect. The ultimate tensile load has been effectively enhanced by electropulsing, which may stem from the fact that elimination of ESH and the healing and arrest of microcracks and microvoids near punched edge.

Effect of CFRP and TRM Strengthening of RC Slabs on Punching Shear Strength

The paper presents experiments involving punching of RC slabs strengthened using externally bonded carbon fiber reinforced polymer (CFRP) sheet and textile reinforced mortar (TRM). Twelve RC slab specimens of two concrete grades (39.9 and 63.2 MPa) and employing two strengthening schemes (CFRP and TRM) were tested. Specimens were supported on two opposite edges. Experimental load-displacement variations show two peak loads in strengthened slabs and one peak followed by a plateau in control. Second peak or the plateau corresponds to the combined action of aggregate interlock and the dowel action of back face rebars and strengthening layers. The dowel action of back face rebars and strengthening layers had no role in ultimate punching load (i.e. first peak). Strengthened slabs showed 9-18% increase in ultimate punching load (i.e. first peak) whereas there was significant increase in the second peak load (190-276% for CFRP; 55-136% for TRM) and energy absorption (~66% for CFRP and 22-56% for TRM). An analytical model was also developed for predicting the punching shear strength (first and second peaks) of strengthened slabs showing good comparison with experiments.

Local Prevention of Hardening for Punching of Hot-Stamped Parts

Punching portions of the sheet are sandwiched between the ceramic billets during rapid resistance heating to prevent hardening of these portions. When the heating temperature is locally lower than that of the austenitic transformation, i.e. below 800 oC, this portion is not hardened without occurrence of martensitic transformation, and thus cold punching of hot-stamped parts becomes easy. The ceramic billets are made of alumina and the heat transfers to the billets. The temperature distribution just after resistance heating, the hardness distribution of the hot-stamped sheet, the cold punching load, the quality of the punched hole, etc. were measured. Hardening of punching portions was successfully prevented by sandwiching between the ceramic billets. The cold punching load for the local prevention of hardening was half of that without local prevention and the delayed fracture was also prevented, whereas the drop in hardness around the sheared edge became larger than that for laser cutting.

Behavior of Concrete Slabs Reinforced with Composite Patch Under Centric Punching Load

Behavior of reinforced concrete slabs with composite patches under centric punching load is presented in this paper. An experimental investigation is being performed. The parameters considered are: the ratio of patch size and the nature of the composite. Several reports are considered to account for the strengthened specific area. The dimensions of the patches are calculated so that the ratio patch XP /YP is proportional to XD/ YD of the slab. Two types of composite CFRP and composite with cork fibers reinforcement are also considered in this study. The main objective is to evaluate the resistance of the concrete slabs reinforced by such composite patches and to define the parameters influencing the stiffness behavior. Using the Kinnunen and Nylander mechanism of rupture, an analytical model to predict the behavior of this type of tiles is proposed. An experimental investigation completed by theoretical modeling is carried out in order to assess the performances of the concrete slabs reinforced with composite patches under centric punching load. The average test and numerical results based on the finite element analysis in terms of overall and observed response are discussed and analyzed. The results show an improvement in terms of strength and mechanical performance function of the ratio of the dimensions of the composite patch which have a direct influence on the deformation before rupture.

Hot semi-punching and cold scrap removing processes for hot stamping of ultra-high strength steel parts

Hot semi-punching and cold scrap removing processes for hot stamping of ultra-high strength steel parts were developed to eliminate laser cutting having low productivity after hot stamping. A quenchable steel sheet is semi-punched without separation of punching scraps during hot stamping, and subsequently, the scraps are removed from the hot-stamped part at room temperature. For hot semi-punching, additional channels for taking the punching scraps out of dies are not required. Minimum remainder without separation of punching scraps and no clearance between the die and punch were optimal for the hot semi-punching process. The hot punching and cold removing loads of the quenched sheet were considerably smaller than the cold punching load, the quality of the hole edge was high and the delayed fracture around the sheared edge was prevented. Hot semi-punching and cold scrap removal were applied to a hot hat-shaped bending process.

The effect of the vertical component of prestress forces on the punching strength of flat slabs

The use of prestress in flat slabs is a common solution, mainly because it allows larger spans and thinner slabs. Nevertheless, smaller thicknesses near the slab-column connections, along with the superposition of high shear and flexural stresses, arise the question of the slab capacity to resist punching. The punching failure results from the superposition of shear and flexural stresses near the column, and is associated to the formation of a pyramidal plug of concrete which punches through the slab. It is a local and brittle failure. The use of prestress can increase the punching capacity of flat slabs-column connections.This work presents the experimental analysis of flat slab specimens with tendons under punching. Nine slabs were tested using unbonded prestress with high strength steel tendons. The influences on the punching capacity of the vertical component of the prestress forces resulting from inclined tendons near the column and their distance to the column are analysed. The in-plane compression force due to prestress was not applied to the slabs, in order to evaluate only the deviation force influence. This work aims to improve the understanding of the behaviour of prestressed flat slabs under punching load in order to properly evaluate the punching resistance of this kind of structures. The experimental punching loads are compared with the provisions of EC2, ACI 318-11 and MC2010.

Hot Semi-punching of Quenchable Steel Sheet

Hot semi-punching of a quenchable steel sheet was carried out to eliminate laser cutting conventionally used for hot stamping of ultra-high strength steel parts. A quenchable steel sheet is semi-punched without separation of punching scraps during hot stamping, and subsequently, the scraps are removed from the hot-stamped part at room temperature. Additional channels for taking punching scraps out of dies are not required. Minimum remainder without detachment of punching scraps and no clearance between the die and punch were optimal for the hot semi-punching process. The hot punching and cold removing loads of the quenched sheet were considerably smaller than the cold punching load, the quality of the hole edge was high and the delayed fracture around the sheared edge was prevented.

ホットスタンピング部材の穴抜きにおける熱間抜残し・冷間抜取り加工と局部焼入れ防止

Hot semi-punching and cold scrap removing processes were developed to make holes in hot-stamped steel parts having high strength. For hot semi-punching, additional channels for taking punching scraps out of dies are not required. Achieving the minimum amount of residue without detachment of punching scraps and no clearance between the die and punch were the best conditions for the hot semi-punching process. The hot punching and cold removing loads were much smaller than the cold punching load of quenched sheets, the quality of the sheared edge was high and the delayed fracture around the sheared edge was prevented. In addition, quenching of punching portions was prevented by sandwiching these portions between ceramic tools during rapid resistance heating to facilitate cold punching of hot-stamped parts. The cold punching load for the local prevention of quenching was half of that without local prevention and the delayed fracture was also prevented, whereas the drop in hardness around the sheared edge became larger than that measured for laser cutting.

Effectiveness of Externally Applied CFRP Stirrups for Rehabilitation of Slab-Column Connections

AbstractThe application of externally bonded carbon fiber-reinforced polymer (CFRP) stirrups is by now proven as an effective technique to strengthen slab-column connections for punching shear. However, there has not been adequate study into its effectiveness for repair and rehabilitation of existing slab-column connections with different levels of prior damage. The punching performance of predamaged slab-column connections rehabilitated with CFRP stirrups was investigated for the first time in this study. Two series of 2/3 scale slab-column connections with two different concrete compressive strengths (low and normal) were rehabilitated using CFRP stirrups at different damage levels. The specimens were all tested under a concentric punching load. Test results showed that externally applied CFRP stirrups could restore the punching shear capacity of fully damaged slabs to levels above their undamaged conditions. The lower concrete strength proved to have no detrimental effect on the effectiveness of the CF...

Comparative Study for Punching of FRP-Reinforced Concrete Slabs

The noncorrosive fiber-reinforced polymer (FRP) composite bars are an excellent alternative reinforcement. In this paper, a review of the available punching load equations of two-way FRP-reinforced slabs and shear strength equations of one-way FRP-reinforced slabs is provided. All the models were verified against results for test specimens from the literature. The equivalent steel ratio model of Theodorakopoulos and Swamy produced the best estimates for the punching load while the model of Deitz, Harik, and Gesund yielded the best predictions of shear capacity. One of the punching strength models was modified in the current research and it gave excellent results. Using this modified model, it is proved that the increase in the reinforcement ratio is accompanied by an increase in the punching strength.

Cold-pressing deformation characteristics of steel–aluminum sheets joining

In order to overcome the deficiencies of the traditional connection of dissimilar sheet metals, a new method “cold-pressing deformation joining” was proposed. By using finite element simulation software DEFORM™-3D, the joining process of the pure aluminium and stainless steel was simulated while the sample size is 100 mm×30 mm and the thicknesses are 1 mm and 0.5 mm, respectively. The results indicate that the punching load presents an increasing trend with the increase of the punch downward displacements. The thickness of stainless steel sheet decreases gradually from the centre to the two sides in the process. In contrast, the thickness of aluminum sheet changes a lot. Experimental scheme and simulation are in good agreement. The unidirectional tensile test demonstrates that with the increase of punch downward displacements, the average thickness of connection area decreases while the connection strength increases before decreasing. The thicknesses in gauge length of connection are not uniformly distributed and the position 15 mm away from the aluminum end shows the largest thickness.

Punching shear tests on flat concrete slabs exposed to fire

Underground parking structures often consist of flat slabs connected by columns, for which punching shear is often the most critical design criterion. In fire conditions, the punching load can increase due to restraint of the thermal curvature of the slab or due to the expansion of the columns. This increase of the punching load is discussed in the paper by means of a literature review. On the other hand, during fire the punching resistance of the slab decreases due to a gradual reduction of the material properties. This reduction in bearing capacity is studied by means of real scale fire tests, consisting of 6 slabs measuring 3.2×3.5×0.25m with a connected column stub and tested for punching shear with a specially designed loading frame. Two reference tests are executed at ambient temperature conditions and four slabs are submitted to ISO 834 curve for 120min. Comparison of the test data with the expected increased axial load due to thermal restraint found in the literature, shows a potential danger for premature punching failure of flat slab-column connections exposed to fire.

Thermo-mechanical analysis of an underground car park structure exposed to fire

Underground concrete structures composed of flat slab-column connections are sensitive to punching failure at ambient temperatures. During fire, however, due to the restraint of thermal curvatures an increase of the punching load can occur, possibly leading to premature collapse. In this paper, this problem is studied numerically by the combination of CFD and mechanical finite element analysis, whereas further considerations are developed by the use of plasticity theory. The basis for the fire scenario used in the analysis is found in the literature as a real fire. The Belgium standard NBN S 21-208-2 for the design of the ventilation system in underground car parks with respect to car fires is used as well. The dimensions of the underground car park are based on a real fire accident that occurred in Gretzenbach, where the structure actually failed due to punching following a rather small fire. In the analysis, special attention is given to the increase of the axial load on the slab-column connection with temperature. For the mechanical properties and boundary conditions assumed, it is found that during fire the axial load may increase with a factor which, at least in the worst cases, can be close to the safety factor found from large scale fire tests.

Punching of small hole of die-quenched steel sheets using local resistance heating

A punching process of a small hole in a die-quenched steel sheet having high strength using local resistance heating of a shearing zone was developed to decrease the punching load. Uniform temperature in the circular shearing zone of the hole was obtained by optimising heating conditions for a pair of rectangular electrodes. The punch load and the burnished surface area for the heating at 500°C were about 1/3 smaller and 2 times larger than those for the cold punching, respectively, and the occurrence of delayed fracture around the punched hole was prevented by the heating above 500°C.

Punching of ultra-high strength steel sheets using local resistance heating of shearing zone

A punching process using local resistance heating of a shearing zone was developed to shear ultra-high strength steel sheets. The shearing zone was heated by passing electric current between the sheet holder and the knockout in order to decrease the flow stress in the shearing, and the heating of the die and punch was prevented by no contact with the sheet during the heating. Electrode pins having an individual spring were employed to attain uniform heating of the shearing zone. The welding resistance of the heads of the electrode pins to the sheet by the heating was examined for Ag–W, Cu–W, Ag + WC and W. The Cu–W pins having the highest welding resistance were employed in a punching experiment of 980 MPa level ultra-high strength steel sheets. The punching load was considerably reduced by the heating, e.g., about 1/5 of the cold punching load at 800 °C. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased.

Influence of geometric factors on the punching load resistance of early-age fibre reinforced shotcrete linings

Understanding the early-age properties of fibre reinforced shotcrete (FRS) is critical to the accurate estimation of the time to safe re-entry under freshly sprayed FRS linings. Previous research in the field has indicated that shear failures dominate early-age load resistance and thereby govern the performance of a lining during the first few hours after spraying. This investigation has examined the influence of several geometric variables on shear resistance in a punching mode of failure. In particular, the research has addressed the influence of the size, shape, eccentricity, and inclination of punching loads on the apparent shear strength of an early-age fibre reinforced shotcrete (FRS) lining.

FRP 시트 및 강섬유 보강 콘크리트의 저속 충격에서의 휨 및 펀칭 파괴 거동

이 연구에서는 FRP와 강섬유로 보강한 콘크리트 시편의 충격하중과 정적하중에서의 거동을 보기 위해 휨 실험과 펀칭 실험을 수행하였다. 1방향 휨 실험과 2방향 펀칭 실험에서 콘크리트 시편은 각각 50 × 100 × 350 mm와 50 × 350 × 350 mm의 크기로 제작하였다. 0.75% 혼입률의 강섬유 보강 콘크리트는 2방향 충격하중 및 정적하중에서 높은 저항 성능을 보였다. 일반 콘크리트와 강섬유 보강 콘크리트에서 FRP 보강은 높은 성능 증가를 보였다. 초고성능 콘크리트는 콘크리트 자체가 가지고 있는 높은 인장강도와 인성으로 인해, CFRP로 보강한 경우 강도와 에너지 소산 능력이 크게 증가하지 않았다.