Pin Penetration Research Articles

Optimization and Analysis of Refill Friction Stir Spot Welding (RFSSW) Parameters of Dissimilar Aluminum Alloy Joints by FE and ANN Methods.

The quality of the refill friction stir spot welding (RFSSW) process is heavily dependent on the selected welding parameters that influence the resultant joint characteristics. Thermomechanical phenomena integral to the process were investigated using finite element (FE) analysis on two dissimilar materials. This FE analysis was subsequently validated through controlled experiments to ensure reliability. An artificial neural network (ANN) was employed to create a neural model based on an experimental setup involving 120 different sets of welding parameters. The parameters adjusted in the experimental plan included pin penetration depth, rotational speed, retention time, and positioning relative to material hardness. To assess the neural model's accuracy, outputs such as maximum temperature and normal stress at the end of the welding process were analyzed and validated by six data sets selected for their uniform distribution across the training domain.

A New Methodology to Estimate the Early-Age Compressive Strength of Concrete before Demolding

Non-destructive testing has many advantages, such as the ability to obtain a large number of data without destroying existing structures. However, the reliability of the estimation accuracy and the limited range of applicable targets remain an issue. This study proposes a novel pin penetration test method to determine the early-age compressive strength of concrete before demolding. The timing of demolding and initial curing is determined according to the strength development of concrete. Therefore, it is important to determine the compressive strength at an early age before demolding at the actual construction site. The applicability of this strength estimation methodology at actual construction is investigated. Small test holes (12 mm in diameter) are prepared on the mold surface in real construction sites and mock-up specimens in advance. The pin is penetrated into these test holes to obtain the relationship between the compressive strength and the penetration depth. As a result, it is confirmed that the pin penetration test method is suitable for measuring the early-age compressive strength at the actual construction site. This allows the benchmark values for compressive strength, necessary to avoid early frost damage, to be directly verified on the concrete structural members at the construction site. For instance, the compressive strengths of greater than 5 MPa and 10 MPa can be confirmed by the penetration depths benchmark values of 8.0 mm and 6.7 mm or less, respectively.

Welding Speed and Pin Penetration Interaction in Mechanical Properties of Friction Stir Welded Lap-Joint AA6061/316 Stainless Steel

Welding Speed and Pin Penetration Interaction in Mechanical Properties of Friction Stir Welded Lap-Joint AA6061/316 Stainless Steel

Pin penetration depths in the neurocranium using a three-pin head fixation device

In estimated 10–15% of neurosurgical interventions employing a conventional three-pin head fixation device (HFD) the patient’s head loses position due to slippage. At present no scientifically based stability criterion exists to potentially prevent the intraoperative loss of head position or skull fractures. Here, data on the skull penetration depth both on the single and two-pin side of a three-pin HFD are presented, providing scientific evidence for a stability criterion for the invasive three-pin head fixation. Eight fresh, chemically untreated human cadaveric heads were sequentially pinned 90 times in total in a noncommercially calibrated clamp screw applying a predefined force of 270 N (approximately 60 lbf) throughout. Three head positions were pinned each in standardized manner for the following approaches: prone, middle fossa, pterional. Titanium-aluminum alloy pins were used, varying the pin-cone angle on the single-pin side from 36° to 55° and on the two-pin side from 25° to 36°. The bone-penetration depths were directly measured by a dial gauge on neurocranium. The penetration depths on the single-pin side ranged from 0.00 mm (i.e., no penetration) to 6.17 mm. The penetration depths on the two-pin side ranged from 0.00 mm (no penetration) to 4.48 mm. We measured a significantly higher penetration depth for the anterior pin in comparison to the posterior pin on the two-pin side in prone position. One pin configuration (50°/25°) resulted in a quasi-homogenous pin depth distribution between the single- and the two-pin side. Emanating from the physical principle that pin depths behave proportionate to pin pressure distribution, a quasi-homogenous pin penetration depth may result in higher resilience against external shear forces or torque, thus reducing potential complications such as slippage and depressed skull fractures. The authors propose that the pin configuration of 50°/25° may be superior to the currently used uniform pin-cone angle distribution in common clinical practice (36°/36°). However, future research may identify additional influencing factors to improve head fixation stability.

Laboratory and in-situ mechanical characterisation of masonry components by comparing destructive and minor destructive testing techniques

The safeguard of existing masonry buildings requires a meticulous characterisation of the component materials (units and mortar) by using in-situ minimally intrusive techniques. This need is especially prominent in the case of heritage buildings in order to minimize their damage and alteration. This paper presents a research on some minor destructive techniques (MDT) for the in-situ evaluation of the mechanical properties of masonry components. In the research, the results derived from MDT were empirically correlated with the compressive strength obtained by using well-known destructive testing (DT) techniques executed in the laboratory. The experimental campaign focused on two different MDT techniques, namely the Pin Penetration Test (PPT) and the Helix screw Pull-out Test (HPT). The PPT is based on the measurement of the depth of penetration of a steel pin. The HPT is based on the measurement of the pull-out load necessary to extract a steel tie inserted into a hole drilled in a brick or in a mortar joint. The experimental campaign also included an evaluation of the Double Punch Test (DPT) by comparison with strength values obtained from mortar specimens prepared and tested following the standard EN 1015–11. The experimental results show a satisfactory agreement between the MDT measurements and the compressive strength obtained with DT. Although both the PPT and HPT are useful for the evaluation of the compressive strength of masonry components, the former exhibits a lower scattering in the experimental measurements.

Experimental investigation of the clinching process for joining the three-layer aluminum/polymer/aluminum composite sheets

Three-layer aluminum/polymer/aluminum composite sheets are among the new materials developed to reduce the weight and fuel consumption of vehicles. Using the conventional methods for joining these materials to other materials is challenging. In this paper, the joinability of the three-layer aluminum/polymer/aluminum to a single-layer 1 mm-thickness aluminum sheet, in the clinching process is investigated. Three-layer sheets of AA5754/polyethylene/AA5754 with thicknesses of 0.5/0.6/0.5 mm were produced under laboratory conditions using two different methods; with and without a local reinforcement piece in the polymer core. The prepared specimens are joined using various geometric parameters of the clinching tools. The joint sections and their geometric parameters including interlock and neck thickness are evaluated in different joint conditions. Also, the strengths of the joints are examined by shear and peel tests. Studies show that it is possible to use the clinching process to join aluminum/polymer/aluminum sheets. Also, with a proper design of tools, the joint strength can be in the same order as the strength of the clinching of single-layer sheets. The maximum shear and peel test strengths, obtained in this study are 1288 N and 540 N, respectively. Increasing the pin penetration depth increases the interlock up to an optimal value. However, further increases in the pin penetration depth will decrease the neck thickness and joint strength. The conical angle of the pin, increasing the die cavity depth, and using a local reinforcement piece reduces the strength of the clinched joint and interlock in these materials. In all the test conditions, the most suitable joint conditions were when the failure mode was combined bottom separation and neck fracture mode.

Neural Networks Applied for Predictive Parameters Analysis of the Refill Friction Stir Spot Welding Process of 6061-T6 Aluminum Alloy Plates

Refill friction stir spot welding (RFSSW) technology is a solid-state joint that can replace conventional welding or riveting processes in aerospace applications. The quality of the new welding process is directly influenced by the welding parameters selected. A finite element analysis was performed to understand the complexity of the thermomechanical phenomena during this welding process, validated by controlled experiments. An optimization model using neural networks was developed based on 98 parameter sets resulting from changing 3 welding parameters, namely pin penetration depth, pin rotation speed, and retention time. Ten parameter sets were used to verify the learning results of the optimization model. The 10 results were drawn to correspond to a uniform distribution over the training domain, with the aim of avoiding areas that might have contained distortions. The maximum temperature and normal stress reached at the end of the welding process were considered output data.

Metallurgical and mechanical investigation on FSSWed dissimilar aluminum alloy

The present work is the novel examination of influence of welding process parameters and material flow on the mechanical and microstructural characteristics during the FSSW of dissimilar AA2024/AA7075 joint were evaluated and compared with similar AA2024/AA2024 and similar AA7075/AA7075 FSSW joints. The microstructural and physical properties of two aluminum alloy joined through friction stir spot welding (FSSW) were examined. The aluminum alloys namely AA2024-T3 and AA7075-T6 were selected for the investigation owing to their typical and potential applications in aeronautical industry. The input parameters were tool rotational speed, plunge depth and dwell time (TRS 1800 rpm, 3.3 mm plunge depth and 45 s dwell time) were utilized. The microstructure of the welded specimen was investigated by scanning electron microscope and their chemical compositions were analyzed by energy-dispersive X-ray spectroscopy. The physical properties were investigated by Vickers microhardness survey and the specimen strength evaluated by tensile tests. The experimental investigation revealed the influence of the tool pin penetration and frictional heat on the microstructure, distribution of the grain structure affecting the microhardness and the tensile properties of the dissimilar joint.

A METHOD TO ESTIMATE THE EARLY AGE COMPRESSIVE STRENGTH OF CONCRETE BEFORE DEMOLDING USING A PIN PENETRATION DEVICE

This study proposed a method to estimate the compressive strength of early-age concrete before demolding using a pin penetration device. Small holes were prepared on molds to apply for this method. A strong correlation was obtained between pin penetration depth and compressive strength despite the materials and mix proportions difference. This method was applicable between 5 and 15 MPa compressive strength, which is the demolding criterion. Within the experimental results, the penetration depths of 12.5, 9.5, and 8.5 mm (Flat), and 13.0, 10.0, and 9.0 mm (Sharp) were sufficient to demolding criteria of 5, 10, and 15 MPa, respectively.

Experimental models of bone marrow lesions in ovine femoral condyles.

To develop an in vivo experimental model for bone marrow lesions (BMLs) in ovine femorotibial joints. Randomized, prospective experimental study. Eighteen healthy, skeletally-mature Dorper cross ewes. One medial femoral condyle was penetrated with a 1.1 mm pin, and the contralateral medial femoral condyle was treated with transcutaneous extracorporeal shockwave (ESW) at 0.39 ± 0.04 mJ/mm2 . Clinical examination, magnetic resonance imaging (MRI), computed tomography (CT), and histopathological analyses were used to detect and characterize the development and progression of BMLs in the medial femoral condyle at 4, 8, and 12 weeks post-surgery. Pin penetration induced a BML detected on MRI within 2 weeks and lasted at least 12 weeks. BMLs were not observed in ESW-treated condyles. Histologically, BMLs were characterized by hemorrhage and inflammatory cellular infiltrate, and progressed to more dense fibrous tissue over time. Pathological changes were not observed in the articular cartilage overlying the region of BMLs. Direct, focal trauma to all layers of the osteochondral unit was sufficient to create an experimentally-induced BML which persisted for at least 90 days. The protocol used for ESW in this study did not induce BMLs. Experimental induction of BMLs is possible and mimicked naturally occurring disease states. Volumetric imaging is a sensitive method for characterization of the dynamic nature of these lesions.

Comparative Evaluation of Investigation Testing Methods in Timber Elements considering Moisture Content

ABSTRACT Wood components in existing buildings suffer from deterioration and damage due to environmental conditions and loading effects. Their preservation strongly depends on the knowledge level of their onsite condition supported by the application of low-intrusive methods of investigation. Estimating the mechanical performance of wood in constructions cannot disregard the evaluation of moisture content. In this paper, 31 wood elements (19 recovered from disassembled buildings and 12 new) underwent a series of non-destructive tests (i.e., ultrasonic and sonic stress wave methods) and semi-destructive tests (i.e., resistance drilling and pin penetration). Red/white firs and pine species constituted the recovered elements; red fir and larch the new ones. The effect of moisture content on the results of low-intrusive methods was estimated by either electric or electromagnetic devices. Destructive tests (in bending and compression) were also carried out on 10 recovered and six new elements, and results were correlated with those of semi-destructive tests.

Accuracy of end-on fluoroscopy in predicting implant position in relation to the vertebral canal in dogs.

To evaluate the accuracy of end-on fluoroscopy in predicting implant position in relation to the vertebral canal in the canine thoracolumbar vertebral column. In vitro imaging and anatomic study. Canine cadaveric thoracolumbar vertebral columns (n = 5). Smooth Steinmann pins were inserted bicortically into the thoracolumbar vertebral columns between T10 and L7 using recommended insertion angles. Penetration of the spinal canal was not strictly avoided. After pin placement, end-on fluoroscopy images were obtained of each pin. Pin position was subsequently assessed by four evaluators and determined to either being out of the vertebral canal or in, with the latter being additionally divided into partially or completely penetrating the canal. To assess potential differences in modalities, fluoroscopy images were gray-scale inverted and evaluated again later by the same four individuals. Correct identification of pin position in relationship to the vertebral canal was assessed for both fluoroscopy images. Anatomic preparation of the spines was used for verification of pin position in relation to the spinal canal. Some data from this study were compared with historical data on accuracy using orthogonal radiography and computed tomography (CT). Overall sensitivity and specificity of F to detect vertebral canal penetration was 98.8 % (95% confidence interval (CI), 96.0-99.6) and 98.0% (95% CI, 77.0-99.9), respectively. For Fi, sensitivity and specificity were 97.0% (95% CI, 91.5-99.0) and 98.5% (95% CI, 81.5-99.9) respectively. F exceeded Fi for the sensitivity of detecting pin penetration into the vertebral canal (p = 0.039) but specificities were not different (p = 0.585). When comparing to historical data, the overall accuracy of end-on fluoroscopy (F) and inverted fluoroscopy (Fi) was statistical better than conventional radiographic assessment (p < 0.001). End-on fluoroscopy is a highly accurate method for the assessment of pin position in relationship to the thoracolumbar spinal canal in cadaveric dogs. End-on fluoroscopy, with or without inversion, is accurate in identifying vertebral canal violation by bicortically placed Steinmann pins. When CT is not available, end-on fluoroscopy might be a valuable imaging modality to determine pin position in the canine vertebral column.

Application of a Wireless and Contactless Ultrasonic System to Evaluate Optimal Sawcut Time for Concrete Pavements.

A recently developed contactless ultrasonic testing scheme is applied to define the optimal saw-cutting time for concrete pavement. The ultrasonic system is improved using wireless data transfer for field applications, and the signal processing and data analysis are proposed to evaluate the modulus of elasticity of early-age concrete. Numerical simulation of leaky Rayleigh wave in joint-half space including air and concrete is performed to demonstrate the proposed data analysis procedure. The hardware and algorithms developed for the ultrasonic system are experimentally validated with a comparison of saw-cutting procedures. In addition, conventional methods for the characterization of early-age concrete, including pin penetration and maturity methods, are applied. The results demonstrated that the developed wireless system presents identical results to the wired system, and the initiation time of leaky Rayleigh wave possibly represents 5% of raveling damage compared to the optimal saw cutting. Further data analysis implies that saw-cutting would be optimally performed at approximately 11.5 GPa elastic modulus of concrete obtained by the wireless and contactless ultrasonic system.

Open technique for supra-acetabular pin placement in pelvic external fixation: a cadaveric study

BackgroundStandard supra-acetabular pin placement still needs fluoroscopic guidance, which is technically demanding for an untrained surgeon due to the unfamiliar fluoroscopic view and the risk of damaging some structures. The risks associated with the open technique without fluoroscopy have not yet been investigated, despite the palpable entry point and large bony corridor for rapid insertion in the limited time available for the management of unstable pelvic fracture patients. The aim of this study was to compare the open technique without fluoroscopy to the fluoroscopically assisted percutaneous technique for the positioning of supra-acetabular screws in cadavers without pelvic instability.Materials and methodsThe open technique for half-pin placement was compared to standard fluoroscopic guidance in 16 hemipelves (8 cadavers). The open technique was first performed on one side in each cadaver after simple randomization, followed by standard fluoroscopic guidance on the other side. In the open technique group, a Schanz pin 5 mm in diameter and 200 mm in length was inserted in the area just above the anteroinferior iliac spine (AIIS) and aimed with a medial inclination of 20° and a cephalad inclination of 10–20° after a 2 cm pilot drill hole had been established. Standard fluoroscopically guided pin placement was performed on the other side. Fluoroscopic assessment was conducted after final pin placement on both sides. The lateral femoral cutaneous nerve of the thigh (LFCN) and the hip capsule were identified via the Smith–Peterson approach. After complete dissection of soft tissue, it was clearly apparent that pin penetration was conducted outside the bony corridor.ResultsThe LFCN was found to be in a risk zone near the pin (mean distance, 15 mm; range, 0–30 mm). One LFCN may have been injured in the fluoroscopic guidance group. The mean medial inclination of the pin was 19.8° (range, 5–40°) and the mean cephalad inclination was 11.5° (range 0–20°) in the open technique group. The mean medial inclination of the pin was 30.4° (range, 20–45°) and the mean cephalad inclination was 19.3° (range, 2–35°) in the fluoroscopic guidance group. The mean distance of the pin entry point from the AIIS was 11.1 mm (range, 0–35 mm) in the open technique group. The mean distance of the entry point of the pin from the AIIS was 15.1 mm (range, 0–25 mm) in the fluoroscopic guidance group. The mean hip joint capsule distance was 12 mm (range, 8–25 mm). No joint penetration was observed in the open technique group, compared to one joint penetration in the fluoroscopic guidance group. No sciatic notch penetration was found in either group, but pin penetration outside the external cortex of the ilium was found only in the open technique group, in 4 hemipelves.ConclusionsThis study shows that the freehand technique performed by experienced trauma surgeons may be as acceptable as controlled pin insertion under image intensification for selecting the proper entry point and stabilizing the anterior pelvic ring.

Fiber response to pin penetration in dry woven fabric using numerical analysis

A new type of washer with integrated pin structures was developed and presented in a previous study for the use in mechanical joints of fiber-reinforced plastics (FRP) and metal sheets. Thereby, the pin-structured washer significantly increases the load-bearing capacity of mechanically joined FRP-metal combinations. To do so, the FRP hole area is to be relieved by transferring the occurring loads into the laminate via the pin structures. In thermoset FRP, the insertion of the pin-structured washer usually leads to fiber breaks. For fiber-reinforced thermoplastics (FRTP) the chance of fiber failures during the penetration process can be reduced by inserting the pin-structured washer via an ultrasonic-based locally invasive heating system. By doing this, the polymer is locally molten, allowing the fibers to move around the pin structures. However, the ultrasonic-based insertion does not allow visual observation of the fiber movement within the process. With the aim of getting information about the fibers, that are moved, stressed or broken depending on the pin tip geometries, a numerical investigation was carried out. Therefore, a 3 × 3 twill model of a dry woven fabric segment based on a Bézier curve for yarn simplifications was developed. With respect to the molten polymer during the penetration process, the effect of the matrix was neglected. With consideration of necessary simplifications regarding the calculation time, the stress distribution on the dry woven fabric and a regular pattern of the damage situation of the fibers could be investigated. To understand the mechanisms of the fiber behavior within the penetration process, three different pin diameters and pin tip angles were examined and compared in numerical simulations.

Investigation of Thermoplastic Polyurethane Finger Cushion with Magnetorheological Fluid for Soft-Rigid Gripper

This paper presents a study of penetrating a pin into a magnetorheological fluid (MR) cushion focused on the force measurement. The research is supported by detailed finite element analysis (FEA) of the magnetic field distributions in several magnetic field exciters applied to control rheological properties of the MR inside the cushion. The cushion is a part of the finger pad of the jaw soft-rigid gripper and was made of thermoplastic polyurethane (TPU) using 3D printing technology. For the pin-penetrating setup, the use of a holding electromagnet and a magnetic holder were considered and verified by simulation as well as experiment. In further simulation studies, two design solutions using permanent magnets as the source of the magnetic field in the cushion volume to control MR fluid viscosity were considered. The primary aim of the study was to analyze the potential of using an MR fluid in a cushion pad and to investigate the potential for changing its viscosity using different magnetic field sources. The analysis included magnetic field simulations and tests of pin penetration in the cushion as an imitation of object grasping. Thus, an innovative application of 3D printing and TPU to work with MR fluid is proposed.

Effect of Rotational Speed, and Dwell Time on the Mechanical Properties and Microstructure of Dissimilar AA5754 and AA7075-T651 Aluminum Sheet Alloys by Friction Stir Spot Welding

In the current study, the effects of dwell time and rotation speed on the mechanical properties and microstructure of friction stir spot welded joints of dissimilar aluminum sheet alloys were investigated. Aluminum AA5754 and AA7075-T651 alloys were selected as the work piece. The joint quality, microstructural evolution and mechanical behavior of the welded regions were considerably affected by the welding parameters. The results obtained that rotation speed and dwell time play an important role on welding quality of aluminum sheet. The microstructure images showed the dwell time and rotation speed has great effects on pin penetration and hook deformation. Maximum tensile shear load 806.3 N was produced at 1000 rpm and 2 s dwell time, while the tensile shear load reduced around 25 % with longer dwell time 5 s and high rotation speed 1400 rpm. Moreover, the welding joint microhardness was improved by the decrease of dwell time and increase of rotation speed.

Evaluation of pin penetration probing technique for the assessment of basic density in air-dried wood

Fast and accurate evaluation of the physical and mechanical properties of engineering ‎materials is of particular importance. The in situ semi-destructive and non-destructive tests versus the ‎static tests for determining the time-consuming physical properties have replaced many traditional ‎methods with reasonable accuracies. Determining the density as one of ‎the most important qualitative and quantitative parameters in the inspection of wood and wood-based ‎products is of great importance. For this purpose, 33 wood specimens from 11 species with varying densities were tested by pin penetration probing. Results were compared with those from ‎the basic density values from traditional methods. The results showed an exponential relationship between the pin penetration depths ‎and the basic density considering the moisture conditions but without any problems. The coefficients of ‎determination while estimating the equality of the basic density via pin penetration probing with the actual ‎basic density for both the testing specimens and the control samples were always over ‎‎0.8. Henceforth, this methodology suggested that the density evaluation could inspire higher precision than what has been achieved in previous efforts.

Experiments on Friction Stir Processing (FSP), with Tilted Parent Metal

Friction stir processing (FSP), with tilted parent metal is a new process, which consists in the use of an inclined base plate with an angle α = 1 - 3 º related to the table of the FSP processing machine, by mounting a calibrated part under one end of the base plate. Two clamping plates are fixed with screws on the base plate, and a sheet (parent metal, PM) or a workpiece is placed between the base plate and the clamping plates. The processing tool has the same tilt angle with respect to the sheet to be processed. Said parts are components of the jig for the process described. The processing tool performs a movement with the speed v(x), correlated with a simultaneous movement with the speed v(z) = v(x) tg α. The correlation is achieved by means of a program developed for the processing machine. Due to the tilt, during the FSP process, the leading edge of the rotating tool shoulder progressively enters the PM, continuously, at the point where the shoulder penetration depth in the PM is minimal. On the multifunctional friction processing machine, MMPF type, the processing by the described process of an aluminium alloy sheet, with the sizes 300 mm x 200 mm x 4 mm was performed. A hardened C45 steel tool was used, having the following technical characteristics: shoulder diameter 14 mm, smooth frusto-conical pin with a large diameter of 2.5 mm, a small diameter of 2.0 mm, and a height of 1.5 mm. The parameters had the following values: tilt angle α = 2° 6'; pin penetration depth h = 1.60 - 1.85 mm; tool speed n = 1500 - 2000 rpm; speed v(x) = 1 - 2 mm / s; speed (software correlated) v(z) = 0.03672181 * (1 - 2) mm / s; the temperature of the sheet behind the tool t = 240 - 420 °C. The appearance of the processed sheet is appropriate. No imperfections are observed. The results are appropriate. The tilted PM brings the following advantages of the process: the mechanical stress to produce burrs and / or chips on the surface of the PM decreases; the way the FSP process is carried out is improved, by reducing gap moving and vibrations; the appearance and mechanical characteristics of the processed areas are improved; the quality level of the executed products rises; wear of processing tools is reduced; energy efficiency increases; the electricity consumption of the machine decreases; productivity increases.

Use of a pin-penetration wood density meter to determine the density of 25 Indonesian species

The pin-penetration device is a minimally destructive instrument that is widely used to estimate the physical properties of wood, e.g., density, with advantages such as reduced testing times, low costs, and fewer defects induced in the wood being tested. In this study, such a device was used on 25 Indonesian hardwood species with a strength class (SC) distribution from SC II to IV (according to the Indonesian classification of wood SCs). Tests were conducted on three different orthogonal planes, i.e., cross-sectional, radial, and tangential planes. The wood density ranged from 0.28 to 0.88 g/cm3, and the specific gravity was 0.25 to 0.76. The cross-sectional plane penetration depth of the wood was significantly greater than that of either the radial or tangential plane, whereas the pin penetration values of the radial and tangential planes were not significantly different. A prediction model for predicting the density and specific gravity of wood via pin penetration showed a significant regression. Thus, the use of a pin-penetration device was found to be suitable for estimating wood density and specific gravity in a range of SCs of tropical wood species regardless of species.