Dynamic Mechanical Characteristics Research Articles

Dynamic Mechanical Characteristics of Limestone Under Cyclic Impact of Various Confining Pressure

Dynamic Mechanical Characteristics of Limestone Under Cyclic Impact of Various Confining Pressure

Numerical study on the anti-collision performance of recycled foam concrete beam guardrail under vehicle collision

Recycled foam concrete (RFC) possesses excellent energy absorption capabilities and can be used in the anti-collision field. To address the insufficient anti-collision performance of existing guardrails, this paper presented a novel RFC beam guardrail by combining RFC with a box-shaped steel shell. The quasi-static compressive tests and drop hammer impact tests were applied to analyze the static and dynamic mechanical characteristics of RFC with different densities and sand-cement ratios. Then, the vehicle collision performance of the RFC beam guardrail (RBG) and standard beam guardrail (SBG) was tested by LS-DYNA. The results showed that, compared with SBG, RBG had better guiding and energy absorption effects, reduced the peak acceleration by 44.15% and the maximum displacement by 73.22% during vehicle collisions, and mitigated the damage to the vehicle. Therefore, it is a viable way to utilize RFC to fabricate RBG, which could not only repurpose the construction and demolition wastes (CDW) but also acquire guardrails with enhanced anti-collision performance.

Dynamic mechanical response and failure characteristics of coal and rock under saltwater immersion conditions

The stability of coal and rock masses in water-rich mines is affected by both mine water erosion and dynamic disturbances. Thus, it is necessary to study the dynamic mechanical response and failure characteristics of coal and rock under the combination of saltwater and a high strain rate. To this end, a split Hopkinson pressure bar device was employed to investigate the effects of impact velocity, water content, and immersion liquid on the dynamic mechanical behaviours of coal and rock. The results revealed that the weakening effect of saltwater on the dynamic mechanical properties of coal and rock is much greater than that of distilled water. With increasing moisture content, the dynamic compressive strength of the coal specimens decreases monotonically, while that of the rock shows a trend of first increasing and then decreasing. The failure process and destruction of coal and rock are comprehensively affected by both the external impact load and the physical and mechanical properties of the material. The degree of damage of the coal and rock specimens increases with increasing impact velocity and water content. Moreover, the influence of various factors on the impact fracture mechanism of coal and rock under saltwater immersion conditions was revealed. These findings are highly important for the design and maintenance of underground coal and rock building structures.

Impact response characteristics and damage mechanism of continuous carbon-fiber-reinforced magnesium matrix composite materials

The continuous carbon-fiber-reinforced magnesium matrix composite (C_f/Mg) is a new type of composite material made of a magnesium alloy sheet and carbon-fiber-reinforced composite material with alternating layers. It has the advantages of high damage tolerance, light weight, and corrosion resistance, and has become the first choice of lightweight materials in aerospace, transportation, and other fields. Because impact-resistance serves as an important index for its structural design and performance stability, a continuous carbon-fiber-reinforced magnesium matrix composite with indirect bonding using a modified epoxy resin was designed in this study. Moreover, C_f/Mg was chosen to be the object of research through the selection of constituent materials such as magnesium alloys, carbon fibers, resins, and other properties, which matched the aim of realizing a lightweight material. With the help of hot-compression molding technology, dynamic impact mechanical behavior, and damage analysis technology, the structural design and preparation process optimization of epoxy resin carbon-fiber prepregs and magnesium alloy layer materials, as well as the dynamic impact mechanical response characteristics and damage evolution process under different impact energy conditions, were studied. Accordingly, through a combination of low-velocity impact experiments and numerical simulations, the effects of continuous multiple impacts at the same location with the same impact energy, as well as impacts with the same impact energy and different punch diameters, on the low-velocity impact damage behavior and dynamic impact response characteristics of C_f/Mg were investigated. The results of this study show that when the 5J impact is applied four times consecutively, the peak impact load gradually increases with the increasing number of impacts. Moreover, the peak impact loads of the second, third, and fourth impacts increase by 6.09%, 10.7%, and 14.5%, respectively, compared with the results of the first impact.

Regulation of the Properties of Polymers based on Thiirane using Mixtures of Amine Hardeners

Abstract: Introduction: The possibility of regulating the curing rate and the complex of adhesive, deformation-strength and dynamic mechanical properties of polymers based on bisphenol A dithioester (thiirane) using a mixture of amine hardeners of various chemical nature is investigated. Method: Diethylenetriamine, diethylenetriaminomethylphenol and aminopolyamide were investigated as hardeners. The ratio of the components of the mixed hardener is selected, which provides the best combination of strength properties. Results: It was found that the rate of adhesion and cohesive strength at the initial stage (during the first hour) of curing compositions containing a mixture hardener significantly exceeds compositions cured by individual components of the mixture. Conclusion: The results of measuring the dynamic mechanical characteristics of the studied polymers indicate that the dynamic modulus of elasticity, measured at temperatures below and above the transition from a glassy state to a high elastic one, for a sample containing a mixed hardener has an intermediate value between the values characteristic of samples containing individual components of a mixed hardener. objective: The ratio of the components of the mixed hardener is selected, which provides the best combination of strength properties.

Dynamic mechanical properties and energy release characteristics of Mg-Na-Al alloy under impact load

To investigate the ignition behavior of Mg-Na-Al alloy under impact load and its mechanical properties under different strain rates, the universal testing machine and Separated Hopkinson Pressure Bar (SHPB) experimental system were used to conduct static and dynamic mechanical performance tests of Mg-Na-Al alloy, and the mechanical properties of Mg-Na-Al alloy under different strain rates were discussed. The light gas gun experimental setup was used to launch Mg-Na-Al alloy projectiles and impact rigid target plates to study the energy release behavior of Mg-Na-Al alloy under impact loads. Finally, based on the 12.7 mm ballistic gun experimental platform, the penetration of 12.7 mm armor-piercing projectiles equipped with Mg-Na-Al alloy wind caps into the homogeneous armor steel plates with a thickness of 15 mm, and the ignition test of the oil box behind the steel plates were conducted. The results indicate that the yield strength of Mg-Na-Al alloy under static compression is 265 MPa, while under high strain rates (from 2490 s−1 to 5640 s−1), the yield strength of Mg-Na-Al alloy is 406–423 MPa, which is 53∼60% higher than that under quasi-static compression. When reaching a certain impact velocity, the Mg-Na-Al alloy projectile breaks and induces ignition behavior, releasing energy. The 12.7 mm armor-piercing projectile equipped with an Mg-Na-Al alloy wind cap penetrated the homogeneous armor steel plate with a thickness of 15 mm and ignited. The released energy and the kinetic energy of the fragments ignited the oil box behind the steel plate.

Dynamic Mechanical Response and Fracture Characteristics of Multi-flawed Rocks Exposed to Hydrostatic Confinements

Dynamic Mechanical Response and Fracture Characteristics of Multi-flawed Rocks Exposed to Hydrostatic Confinements

Dynamic compression intrinsic relationship of W-Ni-Fe-Mn alloys based on J-C modeling

Dynamic compression tests were performed on the sample alloys by using a detached Hopkinson compression bar with a diameter of 14.5 mm caliber for the kinetic behavior of W-Ni-Fe alloy and W-Ni-Fe-Mn alloy materials in the erosion process. The true stress-true strain curves of the two sample alloys were fitted based on the JC eigenstructure model, and the eigenstructure relationships of the two alloys were finally obtained. The obtained constitutive equations were then used to dynamically compress the specimens by using LS-DYNA dynamics software, and the simulated true stress-strain curves of the two alloys at various strain rates agreed well with the experimentally obtained curves, allowing them to be used for simulation calculations under impact loading. When compared toW-Ni-Fe alloys, the dynamic mechanical characteristics of alloys with Mn addition are greatly improved.

Experimental investigation on dynamic mechanical characteristics of casing during hydrate decomposition using depressurization

During marine hydrate production, some potentially undesirable consequences of casing strength failure are prone to occur. However, conventional methods used in casing mechanical analysis is unable to handle casing failure accurately, as the macro constitutive model employed in current simulation could not reflect the real stress-strain relationship of hydrate reservoir. This paper aimed at proposing an experimental method to investigate dynamic mechanical responses of casing during hydrate decomposition based on the self-developed experiment system, in which overlying stress was simulated to construct a similar mechanical environment of casing. By conducting laboratory-scale tests, the effect of overlying stress on casing mechanical behaviors could be revealed, and comparative analysis of casing axial stress obtained from experiment and simulation was carried out. Experimental results indicated that axial stress and extrusion pressure of casing increased 20.91 % and 7.18 % with the increase of overlying stress, respectively. Casing axial stress increased first and then gradually reached stable in different depressurization stage, which agreed well with the results calculated by numerical simulation. The maximum error between experimental data and simulation results was 11.7 %, 11.1 % and 10.2 % in different depressurization stage, respectively. The proposed approach could provide reference for casing safety design of natural gas hydrate production.

Low-cost development of a fully composite fixed-wing hybrid VTOL UAV

Fixed-wing hybrid vertical take-off and landing (VTOL) unmanned aerial vehicles (UAV) are popular due to their interoperability in the military and civilian domains, primarily where significant terrain difficulties exist for humans. Additionally, they can operate without requiring any runway infrastructure and have extended air endurance and efficiency. Since the hybrid UAV operates in distinct flight modes, viz., (a) VTOL and (b) fixed-wing cruise, carrying different payloads, the airframe structure requires careful design and manufacturing to realize sufficient strength. This experimental study aimed to identify the best combinations of various composite materials for manufacturing a lightweight, low-altitude long endurance (LALE) hybrid VTOL UAV. Primary materials include carbon fiber, Kevlar, fiber-reinforced plastic (FRP), resins, etc. Different rectangular test specimens of 120 × 5 mm size were made from ten different grades of carbon fiber, FRP, and resins using vacuum bagging. After properly curing these test specimens, we quantified their dynamic mechanical characteristics using various bending load experiments on a universal testing machine (UTM). An analysis of the experimental data facilitated the identification of the best composite combinations that provide maximum strength while reducing overall weight. Thus, we could understand the dynamic interplay between peak stress and test specimen weight. We also manufactured a UAV prototype using the identified combination and instrumented and flight-tested it to substantiate the experimental findings.

Experimental assessment on the dynamic mechanical characteristics and cracking mechanism of hybrid basalt-sisal fiber reinforced concrete

Hybrid basalt-sisal fiber reinforced concrete (BSFRC) has good application prospects in construction engineering, while the research on its mechanical properties and impact resistance is rather limited. In this study, to systematically investigate the dynamic compressive behavior and cracking characteristics of BSFRC, 9 kinds of concrete specimens containing different fibers and contents were prepared and a series of dynamic compression tests under strain rates of 45–150 s−1 were conducted via the split Hopkinson pressure bar (SHPB) apparatus. For the 9 typical types of concrete prepared in this study, the B1SP2 (i.e., concrete containing 0.1 vol% basalt fibers, 0.1 vol% sisal fibers and 0.1 vol% polypropylene fibers) shows the highest dynamic compressive strength, and the B2S2 (i.e., concrete containing 0.2 vol% basalt fibers and 0.2 vol% sisal fibers) exhibits the highest energy dissipation density. By means of high-speed photography, digital image correlation (DIC) and scanning electron microscope (SEM) techniques, the progressive failure process and microscopic fracture mechanism of BSFRC were further revealed. Fibers in BSFRC effectively inhibit the expansion and penetration of microcracks, and thus delay the initiation and coalescence of macrocracks. Similar to basalt fibers, sisal fibers control cracking behavior of concrete mainly by cutting crack propagation path and exerting bridging effect, and its fiber action is more remarkable for larger cracks. Furthermore, the coupling effect of strain rate and fiber action on the dynamic mechanical behavior of BSFRC were discussed. High loading rate activates more fibers to exert the strength-enhancement and crack-inhibition effect, and the fiber action makes the strain rate dependance of compressive strength more pronounced in BSFRC than that in plain concrete.

Dynamic and Energy Consumption Characteristics of Sandstone Ring Specimens under Dry and Wet Cycling

The aim of this study was to examine the effects of sandstone ring specimens with different inner diameters on dynamic compression mechanical characteristics after dry and wet circulation. To carry out our study, we subjected a sandstone ring specimen with a 50 mm outer diameter and a 0~25 mm inner diameter to 10 cycles of dry and wet circulation. Afterward, we recorded the specimen’s basic physical parameters and used a split-Hopkinson pressure bar (SHPB) test device to perform an impact compression test. Following dry and wet circulation, our results showed that the mass loss rate increased and the volume expansion rates and density decreased with the increase in the inner diameter of the sandstone ring sample. Simultaneously, with the increase in the inner diameter of the specimen ring, the dynamic compressive strength of the specimen presented an exponential negative correlation, the dynamic elastic modulus presented a quadratic negative correlation, and the dynamic peak strain presented a quadratic positive correlation. Concurrently, the average particle size of the specimen decreased, and the degree of breakage increased with the increase in the sandstone sample’s inner diameter. Regarding the energy analysis performed in this study, the sandstone ring sample’s energy dissipation increased, and its kinetic performance evidently weakened with the increase in the ring sample’s inner diameter. The results of this study have certain reference values for the construction and maintenance of natural cavity rock and underground hard rock roadways.

Three-dimensional numerical simulation of dynamic strength and failure mode of a rock mass with cross joints

To study the dynamic mechanical properties and failure characteristics of intersecting jointed rock masses with different joint distributions under confining pressure, considering the cross angle α and joint persistence ratio η, a numerical model of the biaxial Hopkinson bar test system was established using the finite element method–discrete-element model coupling method. The validity of the model was verified by comparing and analyzing it in conjunction with laboratory test results. Dynamics-static combined impact tests were conducted on specimens under various conditions to investigate the strength characteristics and patterns of crack initiation and expansion. The study revealed the predominant factors influencing intersecting joints with different angles and penetrations under impact loading. The results show that the peak stress of the specimens decreases first and then increases with the increase of the cross angle. When α < 60°, regardless of the value of η, the dynamic stress of the specimens is controlled by the main joint. When α ≥ 60°, the peak stress borne by the specimens decreases with increasing η. When α < 60°, the initiation and propagation of cracks in the cross-jointed specimens are mainly controlled by the main joint, and the final failure surface of the specimens is composed of the main joint and wing cracks. When α ≥ 60° or η ≥ 0.67, the secondary joint guides the expansion of the wing cracks, and multiple failure surfaces composed of main and secondary joints, wing cracks, and co-planar cracks are formed. Increasing lateral confinement significantly increases the dynamic peak stress able to be borne by the specimens. Under triaxial conditions, the degree of failure of the intersecting jointed specimens is much lower than that under uniaxial and biaxial conditions.

Experimental Study on Dynamic Tensile Mechanical Behavior and Fracture Mechanical Characteristics of Sandstone with a Single Prefabricated Fissure

The structural stability of engineering rock mass under dynamic disturbance is directly associated with the fracture mechanics properties in engineering practice. Fully understanding the rock’s fracture mechanical behavior and crack evolution caused by stress concentration at the crack tip in engineering rock mass under dynamic load can offer useful insight into the rock’s dynamic fracture mechanism. A dynamic test using split-Hopkinson pressure bar (SHPB) test system was performed on a single prefabricated fissure sandstone centrally cracked Brazilian disk (CCBD) specimens. Based on the theory of fracture mechanics and one-dimensional stress wave theory, the dynamic crack initiation criterion of CCBD specimen is proposed, and the regression model of sandstone’s dynamic fracture toughness under the coupling effect of fissure angle and strain rate is established by using response surface methodology (RSM). The influence of strain rate and fissure angle on stress wave characteristics, dynamic tensile mechanical behavior, and fracture mechanics characteristic was investigated in this study. The findings demonstrate that: (1) The fissure angle plays a pivotal role in determining the failure mode of sandstone. As the fissure angle increases, three distinct failure modes emerge in the sandstone specimens, while variations in strain rate have minimal impact on the fracture mode of these specimens. (2) Alterations in the fissure angle result in changes to the waveform of transmitted waves. When the fissure angle is below 30°, the transmitted wave exhibits “double peak” characteristics; when it exceeds 30°, a “single peak” waveform is observed. This phenomenon can be attributed to diffraction principles governing incident waves. (3) When the impact pressure is 0.2 MPa, the peak load initially exhibits an increase followed by a decrease, with the peak load reaching its maximum at a fracture angle of 60°; when the impact pressures are 0.3 and 0.5 MPa, there exists a negative correlation between the peak load and the fissure angle. (4) The influence of strain rate on sandstone’s fracture resistance is predominant, with alterations in fissure angle exerting an auxiliary effect on this property. The research results can provide a theoretical and experimental basis for dynamic disaster prevention in urban underground space.

DYNAMIC MECHANICAL AND STATIC ANALYSIS OF KEVLAR FIBER REINFORCED EPOXY COMPOSITES INCORPORATED WITH EGG SHELL PARTICLE

Research on composite material has been carried out stringently by using various reinforcing elements in the matrix with an intend to replace traditional monolithic materials eventually. In this regard, the Calcium-rich egg shell, which is abundantly available as a solid waste, can be used as a possible filler material for reducing material cost, improving processability, and refining the requisite properties of composite for specific applications with simultaneous reduction of the waste. In this study, the static and dynamic mechanical characteristics of eggshell particle-filled Kevlar/epoxy composites have been investigated. Through stirring with the aid of an Ultrasonicator, different weight percent of NaOH-treated eggshell particles (0, 10, 20 wt. %) of 90 micron sizes are mixed with epoxy to fabricate the composite using the traditional hand-layup method. In comparison to the hybrid composites, the mechanical properties of Kevlar/epoxy (0 wt. % filler) composite demonstrate superior tensile and flexural strength and modulus. However, when the Thermogravimetric analysis (TGA) and the Dynamic mechanical analysis (DMA) are accomplished for evaluating the thermal deterioration and viscoelastic characteristics, the hybrid composites demonstrated improvement in overall damping properties including storage modulus and loss modulus along with the thermal stability characteristics. Also, the elongation before failure is more for composite with 10 wt. % (71%) as well as 20 wt.% (18%) filler content with substantial improvement in Inter Laminar Shear Strength. Thus, the presently prepared composite is recommended for weight-sensitive and high-performance engineering applications under elevated temperatures and intense dynamic loading.

Progressive damage and fracture behavior of brittle rock under multi-axial prestress constraint and cyclic impact load coupling

To explore the progressive damage and fracture mechanics characteristics of brittle rock materials under combined dynamic-static loading. Taking account of the coupling effect of the constraint states of uniaxial stress (σ1 ≥ σ2 = σ3 = 0), biaxial stress (σ1 ≥ σ2 > σ3 = 0) and true triaxial stress (σ1 ≥ σ2 ≥ σ3 ≠ 0) and impact load, the strain rate effect and prestress constraint effect of dynamic mechanical characteristics of sandstone are studied. The progressive damage evolution law of sandstone under the coupling of true triaxial stress constraint and cyclic impact load is discussed. The results show that with the increase of axial stress σ1, the dynamic compressive strength and peak strain gradually decrease, and the strain rate gradually increases, resulting in crushing failure under high strain rate. When the axial stress is fixed, the lateral stress constraint reduces the damage degree of sandstone and improves the dynamic compressive strength. With the increase of strain rate, the sample changes from slight splitting failure to inclined shear failure mode. Under the true triaxial stress constraint, the intermediate principal stress σ2 obviously enhances the dynamic compressive strength of sandstone. Under the constraints of triaxial stress, biaxial stress and uniaxial stress, the enhancement effect of dynamic compressive strength and the deformation resistance of sandstone are weakened in turn. Under the coupling of true triaxial stress constraint and high strain rate, sandstone samples show obvious progressive damage evolution effect under repeated impacts, and eventually inclined shear failure occurs, resulting in complete loss of bearing capacity.

Double-Row Achilles Insertional Repair With Rip-Stop Increases Construct Strength Compared to Traditional Techniques: A Biomechanical Study.

Although double-row suture-anchored (DRSA) techniques for Achilles insertional tendinosis has proven successful, a reoccurring failure mode not yet addressed is suture tearing through the tendon. This study aims to address suture tearing by incorporating a rip-stop element. Authors hypothesized that the Rip-Stop group would demonstrate increased strength compared with more traditional techniques. 12 paired cadaveric feet were used in this study (n = 24). One sample from each pair was assigned to receive the standard double-row (SDR) Achilles repair with 4.75-mm knotless anchors (n = 12). The control's matched sides were divided between 2 DRSA bridge groups: modified double-row (MDR) bridge with 3.9-mm anchors or rip-stop double-row (RS-DR) bridge repair with soft proximal anchors and 3.9-mm anchored distal row. In neutral position, specimens underwent 1000 cycles (20-100 N) followed by load to failure. Displacements, stiffness, ultimate load, and failure mode were recorded. RS-DR had the lowest initial displacement values followed by SDR and MDR (1.3 ± 0.4, 2.7 ± 1.4, and 3.2 ± 1.3 mm, respectively). Significance was detected when comparing initial displacement of RS-DR to MDR (P = .038). Cyclic displacement was lowest for RS-DR, followed by MDR and SDR (1.6 ± 0.9, 2.2 ± 1.1, and 4.5 ± 3.2 mm, respectively). Cyclic stiffness was similar for RS-DR and MDR (89.1 ± 24.6 and 81.9 ± 5.6 N/mm, respectively). RS-DR ultimate load (1116.8 ± 405.7 N) was statistically greater than SDR (465.6 ± 352.7, P = .003). RS-DR-repaired specimens demonstrated a decrease in displacement values and increased ultimate load and stiffness when compared to other groups. Results of this cadaveric model suggest that the addition of a rip-stop to DRSA Achilles repair is more impactful than anchor size. Limitations include that this was a time-zero biomechanical study, which cannot simulate the performance of the repairs during postoperative healing and recovery. A rip-stop technique for Achilles repair effectively improves dynamic mechanical characteristics and may mitigate suture tearing through tendon in a patient cohort.

Study on the Wind Deviation Characteristics of Y-Type Insulator String under the Action of Strong Wind

Under the action of extreme wind load, the overhead transmission line will generate a wind deflection flashover phenomenon, which seriously affects the normal operation of the transmission system and causes significant losses. Y-type insulator string (hereinafter referred to as Y-string) is an optimized structural form to reduce the wind deflection flashover in windy areas, and the dynamic mechanical characteristics of Y-string under the action of pulsating wind is an important factor that influences the design of the overhead transmission line. The calculation method of pulsating wind load and the static calculation method of wind deflection displacement of Y-string are obtained through theoretical derivation. The mathematical software is used to simulate the time course of pulsating wind speed and convert it into the time course of wind load, establish the finite element model of insulator string, simulate and analyze the wind deflection process of Y-string under the action of pulsating wind by using the finite element method, and calculate the horizontal displacement of Y-string under the excitation of pulsating wind and make a comparative analysis with the results of the static calculations. The results show that the wind deflection displacement of the Y-string under pulsating wind is 1.12–1.28 times that under steady-state wind, which reveals the reason for the wind deflection flashover phenomenon and provides theoretical references for the design and improvement of overhead transmission lines.

Recent Advancements in Copper Infiltration Applied to Sintered Steel

Sintered steel is widely used in various industries, including automotive, aerospace, and construction machinery, due to its notable attributes such as high precision, wear resistance, efficient material utilization, and cost-effectiveness. However, the presence of pores hinders sintered steel from achieving a fully dense state, thereby restricting its broader applications. Copper infiltration enables the reduction or elimination of pores, minimizes stress concentration, and prevents microcrack formation. Consequently, this process significantly enhances the material’s density and mechanical properties, particularly its dynamic mechanical characteristics such as impact toughness and fatigue performance. This paper provides an overview of the copper infiltration development history in sintered steel, summarizes the factors including substrate, infiltrant, and infiltration process, and highlights the research progress in copper infiltration application in sintered steel. Furthermore, challenges associated with copper-infiltrated sintered steel are discussed, and recommendations for future development are presented.

Effect of Ce on microstructure and dynamic compression properties of ZK60 alloy

The Mg–6Zn– xCe–0.6Zr ( x = 1, 2) alloys were subjected to a dynamic compression experiment, and the change in microstructure was noticed and studied. The primary components of two alloys are α-Mg matrix, Mg7Zn3 phase, and (Mg1− xZn x)11Ce phase. The two alloys have grain diameters of 2.8 and 3.2 μm, respectively. The dynamic compressive mechanical characteristics of the two alloys show a positive strain-strengthening effect, with Mg–6Zn–1Ce–0.6Zr alloy having somewhat higher values than Mg–6Zn–2Ce–0.6Zr alloy. At a strain rate of 500 s−1, {10[Formula: see text]2} tensile twin is the dominant deformation mechanism for two alloys, but the Mg–6Zn–2Ce–0.6Zr alloy is more prone to produce {10[Formula: see text]2} tensile twin. With the increase of the strain rate, the dominant deformation mechanism changes from {10[Formula: see text]2} tensile twin to pyramidal slip.