Development of an apparatus for biaxial testing using cruciform specimens

A testing apparatus has been developed to study the behavior of sheet metals and composite materials under monotonic and cyclic biaxial loading conditions. This test facility employs cruciform specimens that are loaded in their plane. Problems encountered while developing the test system are discussed.We also discuss the difficulties common to test methods employing cruciform specimens. These relate to the design of a suitable specimen geometry and to the determination of the stresses throughout the specimen. A method for designing an optimal geometry for these specimens is presented. This method is based on the statistical tools of factorial and response surface designs. The statistical method, coupled with a finite-element analysis of the specimen, was successfully applied to optimize the geometry of a cruciform specimen with a circular reduced central region.Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 28–June 8.     

大当量浅埋地下爆炸抛掷成坑效应的缩比模拟实验装置

针对当前地下爆炸物理模型实验无法模拟大当量地下爆炸抛掷弹坑和疏松鼓包现象的难题,基于相似理论,采用地下爆炸效应真空室模型实验方法,研制了考虑重力影响的大当量地下爆炸效应模拟实验装置。整套装置由容器罐体、快开门密闭机构、爆源系统、真空泵组、量测控制系统等组成,提出的新型爆源模拟装置可以实现精确起爆控制。该装置可模拟0.1~100 kt TNT、埋深20~400 m范围内不同比尺的地下核爆炸成坑和隆起实验,同时也能够模拟不同装药配置方案、不同地质条件下的大当量地下浅埋化爆抛掷实验。典型的核爆抛掷成坑模型实验结果表明,装置实验参数精确可调,实验过程可控,实验结果可信,为钻地核武器地下爆炸毁伤效应分析和大型工程爆破效果预测预报提供了实验室模拟和科学研究设备,填补了爆炸离心机无法模拟大当量地下爆炸抛掷成坑效应的空白。

     

An apparatus for tensile testing of concrete

This paper describes a test fixture and alignment jig used for testing small concrete or rock cylinders in uniaxial tension. The fixture may be easily modified to test cube specimens and is adaptable for use in any standard universal testing machine. A testing procedure is described and results presented for 3 in. × 6 in. (76 mm × 152 mm) concrete cylinders. The main virtues of the fixture are its ease of construction, simple alignment procedure for the test specimen, and accuracy comparable to that obtained from the standard compression test of concrete cylinders.     

A constant-stress,vacuum creep-testing apparatus

A mathematical analysis is presented for a direct-linkage tensile and compression creep-testing mechanism. A particular design, employing a dead weight, a cam and a linkage system, achieves constant stress in a tensile specimen during uniform creep deformation. Superiority has been achieved over previous designs in: (1) compactness with mechanical advantages up to 10:1, (2) higher structural rigidity and (3) higher sensitivity in the region of small displacements. The advantage of compactness has now made possible the successful design of a vacuum system for high-temperature creep testing at constant stress. Typical creep curves on iron specimens are induced.     

Micromechanical fatigue testing

This paper describes the design, modeling, and experimental test results of a single crystal silicon micromechanical device developed to evaluate fracture and fatigue of silicon based micromechanical devices. The structure is a cantilever beam, 300 microns long, with a large silicon plate and gold inertial mass at the free end. Torquing and sensing electrodes extend over the plate, and with associated electronics, drive the structure at resonance. Fatigue crack propagation is measured by detecting the shift in the natural frequency caused by the extension of a preexisting crack introduced near the fixed end of the cantilever. Experimental data are presented demonstrating time-dependent crack growth in silicon. Crack extensions of 10 to 300 nm have been measured with a resolution of approximately 2.5 nm, and crack tip velocities as low as 2.1×10⁻¹⁴ m/s. It is postulated that static fatigue of the native surface silica layer is the mechanism for crack growth. The methodology established here is generic in concept, permitting sensitive measurement of crack growth in larger fatigue specimens as well.     

Resonant-vibration fatigue testing

Techniques are described for the fatigue testing of components, such as turbine buckets, used in large steam turbines and generators. Fatigue of these large structures, which have relatively high unit damping, is obtained through the use of resonant-vibration systems. An electromagnetic drive serves as the source of excitation. The drive system is designed to produce the maximum force from a given-size magnet. Control of the amplitudes of vibration, for very long periods, is obtained through the use of an automatic amplitude control, the stability of which maintains the amplitude with a variation of less than 2 percent. Three specific fatigue-testing techniques described are:

1. 1.
   The fatigue of complete weld sections for large numbers of cycles (500 million), which required dynamic bending moments of greater than 6000 ft-lb to produce failure.     
   
   

High-speed stochastic fatigue testing

Good stochastic fatigue tests are difficult to perform. One of the major reasons is that ordinary servohydraulic loading systems realize the prescribed load history accurately at very low testing speeds only. If the speeds used for constant amplitude testing are applied to stochastic fatigue testing, quite unacceptable errors are introduced. Usually this problem is solved by running the tests at very low speeds and by editing the load history in order to reduce the duration of the test. In this paper a new method for control of stochastic fatigue tests is proposed. It is based on letting the analog control device remain as the basic control mechanism in the system, but distorting the input signal by computer in order to minimize the errors of the load history extremes. The principle proves to be very efficient to reduce all kinds of system errors and has shown to be able to increase the allowable speed by a factor from 10 to 30.     

Vibrational aspects in structural fatigue testing

Structural fatigue tests are required in engineering practice for safety and reliability. Structural bending and torsional fatigue testing machines were developed. The vibrational characteristics of the machines and components to be tested were investigated analytically, numerically and experimentally to determine the safe region of structural fatigue testing frequencies.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

A dropped-weight apparatus for low-speed impact testing of composite structures

A dropped-weight test apparatus has been developed that can be used to perform low-speed impact tests on composite aircraft structures. This vertical drop-weight test apparatus is simple, compact, inexpensive and has precision impact and self-arresting design features similar to the more sophisticated, expensive test machines. The test apparatus has been used to perform low-speed impact response studies on laminated composite plates to understand the influence of impactor and target parameters on structural response and to develop a validated analysis method. Some of the experimental results generated by using this test apparatus for composite laminated plates are presented in the present paper and compared with the corresponding analytical results.     

A methodology for thermomechanical fatigue crack growth testing in MMCs

This paper describes the experimental techniques used in an investigation of the crack growth characteristics of a four-ply, unidirectional, silicon carbide fiber reinforced, titanium matrix composite (SCS-6/Ti–6Al–2Sn–4Zr–2Mo) subjected to thermomechanical fatigue. A mechanical test system was assembled which is capable of conducting fully automated, computer-controlled thermomechanical fatigue crack growth tests. The system is able to simultaneously impose operator-defined arbitrary mechanical and thermal histories on the specimen. Crack lengths in single-edge tension [SE(T)] or middle tension [M(T)] specimens are measured by the direct-current electric potential method and optically using a unique telemicroscope system. A series of isothermal, in-phase and out-of-phase crack growth tests was conducted to obtain baseline data for material modeling purposes. The test temperatures ranged from 150°C to 538°C, and the highest thermal frequency was 0.0083 Hz.     

Resonant fatigue testing of riveted joints

A special test facility was constructed for evaluating the fatigue strength of turbine-blade components under steady pullout and vibratory bending loadings. The use of mechanical resonance for these applications was particularly attractive in that failure in inaccessible areas could be detected by the change in natural frequency of the specimen. The sensitivity of the method allowed determination of a fatigue crack as early in life as possible. The application of this facility to the determination of the fatigue strength of a type of riveted connection employed on certain types of blades is described in this paper. A steady load representing the centrifugal force was applied hydraulically through a long flexible pullrod which in combination with the test specimen was excited laterally at its resonant frequency by an electromagnetic shaker. An accelerometer provided closed-loop control on both vibration amplitude and resonant frequency through special circuitry. Fatigue-strength reduction factors at 10⁷ cycles were determined for three types of riveted test specimens in air at room temperature. These values exceeded the elastic stress-concentration factors estimated from earlier photoelastic tests for all three cases.     

Environmental fatigue testing of molded plastics for prosthetic heart valves

A testing machine was designed specifically to study fatigue behavior of molded polymeric materials by subjecting multiple cantilever specimens to vibratory fluid loading. The fluid loading permits aqueous or other environments, facilitates temperature control, and makes possible high testing frequencies. Samples of fifteen to thirty specimens may be tested simultaneously to obtain statistical data. Example results are presented showing Weibull distributions fitted to polymethyl methacrylate fatigue-life data. A statistical criterion was used to verify that scatter, due to the machine and testing technique, was not excessive.     

A thermal-mechanical fatigue crack-growth testing system

An automated system for crack-growth studies under thermal-mechanical loading conditions utilizing a compact tension specimen has been developed. The system uses quartz lamps and a multiple-zone control system to accurately control the specimen temperature according to a predetermined profile. Further, a cyclic mechanical loading can be applied along with cyclic temperature profile in a prescribed manner.     

超声疲劳试验方法及其应用

超声疲劳是一种加速的疲劳试验方法,它的测试频率(20kHz)远远超过了常规疲劳测试频率(小于200Hz)．超声疲劳试验研究表明50^#车轴钢和40Cr钢直到10^10个应力循环后仍会发生疲劳断裂,并不存在常规疲劳试验曲线所示的“疲劳极限”,因此用10^7周次的疲劳试验数据进行疲劳强度设计并不安全．50^#车轴钢和40Cr钢超声疲劳性能优于常规疲劳性能．扫描电镜分析表明,超长寿命阶段50^#车轴钢裂纹萌生于次表面夹杂．介绍了超声疲劳试验系统、工作原理及超声疲劳试样的设计．     

Isothermal low-cycle fatigue testing of microscale solder interconnections

A series of isothermal low-cycle fatigue studies of small to extremely small-volume solder joints has been conducted. These solder microjoints were designed and fabricated using processing which duplicates the microelectronics interconnection structures that might be used in high density, highly integrated flip-chip packaging and all fatigue tests were conducted in fully reversed simple shear both with and without dwells at maximum strain. Results of low-cycle fatigue tests of both single and double-bump 95/5 Pb/Sn solder microjoints in the form of Manson-Coffin (plastic strain amplitude versus fatigue life) plots and post-test failure mode analysis (FMA) carried out using scanning electron microscope (SEM) fractography are presented and evaluated. Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–12.     

Strain and fatigue testing of large expansion bellows

An experimental study was performed to investigate the strain and fatigue behavior of two 635-mm-diam 316 H stainless-steel convolute bellows. Bellows No. 1 was initially subjected to an axial deflection test and a pressure test, both at ambient temperature. The primary objective of these tests was to acquire strain data utilizing strain-gaging techniques. These strain tests were followed by an elevated-temperature fatigue test. Bellows No. 2 experienced the fatigue test.     

Changes in modal parameters of a bridge during fatigue testing

The effects of high-cycle fatigue on the dynamic modal properties of a full-scale bridge test section are examined in this study. A span 30.5 m (100 ft) in length was salvaged from an interstate bridge for experimental testing to establish the fatigue performance of the retrofit procedures which could be used for future field repairs of the fatigue cracks. Horizontal cracks had formed in the webs of the twin bridge girders in the vicinity of the web-flange weld at or near the connections of the girders to the floor beams. An electrohydraulic vibration shaker was used to excite the test span in the torsional and bending modes to promote fatigue crack growth. In addition to the experimental evaluation of the retrofit procedures, the dynamic response characteristics of the test span during the high-cycle fatigue test were monitored. Experimental data have been used to establish mode shapes and modal damping ratios during the four-million-cycle fatigue test. Damping ratios ranged from 1.09 percent to 0.53 percent. Modal stiffnesses were calculated based on the experimental mode shapes. There is a modest correlation between the changes in stiffness and the observed cracking of the steel girders and the subsequent repairs. Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Porland, OR on June 5–10.