Experimental technique for measurement of stress-acoustic coefficients of Rayleigh waves

Two stress-acoustic coefficients,K ₁ andK ₂, are required to determine the state of biaxial surface stress from ultrasonic Rayleigh-wave velocity or time of flight measurements in elastic, initially isotropic solids. An experimental technique is described for the precise measurement of these two coefficients in uniaxially stressed test specimens. The technique is applied to aluminum 2024-T351 and 6061-T651 alloys. The influence on measurement results of various parameters such as material anisotropy and temperature is considered.     

Fracture prediction based on plastic stress singularity strength

The objective of this study was to predict the fracture behavior of specimens of various cracked configurations and materials based on information from standard fracture specimens of the same thickness. A three-dimensional elastic-plastic finite-element analysis was used to determine the critical value of plastic stress singularity strength and thus characterize specimen fracture behavior. Fracture instability was predicted for specimens of the following structural materials: 7075-T651 and 2024-T351 Aluminum alloys; Type 304 Stainless Steel. This study was one of many contributed by participants in a fracture-predictive round-robin conducted by ASTM Task Group E24.06.02.     

Fatigue and static strength of notched and unnotched aluminum-alloy and steel specimens

This paper describes a method of presentation of fatigue data on three commonly used aircraft materials, 2024-T3 and 7075-T6 aluminum alloys and normalized SAE 4130 steel, such that variations in fatigue strength with stress-concentration factor can be shown. Comparisons of the fatigue strengths of 2024-T3 and 7075-T6 aluminum are made for the most useful range of stress-concentration factors. Static-strength results of notched and unnotched specimens of the three materials are presented to show how the strength varies with some parameters of the stress concentration. Comparison of the data with one theory for the strength of cracked specimens was made.     

Poisson's ratio determined with strain rosettes

Poisson's ratio appears in general stress-strain equations and is essential to experimental stress analysis. An experimental method using bonded electrical-resistance strain rosettes is described in this paper. The standard use of two strain sensing elements at 90 deg was replaced with “rectangular rosettes” installed back-to-back. The third strain-sensing element was used to calculate the gage alignment error and “true” longitudinal and transver e strains. For 2024-T351 sheet material, an average measured value of 0.312 for Poisson's ratio was obtained.     

Evaluation of an applied plane-stress tensor distribution using ultrasonic shear waves

Most acoustoelastic stress measurements using shear waves that have been carried out so far are strongly related to photoelastic experiments: the velocity difference of the birefringent waves is evaluated. Absolute time-of-flight measurements potentially give more information about stresses but are also likely to be inaccurate. A technique is developed enabling time-of-flight and polarization angle measurements in an aluminum plate that are reproducible within 1/3 ns and one-deg respectively. Based on a previous calibration of the acoustoelastic effect, a plane-stress tensor field caused by applying a load to an aluminum 2024-T351 compact-tension specimen is evaluated. The results are compared to those of a finite-element analysis.     

An assessment of residual-stress measurements around cold-worked holes

An X-ray diffraction technique was employed to determine the residual stresses introduced by cold working a fastener hole in a 6-mm thick 2024-T351 aluminum plate. The radial and tangenital residual stresses were measured at both faces of the plate and the measurements compared with the results from a two-dimensional axisymmetric finite-element model. The comparisons were favorable, although modifying the finite-element model to simulate the X-ray process provided better agreement. Experimental determinations of residual stresses showed differences between the two faces of the plate. This feature was attributed to the directional nature of the cold-working process. Paper was presented at the 1994 SEM Spring Conference on Experimental Mechanics held in Baltimore, MD on June 8–10.     

The fatigue of aluminum alloys subjected to random loading

This paper describes an expeirmental investigation which was carried out to determine the fatigue life of two aluminum alloys (2024-T3 and 6061-T6). They were subjected to both constant-strain-amplitude sinusoidal and narrow-band random-strain-amplitude fatigue loadings. The fatigue-life values obtained from the narrow-band random testing were compared with theoretical predictions based on Miner's linear accumulation of damage hypothesis. Cantilever-beam-test specimens fabricated from the aluminum alloys were subjected to either a constant-strain-amplitude sinusoidal or a narrow-band random base excitation by means of an electromagnetic vibrations exciter. It was found that the ε-N curves for both alloys could be approximated by three straight-line segments in the low-, intermediate- and high-cycle fatigue-life ranges. Miner's hypothesis was used to predict the narrow-band random fatigue lives of materials with this type of ε-N behavior. These fatigue-life predictions were found to consistently overestimate the acutal fatigue lives by a factor of 2 or 3. However, the shape of the predicted fatigue-life curves and the high-cycle fatigue behavior of both materials were found to be in good agreement with the experimental results.     

Buckling of an elastic column containing a fatigue crack

Results of an experimental program conducted in order to study the residual buckling strength of an elastic column containing a fatigue crack are presented in this paper. Tests were performed on bars of rectangular cross section, made from 2024-T351 aluminum-alloy plate and containing a fatigue crack at the midsection. Results obtained show a reduction in buckling strength of 8 percent under highed-highed end conditions, in apparent tentative accord with data obtained from columns containing machined notches. Although it is small, the reduction in buckling strength is thought to be significant for design and requires further investigation.     

Prediction of fatigue crack growth life under variable-amplitude loading using finite element analysis

This article presents an elastic-plastic study aiming at predicting the fatigue crack growth (FCG) of 2024-T3 aluminum alloys under variable-amplitude loading. The proposed analysis needs the estimation of the residual stress distribution ahead of the crack tip during propagation. An elastic-plastic FE analysis has been implemented for modeling FCG using Chaboche's model. The FE study has been carried out through consideration of the loading history effect using the memory rules. Three different loading spectra have been applied in this work. The obtained results have been compared to the experimental ones and it has been proved that the suggested model has a better prediction of the FCG lives of cracked 2024-T3 aluminum alloy structures subjected to variable-amplitude loading.     

EVALUATION OF DUCTILE FRACTURE CRITERIA IN A GENERAL THREE-DIMENSIONAL STRESS STATE CONSIDERING THE STRESS TRIAXIALITY AND THE LODE PARAMETER

This paper is concerned with evaluation of various ductile fracture criteria in a general three-dimensional stress state of stress triaxiality, the Lode parameter and the equiva- lent plastic strain to fracture. Evaluation is carried out by comparing fracture loci constructed by fracture criteria to experimental results of A12024-T351. Comparison demonstrates that the Modified Mohr-Coulomb criterion and a newly proposed criterion provide sufficient predictabil- ity of fracture strain. Moreover, evaluation is emphasized on the predicted cut-off value for stress triaxiality. The evaluation demonstrates that the Cockcroft-Latham, Brozzo, Oh, Ko-Huh and the new criteria coupled a reasonable cut-off value for ductile materials.     

Fatigue-life prediction by an order statistics treatment of acoustic-emission signals

In fatigue, both the crack-propagation rates and the cumulative acoustic-emission activity are known to be related to the applied stress-intensity range. By considering the energy balance during crack propagation and the relation of strain energy release to the acoustic-emission characteristics, a formal relation between acoustic emission amplitudes and initial fatigue-crack-propagation rates has been derived. Continuous monitoring of acoustic emission during low cycle (tension-tension) fatigue tests has been conducted on aluminum 2024-T3 and 7075-T6 alloys, until fracture. Initial crack sizes and orientations in the fatigue specimens were randomly distributed. Every few hundred cycles, the acoustic signal having the highest peak amplitude was recorded as the extreme acoustic-emission event for the elapsed period. The extreme peak amplitudes, related to extreme crack-propagation rates, were shown, by an order statistics treatment, to be extremally distributed. Statistical, nondeterministic, approach to fatigue considers that only extreme crack-propagation rates are vital to fatigue lives. Knowledge of the distribution function of propagation rates is therefore essential in design for fatigue. Such knowledge can now be obtained in a nondestructive manner, during service in real time, by analyzing the distribution of amplitudes of acoustic-emission signals emitted during cyclic stressing. The statistical treatment enables the prediction of the number of cycles left until failure. Predictions performeda posteriori, based on results gained early in each fatigue test, were in good agreement with actual fatigue lives. The amplitude distribution analysis of the acoustic signals emitted during fatigue tests has been proven to be a feasible nondestructive method for predicting fatigue life.     

Fatigue-crack propagation in a biaxial-stress field

The effect of biaxiality of stress on the fatigue-crack-propagation rate in 2024-T351 aluminum alloy in completely reversed bending is investigated. Round and elliptical plates, simply supported at the edge, are subjected to bulge bending. The plate dimensions are such that nominal stress-biaxiality ratios of 1∶1, 1∶0.86 and 1∶0.75 are obtained in the plane of the specimen in the region of its geometric center. It is postulated that, in strain-hardening materials, crack-growth rate is a function of nominal biaxial-stress condition in the crack-tip region. Experimental data for different biaxiality ratios fall in a straight line when a modified stress-intensity factor is plotted against the rate of crack propagation.     

Tensile and energy density properties of 2024, 6013, 8090 and 2091 aircraft aluminum alloy after corrosion exposure

Evaluation was made for the corrosion susceptibility of aircraft structure aluminum alloys 2024 T351, 6013 T6, 8090 T81 and 2091 T84. Tensile and energy density data were obtained. Stereoscopic and metallographic corrosion analysis were made as well. The specimens were pre-corroded using accelerated laboratory corrosion tests or out-door atmospheric conditions before testing. Noticeable decrease of yield and ultimate tensile stress were detected when the specimen surface was corroded. Dramatic volumetric embrittlement was observed even after short exposure times that were not sufficient for the appreciable development of surface corrosion attack. Observed material degradation behavior is attributed to hydrogen penetration and absorption.     

Measuring Inaccessible Residual Stresses Using Multiple Methods and Superposition

The traditional contour method maps a single component of residual stress by cutting a body carefully in two and measuring the contour of the cut surface. The cut also exposes previously inaccessible regions of the body to residual stress measurement using a variety of other techniques, but the stresses have been changed by the relaxation after cutting. In this paper, it is shown that superposition of stresses measured post-cutting with results from the contour method analysis can determine the original (pre-cut) residual stresses. The general superposition theory using Bueckner’s principle is developed and limitations are discussed. The procedure is experimentally demonstrated by determining the triaxial residual stress state on a cross section plane. The 2024-T351 aluminum alloy test specimen was a disk plastically indented to produce multiaxial residual stresses. After cutting the disk in half, the stresses on the cut surface of one half were determined with X-ray diffraction and with hole drilling on the other half. To determine the original residual stresses, the measured surface stresses were superimposed with the change stress calculated by the contour method. Within uncertainty, the results agreed with neutron diffraction measurements taken on an uncut disk.     

Wave diffraction-induced crack/cohesive strip growth based on rudimentary non-linear models

The reaction of a Mode I 2D semi-infinite crack to the diffraction of a plane tension wave in an unbounded solid is treated. A vanishingly thin cohesive zone, or strip, represents rudimentary crack edge yield, and two generic response laws that incorporate rudimentary non-linear effects are studied. Simultaneous crack/zone growth at constant rates is treated, with pure zone growth recovered as a limit case.Both models involve multiple solution sets that can be obtained analytically and studied. Attention focuses on those associated with uniform strip fields.Results based on parameters for 2024-T4 aluminum and 4130-HT steel suggest that the two models involve tension loads critical for crack/zone growth lower than standard model predictions, especially for high growth rates. The results also suggest that dynamic crack/zone growth may be more sensitive to multiaxiality near the crack edge, and proceed at lower loads. All of these results are general in the sense that they are obtained without specifying a complete set of growth criteria.     

J resistance curves in aluminum SEN specimens using Moiré interferometry

TheU _y -displacement field obtained by white-light moiré interferometry were used to estimate the approximate far- and near-fieldJ-integral values associated with the subcritical crack growths in fatigue precracked 7075-T6 and blunt notched and fatigue precracked 2024-0 and 5052-H32 aluminum, single-edged notch (SEN) specimens. The initial phases of theJ-resistance curves for the somewhat brittle 7075-T6 and the two ductile 2024-0 and 5052-H32 aluminum SEN specimens are presented. Paper was presented at the 1987 SEM Spring Conference on Experimental Mechanics held in Houston, TX on June 14–19.     

高锁螺栓连接件动态拉伸响应与失效机理

飞机坠撞过程中结构的变形模式和吸能对乘员保护具有重要意义，而连接结构的载荷传递和失效形式是影响飞机结构变形的重要因素之一。为了获取航空高锁螺栓连接件在坠撞载荷下的动态响应和失效机理，基于抗剪型平头高锁螺栓设计了2种材料（2024-T3和7050-T7451）的单钉单搭接连接件，利用高速液压伺服材料试验机进行4种速度（0.01、0.10、1.00和3.00 m/s）下的拉伸测试，得到连接件的动态响应、极限载荷、能量吸收和失效模式随速度的变化规律，并分析了连接件的失效机理。结果表明，连接件的失效模式受母材和高锁螺栓/螺母材料强度影响较大，而受加载速度影响较小；当速度从0.01 m/s增加到3.00 m/s时，2024-T3连接件的极限载荷和能量吸收分别增加了2.17%和34.43%，7050-T7451连接件的极限载荷和能量吸收分别增加了5.53%和6.58%。     

滑轨导向式静/动态双轴拉伸实验技术

基于液压伺服高速加载系统,发展了一种材料双轴拉伸力学性能测试技术。利用锥面接触导向驱动方法,把加载锤竖直方向的驱动力转化为水平方向的双轴驱动力,从而实现对十字形试样平面双轴加载。借助有限元数值模拟手段优化了锥面接触角和十字形试样尺寸。当接触锥角为45°时,既有较好的水平驱动转化效率,同时又保持较小的接触力,确保水平驱动加载各组件在弹性变形范围内,可多次重复使用。确定了加载臂狭缝个数、狭缝与减薄区边缘长度和标距段厚度等试样设计关键参数,在十字形试样测试标距段内实现了均匀变形。设计了测力夹持一体化导杆和非接触光学全场应变测试系统,准确获得了试样的应力和应变。利用此平面双轴拉伸加载装置,开展2024-T351铝合金板单轴拉伸实验和激光探测同步性验证实验,验证装置设计的可行性;开展铝合金板材在不同加载速率下的双轴拉伸实验,得到在双轴加载下铝合金板材应力应变曲线,并与单轴加载下实验结果进行了对比分析。     

超高速碰撞2024-T4铝靶产生的膨胀等离子体云的电磁特性

针对超高速碰撞产生的瞬态等离子体的特点,利用建立的扫描朗缪尔探针、朗缪尔三探针诊断系 统及线圈系统,测量了超高速碰撞喷出物产生的膨胀等离子体云中等离子体的特征参量和磁感应强度。在传 感器布局和方位角固定的前提下,进行了2种碰撞速度、相同入射角度条件下2024-T4铝弹丸超高速碰撞 2024-T4铝靶产生的膨胀等离子体云电磁特性的实验室测量。实验结果表明,碰撞产生的等离子体的平均电 子温度为0.4~0.9eV,电子密度在1012cm-3量级,线圈的磁感应强度幅值为10~20nT 。通过数据处理,获 得了2种实验条件下整个物理过程在给定探针、线圈位置处等离子体的电子温度、电子密度和磁感应强度与 碰撞速度的关系,并对扫描朗缪尔探针和朗缪尔三探针的测量结果进行了比较。