Effect of initial crack length on the measured bridging law of unidirectional E-glass/epoxy double cantilever beam specimens

In this paper, the effect of initial delamination length is experimentally investigated on obtaining the mode I bridging law of unidirectional E-glass/epoxy double cantilever beam (DCB) specimens manufactured by hand layup method. To this end, an experimental test set-up is established for accurate measurement of crack tip opening displacement (CTOD) using digital image processing method. DCB tests are performed for three different delamination lengths and the corresponding bridging laws are calculated using J-integral approach. Results showed that the maximum bridging stress, the shape of bridging law and energy dissipation in bridging zone are slightly affected by changing initial crack length. In other words, the measured bridging law acts independent of initial delamination length. Therefore, the obtained bridging law can be used with the cohesive elements available in the commercial finite element software to simulate the delamination propagation behavior in unidirectional DCB specimens.     

Experimental techniques for fracture instability toughness determination of unidirectional fibre metal laminates

ABSTRACT The aim of this work is to propose procedures for the measurement of the fracture toughness of fibre metal laminates (FMLs) reinforced with unidirectional fibres of aramid or glass. Experimental techniques for fracture toughness evaluation by using Compact (C(T)) and Single‐Edge Bend (SE(B)) specimens obeying ASTM standards are introduced. Procedures from the standard for thick metallic materials were modified in order to overcome problems, which can arise when testing FMLs – that is, specimen buckling, indentations and crack growth in planes other than the plane of the fatigue pre‐crack or notch. The methodology proposed was experimentally tested leading to satisfactory results.     

Determination of energy consumption in the fracture plane of Ultra High Toughness Cementitious Composite with direct tension test

In this paper, the energy consumption in the fracture plane of Ultra High Toughness Cementitious Composite was analyzed with double-edge notched plates under direct tension. The results showed that monitoring elongation and retraction of the notched regions could effectively separate the localized crack from other multiple cracks, and the softening curve could be drawn accordingly. With the softening curve, the fracture energy due to fiber bridging G_b and the fracture energy G_F were calculated. This work provides a thought more than the detailed results to separate the single fracture crack from other multiple fine cracks of pseudo strain hardening materials.     

A size effect law for notched tensile strength of woven carbon/epoxy laminates

Specimen-size effect and notch-size effect on the tensile strength of woven fabric carbon/epoxy laminates are evaluated and modeled. For two different layups of [(0/90)₁₂] and [(±45)₂/(0/90)₅]_S, respectively, static tension tests were performed on two-dimensional geometrically similar unnotched and double-edge notched specimens scaled to three different sizes. Experimental results demonstrate that the notched strength of the woven CFRP laminates depend on the size of specimen as well as the size of notch. The ratio of notched strength to unnotched strength decreases as the length of notch increases, regardless of the size of specimen. For a given size of notch, the notch strength ratio becomes larger with decreasing size of specimen. A notch-size effect law is derived by means of the Neuber interpolation method. A specimen-size effect is embedded into the notch sensitivity parameter involved by the notch-size effect law to establish a size effect law that can cope with these two kinds of size effect. The engineering size effect law proposed can adequately describe the specimen-size effect as well as notch-size effect on the tensile strength of the woven CFRP laminates. It is also demonstrated that the size effect law allows determining the size independent fracture toughness on the basis of notched strengths of small specimens that fail in a quasi-brittle manner.     

Uniaxial tension test for the determination of fracture parameters of concrete: state of the art

An overview is given of methods to determine the strength and fracture energy of concrete subjected to uniaxial tension. The test method is often erroneously--and persistently--referred to as the `direct tension test'. After a brief review of historical, mostly indirect methods, the paper focuses on more recent servo-controlled testing techniques. Assessment of effects like secondary bending, and rotation and/or translations of the loading platens, as well as size/scale effects and environmental effects on fracture seem important for a proper definition of tensile strength and fracture energy. Implications for modeling are debated. The methods can be transferred directly to studies of other quasi-brittle geo-materials like natural rocks, clay and ice, fired clay bricks, plaster and the like.     

超导HWR腔垂直测试低温系统试验研究

中国科学院先导科技专项ADS(Accelerator Driven Suberitical,ADS)嬗变系统中超导HWR(half-wave resonator,HWR)腔垂直测试需低温系统维持4.2 K(液氦)的低温环境,低温系统降温过程包括氮气置换、液氮预冷、氦气置换和液氦冷却。通过实验建立了低温系统降温4个阶段不同测点温度随时间的变化规律,在此基础上,计算了液氦的消耗速率和杜瓦的静态热负荷,分析了低温系统在稳定工作状态时最佳的液氦补液时间间隔。结果表明:该低温系统满足超导HWR腔垂直测试需求,消耗液氮约175 kg、液氦约2 048 L,低温系统稳定工作时液氦体积消耗速率为32 L/h,杜瓦静态热负荷为21.36 W,液氦合理补液时间间隔为4 h,为后续超导HWR腔垂直测试提供了保障。     

Stress concentration formulae useful for any shape of notch in a round test specimen under tension and under bending

In this work stress concentration factors, K_t , for a round bar with a circular-arc or V-shaped notch are considered on the basis of exact solutions for special cases and accurate numerical results. Then, a set of K_t formulae useful for any shape of notch is proposed. The conclusions can be summarized as follows. (i) For the limiting cases of deep (d) and shallow (s) notches, the body force method is used to calculate the K_t values. Then, the formulae are obtained as K_td and K_ts . (ii) On the one hand, upon comparison of K_t and K_td it is found that K_t is nearly equal to K_td if the notch is deep or blunt. (iii) On the other hand, if the notch is sharp or shallow, K_t is mainly controlled by K_ts and the notch depth. (iv) The notch shape is classified into several groups according to the notch radius and notch depth. Then, the least-squares method is applied for the calculation of K_t /K_td and K_t /K_ts . (v) Finally, a set of convenient formulae is proposed that are useful for any shape of notch in a round test specimen. The formulae give SCFs with <1% error for any shape of notch.     

Experimental investigation on test methods for mode II interlaminar fracture testing of carbon fiber reinforced composites

In this paper, experimental investigation on the test methods for mode II interlaminar fracture testing of carbon fiber reinforced composites are carried out. Mode II interlaminar fracture testing of unidirectional composite of carbon fiber reinforced epoxy (T800/#3631) are conducted using four kinds of test methods, namely end notched flexure (ENF) test, end loaded split (ELS) test, four-point bend end notched flexure (4ENF) test, and over notched flexure (ONF) test. An analytical model based on a point-friction assumption and classical beam theory is proposed to evaluate the effect of friction between crack faces on the mode II interlaminar fracture toughness in the 4ENF and ONF tests. The analytical model is validated by the comparison of analytical results with previous ones obtained from finite element analysis. Experimental results show that the ENF test gives reliable initiation value of fracture toughness with a small scatter and that the average value of fracture toughness obtained from 4ENF test is about 2% higher than that obtained from the ENF test. The effect of friction in the 4ENF test is much lower than that in the ONF test in which the effect of friction increases with the crack growing. It is concluded that the 4ENF test method is an effective test method for the experimental evaluation of mode II propagation interlaminar fracture toughness of carbon fiber reinforced composites.     

Determining double-K fracture parameters of concrete for compact tension and wedge splitting tests using weight function

The paper presents use of universal form of weight functions for determining the double-K fracture parameters and on compact test and wedge splitting test specimens. The proposed method enables to obtain a closed form expression of cohesion toughness of concrete specimens. A comparison with existing analytical method shows that the weight function method for determination of double-K fracture parameters yields results without any appreciable error. Significant influence of initial notch to depth (a₀/D) ratio on the double-K fracture parameters is not also observed. Finally, a possible definition of brittleness of concrete using double-K fracture parameters is proposed.     

单丝断裂双树脂法研究碳纤维/环氧树脂界面粘结性能

建立了单丝断裂双树脂体系法,利用外层树脂的韧性使包埋于内层脆性树脂中的纤维单丝断裂达到饱和,解决了断裂伸长率较低的树脂基体采用传统的单丝断裂法无法测得界面剪切强度的问题.分别采用界面剪切强度和界面断裂能作为表征参量,考察了干态及湿热条件下两种T300级和两种T800级碳纤维/环氧树脂的界面性能,并与单丝断裂单树脂体系的界面性能进行比较.结果表明:单丝断裂双树脂体系与单树脂体系在表征碳纤维/环氧树脂的界面性能上定性规律一致;双树脂体系界面断裂能和界面剪切强度均可评价界面的耐湿热性能,且二者得到的变化规律一致;湿热处理后界面粘结性能均呈下降趋势,国外碳纤维体系的界面耐湿热性能明显优于国产碳纤维体系.     

单丝断裂双树脂法研究碳纤维/环氧树脂界面粘结性能

建立了单丝断裂双树脂体系法, 利用外层树脂的韧性使包埋于内层脆性树脂中的纤维单丝断裂达到饱和, 解决了断裂伸长率较低的树脂基体采用传统的单丝断裂法无法测得界面剪切强度的问题。分别采用界面剪切强度和界面断裂能作为表征参量, 考察了干态及湿热条件下两种T300级和两种T800级碳纤维/环氧树脂的界面性能, 并与单丝断裂单树脂体系的界面性能进行比较。结果表明: 单丝断裂双树脂体系与单树脂体系在表征碳纤维/环氧树脂的界面性能上定性规律一致; 双树脂体系界面断裂能和界面剪切强度均可评价界面的耐湿热性能, 且二者得到的变化规律一致; 湿热处理后界面粘结性能均呈下降趋势, 国外碳纤维体系的界面耐湿热性能明显优于国产碳纤维体系。     

Trend analysis of the power law process using Expectation-Maximization algorithm for data censored by inspection intervals

Trend analysis is a common statistical method used to investigate the operation and changes of a repairable system over time. This method takes historical failure data of a system or a group of similar systems and determines whether the recurrent failures exhibit an increasing or decreasing trend. Most trend analysis methods proposed in the literature assume that the failure times are known, so the failure data is statistically complete; however, in many situations, such as hidden failures, failure times are subject to censoring. In this paper we assume that the failure process of a group of similar independent repairable units follows a non-homogenous Poisson process with a power law intensity function. Moreover, the failure data are subject to left, interval and right censoring. The paper proposes using the likelihood ratio test to check for trends in the failure data. It uses the Expectation-Maximization (EM) algorithm to find the parameters, which maximize the data likelihood in the case of null and alternative hypotheses. A recursive procedure is used to solve the main technical problem of calculating the expected values in the Expectation step. The proposed method is applied to a hospital's maintenance data for trend analysis of the components of a general infusion pump.     

Cryogenic through-thickness tensile characterization of plain woven glass/epoxy composite laminates using cross specimens: Experimental test and finite element analysis

This paper investigates the through-thickness tensile behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. Tensile tests were carried out with cross specimens at room temperature and liquid nitrogen temperature (77 K), and the through-thickness elastic and strength properties of the woven GFRP laminates were evaluated. The failure characteristics of the woven GFRP laminates were also studied by optical and laser scanning microscopy observations. A three-dimensional finite element analysis was performed to calculate the stress distributions in the cross specimens, and the failure conditions of the specimens were examined. It is found that the cross specimen is suitable for the cryogenic through-thickness tensile characterization of laminated composite materials. In addition, the through-thickness Young's modulus of the woven GFRP composite laminates is dominated by the properties of the matrix polymer in the given temperature, while the tensile strength is characterized by both, the fiber to matrix interface energy and the cohesion energy of the matrix polymer.