Use of Crack Branching Data for Measuring Near-Surface Residual Stresses in Tempered Glass

An analytical procedure based on fracture mechanics is used to obtain the amount of residual stress in glass from measurements on the fracture surface. The technique utilizes the measurement of microcrack branching distances, known as the mirror — mist boundary, which occur at a critical crack branching stress intensity (K _m ) value. This procedurre shows that σ _A r _m ^1/2 Y _F (θ) =σ _R r ^1/2 _m +Ψ₀, where σ _A is the applied stress, r _m is the microcrack branching radius, σ _R is the residual stress, Y _F ( θ ) is the crack-border correction factor, and Ψ₀ is a material constant based on K _m . Thus, the equation is that of a straight line with the slope equal to the magnitude of the residual stress. Data for tempered glass from the literature are used to demonstrate the applicability of the technique.     

Adhesive Failure and Deformation Behaviour of Polymers

An instrument has been developed to determine the adhesive fracture energy as a function of the most important parameters such as temperature, contact time etc. and to study the stress-strain behaviour during bond separation. Additionally, the deformation processes during debonding were observed by high speed photography. Investigations of two high molecular weight polymers, polyisobutylene (PIB) and polyethylhexylacrylate (PEHA), showed two different types of bond separation: “brittle” behaviour with low adhesive failure energy for PIB and the formation and deformation of fibrillar structures for PEHA leading to much higher strains at break and adhesive failure energies. It follows from mechanical measurements that both polymers differ mainly by their entanglement networks. The much longer entanglement spacing for PEHA leads to the formation of fibrillar structures which, in accordance with a theory of Good, seem to be the reason for strong adhesion.     

Review of Mechanically Related Failures of Ceramic Capacitors and Capacitor Materials

This paper reviews the brittle fracture behavior of dielectric ceramics such as barium titanate, and describes some of the relationships between defects such as cracks and electrical degradation and failure of multilayer capacitors. Stresses arising from the ferroelectric phase transformation in these dielectric materials are shown to play a part as a driving force for crack growth. In addition, possible contributions to failure from stresses arising from thermal excursions in the capacitor are discussed. Low-voltage failures arising from a short between the electrodes in multilayer capacitors are shown to be related to the growth of cracks in the dielectric. A technique for predicting the onset of these types of failures based upon fracture mechanics techniques is described. Possible effects of the electric field itself in promoting or retarding the growth of cracks are discussed.     

Tensile properties and fatigue crack propagation of solution-annealed and aged superaustenitic stainless steel

The effect of aging in air at 650°C for 100~1000 h on the tensile properties of superaustenitic stainless steel in the range RT-750°C and the fatigue crack growth behaviour at RT and 650°C was studied. Yield strength and ultimate tensile strength were almost the same between the as-received and the aged specimen. The fracture strain, however, decreased significantly from aging, and the fracture surface of the aged specimen at RT test was intergranular. The fatigue crack growth rate at RT is enhanced by aging at the high stress intensity factor range. This is due to the occurrence of an intergranular fracture in the aged specimen. At 650°C the fatigue crack growth behaviour of both the as-received and the aged specimen was almost same with no intergranular fracture.     

Assessment of fracture characteristics from revised small punch test using pre-cracked specimen

Small punch (SP) test has been utilized to analyze the neutron irradiation damage of nuclear vessels. Since this technique is easy, simple, and nondestructive, it can be applied to evaluate the mechanical properties and material degradation of in-service components. Conventional SP test has evaluated the ductile-brittle transition temperature and the equivalent fracture strain by the interpretations of load-deflection curve and the change of specimen thickness, respectively. The assumption that fracture occurs at maximum load is, however, not reasonable because the crack initiates at smaller load. In this study, in order to evaluate quantitatively fracture characteristics based on fracture mechanics, the pre-crack is introduced to SP specimen and acoustic emission is used to determine the crack initiation point. Using the load at crack initiation point, the fracture toughness of thin plate is calculated through bending theory. Therefore, the fracture characteristics of thin plate can be evalualed more reliably by using revised SP test.     

用深冷处理消除Cr12MoV钢冲模热处理变形

研究了Ｃｒ１２ＭｏＶ钢冲模经不同工艺处理后的尺寸,冲击韧度和磨损速率,结果表明,Ｃｒ１２ＭｏＶ钢冲模热处理后的尺寸与残留奥氏体的量有关,深冷处理可控制Ｃｒ１２ＭｏＶ钢的尺寸,产可以提高Ｃｒ１２ＭｏＶ钢的冲击韧度,耐磨性能。深冷处理后,Ｃｒ１２ＭｏＶ钢冲击断口韧窝明显细化。     

合金元素对Laves相TiCr2力学性能的影响

研究了合金元素Ｎｂ,Ｍｏ,Ｖ和Ｎｉ对Ｌａｖｅｓ相ＴｉＣｒ２室温力学性能的影响,测量了抗压断裂强度、抗压断裂应变、显微硬度及断裂韧性。结果表明,这些元素都能改善ＴｉＣｒ２的室温力学性能,其中Ｎｂ的作用较小,而Ｖ和Ｎｉ的作用较大,明显地降低抗压断裂强度和显微硬度,提高断裂韧性     

New and Improved Tests for Adhesion

Three new methods are discussed for measuring the work G_a, required to detach unit area of an adhering material from a substrate. The first is a simple modification of the Outwater double-torsion test for long rectangular plates, bonded together. This method is suitable for evaluating aluminum-epoxy bonds, for example, or the transverse strength of fibrous composites. The second is a pull-off test for long strips adhering to a rigid surface. It seems suitable for adhesive tapes and laminates. The third is a reconsideration of the “blister” test for films and coatings, in which a circular debond at the interface is made to grow by internal pressure. The relation obtained between pull-off force F for a strip, or blow-off pressure P for a layer, takes the unusual form:

F⁴ (or P⁴) ∞ KG³_a

where K is the tensile stiffness of the detaching layer. This dependence arises from the non-linear (cubic) relation between load or pressure and deflection in these configurations. Nevertheless, the product Fθ, where θ is the angle of detachment of a strip, or Py, where y is the height of a “blister”, give direct measures of the strength of adhesion G_a, independent of the stiffness of the adhering material and of the extent of detachment.     

Cryo—Cracking方法及其在焊接接头蠕变断裂失效分析中的应用

介绍了Cryo-Cracking方法及其在分析和评定高温下和长期服役的焊管焊接接头蠕变失效中的应用特点,这种方法使用的试样易于截取,制样工艺过程简单,对制样者的技艺无特殊要求,不易产生因试样的制备而引起的误解,通过扫描电镜断口形貌的分析,结合金相定量分析方法,从分析断口的二维图象获得的数据,可以定量地反映在役构件蠕变失效的程度。实践表明,Cryo-Cracking方法是一种简单实用的焊接接头蠕变失效分析方法。