Yves Verreman

Monitoring short fatigue cracks with miniature strain gages

An experimental technique to monitor the length and the opening level of a short fatigue crack is presented. It is based on the progressive decrease with crack length of the response of miniature strain gages installed on the surface near the crack plane. A first gage installed close to the crack plane can monitor cracks from 10 μm in depth to half a millimeter where the response saturates. Other gages at larger distances from the crack plane are less sensitive but can monitor longer cracks. The response is measured so that it is independent of strain-gage calibration, Youngs modulus and Poissons ratio. The paper first presents the basic principles and possibilities of the technique as well as a finite-element analysis performed on automatic welded joints with straight-fronted cracks for which the technique has been developed. The results give a correlation between gage response, crack length and gage location and the conditions of replacement of a gage reaching saturation. The practical exploitation of the technique has required further work to derive a continuous calibration of the gage response that includes corrections to account for the gage finite dimensions and the crack-plane inclination. This calibration is shown to give crack lengths that compare well with fractographic marks and typical results that have been obtained on short crack growth at the weld toe are presented. In particular, the resolution of the technique is put into evidence with results on the initial growth of a 0.1 mm nonpropagating crack. The paper finally points out the distinctive features that appear in current works to adapt the technique to the growth of semi-elliptical cracks of low and high aspect ratio.

Fatigue Behavior of Semi-Solid Cast Aluminum: A Critical Review

Aluminum alloys are increasingly used in automotive and aeronautic applications to produce high performance, lightweight parts. Among the reasons for this, is the emergence of high integrity processes (HIP), which widens the field of application for cast aluminum alloys. In fact, metallurgical quality and consistency that characterize components produced by HIP are necessary for critical safety components. In addition to attaining maximum strength, critical safety components need to be ductile and resistant to cyclic loading. According to the North American Die Casting Association, rheocasting is a high integrity process capable of producing parts with fewer defects than conventional casting process. Rheocast components are known to have better mechanical properties than permanent mold castings. Moreover, they can be heat-treated which is impractical in the case of classical die cast components. However, the fatigue behavior of rheocast aluminum alloys has been investigated since about 2000 and few results have been published on this subject. This paper reviews the studies of fatigue behavior of aluminum semi-solid cast components. Published experimental results on high cycle fatigue resistance (S-N diagrams), long crack propagation, crack closure effects and short crack particularities are presented.

Influence of Welding Residual Stresses on the Closure and Propagation Behavior of Fatigue Cracks Growing at a Fillet Weld Toe

Fatigue tests were performed at three R-ratios (-1, -2 and 0) and different stress amplitudes on cruciform welded joints both in the as-welded and stress relieved conditions. A sensitive system using micro-strain gages installed close to the weld toe allowed monitoring of the crack length and opening level from 10 - 20 ptm. The effect of residual stresses on crack propagation and closure behavior is described for different loading conditions. The experimental data confirm the predictions that can be derived from an elastic analysis based on the crack closure concept, such as a more pronounced effect under partially compressive loading but reaching a saturation below a certain value of R. Further, they give a direct evaluation of the relative residual stress intensity Kr/Kmax and of its progressive decrease as the crack grows, as well as a better understanding of plasticity effects.

Assessment of cold cracking tests for low transformation temperature martensitic stainless steel multipass welds

In the last decades, low-carbon soft‐martensitic stainless steels have been widely used by heavy industries such as hydraulic turbine and pipeline manufacturing. Their good corrosion resistance, high mechanical strength, and good cavitation erosion resistance combined with their relatively good weldability explain this success. However, arc welding operations of thick plates still require significant preheating to avoid cold cracking. From a practical point of view, this requirement can complicate manufacturing operations and limit the use of these alloys for some applications. It thus becomes important to assess their cold cracking susceptibility in order to reduce preheating temperature. The discrepancy between industrial observations in multipass welds and the results obtained from the standardized cold cracking test such as Tekken and GBOP (gapped bead-on-plate) tests led to a modification of the GBOP test as presented here. In order to reproduce the behavior of industrial cold cracks, a comparison between Tekken and GBOP tests was set up for the assessment of the cold cracking susceptibility of multipass 13%Cr‐4%Ni soft martensitic stainless steels welded with 410NiMo filler alloy. A two‐bead GBOP test was proposed. This modification is necessary because the low martensitic transformation temperature of the 13%Cr‐4%Ni stainless steels reverses the residual stress in the weld, preventing the possibility of cold cracking. A second bead is needed to induce tensile stress in the first bead so that crack may be generated as in industrial conditions. The relevance of these modifications is presented and discussed using contour method residual stress measurements and acoustic emission crack detection.

Growth Rates and Closure Development of Short Fatigue Cracks at Notches and at Weld Toes

On the basis of a literature survey and of the author’s own investigations, the paper describes different aspects of short crack behaviour that have been experimentally observed, as well as different explanations or interpretations that have been made. «Microstructurally short cracks» present irregular growth rates; their interaction with grain boundaries determines the fatigue resistance for smooth surfaces and smooth notches. Beyond the microstructural scale, LEFM is not applicable in the «mechanically short crack » regime over a distance equal or comparable to the size of the notch plastic zone. After rapid initial growth, cracks decelerate at severe notches and at fillet welds; they become non-propagating at low stress.