Kang Hai Tan

Microstructure and intermetallic growth effects on shear and fatigue strength of solder joints subjected to thermal cycling aging

Abstract Microstructure development of eutectic solder alloy (63Sn/37Pb) after thermal cycling aging and its impact on the shear and fatigue failure of the solder joint has been investigated. The solder microstructure changes with the reflow process and subsequent thermal cycling environments in solder joint reliability tests from −40 to 125°C. In service, solder joints are subjected to thermal cycling aging, corresponding to power on–off cycling of the electronic equipment or cyclic environmental temperature loading, leading to thermal fatigue failures. Thus, it is important to study the effect of the microstructure changes, mechanical strength and fatigue resistance of solder before and after thermal cycling aging. A new specimen design has been developed to closely resemble the actual electronic packaging assembly condition. The joint is made simply by soldering a solder ball between two FR-4 substrates with copper pads using the reflow process. The study shows that the solder microstructure coarsened and intermetallic compound layers grew after 500, 1000 and 2000 thermal cycles. The shear and fatigue strength of the solder joint decreased with increased exposure to thermal cycling aging effects.

Robustness of Bolted-Angle Connections against Progressive Collapse: Experimental Tests of Beam-Column Joints and Development of Component-Based Models

AbstractSeveral structural collapse incidents indicate that failure usually starts from beam-column joints exposed to abnormal loads, especially for steel and composite structures. If the connections are sufficiently robust and there is adequate axial restraint from adjoining structures, catenary action forms in the beams and slabs, causing alternate load paths when affected columns are severely damaged, and resulting in large deformations in the beams and slabs. This paper presents experimental results of bolted-angle beam-column joints under a middle column–removal scenario. Three types of connections—including (1) web cleat, (2) top and seat angle, and (3) top and seat with web angle connections—were investigated, and three angle thicknesses (8, 10, and 12 mm) were tested. The results of the nine experimental tests conducted demonstrate the ductility and load-carrying capacities of these three connection types with different angle thicknesses in catenary action mode. When the angle thickness increases,...

Structural Behavior of CHS T-Joints Subjected to Brace Axial Compression in Fire Conditions

AbstractThe structural behavior of circular hollow section (CHS) T-joints subjected to axial brace compression in fire conditions was investigated. Five full-scale tubular joints with different brace-to-chord diameter ratios were tested under elevated temperature. The tests were in isothermal heating conditions, where the specimens were heated to the desired temperatures and then subjected to static load to failure. The ultimate strength and failure modes of these joints were investigated. It was observed that both the reduction in material strength and changes in localized plastification area beneath the brace decreased the ultimate strength of the joints as temperature increased. Furthermore, local buckling and ovalisation of the chords were found to be more concentrated around the joint region at elevated temperature. To the authors’ best knowledge, these tests were among the first reported experimental investigations in the ultimate strength and failure mechanisms of tubular joints at elevated tempera...

Rankine approach for fire resistance of axially-and-flexurally restrained steel columns

Traditionally, the Rankine equation has been applied to investigate the load bearing capacities of steel columns or frames at room temperature. Recently, the method has been extended to predict the fire resistance of a simply supported steel column by taking the temperature effects on steel material into consideration. This paper further extends the Rankine equation to predict the fire resistance of an isolated steel column from the surrounding structure. The complicated boundary restraints contributed by the adjoining structure are represented by a linear spring and a pair of rotational springs attached to the column ends. Both the boundary restraints and creep strain are incorporated into the Rankine equation. The predictions of Rankine approach are verified experimentally and numerically. Generally, it is shown that under different external load utilization factors and boundary restraint levels, Rankine approach yields accurate and slightly conservative predictions.

Dynamic Performance of Flush End-Plate Beam-Column Connections and Design Applications in Progressive Collapse

An experimental program was conducted in this study to investigate the dynamic behavior of flush end plate steel beam-column cruciform connections following instantaneous removal of a central supporting column. The dynamic tests were performed by using a quick-release mechanism to simulate sudden column-removal scenarios. In the test program, the geometries of the flush end-plate connections remained unchanged but subjected to five different magnitudes of uniformly distributed load. Another five quasi-static tests were also carried out for comparison with the dynamic response and investigation of the dynamic increase factors (DIF). Test results showed that the release-time durations of the column support force were approximately 30 ms for all five dynamic tests. For the same initial applied dead loads, the maximum dynamic displacement was significantly increased compared to the corresponding quasi-static displacement. In order to observe the full dynamic performance of the tested connection until final failure, a general-purpose finite-element software was used to conduct the three-dimensional numerical simulations. Comparison results showed that the numerical model could accurately simulate the monotonic quasi-static test results and the dynamic test results under sudden column removal. Subsequently, a series of parametric studies was conducted to investigate the connection behavior considering the load-release time, different loading methods, and the magnitude of uniform distributed load. By comparing the full dynamic response of the flush end-plate connection with its corresponding quasi-static response, both the displacement-based DIF and force-based DIF could be respectively determined. The paper also addresses which DIF is more appropriate for the design of structures to resist progressive collapse.

Interaction Formula for Reinforced Concrete Columns in Fire Conditions

This paper presents a simple and rational interaction formula -- the Rankine method -- for predicting the fire resistance of reinforced concrete (RC) columns under fire conditions. Conventionally, the Rankine method has been applied to RC columns and frames subjected to increasing loads but maintained at ambient temperature. Based on a theoretical approach, the Rankine method is extended to RC columns subjected to fire conditions by taking the temperature effects on steel reinforcement and concrete material into consideration. The accuracy of the method is verified through four sets of experimental test results comprising 76 columns. The method yields reasonably accurate predictions for the fire resistance of RC columns. In addition, the paper includes two worked examples to illustrate the flexible use of such a formula for either predicting the load-bearing capacity or the fire exposure time of an RC column subjected to the standard ISO 834 fire curve.

PERFORMANCE OF STRENGTHENED CONCRETE DEEP BEAMS PREDAMAGED IN SHEAR

Finding suitable methods for strengthening or repairing concrete members is essential as the frequency of strengthening and repair work has increased tremendously. This paper describes an experiment in which 18 prestressed and non-prestressed concrete deep beams were tested to failure, strengthened, and then retested to failure for a second time. On the occurrence of the first (shear) failures, the failed shear spans of the beams were strengthened by using steel clamping units that acted as external stirrups. By using the concept of the strut-and-tie approach it can be shown that on retesting, the clamped or strengthened shear span would receive less severe loads and thus would seem to become stronger. This is due to changes in the load transfer mechanism and in the direction of the compression strut that occur in the failed shear span following the introduction of a clamping unit. Fourteen of the externally strengthened deep beams, when tested for the second time, failed at higher failure loads than the first failure loads. This indicates that the full capacities of the damaged deep beams can be restored to the fullest following a strengthening technique using simple clamping units. Comparisons with the American Concrete Institute (ACI) Code and other prediction methods show that the strengthened deep beams failed at conservative ultimate loads.

SIZE EFFECT IN LARGE PRESTRESSED CONCRETE DEEP BEAMS

This paper presents an experimental investigation on the size effect in large prestressed-concrete deep beams. Twelve specimens with a total main-steel-and-strand ratio maintained close to 2.50% were tested to failure. Two parameters were studied: beam height (h) ranging from 500 mm to 1750 mm, and shear-span-to-height ratio (a/h) varying between 0.50 and 1.00. All of the beams had cylinder strengths of approximately 40 MPa. Web reinforcement was omitted to study the effect of beam size on concrete shear strength. In comparison to reinforced concrete deep beams, it is evident that prestressing improves the diagonal cracking and serviceability strengths and generally enhances ultimate shear capacity if bearing failure can be prevented. Test results reveal that the ultimate shear stress is size-dependent, but the diagonal cracking stress is not. With increasing h, prestressed deep beams tend to be more brittle, even though the brittleness is somewhat offset by the location and degree of prestressing. The 12 test results are then compared with predictions based on the American Concrete Institute (ACI) code, the Canadian Standards Association (CSA) code, and the United Kingdoms Construction Industry Research and Information Association (CIRIA). The CSA predictions provide a uniform safety margin for large- and medium-sized beams, but conservatism in both the ACI and CIRIA predictions reduces with increasing h and a/h ratio. An explanation is also given for the uniform safety margin predicted by the CSA code.

Primary creep buckling of steel columns in fire

Abstract This paper proposes an analytical method to predict the fire resistance of a pinned-pinned steel column. Traditionally, the column fire resistance is determined through the standard ISO834 fire test. Under this condition, the external load is kept constant while the member temperature rises monotonically. The complicated creep strain, as well as the degradation of steel mechanical properties is taken into account in this approach. The predictions are verified experimentally and numerically. The extensive numerical verifications show that under different external load levels and different slenderness ratios, the proposed analytical method yields accurate predictions. The procedure can be incorporated into spreadsheet application software.

Horizontal Shear Strength of Indirectly Loaded Composite Concrete Beams

Four composite concrete beams with cylinder compressive strength of approximately 40 MPa were tested to failure under indirect two-point loading. The first beam, CB1, did not have any ties, and it served as a control specimen. The second and third beams, CB2 and CB3, had 0.11% tie content. The main difference is in the shape of ties; ties in CB2 consisted of individual legs, while CB3 had closed ties. Tie content in CB4 was doubled to 0.22%. The first three beams failed in horizontal shear mode, while the fourth beam failed in flexure. Test results clearly showed that, under indirect loading, the ultimate horizontal shear strength of composite beams could be significantly lower than the American Concrete Institute Code predictions; the specified minimum clamping stress of 0.35 MPa appeared to be unsuitable for indirectly loaded beams. The study also demonstrated that the shape of ties does not have significant influence on the horizontal shear strength, provided there is adequate anchorage of ties on both sides of the interface.