Theodore Nicholas

Tensile testing of materials at high rates of strain

A tension version of the split Hopkinson bar or Kolsky apparatus is developed for conducting tests in tension at high rates of strain up to 103 s−1. A number of aluminum, titanium, and steel alloys tested in tension show increasing degrees of rate sensitivity above 10 to 102 s−1. Tests on 6061-T651 and 7075-T6 aluminum show measurable strain-rate sensitivity in tension at the highest strain rates, although similar tests in compression in the literature show essentially no strain-rate sensitivity. Details of the apparatus and instrumentation and guidelines for its use are presented.

Critical issues in high cycle fatigue

High cycle fatigue (HCF) failures in materials used in rotating components of gas turbine engines have often been found to be attributable to fatigue loading on materials which have sustained damage from other sources. Damage can be present in the form of initial material or manufacturing defects, or can develop during service operation. Three major sources of in-service damage have been identified which can alter the HCF resistance individually or in conjunction with one another: low cycle fatigue (LCF), foreign object damage (FOD), and fretting. Methodologies for treating such damage in establishing material allowables are considered. Some recent results on the effects of damage on the Haigh (Goodman) diagram and a discussion of the life management aspects of HCF are presented.

Frequency and stress ratio effects in high cycle fatigue of Ti-6Al-4V

Experiments were conducted to determine the effects of test frequency on the high cycle fatigue (HCF) behavior of Ti-6Al-4V. Specimens were tested at a range of frequencies for three different stress ratios (0.1, 0.5, and 0.8). Results indicate a frequency dependence that varies with stress ratio (R). At low R, the fatigue strength increases with increasing frequency. As R is increased, this trend continues until a transition zone is reached. Above this transition the frequency effect seems to vanish at the lower frequencies, but it is still apparent at high frequency. In addition, the material exhibits ductile behavior above the transition point. The ductile behavior is characterized by void nucleation and growth and the apparent lack of fatigue crack initiation and growth that is evident at low stress ratios. Scanning electron microscopy is used to examine the failure mechanisms and transition regions. Several possible explanations are presented to explain the observed frequency and stress ratio effects. Strain accumulation at high R is observed and is attributed to cyclic strain ratchetting.

Fatigue crack nucleation and growth rate behavior of laser shock peened titanium

Abstract The fatigue nucleation and crack growth rate (FCGR) characteristics of Laser Shocked Peened (LSP) titanium 6Al–4V were examined and compared with unprocessed material. While crack initiation behavior of smooth (Kt=1.0) LSP processed samples is similar to baseline material, a marked improvement is observed in the notched (Kt>1) fatigue strength of LSP processed samples over unprocessed samples. Similar trends were noted for specimens that had been subjected to simulated foreign object damage (FOD) and subsequently fatigue tested. The FCGR resistance of LSP processed samples tested at low stress ratios (R) is likewise shown to be significantly greater than for unprocessed, baseline material and is due to the large residual compressive stresses, which reduce the locally applied stress. Advantages in FCGR resistance diminish at higher R. Differences in growth rate behavior are accounted for by using the effective stress intensity range, ΔKeff. The rationale of using ΔKeff is further demonstrated through fractographic investigations.

Validation of the step test method for generating Haigh diagrams for Ti–6Al–4V

Abstract Step testing provides a method for generating an endurance stress for a particular stress ratio with a single specimen as well as a method of generating endurance limits for one-of-a-kind specimens, e.g. specimens that may have been subjected to pre-test damage that cannot be accurately reproduced. The objective of this paper was to determine the statistical validity of the step test method for specimens excised from Ti–6Al–4V forgings. Room temperature endurance limits and constant-life Haigh (modified-Goodman) diagrams for smooth Ti–6Al–4V specimens generated by both the step method and conventional method (using S–N curves) are analyzed and statistically compared. Using lognormal-based statistical methods, the endurance stress at prescribed reliability limits and a confidence level (50%) is determined for the 10 7 cycle life. It appears that step testing yields results that are within the statistical limits of conventional S–N curve results and therefore is a valid method for generating endurance limits and therefore Haigh diagrams for Ti–6Al–4V specimens. Some questions are raised regarding the validity of the step method when a large number of steps are used.

Notch size effects in HCF behavior of Ti–6Al–4V☆

Abstract The high cycle fatigue (HCF) behavior of Ti–6Al–4V is investigated for cylindrical specimens having three sizes of geometrically similar circumferential V-notches, each with an elastic stress concentration factor, K t , of approximately 2.78. A step loading technique for obtaining a point on a constant life (10 6 cycles) Haigh diagram from a single test specimen was implemented. Tests were performed at stress ratios of R =0.1, 0.5, and 0.8 for specimens machined from two different product forms of Ti–6Al–4V, bar and plate forgings. Results indicate that while there is a definite notch size effect in the Ti–6Al–4V bar within the range of notch sizes tested, little or no such effect exists in the plate.

Strain-rate and strain-rate-history effects in several metals in torsion

A machine for testing thin-walled tubes in torsion at shear-strain rates up to 25/sec is described. Results of constant and variable-strain-rate tests are presented for 1100-0 aluminum, AISI 1020 steel, and 50-A titanium. Results indicate that 1100-0 aluminum is very slightly strain-rate sensitive, but steel and titanium are noticeably sensitive to both strain rate and strain-rate history. Variable-rate tests show that subsequent dynamic loading on a statically prestrained specimen causes an increase in the flow stress in steel and a decrease in the flow stress in titanium.

Effect of various surface conditions on fretting fatigue behavior of Ti–6Al–4V

Abstract An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 107 cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements. An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ~0.3 for the Cu–Ni coated pad against an as-received specimen and ~0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.

Impact damage on titanium leading edges from small hard objects

Impact damage on titanium leading-edge configurations was investigated through a series of hard-particle impact tests. Fatigue tests were used to assess the severity of damage in terms of an equivalent elastic-stress-concentration factor. The concept of geometric scaling was investigated by using various leading-edge thicknesses and projectile sizes in the impact tests. Observations of the type of damage and the use of a critical velocity concept tended to validate the scaling concepts.

Cumulative-damage modeling of fatigue crack growth in turbine engine materials

Abstract Life predictions of turbine engine structural components utilize fracture mechanics principles to determine fatigue crack growth rates. Fatigue cracks grow under conditions of variable temperature, frequency, hold time, stress ratio and stress level. At elevated temperatures, time-dependent material behavior can play a significant role in the material behavior. Cumulative-damage models must account for all these variables as well as interaction effects. The earliest modeling involved interaction schemes and, primarily, time-independent material behavior. More recent work has focused on time-dependence and creep-fatigue interaction effects. A review of current modeling concepts is presented.